Neural variability in responding to identical repeated stimuli has been related to trial-by-trial fluctuations in ongoing activity, yet the neural and perceptual consequences of these fluctuations remain poorly understood. Using functional neuroimaging, we recorded brain activity in subjects who reported perceptual decisions on an ambiguous figure, Rubin's vase-faces picture, which was briefly presented at variable intervals of >20 s. Prestimulus activity in the fusiform face area, a cortical region preferentially responding to faces, was higher when subjects subsequently perceived faces instead of the vase. This finding suggests that endogenous variations in prestimulus neuronal activity biased subsequent perceptual inference. Furnishing evidence that evoked sensory responses, we then went on to show that the pre-and poststimulus activity interact in a nonlinear way and the ensuing perceptual decisions depend upon the prestimulus context in which they occur.fusiform face area ͉ ongoing activity ͉ BOLD fMRI ͉ prestimulus activity ͉ visual perception S ince the earliest neurophysiological recordings, two issues have puzzled brain researchers: why do, trial by trial, cortical responses to identical stimuli vary so much (1), and what is the functional significance of spontaneous neural activity, i.e., activity that cannot be accounted for by the experimental manipulation and that is thus usually discarded as unexplained variance? The two issues have in part been tied together by relating response variability to fluctuations in ongoing prestimulus activity (2, 3). In anesthetized animals, an excellent prediction of the actual response evoked in each trial was achieved by assuming a fixed stimulus-driven or task-related response from averaging and adding, trial by trial, this response to baseline activity (2). Evidence for the perceptual relevance of ongoing neural activity comes from monkey electrophysiology (4) and more recently human imaging studies (5) where periliminal stimuli were perceived when prestimulus activity was at higher levels, thus resembling effects from experimentally instructed allocation of attention. This all-or-none effect of ongoing activity on detection of periliminal stimuli could correspond to the well known behavioral effect of arousal and alertness on perceptual thresholds (6). Here, we address whether ongoing activity only impacts whether something can be perceived or whether it also influences what is perceived. We therefore investigated whether spontaneously occurring cortical activity variations in the seconds before input processing bias subsequent perceptual decisions on a suprathreshold but ambiguous visual input. We then addressed whether observed responses were simply the sum of evoked responses plus the baseline activity or whether baseline activity affected the evoked response, which would indicate an interaction between baseline activity and evoked components).Using functional magnetic resonance imaging (fMRI), we pursued this issue by asking subjects what they perceived each ...

Recent studies have shown that ongoing activity fluctuations influence trial-by-trial perception of identical stimuli. Some brain systems seem to bias toward better perceptual performance and others toward worse. We tested whether these observations generalize to another as of yet unassessed sensory modality, audition, and a nonspatial but memory-dependent paradigm. In a sparse event-related functional magnetic resonance imaging design, we investigated detection of auditory near-threshold stimuli as a function of prestimulus baseline activity in early auditory cortex as well as several distributed networks that were defined on the basis of resting state functional connectivity. In accord with previous studies, hits were associated with higher prestimulus activity in related early sensory cortex as well as in a system comprising anterior insula, anterior cingulate, and thalamus, which other studies have related to processing salience and maintaining task set. In contrast to previous studies, however, higher prestimulus activity in the so-called dorsal attention system of frontal and parietal cortex biased toward misses, whereas higher activity in the so-called default mode network that includes posterior cingulate and precuneus biased toward hits. These results contradict a simple dichotomic view on the function of these two latter brain systems where higher ongoing activity in the dorsal attention network would facilitate perceptual performance, and higher activity in the default mode network would deteriorate perceptual performance. Instead, we show that the way in which ongoing activity fluctuations impact on perception depends on the specific sensory (i.e., nonspatial) and cognitive (i.e., mnemonic) context that is relevant.

Ongoing brain activity has been observed since the earliest neurophysiological recordings and is found over a wide range of temporal and spatial scales. It is characterized by remarkably large spontaneous modulations. Here, we review evidence for the functional role of these ongoing activity fluctuations and argue that they constitute an essential property of the neural architecture underlying cognition. The role of spontaneous activity fluctuations is probably best understood when considering both their spatiotemporal structure and their functional impact on cognition. We first briefly argue against a “segregationist” view on ongoing activity, both in time and space, which would selectively associate certain frequency bands or levels of spatial organization with specific functional roles. Instead, we emphasize the functional importance of the full range, from differentiation to integration, of intrinsic activity within a hierarchical spatiotemporal structure. We then highlight the flexibility and context-sensitivity of intrinsic functional connectivity that suggest its involvement in functionally relevant information processing. This role in information processing is pursued by reviewing how ongoing brain activity interacts with afferent and efferent information exchange of the brain with its environment. We focus on the relationship between the variability of ongoing and evoked brain activity, and review recent reports that tie ongoing brain activity fluctuations to variability in human perception and behavior. Finally, these observations are discussed within the framework of the free-energy principle which – applied to human brain function – provides a theoretical account for a non-random, coordinated interaction of ongoing and evoked activity in perception and behavior.