E. W. Wright

Responses of Human Somatosensory Cortex to Stimuli below Threshold for Conscious Sensation

Averaged evoked responses of somatosensory cortex, recorded subdurally, appeared with stimuli (skin, ventral posterolateral nucleus, cortex) which were subthreshold for sensation. Such responses were deficient in late components. Subthreshold stimuli could elicit sensation with suitable repetition. The primary evoked response was not sufficient for sensation. These facts bear on the problems of neurophysiological correlates of conscious and unconscious experience, and of subliminal perception.

Cortical Representation of Evoked Potentials Relative to Conscious Sensory Responses, and of Somatosensory Qualities - in Man

The study of cortical evoked potential and neuronal unit responses to sensory stimuli can give us essential information about the locations and types of projections to the cortex and about the physiological mechanisms activated by such projections. By themselves, however, such studies cannot establish the relationship between neuronal activities and the production of conscious sensory experiences. Obviously this can be done only by direct validations of the electrophysiological actions against the other independent variable, the subjective experience of the human subject. Indeed, since sensory projections and their cortical responses would be expected to subserve other functions in addition to that of conscious sensory experience, it should be no surprise to find dissociations between the occurrence of certain evoked electrophysiological responses to stimuli and the eliciting of a subjective response. The clearest example of this is seen in the responses to certain muscle afferents. Group I muscle afferents have been found to elicit evoked responses in primary somatosensory or closely related cortex (Amassian and Berlin, 1958; Oscarsson and Rosen, 1963, 1966; Albe-Fessard, 1967; Swett and Bourassa, 1967). Yet these inputs do not elicit any subjective experience of motion, position, muscle length, or any other sensation in man (Brindley and Merton, 1960; Gelfan and Carter, 1967). Also, in contrast to other somatic inputs, the group I afferent inputs could not produce behavioral (Giaquinto, Pompeiano, and Swett, 1963) or conditional learned responses in the cat (Swett and Bourassa, 1967). The projection to the cortex of group I muscle-spindle afferent impulses, as well as of other somatic afferents, may of course serve in the integration and organization of movements mediated by the cortex, without giving rise to any subjective sensory experience (e.g. Oscarsson, 1965; Albe-Fessard, 1967; Kornhuber, 1971). The actual subjective experience of position and movement would depend on other inputs, especially those originating in and about the joints (Skoglund, 1956; Provins, 1958, Mountcastle and Powell, 1959).

Cortical and thalamic activation in conscious sensory experience

Our work has been aimed directly at the question of the nature of the spatio-temporal configurations of cerebral neural activities which elicit, or are at least uniquely correlated with, the conscious awareness of a somatosensory experience in the awake and alert individual. We have focused the study on the differences between cerebral functions associated with stimuli that are at or just above threshold level for eliciting a conscious sensory experience, as opposed to those with stimuli that are below such a threshold. The implication is that there are some unique differences between physiological cerebral states above and below this threshold level. The term threshold is used by us in tF.e broad sense of the just adequate levels for all significant parameters of a stimulus (electrical in our studies) not merely that for intensity (Libet et al., 1964; Libet, 1972). (To avoid confusion with threshold levels for various,other types of responses to stimuli, particularly for evoked potentials (EP’s), the thresholdlevels for eliciting a conscious sensory response will be referred to as „threshold-c.’)