X and Y ganglion cells inform the cat's brain about contrast in the retinal image.

It has been suggested for a number of years that ganglion cells inform the rest of the brain about contrast in the retinal image. The purpose of the work undertaken here was to demonstrate this fact explicitly. Extracellular recordings were made from X- and Y-cell axons of the optic tracts of anesthetized cats. Responses of these cells to gratings that were near optimal in spatial and temporal frequency were measured for a range of contrasts. For each cell, similar measurements were made at a number of light levels, spanning the photopic to high scotopic (inclusive) ranges. A monotonic relationship between response and contrast was found at all light levels studied, and the same relationship was retained to a good approximation across all light levels. A similar result was also found when nonoptimal spatial frequencies were used as stimuli. These results indicate strongly that X and Y cells inform the cat's brain about contrast in the retinal image. It was also observed that the mean discharge rate of X and Y cells did not change with light level, indicating that no information is relayed to the brain by these cells on the mean light level.