The qualities of perception depend not only on the sensory inputs but also on the brain state before stimulus presentation. Although the collective evidence from neuroimaging studies for a relation between prestimulus state and perception is strong, the interpretation in the context of sensory computations or decision processes has remained difficult. In the auditory system, for example, previous studies have reported a wide range of effects in terms of the perceptually relevant frequency bands and state parameters (phase/ power). To dissociate influences of state on earlier sensory representations and higher-level decision processes, we collected behavioral and EEG data in human participants performing two auditory discrimination tasks relying on distinct acoustic features. Using single-trial decoding, we quantified the relation between prestimulus activity, relevant sensory evidence, and choice in different taskrelevant EEG components. Within auditory networks, we found that phase had no direct influence on choice, whereas power in taskspecific frequency bands affected the encoding of sensory evidence. Within later-activated frontoparietal regions, theta and alpha phase had a direct influence on choice, without involving sensory evidence. These results delineate two consistent mechanisms by which prestimulus activity shapes perception. However, the timescales of the relevant neural activity depend on the specific brain regions engaged by the respective task.perception | oscillatory brain activity | EEG | single-trial decoding | prestimulus effects S ensory percepts depend not only on the environmental inputs but also on the internal brain state before stimulus presentation (1). Many studies have shown that the accuracy and speed of sensory performance change with the power and timing (phase) of rhythmic activity during a prestimulus period (2, 3). Studies in the auditory system, for example, have demonstrated that performance in detecting sounds and gaps in noise, or the discrimination of lexical stimuli, varies with the power and phase of rhythmic activity between about 1 and 12 Hz (4-9).Although the collective evidence makes a strong case that prestimulus state shapes the processing and perceptual consequences of sensory inputs, the functional interpretation of these findings in the context of specific sensory computations or higher cognitive processes has remained difficult (7, 10, 11). Electrophysiological studies in animals have described the state dependency of firing rates relative to cortical oscillations (12-15). Hence, it is tempting to interpret the reported prestimulus effects in neuroimaging studies as direct evidence for a link between the neural gain of early sensory cortices and perception. However, this is difficult for two reasons. First, previous studies have used different behavioral protocols (detection and discrimination) and stimuli (tones in silence or noise, gaps in noise, or speech), and each has implied different frequency bands and state parameters as relevant (from 1 to 12 Hz, repo...

As we get older, perception in cluttered environments becomes increasingly difficult as a result of changes in peripheral and central neural processes. Given the aging society, it is important to understand the neural mechanisms constraining perception in the elderly. In young participants, the state of rhythmic brain activity prior to a stimulus has been shown to modulate the neural encoding and perceptual impact of this stimulus – yet it remains unclear whether, and if so, how, the perceptual relevance of pre-stimulus activity changes with age. Using the auditory system as a model, we recorded EEG activity during a frequency discrimination task from younger and older human listeners. By combining single-trial EEG decoding with linear modelling we demonstrate consistent statistical relations between pre-stimulus power and the encoding of sensory evidence in short-latency EEG components, and more variable relations between pre-stimulus phase and subjects’ decisions in longer-latency components. At the same time, we observed a significant slowing of auditory evoked responses and a flattening of the overall EEG frequency spectrum in the older listeners. Our results point to mechanistically consistent relations between rhythmic brain activity and sensory encoding that emerge despite changes in neural response latencies and the relative amplitude of rhythmic brain activity with age.

As we get older, perception in cluttered environments becomes increasingly difficult as a result of changes in peripheral and central neural processes. Given the aging society it is important to understand the neural mechanisms constraining perception in the elderly. In young participants, the state of rhythmic brain activity prior to a stimulus has been shown to modulate the neural encoding and perceptual impact of this stimulusyet it remains unclear whether, and if so, how, the perceptual relevance of pre-stimulus activity changes with age. Using the auditory system as a model, we recorded EEG activity during a frequency discrimination task from younger and older human listeners.By combining single-trial EEG decoding with linear modelling we demonstrate consistent statistical relations between pre-stimulus power and the encoding of sensory evidence in short-latency EEG components, and more variable relations between prestimulus phase and subjects' decisions in fronto-parietal EEG components. At the same time, we observed a significant slowing of auditory evoked responses and a flattening of the overall EEG frequency spectrum in the older listeners. Our results point to mechanistically consistent relations between rhythmic brain activity and sensory encoding that emerge in large despite changes in neural response latencies and the relative amplitude of rhythmic brain activity with age.

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