Few Problems

Admittedly, technical limitations slow progress on the neural bases of interval timing. Current brain imaging methods, however exciting they may be, are unlikely to detect a clock that is supposed to be ticking continuously (see Lewis and Miall, this volume). Or, taken differently, the clock may have been detected long ago, if it consists of aperiodic neural events such as firing bursts in neuronal populations or of oscillatory phenomena that are ubiquitous in organisms (for a review, see Jacklet, 1989) and possibly modulate information processing (Burle and Bonnet, 1999). The point is evidently how these events are selected and stored during the target interval, and how they are repeated on each target occurrence. As a consequence, such processes may be more accessible to experimentation (e.g., encode-decode mechanisms) (see Malapani and Rakitin, this volume). In the same line of reasoning, it is notable that effects on clock speed cannot be detected after genetic manipulations because individuals readapt to their own timing speed (see Cevik, this volume).

Other limitations come from theory rather than methods. Current speculation as expressed in the timing literature often deals with structure and function as if they were inescapably linked. Although such links might be decisive (see Matell et al., this volume), a common timing mechanism may also be subserved by various structures, and various timing mechanisms may coexist in one or several structures. Thus, on many occasions, explicitly disentangling structure and function would help clarify the debate. Taking into account the structural characteristics of the systems discussed may also prove useful. Part of the literature concerning modality effects on timing illustrates these points (see Penney, this volume). Admittedly, postulating that distinct switch-accumulator modules may time signals in each sensory modality implies different neural implementation of core timer components, but does not imply different timing mechanisms. Now, consider, for example, that stimulus coding involves a stage of physico-chemical transduction in visual, but not in auditory receptors; it is, therefore, impossible to decide whether the visual module might be less efficient than the auditory one in encoding time, or whether visual, compared to auditory input, might less efficiently trigger one common timing mechanism implemented in a central structure.

Unfounded distinctions may also be misleading. For example, separating motor tasks from cognitively controlled ones (see Lewis and Miall, this volume) is risky, because a number of motor tasks, even rhythmic ones, require controlled attention or may, at least, be performed with greater accuracy when attention is involved. Cognition is pervasive; it may take place at so-called elementary behavioral levels. Similarly, making a distinction between processes occurring in millisecond vs. multisecond ranges may be questionable, because attentional effects in dual tasks transcend this classification (Macar et al., 1994). A significant outcome of brain imaging reviews on timing is that certain structures are activated irrespective of such distinctions, in particular the supplementary motor area (see Lewis and Miall, this volume; Macar et al., 2002), which, via the thalamus, receives major output from the striatum.

0 0

Post a comment