Electrophysiological and hemodynamic measures of human brain activity have been shown to distinguish between episodes of encoding items that are later recalled versus those that are not recalled (Paller and Wagner, 2002). Using intracranial recordings from 793 widespread cortical and subcortical sites in 10 epileptic patients undergoing invasive monitoring, we compared oscillatory power at frequencies ranging from 2 to 64 Hz as participants studied lists of common nouns. Significant increases in oscillatory power during encoding predicted subsequent recall, with this effect predominantly in the 4-8 Hz (theta) and 28-64 Hz (gamma) frequency bands. Sites exhibiting increased theta activity during successful encoding were clustered in right temporal and frontal cortex, whereas those exhibiting increased gamma activity appeared bilaterally at widespread cortical locations. These findings implicate theta and gamma oscillatory activity, across a widespread network of cortical regions, in the formation of new episodic memories.

Electrode grids on the cortical surface of epileptic patients provide a unique opportunity to observe brain activity with high temporal-spatial resolution and high signal-to-noise ratio during a cognitive task. Previous work showed that largeamplitude theta frequency oscillations occurred intermittently during a maze navigation task, but it was unclear whether theta related to the spatial or working memory components of the task. To determine whether theta occurs during a nonspatial task, we made recordings while subjects performed the Sternberg working memory task. Our results show event-related theta and reveal a new phenomenon, the cognitive "gating" of a brain oscillation: at many cortical sites, the amplitude of theta oscillations increased dramatically at the start of the trial, continued through all phases of the trial, including the delay period, and decreased sharply at the end. Gating could be seen in individual trials and varying the duration of the trial systematically varied the period of gating. These results suggest that theta oscillations could have an important role in organizing multi-item working memory.

Summary The difficulty of visual recognition stems from the need to achieve high selectivity while maintaining robustness to object transformations within hundreds of milliseconds. Theories of visual recognition differ in whether the neuronal circuits invoke recurrent feedback connections or not. The timing of neurophysiological responses in visual cortex plays a key role in distinguishing between bottom-up and top-down theories. Here we quantified at millisecond resolution the amount of visual information conveyed by intracranial field potentials from 912 electrodes in 11 human subjects. We could decode object category information from human visual cortex in single trials as early as 100 ms post-stimulus. Decoding performance was robust to depth rotation and scale changes. The results suggest that physiological activity in the temporal lobe can account for key properties of visual recognition. The fast decoding in single trials is compatible with feed-forward theories and provides strong constraints for computational models of human vision.

Following pial synangiosis, the majority of pediatric patients with moyamoya syndrome stop having strokes and TIAs, and they appear to experience an excellent long-term prognosis.

Epileptic seizures are traditionally characterized as the ultimate expression of monolithic, hypersynchronous neuronal activity arising from unbalanced runaway excitation. Here we report the first examination of spike train patterns in large ensembles of single neurons during seizures in persons with epilepsy. Contrary to the traditional view, neuronal spiking activity during seizure initiation and spread was highly heterogeneous, not hypersynchronous, suggesting complex interactions among different neuronal groups even at the spatial scale of small cortical patches. In contrast to earlier stages, seizure termination is a nearly homogenous phenomenon followed by an almost complete cessation of spiking across recorded neuronal ensembles. Notably, even neurons outside the region of seizure onset showed significant changes in activity minutes before the seizure. These findings suggest a revision of current thinking about seizure mechanisms and point to the possibility of seizure prevention based on spiking activity in neocortical neurons.

The neurophysiological mechanisms by which anesthetic drugs cause loss of consciousness are poorly understood. Anesthetic actions at the molecular, cellular, and systems levels have been studied in detail at steady states of deep general anesthesia. However, little is known about how anesthetics alter neural activity during the transition into unconsciousness. We recorded simultaneous multiscale neural activity from human cortex, including ensembles of single neurons, local field potentials, and intracranial electrocorticograms, during induction of general anesthesia. We analyzed local and global neuronal network changes that occurred simultaneously with loss of consciousness. We show that propofol-induced unconsciousness occurs within seconds of the abrupt onset of a slow (<1 Hz) oscillation in the local field potential. This oscillation marks a state in which cortical neurons maintain local patterns of network activity, but this activity is fragmented across both time and space. Local (<4 mm) neuronal populations maintain the millisecond-scale connectivity patterns observed in the awake state, and spike rates fluctuate and can reach baseline levels. However, neuronal spiking occurs only within a limited slow oscillation-phase window and is silent otherwise, fragmenting the time course of neural activity. Unexpectedly, we found that these slow oscillations occur asynchronously across cortex, disrupting functional connectivity between cortical areas. We conclude that the onset of slow oscillations is a neural correlate of propofol-induced loss of consciousness, marking a shift to cortical dynamics in which local neuronal networks remain intact but become functionally isolated in time and space. electrophysiology | single units | GABA | cortical networks G eneral anesthesia is a drug-induced reversible coma commonly initiated by administering a large dose of a fast-acting drug to induce unconsciousness within seconds (1). This state can be maintained as long as needed to execute surgical and many nonsurgical procedures. One of the most widely used anesthetics is propofol, an i.v. drug that enhances GABAergic inhibitory input to neurons (2-4), with effects in cortex, thalamus, brainstem, and spinal cord (5-7). Despite the understanding of propofol's molecular actions, it is not clear how these effects at molecular targets affect single neurons and larger-scale neural circuits to produce unconsciousness.The effects on macroscopic dynamics are noticeable in the EEG, which contains several stereotyped patterns during maintenance of propofol general anesthesia. These patterns include increased delta (0.5-4 Hz) power (8, 9); increased gamma (25-40 Hz) power (9); an alpha (∼10 Hz) rhythm (10-12) that is coherent across frontal cortex; and burst suppression, an alternation between bursts of high-voltage activity and periods of flat EEG lasting for several seconds (13,14). In addition, slow oscillations (<1 Hz) have been well characterized in deeply anesthetized animals and are associated with an alternation of the neuron...

Theta oscillations (electroencephalographic activity with a frequency of 4-8 Hz) have long been implicated in spatial navigation in rodents; however, the role of theta oscillators in human spatial navigation has not been explored. Here we describe subdural recordings from epileptic patients learning to navigate computer-generated mazes. Visual inspection of the raw intracranial signal revealed striking episodes of high-amplitude slow-wave oscillations at a number of areas of the cortex, including temporal cortex. Spectral analysis showed that these oscillations were in the theta band. These episodes of theta activity, which typically last several cycles, are dependent on task characteristics. Theta oscillations occur more frequently in more complex mazes; they are also more frequent during recall trials than during learning trials.

Recent physiological studies have implicated theta - a high-amplitude 4-8 Hz oscillation that is prominent in rat hippocampus during locomotion, orienting and other voluntary behaviors - in synaptic plasticity, information coding and the function of working memory. Intracranial recordings from human cortex have revealed evidence of high-amplitude theta oscillations throughout the brain, including the neocortex. Although its specific role is largely unknown, the observation of human theta has begun to reveal an intriguing connection between brain oscillations and cognitive processes.