Touching and timing consciousness

William James' metaphor of the "stream" of consciousness has served us well.¹ It captures within a poetic phrase the essence of the transient, shifting nature of our awareness. James' phrase also brings with it flotsam, an implication that our sensory perceptions are dropped into the stream like so many twigs, each flowing at the rate of the stream, isolated from the others, and self-contained, its effect transmitted by its own weight and shape.

In this issue of Neurology, Meador et al. report challenges to both presumptions.² They demonstrate that left-sided somatosensory stimuli are appreciated with a threshold lower than the right(this appreciation cannot be explained by peripheral effects) and that masking stimuli administered 50 to 100 milliseconds after the target are optimal in blocking access of the target stimuli into consciousness.

The beauty of the investigation by Meador et al. comes from its conception rather than technical virtuosity. The technical approach (which any of us who has ever measured a nerve conduction velocity could have done had we only the wisdom to conceive the experiment) was to stimulate the forefinger of each hand with precisely timed and amplitude-modulated electrical stimuli. Meador applied this technique in a large cohort of right-handed and left-handed subjects and determined for each subject the lowest-amplitude pulse that could be reliably detected from each hand. They then examined the effects of masking stimuli on target extinction.

Meador et al. found that for right-handed subjects, the left-sided sensory threshold was approximately 90% of the right. Their evidence that this difference reflected cerebral specialization rather than peripheral effects is persuasive. First, although the sensory thresholds increased with age, the magnitude of the asymmetry was unchanged. This finding is inconsistent with the asymmetry resulting from the accumulated effect …