TEMPORAL SEGREGATION OF MICROSACCADES AND GAMMA SYNCHRONIZATION BY THETA-PHASE LOCKING

Microsaccades (MS) induce a period of perisaccadic suppression (~0.1 s), during which visual processing is impaired. In contrast, rhythmic neuronal activity in the gamma band (40-70 Hz) is strongly associated with stimulus processing. Both MS and gamma-band activity are known to be locked to the phase of the ongoing theta rhythm (3-8 Hz). Thus, the question arises how they are temporally segregated. Moreover, due to their locking to the slow theta rhythm, they might not be optimally aligned to the timing of processing demands imposed by behavioral tasks. We hypothesize that both processes are coupled to different theta phases, enabling temporal segregation, and that the theta-oscillatory cycle is modulated to ensure that they occur at the optimal time. To investigate this, we recorded neuronal activity in area V4 and eye movements from two macaques (Macaca mulatta) performing an attention-demanding shape tracking task. Crucially, behaviorally relevant task events occurred in a fixed temporal sequence that monkeys had learned during training. We found that gamma activity and MS were coupled to different theta phases. Further, the theta cycle was modulated with respect to the current behavioral demand such that MS and gamma activity occurred at the optimal time. Together, these findings show that theta phase, and hence gamma-band activity and MS rate, can be modulated according to task demands. Task-dependent adjustments of the timing of the theta cycle likely enable the precise timing of gamma activity and MS at timepoints when they are required for effective behavior.