Chapter 3 (M. Wolfe)

Question 1

1. What is visual acuity and how can it be measured?

Answer 1

Answer: 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.

Question 2

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

Answer 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.

Question 3

What can we infer from the contrast sensitivity function?

Answer 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.

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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.

Question 6

7. Explain the notion of topographical mapping.

Answer 6

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.

Question 7

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

Answer 7

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.

Question 8

9. What is orientation tuning?

Answer 8

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

Question 9

In what way do striate cortex neurons function as filters?

Answer 9

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.

Question 10

Acuity

Answer 10

The smallest spatial detail that can be resolved at 100% contrast.

Question 11

Adaptation

Answer 11

A reduction in response caused by prior or continuing stimulation.

Question 12

Column

Answer 12

A vertical arrangement of neurons, which tend to have similar receptive fields and similar orientation preferences.

Question 13

Complex cell

Answer 13

A cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions.

Question 14

Cortical magnification

Answer 14

The amount of cortical area (usually specified in millimeters) devoted to a specific region (e.g., 1 degree) in the visual field.

Question 15

Critical period

Answer 15

A phase in the life span during which abnormal early experience can alter normal neuronal development. Proposed for the development of binocular vision and development of a first human language.

Question 16

Cycle

Answer 16

For a grating, a pair consisting of one dark bar and one bright bar.

Question 17

Cycles per degree

Answer 17

The number of pairs of dark and bright bars (cycles of a grating) per degree of visual angle.

Question 18

End stopping

Answer 18

The process by which a cell in the cortex first increases its firing rate as the bar length increases to fill up its receptive field, and then decreases its firing rate as the bar is lengthened further.

Question 19

Magnocellular layer

Answer 19

Either of the bottom two neuron-containing layers of the lateral geniculate nucleus, the cells of which are physically larger than those in the top four layers.

Question 20

Ocular dominance

Answer 20

The property of the receptive fields of striate cortex neurons by which they demonstrate a preference, responding somewhat more rapidly when a stimulus is presented in one eye than when it is presented in the other.

Question 21

Simple cell

Answer 21

A cortical neuron whose receptive field has clearly defined excitatory and inhibitory regions

Question 22

Spatial frequency

Answer 22

The number of cycles of a grating (e.g., dark and bright bars) per unit of visual angle (usually specified in cycles per degree).