Abstract
The processing of a target event is facilitated if its timing is known, e.g. if it appears in phase with a preceding temporally
regular input. This effect has been attributed to automatic entrainment of internal oscillators to the input frequency and
phase (exogenous expectation). However, the timing of an event can also be expected based on voluntary shifting of
attention to an explicitly memorized interval, and the involvement of such endogenous factors in facilitation following
rhythmic input was not examined. Here, we presented a visual target following regularly flickering stimuli, and examined
the facilitative effect of the rhythm (faster responses for target in phase vs. target out of phase) in three block types:
'exogenous', in which rhythm phase was not predictive of target timing; 'endogenous entrainment', in which rhythm phase
validly predicted target timing; 'endogenous-exogenous conflict', in which subjects were endogenously cued to attend to a
memorized interval, while rhythm phase was not predictive. We found a facilitative effect for the rhythm in the all
conditions, with similar magnitude in the exogenous and conflict blocks, but larger than both in the entrainment block.
Importantly, the facilitation effect in the conflict block was independent of the interval expected based on the color cue, or
its validity effect. EEG analysis revealed that the CNV, a potential assumed to reflect expected interval, was driven by the
rhythm, suggesting that the behavioral effect reflected directional bias of temporal expectation by the rhythm, and not
general conflict. In a second experiment subjects did not perform endogenous entrainment blocks, thus eliminating the
possibility that the exogenous effect in the conflict block results from task confusion. Yet, the results for the exogenous
and the endogenous-exogenous conflict blocks were replicated. In conclusion, regular rhythms bias temporal expectation
even in the presence of voluntarily high-level expectation.
