Neurophysiologic correlates of implicit face memory in intracranial visual evoked potentials.

Visual evoked potentials were recorded in the amygdala, hippocampus, mid- and inferotemporal cortex, orbitofrontal cortex, and lateral frontal cortex of seven epileptic patients while they were engaged in a difficult task requiring the discrimination between repeated and nonrepeated faces. The explicit recognition of previously seen faces was at chance levels, as measured by the accuracy of push-button responses. Nevertheless, all subjects showed clear-cut differential evoked responses to repeated versus nonrepeated faces, indicating implicit encoding of the distinction between the two types of stimuli. Differential responses were more frequent in neocortical recording sites (especially in the mid- and inferotemporal leads) than in limbic recording sites such as the amygdala and hippocampus. The authors conclude that implicit encoding processes are modulated by neocortical visual association areas of the temporal lobes.

Evidence for rapid face recognition from human scalp and intracranial electrodes.

It is still generally believed that complex visual analysis is not carried out within the first 100 ms. Here we show that intra- and extracranial visual evoked potentials (VEPs) differentiate previously seen faces from novel faces as early as 50 ms after stimulus onset. EEG was recorded from scalp electrodes in 12 male healthy volunteers (group I) and intracranially from implanted depth electrodes in the temporal and frontal cortex of seven epilepsy patients (group II). Both groups were engaged in a face recognition task. All subjects showed significant differential responses which occurred very early (50-90 ms) and later (190-600 ms). In group II, the early responses were recorded more frequently in the right hemisphere, whereas the late differential VEPs were found in both hemispheres. Both types of VEPs were more frequent in the temporal neocortex, underlining its role as a major contributor to these fast recognition processes.

Technical implementation and clinical findings/results of monitoring oxygen saturation in patients referred for long-term EEG monitoring.

Recent technical developments allow the recording of a patient's oxygen saturation (SpO2) simultaneously with intensive long-term EEG monitoring (LTM). Clinically significant information from this enhanced multi-system physiological monitoring device can contribute to more accurate diagnoses in patients referred for LTM. This report covers the technical usage of combined SpO2/EEG recordings in a small group of patients. Clinically, the findings on the SpO2 monitor helped to define the diagnosis in many of these patients. In a few, the SpO2 changes were diagnostic in their own right and prompted referral to our Sleep Disorders Laboratory. From a research aspect, the details of the morphology and timing of the oxygen desaturations and EEG show several interesting relationships with respect to the dynamics of seizure semiology and respiratory physiology.

Selectively distributed processing of visual object recognition in the temporal and frontal lobes of the human brain.

Evoked potentials to visually driven cognitive tasks were recorded through depth electrodes placed bilaterally within the amygdala, hippocampus, midtemporal and inferotemporal cortex, and lateral frontal cortex of 6 epileptic patients. Task-related differential response patterns were used to identify the recording sites engaged by specific aspects of visual encoding. In this group of 6 patients, the amygdala was most frequently engaged in encoding the familiarity of faces; midtemporal and inferotemporal cortex, in encoding perceptual identity and object categorization; and lateral frontal cortex, in holding visual object information in working memory. The two aspects of encoding that most frequently engaged the hippocampal region were related to working memory and object categorization. The processing of complex visual knowledge is thus anatomically distributed but regionally specialized. These experiments also showed that identical input and output parameters can engage different areas of the brain depending on the nature of the instructional set.

New topographic mapping of temporal lobe seizures.

A unique topographic map has been developed based on EEG data of ictal events originating from the basal/mesiotemporal lobe regions. This technique involves a new mapping method of temporal lobe seizures as opposed to the interictal activity maps of most commercially available software. The map integrates data from sphenoidal electrodes as well as the standard 10-20 surface electrodes recorded with bipolar montages. A basal view is ideal for visualization of onset of temporal lobe ictal discharges recorded with chronic sphenoidal electrodes. We used the last 150 ictal events from 40 patients with basal/mediotemporal lobe epilepsy to develop this technique. Results indicate that a topographic view incorporating sphenoidal and scalp electrodes may provide a useful adjunct for interpretation of EEG recordings and a basis for comparison between and among patient groups for both ictal and interictal epileptic discharges.

Differential neural activity in the human temporal lobe evoked by faces of family members and friends.

In 6 patients, depth electrodes revealed differential evoked responses to familiar versus novel faces. These differential responses were obtained in the amygdala, hippocampus, and temporal neocortex but not in the dorsolateral frontal or cingulate cortex. The limbic and temporal structures that differentiated novel from familiar faces did not respond differentially to variations in luminance. Limbic structures and temporal cortex thus appear to participate in face recognition and in encoding the familiarity of visual experiences.

128-channel cable-telemetry EEG recording system for long-term invasive monitoring.

A 128-channel cable-telemetry EEG monitoring system has been developed for use on patients undergoing intensive neurodiagnostic monitoring with invasive intracranial electrodes. A unique head mounted preamplifier/multiplexor design allows continuous recording from any combination of intracerebral, subdural, epidural or surface electrodes. A computer is used to delay or buffer all 128 channels by 2 min or more. This computer is connected to a fileserver via a local area network and is used exclusively for data acquisition. Seizure files created by the activation of the seizure/event push button or detected by a second computer containing on-line seizure/spike detection algorithms are written directly on to the fileserver by the acquisition computer. The 128 channels of EEG recorded on the fileserver can then be remotely reviewed on a high resolution graphics terminal, printed out on a laser jet printer, archived or played out on paper, 16 channels at a time on any EEG machine. A time of day signal is also generated by the computer to permit time correlation with a continuous audio/video VCR unit which permits comparison of the EEG and clinical events. This system allows continuous EEG monitoring of extensive multichannel arrays not previously possible with conventional 16-, 32- or even 64-channel systems.

High resolution EEG output using a laser printer.

With increasing computerization, digitization and complexity of 64- to 128-channel long-term monitoring (LTM), new problems have arisen with data presentation. We describe customized software that allows for high resolution EEG output on a standard laser printer. This system can display from 1 to 128 channels of EEG on standard or legal size paper with freedom to alter both time and amplitude scales as desired. This technique is inexpensive and provides a high resolution rectilinear paper record of selected EEG events that is convenient for review and storage.