Figure 1. Example responses to multipoint correlations in V1 and V2.
Top row: examples of the stimulus sets used to isolate the different kinds of multipoint correlations. Six sets consist of 1024 16 × 16 binary checkerboards, each with a different statistical structure (left columns); the seventh set consists of 1024 16 × 16 random checkerboards (right column); see ‘Materials and methods’ for details. In each column, the row of PSTH's shows responses of a single neuron to 1024 examples of stimuli drawn from the seven sets. Responses are generally dominated by a transient increase or decrease in firing, occurring 70 to 100 ms after the onset of each stimulus. In some cases, the size or configuration of this transient depends on the type of multipoint correlation (for example, the units in the second row). The asterisks indicate responses to the structured stimulus sets (black) that are significantly different (see ‘Materials and methods’) from the responses to the random stimuli (light gray, beginning of each row). Decremental responses following contrast onset were present in both areas, but more often in V2. However, a decremental response was not a requirement for discriminating among multipoint correlations: outside of supragranular V2, there were many neurons that had incremental responses to the stimulus transient and distinguished among the types of multipoint correlation (for example, the third and fourth rows on the right).