Three experiments examined the development of the acoustic startle reflex and its modification by a preliminary stimulus in the infant rat during the 2nd and 3rd postnatal weeks. The 1st experiment employed a white noise S1 (20 msec, 70 dB), the 2nd a cutaneous S1 (.5 msec, .5 mA and 1.0 mA shock), and the 3rd identical S1-S2 pairs (20 msec, 10 kHz, 110 dB tones). The results demonstrate a similar maturation of the prepulse modification pattern over days in the 3 experiments, evidenced mainly in the growth of inhibition. The findings indicate peripheral and central mechanisms that are maturing during the period of life under observation and that contribute to the developmental patterns of modification.

Repetitive elicitation of startle-like responses by electrical stimulation of the cochlear nucleus led to sensitization followed by habituation. In contrast, repetitive elicitation of startle-like responses by electrical stimulation of the reticular formation led only to sensitization. Since these different locations represent different points along the acoustic startle circuit, the data suggest that sensitization may be related to the motor side of reflex arcs, whereas habituation may be related to the sensory side.

Characteristic potentiation of rat locomotion responses and acoustic startle reflexes that normally appear in the third postnatal week was absent in rats exposed to diazepam during the third week of gestation. Loss of these behaviors suggests a long-term effect that may result from changes in cellular development. Tissue undergoing neuronal differentation may be especially sensitive to drugs that act on the central nervous system, and the period in which differentiation occurs is perhaps critical for the induction of changes that are later expressed as altered behavior.

Using functional magnetic resonance imaging (fMRI), finger representations were characterized in the precentral cortex of 11 normal musicians and 14 musicians with focal task-specific dystonia. Finger representations were identified from differential activation during repetitive movements of each finger relative to others. Despite group similarities in topography, abnormalities in representations of affected fingers were identified. For the finger showing chronic flexion (primary dystonic finger or PDF), the cortical "disparity" from its normal location and the distance to the adjacent finger were increased. By contrast, representational characteristics of the finger showing chronic extension (primary compensatory finger or PCF) did not differ significantly from the control group, but did differ from those of the PDF. Regardless of whether either finger's representation differed substantively from normal, the PCF consistently showed greater volume of activation than the PDF or other fingers. These findings reflect dysfunctional interactions between at least two fingers and their cortical representations.

In Experiment I, rats 13-35 days of age were presented with acoustic startle stimuli in conjunction with light flashes or noise bursts at 4-320 msec lead times. Inhibition of the startle reflex by leading noise bursts was present on Days 13-15, whereas light flashes were not effective until Days 21-23. In Experiment II, 3 intensities of noise and 3 intensities of light preceded the reflex-eliciting stimuli on Days 16 and 35. All intensities of noise were effective on Day 16, whereas no visual stimuli were. On Day 35, all stimuli were effective. These data reveal that intrinsic visual structures and/or connections between these structures and reflex-control mechanisms mature relatively late in the rat.