Answer to Question 1
Answer: Photoreceptors are partially depolarized and release glutamate in the dark. Light
hyperpolarizes the photoreceptor and reduces the release of glutamate. Photoreceptors synapse onto bipolar cells, which in turn synapse onto ganglion cells. Glutamate released from the photoreceptor inhibits the bipolar cell in the dark; an IPSP in the photoreceptor would thus produce less inhibition of the bipolar cell, and this would lead to more release of an excitatory transmitter onto the ganglion cell, producing action potentials in these cells.
Answer to Question 2
Answer: The striate cortex is organized into about 2500 modules, each containing about 150,000 neurons. Each module has two segments that surround a central regiontermed a blobthat contains cells that stain positively for cytochrome oxidase. Cells within the blob receive information about color and low spatial frequencies. Cells outside the blob are sensitive to different orientations, movement, spatial frequency, and texture, but not color. Each neuron in the module receives information from the same region of the visual field.
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