Stimulus-evoked resetting of the dentate theta rhythm: relation to working memory

Theta and gamma oscillations during encoding predict subsequent recall

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.

Event-related potentials in the frontal cortex, hippocampus, and cerebellum during a temporal discrimination task in rats

The purpose of this study was to evaluate the roles of the frontal cortex, hippocampus, and cerebellum in timing and time perception using event-related potentials (ERPs) in rats. Rats were trained to discriminate between 2-s and 8-s auditory signal durations using a choice procedure. A simple reaction-time task using only the 2-s signal served as the control condition. In both tasks, ERPs were computed at stimulus onset and offset. Auditory ERPs were recorded from the frontal cortex, hippocampus, and cerebellum. The stimulus onset ERPs consisted of P2, N2, and P3-like components. Significantly greater amplitudes of the P3-like components were observed during the temporal-discrimination task when compared to the simple reaction-time task. At the stimulus offset, P2, N2, and P3-like components were elicited as well as the stimulus onset. Only in the frontal cortex was the P3-like component that appeared at the stimulus offset larger for the temporal-discrimination task than for the simple reaction-time task. These results suggest that the frontal cortex, hippocampus, and cerebellum contribute to interval timing in the seconds range.

Human theta oscillations related to sensorimotor integration and spatial learning

oscillations in the rat hippocampus have been implicated in sensorimotor integration (Bland, 1986), especially during exploratory and wayfinding behavior. We propose that human cortical activity coordinates sensory information with a motor plan to guide wayfinding behavior to known goal locations. To test this hypothesis, we analyzed invasive recordings from epileptic patients while they performed a spatially immersive, virtual taxi driver task. Consistent with this hypothesis, we found oscillations during both exploratory search and goal-seeking behavior and, in particular, during virtual movement, when sensory information and motor planning were both in flux, compared with periods of self-initiated stillness. oscillations had different topographic and spectral characteristics during searching than during goal-seeking, suggesting that different cortical networks exhibit depending on which cognitive functions are driving behavior (spatial learning during exploration vs orienting to a learned representation during goal-seeking). In contrast, oscillations in the beta band appeared to be related to simple motor planning, likely a variant of the Rolandic mu rhythm. These findings suggest that human cortical oscillations act to coordinate sensory and motor brain activity in various brain regions to facilitate exploratory learning and navigational planning.

Reset of human neocortical oscillations during a working memory task

Both amplitude and phase of rhythmic slow-wave electroencephalographic activity are physiological correlates of learning and memory in rodents. In humans, oscillatory amplitude has been shown to correlate with memory; however, the role of oscillatory phase in human memory is unknown. We recorded intracranial electroencephalogram from human cortical and hippocampal areas while subjects performed a short-term recognition memory task. On each trial, a series of four list items was presented followed by a memory probe. We found agreement across trials of the phase of oscillations in the 7- to 16-Hz range after randomly timed stimulus events, evidence that these events either caused a phase shift in the underlying oscillation or initiated a new oscillation. Phase locking in this frequency range was not generally associated with increased poststimulus power, suggesting that stimulus events reset the phase of ongoing oscillations. Different stimulus classes selectively modulated this phase reset effect, with topographically distinct sets of recording sites exhibiting preferential reset to either probe items or to list items. These findings implicate the reset of brain oscillations in human working memory.

Theta- and movement velocity-related firing of hippocampal neurons is disrupted by lesions centered on the perirhinal cortex

The hippocampus is critically involved in spatial memory and navigation. It has previously been proposed that, as part of this process, the hippocampus might have access to self-motion information. The possibility that some of this information may originate from the perirhinal cortex, a region involved in high-order multimodal processing, was tested in the present study by recording the responses of hippocampal complex-spike (place cells) and theta cells (putative interneurons) to movement velocity and to the movement-related theta rhythm EEG while rats with bilateral ibotenic acid lesions centered on the perirhinal cortex (n = 5), or control surgeries (n = 5), foraged in a rectangular environment. Perirhinal cortex lesions altered several characteristics of place and theta cell firing. First, the proportion of theta cells recorded was significantly lower in perirhinal lesion animals (8/39 units) compared to controls (22/53 units). Second, the firing of place cells recorded from lesion animals was phase-shifted so as to occur significantly earlier during the theta rhythm cycle than in place cells from controls (mean difference = 48.73 degrees). Third, the firing rates of a significantly lower proportion of place cells from lesion animals were modulated by the movement velocity of the animal compared to place cells from controls. These results indicate that the perirhinal cortex contributes to the responses of hippocampal CA1 place cells by providing information about self-movement and by controlling the timing of firing of these cells. This information may normally be utilized by the hippocampus during spatial memory and navigation processes.

Stimulation-induced reset of hippocampal theta in the freely performing rat

Previous research has suggested that visual and auditory stimuli in a working memory task have the ability to reset hippocampal theta, perhaps allowing an organism to encode the incoming information optimally. The present study examined two possible neural pathways involved in theta resetting. Rats were trained on a visual discrimination task in an operant chamber. At the beginning of a trial, a light appeared over a centrally located lever that the rat was required to press to receive a water reward. There was a 30-s intertrial interval before the next light stimulus appeared. After learning the task, all rats received surgical implantation of stimulating electrodes in both the fornix and the perforant path and recording electrodes, bilaterally in the hippocampus. After surgery, theta was recorded before and after the light stimulus to determine whether resetting to the visual stimulus occurred. During the intertrial interval, rats received single-pulse electrical stimulation of either the fornix or perforant path. Theta was recorded both before and after the electrical stimulation to determine whether resetting occurred. In this experiment, hippocampal theta was reset after all three stimulus conditions (light, perforant path, and fornix stimulation), with the greatest degree of reset occurring after the fornix stimulation. The results suggest that activation of the perforant path and fornix may underlie theta reset and provide a mechanism by which the hippocampus may enhance cognitive processing.

Enhancement of temporal and spatial synchronization of entorhinal gamma activity by phase reset

The synchronization of cortical gamma oscillatory activity (25-80 Hz) is thought to coordinate neuronal assemblies in the processing and storage of information. The mechanism by which independently oscillating and distantly located cortical zones become synchronized is presumed to involve activity in corticocortical connections, although evidence supporting this conjecture has only been indirect. In the present study, we show that activation of synaptic inputs within and to the medial entorhinal cortex (mEC) of the in vitro isolated guinea pig brain preparation resets the phase of ongoing gamma activity induced by muscarinic receptor agonism with carbachol (frequency: 24 +/- 2 Hz at 32 degrees C). Phase reset was associated with a transient enhancement of the synchronization of gamma activity recorded at distant (>1 mm) mEC sites, across which low coherence (>0.75) was observed before stimulation. This increase in synchronization, as measured by cross-correlation analysis, was restricted to a maximal period of 200 ms after either local mEC or CA1 afferent stimulation. The results provide direct evidence that synaptic activation can enhance the rhythmic synchronization of spatially remote, independently oscillating neuronal assemblies in the mEC through a mechanism of synaptically evoked phase reset. Dynamic functional grouping of oscillatory discharges across long distances in the mEC may underlie coding processes involved in the integration and storage of incoming information and thus may be important for the role of this region in memory processes.

A proposed function for hippocampal theta rhythm: separate phases of encoding and retrieval enhance reversal of prior learning

The theta rhythm appears in the rat hippocampal electroencephalogram during exploration and shows phase locking to stimulus acquisition. Lesions that block theta rhythm impair performance in tasks requiring reversal of prior learning, including reversal in a T-maze, where associations between one arm location and food reward need to be extinguished in favor of associations between the opposite arm location and food reward. Here, a hippocampal model shows how theta rhythm could be important for reversal in this task by providing separate functional phases during each 100-300 msec cycle, consistent with physiological data. In the model, effective encoding of new associations occurs in the phase when synaptic input from entorhinal cortex is strong and long-term potentiation (LTP) of excitatory connections arising from hippocampal region CA3 is strong, but synaptic currents arising from region CA3 input are weak (to prevent interference from prior learned associations). Retrieval of old associations occurs in the phase when entorhinal input is weak and synaptic input from region CA3 is strong, but when depotentiation occurs at synapses from CA3 (to allow extinction of prior learned associations that do not match current input). These phasic changes require that LTP at synapses arising from region CA3 should be strongest at the phase when synaptic transmission at these synapses is weakest. Consistent with these requirements, our recent data show that synaptic transmission in stratum radiatum is weakest at the positive peak of local theta, which is when previous data show that induction of LTP is strongest in this layer.

Theta oscillations in the hippocampus

Theta oscillations represent the "on-line" state of the hippocampus. The extracellular currents underlying theta waves are generated mainly by the entorhinal input, CA3 (Schaffer) collaterals, and voltage-dependent Ca(2+) currents in pyramidal cell dendrites. The rhythm is believed to be critical for temporal coding/decoding of active neuronal ensembles and the modification of synaptic weights. Nevertheless, numerous critical issues regarding both the generation of theta oscillations and their functional significance remain challenges for future research.

Theta returns

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.

Hippocampal theta oscillations and classical conditioning

Studies are reviewed that support a hypothesized role for hippocampal theta oscillations in the neural plasticity underlying behavioral learning. Begun in Richard F. Thompson's laboratory in the 1970s, these experiments have documented a relationship between free-running 3- to 7-Hz hippocampal slow waves (theta) and rates of acquisition in rabbit classical nictitating membrane (NM) conditioning. Lesion and drug manipulations of septohippocampal projections have affected NM and jaw movement conditioning in ways consistent with a theta-related brain state being an important modulator of behavioral acquisition. These findings provide essential empirical support for the recently developed neurobiological and computational models that posit an important role for rhythmic oscillations (such as theta) in cellular plasticity and behavioral learning.

Theta synchronization during episodic retrieval: neural correlates of conscious awareness

The neural correlates of conscious awareness during successful memory retrieval were examined. In a recognition test, subjects indicated whether they consciously recalled the event in which a word was earlier presented (Remembering), or whether they recognized it on the basis that it was familiar in the absence of recollection (Knowing). An early EEG synchronization in the theta band predicted knowing, and a later remembering. Moreover, early and late event-related potentials were also found to predict knowing and remembering, respectively. The results indicate that the temporal dynamics of theta synchronization are related to the particular conscious experiences associated with memory retrieval.

Distinct patterns of brain oscillations underlie two basic parameters of human maze learning

We examine how oscillations in the intracranial electroencephalogram (iEEG) relate to human maze learning. Theta- band activity (4-12 Hz in rodents; 4-8 Hz in humans) plays a significant role in memory function in rodents and in humans. Recording intracranially in humans, we have reported task-related, theta-band rhythmic activity in the raw trace during virtual maze learning and during a nonspatial working memory task. Here we analyze oscillations during virtual maze learning across a much broader range of frequencies and analyze their relationship to two task variables relevant to learning. We describe a new algorithm for detecting oscillatory episodes that takes advantage of the high signal-to-noise ratio and high temporal resolution of the iEEG. Accounting for the background power spectrum of the iEEG, the algorithm allows us to directly compare levels of oscillatory activity across frequencies within the 2- to 45-Hz band. We report that while episodes of oscillatory activity are found at various frequencies, most of the rhythmic activity during virtual maze learning occurs within the theta band. Theta oscillations are more prevalent when the task is made more difficult (manipulation of maze length). However, these oscillations do not tend to covary significantly with decision time, a good index of encoding and retrieval operations. In contrast, lower- and higher-frequency oscillations do covary with this variable. These results suggest that while human cortically recorded theta might play a role in encoding, the overall levels of theta oscillations tell us little about the immediate demands on encoding or retrieval. Finally, different patterns of oscillations may reflect distinct underlying aspects of memory function.

Gating of human theta oscillations by a working memory task

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 large-amplitude 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.

Effects of metrifonate on the hippocampal theta rhythm of freely moving intact and MS-lesioned mice

Changes in hippocampal electroencephalogram (EEG) have been suggested to be closely associated with spatial learning ability. Spatial learning can be improved in medial septal (MS)-lesioned mice by metrifonate, a cholinesterase inhibitor. We designed this study to investigate the effects of metrifonate on the hippocampal theta oscillation of intact and MS-lesioned mice. Intact and MS-lesioned C57BL mice were treated with acute injections of metrifonate (doses: 15, 50 and 100 mg/kg ip). These included a dose that considerably improved spatial memory of MS-lesioned mice in our earlier study. In addition, subtype selective muscarinic agents, BIBN-99, AF267B and AF150(S) were used. Recordings of hippocampal theta during movement and awake immobility revealed a dramatic reduction of theta in the lesioned animals. Metrifonate induced prominent changes in the EEG of intact mice, but not of MS-lesioned mice. The effect of metrifonate was not mimicked by two selective M(1)-agonists and was augmented by a combined injection of a selective M(2)-antagonist. These data suggest that improved spatial learning by the cholinesterase inhibitor metrifonate is unrelated to its effects on the hippocampal EEG. These two effects may be mediated through different muscarinic receptor subtypes.

Computational modeling of entorhinal cortex

Computational modeling provides a means for linking the physiological and anatomical characteristics of entorhinal cortex at a cellular level to the functional role of this region in behavior. We have developed detailed simulations of entorhinal cortical neurons and networks, with an emphasis on the role of acetylcholine in entorhinal cortical function. Computational modeling suggests that when acetylcholine levels are high, this sets appropriate dynamics for the storage of stimuli during performance of delayed matching tasks. In particular, acetylcholine activates a calcium-sensitive nonspecific cation current which provides an intrinsic cellular mechanism which could maintain neuronal activity across a delay period. Simulations demonstrate how this phenomena could underlie entorhinal cortex delay activity as described in previous unit recordings. Acetylcholine also induces theta rhythm oscillations which may be appropriate for timing of afferent input to be encoded in hippocampus and for extraction of individual stored sequences from multiple stored sequences. Lower levels of acetylcholine may allow sharp wave dynamics which can reactivate associations encoded in hippocampus and drive the formation of additional traces in hippocampus and entorhinal cortex during consolidation.

Theta oscillations and the ERP old/new effect: independent phenomena?

OBJECTIVES

The hypothesis is examined whether a memory-related change in induced band power (oscillatory old/new effect) is functionally related to a memory-related increase in ERP positivity (ERP old/new effect).

METHODS

In order to avoid a confounding on the measurement level, induced band power (IBP) was used as a measure that is devoid of the influence of evoked components. The EEG was recorded during a recognition memory task.

RESULTS

The results show that compared to correctly rejected words, targets (remembered words) elicit a significantly larger P300. An oscillatory old/new effect was found for the delta and theta but not for the alpha band. It is manifested by an increase in delta and theta IBP which is significantly larger for targets than for correctly rejected words. It can be observed during the same time interval and shows the same topographic distribution as the ERP old/new effect. Most importantly, however, the ERP old/new effect (as well as the P300 itself) is generated by very slow frequencies which lie below the delta band.

CONCLUSIONS

These findings demonstrate that the two types of old/new effects are functionally related. Possible physiological mechanisms underlying this relationship are discussed in terms of a threshold change in the cortex (generating the P300) that occurs during an increase in hippocampal theta activity (generating an increase in induced theta power).

Septohippocampal pathway as a site for the memory-impairing effects of ethanol

Ethanol affects behavior by interacting with synaptic sites at many levels of the nervous system. However, it targets most readily and at the lowest concentrations those sites mediating higher cognitive functions such as attention and memory. The memory-impairing effects of ethanol are thought to involve the hippocampus, a structure particularly vulnerable to the effects ethanol at low concentrations and early in the rising phase of the blood ethanol concentration curve. One of the early, low-dose effects of ethanol is an interruption of the normal physiological regulation of the hippocampus by the ascending septohippocampal pathway originating in the medial septal area (MSA). Ethanol enhances GABAergic transmission in the MSA, thereby reducing the regularity and vigor with which rhythmically bursting neurons of the MSA drive the hippocampal theta rhythm. Disruption of septohippocampal activity also has consequences on the response of the hippocampus to cortical inputs. Ethanol produces a loss of hippocampal responsivity that reduces the ability of the hippocampus to encode and retrieve relevant stimulus information necessary for accurate memory. This paper examines the behavioral and neural evidence for hippocampal vulnerability to ethanol and explores the hypothesis that these effects are due to ethanol disrupting septohippocampal modulation of the hippocampus, resulting in impairments of memory.

Theta oscillations index human hippocampal activation during a working memory task

Working memory (WM) is the ability to retain and associate information over brief time intervals. Functional imaging studies demonstrate that WM is mediated by a distributed network including frontal and posterior cortices, hippocampus, and cerebellum. In rodents, the presentation of stimuli in a WM task is followed by a reset of the phase of hippocampal theta. In this paper we report the observation of a similar phenomenon in normal human subjects. Neuromagnetic responses were recorded during presentation of a set of digits and a subsequent probe of the retained items. All stimuli were presented with a fixed temporal pattern. We observed phase reset of approximately 7 Hz theta in left hippocampus approximately 120 ms after probe stimuli, whereas reset of theta in right hippocampus was visible approximately 80 ms prior to these anticipated stimuli. The duration of stimulus-locked theta increased with memory load, with a limiting value of approximately 600 ms for 5-7 retained items. We suggest that, as in rats, stimulus-locked theta may index involvement of human hippocampal networks in the cognitive processing of sensory input. The anticipatory phase reset of theta indicates involvement of hippocampus in right hemisphere and cerebellar timing networks. Hippocampal structures are essential for orientation to perturbations in the sensory scene, a function that requires use of a context established by a constellation of stimuli. We suggest that the initiation and maintenance of stimulus-locked hippocampal theta observed here may facilitate processing of potentially salient and/or novel input with respect to a context established by the contents of WM.

The time locked theta response reflects interindividual differences in human memory performance

Recent research indicates that an increase in theta band power is related to episodic memory performance. In this study with human subjects, the evoked (time locked) and induced (not time locked) theta response is analyzed in a recognition task. The results show a strong evoked theta response during an early retrieval period of up to 400 ms. Only for good memory performers theta is strictly time locked, indicating that theta peaks appear in preferred time windows after a target is presented. This effect--which coincides with a large P3--suggests that good performance requires a strict timing of different processing stages that correspond to cycles of theta activity.

Hippocampal theta rhythm in anesthetized rats: role of AMPA glutamate receptors

The present study describes the effects of intraseptal infusions of 1 nmol AMPA and 12 nmol NBQX on both frequency and amplitude of physostigmine-induced theta rhythm in urethane-anesthetized rats. Infusion of AMPA increased the theta frequency. This effect was blocked by a prior infusion of NBQX. Infusion of NBQX decreased the theta amplitude, and this effect was not altered by AMPA. These results suggest that the septal AMPA/glutamate receptors exert subtle modulatory influences on septohippocampal cells involved in theta rhythm generation.

EEG alpha and theta oscillations reflect cognitive and memory performance: a review and analysis

Evidence is presented that EEG oscillations in the alpha and theta band reflect cognitive and memory performance in particular. Good performance is related to two types of EEG phenomena (i) a tonic increase in alpha but a decrease in theta power, and (ii) a large phasic (event-related) decrease in alpha but increase in theta, depending on the type of memory demands. Because alpha frequency shows large interindividual differences which are related to age and memory performance, this double dissociation between alpha vs. theta and tonic vs. phasic changes can be observed only if fixed frequency bands are abandoned. It is suggested to adjust the frequency windows of alpha and theta for each subject by using individual alpha frequency as an anchor point. Based on this procedure, a consistent interpretation of a variety of findings is made possible. As an example, in a similar way as brain volume does, upper alpha power increases (but theta power decreases) from early childhood to adulthood, whereas the opposite holds true for the late part of the lifespan. Alpha power is lowered and theta power enhanced in subjects with a variety of different neurological disorders. Furthermore, after sustained wakefulness and during the transition from waking to sleeping when the ability to respond to external stimuli ceases, upper alpha power decreases, whereas theta increases. Event-related changes indicate that the extent of upper alpha desynchronization is positively correlated with (semantic) long-term memory performance, whereas theta synchronization is positively correlated with the ability to encode new information. The reviewed findings are interpreted on the basis of brain oscillations. It is suggested that the encoding of new information is reflected by theta oscillations in hippocampo-cortical feedback loops, whereas search and retrieval processes in (semantic) long-term memory are reflected by upper alpha oscillations in thalamo-cortical feedback loops.