Abstract
A vast volume of psychophysical studies on interval timing (IT) point to its omnibus involvement in an extensive repertoire of sensory, executive, and motor behaviors. This ubiquitous association of IT with other cognitive events makes it difficult to elucidate neural underpinnings of IT because neural responses pertaining to IT and those non-IT events are intimately intermingled. Recently, we developed an implicit interval timing (IT) task, in which subjects watched an object move at a constant speed and become invisible unpredictably, and guessed the arrival time of the invisible motion at a specified position. Capitalizing on the invisibility of the object that carries time intervals, we adapted this task to fMRI experiments to explore cortical mechanisms that substantiate IT in humans. We found that area MT and superior parietal region (SPR), which has been proposed as a human homologue of the macaque LIP, were activated during the visible-motion period, consistent with their well-known responses to motion stimuli. Unlike MT, SPR remained active during the following invisible-motion period, along with other motor regions including putamen and cerebellum. The SPRs sustained activity overarching the visible-motion, invisible-motion, and motor periods may be construed as (i) the cortical activity of accessing the spatiotemporal information available in sensory or motor areas, (ii) the neural accumulation of elapsed time at timescale of seconds as suggested by the pacemaker-accumulator model, or, alternatively, (iii) a neural correlate of the hazard rate of ecologically relevant events, in line with previous findings in the primate brain.