Chapter 6.3 Flashcards
Retina-geniculate-striate pathways
conduct signals from each retina to the primary visual cortex, or striate cortex, via the lateral geniculate nuclei of the thalamus.
About 90% of the axons of retinal ganglion cells
become part of the retina-geniculate-striate pathways.
All signals from the left visual field reach
the right primary visual cortex, either ipsilaterally from the temporal hemiretina of the right eye or contralaterally (via the optic chiasm) from the nasal hemiretina of the left eye. Opposite is true of all signals from the right visual field.
Each lateral geniculate nucleus has
six layers, and each layer of each nucleus receives input from all parts of the contralateral visual field of one eye.
each lateral geniculate nucleus receives visual input only from
the contralateral visual field; three layers receive input from eye and three from the other.
Most of the lateral geniculate neurons that project to the primary visual cortex
terminate in the lower part of the cortical layer IV, producing a characteristic stripe, or striation, when viewed in cross section.
Retinotopic
includes the retina-geniculate-striate system; each level of the system is organized like a map of the retina. This means two stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the system.
The retinotopic layout of the primary visual cortex
a disproportionate representation of the fovea. Even though it is only a small part of the retina, a relatively large proportion of the primary visual cortex (about 25%) is dedicated to the analysis of its input.
Dobelle, Mladejovsky, and Girvin
implanted an array of electrodes in the primary visual cortex of patients who were blind because of damage to their eyes. If electrical current was administered simultaneously through an array of electrodes forming a shape on the patient’s cortex, the patient reported “seeing” a glowing image of that shape. Could be the basis for the development of a visual prosthesis.
At least two parallel channels of communication flow through each lateral geniculate nucleus.
One channel runs through the top four layers (parvocellular layers; P layers). Another channel runs through the bottom two layers (magnocellular layers; M layers).
parvocellular layers
composed of neurons with small cell bodies. Responsive to color, fine pattern details, and stationary or slowly moving objects. Cones provide the majority of the input.
magnocellular layers
composed of neurons with large cell bodies. Responsive to movement. Rods provide the majority of the input.
The parvocellular and magnocellular neurons project
to different areas in the lower part of layer IV of the striate cortex. In turn, these M and P areas of the lower layer IV project to different areas of the visual cortex.
