Visual Pathways and Cortical Cells

Optic Chiasm

• Some nerve fibers cross to the opposite side of the brain from the eye.
• The right visual field corresponds to the left side/hemisphere of the brain, and vice versa.

Visual Pathway

• 90% of signals from the retina go to the lateral geniculate nucleus (LGN).
• The signal from the LGN to the cortex is smaller than the input the LGN receives from the retina.
• LGN receives more signals from the cortex than from the retina.
• The visual signal travels to the striate cortex in the occipital lobe, which is the visual receiving area
• Effects of processing at different levels of the visual system were studied by David Hubel and Torsten Wiesel.

Cortical Cells

• Simple cortical cells exist.
• Orientation tuning curve is determined by measuring a simple cortical cell's responses to bars with varying orientations.
• Complex cells, like simple cells, respond best to bars of a particular orientation.
• End-stopped cells fire to moving lines of a specific length or to moving corners/angles.
• Feature detectors are present.

Selective Rearing

• Neurons responding to stimuli become more prevalent if an animal is raised in an environment with only certain types of stimuli
• There is a "use it or lose it" principle.
• Neural plasticity/experience-dependent plasticity means neuron response properties can be shaped by perceptual experience.
• Selective adaptation occurs.

Neural Maps

• Retinotopic map: points close together on an object and the retina activate neurons that are close together in the brain.
• Electrical signals from the part of a tree being looked at occupy more space on the cortex than signals from parts off to the side.
• Cortical magnification: stimulation of the small area near the fovea activates a greater area on the cortex (red) than stimulation of the larger area in the periphery (blue).
• The striate cortex is organized into location columns where neurons within a column have receptive fields at the same location on the retina.
• Orientation columns have neurons along a track with receptive fields at the same location on the retina, but with preferences for different stimulus orientations.
• Hypercolumn: one Location Column contains Many Orientation Columns

Anatomy

• Optic nerve
• Optic chiasm
• Lateral geniculate nucleus
• Superior colliculus

Visual Cortex

• After processing in the striate cortex (V1), the visual signal goes to other visual areas in the occipital lobe and beyond
• These areas are known as V2, V3, V4, and V5
• The receptive neurons progressively become larger from V1 through V5