chapter 6.4

Question 1

Mach bands

Answer 1

the nonexistent stripes of brightness and darkness running adjacent to the edges. Enhance the contrast at each edge and make the edge easier to see.

Question 2

Ommatidia

Answer 2

large receptors, each with its own large axon. Found in horeshoe crabs. The axons of these are interconnected by a lateral neural network.

Question 3

When a ommatidium receptor fires

Answer 3

it inhibits its neighbors via the lateral neural network; this inhibition is called lateral inhibition because it spreads laterally across the array of receptors.

Question 4

The amount of lateral inhibition produced by a receptor is greatest

Answer 4

when the receptor is most intensely illuminated, and the inhibition has its greatest effect on the receptor’s immediate neighbors.

Question 5

Hubel and Wisel

Answer 5

technique for studying single neurons in the visual systems of laboratory animals. First the tip of a microelectrode is positioned near a single neuron in the part of the visual system under investigation. During testing, eye movements are blocked by paralyzing the eye muscles, and the images on a screen in front of the subject are focused sharply on the retina by an adjustable lens. Next step is to identify the receptive field of the neuron. The final step is to record the responses of the neuron to various simple stimuli within its receptive field in order to characterize the types of stimuli that most influence its activity.

Question 6

Receptive field of a visual neuron

Answer 6

the area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron.

Question 7

When Hubel and Wisel compared the receptive fields recorded from retinal ganglion cells, lateral geniculate nuclei, and lower layer IV neurons, four commonalities were readily apparent:

Answer 7

(1) At each level, the receptive fields in the foveal area of the retina were smaller than those at the periphery; this is consistent with the fact that the fovea mediates fine-grained (high-acuity) vision.
(2) All the neurons (retinal ganglion cells, lateral geniculate neurons, and lower layer IV neurons) had receptive fields that were circular.
(3) All the neurons were monocular; each neuron had a receptive field in one eye but not the other.
(4) Many neurons at each of the three levels of the retina-geniculate-striate system had receptive fields that comprised an excitatory area and an inhibitory area separated by a circular boundary.

Question 8

“on” firing

Answer 8

neuron showed a burst of firing when the light was turned on.

Question 9

“off” firing

Answer 9

neuron displayed an inhibition of firing when the light was turned on and a burst of firing when it was turned off.

Question 10

on-center cells

Answer 10

respond to lights shone in the central region of their receptive fields with “on” firing and to lights shone in the periphery of their receptive fields with inhibitions, followed by “off” firing when the light is turned off.

Question 11

off-center cells

Answer 11

respond with inhibition and “off” firing in response to lights in the center of their receptive fields and with “on” firing to lights in the periphery of their receptive fields.

Question 12

On-center and off-center cells respond best to

Answer 12

contrast. The most effective way to influence the firing rates of these cells is to maximize the contrast between the center and the periphery of its receptive field by illuminating either the entire center or the entire surround (periphery), while leaving the other region completely dark. Diffusely illuminating the entire receptive field had little effect on firing.

Question 13

Spontaneous activity is a characteristic of most

Answer 13

cerebral neurons and response to external stimuli consume only a small portion of the constant energy required by ongoing brain activity.

Question 14

simple cells

Answer 14

have receptive fields that can be divided into antagonistic “on” and “off” regions and are thus unresponsive to diffuse light. All monocular. Although, the borders between the “on” and “off” regions of the cortical receptive fields are straight lines rather than circles. Responds maximally when its preferred straight-edge stimulus is in a particular position and in a particular orientation.

Question 15

complex cells

Answer 15

more numerous than simple cells; have rectangular receptive fields, respond best to straight-line stimuli in a specific orientation, and are unresponsive to diffuse light.

Question 16

Complex cells differ from simple cells by:

Answer 16

(1) Complex cells have larger receptive fields.
(2) Not possible to divide the receptive fields of complex cells into static “on” and “off” regions. It responds to a particular straight-edge stimulus of a particular orientation regardless of its position within the receptive field of that cell.
(3) Many complex cells are binocular, which means they respond to stimulation of either eye.

Question 17

If the receptive field of a binocular complex cell is measured through one eye and then through the other

Answer 17

the receptive fields in each eye turn out to have almost exactly the same position in the visual field as well as the same orientation preference.

Question 18

A binocular cell usually fires more robustly

Answer 18

if the appropriate stimulation is applied through both eyes simultaneously.

Question 19

Ocular dominance

Answer 19

respond more robustly to stimulation of one eye than they do to the same stimulation of the other.

Question 20

Some binocular cells fire best when the preferred stimulus is presented

Answer 20

to both eyes at the same time but in slightly different positions on the two retinas. These cells respond best to retinal disparity.

Question 21

first conclusion Hubel and Wisel had regarding the organization of the primate visual cortex

Answer 21

Primary visual cortex was organized into functional vertical (right angles to the cortical layers) columns: all of the neurons in the same vertical column respond to stimuli applied to the same area of the retina, are dominated by the same eye (if they display dominance or monocularity), and “prefer” the same straight-line angles.

Question 22

second conclusion Hubel and Wisel had regarding the organization of the primate visual cortex

Answer 22

The location of various functional columns in primary visual cortex is influenced by the location on the retina of the column’s visual fields, by the dominant eye of the column, and by the column’s preferred straight-line angle. All of the functional columns in the primary visual cortex that analyze input from one area of the retina are cluster together, that half of a cluster receives input from the left eye and the other half from the right eye, and that each cluster includes neurons with preferences for straight-line stimuli of various orientations.

Question 23

third conclusion Hubel and Wisel had regarding the organization of the primate visual cortex

Answer 23

Neurons with simpler preferences converged on neurons with more complex preferences.

Question 24

Contextual influences

Answer 24

influences on a visual neuron’s activity that are caused by stimuli outside the neuron’s receptive field. Most are inhibitory.

Question 25

A neuron’s receptive field is viewed as a

Answer 25

plastic property of the neuron that is continually fine-tuned on the basis of changing signals from the context.