Ramesh Kushwaha

A direct brain interface based on event-related potentials

Cross-correlation between a trigger-averaged event-related potential (ERP) template and continuous electrocorticogram was used to detect movement-related ERPs. The accuracy of ERP detection for the five best subjects (of 17 studied), had hit percentages >90% and false positive percentages <10%. These cases were considered appropriate for operation of a direct brain interface.

Interictal spiking increases with sleep depth in temporal lobe epilepsy

Summary: Purpose: To test the hypothesis that deepening sleep activates focal interictal epileptiform discharges (IEDs), we performed EEG‐polysomnography in 21 subjects with medically refractory temporal lobe epilepsy.

Relationship of interictal epileptiform discharges to sleep depth in partial epilepsy

Non-rapid eye movement (NREM) sleep activates interictal epileptiform discharges (spikes) in many epileptic syndromes. To define this phenomenon more precisely, we studied the relationship of spikes to absolute log delta power (LDP), a continuous measure of sleep depth, in 8 patients with partial epilepsy. LDP differed significantly across visually scored sleep stages. Logistic regression analyses of spike occurrence in relation to LDP were carried out on the central-occipital channel contralateral to the dominant spike focus (C4-O2 for left and C3-O1 for right temporal focus). Within NREM sleep, spikes were more likely to occur: (1) at higher levels of LDP, (2) on the ascending limb of LDP, and (3) with more rapid rises in LDP. Spike frequency per minute was 4.6 times higher in NREM than in rapid-eye movement (REM) sleep and diminished with time from sleep onset. When the effect of LDP was controlled for in the analysis, however, there was no significant effect of REM sleep stage or time on spike occurrence. Only 1% of spikes occurred within 10 s of an arousal. These findings suggest that processes underlying the deepening of NREM sleep may contribute to spike activation in partial epilepsy.

Identification of electrocorticogram patterns as the basis for a direct brain interface.

This study reports on the first step in the development of a direct brain interface based on the identification of event-related potentials (ERPs) from an electrocorticogram obtained from the surface of the cortex. Ten epilepsy surgery patients, undergoing monitoring with subdural electrode strips and grid arrays, participated in this study. Electrocorticograms were continuously recorded while subjects performed multiple repetitions for each of several motor actions. ERP templates were identified from action-triggered electrocorticogram averages using an amplitude criterion. At least one ERP template was identified for all 10 subjects and in 56% of all electrode-recording sets resulting from a subject performing an action. These results were obtained with electrodes placed solely for clinical purposes and not for research needs. Eighty-two percent of the identified ERPs began before the trigger, indicating the presence of premovement ERP components. The regions yielding the highest probability of valid ERP identification were the sensorimotor cortex (precentral and postcentral gyri) and anterior frontal lobe, although a number were recorded from other areas as well. The recording locations for multiple ERPs arising from the performance of a specific action were usually found on close-by electrodes. ERPs associated with different actions were occasionally identified from the same recording site but often had noticeably different characteristics. The results of this study support the use of ERPs recorded from the cortical surface as a basis for a direct brain interface.

Objective Measures of Disordered Sleep in Fibromyalgia

Objective. Patients with fibromyalgia syndrome (FM) complain of inadequate sleep, which could contribute to common symptoms including sleepiness, fatigue, or pain. However, measures that consistently and objectively distinguish FM patients remain elusive. Methods. Fifteen women with FM and 15 age- and gender-matched controls underwent 3 nights of polysomnography; Multiple Sleep Latency Tests to assess sleepiness; testing of auditory arousal thresholds during non-REM stage 2 and stage 4 sleep; overnight assessment of urinary free cortisol; and analysis of 24-hour heart rate variability. Results. On the second night of polysomnography, women with FM in comparison to controls showed more stage shifts (p = 0.04) but did not differ significantly on any other standard polysomnographic measure or on the Multiple Sleep Latency Tests. Alpha EEG power during deep non-REM sleep, alone or as a proportion of alpha power during remaining sleep stages, also failed to distinguish the groups, as did auditory arousal thresholds. Urinary free cortisol did not differ between FM and control subjects in a consistent manner. However, decreased short-term heart rate variability (HRV) and especially ratio-based HRV among FM subjects suggested diminished parasympathetic and increased sympathetic activity, respectively. Other HRV measures suggested decreased complexity of HRV among the FM subjects. Conclusion. Standard measures of sleep, a gold-standard measure of sleepiness, quantified alpha-delta EEG power, auditory arousal thresholds, and urinary free cortisol largely failed to distinguish FM and control subjects. However, HRV analyses showed more promise, as they suggested both increased sympathetic activity and decreased complexity of autonomic nervous system function in FM.

P3b, consciousness, and complex unconscious processing.

How can perceptual consciousness be indexed in humans? Recent work with ERPs suggests that P3b, a relatively late component, may be a neural correlate of consciousness (NCC). This proposal dovetails with currently prevailing cognitive theory regarding the nature of conscious versus unconscious processes, which holds that the latter are simple and very brief, whereas consciousness is ostensibly required for more durable, complex cognitive processing. Using a P3b oddball paradigm, we instead show that P3b and even later, related slow wave activity occur under rigorously subliminal conditions. Additional principal component analysis (PCA) further differentiated the presence of both P3a and P3b components, demonstrating that the latter indeed occurred subliminally. Collectively, our results suggest that complex, sustained cognitive processing can occur unconsciously and that P3b is not an NCC after all.

Hippocampal sleep spindles revisited: physiologic or epileptic activity?

OBJECTIVE Few reports have described sleep spindles in intracranial electrode recordings from human hippocampus. Controversy exists regarding whether hippocampal spindles represent a physiologic or epileptic phenomenon. METHODS We reviewed hippocampal recordings in 8 subjects to characterize events resembling sleep spindles. RESULTS In 6 subjects, events occurred exclusively during non-rapid eye movement (NREM) sleep, were similar in morphology to surface spindles, occurred simultaneously or independently of surface spindles, and did not show a consistent relationship to the epileptic region. In an additional subject, a proportion of the hippocampal activity recorded differed slightly in morphology from surface spindles, was present during both NREM and rapid eye movement (REM) sleep, occurred in the same channels as isolated interictal epileptiform discharges, attenuated just prior to seizures, and occurred postictally as repetitive discharges. This activity occurred simultaneously or independently of surface spindles, but differed from surface spindles by both visual and signal analysis measures. CONCLUSIONS Most examples of hippocampal activity resembling spindles are probably physiologic, originating within the hippocampus or propagated from neighboring regions. However, in one subject, spindle activity and epileptiform discharges may have coincided, supporting experimental evidence that neurophysiological processes associated with spindle generation and NREM sleep contribute to the activation of epileptiform discharges.

Characterization of event related potentials using information theoretic distance measures

Analysis of event-related potentials (ERPs) using signal processing tools has become extremely widespread in recent years. Nonstationary signal processing tools such as wavelets and time-frequency distributions have proven to be especially effective in characterizing the transient phenomena encountered in event-related potentials. In this paper, we focus on the analysis of event-related potentials collected during a psychological experiment where two groups of subjects, spider phobics and snake phobics, are shown the same set of stimulus: A blank stimulus, a neutral stimulus and a spider stimulus. We introduce a new approach, based on time-frequency distributions, for analyzing the ERPs. The difference in brain activity before and after a stimulus is presented is quantified using distance measures as adapted to the time-frequency plane. Three different distance measures, including a new information theoretic distance measure, are applied on the time-frequency plane to discriminate between the responses of the two groups of subjects. The results illustrate the effectiveness of using distance measures combined with time-frequency distributions in differentiating between the two classes of subjects and the different regions of the brain.

An information flow technique for category related evoked potentials

A general mathematical technique is developed for information flow among various subsystems of a system when two or more classes of stimuli are presented to the system. The technique is validated by various simulation studies and then applied to a brain system. The usefulness of the technique is demonstrated for visual event related potentials (ERPs) obtained from human subjects suffering from phobias. The stimuli are briefly flashed words and phrases. The world classes are pleasant, unpleasant, conscious, and unconscious. It demonstrated that the information flow is suppressed under supraliminal presentation of the unconscious class, but is strong under subliminal presentation. The technique has the potential of being an objective indicator of conflictual relationships in these patients. The principle of the technique can be applied to any system in which interactions among subsystems are to be analyzed.<<ETX>>

An information flow technique in ERP application

A directional information flow technique is applied to investigate interactions among different parts of the brain during a conditioning paradigm using visually masked stimuli. Data of three phases, preconditioning, conditioning, and postconditioning for 17 subjects were analysed. The number of points above the mean+3 standard deviations of information flow profiles were used in a MANOVA analysis for comparing information flows in the three experimental phases. Significant differences in information flow were found between the P3 and P4 electrode pair while no such differences were found for the Oz and Cz electrode pair. Significant directional difference between P3 and P4 electrode pair does not exist.