Neuroscience of Visual Processing

Feature Detectors

• Feature detectors are involved in identifying lines, edges, and shapes.
• Visual illusions activate the same brain regions as non-illusory perceptions.

Organisation of Visual Processing

• Dorsal stream (Parietal cortex): Processes object location and visual-guided actions.
• Ventral stream (Inferotemporal cortex): Identifies objects.
  • Category selectivity increases from posterior to anterior regions of the ventral stream.

Double Dissociation

• Lesion studies (humans) and ablation/single-unit recording studies (monkeys) show double dissociation.
• Visual agnosia: Temporal lobe lesion; difficulty recognising shapes.
• Optic ataxia: Parietal lobe lesion; difficulty coordinating visual input and hand movements.
  • Primarily affects upper visual field.
  • Reaching movements appear wide-open.

PET study - Zeki et al. 1991

• V4: Primarily involved in color perception (comparing coloured to grayscale images).
• V5: Involved in motion perception (comparing moving and static dots).

Patient LM - Zihl et al. 1983 (Akinetopsia)

• Inability to perceive motion.
• Difficulty in identifying moving and stationary objects.
• Can perceive motion through other senses (hearing, touch).

Retina vs. V4

• Retina (cones): Detect different wavelengths of light.
• V4: Accounts for variations in lighting conditions, preserving colour constancy.
  • V4 cells respond to the same surface color even under varying light sources.
  • V1 cells do not demonstrate colour constancy.

De Renzi 1986

• Patient failed to recognize family members; this ability was present with voice or clothes.
• Could compare different views of faces and identify other objects.

Fusiform Face Area (FFA)

• Located in the right hemisphere.
• Actively responds to faces more than other objects in functional imaging experiments (Kanwisher).
• FFA activity might be linked to expertise in recognizing faces.
• There is a relative difference in face vs object recognition in some cases.
• Prosopagnosia: Damage to the FFA results in prosopagnosia, also known as "face blindness," where individuals
  cannot recognize familiar faces but may still identify objects or voices

Expertise and FFA - Gauthier et al. 1999

• Greeble recognition training demonstrates holistic face-like perception in recognising Greebles.
• Greeble recognition shows similar behavioral patterns, following face recognition.
• Increased ability to process Greebles after extensive training, sometimes at the cost of face recognition ability.

Domain Specificity and Innateness

• Difficulties in definitively proving domain specificity/innateness of face recognition mechanisms.

Visual Word Form Area (VWFA)

• Located in the left hemisphere.
• Displays stronger response to written words than consonant strings.
• Pure alexia: Difficulty in reading letters individually, showing a letter-by-letter reading pattern.
• Recovery in reading abilities is possible over time, though full recovery is not guaranteed.

VWFA vs. FFA

• Differing locations in left and right hemispheres.
• Word and face recognition have similar visual and computational demands.
• Theory: Functional specialization is a consequence of task and visual system interactions.
• Neuronal recycling: Reading may repurpose pre-existing object or face recognition neural networks (Dehaene et al. 2015).

Emergent Specialisation

• Distinct tasks within the visual system exist and have unique computational demands.
• The What Pathway (Inferotemporal cortex): Object recognition (location, size invariant).
• The Where Pathway (Parietal cortex): Object localisation (dependent on location, shape, slightly on size).
• Deconstruction of complex tasks into specialized cognitive systems (e.g., VWFA and FFA).

Ventral-Dorsal Streams

• Connections between dorsal and ventral streams exist.
• Important for skilled object use.

Tool Use in Chimpanzees

• Tool use ability correlates with the grey matter volume in the ventral-dorsal stream.