Abstract
At least two strategies are available to humans for estimating multisecond
intervals. One depends on an interval timing system that is common
to many species. The other is the language-based strategy of chronometric
counting. These two strategies are easily distinguished by the psychophysical
properties of their behavioral correlates: counting supports substantially
more precise estimates than are possible using the more general interval
timing system. The present study investigates the neural systems
that underlie the execution of these different strategies. Eighteen
adults reproduced a 16-s interval either by internally timing or
covertly counting the duration. Comparison of counting and timing
to a resting baseline suggested that these strategies engage some
nonoverlapping neural systems. Counting, but not timing, strongly
activated Broca's area, primary motor cortex in the mouth region,
and right cerebellum, all of which are associated with internal speech.
Counting also activated parts of the medial premotor circuit, including
the putamen, supplementary motor area (SMA) proper, and cingulate
motor area (CMA), that have been associated with reproducing isochronous
and syncopated rhythms of elements lasting hundreds of milliseconds.
During timing, only a portion of this circuit, the SMA proper and
CMA, was engaged. Both timing and counting interfered with semantic
processing during the resting state, evidenced by task-related decreases
in the left inferior and middle frontal gyri, right superior frontal
gyrus, left angular gyrus, and bilateral posterior cingulate cortex.
This study suggests that counting activates a corticostriatal network
associated with millisecond, rhythmic timing. In contrast, timing
long durations without the benefit of linguistic strategies for subdividing
counts reduces activity in this circuitry.
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