Mechanisms underlying somatosensory cortical dynamics: I. In vivo studies

The experiments of this study demonstrate that relatively modest rates of repetitive tactile stimulation are accompanied by rapid and reversible modifications (either increases or decreases) in the response of SI neurons. Complete recovery occurs in a few minutes following cessation of stimulation. The modifications are reproducible (1) if stimulus parameters remain the same and (2) if time for recovery is provided between successive exposures. In contrast, repetitive tactile stimuli identical to those that modify SI neuron response rarely lead to changes in the response of cutaneous mechanoreceptive afferents. SI neuron functional properties conventionally regarded as immutable [e.g., directional selectivity, and distribution of sensitivity within the receptive field (RF)] also modify with repetitive stimulation. While the changes in RF organization differ in detail from one neuron to the next, they are similar in form: the response generated by stimulus contact with one (or more rarely, several) RF region(s) becomes enhanced relative to the response the same stimulus evokes from neighboring regions. Neurons in the same column (sampled in the same radial penetration) exhibit very similar changes in the distribution of sensitivity within the RF, whereas neurons sampled in tangential penetrations exhibit diverse, apparently unrelated changes in RF organization in response to the same repetitive stimulus. Simultaneous multichannel recordings reveal that a repetitive tactile stimulus exerts similar effects on the response and RFs of the neurons within local (no more than 100 microns) neuron groupings. A model that incorporates a manner of SI topographical organization (segregate organization) and well-known aspects of neocortical cellular, neurotransmitter/receptor, and connectional architecture accounts for the changes in SI neuron behavior observed during repetitive stimulation.