Sensation and Perception Problem Explanations

Question 1

Why is your vision blurry underwater? Why is it not blurry when you wear swimming
goggles underwater?

Answer 1

When you open your eyes underwater, there is water both outside and inside your cornea, so the cornea loses its focusing power and your vision becomes blurry. However, when you wear goggles, you preserve this air-water boundary, and the cornea keeps its focusing power

Question 2

You are doing an experiment to find out how dim the faintest light a person can
perceive is. Using what you know about the human eye, where should you position the
light?

Answer 2

Rods are more sensitive to faint light than cones are, so we should design the stimulus
so that the observer relies on rod vision. Rod density peaks away from the fovea, except at the optic disc, on the nasal side, where there are no photoreceptors at all. We should position the light so that it falls on this high-density region of the rods. (In fact this is about 15 degrees from the fovea.) To be safe we should put the light to the nasal side of straight ahead, so that the photons fall on the temporal side of the retina and avoid the optic disc

Question 3

Pigeons have unusual eyes: the bottom half of their retina is much closer to the lens
than the top half of the retina. Why do you think pigeons have developed such strange
eyes

Answer 3

The image falling on the top part of the pigeon’s retina comes from nearby objects on the ground, and the image on the bottom part of the retina comes from distant objects in the sky. By having the top half it its retina farther from the lens than the bottom half of its retina, the pigeon is able to focus on both nearby and distant objects simultaneously

Question 4

How does adapting to a 5 cycle/degree sine wave grating change the contrast
sensitivity function of an observer with many spatial frequency channels? Why?

Answer 4

If the observer has many spatial frequency channels, then adapting to a 5
cycle/degree grating will fatigue only channels sensitive to the 5 cycle/degree grating
and leave others unaffected. This will produce a dip in the contrast sensitivity function
around 5 cycles/degree, but leave sensitivity the same at other spatial frequencies.

Question 5

How would adapting to a 5 cycle/degree sine wave grating change the contrast sensitivity
function of an observer who had just one spatial frequency channel responsible for
perceiving all spatial frequencies? Why?

Answer 5

If the observer has only one spatial frequency channel, then adapting to a
5 cycle/degree grating will fatigue that channel, and will lower sensitivity at all spatial
frequencies.

Question 6

5. Suppose you adapt to a pattern that contains a 2.0 cycle/degree sine wave and also a 6.0 cycle/degree sine wave. Sketch a plot showing what your contrast sensitivity function would look like after adapting to this stimulus.

Answer 6

When we adapt to 2 cycle/degree and 6 cycle/degree sine waves, the neurons that detect those spatial frequencies become fatigued, and so they become worse at detection. Thus our sensitivity at 2 and 6 cycles/degree declines, and our contrast sensitivity function has dips at these spatial frequencies.