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E Halgren

Publications and source records attributed to E Halgren.

At least 73 records · Page 4Linked to original sources
PubMed1987

Cognitive evoked potentials as modulatory processes in human memory formation and retrieval.

The complex experiential nature of human cognitive memories implies that their formation and retrieval result from neural activity that is widespread and patterned. This integration of widespread activity into coherently patterned gestalts is accompanied by event related brain potentials (ERPs). ERP component N4, reflecting associative activation, declines as words or faces become more familiar. ERP component P3, reflecting cognitive closure, generally grows larger with familiarity. Similar changes in the N4/P3 are seen to familiarity in remote semantic memory. The N4/P3 may be generated directly by the multitude of specific neuronal interactions that constitute associative activation and contextual closure. Alternatively, the N4/P3 may be generated by diffuse modulatory pathways exerting non-specific control over these same interactions. In any case, the N4 and P3 appear to be generated simultaneously in several neocortical and subcortical structures, including the medial temporal lobe (MTL), where lesions are known to produce a profound deficit in learning new cognitive material. The MTL in the dominant hemisphere is necessary for the extratemporal N4/P3 generators to alter their response to words that are familiar in context. The mechanism whereby the MTL controls cortical activation is unknown. However, some hippocampal neurons are activated by particular words, suggesting that the MTL contributes specific information to the cortex during memory retrieval. These properties of the N4/P3 bear a close similarity to modulatory processes which have been found to aid formal associative networks in creating patterns that best represent current input in light of past experience. These modulatory processes consist of an initial generalized activation that encourages partially specified widespread elements to emerge as a network, followed by inhibition that focuses the emerging pattern into a coherent gestalt.

Brain↗
PubMed1986

Human medial temporal lobe potentials evoked in memory and language tasks.

Lesion evidence indicates that the human medial temporal lobe (MTL) makes an important contribution to Recent Memory formation and retrieval very soon after a stimulus item is encountered. For verbal materials, this contribution is lateralized to the language dominant hemisphere. Evoked potentials recorded from the MTL during verbal recognition memory display two late endogenous components. Both show differences between repeated target words and non-repeated distractor words. The first component is usually negative and has an average latency of 460 msec. It is also observed in lexical decision and picture naming tasks. This component is similar in latency, morphology, and task correlates to the scalp-recorded N4 potential. This 'MTL-N4' is smaller in amplitude to words recognized as repeats and is largest in amplitude in the left MTL. The second component is usually positive and has an average latency of 620 msec. It is similar in morphology and MTL topography to the P3-like component evoked at 360 msec to rare tones in auditory discrimination tasks. This 'MTL-P3' is larger in amplitude to words recognized as repeats. Both components are of very large amplitude and invert polarity over short distances within the MTL. Hence, they appear to be locally generated in the MTL. The extent to which volume conduction of these MTL potentials contribute to scalp-recorded EPs is unclear. The MTL-N4 might be involved with memory formation and retrieval processes, and the MTL-P3 might index completion of the detection-recognition cycle.

Adult↗
PubMed1986

Interictal spike-wave complexes in the human medial temporal lobe: typical topography and comparisons with cognitive potentials.

Ten of 16 patients with complex partial epilepsy displayed an interictal spike-slow wave sequence with characteristic morphology and depth voltage topography. This 'typical slow wave' (TSW) lasted 300-600 msec, was usually largest and negative in the anterior hippocampus, and positive in the amygdala. Simultaneous recordings from ipsilateral cingulate, supplementary motor, orbitofrontal, and lateral temporal cortices, as well as from the contralateral medial temporal lobe (MTL), revealed only small, apparently volume-conducted, wave forms. Simultaneously recorded multiunit activity within the focal MTL was profoundly inhibited during the TSW. The TSW propagated to the scalp, producing a large widespread positivity. A large endogenous potential with similar latency range and task correlates as the scalp-P3 was recorded from the MTL to infrequent tones in a simple discrimination task. This 'depth-P3' had very similar polarity and relative amplitude across MTL sites, as was observed for the TSW at the same electrode contacts. However, at more superficial intracranial sites, the TSW was relatively smaller than the P3. Similarly, from MTL to surface, the P3 was found to decrement about half as much as the TSW decrements. This evidence suggests that the surface P3 is generated, but in part only, by the MTL.

Adolescent↗
PubMed1985

Recall deficits produced by afterdischarges in the human hippocampal formation and amygdala.

The human amygdala, hippocampus and parahippocampal gyrus were stimulated in 3 epileptic patients with low-level electrical current during a paired associates learning task. Delayed recall of the response-words was severely impaired if, and only if, a unilateral afterdischarge was evoked. Stimulation at the same strength produced no deficit in immediate recall, nor in mental arithmetic, even when auras and afterdischarges were evoked. These data support the association of amnesia with medial temporal lobe paroxysms during complex partial seizures.

Amygdala↗
PubMed1985

Human medial temporal-lobe stimulation disrupts both formation and retrieval of recent memories.

Disrupting medial temporal-lobe (MTL) activity for less than a second, either during the initial presentation of a complex scene or when it is presented again 1 min later, severely impairs recognition at the second presentation. MTL disruption during both presentation and recognition periods produces further impairment. These results are not consistent with views limiting MTL function solely to encoding or solely to retrieval mechanisms.

Amygdala↗
PubMed1984

Habituation of human limbic neuronal response to sensory stimulation.

Hippocampal, parahippocampal gyrus, and amygdalar neuronal responses to visual and acoustic stimuli were analyzed during trains of several hundred stimulus repetitions as part of an investigation of sensory pathways to medial temporal lobe structures in complex-partial epilepsy patients who were being monitored with depth electrodes. Ten percent of more than 500 single and multiple units tested were responsive to simple sensory stimuli. The majority of the responsive units were recorded in the posterior parahippocampal gyrus (HG) during visual stimulation. Although neurons in pes hippocampi (PH; Ammons's horn) were also responsive to photic stimuli, no visually responsive units were found in amygdala. Very few units were responsive to acoustic stimuli, and these were found only in PH and amygdala, and not in HG. Significant trends of increase or decrease in response amplitude during trains of stimuli were found in all acoustically responsive units. Significant trends of visual response amplitude increase or decrease were found in 20% of PH units, and in 44% of HG units. Mean latencies of acoustically responsive units were longer than those of visually responsive units, and latencies of PH sensory units showing decremental response were longer than nondecremental PH units. Rate of response decrement was usually linear for acoustic responses and exponential for visual responses. The response dynamics of medial temporal lobe neurons are compared with those described in the animal limbic system and are related to habituation of human sensory evoked scalp potentials.

Acoustic Stimulation↗
PubMed1983

Visual receptive fields and response properties of neurons in human temporal lobe and visual pathways.

Recordings were made from depth electrodes placed in medial temporal and occipital lobes for the localization of seizure foci in patients with medically intractable psychomotor epilepsy. Electrodes were located in the amygdala and at three rostrocaudal levels of the hippocampal formation including the posterior hippocampal gyrus, which lies medial to the portion of the geniculostriate pathway which passes through the temporal lobe. 1. Approximately 10 per cent of single units recorded from microelectrodes chronically implanted in medial temporal sites were visually responsive. Some of the units in the posterior hippocampal gyrus displayed receptive field characteristics similar to those reported in lower primates, including circular or linear shape, monocular or binocular response, variable (1 to 10 deg) receptive field size, retinotopic organization and sustained or transient response. 2. Visually responsive units were also recorded from lateral geniculate nucleus, pulvinar nucleus and occipital cortex, and their basic response form and latencies compared with the short latency visually responsive cells in medial temporal lobe. 3. The heterogeneity of receptive field characteristics in the medial temporal lobe is consistent with the existence of more than one visual input to this region, while their retinotopic relationship differentiates these receptive fields from those of units studied in the inferior temporal lobe of lower primates. 4. Properties of visual pathways in this region are discussed in relation to several types of temporal lobe function and to memory. Recordings from these neurons offer a rare opportunity for comparison of central visual response properties and receptive field characteristics of humans with those reported in animal studies.

Adult↗
PubMed1983

Local cerebral glucose metabolic response to audiovisual stimulation and deprivation: studies in human subjects with positron CT.

Positron computed tomography (CT) and 18-F fluorodeoxyglucose (FDG) were used to measure local cerebral glucose metabolism (LCMRGlc) in patients and normal subjects during auditory and visual stimulation and deprivation. Sixty-six studies were performed in 42 individuals. In normal subjects metabolic left-right symmetry was found in states of partial (eyes patched or ears plugged) sensory deprivation. Visual stimuli of increasing complexity produced symmetric increases in LCMRGlc of the primary and associative visual cortices. Patients with lesions of the visual pathway but sparing the visual cortex demonstrated abnormalities in visual cortical LCMRGlc that correlated with clinical symptoms. Auditory stimulation studies in normal subjects demonstrated metabolic evidence of hemispheric specialization. The side (left versus right) and site of maximal metabolic response correlated with the type (verbal versus nonverbal) and content (factual story, chords, tone sequences) of the stimulus as well as the strategy used by the subject to solve the listening task. There was no correlation between the site of metabolic response and side of stimulus presentation. The results of these studies demonstrate that positron CT provides new and previously unattainable information about local human brain physiology in normal and pathological states. The limitations, advantages and future prospects of using these methods to study human brain function are discussed.

Aged↗
PubMed1982

Brain generators of evoked potentials: the late (endogenous) components.

Locating the synapses which generate endogenous potential components recorded at the scalp would permit some aspects of the neural activity directly underlying cognition to be probed non-invasively. Knowing which synapses are functioning abnormally in certain neurological and psychiatric diseases may suggest rational treatment strategies aimed at local brain regions. Although it is not possible to localize the generating synapses using scalp topography, large potentials can be recorded in the hippocampal formation during the same task conditions which evoke scalp endogenous potentials. Phase-reversal of these hippocampal potentials over short distances, accompanied by changes in simultaneously recorded unit-activity, indicates that they are generated locally. Demonstration that they volume-conduct to the scalp would, however, require observation of lesion effects, and recordings from other candidate structures, as well as a realistic model for propagation.

Afferent Pathways↗
PubMed1982

Mental phenomena induced by stimulation in the limbic system.

Direct electrical stimulation of any limbic sector may evoke a visceral sensation or an emotion, usually fear or anxiety. Vivid formed dream- or memory-like hallucinations, or intense feelings of familiarity, may be evoked from the hippocampal formation and amygdala. Conversely, amnesia may result from stimulation-induced bilateral disruption of the same region. Cingulate gyrus stimulation near the supplementary motor cortex may evoke partially adaptive movement sequences, or may interfere with the performance of movements. In general, those phenomena are not due to epileptic pathology, nor to gross spread of activation. The particular response evoked is not related to the precise electrode location, but rather to the patient's psychological traits and concerns. Thus, there is no direct relationship between specific mental contents and the activation of particular limbic neurons. Limbic stimulation appears to produce deep mental alterations whose manifestation at the surface of awareness, or in specific movements, is defined by the ongoing context.

Affect↗
PubMed1981

Neuronal firing patterns during the spread of an occipital lobe seizure to the temporal lobes in man.

Neuronal recordings from occipital cortex and hippocampus during a seizure originating in the occipital lobe of a patient with an occipito-temporal tumor demonstrated that the visual aura resulted from neuronal activation of medial peristriate and possibly other occipital lobe neurons while the psychomotor automatisms followed 10-20 sec later and were caused by recruitment of hippocampal neurons bilaterally. This confirmed that the complex seizure symptoms were caused by propagation from an occipital lobe focus. The psychomotor symptoms have not recurred for the 6 months since interruption of occipito-temporal connections; however, the spared medial occipital neurons still produce visual auras.

Astrocytoma↗
PubMed1980

A circuit for safe diagnostic electrical stimulation of the human brain.

An electronic circuit for controlled electrical stimulation of the human brain has been designed to optimize safety in charge transfer from electrodes to brain and to eliminate the likelihood of unwanted currents from the neurostimulator resulting from component failure. The circuit schematics feature the following designs: (1) a highly accurate and versatile rate generator 0.5 p.p.s. to 99 in increments of 0.5 p.p.s., (2) symmetrically-biphasic lead and lag pulses of 100 musec duration, (3) photo-isolated driver amplifiers with accurate waveform, reproduction, (4) true biphasic passive-current regulators driven by an isolated battery supply voltage for switch-selectable currents of 0.25 to 5 ma or variable current regulation, (5) accurate current and voltage waveform monitors that isolate the stimulating electrodes from the monitors, and (6) capacity-coupled outputs to guarantee no net D.C. component at the end of each biphasic pulse. Relay-switching circuits are also shown that allow sequential stimulation and recording from one or several electrodes. This neurostimulator has been in use for over two years without evidence of electrolytic damage at identifiable electrode tips. The utility of simultaneous stimulation of several different electrodes at safe charge levels is described.

Amygdala↗
PubMed1979

Activity of human hippocampal formation and amygdala neurons during sleep.

Fine wire microelectrodes were implanted for diagnostic purposes in 17 patients suffering from psychomotor epilepsy. Single- and multiunit activity during waking and natural nocturnal slow wave sleep and REM sleep was recorded in the hippocampus (n = 42), hippocampal gyrus (n = 53), and amygdala (n = 32). The firing rates of hippocampal gyrus units usually decreased during slow wave sleep and then increased to levels equal to or above waking during REM. In contrast, the firing rates of hippocampal neurons generally increased during slow wave sleep and fell to very low levels during REM. The amygdala presented a more mixed pattern. Since the projection from the hippocampal gyrus to hippocampus is excitatory, their opposite patterns during sleep suggest that the tonic firing patterns of HC neurons may be mainly the result of subcortical afferents.

Afferent Pathways↗
PubMed1978

Activity of human hippocampal formation and amygdala neurons during memory testing.

Single and multiple unit recordings were made from fine wires stereotaxically implanted in the hippocampus (HC), hippocampal gyrus (HCG), and amygdala (Am) of psychomotor epileptics. During a series of memory and control tests presented on slides, 21 of 155 HCG units, 15 of 59 HC units, and 2 of 54 Am units showed what appeared to be simple phasic or tonic visual responses. Twenty-seven other units, found only in the HCG, changed firing only during slides requiring a choice ('choice units'). A given choice unit responded during choices indicated verbally or manually, and during tasks requiring recall of Recent Memory, various visual discriminations, and expressions of preference. Choice units were not affected by sensory stimulation or motor activity in contexts not requiring choice. Phasically inhibited choice units had higher firing rates and lower signal-to-noise ratios than tonically excited units. Whether an electrode recorded a choice unit was unrelated to if it recorded a response to hyperventilation, or was in an area of epileptic pathology. Recordings were also made during an interview lasting several hours and eliciting a wide range of behaviors. Five of the 131 HCG units fired in repeated extended bursts, at least 50 times background during recall of word pairs or of the patient's hospital room. The unit response did not occur during numerous control tasks possessing similar overt sensory, motor, and social concomitants, but not requiring Recent Memory.

Action Potentials↗