Reinterpretation Of Some Experimental Results

An explicit separation of the auditory template from the vocal learning nuclei is agreeable with some of the findings in the birdsong literature. Electrolytic lesions of Area X or LMAN do not affect the zebra finch's capacity to learn discriminations between conspecific songs signaled by a motor behavior in an operant go/no-go conditioning paradigm. The lesioned zebra finch does take longer than controls to acquire the BOS discrimination (Scharff et al., 1998). Within the framework of our proposal, the auditory template would still be intact after an AFP lesion. However, a disruption in learning may be expected if the motor system makes use of some information residing within the AFP, or through cells that traverse the AFP, to benefit from song-related, perceptual information. If this is the case, the results suggest that the songbird could discriminate between songs, but was hindered in its ability to convey this to the observer because of difficulty in integrating the template's perceptual information with the operant response. This would imply that the AFP is not entirely devoted to vocal production of birdsong.

Another result worth bringing up again is the observation that lesioning the AFP circuit in the adult songbird prevents song deterioration that is induced by deafening. From our perspective, a song decrystallizes in a deafened bird due to "drift" introduced into the system from the interaction between the auditory template and the AFP. The entry point of this drift is unknown, though it is tempting to think that it is a memory effect such that the removal of auditory feedback deprives the auditory template of a consistent "update." With this in mind, one might expect different types of song deterioration depending on the nature of the perturbation introduced to the songbird. Perturbations that actively distort the auditory feedback, presumably effecting a change to the auditory template, in an otherwise normal songbird might be expected to induce a more profound change in song relative to those songbirds that are deafened. Indeed, the distortion of temporal cues by providing delayed feedback (Leonardo and Konishi, 1999) to the adult songbird induces a more profound degradation than deafening the adult songbird, which leads to a gradual loss of song stability (Nordeen and Nordeen, 1992).

Alternatively, it may be useful to interpret results from interval timing experiments using our proposal as an outline. It was revealed that lesions in LMAN early in vocal development disrupted song significantly (Bottjer et al., 1984; Scharff and Nottebohm, 1991), though these same lesions in the adult songbird have no effect (Scharff and Nottebohm, 1991). Indeed, these lesion effects revealed that songbirds seemed to prematurely crystallize songs to the extent that the song was rendered a sequence of simplistic, often-recurring syllables. Also, the intervals between syllables became more variable. In light of our current proposal, lesions of this sort would have eradicated the capacity to form stabilized activations of spatial patterns within LMAN, thus preventing their association with descending input to RA via HVC[RA] projection neurons. In this case, the song system was forced to stabilize the rudimentary pattern of activity in RA that was developed up until that point.

A similar distributed, interactive cortico-striatal motif may be present in mammals insofar that a sensory module may influence the acquisition of behavioral output through a motor module. Thus, one would presume that frontal cortical lesions in rats that were previously trained in an interval timing task such as the peak-interval (PI) procedure would not show a significant deficit in the temporal control of behavior. Indeed, after training the animal in the PI procedure, the sensory module would have had ample time to impose its temporal content on the motor units constituting the behavior. Interestingly, lesions within the dlPFC homologue of the rat after it has reached steady-state timing performance do not drastically affect its ability to discriminate time, although slight rightward shifts in the timing functions are sometimes observed (Dietrich et al., 1997; Meck et al., 1987).

However, given our interpretation of the effects of LMAN lesions in the juvenile songbird, the largest effects of frontal cortical lesions in rats would best be observed when learning a behavioral output that required temporal reorganization. That is, frontal cortical lesions may not bring about substantial effects on a temporally organized behavior that previously had been established, but would disrupt the acquisition of the same behavior. To be sure, frontal lesions in rats drastically impair their ability to learn a temporal discrimination, indicated by significantly more trials until acquisition. Moreover, the behavioral output as a function of time is significantly flattened in the rats that did acquire the behavior, although 33% of the rats never learned the discrimination (Dietrich and Allen, 1998).

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