#mountcastle

Talbot, W.H., Darian-Smith, I., Kornhuber, H.H., and Mountcastle, V.B. (1968). The sense of flutter-vibration: comparison of the human capacity with response patterns of mechanoreceptive afferents from the monkey hand. Journal of Neurophysiology 31, 301-334.

In the mid-1960’s, dudes all over the country were asking the same tough question(s): what the fuck is up with my hands? Why they feelin’ so crazy? On the west coast, thousands of grubby hippies closed in on San Francisco to sit around and stare at their hands opening and closing like drowning fish; the resulting acid-addled boink-fest came to be known as the Summer of Love. Meanwhile, on the east coast, Vernon Mountcastle and his team of ultra-serious collabo-men locked themselves into a Baltimore basement with a brigade of angry monkeys and refused to come out until they had figured out what the fuck was really going on (with hands). The value of these two complimentary efforts is still a point of intense debate: although Mountcastle’s  J Neurophys pape has been cited over 650 times since its publication in 1968, aging suburban Deadheads cite the Summer of Love at least once every 3 seconds (on average). But with all due respect to Donovan and his army of burrito-worshipping hedonists, we at Sick Papes fall firmly on the side of the living legend, Professor Vernon Mountcastle (who turns 94 on July 15— Happy Birthday, Vern!).

In the 1950’s, Mountcastle and his buds were the first to demonstrate that the sensation of touch is mediated by specialized mechanosensory neurons located within the skin. Recording from rabbits, cats, and monkeys, they showed that the activity of primary mechanosensory neurons is transmitted through the spinal column, ascending to the thalamus, and eventually the cortex. However, the fact that neurons in the skin responded to mechanical forces said little about how they give rise to touch perception. What hand-freaks really wanted to know is how these simple mechanical sensors in the hand contribute to the central neural representation of touch.

In their 1968 paper, Mountcastle and his cronies addressed this question by directly comparing the activity of peripheral mechanosensory neurons with the perceptual limits of touch sensation. The physiology experiments were done with monkeys, recording extracellularly from cutaneous mechanoreceptors that innervated the hand, while the psychophysical experiments were done with humans (they assumed that any differences between the two species were negligible). They focused on the sense of “flutter-vibration”, or the ability to detect a rapidly oscillating probe applied to the skin. In humans, they found that low frequency stimulation (5-50 Hz) evoked a fluttering sensation, while higher frequencies (5-300 Hz) evoked a more diffuse feeling of vibration. They constructed psychometric tuning curves of these sensations by varying the amplitude and frequency of the oscillating stimulus, and asking people to estimate the strength of the stimulus. Surprisingly, the two sensations, flutter and vibration, had distinct frequency-intensity tuning functions, and applying liquid cocaine to the skin (as a superficial anesthetic) altered perceptual thresholds for flutter alone. These two clues suggested that flutter and vibration are served by distinct populations of primary mechanosensory neurons.

Recording from hundreds of mechanosensory neurons, Mountcastle’s squad found three basic types of responses: slowly-adapting (now known as Merkel cells), fast-adapting (Meissner cells), and the previously described Pacinian corpuscles. Similar to the psychophysics experiments, they constructed stimulus-response tuning curves to determine which oscillation frequencies each nerve type responded to optimally. At the optimal oscillation frequency, a mechanosensory neuron fired one action potential per stimulus cycle, so that the stimulus was coded unambiguously. By comparing these neurophysiological tuning curves to the human psychophysics data, they found that the response properties of fast-adapting neurons neatly matched the perceptual sensation of flutter, while the responses of Pacinian corpuscles fit with the perception of higher frequencies (i.e., vibration). Therefore, they concluded, the dual nature of human touch perception (flutter vs. vibration) is due to the fact that two distinct populations of mechanosensory neurons contribute to our sensation of touch.