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E Halgren

Publications and source records attributed to E Halgren.

At least 55 records · Page 3Linked to original sources
PubMed1994

Role of inhibition in memory retrieval by hippocampal area CA3.

A network model with some general properties of hippocampal area CA3, and the results of its simulation on a massively parallel processor, are described. This network performs the tasks of recent declarative memory including recovery of complete traces from partial cues and recognition of familiarity. Immediate recurrent inhibition is essential for providing sensitivity to small cues while preventing spurious recall. Tonic inhibition seems to set the retrieval speed/accuracy trade-off. Delayed inhibition resets hippocampal activity. The behavior under excessive or deficient inhibition resembles that of amnesics with lesions in brainstem areas known to modulate hippocampal inhibition. The rate of recall and dynamics of inhibition by the model are similar to those inferred to occur in the human hippocampus from unit and evoked potential recordings. The model suggests a mechanism whereby the hippocampus can control its own plasticity. These simulations demonstrate that the retrieval mechanism in several hippocampal models is feasible and that the theta rhythm and the cognitive evoked potentials may be generated by synaptic events modulating network parameters.

Cognition↗
PubMed1994

Spatio-temporal stages in face and word processing. I. Depth-recorded potentials in the human occipital, temporal and parietal lobes [corrected].

Evoked potentials (EPs) were used to help identify the timing, location, and intensity of the information-processing stages applied to faces and words in humans. EP generators were localized using intracranial recordings in 33 patients with depth electrodes implanted in order to direct surgical treatment of drug-resistant epilepsy. While awaiting spontaneous seizure onset, the patients gave their fully informed consent to perform cognitive tasks. Depth recordings were obtained from 1198 sites in the occipital, temporal and parietal cortices, and in the limbic system (amygdala, hippocampal formation and posterior cingulate gyrus). Twenty-three patients received a declarative memory recognition task in which faces of previously unfamiliar young adults without verbalizable distinguishing features were exposed for 300 ms every 3 s; 25 patients received an analogous task using words. For component identification, some patients also received simple auditory (21 patients) or visual (12 patients) discrimination tasks. Eight successive EP stages preceding the behavioral response (at about 600 ms) could be distinguished by latency, and each of 14 anatomical structures was found to participate in 2-8 of these stages. The earliest response, an N75-P105, focal in the most medial and posterior of the leads implanted in the occipital lobe (lingual g), was probably generated in visual cortical areas 17 and 18. These components were not visible in response to words, presumably because words were presented foveally. A focal evoked alpha rhythm to both words and faces was also noted in the lingual g. This was followed by an N130-P180-N240 focal and polarity-inverting in the basal occipitotemporal cortex (fusiform g, probably areas 19 and 37). In most cases, the P180 was evoked only by faces, and not by words, letters or symbols. Although largest in the fusiform g this sequence of potentials (especially the N240) was also observed in the supramarginal g, posterior superior and middle temporal g, posterior cingulate g, and posterior hippocampal formation. The N130, but not later components of this complex, was observed in the anterior hippocampus and amygdala. Faces only also evoked longer-latency potentials up to 600 ms in the right fusiform g. Words only evoked a series of potentials beginning at 190 ms and extending to 600 ms in the fusiform g and near the angular g (especially left). Both words and faces evoked a N150-P200-PN260 in the lingual g, and posterior inferior and middle temporal g.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1994

Spatio-temporal stages in face and word processing. 2. Depth-recorded potentials in the human frontal and Rolandic cortices.

Evoked potentials (EPs) were recorded directly from 650 frontal and peri-Rolandic sites in 26 subjects during face and/or word recognition, as well as during control tasks (simple auditory and visual discrimination). Electrodes were implanted in order to localize epileptogenic foci resistant to medication, and thus direct their surgical removal. While awaiting spontaneous seizure onset, the patients gave informed consent to perform cognitive tasks during intracerebral EEG recording. The earliest potentials appeared to be related to sensory stimulation, were prominent in lateral prefrontal cortex, and occurred at peak latencies of about 150 and 190 ms. A small triphasic complex beginning slightly later (peak latencies about 200-285-350 ms) appeared to correspond to the scalp N2-P3a-slow wave, associated with non-specific orienting. Multiple components peaking from 280 to 900 ms, and apparently specific to words were occasionally recorded in the left inferior frontal g, pars triangularis (Broca's area). Components peaking at about 430 and 600 ms were recorded in all parts of the prefrontal cortex, but were largest (up to 180 microV) and frequently polarity-inverted in the ventro-lateral prefrontal cortex. These components appeared to represent the N4-P3b, which have been associated with contextual integration and cognitive closure. Finally, a late negativity (650-900 ms) was recorded in precentral and premotor cortices, probably corresponding to a peri-movement readiness potential. In summary, EP components related to early sensory processing were most prominent in lateral prefrontal, to orienting in medial limbic, to word-specific processing in Broca's area, to cognitive integration in ventro-lateral prefrontal, and to response organization in premotor cortices.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1994

Anatomical origin of déjà vu and vivid 'memories' in human temporal lobe epilepsy.

Jackson (Brain 1898; 21: 580-90) observed that seizures arising in the medial temporal lobe may result in a 'dreamy state', consisting of vivid memory-like hallucinations, and/or the sense of having previously lived through exactly the same situation (déjà vu). Penfield demonstrated that the dreamy state can sometimes be evoked by electrical stimulation of the lateral temporal neocortex, especially the superior temporal gyrus. Halgren et al. (Brain 1978; 101: 83-117) showed that the dreamy state can be evoked by stimulation of the hippocampal formation and amygdala and Gloor (Brain 1990; 113: 1673-94) has suggested that it is evoked by lateral stimulation only when the resulting after-discharge spreads medially. In order to resolve the relative importance of these areas, we considered the mental phenomena observed in epileptic patients with electrodes stereotaxically implanted into different brain areas for seizure localization prior to surgical treatment. Sixteen patients, all with seizures involving the temporal lobe, experienced the dreamy state either as a result of spontaneous seizures (nine dreamy states in six patients), or due to electrical stimulation (43 in 14) or to chemical activation (five in three). Déjà vu and hallucinations of scenes were often evoked by different stimulations of the same electrode in the same patient. As Jackson had also observed, the dreamy state could occur alone but was often associated with epigastric phenomena and fear, and followed by loss of contact and oro-alimentary automatisms, and then by simple gestural automatisms, all characteristic of partial seizures beginning in the medial temporal lobe. Furthermore, as also emphasized by Jackson, the dreamy state was seldom associated with sensory illusions. Stimulation of either the neocortex (15 occurrences), anterior hippocampus (17) or amygdala (10) could evoke a dreamy state. However, since fewer hippocampal and amygdala leads were stimulated than temporal neocortical, the proportion of medial temporal electrodes where dreamy states could be evoked was much higher than in the neocortex. Most responsive lateral temporal sites were located in the superior temporal gyrus, rather than the middle temporal gyrus which was significantly less responsive. In 85% of dreamy states evoked by medial temporal lobe stimulation, the discharge spread to the temporal neocortex; and in 53% of dreamy states evoked by lateral temporal stimulation, the discharge spread medially. Considering all dreamy states, the amygdala was involved (as the stimulated structure, or as the site of ictal- or after-discharge) in 73% of cases, the anterior hippocampus in 83% and the temporal neocortex in 88%.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1991

Localized brain metabolic response correlated with potentials evoked by words.

Evoked potentials (EPs) measure synaptic current flows that propagate from brain to scalp, Alternatively, positron emission tomography (PET) using fluoro-deoxyglucose (FDG) can measure the increased glucose metabolism supporting this synaptic activation. It is difficult to localize the brain activity-generating EPs from their scalp distribution, because activity originating in different regions tends to produce overlapping scalp topographies. In contrast, FDG-PET provides better spatial resolution for activity throughout the brain, but shows only the total metabolism integrated over a 30-min uptake period. We combined the temporal and psychological resolution of EPs with the spatial resolution of PET to help define when and where in the brain words are encoded for meaning.

Adult↗
PubMed1991

Firing of human hippocampal units in relation to voluntary movements.

During certain movements (termed "type I," "instrumental," or "voluntary"), the rodent hippocampal EEG is dominated by regular 7-10 Hz waves. This "theta rhythm" is accompanied by increased firing of hippocampal interneurons and dentate gyrus granule cells. No obvious theta rhythm is present in comparable situations in humans or other primates. However, a widespread scalp negativity (the "readiness potential") starts approximately 1 second before spontaneous key presses at long intervals. The readiness potential has been recorded in the monkey hippocampus. In this study, action potentials were recorded in the human hippocampus in relation to various movements. During a broadly ranging interview including various movements and memory tests, hippocampal units were found that fired during movements of the tongue and/or hands. Only movements that required a high degree of effort were effective. Other hippocampal units appeared to be correlated with either the transitions between tasks or the interruptions within tasks. In a second experiment, hippocampal units were found to change their firing in the seconds preceding spontaneous key presses. These data indicate that, like the rodent hippocampus, human hippocampal neuronal activity is strongly influenced by movement.

Action Potentials↗
PubMed1991

Memory dysfunction in epilepsy patients as a derangement of normal physiology.

Patients with CPS often display recent memory deficits. Typically, general intelligence, perceptual skills, language, remote memory, and primary memory are all normal. However, the ability to learn new combinations of cognitively complex material is deficient. This deficit may be specific for verbal material (e.g., as a difficulty with learning to recall a response word given an unrelated cue word), for nonverbal material (e.g., as a difficulty in drawing a complex figure from memory), or for both verbal and nonverbal material. Because these characteristics are typical of memory deficits after MTL damage, it is reasonable to suspect that these deficits in patients with epilepsy also reflect MTL damage. In many cases, MTL damage is apparent from neuroimaging studies, whereas seizure semiology suggests MTL onset. In these patients, the same pathology might be the cause of both the ictus and memory deficits. In other cases, memory impairment appears to be secondary to seizures. This suggestion is supported by cases where prolonged complex partial status resulted in a permanent global amnesia. Cases with shorter-lasting memory deficits were also presented. Neuropsychological testing revealed specific recent-memory deficits that cleared 2 weeks after a flurry of CPS and 24 hr after a single seizure. Depth recordings have demonstrated that MTL electrographic seizures can occur without subjective manifestations. When these are evoked by local electrical stimulation, a profound inability to learn new material may be observed during the afterdischarge. Similarly, artificially induced MTL spike-and-wave complexes interfere with the memory for simultaneously presented complex visual scenes. Recent evidence suggests that all of the above phenomena may reflect the engagement by epileptiform processes of the association-cortex (AC)-MTL circuits used in normal human memory. In recent memory tasks, cognitive evoked-potential components N4 and P3 are generated in the MTL and to a lesser degree in related AC regions. The N4/P3 are strongly modulated by familiarity in recent memory. This modulation is eliminated by anterior temporal lobectomy. The typical slow wave following spontaneous MTL interictal spikes has the same MTL voltage topography, and thus probably similar synaptic generators, as the cognitive P3 potential. Furthermore, MTL spike-and-wave complexes can be evoked in recent memory tasks at a fixed latency equal to that of the N4.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1990

The intracranial topography of the P3 event-related potential elicited during auditory oddball.

In order to isolate the anatomical locus of neural activity primarily responsible for generating the scalp-recorded P3 (or P300), the topography of event-related potentials (ERPs) elicited during an auditory oddball task was compared between medial-to-lateral aspects of the frontal, parietal, and temporal lobes in 10 epileptic patients undergoing stereoelectroencephalography for seizure localization. Evidence of local ERP generation was obtained from each of these areas. Small amplitude P3-type potentials were sometimes observed to invert polarity across recording contacts in the frontal lobe. Large amplitude positive polarity P3-type components were observed in the lateral neocortex of the inferior parietal lobule (IPL), that rapidly attenuated in amplitude at more anterior, posterior, superior, inferior, and medial recording contacts. Large amplitude polarity inverting P3-type components were also observed to be highly localized to hippocampal contacts of temporal lobe electrodes. These data are discussed in the context of other recent studies of lesion effects, scalp topography, and intracranial recordings, and it is concluded that activity generated in the IPL is likely to make the major contribution to the scalp-recorded P3, with smaller contributions from these other sources. Finally, salient topographical differences between the intracranial distribution of the P3 and those of the N2 (or N200) and slow wave (SW) suggest that the generators of these components are not identical.

Acoustic Stimulation↗
PubMed1990

Multidisciplinary analysis of patients with extratemporal complex partial seizures. II. Predictive value of semiology.

Eight patients were diagnosed as having extratemporal seizures based on their ictal electroclinical manifestations. The correlation between the focus based on semiology and that based on intracranial electrographic recording and surgical excision was excellent in five, good in three. The electroclinical manifestations of seizures are important data to consider in the diagnosis and treatment of extratemporal seizure foci.

Electroencephalography↗
PubMed1990

Neuronal activity in the human medial temporal lobe during recognition memory.

Human medial temporal lobe neuronal activity and event-related potentials were recorded during the following behaviours: contextual recognition of words and faces, semantic discrimination of nonwords from words, and discrimination of stimulus classes based on perceptual attributes. Three distinct classes of behavioural correlates of unit activity were demonstrated by visual inspection of peristimulus histograms and by nonparametric statistics: (1) neuronal excitation during a keypress related to the subject's choice; (2) specific and nonspecific excitation to words; (3) excitation or inhibition to rare stimuli in a sensory discrimination task. Responses specifically to familiar (as opposed to unfamiliar) words or faces, or to tasks requiring recent memory per se were never seen. Keypress excitation was relatively common (32/76 units) and occurred regardless of whether the keypress target was a repeated or nonrepeated word, or the task required recent or remote semantic memory. In a more complex recognition task utilizing two responses and an imperative cue for the patient's response, units with prior keypress excitation failed to generate the response. This suggests that keypress excitation is not strictly tied either to response choice or to generation. The onset latencies and temporal relationship to event-related potentials of the nonspecific and specific excitation to words and the excitation to rare stimuli suggest that they represent contextual encoding of stimuli. Similar evidence suggests that the inhibition to rare stimuli represents inhibitory processes terminating contextual encoding. Thus human medial temporal lobe neurons seem to contribute information during successive stages of a cognitive stimulus-response task: contextual encoding, closure and response-selection.

Action Potentials↗
PubMed1989

Dissociation of recognition memory components following temporal lobe lesions.

It has been proposed that recognition decisions are based on contextual retrieval of specific trace information, in addition to an assessment of item strength. The retrieval component is maximal after a single presentation, whereas the strength component increases with multiple repetition. We report that unilateral anterior temporal lobectomy (ATL) in the language dominant (left) hemisphere impairs initial recognition accuracy without affecting the rate at which repetition improves performance. The implication that the temporal lobe contributes to retrieval rather than strength during recognition is supported by simultaneous event-related potential (ERP) recordings. In normal subjects, the large ERP difference between repeated and nonrepeated words does not increase with increasing study and is associated with contextual integration in other tasks. Thus, the lack of a repetition-induced ERP difference after left-ATL reported here provides converging evidence for a critical role of the temporal lobe in contextual retrieval during recognition.

Adult↗
PubMed1988

Neural encoding of individual words and faces by the human hippocampus and amygdala.

Patients with lesions in the medial temporal lobe (MTL) of the brain, which includes the hippocampus, amygdala and parahippocampal gyrus, are severely impaired in their ability to remember and recognize words or faces which they saw only a short time ago. These lesions also prevent the effect of word repetition on cortical event-related potentials that are associated with these tasks. We have been able to study the response of individual neurons in the human medial temporal lobe to such delayed recognition tasks in epileptic patients undergoing neurosurgery. We found that some MTL neurons preferentially fired on sight of one particular word from a set of ten words used in a memory task, and others fired in response to one particular face. This stimulus-specific firing was maximal during the time that the neocortical event potentials are most sensitive to stimulus repetition, suggesting that the MTL contributes specific information to the cortex during the retrieval of recent memories.

Action Potentials↗
PubMed1988

Attenuation of a sustained visual processing negativity after lesions that include the inferotemporal cortex.

Prior studies have identified a sustained, posteriorly distributed 'processing negativity' related to visual attention and pattern recognition processes. In the present report, we document that such a negativity is elicited in a visual word classification task, and that it is disrupted following unilateral anterior temporal lobectomy for the treatment of epilepsy. For both right- and left-sided excisions the disruption is greatest on the side of surgery. These results are interpreted within theories of visual processing which attribute a role for the inferotemporal cortex (which is affected by anterior lobectomy) in pattern recognition, and they suggest that this region plays an important role in the generation of the visual processing negativity.

Adult↗
PubMed1987

Endogenous potentials evoked in simple cognitive tasks: depth components and task correlates.

Depth event-related potentials in humans were studied during a series of simple cognitive tasks. The previously reported finding of large, polarity-reversing potentials in medial temporal lobe (MTL) during tasks which evoke P3 at the scalp was confirmed. Within the latency range of the scalp N2/P3, at least two depth components were identified which are consistent across patients, and topographically distinct. They have task correlates similar to P3 in that they are larger to rare attended tones than to frequent or ignored tones and are present to rare omitted stimuli.

Cognition↗
PubMed1987

Endogenous potentials after anterior temporal lobectomy.

The scalp topography of endogenous potentials was studied in patients who had previously undergone unilateral anterior temporal lobectomy (ATL). These excisions include medial temporal lobe (MTL) structures that have been shown to generate large potentials during tasks that evoke P3 at the scalp. Following right or left ATL, patients showed no differences from unoperated control subjects in overall amplitude of P3 or any other potential measured. The topography of P3 was very similar in both ATL groups and the control subjects, with no differences in laterality. These results suggest that the MTL is not the major generator of the P3 recorded at the scalp in the tasks studied here.

Adult↗
PubMed1987

Endogenous potentials evoked in simple cognitive tasks: scalp topography.

The scalp topography of endogeneous potentials was studied during a series of simple cognitive tasks in the auditory modality. P3 and Slow Wave showed the expected task correlates of being larger to rare than to frequent tones, and larger when attended than ignored, but N2 was not clearly measurable due to overlap with P2 and P3. Overall, across all task conditions, P3 was larger at sites over the right hemisphere than the left, particularly for sites near the midline (F3, F4, C3, C4, P3, P4). P3 was present (though smaller) to rare omitted stimuli in a tone series, as well as to tones presented at long, random intervals (3 to 9 s) in a simple counting task. P3 was very large to novel, unique, nontarget auditory stimuli, and had a somewhat different distribution to these stimuli than to targets, suggesting a difference in the underlying intracranial generators. Except for these novel nontarget sounds, P3 showed a consistent topography across tasks in spite of differences in amplitude.

Adolescent↗