PRODUCTION OF THRESHOLD LEVELS OF CONSCIOUS SENSATION BY ELECTRICAL STIMULATION OF HUMAN SOMATOSENSORY CORTEX

Abstract

Parameters of electrical stimuli (rectangular pulses) eliciting a conscious sensation when applied directy to somatosensory cortex of human subjects are characterized. Threshold parametric points can be established within usable limits under suitable experimental conditions. Parameters studied include peak current (I), pulse polarity, pulse duration (PD), pulse repetition frequency (PF), train duration (TD), train repetition rate (TRR), uni- and bipolar electrode arrangement, and electrode area. Threshold parametric points can be grouped into 3 parametric regions. Region A encompasses threshold stimuli with relatively long TDs (> 0.5 sec) and with pulse frequencies 15 pulses/sec and up. Responses in this region are purely somatosensory. Region B encompasses threshold stimuli requiring relatively high I and short TDs (including single pulses). Threshold response in region B is motor. (Motor responses obtained from post central gyrus differ from pre-central in relative threshold I and in type of motor response.) Parametric region C encompasses threshold stimuli and responses intermediate between A and B. TD may be reduced to about 0.5-1 sec with very little rise in threshold I (PF [greater than or equ 15 pulses/sec). This minimum TD (required of stimulus trains having the "liminal" or lowest threshold intensity) is named "utilization TD." In contrast to cortical stimulation, utilization TDs with stimuli applied to skin are very short (2 to 4 pulses). This leads to inferences about a latent period for conscious awareness of sensory input at near threshold levels. PF is inversely related to liminal L This, and the comparative absence of effect of PF from about 15 pulses/sec up on utilization TD, suggests that an excitatory change is developed by each stimulus pulse in a train, having a decay time of 50-100 msec. Stimuli in regions B and C produce a longer-lasting facilitory change, with a decay time greater than 30 sec. Surface cathodal stimuli are more effective than anodal, more so on somatosensory than motor cortex. Bipolar stimuli, 2-mm inter-electrode distance, require threshold I levels between those for unipolar cathodal and anodal ones. This suggests that relevant neural components lie superficially in the cortex and are oriented perpendicular to the surface. A 10-mm diameter electrode elicits a threshold conscious sensation similar to a 1-mm electrode, but with lower current density; thus spatial facilitation can be an important factor in eliciting threshold sensations. Coulombic content of threshold stimulus trains is markedly different for differing parametric points. It is lower the lower the PF, the shorter the PD, and the shorter the TD, even though the threshold I values change in the opposite direction with such changes in these variables. When the usual alerting procedure (ready signal, eeg-alpha blockade obtained) is changed to another (no ready signal, eeg alpha present), there is no significant difference in liminal I or utilization TD, either for cortical or skin stimulation. Mechanisms that modify eeg-alpha state in the so-called "arousal" response do not necessarily modify the state of cortical responsiveness or the attentive state, in so far as these states are relevant to eliciting conscious sensation.