Abstracts

Rationale: Functional brain imaging studies of the human inferior temporal cortex (ITC) have shown regions that are selective to specific visual forms, such as faces, tools, places or words. The topographic location of these regions shows consistent organization across individuals. Existing models of human visual perception posit that this cluster of regions is important for the early stages of processing written words, faces, or objects. However, evidence for selective response to mathematical objects, such as numbers or mathematical symbols has not been observed in the ITC. Using intracranial electrophysiological recordings in humans, we studied the evoked responses in the ITC to visual mathematical and non-mathematical stimuli.Methods: We recorded intracranial electrocorticography (ECoG) from 5 patients undergoing surgical evaluation for the treatment of refractory epilepsy. ECoG responses were measured in multiple subjects during several different tasks of visual perception. Subjects performed conventional tasks of visual processing for faces, single words, houses, limbs, houses, numbers, and mathematical symbols and equations. Results: Time-frequency analysis revealed selective and exclusive response in the broadband gamma range (40 150 Hz) in a specific site in the left inferior-temporal gyrus to numbers and mathematical equations. In these sites, no responses were seen to faces, words, other objects, or to addition equations in the linguistic or symbolic forms, nor to written statements. The responsive site in each subject was located lateral to the previously reported visual word form area (VWFA), which was identified by selective and exclusive response to all linguistic (i.e. written statements and addition equations in linguistic form) but not to numerical conditions. The relative location of both areas was consistent between subjects. Conclusions: We have identified an area in the inferior temporal cortex that is tuned exclusively to visual numerical forms. This area is located laterally adjacent to the VWFA, and displays similar selective responses but only to numerical stimuli. We hypothesize that this brain region exists to assist early visual parsing of number forms for numerical processing.