Some visual illusions fool our judgement but apparently not our grasping hands
Show people the Ebbinghaus illusion and their perceptual judgements go wrong, yet some studies find that when they reach out and grasp, no error appears. That split fuels a theory that the brain runs vision down two streams, one for recognising things and one for guiding action, though other researchers find both are equally fooled.
At the back of the head, in the occipital lobe, the visual cortex receives signals that travel from the eyes through the lateral geniculate nucleus of the thalamus. Each hemisphere handles the opposite half of the visual field. The first stop is the primary visual cortex, V1, also called Brodmann area 17 or the striate cortex, a name taken from the line of Gennari, a stripe of myelinated fibres visible to the naked eye. Areas V2 to V5 lie beyond it, and the size of V1 to V3 can differ threefold between people, partly through inheritance.
V1 is laid out like a map: neighbouring cells respond to neighbouring patches of the visual field. It has six layers, with the upper ones handling local processing and the deeper ones sending output to regions involved in higher-level decisions. Many of its cells are orientation-selective, firing for edges at a particular angle, so a V1 neuron might react to any vertical shape. Further along, tuning becomes far more specific; in the inferior temporal cortex a neuron may respond only to one particular face. V1 is also plastic, reorganising after sensory deprivation or enriched experience.
From V1, information splits. The ventral stream runs through V2 and V4 to the inferior temporal cortex and handles form and object recognition, earning the nickname What Pathway. The dorsal stream passes through V2 to areas V6 and V5 and on to the posterior parietal cortex, dealing with motion, location and the steering of eye and arm movements. Ungerleider and Mishkin first framed this what-versus-where division.
Goodale and Milner later recast it as perception versus action, pointing to illusions that distort judgement but leave grasping intact. Work by Franz and colleagues suggests both systems are equally deceived, while other studies strongly back the dissociation, so the question is unsettled.
Source: Visual cortex
