Spike Timing Matters in Novel Neuronal Code Involved in Vibrotactile Frequency Perception  Ingvars Birznieks, Richard M. Vickery  Current Biology  Volume 27, Issue 10, Pages 1485-1490.e2 (May 2017) DOI: 10.1016/j.cub.2017.04.011 Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 1 Time-Controlled Afferent Spike Trains Created by Short Pulsatile Mechanical Stimuli (A) Schematic representation of a set of four pulsatile vibrotactile stimuli intended to evoke spike trains with the same repetition rate (base periodicity) but strikingly different mean spike rates. Each vertical line indicates the time of the mechanical pulse. (B) Illustrative recording of spike trains evoked in a single human FA-I tactile afferent, demonstrating a perfect match with the temporal pattern of pulsatile stimuli as shown in (A) (one spike per stimulus pulse). Identified single spikes are highlighted in red. Cycle histograms on the right show binned spike count over the 1-s stimulation period, recorded in four consecutive 1-s stimulation periods each consisting of 23 bursts. Each mechanical pulse generated a single action potential without failing on a single occasion. The dashed horizontal line corresponds to the count of 23 spikes. One bin corresponds to 1/40th of the period (∼1.1 ms; the cycle histogram is truncated at 30 ms). The onset latency is the time from when the command was sent to the stimulator at the beginning of each burst to when the spike was recorded from the median nerve at the wrist level; spike jitter was less than 1 ms. See also Figure S1. Current Biology 2017 27, 1485-1490.e2DOI: (10.1016/j.cub.2017.04.011) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 2 Spike Rate Does Not Predict the Perceived Frequency in Psychophysical Tests of Complex Tactile Stimuli (A) Boxplots with whiskers represent the point of subjective equality (PSE) data obtained with stimuli illustrated in Figure 1. The box extends from the 25th to 75th percentiles; the whiskers extend to minimum and maximum values, and circle symbol indicates median. The horizontal dashed line indicates mean (n = 12). Predicted PSE values if the frequency judgment was based on mean spike rate code are indicated using green arrowheads; burst rate, purple arrowheads, reciprocal of burst gap 1/tL, blue arrowheads. The experimentally obtained PSE values indicate that neither the mean spike rate code nor burst rate code can explain the perceived frequency; the closest match is the reciprocal of tL. (B) The reciprocal of burst gap tL provides good match with PSEs. The solid black line connects data points obtained by averaging all of the individual subject data, which are shown by thin gray lines (n = 12). The dotted red line represents the line of identity. See also Figure S2. Current Biology 2017 27, 1485-1490.e2DOI: (10.1016/j.cub.2017.04.011) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 3 Number of Spikes within the Burst Does Not Affect the Perceived Frequency (A) Schematic representation of the stimuli used in the second experiment. The index “d,” standing for “doublet,” is added to the stimulus number to distinguish corresponding stimuli from those in experiment 1. (B) PSE values for the perceived frequency of stimuli shown in (A) (blue line with diamonds; with 95% confidence intervals are shown; n = 12). PSEs for the stimuli depicted in Figure 1 are plotted for comparison (orange line with squares, slightly horizontally shifted). The dashed black lines indicate PSE values predicted by the burst gap code model. Despite the different number of spikes in the two sets of corresponding stimuli, the perceived frequencies were similar. See also Table S1. Current Biology 2017 27, 1485-1490.e2DOI: (10.1016/j.cub.2017.04.011) Copyright © 2017 Elsevier Ltd Terms and Conditions

Figure 4 Relationship between the Shortest Inter-spike Interval Duration and the Perceived Vibrotactile Frequency (A) Schematic representation of pulsatile vibrotactile stimuli consisting of a repeating pattern of one shorter interval constituting a doublet followed by an 87-ms-long interval. (B) PSE values obtained in psychophysical tests shown with 95% confidence level shaded in gray (n = 12). Individual data points indicated by orange squares and three linear segments as determined by joinpoint regression model indicated by three connected orange lines. Predicted PSE values if the frequency judgment was based on: mean spike rate code (green line); the burst gap code (blue line). Current Biology 2017 27, 1485-1490.e2DOI: (10.1016/j.cub.2017.04.011) Copyright © 2017 Elsevier Ltd Terms and Conditions