Figure 2.

Auditory neurons show a range of invariant discrimination in field L, the primary auditory cortex analog. A, Spike distance matrix from the sensitive neuron shown in Figure 1 (left). All pairwise spike distances are shown for each of 10 trials, seven stimulus amplitudes, and two songs for a total of 140 spike trains. Spike trains 1–70 correspond to song A and 71–140 to song B. Spike distances were calculated using the van Rossum metric with a τ value of 10 ms using the first 1 s of each song. Warmer colors correspond to larger distances or more different spike trains. B, Spike distance matrix from invariant neuron shown in Figure 1 (right) using the same parameters as in A. C, MDS plot of relative spike distances in two dimensions for the sensitive neuron. D, MDS plot for the invariant neuron in the same MDS axes as C. E, Percentage of correct classification of two songs as a function of song amplitude for both an invariant (neuron 2) and a sensitive (neuron 1) neuron. Classification was done by measuring spike distance to two randomly chosen templates from the middle intensity (one from each song) and assigning the trial to the template with the shortest distance. F, Histogram of measure of intensity invariance based on flatness of percentage correct curve for each of the 34 sites in this study from field L that showed significant discrimination (better than halfway between chance and perfect) at any stimulus intensity. Larger numbers indicate more invariant discrimination.