FINE-GRAIN OF THE NEURAL REPRESENTATION OF HUMAN SPATIAL VISION

It is widely held that in human spatial vision the visual scene is ini tially processed through visual filters, each of which is responsive t o narrow ranges of image spatial frequencies. The physiological basis of these filters are thought to be cortical neurons with receptive fie lds of different sizes. The grain of the neural representation of spat ial vision is much finer than had been supposed. Using laser interfero metry, which effectively bypasses the demodulation of the optics of th e eye, we measured discrimination of, and adaptation to, high spatial frequency laser interference fringe patterns. Spatial frequency discri mination was good right up to the visual resolution limit (average Web er fractions of 0.13 at 50 c/deg). Both contrast and spatial frequency matches made after adapting to extremely fine interference fringes st rongly suggested that there existed even finer, relatively unadapted, filters (mechanisms with small receptive fields). The smallest cortica l receptive fields processing spatial information in human vision are so small that they can possess receptive field centers hardly wider th an single cone photoreceptors.