Environmental rhythms potently drive predictive resource allocation in time, typically leading to perceptual and motor benefits for on-beat, relative to off-beat, times, even if the rhythmic stream is not intentionally used. In two human EEG experiments, we investigated the behavioral and electrophysiological expressions of using rhythms to direct resources away from on-beat times. This allowed us to distinguish goal-directed attention from the automatic capture of attention by rhythms. The following three conditions were compared: (1) a rhythmic stream with targets appearing frequently at a fixed off-beat position; (2) a rhythmic stream with targets appearing frequently at on-beat times; and (3) a nonrhythmic stream with matched target intervals. Shifting resources away from on-beat times was expressed in the slowing of responses to on-beat targets, but not in the facilitation of off-beat targets. The shifting of resources was accompanied by anticipatory adjustment of the contingent negative variation (CNV) buildup toward the expected off-beat time. In the second experiment, off-beat times were jittered, resulting in a similar CNV adjustment and also in preparatory amplitude reduction of beta-band activity. Thus, the CNV and beta activity track the relevance of time points and not the rhythm, given sufficient incentive. Furthermore, the effects of task relevance (appearing in a task-relevant vs irrelevant time) and rhythm (appearing on beat vs off beat) had additive behavioral effects and also dissociable neural manifestations in target-evoked activity: rhythm affected the target response as early as the P1 component, while relevance affected only the later N2 and P3. Thus, these two factors operate by distinct mechanisms.
SIGNIFICANCE STATEMENT:
Rhythmic streams are widespread in our environment, and are typically conceptualized as automatic, bottom-up resource attractors to on-beat times-preparatory neural activity peaks at rhythm-on-beat times and behavioral benefits are seen to on-beat compared with off-beat targets. We show that this behavioral benefit is reversed when targets are more frequent at off-beat compared with on-beat times, and that preparatory neural activity, previously thought to be driven by the rhythm to on-beat times, is adjusted toward off-beat times. Furthermore, the effect of this relevance-based shifting on target-evoked brain activity was dissociable from the automatic effect of rhythms. Thus, rhythms can act as cues for flexible resource allocation according to the goal relevance of each time point, instead of being obligatory resource attractors.
