Discriminative whisking in the head-fixed rat: optoelectronic monitoring during tactile detection and discrimination tasks

We compared whisking movement patterns during acquisition of tactile detect ion and object discrimination under conditions in which (a) head movements are excluded and (b) exposure to tactile discriminanda is confined to the l arge, moveable vibrissae (macrovibrissae). We used optoelectronic instrumen tation to track the movements of an individual whisker with high spatio-tem poral resolution and a testing paradigm, which allowed us to dissociate per formance on an "indicator" response (lever pressing) from the rat's "observ ing" responses (discriminative whisking). We analyzed the relation between discrimination performance and whisking movement patterns in order to clari fy the process by which the indicator response comes under the stimulus con trol of information acquired by the rat's whisking behavior. Whisking patte rns over the course of task acquisition differed with task demands. Acquisi tion of the Detection task was correlated with modulation of only one whisk ing movement parameter-total number of whisks emitted, and more whisking wa s seen on trials in which the discriminandum was absent. Discrimination bet ween a sphere and cube differing in size and texture was correlated with a reduction in whisk duration and protraction amplitude and with a shift towa rds higher whisking frequencies. Our findings confirm previous reports that acquisition of tactile discriminations involves modulation by the animal o f both the amount and the type of whisking. In contrast with a previous rep ort (Brecht et al., 1997), they indicate that rats can solve tactile object detection and discrimination tasks (a) using only the large, motile mystac ial vibrissae (macrovibrissae) and (b) without engaging in head movements. We conclude that the functional contribution of the macrovibrissae will var y with the nature of the task and the conditions of testing.