Conclusion And Future Perspectives

This proposal likens the process of song learning to trial and error. The system progressively transforms the prototype syllables into a sequence that matches an auditory representation of song that originally entailed hedonic feedback. Indeed, because the MDNs are sensitive to temporal discrepancy, they may contribute to acquiring a spatial representation of serially arranged events (e.g., the syllable) that also encode the correct temporal relationships among the constituent elements in the sequence (J. Houk, personal communication, August, 2002). Our proposal explains the nature of this sensitivity to temporal discrepancy as a product of SBF mechanisms. One goal of this chapter was to integrate the fields of timing and time perception and songbird neuroethology in a manner that could benefit the experimental approaches of both fields. To this end, our proposal of integrated, parallel cortico-striatal modules has, at the very least, introduced some potential avenues of study for both sides. In particular, the role of the MDNs in the songbird needs to be elucidated along with the role of the subpallial region PC.

The hierarchical structure found in songbird behavior need not end at the level of the song. Many experiments have observed the songbird for temporal patterning in song delivery during a bout. Indeed, there is a hierarchical organization in the song types that nightingales copy that, remarkably, may exceed 200. This hierarchical organization is reflected in the song bout so that some song types in the bout are consistently found in given positions (for a review, see Todt and Hultsch, 1998). The nature of song type switching during a bout (i.e., A, A, A, B, B, B, C, C, ...) may reflect an interval timing process. In the chaffinch, bouts with many repetitions are consistently delivered at a high rate, while bouts with few repetitions are delivered at a low rate. This is suggestive of a temporal window that is allocated and for which the bird delivers a song type sequence (Riebel and Slater, 1999). Given that there is evidence that the songbird discriminates song types by means of categorical perception (Searcy et al., 1999), a startling possibility begins to unfold. Could interval timing mediate birdsong to the extent that it contributes to a hierarchical structure ranging from seconds to minutes?

Our knowledge of the neurobiological underpinnings of interval timing is in its infancy, though enormous strides have been taken within the last few years (see chapters in this volume by Diedrichsen et al.; Malapani and Rakitin; Matell et al.; Pang and McAuley; Pouthas; Sakata and Onoda). One uncertainty in the field concerns the extent to which the neurophysiological substrate for interval timing is confined to a specified area in the brain. The proposal in this chapter clearly implicates a distributed system (see Matell et al., this volume) such that sensorimotor behavior is the product of an interaction between temporal processing occurring in the auditory system and the motor system. Both systems share the utility of a cortico-striatal module, and the interaction is permitted by an interface through stri-ato-nigral-striatal pathways. The implication is that there may be separate pallial-striatal modules for different stimulus modalities and that "clocks" may be both centralized (e.g., amodal) and distributed (e.g., modality specific), depending on the need for global synchronization and reduction in variability (see chapters in this volume by Diedrichsen et al.; Hopson; Malapani and Rakitin; Matell et al.; Penney).

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