Module 7 Flashcards
True or False? Some animals can see in complete darkness.
False. The light reflected into your eyes from the objects around you is the basis for your ability to see them; if there is no light, there is no vision.
Light can be thought of in what 2 different ways?
1) As discrete particles of energy, called photons, traveling through space at about 300,000 kilometers (186,000 miles) per second
2) As waves of energy. Both theories are useful; in some ways, light behaves like particles; and in others, it behaves like waves.
Light is sometimes defined as ________ of electromagnetic energy between 380 and 760 nanometers (billionths of a meter) in length.
waves
True or False? Animals can see wavelengths that we cannot.
True. For example, rattlesnakes can see infrared waves, which are too long for humans to see; as a result, they can see warm-blooded prey in what for us would be complete darkness. So, if we were writing this text for rattlesnakes, we would be forced to provide a different definition of light for them.
What 2 properties of light are of particular interest/importance?
1) Wavelength - it plays an important role in the perception of color.
2) Intensity - because it plays an important role in the perception of brightness.
The amount of light reaching the retinas is regulated by what?
The donut-shaped bands of contractile tissue, the irises, which give our eyes their characteristic color.
Light enters the eye through what?
The pupil, the hole in the iris.
The adjustment of pupil size in response to changes in illumination represents a compromise between what 2 things?
Sensitivity (the ability to detect the presence of dimly lit objects) and acuity (the ability to see the details of objects).
When the level of illumination is high and sensitivity is thus not important, the visual system takes advantage of the situation by doing what?
Constricting the pupils. When the pupils are constricted, the image falling on each retina is sharper and there is a greater depth of focus; that is, a greater range of depths is simultaneously kept in focus on the retinas. However, when the level of illumination is too low to adequately activate the receptors, the pupils dilate to let in more light, thereby sacrificing acuity and depth of focus.
Behind each pupil is a________, which focuses incoming light on the retina.
lens
When we direct our gaze at something near, the tension on the ligaments holding each lens in place is adjusted by the _________ muscles, and the lens assumes its natural cylindrical shape.
ciliary
(This increases the ability of the lens to refract (bend) light and thus brings close objects into sharp focus.)
When we focus on a distant object, the lens is flattened. The process of adjusting the configuration of the lenses to bring images into focus on the retina is called ______________.
accomodation
What is one reason vertebrates have two eyes?
Vertebrates have two sides: left and right. By having one eye on each side, which is by far the most common arrangement, vertebrates can see in almost every direction without moving their heads.
But why do some vertebrates, including humans, have their eyes mounted side-by-side on the front of their heads?
This arrangement sacrifices the ability to see behind so that what is in front can be viewed through both eyes simultaneously—an arrangement that is an important basis for our visual system’s ability to create three-dimensional perceptions (to see depth) from two- dimensional retinal images.
The movements of your eyes are coordinated so that each point in your visual world is projected to corresponding points on your two retinas. To accomplish this, your eyes must _______.
converge (turn slightly inward).
Convergence (eyes turning slightly inward) is greatest when you are inspecting things that are________.
close
(But the positions of the images on your two retinas can never correspond exactly because your two eyes do not view the world from exactly the same position.)
What is binocular disparity?
The difference in the position of the same image on the two retinas—is greater for close objects than for distant objects; therefore, your visual system can use the degree of binocular disparity to construct one three- dimensional perception from two two-dimensional retinal images.
Why do a few vertebrate species have their eyes side-by-side on the front of the head while most species have one eye on each side?
Predators tend to have the front-facing eyes because this enables them to accurately perceive how far away prey animals are;
prey animals tend to have side-facing eyes because this gives them a larger field of vision and the ability to see predators approaching from most directions.
After light passes through the pupil and the lens, it reaches the ________.
retina
(The retina converts light to neural signals, conducts them toward the CNS, and participates in the processing of the signals.
The retina is composed of what 5 different types of neurons?
Receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
In the retina, amacrine cells and the horizontal cells are specialized for what?
lateral communication (communication across the major channels of sensory input).
True or False? Retinal neurons communicate both chemically via synapses and electrically via gap junctions.
True.
The retina is in a sense inside-out. Why?
Light reaches the receptor layer only after passing through the other layers. Then, once the receptors have been activated, the neural message is transmitted back out through the retinal layers to the retinal ganglion cells, whose axons project across the outside of the retina before gathering together in a bundle and exiting the eyeball.
This inside-out arrangement of the retina creates what 2 visual problems?
1) The incoming light is distorted by the retinal tissue through which it must pass before reaching the receptors.
2) For the bundle of retinal ganglion cell axons to leave the eye, there must be a gap in the receptor layer; this gap is called the blind spot. The first of these two problems is minimized by the fovea.
What is the fovea?
An indentation at the center of the retina that is specialized for high-acuity vision (for seeing fine details). The thinning of the retinal ganglion cell layer at the fovea reduces the distortion of incoming light.
The blind spot requires a more creative solution called what?
Completion (or filling in). The visual system uses information provided by the receptors around the blind spot to fill in the gaps in your retinal images.
The __________ phenomenon is one of the most compelling demonstrations that the visual system does much more than make a faithful copy of the external world.
Completion
True or False? The completion phenomenon is just a response to blind spots.
False.
It also plays an important role in normal vision. When you look at an object, your visual system does not conduct an image of that object from your retina to your cortex. Instead, it extracts key information about the object—primarily information about its edges and their location—and conducts that information to the cortex, where a perception of the entire object is created from that partial information.
The color and brightness of large unpatterned surfaces are not perceived directly but are filled in (completed) by a completion process called ____________.
surface interpolation
What is surface interpolation?
The process by which we perceive surfaces; the visual system extracts information about edges and from it infers the appearance of large surfaces.
What are the two different types of receptors in the human retina?
Cone-shaped receptors called cones and rod-shaped receptors called rods.
What is the duplexity theory of vision?
The theory that cones and rods mediate different kinds of vision.
How did the duplexity theory of vision emerge?
From the observation that that species active only in the day tend to have cone-only retinas, and species active only at night tend to have rod-only retinas.
What type of vision predominates in good lighting conditions and provides high-acuity colored perceptions of the world?
Photopic vision (cone-mediated vision)
What type of vision predominates in dim illumination and lacks both detail and color?
Scotopic vision (rod-mediated vision)
In which system, photopic or scotopic, does the output of several hundred receptors converge on a single retinal ganglion cell?
Scotopic system
What is the result of convergence in the scotopic system?
Sensitivity at the cost of acuity
What is the region in the retina where there are no rods, only cones?
Fovea
What is the effect observed when lights of the same intensity but different wavelengths are perceived with different brightness?
Spectral sensitivity
What are the two spectral sensitivity curves found in humans and other animals with both cones and rods?
Photopic and scotopic spectral sensitivity curves
Under photopic conditions, the visual system is maximally sensitive to wavelengths of about how many nanometers?
About 560 nanometers
Under scotopic conditions, the visual system is maximally sensitive to wavelengths of about how many nanometers?
About 500 nanometers
What visual effect describes the shift in relative brightness observed during the transition from photopic to scotopic vision?
Purkinje effect
Describe the Purkinje effect.
The relative brightness of objects shifts during the transition from photopic to scotopic vision, causing some colors to appear brighter in different lighting conditions.
How can cones accomplish their task of mediating high-acuity color vision when most of them are crammed into the fovea?
Through temporal integration and eye movements that continuously scan the visual field.
What are the three kinds of involuntary fixational eye movements?
Tremor, drifts, and saccades.
What is the purpose of fixational eye movements?
To keep images moving on the retina, enabling us to see during fixation.
What is transduction?
The conversion of one form of energy to another.
What is visual transduction?
The conversion of light to neural signals by the visual receptors.
What is the name of the red pigment extracted from rods in 1876?
Rhodopsin.
What is the first step in rod-mediated vision?
Rhodopsin’s absorption of light and bleaching.
What is the relationship between rhodopsin’s absorption spectrum and the human scotopic spectral sensitivity curve?
They are nearly identical, indicating that sensitivity to various wavelengths under scotopic conditions is directly influenced by rhodopsin’s ability to absorb them.
What type of receptor is rhodopsin?
A G-protein-coupled receptor.
What happens to rods when rhodopsin receptors are bleached by light?
Sodium channels close, rods hyperpolarize, and the release of glutamate reduces.
Although we are not aware of it, the eyes continually scan the visual field, and our visual perception at any instant is a summation of recent visual information. It is because of this __________________ that the world does not vanish momentarily each time we blink.
temporal integration
What is a pigment?
any substance that absorbs light
The transduction of light by rods exemplifies an important point: Signals are often transmitted through neural systems by __________________.
decreases in activity.
True or False: The retina-geniculate-striate pathways are the largest and most thoroughly studied visual pathways in the brain.
True
How do signals from the left visual field reach the right primary visual cortex?
Answer: Signals from the left visual field reach the right primary visual cortex either ipsilaterally from the temporal hemiretina of the right eye or contralaterally via the optic chiasm from the nasal hemiretina of the left eye.
What is the primary visual cortex also known as?
Striate cortex or V1
How many layers does each lateral geniculate nucleus have?
Six layers
Which part of the lateral geniculate nucleus receives visual input from the contralateral visual field?
Each lateral geniculate nucleus receives visual input only from the contralateral visual field, with three layers receiving input from one eye and three layers receiving input from the other.
Where do most lateral geniculate neurons that project to the primary visual cortex terminate?
In the lower part of cortical layer IV, producing a characteristic stripe or striation.
What is the primary visual cortex often referred to as due to its characteristic stripe when viewed in cross-section?
Striate cortex
What does it mean for the retina-geniculate-striate system to be retinotopic?
It means that each level of the system is organized like a map of the retina, where two stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system.
What proportion of the primary visual cortex is dedicated to the analysis of input from the fovea?
About 25 percent
What was the dramatic demonstration of the retinotopic organization of the primary visual cortex provided by Dobelle, Mladejovsky, and Girvin in 1974?
They implanted an array of electrodes in the primary visual cortex of blind patients, and by administering electrical current through specific electrodes, they elicited the perception of shapes.
What could recent research on retinal implants and the findings of Dobelle, Mladejovsky, and Girvin in 1974 potentially lead to?
The development of visual prostheses that could benefit blind individuals.
How many parallel channels of communication flow through each lateral geniculate nucleus?
At least two parallel channels.
What are the top four layers of the lateral geniculate nucleus called?
Parvocellular layers (P layers).
What type of neurons are found in the parvocellular layers?
Neurons with small cell bodies.
What are the bottom two layers of the lateral geniculate nucleus called?
Magnocellular layers (M layers).
What type of neurons are found in the magnocellular layers?
Neurons with large cell bodies.
What are parvocellular neurons particularly responsive to?
Color, fine pattern details, and stationary or slowly moving objects.
What are magnocellular neurons particularly responsive to?
Movement.
Which visual receptors provide the majority of input to the parvocellular layers?
Cones.
Which visual receptors provide the majority of input to the magnocellular layers?
Rods.
What is the receptive field of a visual neuron?
The area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron.
What is the method used by David Hubel and Torsten Wiesel to study single neurons in the visual systems of laboratory animals?
A technique involving microelectrodes to study receptive fields and record neuron responses to stimuli.
(the tip of a microelectrode is positioned near a single neuron in the part of the visual system under investi- gation. During testing, eye movements are blocked by para- lyzing the eye muscles, and the images on a screen in front of the subject are focused sharply on the retina by an adjust- able lens. The next step in the procedure is to identify the receptive field of the neuron. The receptive field of a visual neuron is the area of the visual field within which it is pos- sible for a visual stimulus to influence the firing of that neu- ron. The final step in the method is to record the responses of the neuron to various simple stimuli within its recep- tive field in order to characterize the types of stimuli that most influence its activity. Then the electrode is advanced slightly, and the entire process of identifying and character- izing the receptive field properties is repeated for another neuron, and then for another, and another, and so on. The general strategy is to begin by studying neurons near the receptors and gradually work up through “higher” and “higher” levels of the system in an effort to understand the increasing complexity of the neural responses at each level.)
What is the receptive field of a visual neuron?
The area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron.
How does the research of David Hubel and Torsten Wiesel contribute to understanding vision?
Their research revealed much about the neural mechanisms of vision, particularly in understanding how visual neurons respond to stimuli.
Light reflected from objects enters the eye though the _______.
pupil
Depending on how close or far away an object is, the lens is adjusted using the _______ muscles.
ciliary
About 25 percent of the primary visual cortex is dedicated to analyzing input from the _______.
fovea
