FIG. 6.

The effect of electrical microstimulation on the classification choices for novel stimuli composed of image mixtures. A: psychophysical performance from a single behavioral experiment shows the systematic effect of varying the stimulus content from 100% left stimulus to 100% right stimulus. After a learning phase consisting of 100 repetitions for each of the individual stamps, stimulus blends were created by mixing the 2 stamps from the same orientation condition (upright or rotated) in various proportions. A brief pretest with fixed blend ratios (see methods) was administered, and the data were fit to a sigmoid function to estimate the midpoint for response selection (which was not always at the 50% blend level). Three levels of mixing around this subjective midpoint were then used for the main test condition. The abscissa for these plots represents the proportion of the right stimulus at each point, and the ordinate shows the proportion of right responses chosen. At the extremes, the animal's choices were close to perfect and choices between the 2 extremes varied smoothly and systematically as a function of blend proportion. B, left: comparison of stimulated (filled circles) and unstimulated (open circles) trials shows a systematic shift in response proportion upward in the direction of the response associated with the stimulus paired with microstimulation during learning. The microstimulation induced shift was measured as the average response difference between the stimulated and unstimulated conditions in the ambiguous region of the response curve (data appeared on gray background). Right: effect of microstimulation on trials from the same experimental session containing mixtures of images neither of which had been been previously been associated with microstimulation. There is transfer of the effect of microstimulation (for this experiment, microstimulation biased choices to favor the right hand response), but the magnitude of this shift was systematically smaller than that observed for the visual mixtures that included the previously associated image (see Fig. 7).