presbyopia
laser surgery and perfect vision…
pupil size control….from brain, or in the eye?
Advantage: less aberration, better image, more depth of field if sensitivity isn’t vital. Plus: avoids dazzle.
Why do we fail to perceive every photon of light that arrives at the retina in near-darkness?
Suppose that a drop of some chemical were placed on a small part of your retina, and that the chemical attached to rods and cones and prevented sodium ions (Na+) from entering them. What would be the effect?
a. receptors in that area would hyperpolarize, and a bright spot of light would appear (and then fade)
b. receptors in that area would depolarize, and a bright spot of light would appear (and then fade)
c. receptors in that area would depolarize, and a black spot would appear (and then fade)
When a single photon is absorbed by the rhodopsin in a rod, it initiates a complex chain of chemical events. This has the ultimate effect of
a. closing a large number of sodium channels
b. releasing a single vesicle of the transmitter GABA
c. opening a large number of channels for various ionic species
d. contracting the pedicle and squeezing out sodium
Horizontal cells are apparently the first stage in the process of
a. lateral antagonism
b. accentuation of temporal changes
c. motion detection
d. absorption of photons
e. orientation selectivity
The responses of bipolar cells are determined mainly by
a. total amount of light in their receptive fields
b. light in the center of their fields, moderated by that in the surround
c. rapidity of change of the lighting conditions
d. location in the retina: bipolars in the fovea hyperpolarize, while those in the periphery depolarize