When faced with ambiguous sensory inputs, subjective perception alternates between the different interpretations in a stochastic manner. Such multistable perception phenomena have intrigued scientists and laymen alike for over a century. Despite rigorous investigations, the underlying mechanisms of multistable perception remain elusive. Recent studies using multivariate pattern analysis revealed that activity patterns in posterior visual areas correlate with fluctuating percepts. However, increasing evidence suggests that vision-and perception at large-is an active inferential process involving hierarchical brain systems. We applied searchlight multivariate pattern analysis to functional magnetic resonance imaging signals across the human brain to decode perceptual content during bistable perception and simple unambiguous perception. Although perceptually reflective activity patterns during simple perception localized predominantly to posterior visual regions, bistable perception involved additionally many higher-order frontoparietal and temporal regions. Moreover, compared with simple perception, both top-down and bottom-up influences were dramatically enhanced during bistable perception. We further studied the intermittent presentation of ambiguous images-a condition that is known to elicit perceptual memory. Compared with continuous presentation, intermittent presentation recruited even more higher-order regions and was accompanied by further strengthened top-down influences but relatively weakened bottom-up influences. Taken together, these results strongly support an active top-down inferential process in perception.visual perception | fMRI | MVPA | Granger causality | ambiguous images T he problem of vision entails the constant interpretation of inherently ambiguous local components of a complex scene. In contrast to reduced visual stimuli routinely used in laboratory research such as Gabor patches and isolated faces, natural scenes contain many ambiguities caused by clutter, occlusion, shading, and the inherent complexity of natural objects (1, 2). Similarly, simple daily tasks, such as interpreting the handwriting of another individual, require a level of cognitive capability surmounting that of modern-day computers. The ease with which we are able to rapidly perform such tasks attests to the remarkable capacity of the human visual system, or alternatively, to the vast knowledge and templates stored in the human brain aiding in visual perception (3).Ambiguous images such as the Necker cube and Rubin facevase illusion provide a well-controlled experimental approach to studying the brain's processing when it is faced with ambiguities in sensory inputs. When multiple interpretations of the same sensory inputs are possible, subjective perception alternates between the different interpretations in a stochastic manner (for reviews, see refs. 2 and 4-6). In the case of ambiguous images containing two possible interpretations, this phenomenon is referred to as "bistable perception."Neuroscientific studies of bis...

What neural mechanisms underlie the seamless flow of our waking consciousness? A necessary albeit insufficient condition for such neural mechanisms is that they should be consistently modulated across time were a segment of the conscious stream to be repeated twice. In this study, we experimentally manipulated the content of a story followed by subjects during functional magnetic resonance imaging (fMRI) independently from the modality of sensory input (as visual text or auditory speech) as well as attentional focus. We then extracted brain activity patterns consistently modulated across subjects by the evolving content of the story regardless of whether it was presented visually or auditorily. Specifically, in one experiment we presented the same story to different subjects via either auditory or visual modality. In a second experiment, we presented two different stories simultaneously, one auditorily, one visually, and manipulated the subjects' attentional focus. This experimental design allowed us to dissociate brain activities underlying modality-specific sensory processing from modality-independent story processing. We uncovered a network of brain regions consistently modulated by the evolving content of a story regardless of the sensory modality used for stimulus input, including the superior temporal sulcus/gyrus (STS/STG), the inferior frontal gyrus (IFG), the posterior cingulate cortex (PCC), the medial frontal cortex (MFC), the temporal pole (TP), and the temporoparietal junction (TPJ). Many of these regions have previously been implicated in semantic processing. Interestingly, different stories elicited similar brain activity patterns, but with subtle differences potentially attributable to varying degrees of emotional valence and self-relevance.