Vibrotactile intensity perception: Predominant influence of afferents recruited remote from stimulus site

Sinusoidal vibratory stimuli are frequently used to study human sensory perception but have the limitation that changes in vibration frequency are accompanied by changes in the number and type of activated mechanoreceptive afferents. Here we used trains of brief mechanical pulses to investigate the neural coding of vibrotactile perceived intensity by grouping pulses into bursts. These pulse trains evoke the same perceived frequency, determined by the interval between bursts, as we have previously demonstrated, and held constant across conditions. Subjects rated the perceived intensity using a magnitude estimation task for stimuli varying in the number of pulses per burst (up to four) and stimulation amplitude (5-150 µm). In marked contrast to our previous findings using electrical stimulation, increasing the number of pulses per burst had only a minimal and inconsistent effect on perceived intensity. To explain this we simulated the responses of the afferent population across the hand using the TouchSim computational model. The model revealed that increasing pulse number, without changing amplitude, produced only a modest increase in total population spike count. This occurred because the population response was dominated by large numbers of afferents remote from the stimulation site, many of which failed to respond reliably to each pulse within a burst. In contrast increasing stimulus amplitude enhanced spatial recruitment, leading to greater population spike counts and increased perceived intensity. Together these results highlight the importance of both temporal and spatial summation in shaping tactile intensity perception and argue against a 'hot zone' model of intensity encoding. KEY POINTS: Electrical stimulation of the finger has been shown to change perceived intensity when varying the number of pulses within a stimulus burst, indicating that touch-sensitive nerve fibres encode intensity through the number of impulses they generate within bursts of activity. Here we used mechanical pulses applied to the skin to induce similar bursts of activity. In contrast to expectations, increasing the number of pulses within a burst did not consistently increase perceived intensity. We simulated the neural responses of the entire population of tactile nerve fibres in the hand, revealing that the burst pattern had only a small effect on nerve activity, which was dominated by large numbers of remote fibres that did not reliably follow bursts. These findings argue against a 'hot zone' model, where intensity is determined by nerve activity near the stimulus site, and instead suggest that the number of active fibres and overall activity are most significant.