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David T. Field, John P. Wann; Perceiving time to collision activates sensorimotor cortex. Journal of Vision 2005;5(8):733. doi: 10.1167/5.8.733.
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The survival of many animals hinges upon their ability to avoid collisions with other animals or objects, or to precisely control the timing of collisions. Judgements of time-to-collision (TTC) can be made using optical expansion information (Lee, 1976) and this may be combined with binocular information (Rushton & Wann, 1999). Neural systems underlying TTC judgement from optic expansion have been identified in pigeons (Wang & Frost, 1992) and houseflies (Wagner, 1982). In the case of humans, what cortical areas process judgments of TTC from optical expansion or how these are linked to co-ordinated action is unknown. We investigated this question using fMRI. Observers judged which of two approaching objects would strike them first (Todd, 1981), using optical expansion information. The main control task involved judgements of objects that inflated while remaining at a fixed perceived depth. A second control task utilised TTC judgements for objects translating in the frontoparallel plane. We identified areas of superior parietal and motor cortex, which are selectively active during perceptual judgments of TTC with the point of observation, some of which are normally involved in producing reach-to-grasp responses. These activations could not be attributed to actual movement of participants. We demonstrate that networks involved in the computational problem of extracting TTC from expansion information have close correspondence with the sensorimotor systems that would be involved in preparing a timed motor response, such as catching a ball.
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