Same-different task and behavioral performance. (A). Following 2 – 4 presentations of the reference stimulus, a test stimulus was presented. The reference stimulus was always one of the two prototype spoken words (bad or dad). The test stimulus was a morphed version of the prototypes. If the monkeys perceived that the reference and test stimuli were the same, they made a saccade to a leftward target. If the monkeys perceived that the reference and test stimuli were different, they made a saccade to a rightward target. (B). Spectrographic representations of the prototype spoken words and two of the morphs. In this example, the reference stimulus is bad. Consequently, it is the 100% morph, whereas dad is the 0% morph; see Experimental Procedures for more details. When the reference stimulus is the spoken word dad, the morph percentages are reversed (e.g., the 0% morph is bad and the 100% morph is dad). The axes for all of the spectrograms are seen in the leftmost spectrogram. (C). The average performance of the monkeys from those recording sessions reported in this manuscript. The monkeys’ performance is shown as a function of the reference stimulus: the prototype spoken word bad (left column) or dad (right column). A 0% morph means that the test stimulus was a different prototype than the reference stimulus (e.g., the reference stimulus was the prototype bad and the test stimulus was the prototype dad). A 100% morph means that the test and reference stimuli were the same (e.g., both were the prototype dad). Other values represent morphed stimuli between these two extremes. The data that are plotted with a solid line are from monkey H; the data that are plotted with a dashed line are from Monkey P. Error bars are standard error of the means.

Examples of vPFC activity during the same-different task. In A and B, the reference stimulus was the prototype spoken word bad. In each panel, the rasters and spike-density histograms are aligned relative to the onset of the test stimulus. The morph value of the test stimulus is indicated by color as shown by the color bar: 0% morphs are the lightest blue color and 100% morphs are the purple color. When the test stimulus was a 100% morph, it was identical to the reference stimulus. The arrows in A indicate the approximate times of each of the reference stimuli. In these two panels, only successful trials are shown.

Choice probability. The distribution of grand choice-probability values for each neuron in our population of vPFC neurons when (A) the reference stimulus is the prototype word bad and when (B) the reference stimulus is dad. Panel C shows the distribution of the CP values generated from those trials when the monkeys’ decisions were “easy”; that is, those test-stimulus morphs that were very different or identical to the reference stimulus (i.e., 0%, 20%, 80%, and 100%). Panel D shows distribution of the CP values generated from those trials when the monkeys’ decisions were “hard”; that is, those test-stimulus morphs that were very similar to the reference stimulus (i.e., 40%, 50%, and 60%). For panels C and D, we grouped together data collected for the two different reference stimuli.

Time course of choice probability. A, B: The CP values were calculated from non-overlapping 200-msec epochs of neural activity. The data in A are aligned relative to the onset of the test stimulus, whereas the data in B are aligned relative to onset of the LEDs. In both panels, the data in green illustrate the CP values generated when the reference stimulus was bad, whereas the data in purple illustrate the CP values generated when the reference stimulus was dad. The data in black illustrate the CP values generated when the two reference stimuli were combined. The error bars represent the standard error.