Optical imaging of intrinsic signals in somatosensory cortex

Summary

The method of optical imaging of intrinsic signals emitted in the near-infrared (IR; 820-846 nm) was used to study the response of somatosensory cortex (SI) of anesthetized cats and squirrel monkeys to precisely controlled vibrotactile stimuli. The cortical territory exhibiting decreased average IR reflectance in response to a low frequency, modest amplitude (25 Hz; 200 - 400 microns peak-to-peak, sinusoidal) vibrotactile stimulus shifted location in a manner consistent with a published map of SI topographical organization generated from receptive field mapping observations. The size of the SI territory responding with a decrease in reflectance was greatly reduced by intravenous administration of even small doses of a selective NMDA receptor blocker (ketamine). Extracellular single and multiunit recordings obtained during microelectrode penetrations of the same SI region studied with the IR imaging method revealed that (i) the spike discharge activity of neurons sampled within territories exhibiting decreased average IR reflectance values in response to a vibrotactile stimulus was consistently elevated by the stimulus, and (ii) the discharge activity of neurons sampled in regions exhibiting stimulus-evoked increases in average IR reflectance was consistently depressed. Neurons located in regions which did not exhibit a reflectance change rarely were influenced by the stimulus. Analysis of the time course of the IR reflectance changes evoked by both short (500-1000 msec) and long-duration (6-10 sec) stimuli revealed considerable dynamics in the two types of SI territories (excitatory vs. inhibitory) that modify their average IR reflectance in response to vibrotactile stimulation. In a final series of experiments we evaluated the IR intrinsic signals evoked in SI of anesthetized subjects in whom the spinal dorsal column pathway had been surgically transected. The findings show that in the absence of the spinal dorsal column pathway, the dynamics of the IR intrinsic signals evoked in SI by a low frequency vibrotactile stimulus are strikingly different than observed in normal subjects. In a number of respects, the abnormal dynamics of the IR intrinsic signals observed in SI after dorsal column transection appear to parallel the disturbances in vibrotactile detection and discriminative capacity reported for monkeys with a selective dorsal column lesion.