Biol 317 The Visual System In our brain, the visual field is presented by a right one and a left one, two individual maps The right side of the field is projected on the left side of the retina for both eyes, but the left side neuron?s ganglion cells only project to the left side of the brain; the left side of the visual field is projected onto the right side of each retina that is then projected only to the right side of the brain. If the visual cortex is damaged, loss of sight occurs In most vertebrate animals, from fish to mammals, the baseline design for eyes is a lateral view, left eye looks at left wall, right eye looks at right wall, in the middle there is an overlap of the visual field, but it?s very small. In primates, the frontal vision became more important than the lateral vision, causing the eyes to move forward Need frontal vision for depth perception Fovea is the area where the specific images are brought to The output cells from the retina send them up (or down) into the optic nerve The outermost layer is the photoreceptor layer, it is thickened close to the center of the fovea Has very high special resolution, the receptive fields for the processing neurons are very small A singe photoreceptor is connected to a single ganglion cell (normally ganglion cells are connected to many photoreceptors) -blind spot correlates to the area in the retina where there are no photoreceptors Retinal Structure Photoreceptors: cell that receive photons and transfuse the light information into neural signal, cones and rods Cones are less sensitive, rods are highly sensitive, we switch between them in night and day vision If all the lights were turned off, we would use rods, if we went outside at noon, we completely rely on cone The next layer is horizontals: this cell connects horizontally between photoreceptors (lateral connections) Bipolar cells: very important in the retina, connects information from the photoreceptor to the ganglion cell Amacrine cells connect bipolar cells and ganglion cells laterally Ganglion cells contain photoreceptors as well (this is newly discovered), horizontal cells are also intrinsically neuro-responsive/photoreceptive. some retinal neurons are nonspiking neurons, they do NOT generate action potentials, -spikes arrive at the synapse and ion channels are opened or closed There are neurons that respond to slow graded potentials Nonspiking neurons are found in the retina and in the invertebrate nervous system Cones and rods, bipolar and horizontal cells are completely non-spiking, Graded potential does not propagate at long distance, they signal locally only Amacrine cells and ganglion cells do spike(for the most part); ganglion cells must spike to send information to the brain from the optic nerve photoreceptors and horizontal cells respond to light with hyperpolarization; all neurons everywhere, depolarization means excited neuron and signaling When light hits cones and rods, the membrane potential goes down Color Vision - Light comes with different wave lengths, we can produce monochromatic light to see one color at a time, or we can see many: there is NO white light, it is a mixture of light from all wavelengths Our ability to see color is our ability to see spectral distribution Photo pigments or color filters are necessary to tune into specific wavelengths to see specific light mammals default to have two photoreceptors, only primates developed a third, but reptiles, birds, and fish sometimes have many more than two or three Color perception is recognition of spectral distribution. Presence of photo pigments or color filters is the basis of color vision. At least two types of photoreceptors are necessary. We have three, some organisms have as many as eleven. Presence of a color filter, however, does not ensure color vision. A color-opponent process is required.
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