Chapter 6.2

Question 1

the retina converts light…

Answer 1

to neural signals, conducts them to the CNS, and participates in the processing of the signals.

Question 2

The retina is composed of five layers of different types of neurons

Answer 2

receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.

Question 3

Amacrine and horizontal cells

Answer 3

specialized for lateral communication (communication across the major channels of sensory input).

Question 4

Retinal neurons

Answer 4

communicate both chemically via synapses and electrically via gap junctions.

Question 5

the retina is inside out

Answer 5

has to pass through four layers to reach the receptor layer, 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 inside of the retina before gathering together in a bundle and exiting the eyeball. 2 problems:

Question 6

inside out prob 1

Answer 6

incoming light is distorted by the retinal tissue through which it must pass before reaching the receptors

minimized by the fovea, which is an indentation, about .33 cm in diameter, at the center of the retina; it is the area of the retina that is specialized for high-acuity vision. The thinning of the retinal ganglion cell layer here reduces the distortion of incoming light

Question 7

inside out prob 2

Answer 7

for the bundle of retinal ganglion cell axons to leave the cell axons to leave the eye, there must be a gap in the receptor layer; this is called the blind spot.

Question 8

completion (blind spot)

Answer 8

visual system uses info provided by the receptors around the blind spot to fill in the gaps in your retinal images.

Question 9

Surface interpolation

Answer 9

the process by which we perceive surfaces; the visual system extracts info about edges and from it infers the appearance of large surfaces

Question 10

when you look at an object

Answer 10

your visual system extracts key info about the object, primarily info about its edges and their location, and conducts that info to the cortex, where a perception of the entire object is created from that partial info.

Question 11

Duplexity theory of vision

Answer 11

the theory that cones and rods mediate different kinds of vision.

Question 12

Photopic vision (cone-mediated vision)

Answer 12

predominates in good lighting and provides high acuity colored perceptions of the world.

Question 13

Scotopic vision (rod-mediated vision)

Answer 13

lacks both detail and color that photopic vision provides.

Question 14

differences between photopic and scotopic

Answer 14

in scotopic system, the output of several hundred rods converge on a single retinal ganglion cell, whereas in the photopic system, only a few cones converge on the retina ganglion cell.

Question 15

dim light

Answer 15

simultaneously stimulating many rods can summate (add) to influence the firing of the retinal ganglion cell onto which the output of the stimulated rods converges whereas the effects of the same dim light applied to a sheet of cones cannot summate to the same degree and the retinal ganglion cells may not respond at all to the light.

Question 16

less ambiguity in photopic system

Answer 16

of the location of a stimulus in the photopic system over the scotopic system where input from hundreds of rods makes it impossible to know which portion contributed to the change.

Question 17

fovea

Answer 17

no rods; only cones. Density of rods reach a maximum at 20 degrees from the center of the fovea; more rods in the nasal hemiretina (half of each retina next to the nose) than in the temporal hemiretina (half next to the temples)

Question 18

Spectral sensitivity curve

Answer 18

graph of relative brightness of lights of the same intensity presented at different wavelengths

Question 19

Photopic spectral sensitivity curve

Answer 19

can be determined by having subjects judge the relative brightness of different wavelengths of light shone on the fovea. Maximally sensitive to wavelengths of about 560 nm so a light at 500 nm would have to be much more intense than one at 560 nm to be seen as equally bright.

Question 20

Scotopic spectral sensitivity curve

Answer 20

can be determined by asking subjects to judge the relative brightness of different wavelengths of light shone on the periphery of the retina at an intensity to low to activate the few peripheral cones located there. Maximally sensitive to wavelengths of about 500 nm so a light of 560 nm would have to be much more intense than one at 500 nm to be seen as equally bright.

Question 21

Purkinje effect

Answer 21

red flowers appear brighter in daylight over blue ones and blue ones appear brighter (brighter shade of grey) than red ones at night

Question 22

why don’t we see a colored dot at the center of our visual field surrounded by grey?

Answer 22

our visual perception at any instant is a summation of recent visual info because of temporal integration.

Question 23

fixational eye movements

Answer 23

involuntary; eyes continuously move even when we try to keep them still; three kinds: tremor, drifts, and saccades.

Important because when eye movements are blocked, visual objects begin to fade and disappear as most visual neurons respond only to changing images.

Question 24

transduction

Answer 24

the conversion of one form of energy to another. Visual transduction is the conversion of light to neural signals by the visual receptors.

Question 25

rhodopsin

Answer 25

red pigment; was exposed to continuous intense light and was bleached (lost its color) and lost its ability to absorb light; but when it was returned to the dark, it regained both its redness and its light-absorbing capacity.

Question 26

First step in rod-mediated vision

Answer 26

the degree to which rhodopsin absorbs light in various situations predicts how humans see and detect different wavelengths under the very same conditions.

Question 27

Relationship between the absorption spectrum of rhodopsin and the human scotopic spectral sensitivity curve

Answer 27

Goodness of fit indicates that our sensitivity to various wavelengths is a direct consequence of rhodopsin’s ability to absorb them

Question 28

rhodopsin is a g-protein-coupled receptor

Answer 28

responds to light rather than to neurotransmitter molecules; they initiate a cascade of intracellular chemical events when they are activated.

Question 29

rods in the dark

Answer 29

their sodium channels are partially open, thus keeping the rods slightly depolarized and allowing a steady flow of excitatory glutamate neurotransmitter molecules to emanate from them.

Question 30

rods bleached from light

Answer 30

the resulting cascade of intracellular chemical events closes the sodium channels, hyperpolarizes the rods, and reduces the release of glutamate.

Question 31

The transduction of light by rods exemplifies an important point

Answer 31

Signals are often transmitted through neural systems by inhibition.