Visual Processing Review Notes

Visual Processing Overview

• Light enters through the cornea; the pupil regulates light entry; the proximal stimulus is the retinal image, which is upside-down and reversed on the retina.

• Retina contains photoreceptors: rods (low light, motion) and cones (color, high acuity); chemical changes in photoreceptors trigger signals to bipolar cells, then to ganglion cells.

• Ganglion cells form the output layer; their cell bodies reside in the retina and their axons bundle to form the optic nerve.

• Visual information from the two eyes travels via the optic nerve; at the optic chiasm, $\tfrac{1}{2}$ of the information crosses to the opposite hemisphere.

• Signals proceed to the LGN (lateral geniculate nucleus) in the thalamus, then onward to the occipital lobe for processing.

• Visual processing uses parallel processing, meaning multiple features are processed in different locations simultaneously.

• After initial processing in the occipital lobe, information exits via two pathways:

  • ## Ventral Stream (What Pathway)

  • connects to the inferotemporal cortex in the temporal lobe; responsible for object identification.

  • ## Dorsal Stream (Where/How Pathway)

  • connects to the parietal lobe; responsible for spatial location, movement, and guiding actions.

Retina and Early Visual Processing

• Photoreceptors (rods and cones) convert light into chemical signals that excite next-layer neurons (bipolar cells).

• Bipolar cells transmit to ganglion cells; ganglion cell axons form the optic nerve that carries signals toward the brain.

• The retina serves as both the initial input stage and the first site of neural computation before transmission to the brain.

Visual Pathway to the Brain

• Optic nerve carries signals from the retina toward the brain.

• At the optic chiasm, $\tfrac{1}{2}$ of the information crosses to the opposite hemisphere, enabling contralateral processing.

• Signals reach the LGN (thalamus) and are relayed to the occipital lobe for initial cortical processing.

Parallel Processing in Vision

• Different visual features (color, motion, form, depth) are processed simultaneously in specialized pathways.

• This distributed processing enables rapid, integrated perception across multiple attributes.

Ventral Stream (What Pathway)

• Purpose: object identification and recognition.

• Pathway: Occipital -> Inferotemporal cortex (in the temporal lobe).

• Key function: Determine "what" an object is.

Dorsal Stream (Where/How Pathway)

• Purpose: spatial location, movement, and action guidance.

• Pathway: Occipital -> Parietal lobe.

• Key function: Determine "where" an object is and how to interact with it (guides movements).

Key Terms

• Cornea: the transparent front surface of the eye that begins refraction of light.

• Retina: the light-sensitive layer at the back of the eye containing rods and cones.

• Proximal Stimulus: the retinal image formed by external light; the image is inverted and reversed on the retina.

• Parallel Processing: the simultaneous processing of multiple aspects of a visual scene.

• Optic Nerve: bundle of ganglion cell axons that carries visual information to the brain.

• Optic Chiasm: the crossing point where some optic nerve fibers cross to the opposite hemisphere; contributes to contralateral processing.

• LGN (Lateral Geniculate Nucleus): thalamic relay that forwards visual information to the visual cortex.

• Occipital Lobe: primary cortical region for visual processing.

• Parietal Lobe: brain region involved in spatial processing and action guidance.

• Inferotemporal Cortex: region in the temporal lobe involved in object recognition.