Figure 5.

Diversity of summation effects by synchronizing feedforward and horizontal activations. Single-cell examples of responses to 2S-AM trajectories (directions 0°, left, and 180°, right) along the preferred orientation axis. Top diagrams use the same conventions as in Figure 1C and represent the link between the hexagonal tiles and the trajectories (arrows) for the trajectories along the main axis. In each square inset, the black trace represents the response to the corresponding trajectory, overlaid on its respective linear prediction (LP), shown in gray. Black dashed boxes indicate the two trajectories where one of the two flashed Gabor patches stimulates directly the RF (henceforth, “central trajectories”). As in Figure 2, the one-tailed response significance thresholds at p < 0.05 are shown in blue. Asterisks indicate the statistical significance level of the nonlinear summation (all one-tailed, paired permutation test), measured in the 20 ms preceding the peak of significant responses (see Materials and Methods). Red asterisks indicate when the nonlinearity is facilitatory. Blue asterisks indicate when the nonlinearity is suppressive. We found that suppressive nonlinearity is the exception, not the rule (see also Fig. 6). A–C, Same three example cells as shown in Figure 2. A, Facilitatory interaction can be found for various trajectories across the visual field, and not only on the main axis. B, Occasionally, de novo responses could be revealed in the far periphery, as is the case for the 180° centripetal distal trajectory (rightmost trajectory in the figure). C, D, Spiking responses are shown in the row below the synaptic responses. C, Even relatively small facilitation at the subthreshold level can translate into much larger facilitation at the spiking level. D, Example of highly localized nonlinearity (second left). Note also another de novo distal synaptic response (rightmost trajectory in the figure). For this cell, n = 38 trials.