the disorders













visual form agnosia


"a neurological virus"

"a unifying vision"

"an active process"

The pioneering studies of visual neurophysiology by David Hubel and Torsten Weisel opened up a new field that now engages hundreds of talented researchers. The efforts of these researchers have revealed the details of the complex circuitry and computational processes that underlie the simple act of looking. Billions of neurons and trillions of synapses are engaged with every glance. The computations performed by these neurons are now being understood with mathematical precision, and are being used to explain well-know, but heretofore puzzling, visual effects. For instance, mathematical analyses of the so-called "aperture problem" have led to detailed understanding of the neural circuits that underlie our perception of visual motion. Individual neurons cannot, by themselves, determine the correct interpretation of the motion patterns on the retina but, working together to solve the aperture problem, they build for us a perception of motion that usually is quite accurate. The neurons involved in these computations are, of course, cells subject to the normal limits of biology, including limits in energy and other nutritional resources. As a result, if these neurons are used for too long a period in a repetitive fashion, they can exhaust their resources and produce inaccurate computations.

This is the source of the motion aftereffects that are normally seen if one views, e.g., a rotating spiral display for an extended period. The neurons solving the aperture problem become adapted to, say, an inward motion of the spiral. In consequence, if the observer is shown a static view of the spiral, or indeed a static view of any object, the image will appear to grow. This is a normal effect, and usually wears off after a few seconds, the time it takes for the neurons to renew their energetic resources. In rare cases, the depletion of neural resources when viewing a rotating spiral can be so severe that neural computations cease altogether while the neurons try to recover. The result is a brief, and disconcerting, disappearance of the spiral altogether, a condition known technically as spiralepsy, since the neurological conditions which cause it are similar to those responsible for epilepsy induced by flashing lights. Published reports show a rise in cases of spiralepsy, and attribute it, in part, to the neural impairments induced by watching excessive media, particularly media that use rapid cuts and flashing graphics to grab viewer attention. Apparently, human vision evolved in a niche that did not contain such stimuli, and can be overloaded when pushed sufficiently outside its visual niche. Fortunately, spiraleptic episodes are usually brief. The spiraleptic reports that, within seconds, the visual world "reboots", and they can once again see normally. This is, however, an early warning sign that the visual system is being pushed beyond neurologically healthy limits.