Electroencephalographic evidence of sensory gating in the occipital visual cortex.

Sensory gating refers to the suppression of the neuronal response to a repeating stimulus and is considered a protection mechanism in the brain. In this study, we assessed gating of the mid-latency components of the visual evoked potentials (N75, P100, N150) in 11 healthy individuals using a paired-flash paradigm. A significant decrease of P100 and N150 amplitudes was shown; additionally, a significant increase in the latency of N75 and P100 for the second stimulus of the pair compared with the first one was also observed. Absolute power of the stimulus 2 signal at theta frequency was significantly suppressed as compared with the stimulus 1 signal. These results indicate a gating effect in the visual modality, reflected in both time-domain and frequency-domain measures.

Cortical inhibition and excitation in abstinent cocaine-dependent patients: a transcranial magnetic stimulation study.

Prior transcranial magnetic stimulation studies showed that resting motor threshold is elevated in abstinent cocaine-dependent patients, suggesting a decrease in axonal excitability. In contrast, the increased incidence of seizures and psychosis in this group suggests increased excitability or decreased inhibition. Here, we studied long-interval intracortical facilitation and long-interval intracortical inhibition, paired-pulse transcranial magnetic stimulation measures that are more directly linked to glutamatergic cortical facilitation and GABAergic inhibition, respectively. Ten cocaine-dependent and 10 healthy controls were examined. Resting motor threshold, long-interval intracortical facilitation and long-interval intracortical inhibition were tested from the left motor cortex. The cocaine group showed an elevated resting motor threshold and an increased long-interval intracortical facilitation, whereas long-interval intracortical inhibition was normal. Although the increase in long-interval intracortical facilitation suggests exaggerated cortical glutamatergic excitability, the increase in resting motor threshold may signify a protective mechanism against seizures and psychosis.

Generators of the intracranial P50 response in auditory sensory gating.

Clarification of the cortical mechanisms underlying auditory sensory gating may advance our understanding of brain dysfunctions associated with schizophrenia. To this end, data from nine epilepsy patients who participated in an auditory paired-click paradigm during pre-surgical evaluation and had grids of electrodes covering temporal and frontal lobe were analyzed. A distributed source localization approach was applied to the intracranial P50 response and the Gating Difference Wave obtained by subtracting the response to the second stimuli from the response to the first stimuli. Source reconstruction of the P50 showed that the main generators of the response were localized in the temporal lobes. The analysis also suggested that the maximum neuronal activity contributing to the amplitude reduction in the P50 time range (phenomenon of auditory sensory gating) is localized at the frontal lobe. Present findings suggest that while the temporal lobe is the main generator of the P50 component, the frontal lobe seems to be a substantial contributor to the process of sensory gating as observed from scalp recordings.

Cocaine-dependence and cocaine-induced paranoia and mid-latency auditory evoked responses and sensory gating.

Cocaine-dependence has been shown to affect the amplitudes of the P50 mid-latency auditory evoked response (MLAER) as well as P50 sensory gating. The effects on subsequent MLAERs (N100 and P200) have not been examined. The objective of the current study was to further assess the effects of chronic cocaine use on the P50, N100, and P200 components. Thirty-four, at least three weeks abstinent, cocaine-dependent individuals and 34 age and gender matched healthy controls were examined. The amplitudes, latencies and gating measures were calculated and compared between the groups. The N100 and P200 were significantly smaller in patients as compared to control subjects. Sensory gating of the P50, the N100, and the P200 were deficient in cocaine-dependent subjects. Latencies of all measured components were prolonged in subjects who reported developing paranoia while intoxicated. Finally, a positive correlation was found between length of abstinence and evoked response amplitudes. We conclude that the effects of cocaine on sensory gating extend beyond the P50 to the N100 and the P200 components. The data also suggest that prolonged latency of the evoked potentials may be a correlate of cocaine-induced psychosis. Finally, the data suggest that some recovery of amplitude and gating occurs with abstinence.

Artifacts, habituation and p300-based brain machine interfaces.

We investigated to what degree the detection rate of the P300 in single trial event-related potentials is affected by short-term and long-term habituation effects, and we present an algorithm to eliminate eye-movement artifacts. Data from 26 subjects were collected using a visual oddball paradigm. P300 components were detected using a threshold algorithm operating on the delta band (0-4 Hz). Using data from four subjects, collected over a 7 to 12 week period, it was observed that the P300 amplitude tended to decrease within a session, and also between successive sessions. However, this decrease did not affect the detection rate. The eye-movement removal algorithm was tested on simulated and actual data, and resulted in a significant increase in detection rate.

An exploratory study of factors affecting single trial P300 detection.

A threshold detector for single-trial P300 detection has been evaluated. The detector operates on the 0-4 Hz band, isolated from the raw electroencephalogram using low-pass filtering, wavelet transforms, or the piecewise prony method (PPM). A detection rate around 70% was found, irregardless of stimulus type, interstimulus interval (ISI), probability of occurrence (Pr) of the target stimuli, intrasession and intersession effects, or filtering method. This suggests that P300-based brain-machine interfaces can use an ISI as short as 1 s and a Pr of 45%, to increase throughput.