OFF response transformations in the whisker/barrel system

1. Previous studies have demonstrated marked differences in the relative sizes of ON and OFF responses of neurons in the whisker/barrel system. In particular, OFF responses are unexpectedly large in thalamic neurons. Extracellular unit recordings were used to examine whether varying the time between stimulus onset and offset differently affects OFF responses of neurons in the trigeminal ganglion, ventrobasal thalamus, and somatosensory cortical layer IV. Controlled whisker stimuli were used to deflect individual vibrissal hairs in different directions. We hypothesized that, in part because of the gradual waning of central inhibition evoked by stimulus onset, OFF responses of thalamic and cortical neurons but not trigeminal ganglion cells would increase in size with longer duration stimuli, with relative changes being greatest in the cortex. 2. OFF response magnitudes for thalamic and cortical neuronal populations increased as the stimulus duration was increased from 200 to 1,400 ms. Increases were greater at nonoptimal deflection angles. Similarly, individual cells having smaller OFF responses for the short-duration stimulus tended to display proportionately greater increases when the stimulus was lengthened. OFF responses of trigeminal ganglion cells were largely unaffected by stimulus duration. 3. Barrel neurons were subclassified as regular-spike units (RSUs) or fast-spike units (FSUs) on the basis of the time course of their action potentials. ON and OFF responses were smaller in the former and, when the stimulus was lengthened, percentage increases in their OFF responses were greater than those in FSUs. Results illustrate nonlinear transformations of the thalamic input signal by RSUs, which are presumed to be excitatory barrel neurons, and extend previous findings of response similarities between thalamocortical units (TCUs) and FSUs, the latter of which are thought to be inhibitory. 4. The time course of OFF response suppression in cortical neurons suggests that stimulus onset evokes central inhibition having two components, a potent one lasting several tens of milliseconds and a weaker one lasting many hundreds of milliseconds. Background activity levels in cortex and thalamus were diminished for > or = 1,800 ms after whisker movement. 5. For TCUs, 200-ms stimuli were less likely than 1,400-ms stimuli to elicit an OFF response, but when responses occurred they consisted of a greater number of spikes timed closer together. By contrast, the 200-ms stimulus OFF responses of the RSUs and FSUs displayed longer interspike intervals than did their 1400-ms responses, with no change in the number of spikes per response.(ABSTRACT TRUNCATED AT 400 WORDS)