In neural systems, information processing can be facilitated by adding an optimal level of white noise. Although this phenomenon, the so-called stochastic resonance, has traditionally been linked with perception, recent evidence indicates that white noise may also exert positive effects on cognitive functions, such as learning and memory. The underlying neural mechanisms, however, remain unclear. Here, on the basis of recent theories, we tested the hypothesis that auditory white noise, when presented during the encoding of scene images, enhances subsequent recognition memory performance and modulates activity within the dopaminergic midbrain (i.e., substantia nigra/ventral tegmental area, SN/VTA). Indeed, in a behavioral experiment, we can show in healthy humans that auditory white noise-but not control sounds, such as a sinus tone-slightly improves recognition memory. In an fMRI experiment, white noise selectively enhances stimulus-driven phasic activity in the SN/VTA and auditory cortex. Moreover, it induces stronger connectivity between SN/VTA and right STS, which, in addition, exhibited a positive correlation with subsequent memory improvement by white noise. Our results suggest that the beneficial effects of auditory white noise on learning depend on dopaminergic neuromodulation and enhanced connectivity between midbrain regions and the STS-a key player in attention modulation. Moreover, they indicate that white noise could be particularly useful to facilitate learning in conditions where changes of the mesolimbic system are causally related to memory deficits including healthy and pathological aging.
Theoretical models and empirical work indicate a critical role of the NAcc in salience processing. For instance, the NAcc not only responds to appetitive and aversive information, but it also signals novelty, contextual deviance, and action monitoring. However, because most studies have investigated only one specific type of salience independently, it remains unclear how the NAcc concurrently differentiates between different forms of salience. To investigate this issue, we used intracranial electroencephalography in human epilepsy patients together with a previously established visual oddball paradigm. Here, three different oddball categories (novel, neutral, and target images) were infrequently presented among a standard scene image, and subjects responded to the target via button press. This task allowed us to differentiate "item novelty" (new vs neutral oddballs) from "contextual deviance" (neutral oddballs vs standard images) and "targetness" (target vs neutral oddballs). Time-frequency analysis revealed a dissociation between item novelty and contextual deviance on the basis of decreases in either (4 -8 Hz) or  power (20 -30 Hz). Targetness, on the other hand, was signaled by positive deflections in the stimulus-locked local field potentials, which, importantly, correlated with subjects' reaction times. These findings indicate that, in an ongoing stream of information, the NAcc differentiates between types of salience by distinct neural mechanisms to guide goal-directed behavior.
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