- in the mouse, everything crosses at the chiasm, in humans it’s not the case
- axons (of RGCs) of the optic tract terminate in 4 nuclei w/in the brain:
1.) LGN: of the thalamus – for visual perception
2.) superior colliculus: of the midbrain, non-image forming – for control of eye movements
3.) SCN: of the hypothalamus, non-image forming – for control of diurnal rhythms and hormonal changes
4.) pretectum: of the midbrain, non-image forming – for control of the pupillary reflex
- LGN has 6 layers, 3-6 are v. alike and 1+2 are similar to each other (Nissl stain of ER ribosomes)
1&2 = magnocellular layer, cells are larger, contain more Nissl stain substance
3-6 = parvocellular layer
- just a relay station, inputs = outputs of the ganglion cells that then project to the primary cortex/v1/striate cortex
- alternating layers get input from RGCs
- 18 types of RGCs (ON and OFF RGCs, ON/OFF center surround RGCs, etc)
- P&M ganglion cells are specific to the primate high visual acuity and color processing systems
- classes of ganglion cells called P and M ganglion cells in primates, encode two important features of vision
- P and M project to two different parts of the LGN
- size of RFs of P and M are very different, structure related to function
M cells: (like alpha cells in cats) are important for detecting motion
- M-cells project to magnocellular layers
- M-cells are color-blind, direction sensitive, adapt to a maintained stimulus, best for detecting movement across a RF
- much larger than P-ganglion cells
- synapses w/ many bipolar cells
- large concentric RF and more sensitive to small center-surround brightness differences
- responds w/ a transient, RA response to a maintained stimulus
- responds maximally, w/ high discharge rates, to stimuli moving across its RF
P cells – high acuity b/w neighboring points in the RFs/retinas, and color sensitive (sensitive to wavelengths of light), like beta cells in cats
- not direction selective (produce weak responses), midget RGCs
- pavocellular layer receive input from the P-cells
- out# the M-ganglion cells by ~100 to 1 in the primate retina
- make synaptic contact w/ one to a few cone BCs that are innervated by foveal cones
- small concentric RF
- produces a stustained, SA response that lasts as long as a stimulus is centere in its RF
- this type of response is best suited for singlaing the presence, color ad duration of a visual stimulus and is poor for signaling
stimulus movement
Recent comments