The Visual Cortex

The neural impulses coming from the eye are heavily processed by the visual cortex. The main task of the visual cortex is to group the 1.5 million inputs from each eye into larger spatially coherent groupings with shared qualities (for example of shared colour, or shared movement). The details of the processing in the cortex are beyond the scope of this exploration. However, there are some details that are relevant to this inquiry.

The visual cortex maintains the spatial arrangement of the sensors in the retina. The space of the visual field is for the most part physically reflected within the structure of the visual cortex, although this space is not linear like the space in a photographic image, but tends to reflect the varying density of sensors in the retina itself.

A very large proportion of the visual cortex is devoted to the processing of the fovea and the area immediately around it.

The neural patterns are initially received and processed as local contrast information rather than absolute colour values.

The visual cortex builds its interpretation of the scene heirachically. The processing starts by looking at characteristics local to small sections of the visual field. These characteristics include orientation, colour, spatial frequency, disparity and movement. As the visual input moves through the layers of the cortex, these local characteristics get integrated into larger scale coherent patterns and fields, eventually resulting in our perception of objects, surfaces and backgrounds.

There appear to be 2 parallel streams of processing known as the "Where" pathway and the "What" pathway. The "Where" pathway determines motion and spatial relationships and the "What" pathway determines size, colour, texture and shape.

The processing at lower levels can be modulated by prior observations at a higher level. This means that the lower level processing can be tuned to suit what the cortex thinks it is looking at.

There is a time lag to visual perception. For the first 40 milliseconds, the lower levels of the cortex are busy processing local detail. The higher level tasks of making sense of the image do not begin until about 100 milliseconds after the impulses reach the cortex from the eye.