NEUR2020 Neuroscience for Psychologists: Visual System

Visual System Overview

Visual Pathway Overview

  • Pathway: Eye -> Retina -> Thalamus (LGN) -> Primary Visual Cortex (V1) -> Extrastriate Cortex -> Extended Cortex (Temporal/Parietal).

  • Superior Colliculus (SC): 10\% of visual input; involved in exogenous (stimulus-driven) orienting and emotion via SC-pulvinar-amygdala pathway.

Key Concepts

  • Decussation: Partial crossing of optic nerves at the optic chiasm (50\% in humans). Left visual field maps to right cortex, right to left.

  • Retinotopic Organisation: Adjacent points in the visual field map to adjacent points on the retina; this mapping is maintained through the processing hierarchy.

  • Cortical Magnification: A disproportionately larger area of cortex is dedicated to processing the central visual field (fovea) than the periphery.

  • Receptive Fields (RFs): Regions on the retina (and visual field) where light must fall to change a neuron's firing rate. They have excitatory and inhibitory regions. Small RFs provide high spatial resolution; large RFs provide low spatial resolution.

The Eye

  • Functions: Image formation, transduction of light to neural signals, early neural processing, and transmission to the brain.

  • Anatomy for Image Formation:

    • Cornea: Transparent outer layer; performs most light bending (refraction).

    • Lens: Fine-tunes image formation, adjustable via accommodation reflex.

    • Iris and Pupil: Iris regulates the pupil's size, controlling light entry and focal length.

  • Retina: Contains photoreceptors for transduction (rods and cones) and layers of neurons for early processing. Retinal Ganglion Cells (RGCs) are the final layer, whose axons form the optic nerve.

  • Fovea: Small, specialised area for high-acuity central vision; high density of cones.

  • Optic Disc (Blind Spot): Point where RGC axons leave the retina to form the optic nerve; lacks photoreceptors. Vision is