Wolfe Chapter 3 Review Questions - Spatial Vision: From Spots To Stripes

Front 1

What is visual acuity and how can it be measured?

Back 1

Visual acuity is the smallest spatial detail that can be seen accurately. It can be measured by doing a visual acuity test, which requires looking at figures from a distance and identifying them.

Front 2

Explain the meaning of being able to see 20/20.

Back 2

Being able to see 20/20 means that the person being tested can identify an object at 20 feet as well as somebody with normal vision would be able to identify it at 20 feet. If the observer’s vision is 20/40, that means that the observer can see at 20 feet what somebody with normal vision can see at 40 feet (meaning the observer needs glasses!).

Front 3

What can we infer from the contrast sensitivity function?

Back 3

The contrast sensitivity function describes our window of visibility. Any object whose spatial frequencies and contrast fall within the region specified by the contrast sensitivity function will be visible. Those objects outside the region are outside our window of visibility. We can infer from this function that sensitivity to contrast depends on the spatial frequency of the stimulus.

Front 4

Explain how retinal ganglion cells respond to stripes.

Back 4

Each ganglion cell responds to certain types of stripes or gratings. For instance, an ON ganglion cell responds to gratings with spatial frequencies and phases that make the lightest part of the grating fall on the center of the cell and the darkest part of the grating fall on the surround. When the spatial frequency of the grating is too low, the ganglion cell responds weakly because part of the bar of the grating lands in the inhibitory surround, dampening the cell’s response. Similarly, when the grating’s spatial frequency is too high, the ganglion cell responds weakly because both dark and light stripes fall within the receptive field’s center and surround, washing out the response. When the frequency is just right, the cell responds vigorously.

Front 5

What is the role of the lateral geniculate nucleus (LGN)?

Back 5

The lateral geniculate nucleus is a structure in the thalamus (part of the midbrain) that shares connections with both the retina and visual cortex.

Front 6

What are the two types of layers in the LGN and how are they different from each other?

Back 6

The two types of layers in the LGN are the magnocellular layers and the parvocellular layers. The magnocellular layers are the two bottom layers of the LGN, and contain neurons that are physically larger than those in the parvocellular layers. Neurons in these layers respond to large, fast-moving objects and do not represent color information. The parvocellular layers are the top four layers of the LGN. They contain neurons that respond to the fine spatial details and color of stationary objects.

Front 7

Explain the notion of topographical mapping

Back 7

Topographical mapping is the orderly mapping of the world in the lateral geniculate nucleus and the visual cortex. Points of light that are near each other in the world fall on parts of the retina that are near each other and will be processed by neurons that are near each other in the brain. This orderly representation provides us with a neural basis of knowing where things are in space.

Front 8

What are two important features of the visual cortex? Explain.

Back 8

One important feature of the visual cortex is topographical mapping, which is the orderly mapping of the world in the brain. The second feature is magnification, or the dramatic scaling of information from different parts of the visual field. Objects on or near the fovea are processed by neurons in a large part of the striate cortex, while objects imaged in the periphery are allocated a much smaller portion of the striate cortex.

Front 9

What is orientation tuning?

Back 9

Orientation tuning is the tendency of neurons in striate cortex to respond optimally to certain orientations, and less to others.

Front 10

In what way do striate cortex neurons function as filters?

Back 10

Each striate cortex neuron responds to a particular location and is tuned to a particular spatial frequency, orientation, and phase. These narrow tuning functions mean that each striate cortex neuron functions as a filter for the portion of the image that excites the cell.

Front 11

What is ocular dominance?

Back 11

Ocular dominance is the property of the receptive fields of striate cortex neurons by which they respond more vigorously when a stimulus is presented in one eye than when it is presented in the other.

Front 12

What is the difference between simple and complex cells?

Back 12

Simple cells are cortical neurons with clearly defined excitatory and inhibitory regions, while complex cells are neurons whose receptive field responds to any properly oriented bar of light, regardless of whether it is light or dark.

Front 13

What is the role of end stopping?

Back 13

End stopping refers to a property of certain cortical neurons in which they respond vigorously when the end of a bar of light falls within their receptive field and weakly when it doesn’t. End stopping plays an important role in our ability to detect luminance boundaries and discontinuities.

Front 14

What is a column in primary visual cortex?

Back 14

A column is a vertical arrangement of neurons in striate cortex that tend to have similar receptive fields and similar orientation preferences.

Front 15

What does a hypercolumn contain?

Back 15

A hypercolumn is a 1-mm block of striate cortex contains two sets of columns, each covering every possible orientation (0–180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye.

Front 16

What is the enzyme cytochrome oxidase (CO) used for?

Back 16

This enzyme is used to reveal the regular array of “CO blobs,” which are spaced about 0.5 mm apart in the primary visual cortex. These blobs have been implicated in processing color, motion, and spatial structure.

Front 17

How can adaptation provide insights into the properties of cortical neurons?

Back 17

Adaptation is the diminishing response of a sense organ to a sustained stimulus. It is helpful in learning about the properties of cortical neurons because, by exposing an observer to a particular stimulus for an extended period of time, the experimenter can make inferences about the visual system due to the observer’s changing responses. If two stimuli are processed by unrelated sets of neurons, then selectively adapting one set of neurons should have no effect on the other set.

Front 18

What idea does the tilt aftereffect support?

Back 18

The tilt aftereffect supports the idea that the human visual system contains individual neurons selective for different orientations.

Front 19

What are spatial-frequency channels?

Back 19

Spatial-frequency channels are pattern analyzers, implemented by ensembles of cortical neurons, with each set of neurons tuned to a limited range of spatial frequencies.

Front 20

How do psychologists study visual processing in infants?

Back 20

Infants tend to look more at complex scenes than at simple scenes. If presented with the choice of looking at a series of stripes or a uniform gray field, infants will look more often and longer at the stripes. If the stripes are low contrast and the baby cannot see the difference between the stripes and the gray field, he or she will stare equally often at the two stimuli. Thus, through careful observation of infants’ preferential looking, psychologists can tell which stimuli they can see and which they can’t.

Front 21

Why is it crucial to find visual impairments early in life?

Back 21

It is crucial to find visual impairments early because there is a critical period of development for the visual system (between 3 and 8 years of age), during which cortical neurons are still being wired to their inputs from the two eyes. Many impairments found during this time can be medically fixed so that visual development will proceed normally.