Vibrotactile discrimination task.

<p><b><i>A</i></b>, Top line shows vibrotactile stimuli, bottom colored lines show target actions for the example trial (F1 < F2). H, hold key; L, press left button to indicate F2 < F1; R, press right button to indicate F2 > F1. <b><i>B</i></b>, Network model. The units in the sensory layer are tuned for the tactile frequency with monotonically increasing or decreasing sigmoidal tuning curves. The binary ‘S’ neuron codes for skin contact of the vibrotactile probe and becomes active at ‘Contact’ in A. <b><i>C</i></b>, Average psychometric curves for 100 networks trained on the variable F1 task. Each set of data points (grouped by color) shows responses for the F1 stimulus that is indicated with a vertical line for flanking F2 stimuli; blue: F1 = 20Hz, yellow: F1 = 30Hz and pink: F1 = 40Hz. Y-axis shows the mean proportion of trials where networks indicated that F2 > F1 (each comparison was evaluated 100 times for every network). Error bars show s.d. over networks. Curves are logistic fits to the model responses. <b><i>D</i></b>, Tuning of two example memory units to F1 frequency during the delay phase. <b><i>E</i></b>, Histogram of linear correlations between F1 frequency and memory unit activations during the delay phase for 100 networks (N = 400). <b><i>F</i></b>, Example activity traces for two memory units and the three Q-value units. Left panel shows the response for F1 = 20Hz and F2 = F1±5Hz (solid +5Hz, dashed -5Hz). The response of the <i>Q</i>-value units is coded following the color scheme in panels A and B. Right panel shows activity of these units when F1 was 30 Hz. F2 indicates onset of second vibration stimulus. D: Memory delay phase. Note that F2 is 25Hz for the continuous lines in the left panel and also for the dashed lines in the right panel, but that these trials require different responses (right button if F1 = 20Hz and left button if F1 = 30Hz). <b><i>G</i></b>, Scatter plot of linear regression parameters of various unit types when F2 was presented (as explained in the main text). A positive A1 (A2) parameter indicates that a unit becomes more active for higher F1 (F2). Green line shows y = x and the activity of units on this line is related to the sum of F1 and F2. The red line represents y = -x, and the activity of units on this line represents the difference between F1 and F2. The color scheme for the Q-value units is the same as in (A) and (B). <b><i>H</i></b>, Scatter plot of linear regression parameters at the time of F2 presentation for networks trained on the version of the task with fixed F1. <b><i>I</i></b>, Psychometric curves for block-trained fixed F1 networks (see main text). Same conventions as for (C). Only the logistic fit (black line) for F1 = 30 Hz is drawn.