Module 9 How Do We Sense, Perceive, and See the World? Flashcards
(190 cards)
1
Q
Migraines
A
- They were caused by the dilation of cerebral blood vessels that occurs during an aura
- Usually vary in severity, frequency, and duration (left untreated, some may last for hours or even days) and are often accompanied by nausea and vomiting
- Most common of all neurological disorders, affecting some 5 to 20% of the pi=opulation at some time in their lives
2
Q
Auras
A
-May be auditory, tactile, or visual, and they may result in an inability to move or talk
3
Q
Scotoma
A
-A small blind spot
4
Q
Blindsight
A
- When a light blinked and where it appeared
- Which the brain knew more that they are aware of consciously
5
Q
Selective Awareness
A
-An important working principle behind human sensation and perception
6
Q
Sensory Receptors Neurons
A
- Are specialized to transduce (convert) environmental energy-light
- Are designed to respond only to a narrow band of energy analogous to particles of certain sizes-such as specific wavelengths of electromagnetic energy that form the basis of our vision
7
Q
Each sensory system’s receptors are specialized to filter a different form of energy
A
-For vision
~The photoreceptors in the retina convert light energy into chemical energy, which is, in turn, converted into action potentials
-The auditory system
~Air pressure waves are first converted into mechanical energy, which activates the auditory receptors that produce action potentials in auditory receptor neurons
-The somatosensory system
~mechanical energy activates receptors sensitive to touch, pressure, or pain; these receptors, in turn, generate action potentials in somatosensory neurons
-Taste and olfaction
~Various chemical molecules in the air or in food fit themselves into receptors of various shapes to activate action potentials in the respective receptor neurons
8
Q
Receptive Field
A
- Region of sensory space (example, skin surface) in which a stimulus modifies a receptor’s activity
- Not only to identify sensory information but also to contrast the information each receptor field is providing
- Not only sample sensory information but also help locate events in space, because adjacent receptive fields may overlap, the contrast between their responses to event help us localize sensations
- The spatial dimension of sensory information produces cortical patterns and maps that form each person’s sensory reality
9
Q
Photoreceptor Cells
A
- About 120 million
- The eye points in a slightly different direction and so has a unique receptor field
10
Q
Visual Receptors
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-Are more numerous in the center of our visual field than toward the edges
11
Q
Density of Receptors
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- Is related to sensory sensitivity
- Our sensory systems used different types of receptors to enhance our perceptual experience
12
Q
Color Photoreceptors
A
- Are small and densely packed to make sensitive color discriminations in bright light
- receptors for black-white vision are larger and more scattered, but their sensitivity to light-say, a lighted match as a distance of 2 miles on a dark night-is truly remarkable
13
Q
Receptors Connect
A
- To the cortex through a sequence of intervening neurons
- The number of these neural relays varies across different sensory systems
14
Q
Sensory Information
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-Is modified at each stage in the relay, allowing each region to construct different aspects of the sensory experience
15
Q
Visual system
A
-Each of our eyes has a separate view of the world; the information from the two views is combined in the thalamus such that the input from the left side and right side of each field is superimposed to produce two visual fields one from the left and one from the right
-The brain begins to separate different aspects if the visual input such as shape and color
~Also a second visual pathway that goes from the retina to the superior colliculus and then to the thalamus and cortex
*This pathway is involved in the perception of movement
16
Q
Interaction Effect is Potent
A
-It highlights the fact that a speaker’s facial gestures influence our perception of speech sounds
-Synchrony of gestures and sounds is an important aspect of language acquisition
~The difficulty of learning a foreign language can relate to the difficulty of blending a speaker’s articulation movements with the sounds the speaker produce
17
Q
All information from all sensory systems is encoded
A
- Action potentials that travel along nerves until they enter the spinal cord or brain
- From there action potentials that travel on nerve tracts within the CNS; every bundle carries the same kind of signal
18
Q
Presence of Stimulus
A
-Can be encoded by an increase or decrease in a neuron’s discharge rate, and the amount of increase or decrease can encode stimulus intensity
19
Q
Qualitave visual chagnes
A
-Such as from red to green, can be encoded by activity in different neurons or even by different levels of discharge in the same neuron
20
Q
Synesthesia
A
- This mixing of the senses
- Some people hear in color or identify smells by how the smells sound to them
- Anyone who shivers when hearing a piece of music or cringed at the noise fingernails make when scraping a blackboard has “felt” sound
21
Q
Topographic Map
A
-Spatially organized neural representation of the external world
-A neural-spatial representation of the body or of the areas of the sensory world perceived by a sensory organ
~All mammals have at least one primary cortical area for each sensory system areas are usually referred to as secondary because most of the information that reaches these areas is relayed through the primary area
-Each additional representation is probably dedicated to encoding one specific aspect of the sensory modality
22
Q
Sensation
A
- Registration by the sensory organs pf physical stimuli from the environment
- Is far more than the simple registration of physical stimuli from the environment by the sensory organs
- Our sensory impressions are affected by the cortex in which they take place, by our emotional state
23
Q
Perception
A
- Subjective interpretation of sensations by the brain
- How we interpret what we sense
- Is more than sensation lies in the fact that different people transform the same sensory stimulation into totally different perceptions
24
Q
Retina
A
- Light-sensitive surface at the back of the eye consisting of neurons and photoreceptors
- Light energy travels from the outside world through the pupil and into the eye, where it strikes a light-sensitive surface
- Unevenly distributed between cones and rods
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Photoreceptors
-Specialized retinal neuron that transduces light into neural activity
-Stimulation of cells on the retina, we begin to construct a visual world
-The neurons lie in front of the photoreceptors beneath a layer of neurons connected to them; the neurons lie in front of the photoreceptors because they do not prevent incoming light from being absorbed by those receptors because the neurons are transparent and the photoreceptors are extremely sensitive to light
-The photoreceptors and the retinal neurons perform some amazing functions
~translate light into action potentials, discriminate wavelengths so that we can distinguish colors, and work in a range of light intensities from bright to dim; these cells afford visual precision sufficient for us to see a human hair lying on the page of this book from a distance of 18 inches
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As in a camera
-The image of objects project onto the retina is upside down and backward
-This flip-flopped orientation poses no problem for the brain
~Remember that the brain is constructing the outside world, so it does not really care how the image is oriented initially
-The brain can make adjustments regardless of the orientation of the images that it receives
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For several days
- You were to wear your glasses that invert visual images, the world would first appear upside down but then would suddenly appear right side up again because your brain would correct the distortion
- Upon removing the glasses, the world would temporarily seem upside down once again because your brain at first would be unaware that you had tricked it again; eventually, your brain would solve this puzzle, and the world would flip back to the correct orientation
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Periphery
-Letters at the periphery must be much larger than those in the center for us to see them well
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Fovea
-Central region of the retina is specialized for high visual acuity; its receptive fields are at the center of the eye's visual field
-The difference is partly due to the fact that photoreceptors are more densely packed at the center of the retina
-This depression is formed because many optic nerve fibers skirt the fovea to facilitate light access to its receptors
-Only has cones, but their density drops dramatically outside this area
~For this reason , our vision is not so sharp at the edges of the visual field
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Blind Spot
- The retinal region where axons forming the optic nerve leave the eye and where blood vessels enter and leave; has no photoreceptors and is thus said to be blind
- A small area of the retina known as the optic disc
- This is the area where blood vessels enter and exit the eye and where fibers leading from retinal neurons from the optic nerve, which goes to the brain
- Therefore are no photoreceptors in this part of the retina
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Visual System solves the blind spot problem
-Your optic disc is in a different location in each eye; the optic disc is lateral to the fovea in the left eye and to the right of the fovea in the right eye; because the two eyes' visual fields overlap, the right eye can see the left eye's blind spot and vice versa
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Blind Spot is important in neurology
- Is allows neurologists to indirectly view the condition of the optic nerve while providing a window on events in the brain
- If intracranial pressure increases, as occurs with a tumor or brain abscess (an infection), the optic disc swells, leading to papilledema (swollen disc)
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Papilledema
-the swelling occurs in part because, like all other neural tissue, the optic nerve is surrounded by cerebrospinal fluid (CSF)
-Pressure inside the cranium can displace CSF around the optic nerve, causing swelling at the optic nerve
-Is inflammation of the optic nerve itself, a condition known as optic neuritis
-Whatever the cause, a person with a swollen optic disc usually loses vision due to pressure on the optic nerve
~If the swelling is a result of optic neuritis, the prognosis for recovery is good
34
Rods
- Photoreceptors specialized for functioning at low light levels
- Are longer than cones and cylindrical at one end
- Are more numerous than cones; are sensitive to low levels of brightness (luminance), especially in dim light; and function mainly for night vision
- All rods have the same pigment
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Cones
-Photoreceptors specialized for color and high visual acuity
-Have a tapered end
-Do not respond to dim light, but they are highly responsive to bright lights
-Mediate both color vision and our ability to see fine detail (visual activity)
-Three pigments in the cones
~Absorb light across a range of visible frequencies, but each is most responsive to a small range of wavelenghts- short (bluish light), medium (greenish light), and long (redish light)
-That you are looking at the lights with all three of the cone tyoes and that each cone pigment responds to light across a range of frequencies, not just to its frequency of maximum absorption
-Both the present of three cone receptor types and their relative numbers and distribution across the retina contribute to our perception of color
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Visual Illuminance
-The eye works correctly only when sufficient light passes through the lens and is focused on the receptor surface- the retina of the eye or the light-sensitive surface in the camera
-Too little light entering the eye produces a problem of visual illuminance, in which objects are too dim and it is hard to see any image at all
-The reason objects appear blurry in low illuminance is likely that we are mostly using rods, which provide a less sharp image
-Is typically a complication of aging eyes; it cannot be cured by corrective lenses
-As we age, the eye's lens and cornea allow less light through, so less strikes the retina
-Estimated that between ages 20 and 40, people's ability to see in dim light drops by 50%-and over each additional 20 years, by a further 50%
~Seeing in dim light becomes increasingly difficult, especially at night
-The only way to compensate for visual illuminance is to increase light
-Statistics show a marked drop in the number of people who drive at night in each successive decade after age 40
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The Three Cone Types
-Are distributed more or less randomly across the retina, making our ability to perceive different colors fairly constant across the visual field
-The number of red and green cones are approximately equal, but blue cones are fewer
~As a result, we are not as sensitive to wavelengths in the blue part of the visible spectrum as we are, to red and green wavelengths
38
The Differences in these two red coned
-Appear minuscule, but it does make a functional difference in some human females' color perception
-The gene for the red cone is carried on the X chromosome
~Males have only one X chromosome, so they have only one of these genes and only one type of red cone
-The situation is more complicated for females, who possess two X chromosomes; although most women have only one type of red cone, those who have both are more sensitive than the rest of us to color differences at the red end of the spectrum
-Could say that women who have both red cone types have a slightly rosier view of the world: their color receptors construct a world with a richer range of red experience, but they also have to contend with seemingly peculiar color coordination by other
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Photoreceptors
-Are connected by two layers of retinal neurons
-The first layer contains three cell types
~Bipolar
~Horizonal
~Amacrine
-Horizontal cells link photoreceptors to bipolar cells
-Amacrine cells link bipolar cells with cells in the second neural layer
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Retinal Ganglion Cell (RGC)
-One of the groups of retinal neurons with axons that give rise to the option nerve
-Axons collect in a bundle at the optic disc and leave the eye to form the optic nerve
-Are especially sensitive to increased intraocular pressure, which can lead to blindness
-A tiny subset about 1% contain melanopsin, a light-sensitive protein, and thus form a third type of photoreceptor in the eye
~These photoreceptors function to synchronize circadian rhythms, regulate pupil size, and regulate melatonin release
-Form the optic nerve, the road into the brain
~Road forks off to several places; destinations of these branches give us clues to what the brain is doing with visual input and how the brain constructs our visual world
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Retinal Ganglion Cells Two Major Categories
- Magnocellular cells (M cells)
| - Parvocellular cells (P cells)
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M cells
- Large visual system neuron sensitive to moving stimuli
- Receive their input primarily from rods and so are sensitive to light but not color
- Found throughout the retina, including the periphery, where we are sensitive to movement but not to the color of fine detail
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P cells
- Small visual system neuron sensitive to differences in form and color
- Receive their input primarily from cones and so are sensitive to color
- Found largely in the region of the fovea, where we are sensitive to color and fine details
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Muller Cells
-That span from the retina's inner membrane at the front to the photoreceptors at the back of the retina and act as optical fibers, channeling light to the buried photoreceptors
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Glaucoma
-An eye disease that destroys the optic nerve is the most common cause of irreversible blindness and a prime target for research to restore vision
-The optic nerve begins with the axons of the retinal ganglion cells (RGCs), and if they are dead or dysfunctional, vision is impossible
-Clinical goal is to repair or replace RGCs after injury
-Challenge is that RGC axons do not spontaneously regenerate after injury, and the RGC die, they are not replaced
-One strategy for restoring vision is to stimulate RGCs to regenerate axons
~Huberman used both genetic and visual stimulation to enhance neural activity in the RGC of mice with severed RGC axons
~mTOR Signaling Pathways
-The second strategy for vision restoration is to replace RGCs by transplanting health RGCs from recently deceased donors
~Studies in rats have shown that transplanted RGCs thrive, respond to light signals, and extend axons into the brain to reach usual targets
*Although this is not ready to head to the clinic, it appears to offer a clinically viable strategy for curing blindness related to RGC death
-Not all blindness originating in the eye is related directly to lost RGC
~If photoreceptors are dysfunctioning or dying, as happens in retinitis pigmentosa (RP), blindness will occur
*To implant prosthetic devices into the eye to convert light to electrical signals and then pass to into RGCs
*To introduce light-senstive ion-gated channels to repair the receptors
*Based on independent parallel work by two different research groups on mouse models of RP, which experiments used CRISPR to reprogram genes expressed in rods, leading to an increase in conelike cells, with the restoration of visual function
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mTOR Signaling Pathways
-While at the same time repeatedly exposing the eye to high-contrast black-and-white images; this procedure stimulated the RGC to re-establish many of the lost connections to their correct target and allow a partial restoration of vision
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Optic Chiasm
- Junction of the optic nerves, one from each eye, at which the axons from the nasal halves of the retinas cross the brain's opposite sides
- About half the fibers in each eye cross in such a way that the left half of each optic nerve goes to the left side of the brain, and the right half goes to the brain's right side
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Nasal Retina
-Middle path crosses the opposite side
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Temporal Retina
-Lateral path travels straight back on the same side
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Vision Field
- Light falls on the right half of each retina actually comes from the left side of the visual field
- Information from the left visual field goes to the brain' right hemisphere
- Information from the right visual field goes to the left hemisphere
- Half of each retina's visual field is represented on each side of the brain
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Geniculostriate System
-Projections from the retina to the lateral geniculate nucleus to the visual cortex
-All of the ganglion and some of the M ganglion axons form a pathway
~This pathway goes from the retina to the lateral geniculate nucleus (LGN) of the thalamus and then to layer IV of the primary visual cortex in the occipital lobe
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Striate Cortex
-Primary visual cortex (V1) in the occipital lobe; shows stripes (striations) on staining
-The primary visual cortex shows a broad strip across it in layer IV
-The geniculostriate system, therefore, bridges the thalamus (geniculate) and the striate cortex
~The striate cortex, the axon pathway divides
*One route goes to vision-related regions of the parietal lobe
*The other route goes to vision-related regions of the temporal lobe
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Tectopulvinar System
-Projections from the retina to the superior colliculus to the pulvinar (thalamus) to the parietal and temporal visual areas
-Second pathway leading from the eyes is formed by the axons of the remaining M ganglion cells
~These cells send their axons to the midbrain's superior colliculus, which send connections to the pulvinar region of the thalamus
-It runs from the eye through the midbrain tectum to the pulvinar
~The pulvinar sends connections to the parietal and temporal lobes, bypassing the occipital visual area
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Retinohypothalamic Tract
- Neural route formed by axons of photosensitive retinal ganglion cells (pRGCs) from the retina to the suprachiasmatic nucleus; allows light to entrain the SCN' rhythmic activity
- Between 1 and 3% of RGCs are unique in that they are photosensitive: they act as photoreceptors
- pRGCs, contain the pigment melanopsin, absorb blue light at a wavelength different from the wavelengths of rods or cones
- Axons of pRGCs form a small third visual pathway
- Retinohypthalamic tract synapses in the tiny suprachiasmatic nucleus (SCN) in the hypothalamus, next to the optic chiasm
- Photosensitive RGCs participate both in regulating circadian rhythms and in the pupillary reflex that expands and contracts the pupil in response to the amount of light falling on the retina
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Two Distinct Visual Pathways that Originate in the Striate Cortex
- Ventral Stream
| - Dorsal Stream
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Ventral Stream
- Visual processing pathway from V1 to the temporal lobe for object identification and perceiving related movements
- Pathway to the temporal lobe
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Dorsal Stream
- Visual processing pathway from V1 to the parietal lobe; guides movements relative to objects
- Pathway to the parietal lobe
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Geniculostriate Pathway
-RGC fibers from the two eyes distribute their connections to the two lateral geniculate nuclei (left and right) of the thalamus
-The fibers from the left half of each retina go to the left LGN; those from the right half of each retina go to the right LGN
~Fibers from each eye do not go to exactly the same LGN location
-Each LGN has six layers, and the projections from the two eyes go to different layers
~Layers 2,3, and 5 receive fibers from the ipsilateral eye (eye on the same side)
~Layers 1,4, and 6 receive fibers from the contralateral eye (eye on the opposite side)
*This arrangement provides for combining the information from the Pand M ganglion cells
-Axons from the P cells are responsive to color and fine details, LGN layers 3 through 6 must be processing information about color and form
-M cells mostly process information about movement, so layers 1 and 2 must deal with movement
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Visual Cortex
-Where the LGN cells from the thalamus send their connection
-Layer IV is the main afferent (incoming) layer of the cortex
~Layer Iv has several sublayers, two of which are known as IVSa and IVCb
*LGN layers 1 and 2 go to IVCa
*LGN layers 3 through 6 go to IVCb
-A distinction between P and M functions thus continues in the striate cortex
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Cortical Columns
- Anatomic organization that represents a functional unit six cortical layers deen and approximately 0.5 mm square, perpendicular to the cortical surface
- The input from the ipsilaterally and contralateral connected parts of the LGN go to adjacent strips of the occipital cortex; these trips, which are about 0.5mm across
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Primary Visual Cortex (V1)
- Striate cortex in the occipital love that receives input from the lateral geniculate nucleus
- Is its striations-its distinctly visible layers
- Wong-Riley stained region VI for the enzyme cytochrome oxidase, which has a role in cell metabolism, they found an unexpected heterogeneity
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Extrastriate (Secondary visual) Cortex (V2-V5)
- Visual cortical areas in the occipital lobe outside the striate cortex
- With each region processing specific features f visual information
- Each occipital region has a unique cytoarchitecture (cellular structure) and unique inputs and outputs, we can infer that each must be doing something different from the others
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Blob
- Region in V1 that contains color-sensitive neurons, ae revealed by standing for cytochrome oxidase
- The darkened region in the V1 layer
- Neurons take part in color perception
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Interblobs
- The less-dark region in the V1 layer
| - Neurons participate in the perception of form and motion
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V1 input arriving from P cells and M Cell Pathway of the Grniculostriate System is Segregated into Three Separate Types of Information
-Color
-Form
-Motion
~All three types of information moves from V1 to the adjoining region V2
-The color, form, and motion input remain segregated, again seen through the pattern of cytochrome oxidase staining
~The staining pattern in region V2 differed from that in region V1
*Region V2 has a pattern of thick and thin stripes intermixed with pale zones
**The think strips receive input from the movement-sensitive neurons in region V1; the thin stripes receive input from V1's color-sensitive neurons, and the pale zones receive input from V1's form-sensitive neurons
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The Visual Pathway
- Proceed from region V2 to the other occipital region and then to the parietal and temporal lobes, forming the dorsal and ventral stream
- Many parietal and temporal regions take part, the major ones are region G in the parietal lobe (thus called PG) and region E in the temporal lobe (thus called TE)
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The simple records of color, form, and motion from the occipital regions
- Assembled in the dorsal and ventral streams to produce a rich, unified visual world of a complex object (such as faces and paintings) and complex skills (such as bike riding and ball catching)
- Think of the complex representations of the dorsal and ventral streams as consisting of how functions and what functions
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Two Regions on the ventral surface of the temporal lobes
- One is specialized for recognizing face (fusiform face area (FFA))
- The other for analyzing landmarks such a building or trees (parahippocampal place area (PPA))
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Three regions in the parietal lobe
```
-Eye movement
~Lateral intraparietal area (LIP)
-Visual control
~Anterior intraparietal area (AIP)
-Visual guided reaching movements
~Parietal reach region (PRR)
-Damage to these regions can produce surprising specific deficits
```
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Facial Agnosia (Face blindness)
-The inability to recognize faces
~Also called prosopagnosia
-Damage to the FFA
-A condition in which an individual cannot recognize faces
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Visual Field
-A region of the visual world seen by the eyes
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Ganglion Cell's Receptors Fields
- By shining a light on the receptors the cells respond to stimulation on just a small circular patch of the retina
- The retina region on which it is possible to influence that cell's firing
- Represents the outer world as seen by a single cell
- Each RGC only sees a small bit of the world, much as you would if you looked through a narrow cardboard tube
- Is composed of thousands of such receptive fields
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How Receptive Fields Enable the Visual system to Interpret an Object's Location
-Whan a tiny light shines on different parts of the retina, different ganglion cells respond
~Example
*When light shines on the top-left corner of the flattened retina, a particular RGC responds because that light is in its receptive field
*When light shines on the top-right corner, a different RGC responds
-Light comes above hits the bottom of the retina after passing through the eye's lens, and the light from below hits the top of the retina
~Information at the top of the visual field stimulates ganglion cells on the bottom of the retina; information at the bottom of the field stimulates ganglion cells on the top of the retina
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Connection from the Ganglion cells to the Lateral Geniculate Nucleus
-The LGN is not a thin sheet; it is shaped more like a sausage
~Each slice represents a layer of cells
-RGC that responds to light in the top-left region of the retina connects to the left side of the first slice
-RGC that responds to light in the bottom-right region of the retina connects to the right side of the last slice
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Each LGN cell has a receptive field
- The region of the retina that influences its activity
- Two adjacent retinal ganglion cells synapse on a single LGN cell, the receptive field of that LGNcell will be the bum of two ganglion cell's receptive fields
- Results from the receptive fields of LGN cells are bigger than those of RGCs
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LGN projection to the striate cortex
- Region V1 also maintains spatial information
- Each cell representing a particular place, projects to region V1, a spatially organized neural representation- a topographic map-is produced in the cortex
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Central Part of the Visual Field
- Represented at the back of the brain, whereas the periphery is represented more anteriorly
- The upper part of the field is represented at the bottom of region V1 and the lower part at the top of V1
- Other regions (V3, V4, V50 have topographic maps similar to V1
- V1 neurons must project to the other regions in an orderly manner, just as the LGN neurons project to region V1 in an orderly fashion
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Visual Cortex Area
-Each neuron's receptive field corresponds to the part of the retina to which the neurons connect
-Rule of thumb, cells in the cortex must have much larger receptive fields than do RGCs
~This larger field corresponds to the part of a cortical neuron that must be composed of the receptive fields of many RGCs
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Harry Jersion proposed the principle of mass
-That the amount of neural tissue responsible for a particular function is proportional to the amount of neural processing that function requires
~The more complex a function is, the larger a specific region performing that function must be
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Topographic Mapping
-Based on neuronal receptive fields is an effective way for the brain to code an object location
-If the left visual field is represented in the right cerebral hemisphere and the right visual field is represented in the left cerebral hemisphere
~How are the two halves of the visual field ultimately merge
*Lies in the corpus callosum
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Corpus Callosum
-Binds the two sides of the visual field together at the midline
-Cleary links the two hemispheres of the brain, but exactly which parts were connected were not yet known in the 1050s
-Now we know it connects only certain brain structure
~The frontal lobes have many callosal connections
~Occipital lobe have almost none
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Microelectrode
-Records changes in the neuron's firing rate
-Cells occasionally fires spontaneously, producing action potentials with each discharge
-Neurons discharge, on average, once every 0.008 seconds
~Each action potential is brief, on the order of 1 millisecond
-A single-cell recording of 12 pikes in the spend of 1 second
-If firing rates of the cell increase, we see more spikes
-If firing rates decrease, we see fewer spikes
-Increasing in firing is the result of neuronal excitement, whereas a decrease indicated inhibition
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Neurons in three areas of Ganglion cells layer
- Retinal
- Primary visual cortex
- Temporal cortex
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Receptive Field of a ganglion cell
| -On-center cells
- Has a concentric circle arrangement
- A spot of lighting in the receptive field's central circle excites some of these cells, whereas a spot of light falling in the receptive field's surrounding area (periphery) inhibits the cell
- A spot of light falling across the entire receptive field weakly increases the cell's firing rate
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Receptive field of a ganglion cell
| -Off-center cells
-With light in the center of the receptive field inhibiting, light in the surrounding area exciting, and light across the entire field producing weak inhibition
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The on-off arrangement of RGC receptive fields
-Make these cells especially responsive to tiny spots of light
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ganglion cell receptive fields
- Might mislead you into thinking that they form a mosaic of discrete little circles on the retina
- Neighboring RGC receive their input from an overlapping set of photoreceptors
- A small spot of light shining on the retina is likely to produce activity in both on-center and off-center RGCs
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Luminance Contrast
- Amount of light on an object reflects relative to its surroundings
- The amount of visible light reflected the eye from a surface, and in contrast is the difference in luminance between adjacent parts of the surface
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The hypothetical population of on-center ganglion cells
-Their receptor fields are disturbed across the retinal image of a light-dark edge
-Some of the ganglion cells' receptive fields are in the dark area, others are in the light area, and still, other's fields straddle the edge of the light
-Ganglion cells with receptive fields in the dark and light areas are least affected because they receive either no stimulation or stimulation of both the excitatory and inhibitory regions of their receptive fields
-Ganglion cell B is inhibited because the light falls mostly on the inhibitory surrounding area; ganglion cell D is excited because its entire excitatory center is stimulated, but only part of its inhibitory surrounding is
-Information transmitter from RGCs to the visual areas in the brain does not give equal weight to all visual field regions
~If emphasizes regions containing differences in luminance-areas along the edge
*RGC are really sending signals about edges, and edges form shapes
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Orientation Detectors
-Each V1 cell receives input from multiple RGcs, the receptive fields of the V1 neurons are much larger than those of retinal neurons
-Consequently, V1 cells respond to stimuli more complex than simply light on of lights off
~These cells are maximally excited by bars of light oriented in a particular direction rather than by spots on light
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Simple Cells
- Some orientation doctors have the on-off receptive-fields arrangements, but the arrangement is rectangular rather than circular
- Are not the only kind of detectors in the primary cortex; several functionally distinct types of neurons populate region V1
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Complex Cells
-Are maximally excited by bars of light moving in a particular direction through the visual field
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Hypercomplex Cells
-Like a complex cell, is maximally responsive to moving bars but also has a strong inhibitory area at one end of its receptive field
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Ocular Dominance Columns
-Functional columns in the visual cortex are maximally responsive to information coming from one eye
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Keiji Tanaka
- Found that most neurons in area Te require rather complex features for their activation
- These features include a combination of characteristics such as orientation, size, color, and texture
- Neurons with similar but not identical responsiveness to particular features tend to cluster in columns
- An object is represented not by the activity of a single neuron but rather by the activity of many neurons with slightly varying stimulus specificities
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Stimulus Equivalence
-Recognizing an object as remaining the same despite being viewed from different orientations
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Subtractive Color Mixing
- Painter of the Renaissance era 600 years ago discovered that they could obtain the entire range of colors in the visual world by mixing only three colors of paint (red, blue, and yellow)
- Works by removing light from the mix
- Is why matte black surfaces reflect no light: the darker the color, the less the light it contains
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additive color mixing
- Increases light to make color
- The lighter the color, the more the light it contains, which is why a white surface reflects the entire visible spectrum
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Trichromatic Theory
-Explanation of color vision based on the coding of three primary colors: red, green, and blue
-The color we see
~Blue at short 400-nm wavelengths
~Green at medium 500-nm weavelenghts
~Red at long 600-nm wavelengths
*Is determined by their relative responses of the corresponding cone types
**If all three types are equally active, we see white
-If we lack one cone receptor type, we cannot process as many colors as we could with all three
-When a person is born with only two cone types, the colors this person cannot perceive depends on which receptor type is missing
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Color-Deficient Vision
-Some people are missing one or more cone types and are often mistakenly said to be color-bind-mistakenly because people who have two types of cones can distinguish lots of colors, though not as many as people with three cones
-To have no color vision, one would have to have only one type of photoreceptor, rods
~This is a rare occurrence
-The complete lack of red cones lead to a condition called Protanopia
-The complete lack of green cones lead to a condition called Deuteranopia
-The complete lack of blue cones lead to a condition called Tritanopia
-Fewer cones of one type, most commonly the green cone
-This condition afflicts about 5% of men and 0.4% of women
-Military forces often use humans with deuteranopia to help see through camouflage
-There are a few treatments in which people wear glasses that change the wavelength of light entering the eye, allowing the wearer to perceive color as it is seen by people with normal vision
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RGC color-sensitive
- About 60 % of humans
| - With the center responsive to one wavelength and the surrounding area to another
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Opponent-process
Explanation of color vision that emphasizes the importance of the apparently opposing colo pairs: red vs. green and blue vs. yellow
- Medium-wavelength (green) vs. long-wavelength (red), but we also have blue vs. yellow
- Cell evolved to enhance the relatively small differences in spacial absorption among the three-cone types
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Color Constancy
- Phenomenon whereby an object's perceived color tends to remain constant relative to other colors, regardless of changes in illumination
- Were you to look at a bowl of fruit through light-green glasses, the fruit would take on a greenish tinge, but bananas would still look yellow relative to red apples
- If you remove all the fruit except the bananas and look at them through the tinted glasses, the bananas would appear green because the color you perceive is isolated relative to any other
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fMRI Study
- Shown that the dorsal pathway can also perceive and process objects, although studies of patients show that it cannot do this without the presence of the ventral stream
- It is still not known exactly what role the dorsal stream plays in object perception, but the result suggests that shape processing may be more distributed than previously believed
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Monocular Blindness
- Destruction of the retina or optic nerve of one eye, the loss of sight in that eye
- Partial destruction of the retina or optic nerve produces a partial loss of sight in one eye, restricted to the visual field region that has severed connections to the brain
- Injury to the visual pathway beyond the eye also produce blindness
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Homonymous Hemianopia
-Blindness of the entire left or right visual field
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Quadrantanpia
- Blindness of one quadrant of the visual field
- Injuries affecting vision are rarely localized to gray matter but also include white matter, leading to disconnection of different brain regions
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Scotomas
-Small blind spot in the visual field caused by migraine or by a small lesion of the visual cortex
-Can be a warning symptom for migraine sufferers
-But brain-injured people are often totally unaware of them
~One reason is that the eyes are usually moving
-Moves about the visual field, allowing the intact brain region to perceive all the information in that field
-If eyes are temporarily held still, the visual system actually compensates for a scotoma through pattern completion-filling in the hole- so that the people and objects in the visual world are perceived as whole
-The visual system may cover up a scotoma so successfully that its presence can be demonstrated to the patient only by tricking the visual system
~The trick is to place an object entirely within the scotoma, and without allowing the patient to shift gaze asking what the object is
*If the patient sees nothing, to confirm the existence of a blind area, the examiner moves the object out of the scotoma so that it suddenly appears in the intact region of the visual field
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Nystagmus
-Constant eye movement
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Visual-form Agnosia
-Inability to recognize objects or drawing of objects
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Optic Ataxia
- Deficit in the visual control of reaching and other movements
- A severe deficit in visually guide reaching, even though he would still make accurate movements directed towards his own body
- Associated with parietal injury have been recorded
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How do we sense, perceive, and see the world?
- Nature of sensation and perception
- Functional anatomy of the visual system
- Location in the visual world
- neuronal activity
- The visual brain in action
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Nature of sensation and perception
-The only input our brains receive from the "real" world is a series of action potentials passed along the neurons of our various sensory pathways
~How nerves can turn energy, such as light waves, into nerve impulses is understood
~The pathway those nerve impulses take to reach the brain are also known
-Less known is how we end up perceiving one set of nerve impulses as a representation of the world
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Sensory receptors
-Specialized cells that transduce (convert) sensory energy (light) into neural activity
-Each sensory system's receptors are designed to respond only to a narrow band of energy
~Vision
*Light energy to chemical energy
~Auditory
*Air pressure to mechanical energy
~Somatosensory
*Mechanical energy
~Taste and olfaction
*Chemical molecules
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Sensory Receptors
| -Receptive fields
-Specific part of the world to which a sensory receptor organ responds
~Sample sensory information and help locate sensory events in space
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Sensory Receptors
| -Optic flow
-stream of visual stimuli that accompanies an observer's forward movement through space
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Sensory Receptors
| -Auditory flow
-Change in sound heard as a person moves past a sound source or as a sound source moves past a person
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Receptor Density and sensitivity
-Density is important in degerming the sensitivity of a sensory system
~More tactile receptors on the fingers than on the arm
-Differences in receptor density determine the special abilities of many animals
~Olfactory ability of dogs
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Neural Relays
-All receptors connect to the cortex through a sequence of 3 to 4 intervening neurons
-Information can be modified at different stages in the relay, allowing the sensory system to mediate different responses
-There is no straight-through, point-to-point correspondence between one neural relay and the next
~A recording of activity in each successive relay
~Sensory neural relay are central to the hierarchy of motor responses in the brain
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Sensory Coding and Representation
-All sensory information from all sensory systems in encoded by action potentials that travel along peripheral nerves in the somatic nervous system to the CNS
-How do action potentials code the different kinds of sensations (e.g. vision vs. touch)?
-How do they encode the features of particular sensations (e.g. purple vs. blue)?
-How do action potentials encode the features of particular sensations (e.g. purple vs. blue)?
~The presence of a stimulus can be encoded as an increase or decrease in a neuron’s firing rate.
~The amount of increase or decrease can encode the stimulus intensity.
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Sensory Coding and Representation
| -How do action potentials code the different kinds of sensations (e.g. vision vs. touch)?
-The answer to this is more complex:
~Different sensations are processed in different areas of the cortex.
~We learn to distinguish the senses through experience.
~Each system has a distinct wiring set up at all levels of neural organization
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Sensory Coding and Representation
-The neocortex represents the sensory field of each modality—vision, hearing, touch, smell, or taste—as a spatially organized neural representation of the external world.
- Topographic map is a neural–spatial representation of the body or of the areas of the sensory world perceived by a sensory organ
- In mammals, each sensory system has at least one primary cortical area.
- These may project to secondary areas.
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Perception
| -Sensation
-The registration of physical stimuli from the environment by the sensory organs
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Perception
| -Perception
- Subjective interpretation of sensations by the brain
- Our visual experience is not an objective reproduction of what is “out there,” but rather is a subjective construction of reality that is manufactured by the brain.
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Functional Anatomy of the Visual System
- Vision is our primary sensory experience.
- Far more of the human brain is dedicated to vision than to any other sense.
- Understanding the visual system’s organization is therefore key to understanding human brain function.
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Visible Light and the Structure of the Eye
-Light is electromagnetic energy that we see.
-Range of electromagnetic energy that is visible to humans:
~About 400 nanometers (violet) to 700 nanometers (red)
~Nanometer (nm): one-billionth of a meter
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The Basics: Visible Light and the Structure of the Eye
| -Cornea
-Clear outer covering
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The Basics: Visible Light and the Structure of the Eye
| -Iris
- Opens and closes to allow more or less light in
| - The hole in the iris is called the pupil.
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The Basics: Visible Light and the Structure of the Eye
| -Lens
- Focuses light
| - Bends to accommodate near and far objects
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The Basics: Visible Light and the Structure of the Eye
| -Retina
-Where light energy initiates neural activity
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The Basics: Visible Light and the Structure of the Eye
| -Myopia (nearsightedness)
- Inability to bring distant objects into clear focus
| - Focal point of light falls short of the retina
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The Basics: Visible Light and the Structure of the Eye
| -Hyperopia (farsightedness)
- Inability to focus on near objects
| - Focal point of light falls beyond the retina
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The Basics: Visible Light and the Structure of the Eye
| -Presbyopia
-Common form of hyperopia seen in older adults
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Structure of the Retina
| -Retina
-Light-sensitive surface at the back of the eye consisting of neurons and photoreceptor cells
~Translates light into action potentials
~Discriminates wavelengths (colors)
~Works in a wide range of light intensities
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Structure of the Retina
| -Fovea
- Region at the center of the retina that is specialized for high acuity
- Receptive field at the center of the eye’s visual field
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Structure of the Retina
| -Blind Spot
- Region of the retina (known as the optic disc) where axons forming the optic nerve leave the eye and where blood vessels enter and leave
- This region has no photoreceptors.
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Structure of the Retina
| -Papilloedema
- Swollen optic disc
- May be due to increased intracranial pressure (tumor or brain infection) or inflammation of the optic nerve (optic neuritis)
- Can cause loss of vision
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Photoreceptors
| -Rods
- More numerous than cones
- Sensitive to low levels of light (dim light)
- Used mainly for night vision
- One type of pigment only
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Photoreceptors
| -Cones
- Highly responsive to bright light
- Specialized for color and high visual acuity
- Located in the fovea only
- Three types of pigment
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Photoreceptors
| -Three Types of Cone Pigments
-Absorb light over a range of frequencies, but their maximal absorptions are:
~419 nm (“blue” or short wavelength)
~531 nm (“green” or middle wavelength)
~559 nm (“red” or long wavelength)
-There are approximately equal numbers of red and green cones, but fewer blue cones.
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Retinal-Neuron Types
| -Bipolar cells
-Receives input from photoreceptors
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Retinal-Neuron Types
| -Horizontal cell
-Links photoreceptors and bipolar cells
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Retinal-Neuron Types
| -Amacrine cell
-Links bipolar cells and ganglion cells
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Retinal-Neuron Types
| -Retinal ganglion cell (RGC)
-Gives rise to the optic nerve
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Retinal-Neuron Types
-Two Types of Ganglion Cells
~Magnocellular cell (M-cell)
- Magno = large
- Receives input primarily from rods
- Sensitive to light and moving stimuli
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Retinal-Neuron Types
-Two Types of Ganglion Cells
~Parvocellular cell (P-cell)
- Parvo = small
- Receives input primarily from cones
- Sensitive to color
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Visual Pathways
| -Optic Chiasm
- Junction of the optic nerves from each eye
- Axons from the nasal (inside) halves of each retina cross over to the opposite side of the brain.
- Axons from the temporal (outer) halves of each retina remain on the same side of the brain.
- Information from left visual field goes to right side of brain; information from the right visual field goes to the left side of the brain.
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Visual Pathways
| -Geniculostriate System
-Projections from the retina to the lateral geniculate nucleus to the visual cortex
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Visual Pathways
| -Tectopulvinar System
-Projections from the retina to the superior colliculus to the pulvinar (thalamus) to the parietal and temporal visual areas
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Visual Pathways
| -Retinohypothalamic Tract
- Synapses in the tiny suprachiasmatic nucleus (SCN) in the hypothalamus
- Role in regulating circadian rhythms and in the pupillary reflex
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Striate Cortex
-The primary visual cortex
-Two visual paths emerge from striate cortex:
~One route goes to vision-related regions of the parietal lobe
~One route goes to vision-related regions of the temporal lobe
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Dorsal and Ventral Visual Streams
| -Dorsal Visual Stream
- Pathway that originates in the occipital cortex and projects to the parietal cortex
- The “how” pathway (how action is to be guided toward objects)
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Dorsal and Ventral Visual Streams
| -Ventral Visual Stream
- Pathway that originates in the occipital cortex and projects to the temporal cortex
- The “what” pathway (identifies what an object is)
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Geniculostriate Pathway
-Lateral Geniculate Nucleus (thalamus)
~Right LGN: input from right halves of each retina
~Left LGN: input from left halves of each retina
~Six layers
*Layers 1, 4, and 6: input from contralateral retina
*Layers 2, 3, and 5: input from ipsilateral retina
*Layers 1 and 2: input from magnocellular cells
Layers 3 to 6: input from parvocellular cells
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Geniculostriate Pathway
| -Cortical column
- Organization that represents a functional unit of six cortical layers deep and approximately 0.5 mm square; perpendicular to the cortical surface
- From each LGN, information from each retina is sent to adjacent cortical columns, thus maintaining the separation of information from each retina.
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Tectopulvinar Pathway
-Magnocellular cells from the retina project to the superior colliculus, which then sends information to the pulvinar (thalamus).
~Medial Pulvinar
*Sends connections to the parietal lobe
~Lateral Pulvinar
*Sends connections to the temporal lobe
-Provides information regarding location (“where”)
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Occipital Cortex
-Composed of at least six different visual regions
-Primary Visual Cortex (V1; Striate Cortex)
~Striate cortex that receives input from the lateral geniculate nucleus
-Secondary Visual Cortex (V2-V5; Extrastriate Cortex)
~Visual cortical areas outside the striate cortex
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Heterogeneous Layering
| -Blob (V1)
- Region in the visual cortex that contains color-sensitive neurons
- Revealed by staining for cytochrome oxidase
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Heterogeneous Layering
| -Interblob (V1)
- Region that separates blobs
| - Participates in perception of form and motion
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Heterogeneous Layering
-Segregation of information about movement, color, and form is maintained at V1 and V2.
-Thick Stripes (V2)
~Receive information from movement-sensitive neurons
-Thin Stripes (V2)
~Receive information from color-sensitive neurons
-Pale zones (V2)
~Receive information from form-sensitive neurons
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Vision Beyond the Occipital Cortex
- Fusiform face area (FFA): recognizing faces
- Parahippocampal place area (PPA): analyzing landmarks
- Lateral intraparietal area (LIP): related to eye movements
- Anterior intraparietal area (AIP): visual control of grasping
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Location in the Visual World
-Visual Field
~Region of the visual world that is seen by the eyes
~Divided into left and right halves
~Information in left visual field goes to the right hemisphere
~Information in the right visual field goes to the left hemisphere
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Coding Location in the Retina
-Each retinal ganglion cell (RGC) responds to stimulation on just a small circular patch of the retina – the cell’s receptive field.
-Coding location
~Light shone in one place on the retina will activate one ganglion cell, and light shone in another place will activate a different ganglion cell.
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Location in the LGN and Region V1
-Cells in the lateral geniculate nucleus (LGN) also have visual fields.
-Each LGN cell represents a particular place.
-Projects to V1, forming a topographic map
~Receptive fields of cells in the cortex are typically larger than those of RGCs.
~More cortical tissue is devoted to cells in the fovea than in the periphery.
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Topographic Organization ofRegion V1
- The central part of the visual field is represented at the back of the brain; the periphery is represented more anteriorly.
- The upper part of the visual field is represented at the bottom of region V1 and the lower part at the top of V1.
- The other regions of the visual cortex (such as V3, V4, and V5) have topographical maps similar to that of V1.
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Receptive-Field Hierarchy
- Cells in the cortex have much larger receptive fields than those of retinal ganglion cells.
- The receptive field of a cortical neuron must be composed of the receptive fields of many RGCs.
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The Visual Corpus Callosum
-The corpus callosum connects the two hemispheres, but only certain brain structures.
~Most of the frontal lobes are connected to one another.
~The occipital lobes have almost no callosal connections
*Exception: Cells that lie along the midline of the visual field are connected to one another via the callosum so that their receptive fields overlap.
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Neuronal Activity
| -Seeing Shape
-Place before the eye a straight line positioned at a 45°angle.
-The cell could respond to this stimulus either by increasing or decreasing its firing rate
~Cell could show excitation to one stimulus, inhibition to another stimulus or no reaction at all
-In either case, we would conclude that the cell is creating information about the line
-Cell’s response to a particular stimulus is selective
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Processing in Retinal Ganglion Cells
| -Retinal Ganglion Cells
-Respond only to the presence or absence of light in their receptive field, not to shape
-Concentric circle arrangement
~Center and surround (periphery)
-On-center cells
~Excited when light falls on the center portion of the receptive field; inhibited when light falls on the surround (periphery) of the receptive field
~Light across whole receptive field produces weak excitation
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Processing in Retinal Ganglion Cells
| -Retinal Ganglion Cells
-Off-center cells
~Excited when light falls on the surround portion of the receptive field; inhibited when light falls on the center of the receptive field
~Light across the whole receptive field produces weak inhibition
-Each ganglion cell tells the brain about the amount of light hitting a certain spot on the retina compared with the rest of the retina
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Overlapping ReceptiveFields
-Neighboring retinal ganglion cells receive their inputs from an overlapping set of photoreceptors.
~Receptive fields overlap
-A small spot of light shining on the retina is likely to produce activity in both on-center and off-center ganglion cells.
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Processing in Retinal Ganglion Cells
-Luminance Contrast
~The amount of light reflected by an object relative to its surroundings
-This allows input from RGCs to tell the brain about shape.
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Processing Shape in the Primary Visual Cortex (V1)
-V1 neurons receive input from multiple RGCs
-V1 neurons have much larger receptive fields than RGCs
-Cells behave like orientation detectors.
~Excited by bars of light oriented in particular directions
-Simple Cells
~Receptive field with a rectangular on-off arrangement
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Processing Shape in the Primary Visual Cortex
| -Complex Cells
-Maximally excited by bars of light moving in a particular direction through the receptive field
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Processing Shape in the Primary Visual Cortex
| -Hypercomplex Cells
-Like complex cells, they are maximally responsive to moving bars but they also have a strong inhibitory area at one end of its receptive field.
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V1 Receptivity
- Bar of light strikes the receptive fields of perhaps dozens of retinal ganglion cells
- Input to a V1 neuron comes from a group of ganglion cells that happen to be aligned in a row.
- V1 neuron is activated (or inhibited) only when a bar of light hitting the retina strikes that particular row of ganglion cells.
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Processing Shape in V1
-Neurons are organized into functional columns (0.5 mm strip of cortex).
~Comprises neurons with similar functions
~Inputs arrive in layer IV and then connect vertically with other layers.
-Ocular-Dominance Columns
~Receive input from the left or right eye
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Processing Shape in the Temporal Cortex
-Cells are maximally excited by complex visual stimuli (e.g., faces or hands).
~May even be selective to particular faces seen head-on, to faces viewed in profile, to the posture of the head, or even to certain facial expressions
-Stimulus equivalence
~Recognizing that an object is the same across different viewing orientations
-Whereas the preferred stimuli of neurons in the primary visual cortex (V1) are fixed (genetically programmed), the neurons in the temporal lobe are able to alter their preference with experience
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Seeing Color
| -Trichromatic Theory
-Explanation of color vision based on the coding of three primary colors: red, green, and blue
-The color we see is determined by the relative responses of the different cone types
-Can explain different types of color blindness
-Limitation:
~Four basic colors: red, green, yellow, and blue
~Cannot explain afterimages
*Red-green; blue-yellow
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Seeing Color
| -Opponent-Process Theory
-Ewald Hering (1874)
-Explanation of color vision that emphasizes the importance of the opposition of colors
~Red versus green
~Blue versus yellow
-Opponent processing occurs in retinal ganglion cells
~On-off/center-surround receptive fields
~60% of retinal ganglion cells
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Opponent-Color-Contrast Response
-Center of receptive field is excitatory (in some cells) or inhibitory (in other cells)
-Stimulation to the periphery has the opposite effect
~Center responsive to one wavelength and the surround to another
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Opponent-Process Theory
| -Neurons in Cortical Region V1
-Color-sensitive cells are found in blobs, which also respond in an opponent-process manner.
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Opponent-Process Theory
| -Neurons in Cortical Region V4
-Do not respond to particular wavelengths, but are responsive to different perceived colors
~Center of the receptive field is excited by a certain color and the surround is inhibited
-May be important for color constancy
~Perceived color is constant relative to other colors, regardless of changes in illumination
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Neuronal Activity in the Dorsal Stream
-Posterior Parietal Cortex
~Involved in processing visual information for action: the “how” stream
*Neurons in this area are silent to visual stimulation when a person is under anesthesia.
-Some cells in this area process the visual appearance of an object to be grasped.
~These cells will fire when a monkey simply watches another monkey picking up an object
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Injury to the Pathway Leading to the Cortex
| -Monocular Blindness
-Destruction of the retina or optic nerve of one eye that produces loss of sight in that eye
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Injury to the Pathway Leading to the Cortex
| -Homonymous Hemianopia
-Blindness of an entire left or right visual field
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Injury to the Pathway Leading to the Cortex
| -Quadrantanopia
-Blindness of one quadrant of the visual field
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Injury to the Pathway Leading to the Cortex
| -Scotoma
-Small blind spot in the visual field caused by a small lesion or migraines of the visual cortex
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Injury to the “What” Pathway
-Agnosia = not knowing
-Visual-Form Agnosia
~Inability to recognize objects or drawings of objects
-Color Agnosia (achromatopsia)
~Inability to recognize colors
-Face Agnosia (prosopagnosia)
~Inability to recognize faces
-Although patients with visual-form agnosia cannot recognize objects, they can:
~Copy objects and even draw objects from memory, but they cannot later recognize these copied objects
~Still appropriately shape their hands when grasping for objects, despite not being able to recognize those objects
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Injury to the “How” Pathway
| -Optic Ataxia
- Deficit in the visual control of reaching and other movements
- Damage to parietal cortex
- Can recognize objects normally
