EXAM 2 STUDY GUIDE
1. How might the phenomenon of lateral inhibition help to accentuate the presence of a border between
light and dark areas?
Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring
neurons in the lateral direction. This creates a contrast in stimulation that allows increased sensory
perception. Lateral inhibition increases the
contrast and sharpness in visual response.
In the dark, a small light stimulus will be
enhanced by the different photoreceptors.
This contrast between the light and dark
creates a sharper image.
The way in which lateral inhibition could
produce the perception of Mach bands is
shown to the left. If the receptive field of a
retinal ganglion cell lies entirely within the
boarders of a band (e.g., cells 1 and 2
below), then both the excitatory center and
inhibitory surround are uniformly illuminated
with little net effect on their firing (i.e., the excitatory and inhibitory firing cancel each other). Compare
this to cell 3, whose inhibitory surround is partially in the darker area to its left. As a result, cell 3 will be
less inhibited by the surround and fire at a faster rate, resulting in the brain interpreting this as brighter.
2. (PART A) What effects to rods and cones have on dark adaptation? (Understand Figure 2.16).
Photopic: Light intensities that are bright enough to stimulate the cone receptors and
bright enough to “saturate” the rod receptors. Sunlight and bright indoor lighting are
both photopic lighting conditions.
Scotopic: Light intensities that are bright enough to stimulate the rod receptors but
too dim to stimulate the cone receptors. Moonlight and extremely dim indoor lighting
are both scotopic lighting conditions.
Rods are sensitive to scotopic light levels. All rods contain the same photopigment
molecule: Rhodopsin. All rods have the same sensitivity to various wavelengths of
light. Therefore, rods suffer from the problem of invariance and cannot sense
differences in color. Under scotopic conditions, only rods are active, so that is why the
world seems drained of color.
Light Adaptation: Light adaptation is MUCH FASTER than dark adaptation. When you step
out from the dark into bright light and the cones become hyperpolarized, the pupils constrict to
help reduce the light entering the eye
Dark Adaptation: The transition from all-cone daytime vision to all-rod nighttime vision. The
process in which the eye increases its sensitivity in the dark in two distinct stages.
Stage 1 of dark adaptation involves the cone receptors and is very rapid. During stage 1 The
Rods begin adapting to the dark first, followed by the cones, thus enabling them to control our
vision at this stage. The cones rapidly adapt to the dark, reaching the max cone sensitivity (to
darkness) before the Rods reach their max rod sensitivity.
Stage 2 of this type of adaptation involves the rod receptors and is slower than stage 1. The
rods continue to adapt to the dark at a much slower pace, eventually becoming much more
sensitive in the dark than the cones.
2. (PART B) What mechanisms does the visual system have for light adaptation?
During dark adaptation, the pupils dilate and unbleached rhodopsin is regenerated.
During dark adaptation, the functional circuitry of the retina is adjusted so that
information from more rods is available to each ganglion cell.
Other changes that occur are increased sensitivity of photoreceptors, and increasing
dominance of rod activity—purkinje shift (most important). Purkinje shift is the relative increase
in the brightness of longer wavelength stimuli as lighting conditions change from scotopic to
photopic.
