A fundamental question in neuroscience concerns the relation between the spiking of individual neurons and the aggregate electrical activity of neuronal ensembles as seen in local field potentials (LFPs). Because LFPs reflect both spiking activity and subthreshold events, this question is not simply one of data aggregation. Recording from 20 neurosurgical patients, we directly examined the relation between LFPs and neuronal spiking. Examining 2030 neurons in widespread brain regions, we found that firing rates were positively correlated with broadband (2-150 Hz) shifts in the LFP power spectrum. In contrast, narrowband oscillations correlated both positively and negatively with firing rates at different recording sites. Broadband power shifts were a more reliable predictor of neuronal spiking than narrowband power shifts. These findings suggest that broadband LFP power provides valuable information concerning neuronal activity beyond that contained in narrowband oscillations.

Psychological theories of memory posit that when people recall a past event, they not only recover the features of the event itself, but also recover information associated with other events that occurred nearby in time. The events surrounding a target event, and the thoughts they evoke, may be considered to represent a context for the target event, helping to distinguish that event from similar events experienced at different times. The ability to reinstate this contextual information during memory search has been considered a hallmark of episodic, or event-based, memory. We sought to determine whether context reinstatement may be observed in electrical signals recorded from the human brain during episodic recall. Analyzing electrocorticographic recordings taken as 69 neurosurgical patients studied and recalled lists of words, we uncovered a neural signature of context reinstatement. Upon recalling a studied item, we found that the recorded patterns of brain activity were not only similar to the patterns observed when the item was studied, but were also similar to the patterns observed during study of neighboring list items, with similarity decreasing reliably with positional distance. The degree to which individual patients displayed this neural signature of context reinstatement was correlated with their tendency to recall neighboring list items successively. These effects were particularly strong in temporal lobe recordings. Our findings show that recalling a past event evokes a neural signature of the temporal context in which the event occurred, thus pointing to a neural basis for episodic memory.EEG | electrocorticography | oscillations | free recall | contiguity T he pivotal distinction between memory for facts (semantic memory) and memory for episodes or experiences (episodic memory) has been argued to reflect, at least in part, the reinstatement of a gradually changing context representation that reflects not only external conditions, but also an ever-changing internal context state (1, 2). According to this view, the unique quality of episodic memory is that in remembering an episode, we partially recover its associated mental context, and that this context information conveys some sense of when the experience took place, in terms of its relative position along our autobiographical time line.A number of laboratory memory tasks rely on episodic memory, including experimenter-cued tasks (e.g., item recognition and cued recall) and self-cued tasks (e.g., free recall). Performing these episodic memory tasks requires distinguishing the current list item from the rest of one's experience. According to early theories of episodic memory (e.g., 3, 4), context representations are composed of many features that fluctuate from moment to moment, gradually drifting through a multidimensional feature space. These contextual features may reflect environmental cues, recently studied items, participants' internal mental states, or may evolve randomly over time. During recall, the context representation forms part of th...

How we process ongoing experiences is shaped by our personal history, current needs, and future goals. Consequently, ventromedial prefrontal cortex (vmPFC) activity involved in processing these subjective appraisals appears to be highly idiosyncratic across individuals. To elucidate the role of the vmPFC in processing our ongoing experiences, we developed a computational framework and analysis pipeline to characterize the spatiotemporal dynamics of individual vmPFC responses as participants viewed a 45-minute television drama. Through a combination of functional magnetic resonance imaging, facial expression tracking, and self-reported emotional experiences across four studies, our data suggest that the vmPFC slowly transitions through a series of discretized states that broadly map onto affective experiences. Although these transitions typically occur at idiosyncratic times across people, participants exhibited a marked increase in state alignment during high affectively valenced events in the show. Our work suggests that the vmPFC ascribes affective meaning to our ongoing experiences.

Word count: 4872Abstract How we process ongoing experiences is shaped by our personal history, current needs, and future goals. Consequently, brain regions involved in generating these subjective appraisals, such as the vmPFC, often appear to be heterogeneous across individuals even in response to the same external information. To elucidate the role of the vmPFC in processing our ongoing experiences, we developed a computational framework and analysis pipeline to characterize the spatiotemporal dynamics of individual vmPFC responses as participants viewed a 45-minute television drama. Through a combination of functional magnetic resonance imaging, facial expression tracking, and self-reported emotional experiences across four studies, our data suggest that the vmPFC slowly transitions through a series of discretized states that broadly map onto affective experiences. Although these transitions typically occur at idiosyncratic times across people, participants exhibited a marked increase in state alignment during high affectively valenced events in the show. Our work suggests that the vmPFC ascribes affective meaning to our ongoing experiences.

Although it is well established that remembering an item will bring to mind memories of other semantically related items (Bousfield, 1953), the neural basis of this phenomenon is poorly understood. We studied how the similarity relations among items influence their retrieval by analyzing electrocorticographic (ECoG) recordings taken as 46 human neurosurgical patients studied and freely recalled lists of words. We first identified semantic components of neural activity that varied systematically with the meanings of each studied word, as defined by latent semantic analysis (LSA; Landauer and Dumais, 1997). We then examined the dynamics of these semantic components as participants attempted to recall the previously studied words. Our analyses revealed that the semantic components of neural activity were spontaneously reactivated during memory search, just prior to recall of the studied words. Further, the degree to which neural activity correlated with semantic similarity during recall predicted participants’ tendencies to organize the sequences of their responses on the basis of semantic similarity. Thus, our work shows that differences in the neural correlates of semantic information, and how they are reactivated prior to recall, reveal how individuals organize and retrieve memories of words.