Visual System: An Extensive Overview

University of Queensland Acknowledgment

  • The University of Queensland (UQ) acknowledges the Traditional Owners and their custodianship of the lands on which they meet.

  • Respect is paid to their Ancestors and their descendants, who maintain cultural and spiritual connections to Country.

  • Their valuable contributions to Australian and global society are recognised.

NEUR2020: Neuroscience for Psychologists - Lecture 5: Visual System

  • This lecture, delivered by Mick Zeljko in Semester 2, 2025, focuses on the Visual System.

Visual System Overview

  • The visual pathway includes: Eye, Retina, Thalamus, Primary Visual Cortex (Occipital Lobe), Extrastriate Cortex (Occipital Lobe), and Extended Cortex (Temporal and Parietal).

  • Initial Stages:

    1. Image Formation occurs in the eye.

    2. Transduction (light to neural signal) occurs in the retina.

    3. Visual Processing begins in the retina.

  • Subcortical Pathways:

    • About 10\% of visual information goes to the Superior Colliculus (SC), involved in exogenous (stimulus-driven) orienting.

    • A pathway from SC to pulvinar to amygdala is linked to emotion.

  • Main Cortical Pathway: The primary pathway is the retino-geniculate-striate pathway.

  • Key Concepts:

    • Decussation: Partial crossing of optic nerves at the optic chiasm.

    • Retinotopic organisation: Adjacent points in the visual field map onto adjacent points on the retina, maintained throughout processing.

    • Cortical magnification: More cortical area is dedicated to processing the central visual field (fovea) than the periphery, due to convergence.

    • Receptive fields (RFs): Regions on the retina where light must fall to change a particular neuron's firing rate, characterising a cell's function and features it detects.

Decussation

  • Partial decussation means that optic nerve fibres partially cross over.

  • Information from the left visual field projects to the right cortex, and the right visual field projects to the left cortex.

  • Approximately 50\% of optic nerve fibres cross at the optic chiasm in humans.

  • Optic nerves transmit information from bilateral visual fields.

  • Optic tracts transmit information from unilateral visual fields (the contralateral visual field).

Retinotopic Organisation

  • This principle states that the spatial arrangement of light on the retina is preserved as neural signals are mapped onto the brain.

  • Adjacent points on the retina correspond to adjacent points in the visual field, and this adjacency is maintained in the cortex.

Cortical Magnification

  • A disproportionately large area of the visual cortex is dedicated to processing information from the fovea (central vision).

  • This allows for high spatial resolution in the central visual field, despite its small physical size on the retina.

Receptive Fields (RFs)

  • A neuron's RF is the specific area of the retina (and thus the visual field) where light stimulation influences its firing rate.

  • RFs typically have both excitatory regions (where light increases firing) and inhibitory regions (where light decreases firing).

  • The arrangement of these regions provides clues about what specific features a cell is detecting (e.g., edges, orientations).

  • Size and Function:

    • Small RFs are found in areas like the fovea and are associated with high spatial resolution, allowing for fine detail perception.

    • Large RFs are found in the periphery and are associated with low spatial resolution, processing coarser information.

  • Examples: Centre-surround cells and simple cells are noted as having specific RF types.

The Eye

  • Definition: A receptive field is the part of the retina (and therefore the visual field) where light must fall to change the firing of a particular neuron.

  • Primary Functions:

    1. Form an image.

    2. Generate a neural signal (transduction).

    3. Perform early neural processing of the signal.

    4. Transmit the visual signal to the brain.

Anatomy of the Eye

  • Cornea: Transparent outer layer, responsible for most light bending (refraction).

  • Lens: Fine-tunes image formation, adjustable via the accommodation reflex, stiffens with age.

  • Iris and Pupil: The iris is contractile tissue that regulates the size of the pupil (the opening), controlling light entry and, importantly, focal length.

  • Retina: Contains receptors for light transduction and layers of neurons for early signal processing. Retinal ganglion cells (RGCs) are the final layer, whose axons form the optic nerve.

  • Fovea: A small, specialised area for high-acuity central vision, which solves the