MODULE 5

• the brain codes information largely in terms of which neurons are active, and how active they are at any moment • impulses in certain neurons indicate light, whereas impulses in others indicate sound, touch, or other sensations • whatever excites a particular nerve establishes a special kind of energy unique to that nerve

light enters the eye through an opening in the center of the iris

• back of the surface of the eye that is lined by visual receptors • light from above strikes the bottom half of the retina, and light from below strikes the top half • the inversion of the image poses no problem for the nervous system

• messages go from the receptors at the back of the eye to bipolar cells, located closer to the center of the eye • bipolar cells send their messages to the ganglion cells, located still closer to the center of the eye • the ganglion cell’s axons join together and travel back to the brain • additional cells called amacrine cells to get information from bipolar cells and send it to other bipolar, amacrine, and ganglion cells • amacrine cells refine the input to ganglion cells, enabling certain ones to respond mainly to particular shapes, directions of movements, changes in lighting, color, and other visual features

they are transparent, and light passes through w/o distortion

the ganglion cell axons join to form the optic nerve that exits through the back of the eye. the point which it leaves is a blind spot because it has no receptors

3mm x 5mm center of the retina w/ greatest ability to resolve detail

• center of macula • a tiny area specialized for acute, detailed vision

• each receptor in the fovea connects to a single bipolar cell, which in turn connects to a single ganglion cell that has an axon to the brain. the ganglion cells in the fovea of humans and other primates are called this because each is small and responds to just a single cone • each cone in the fovea has a direct route to the brain • provides 70% of input to the brain–your vision is dominated by what you see in and near the fovea

toward the periphery of the retina, more and more receptors converge onto the bipolar and ganglion cells

• receptors are cones only • each ganglion cell excited by a single cone • distinguishes among bright lights; responds poorly to dim light • good detail vision because each cone’s own ganglion cell sends a message to the brain • good color vision because of many cones

• proportion of rods increases toward the periphery • each ganglion cell excited by many receptors • responds to dim light; poor for distinguishing among bright lights • poor detail vision because many receptors converge their input onto a given ganglion cell • color vision is poor because of few cones

• abundant in the periphery of the human retina, responds to faint light but are not useful in daylight because bright light bleaches them • outnumber cones by about 20 to 1 in the human retina, though cones provide about 90% of the brain’s input

abundant in and near the fovea, are less active in dim light, more useful in bright light, and essential for color vision

photopigments: chemicals that release energy when struck by light

• perceive color through the relative rates of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths • short, medium, and long-wavelength • we discriminate among wavelengths by the ratio of activity across the three types of cones

• perceive color in terms of opposites • the brain has a mechanism that perceives color on a continuum from red-green, yellow-blue, and white-black • after you stare at one color in one location long enough, you fatigue that response and swing to the opposite

a replacement of the red you had been staring at with green, green with red, yellow and blue with each other

• color perception requires reasoning • color constancy: the ability to recognize colors despite changes in the lighting • the cortex compares information from various parts of the retina to determine the brightness and color for each area

• colorblindness–inability to perceive color differences • when people to fail to develop one type of cone • cones have the same photopigments instead of different ones

• synapse with rods and cones of the retina along with bipolar cells • makes inhibitory contact with bipolar cells, which in turn make synapses onto amacrine and ganglion cells

optic nerves from the two eyes meet here, where, in humans, half of the axons from each eye cross to the opposite side of the brain

• part of the thalamus where most ganglion cells go to • sends axons to other parts of the thalamus and the visual cortex

• the retina’s way of sharpening contrasts emphasizes the borders of objects • the reduction of activity in neighboring neurons