Temporal integration of multiple-point stimuli in primary somatosensory cortical receptive fields of alert monkeys

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Abstract
In-field inhibition in somatosensory cortex (SI) of alert monkeys, was studied in 67 cortical neurons with a conditioning-test paradigm. Brief (10 ms) air puffs exerting a peak force of 975 dyn were used as conditioning and test stimuli. The amount of suppression of the response to the test stimulus defined the amount of inhibition generated by the conditioning stimulus. The effect of altering conditioning-test (C-T) intervals, the number of air puffs in the conditioning stimulus and the position of the conditioning stimulus within the receptive field was measured. Test-response suppression was greatest 10-20 ms following the excitatory response produced by the conditioning stimulus, and decayed to control levels within 60-100 ms. The spatial distribution of inhibition produced by single-point stimuli was determined by presenting conditioning air puffs at specified positions within the receptive field. Three-point simultaneously applied stimuli, which produce a single broad distribution of excitation in the cortex, also evoke unimodally distributed in-field inhibition in the cortex lasting 60-100 ms. Maximum inhibition, like maximum excitation, occurs when 1 of the air puffs is positioned over the field center or when 2 adjacent air puffs straddle the field center. Three air puffs presented simultaneously inhibit a larger population of neurons than does any air puff presented alone. In at least 1/3 of the neurons tested, significantly more inhibition was evoked by 3 air puffs than by single air puffs, showing that inhibition sums from multiple-stimulus sites. Simultaneous presentation of multiple-point stimuli evokes fewer impulses than the sum of the responses to their separate presentation. This nonlinear summation of excitation may be the result of in-field inhibition, which restricts the duration of the excitatory burst to 20-25 ms. This reduced duration limits the amount of summation. Three-point stimuli with the lateral air puff on the field center produce less excitation, but greater in-field inhibition than the lateral air puff alone. In-field inhibition, which foreshortens the excitatory response, may produce loss of resolution of separate stimuli and funneled sensations.