Summary

This section has explained why we believe that different mechanisms are recruited for the measurement of time in different tasks. Both an automatic timing system, which is used to measure subsecond intervals when these are measured continuously via movement, and a cognitively controlled system, which is recruited for temporal measurements that cannot easily be performed by the automatic system (i.e., those of suprasecond durations, measured discontinuously, and not via movement), have been described. Evidence from lesion studies as well as from studies of motor circuitry suggests that the motor system could perform the task of the automatic system, while the flexible cognitive modules of the prefrontal and parietal cortices are more suited to the task of the cognitively controlled system. Hence, we have hypothesized that there may be a dissociation in functional locus for these two systems. A meta-analysis of existing neuroimaging studies of time measurement has shown that when the literature is taken as a whole, there is no strong consensus regarding the areas most commonly involved. If the studies are divided based on the characteristics of the task performed, however, a clear dissociation is seen between areas activated by automatic-associated and cognitive control-associated tasks. The former frequently activate parts of the motor system (SMA, sensorimotor cortex, cerebellar hemisphere, PMC, and basal ganglia) as well as the superior temporal gyrus, but only rarely activate the prefrontal or parietal cortices. The latter frequently activate the prefrontal and parietal cortices (DLPFC, VLPFC, inferior parietal, and IPS), with additional activity in the cerebellum and SMA and PMC, among other areas. This analysis supports the possibility that functionally and anatomically distinct systems for time measurement exist within the human brain and illustrates how failure to recognize this multiplicity can lead to confusion in the literature. Future attempts to investigate the neural locus of time measurement should therefore take the possibility of multiple systems into account, both when choosing a task to study and when interpreting their findings or the findings of others.

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