e., the rate of evidence accumulation) is allowed to vary across trials. Varying drift rate randomly across successive trials allows different admixtures of trials with high and low drift rates to drive correct and incorrect choices, Compound C leading to the widely observed phenomenon that decision latencies for errors typically exceed those for correct choices (Ratcliff and Rouder, 1998). Even within each trial, the gradual buildup of neuronal firing rates in sensorimotor cortex is known to vary stochastically
in a fashion that predicts the dynamics of the eventual movement (Hanes and Schall, 1996). Our findings suggest a neurophysiological explanation for these two phenomena—that the rate of evidence accumulation varies within the course of a single trial according to the phase of ongoing slow cortical oscillations. Measuring the temporal spread of this cyclic modulation of information processing revealed that it lasted for several delta cycles, and was thus not simply a transient, discrete activity evoked by each element. Nevertheless, the influence of parietal delta phase on decision weighting tapered off quite rapidly, indicating that this rhythmic mechanism is not a rigid oscillatory mechanism but rather can be flexibly aligned to account
for the changing demands of information processing—e.g., become entrained to the onset of relevant stimuli when they are presented at predictable times at delta-band (<3 Hz) stimulation frequencies (Lakatos et al., 2008; Schroeder and Lakatos, 2009; Stefanics selleck chemicals et al., 2010). However, the strongest competition was observed between a given element and its immediate neighbors—within
each delta cycle—as expected if successive samples of information were competing to pass through a serial processing bottleneck. Finally, these findings shed light on the role of slow cortical oscillations in sensory selection (Lakatos et al., 2008; Schroeder and Lakatos, 2009; Stefanics et al., 2010). Previous research has demonstrated that the encoding of sensory information depends on the phase of delta oscillations in sensory cortex, but because stimulation occurred in the delta Moxisylyte band, it is hard to tell whether this modulation was dependent on being driven exogenously (entrained) by the external stimulation rhythm. Here we show that the neural encoding of both perceptual and categorical information in the human EEG was modulated by an internal rhythm distinct from the stimulation frequency. This finding demonstrates unambiguously that the selection of perceptual and categorical information is dependent on endogenous delta oscillations, not on the entrainment of neural oscillations to any stimulation frequency.
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