Episode 47: Vision Part 3

Two-Stream Theory

The two-stream hypothesis in visual processing distinguishes between the 'what' and 'where' pathways, each serving unique functions. James Fodor explains that the 'what' pathway is responsible for object recognition, while the 'where' pathway handles spatial location and movement coordination 1. This distinction is evident in conditions like optic ataxia, where individuals can recognize objects but struggle with using them due to impaired spatial processing 2.

The double dissociation of these two conditions allows us to infer that there are two separate regions of the brain, or streams of processing, one for object location and sort of motor function. And the other one focusing on object recognition.

--- James Fodor

These pathways highlight the brain's specialized approach to processing visual information.

   

Dorsal Pathway

The dorsal pathway, or 'where' pathway, is crucial for motion and spatial processing. Fodor describes area V5, also known as MT, as a key region that specializes in detecting motion, with neurons responding to specific directions of movement 3. Experiments with monkeys have shown that stimulating these neurons can alter motion perception, demonstrating the pathway's role in interpreting movement 4.

Scientists have been able to reliably alter the direction of motion, perception of monkeys, generally macaque monkeys, by selectively stimulating parts of v five.

--- James Fodor

This pathway's function is further evidenced by cases where damage to V5 results in an inability to perceive continuous motion, underscoring its importance in visual processing.

   

Ventral Pathway

The ventral pathway, or 'what' pathway, is essential for object and color recognition. Fodor highlights area V4's role in color processing and the inferior temporal cortex's specialization in recognizing complex shapes and faces 5. The fusiform face area within this pathway is particularly responsive to faces, supporting the idea of specialized brain regions for facial recognition 6.

Human faces are also so very similar for the most part, but we don't perceive it that way. We perceive different faces as being very different.

--- James Fodor

This specialization is crucial for social interactions, as evidenced by conditions like prosopagnosia, where individuals struggle to recognize faces despite normal vision.