Concluding Remarks

In summary, we can envisage some specific problems in functional imaging of human timing systems at present. Many of these are rooted in the intrinsic limits of PET and fMRI, with their dependency on sluggish and indirect measures of relative change in blood flow and oxygenation levels, rather than direct neural measures. EEG and MEG techniques in turn have limits in spatial resolution and are relatively insensitive to central brain structures. However, several features of the review pre sented in Table 21.1 encourage us. First, it is now clear that functional imaging can detect multiple areas of the human brain associated with time measurement tasks, and while the variation among different experiments is high, some areas activate consistently in association with timing tasks having specific characteristics and are thus strong candidates for further exploration. Second, we have argued that much of the variation between the experiments may be due to the diversity of timing tasks used — these tasks may even be drawing on quite separate systems. We can draw an analogy here with functional localization within the visual cortex. When an obvious, but inappropriate stimulus (such as a natural scene) is used, one gets the impression that the visual areas of the brain are horribly difficult to distinguish functionally. When a more appropriate stimulus is used (moving bars of light or drifting fields of dots), the organization becomes apparent. Thus, when appropriate timing tasks are used, the relationships between neural locus, neural mechanism, and timing behavior may become clear. Third, the enormous expansion of techniques now available to tackle the problem of time measurement, including imaging, mul-tielectrode unit recording, drug studies, gene knockouts, transcranial magnetic stimulation, and the rest, means that a combined approach is both feasible and fruitful.

We believe that the most immediate future goal in the study of time measurement is to determine which timing systems are used under which specific circumstances. Once this has been established, the imposing arsenal of techniques at our command will facilitate further examination of the detailed functioning of each time measurement system. We are therefore optimistically confident that with the aid of these methods, we will soon succeed not only in finding the timers for which we search, but also in understanding how they work.

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