Surround suppression in the responses of primate SA1 and RA mechanoreceptive afferents mapped with a probe array

Surround suppression in the responses of primate SA1 and RA mechanoreceptiv e afferents mapped with a probe array. J. Neurophysiol. 81: 2711-2719, 1999 . Twenty-four slowly adapting type 1 (SAI) and 26 rapidly adapting (RA) cut aneous mechanoreceptive afferents in the rhesus monkey were studied with an array of independently controlled, punctate probes that covered an entire fingerpad. Each afferent had a receptive field (RF) on a single fingerpad a nd was studied at 73 skin sites (50 mm(2)). The entire array was lowered to 1.6 to 3.0 mm below the point of initial skin contact (the background inde ntation) before delivering indentations with one to seven probes. Indentati ons were generally limited to 100 mu m to minimize gross mechanical interac tions. There were two major, new findings. 1) The discharge rates of both S A1 and RA afferents were strongly affected by the number of probes indentin g the RF simultaneously. The effect was exponential. Each increase in probe number reduced the response by 24% in SAI and 12% in RA afferents on avera ge. When seven probes indented the skin simultaneously, the impulse rates i n SA1 and RA afferents were reduced to 20 and 40% of the rates evoked by a single probe at the hot spot tall indentations were 100 mu m). This shows t hat before any synaptic interaction in the CNS there is already a mechanism analogous to surround inhibition that suppresses an afferent's responses t o uniform indentation and makes it especially sensitive to deviations from spatial uniformity. 2) The responses of both SA1 and RA afferents were inde pendent of background array depth over the range from 1.6 to 3.0 mm below t he point of initial skin contact. This shows that the neural responses to e lements raised above a background are independent of the applied force over a wide range of forces. To relate the background depths to indentation for ce and to compare humans and monkeys, we studied the biomechanics of indent ation with a uniform surface. A remarkable result is that the force-displac ement relationships in humans and monkeys were the same; the skin is highly compliant for the first 2-3 mm of indentation and then becomes much stiffe r. The results were the same in alert humans and monkeys and in monkeys ane sthetized with pentobarbital. Ketamine anesthesia made the skin much stiffe r and reduced the compliant range substantially.