PSY 3061 Lecture Notes - Lecture 11: Optic Chiasm, Visual Acuity, Ganglion Cell
Biopsychology lecture 3/7/18
Rods and light sensitivity
• Rods concentrated in the periphery of the retina, cones in fovea (center)
• Rhodopsin is a pigment (chemical that absorbs light) which is found in rods
o When exposed to light, rhodopsin changes shape
Convergence: Reduction of signal
• Cones have less convergence (important for visual acuity)
From retina to brain…
1. Rods and cones hyperpolarize in light
a. Rhodopsin changes shape, release G-protein, obscures Na+ channels, Na+ influx
impossible
b. Photoreceptors release less excitatory neurotransmitter Glutamate
c. Less Glutamate = less excitation = less acuity
2. Bipolar cells relay the message to ganglion cells
3. Ganglion cells transmit message to the brain via the optic nerve
Retinogeniculate visual pathway
• Visual messages are transmitted from ganglion cells of the retina, through the optic chiasm and
lateral geniculate (LGN) and to the primary visual cortex
• Visual fields are determined by the nasal (medial) and temporal (lateral) hemiretinas
• The optic chiasm sorts axons from the right visual field to the left hemisphere of the brain, and
vice versa
• Visual fields overlap
• Left visual field projects onto right side of retina, right visual field projects onto left side of retina
• Shared visual field gets projected onto the right primary visual cortex
• Right visual field projected onto left primary visual cortex, left visual field projected onto right
primary visual cortex
• Visual cortex mimics a map of the retina
o Retinotopic map
• The retina-geniculate-striate pathway is organized retinotopically (like a map of the retina)
o Fovea represented by greatest area in primary visual cortex
Lateral inhibition enhances contrast
• Lateral inhibition:
o Cone transduces signal to a ganglion cell
o Neighbor ganglion cells will not become active (inhibited)
o As a result, some parts of the stimulus are brighter than their adjacent parts in the visual
field
o Illusion that edges are brighter than they actually are
