Week 4-Low Level/Visual Perception Flashcards
What is the structure of the eye?
- Cornea: focuses light, fixed
- Pupil: the aperture in the iris, changes size according to light levels
- Lens: focuses light, adjustable ——> accommodates
- Retina: rods and cones are cells in the retina that convert light into an electrochemical signal
What is the Retinal Structure?
-Light-sensitive receptors (rod and cone cells) that convert light into electrical activity ~125 million rods per eye and ~ 6 million cones per eye
-Various types of nerve cells (neurons)
-Nerve fibres (axons of neurons) - around 800,000 per eye; these POOL (i.e., combine) signals from multiple rod or cone receptors
How do the rod and cone receptors send signals?
-When enough light falls on a rod
or cone cell in the retina, that cell
sends an electrical signal down
its nerve fibre (an axon).
-This signal is like an action
potential. It triggers a chemical
signal to another neuron which,
in turn, can send signals to other
neurons.
-The rods and cones convert light
into neural signals - the language
of the brain.
What is the electromagnetic spectrum?
-Is a spectrum containing short, medium and long wavelengths of varying kinds e.g., X-rays, UV rays, TV rays etc.,
-Visible light is only a tiny part of it (roughly 380-700nm)
-nm = nanometres = 10-9 metres or one billionth of a metre, so very, very small; 1000 nm = one millionth of a metre
-400nm=short wavelengths
-500nm=medium wavelengths
-700nm=long wavelengths
What are the roles of rod cells?
-Achromatic night vision
-1 type
*Are mostly in the periphery, outside the fovea.
*Are specialized for vision under low light conditions.
*Do not distinguish between different colours (check for yourself at night). The periphery is
low acuity (function at low light levels) as outputs from hundreds of rod cells are pooled together.
What are the roles of cone cells?
-Daytime, achromatic and chromatic vision
3 types:
1. Long-wavelength-sensitive (L) or “red” cone
- Middle-wavelength-sensitive (M) or “green” cone
- Short-wavelength-sensitive (S) or “blue” cone
*Are mostly in the fovea (little dent in the eye)
- The eye has short, middle and long wavelength cones, which detect different colours.
- The fovea is high acuity as outputs from only a few cone cells are pooled together (pupil is smaller with light and larger in the dark)
- Cones are used in normal lighting conditions and in most situations. We move our eyes to bring important things onto the
fovea (as we don’t tend to look at things peripherally unless sneaking a peek!)
Define what is meant by the Blind spot
Where the optic nerve leaves the retina so no rod or cone cells to detect light (Brain FILLS IN the gap, so we don’t SEE a black spot!)
What is meant by acuity high?
-Acuity high: fine detail visible at fovea Information processed by CONE cells
-The central dot shows the fixation point in the image, which is where information in the image would reach retinal receptors in the fovea.
-We are constantly moving our eyes to control what information falls on the fovea.
-Stimuli need to be much larger to be identified at periphery
-Information processed by ROD cells
-This shows the SIZE that letters need to be so they are recognised
equally well at different distances from the fixation point.
How does information go from the eye to the cortex?
- Most of the back of the brain (‘posterior regions’) are dedicated to visual processing.
- Nerve fibers go from the eye to the Lateral Geniculate Nucleus in the Thalamus (LGN), and then to the primary visual cortex (V1)
- The right visual field projects to the left cerebral hemisphere
- And the left visual field projects to the right cerebral hemisphere.
What are the rod and cone pathways to the visual cortex? (Solomon, 2021)
- Cone cells (L & M)=Parvocellular pathway (sensitive to colour and fine detail)
- Cones and rod=Magnocellular pathway (sensitive to luminance and motion in the periphery)
- Cones (S & M)=Koniocellular pathway (mediates mostly colour vision: Yellow-blue pathway)
How is the visual brain divided?
-The visual brain is divided into different anatomical regions, with each area having a distinct function.
-Most cells in V1 and V2 only detect simple information (eg oriented lines) whereas cells at
later stages may, for example, respond to complex shapes and motion patterns:
V4=Responds to Colour
V5=Responds to Motion
LO=Responds to Shape
How does the visual information travel to the brain? (Mather, 2009)
-Processing cascades up through the different areas of the brain, starting from the LGN and then to the visual cortex (V1, V2, V3, V5 and V5) and to IT (inferotemporal cortex).
-Information from the LGN to the V1 takes 10ms.
What are some methods which have allowed us to learn everything we know about the visual system?
- We can disrupt processing in brain areas with Transcranial Magnetic Stimulation (TMS).
- We can record brain activity using functional Magnetic Resonance Imaging (fMRI).
- We can measure the effect of accidental damage to specific brain regions in humans.
- We can probe individual neurons in live animal brains with microelectrodes. When the ‘preferred’ stimulus is presented, the neurons fire.
- When it comes to the visual system, all these techniques provide converging evidence about the specialization of function.
What can we disrupt with TMS?
-With Transcranial Magnetic Stimulation, we can temporarily disrupt neural processing in
specific visual maps.
-TMS disrupting V5 produces a
sense of motion.
-TMS disrupting V4 produces
colored phosphines.
-TMS is non-invasive, reversible and only disrupts the brain temporarily
Define what is meant by Brain lesions
- Brain lesions = holes in the brain
caused by accidents, strokes, tumors etc., - Lesions often have specific effects, loss of language, difficulty seeing faces.
- Some lesions can change personality dramatically (As in the famous case of Phineas Gage)
What is Akinetopsia? (Zihl et al., 1983).
-Akinetopsia means difficulty seeing and perceiving motion. It is caused by discrete damage to area MT (also known as V5). This is a description of a patient with Akinetopsia:
-“She had difficulty .. In pouring tea or coffee into a cup because
the fluid appeared to be frozen, like a glacier. In addition, she
could not stop pouring at the right time since she was unable to
perceive the movement in the cup or pot when the fluid rose”.
-In a room where more than two people were walking… “people
were suddenly here or there but I have not seen them moving.”
What is the consequence when the MST is damaged?
- Damage to area MST selectively impairs optic flow processing which we use for walking (Vaina, 1998).
- First order motion and second order motion are different. First order motion = luminance (shadows) defined motion. Second order motion = Contrast defined motion
What happens with V4 lesions?
-Cells in V4 fire are to some extent selective to colours (but broadly tuned).
-There are no neurons that are only driven by red, yellow, green etc.,
-Bouvier and Engel (2006) ran a “meta analysis” of all lesions which create achromatopsia (trouble seeing colours).
-The most common lesion site was V4. Some colour processing is spared after V4 lesions – so this is not the only colour center in the brain.
What can we measure with microelectrode recordings?
- We can actually pick up various types of neural activity with
microelectrodes. - Most simply, we can measure action potentials = neurons firing electrical pulses down the long axon.
- Firing rate increases when the neuron is active.
-When we say a neuron ‘responds’ to something, we mean there is an increase in firing rate when an animal looks at one stimulus, but not at another.
When do neurons fire in the inferior temporal cortex?
-In the inferior temporal cortex, there are neurons that fire when we see certain shapes but not others.
-A cell might fire very strongly when a monkey is looking at a star, but not at a circle.
What is the “binding problem”?
- If we had different groups of neurons responding to different features, and these neurons are in different neuroanatomical regions, how we group the features which belong to a particular object.
- We have color cells in V4 and shape cells in the inferotemporal cortex.
- How do we know that the blue goes with the circle shape, yellow goes with the star, and red goes with the square?
- Neurons switch between intervals of low and high excitability rapidly. All neurons that code features of a single object oscillate in synchrony (Singer and Gray, 1995). There is a synchronized assembly of thousands of neurons.
- But how does the brain synchronize the right neurons?
What is the ventral and dorsal stream?
- Ventral stream is also called the ‘what stream’. It is involved in identifying objects. (Is it a car? Is it a plane?)
- The Dorsal stream is called the ‘where stream’ (It controls actions).
What is meant by double dissociation? (Milner and Goodale, 1995)
- Milner and Goodale (1995) are famous for work on the dissociation between ventral and dorsal streams:
Double dissociation: Damage to ventral stream should impair
object recognition but not action planning, while damage to dorsal stream should impair action planning but not object recognition.
What is Optic ataxia?
Damage to the posterior parietal lobe (part of the dorsal stream). Patient profile partially fits with
the theory as they have some difficult action planning.
E.g., inserting a card through an oriented slot (although they are not a simple group). Can label objects
reasonably well.
