Visual System Structure and Processing (6A) - MCAT Psychological and Social Foundations

Light-sensitive pigment in rods (opsin + retinal). The visual pigment undergoes conformational change when light hits retinal.

Both: nasal retinal fibers cross, carrying the contralateral visual field. Nasal fibers from each eye cross to process opposite visual fields.

Relay signals from photoreceptors to ganglion cells. They form the middle layer of the retinal neural circuit.

Difference between the two retinal images; a binocular depth cue. Brain computes distance from horizontal image displacement between eyes.

The optic nerve (cranial nerve II). Ganglion cells are the output neurons whose axons carry visual signals to the brain.

Optic disc (optic nerve head). Where ganglion cell axons converge to form the optic nerve, no photoreceptors exist.

Photoreceptors hyperpolarize and reduce glutamate release. Light closes cation channels, causing hyperpolarization and decreased neurotransmitter.

Occipital-to-parietal “where/how” pathway for spatial location and motion. Guides actions and tracks movement through posterior parietal cortex.

Occipital-to-temporal “what” pathway for object identity and form. Processes visual features for recognition in inferior temporal cortex.

Primary visual cortex (V1, striate cortex) in the occipital lobe. V1 contains orientation-selective cells organized in columns.

Lateral inhibition to enhance contrast via photoreceptor-bipolar modulation. They inhibit neighboring photoreceptors to sharpen edges and boundaries.

Intermediate neurons between photoreceptors and ganglion cells. They relay signals vertically through the retinal layers.

Fovea (within the macula). Dense cone concentration provides sharp central vision.

Color (photopic) vision; lower sensitivity, high acuity. Cones contain opsins for detecting different wavelengths.

Dim-light (scotopic) vision; high sensitivity, low acuity, no color. Rods contain rhodopsin for detecting low light levels.

Light converts signals in photoreceptors into neural activity. Photons trigger molecular cascades that hyperpolarize cells.

Glutamate. Darkness depolarizes photoreceptors, triggering release.

ON: fire to light in center; OFF: fire to darkness in center. Opposite responses create contrast detection mechanisms.

Retina → optic nerve → optic chiasm → LGN → optic radiations → V1. Visual signals relay through thalamus before reaching cortex.

Nasal retinal fibers decussate; temporal fibers remain ipsilateral. This crossing allows each hemisphere to process both eyes.

Right visual field. Contralateral processing: each hemisphere sees opposite field.

Left visual field. Visual fields project contralaterally after chiasm crossing.

Thalamic relay nucleus for visual input from retina to V1. Part of thalamus that processes visual signals before V1.

First cortical visual area in occipital lobe (calcarine cortex). Receives LGN input for initial cortical visual processing.

Spatial layout of retina is preserved in V1 cortical representation. Neighboring retinal points map to neighboring cortical areas.

Object identity processing (form, color); projects to temporal lobe. Processes visual features to recognize objects and faces.

Spatial location and motion processing; projects to parietal lobe. Analyzes movement and guides visually-directed actions.

Modulate bipolar-to-ganglion signaling; motion/temporal processing. They integrate signals laterally for complex processing.

Exit of optic nerve; creates the blind spot (no photoreceptors). Axons converge here, leaving no room for photoreceptors.