Lecture 14- Visual Pathways

Understanding visual perception involves different pathways and processes within the brain related to how we see and interpret color, shape, and movement.

Cone Types and Metamers:
- Humans typically have three types of cones (S, M, L) that detect short, medium, and long wavelengths of light.
- Metamers are colors that appear the same but are made up of different spectral power distributions.
- Having only two types of cones can lead to an increase in the variety of metamers due to fewer distinguishable frequencies of light being detected.
Color Blindness:
- Caused by the lack of one type of cone.
- Usually more similar color responses are observed in the presence of two functioning cones compared to the third.
Opponent-Process Theory:
- This theory explains certain visual phenomena better than the trichromatic theory. It accounts for features like negative afterimages, color combinations, and color contrast effects through the interaction of opposing color channels.
- Cells in the Lateral Geniculate Nucleus (LGN) demonstrate opponent-processing by being activated by one cone combination while being inhibited by another.

Visual Pathway: The visual pathway comprises components such as the optic nerve, optic chiasm, optic tract, LGN, and visual cortex.
- Optic Chiasm: Where the optic nerves cross; this means that the left visual field is processed in the right hemisphere and vice versa.
Contralateral Control: Each side of the brain processes visual information coming from the opposite visual field.

Visual processing begins at the:
- Optic Nerve: Composed of axons from the retinal ganglion cells.
- Optic Chiasm: Where half of the axons cross, leading to the formation of the optic tract.
- Lateral Geniculate Nucleus (LGN): A relay center in the thalamus that directs information to the visual cortex.
- Primary Visual Cortex (V1): The area where initial processing occurs and where information first reaches the cortex.

V1 (Primary Visual Cortex): First cortical relay; handles basic visual processing such as line orientation and shape detection.
- Discoveries by Hubel and Wiesel identified types of cells within V1, such as simple, complex, and end-stopped cells which play distinct roles in vision.
V2 and Higher Areas:
- V2: Enhances shape processing by interpreting complex shapes and featuring more detail than those processed in V1.
- V4: Responsible for color perception; damage can lead to conditions like achromatopsia or hemi-achromatopsia.
- V5/MT: Critical for motion detection; requires analysis of patterns of neural activation to interpret motion accurately.

What Pathway (Ventral Pathway): Deals with object recognition and identification, following V1 to V4 and into the inferior temporal cortex.
Where Pathway (Dorsal Pathway): Involved in spatial awareness and the location of objects, extending from V1 to V5/MT and into the posterior parietal cortex.
Empirical Evidence:
- Studies involving lesioning specific areas in monkeys demonstrated that damage to the temporal lobe impaired object recognition (What Pathway), while damage to the parietal lobe impaired spatial task performance (Where Pathway).

Magnocellular Cells (M-cells): Support motion perception; primarily fed by rods and activated in low-light conditions.
Parvocellular Cells (P-cells): Associated with color, texture, and depth perception; predominantly connected to cones.
Interconnections: Both pathways have mutual interactions and feedback loops involving higher brain areas, which refine visual processing and perception.

Understanding visual pathways is crucial to grasp how the brain processes different visual stimuli for coherent perception and understanding of our environment. This knowledge has implications in areas such as neuropsychology and rehabilitation for visual deficits.