Responses of Contralateral SI and SII in Cat to Same-Site Cutaneous Flutter Versus Vibration

Abstract

The methods of14C-2-deoxyglucose (14C-2DG) metabolic mapping and optical intrinsic signal (OIS) imaging were used to evaluate the response evoked in the contralateral primary somatosensory receiving areas (SI and SII) of anesthetized cats by either 25 Hz (“flutter”) or 200 Hz (“vibration”) sinusoidal vertical skin displacement stimulation of the central pad on the distal forepaw. Unilateral 25-Hz stimulation consistently evoked a localized region of elevated14C-2DG uptake in both SI and SII in the contralateral hemisphere. In contrast, 200-Hz stimulation did not evoke elevated14C-2DG uptake in the contralateral SI but evoked a prominent, localized region of increased 14C-2DG uptake in the contralateral SII. Experiments in which the OIS was recorded yielded results that complemented and extended the findings obtained with the 2DG method. First, 25-Hz central-pad stimulation evoked an increase in absorbance in a region in the contralateral SI and SII that corresponded closely to the region in which a similar stimulus evoked increased 14C-2DG uptake. Second, 200-Hz stimulation of the central pad consistently evoked a substantial increase in absorbance in the contralateral SII but very little or no increase in absorbance in the contralateral SI. And third, 200-Hz central-pad stimulation usually evoked a decrease in absorbance in the same contralateral SI region that underwent an increase in absorbance during same-site 25-Hz stimulation. Experiments in which the OIS responses of both SI and SII were recorded simultaneously demonstrated that continuous (>1 s) 25-Hz central-pad stimulation evokes a prominent increase in absorbance in both SI and SII in the contralateral hemisphere, whereas only SII undergoes a sustained prominent increase in absorbance in response to 200-Hz stimulation to the same central-pad site. SI exhibits an initial, transient increase in absorbance in response to 200-Hz stimulation and at durations of stimulation >1 s, undergoes a decrease in absorbance. It was found that the stimulus-evoked absorbance changes in the contralateral SI and SII are correlated significantly during vibrotactile stimulation of the central pad—positively with 25-Hz stimulation and negatively with 200-Hz stimulation. The findings are interpreted to indicate that 25-Hz central-pad stimulation of the central pad evokes spatially localized and vigorous neuronal activation within both SI and SII in the contralateral hemisphere and that although 200-Hz stimulation evokes vigorous and well maintained neuronal activation within the contralateral SII, the principal effect on the contralateral SI of a 200-Hz stimulus lasting >1 s is inhibitory.