Holistic versus feature-based binding in the medial temporal lobe.

A central question for cognitive neuroscience is how feature-combinations that give rise to episodic/source memories are encoded in the brain. Although there is much evidence that the hippocampus (HIP) is involved in feature binding, and some evidence that other brain regions are as well, there is relatively little evidence about the nature of the resulting representations in different brain regions. We used multivoxel pattern analysis (MVPA) to investigate how feature combinations might be represented, contrasting two possibilities, feature-based versus holistic. Participants viewed stimuli that were composed of three source features - a person (face or body), a scene (indoor or outdoor), and an object (bike or luggage) - which were combined to make eight unique stimulus identities. We reasoned that regions that can classify the eight identities (a multiclass classification) but not the individual features (a binary classification) likely have a holistic representation of each identity. In contrast, regions that can classify the eight identities and can classify each feature are likely to contain feature-based representations of these identities. To further probe the extent of feature-based or holistic classification in each region, we developed and validated a novel approach that directly compares binary and multiclass classification. We found clear evidence for holistic representation in the parahippocampal cortex (PHC), consistent with theories that posit that pattern-separation-like binding mechanisms are not unique to the HIP. Further clarifying the mechanisms of feature binding should benefit from systematic comparisons of multi-feature representations and whether they vary with task, type of stimulus, and/or experience.

Aberrant Oscillatory Synchrony Is Biased Toward Specific Frequencies and Processing Domains in the Autistic Brain.

BACKGROUND: Prevailing theories suggest that autism spectrum disorder (ASD) results from impaired brain communication, causing aberrant synchrony among neuronal populations. However, it remains debated whether synchrony abnormalities are among local or long-range circuits, are circuit specific or are generalized, reflect hypersynchrony or reflect hyposynchrony, and are frequency band-specific or are distributed across the frequency spectrum. METHODS: To help clarify these unresolved questions, we recorded spontaneous magnetoencephalography data and used a data-driven, whole-brain analysis of frequency-specific interregional synchrony in higher-functioning adolescents and adults, with 17 ASD and 18 control subjects matched on age, IQ, and sex, and equal for motion. RESULTS: Individuals with ASD showed local hypersynchrony in the theta band (4-7 Hz) in the lateral occipitotemporal cortex. Long-range hyposynchrony was seen in the alpha band (10-13 Hz), which was most prominent in neural circuitry underpinning social processing. The magnitude of this alpha band hyposynchrony was correlated with social symptom severity. CONCLUSIONS: These results suggest that although ASD is associated with both decreased long-range synchrony and increased posterior local synchrony, with each effect limited to a specific frequency band, impairments in social functioning may be most related to decreased alpha band synchronization between critical nodes of the social processing network.

Reactivation during encoding supports the later discrimination of similar episodic memories.

Episodic memory is characterized by remembering events as unique combinations of features. Even when some features of events overlap, we are later often able to discriminate among them. Here we ask whether hippocampally mediated reactivation of an earlier event when a similar one occurs supports subsequent memory that two similar but not identical events occurred (mnemonic discrimination). In two experiments, participants viewed objects (Experiment 1) or scenes (Experiment 2) during functional MRI (fMRI). After scanning, participants had to remember whether repeated items had been identical or similar. In Experiment 2, representational similarity between the 1st and 2nd presentation predicted participants' ability to remember that the presentations were different, suggesting that the first item was reactivated while viewing the second. A similar but weaker result was found in Experiment 1 that did not survive correction for multiple comparisons. Furthermore, both experiments yielded evidence that the hippocampus was involved in reactivation; hippocampal pattern similarity (and, in Experiment 2, hippocampal activity during the 2nd presentation) correlated with pattern similarity in several regions of visual cortex. These results provide the first fMRI evidence that hippocampally mediated reactivation contributes to the later memory that two similar, but different events occurred. © 2016 Wiley Periodicals, Inc.

Stimulus-induced reversal of information flow through a cortical network for animacy perception.

Decades of research have demonstrated that a region of the right fusiform gyrus (FG) and right posterior superior temporal sulcus (pSTS) responds preferentially to static faces and biological motion, respectively. Despite this view, both regions activate in response to both stimulus categories and to a range of other stimuli, such as goal-directed actions, suggesting that these regions respond to characteristics of animate agents more generally. Here we propose a neural model for animacy detection composed of processing streams that are initially differentially sensitive to cues signaling animacy, but that ultimately act in concert to support reasoning about animate agents. We use dynamic causal modeling, a measure of effective connectivity, to demonstrate that the directional flow of information between the FG and pSTS is initially dependent on the characteristics of the animate agent presented, a key prediction of our proposed network for animacy detection.

Interregional neural synchrony has similar dynamics during spontaneous and stimulus-driven states.

Assessing the correspondence between spontaneous and stimulus-driven neural activity can reveal intrinsic properties of the brain. Recent studies have demonstrated that many large-scale functional networks have a similar spatial structure during spontaneous and stimulus-driven states. However, it is unknown whether the temporal dynamics of network activity are also similar across these states. Here we demonstrate that, in the human brain, interhemispheric coupling of somatosensory regions is preferentially synchronized in the high beta frequency band (~20-30 Hz) in response to somatosensory stimulation and interhemispheric coupling of auditory cortices is preferentially synchronized in the alpha frequency band (~7-12 Hz) in response to auditory stimulation. Critically, these stimulus-driven synchronization frequencies were also selective to these interregional interactions during spontaneous activity. This similarity between stimulus-driven and spontaneous states suggests that frequency-specific oscillatory dynamics are intrinsic to the interactions between the nodes of these brain networks.