Physiology of somatosensory perception

Objective: To demonstrate the effects of cerebral lateralization and temporal dynamics on somatosensory perception. Background: We postulated that perceptual thresholds for simple somatosensory stimuli would be less in the left than the right hand, and that a left/right asymmetry in extinction would exist in healthy right-handed subjects (but not in left-handed subjects). During the course of these experiments we also examined the controversy concerning the temporal dynamics of somatosensory perception. Methods: A total of 126 healthy subjects (age range, 6 to 73 years) participated in the study. Effects of handedness, age, vigilance, gaze, and temporal interval on somatosensory perception were examined in a series of experiments. Brief electric pulses were applied to the index finger of one or both hands. Results: Perceptual thresholds are lower in the left than the right hand of healthy right-handed subjects in a large cohort across a wide age range. Left-handed subjects have no overall asymmetry. Even after compensation for baseline threshold differences, single stimuli in right-handed subjects are perceived more readily in the left than the right hand, and left-hand targets are more difficult to mask. Leftward eye/head gaze lowers thresholds in both hands of right-handed subjects (compared with right or straight gaze). Extinction was consistently maximal when the mask followed the target by 50 to 100 msec. Conclusions: The findings demonstrate clearly that left/right perceptual thresholds for simple somatosensory stimuli are asymmetric in healthy right-handed subjects. Both central and peripheral asymmetries exist. The central asymmetry and gaze effects are consistent with right cerebral dominance for externally directed attention. Access of somatosensory stimuli to conscious awareness is delayed and particularly vulnerable to disruption at 50 to 100 msec after onset of the stimulus.