Julien Debecker

Wave form and neural mechanism of the decision P350 elicited without pre-stimulus CNV or readiness potential in random sequences of near-threshold auditory clicks and finger stimuli

Abstract In 17 successful experiments on normal human adults random sequences of equiprobable acoustic clicks or electrical stimuli to the index finger were delivered at intervals varying at random from 1 to 15 sec. The stimuli were of low intensity and provided a feasible, but exacting task. No motor activity was involved. In alternate runs either the clicks or the finger stimuli were designated as targets to be identified and mentally counted by the subject. The accuracy of the counts was checked after each run. Scalp recorded brain potentials were stored on FM tape, edited for removal of sections with artifacts and averaged. Large P350 components were elicited by the targets, but not by identical stimuli when they were non-targets. The reciprocal experimental design provided consistent controls. No pre-stimulus negative shift of the CNV type was observed in spite of the use of long time constants (8 sec). The background EEG did not affect the results. The P350 to targets was maximal in the centro-parietal region and of equal amplitude on the left and right hemispheres. Because there was little overlap from other ERP components, the wave form of P350 could be analyzed, namely the latency to onset, the time from onset to peak and the total duration. The primary components of ERPs to finger stimuli were not modified by the cognitive task, which excludes centrifugal gating of corticipetal input as a mechanism. The ERPs to targets presented an increased N120. The latter was symmetrically distributed over the central region for target clicks, but was larger in the contralateral parietal region for target finger stimuli. The neural basis of CNV, P350 and N120 is discussed in detail. It is suggested that the mesencephalic reticular formation (MRF) exerts a diffuse bilateral facilitatory neuromodulation of cortical circuits which can be transiently adjusted (to produce CNV) by MRF control from prefrontal granular cortex. P350 is a post-decision event with bilateral distribution and results from a phasic inhibition of MRF at the closure of a cognitive processing epoch. P350 and CNV are dissociable: they are distinct events that involve roughly coextensive brain generators, but result from the operation of different prefrontal cortex controls on the MRF output to the telencephalon, in cognitive behavior. N120 indexes modality-specific focal processors engaged for stimulus identification before a decision is reached. N120 may involve the more specific thalamic reticular system that is also controlled by prefrontal cortex.

The system bandpass required to avoid distortion of early components when averaging somatosensory evoked potentials

Abstract The effects of 14 filter conditions with cutoff ranging from 3 kc/sec to 50 c/sec and with either 6 or 24 dB/octave roll off were studied when averaging the somatosensory cerebral potentials evoked by electrical stimulation of fingers in normal adult men. Severe distortions of latencies and voltages were observed for low pass filtering below 1 kc/sec. It is emphasized that the overall system bandpass should extend to 3 kc/sec when studying the early components and when estimating the onset latency of the surface negative N 1 component. Precise information about the system bandpass used should be included in published reports.

Slow potential shifts and decision P350 interactions in tasks with random sequences of near-threshold clicks and finger stimuli delivered at regular intervals

Abstract In 12 successful experiments on normal human adults, random sequences of equiprobable acoustic clicks or electrical stimuli to the index finger were delivered at regular intervals of 2.5 sec. In alternate runs either the clicks or the finger stimuli were designated as targets to be identified and mentally counted by the subject. The stimulus intensities were carefully adjusted near threshold to provide a feasible, but exacting, task. Scalp-recorded brain potentials were recorded with long time constant (8 sec) on FM tape, edited for removal of sections with artifacts, and averaged. Potentials associated with motor activities were excluded. Large P350 components were elicited by the stimuli designated as targets, but not by identical stimuli when they were non-targets. In many experiments the stimuli were preceded by a transient negative shift of up to 3 μV. The size of P350 to targets correlated with the size of this negative shift. After non-targets a slow transient positive shift could be observed, but no P350 component. The P350 and negative shift were maximal at the vertex and symmetrically distributed on the two sides. The primary components of ERPs to finger stimuli were not modified by the cognitive task. It is hypothesized that both the transient shifts and the P350 are produced by variations in the level of neuromodulation of the cortical circuits by the mesencephalic reticular formation (MRF). These different cortical events reflect corresponding variations in the MRF control by efferent pathways from the prefrontal granular cortex.

Somatosensory decision tasks in man: Early and late components of the cerebral potentials evoked by stimulation of different fingers in random sequences☆

Abstract Twenty experiments involving an intramodality somatosensory selective attention tasks were carried out in 12 normal adult motivated subjects. A random sequence of subjectively equivalent electrical stimuli was delivered to 4 fingers in the two hands. The intervals between shocks varied at random between 0.5 and 15 sec (mean 6 sec) in any one finger. The subject was instructed to mentally count the target shocks in a designated finger which was different in alternate runs. The target shocks about 1 mA above subjective threshold elicited large P400 components in the SEP, whereas non-target shocks elicited little, if any, P400. The somewhat more complex findings when strong shocks were delivered according to the same random programme have been discussed. With sequences of weaker shocks, the unitary P400 phenomenon could be studied in single trial SEPs in the best subjects. The early components N20 and P45 of the SEP were not affected by the task conditions, and this was also true for the N140 component. The large P400 to target shocks were of the same voltage at symmetrical parietal locations corresponding to the hand projections.

Automatic suppression of eye movement and muscle artifacts when averaging tape recorded cerebral evoked potentials

Abstract A method is described which allows the elimitation of eye blinks and other artifacts from the evoked potentials averaged from tape-stored EEG data. The method includes the reading at faster speed of the recorded tape in reverse (with respect to the initial record) into comparator and gate functions which write on a separate track a step function-during and before baseline shifts. The method consistently eliminates the inclusion of tape sections with spurious baseline shifts in the multichannel EEG samples subsequently averaged.

A random interval generator using beta ray emission

Abstract A solid generator of random intervals, based on a beta ray emitter, is described. The instrument provides for independent control of the minimal interval between successive output plus and of the dispersion of the random intervals. It has been used for delivering sensory stimuli at random intervals or in random sequences.

Wavelength sensitivity of the two components of the early receptor potential (ERP) of the human eye

Abstract The ERP of the human eye is studied for violet, blue, green, yellow and red flashes produced with absorption filters. For isoenergetic flashes the amplitude of R 1 and of R 2 is maximum for the green flash. The R 1 R 2 ratio however increases progressively from 0 to 13 from violet to red flashes. The dark-adaptation half-time of R 2 is 210 sec for violet flashes, suggesting a dominant rod contribution for this color, 90 sec for red flashes, indicating a dominant cone contribution, and 135 sec for green flashes, corresponding to a mixed origin. The dark-adaptation half-time of R 1 is about 100 sec, whatever the wavelength of the flash, which suggests a cone origin for this wave in the human eye.

Intensity functions of the early receptor potential and of the melanin fast photovoltage in the human eye

The early potential of the human eye is studied for a wide range of flash intensities, inducing early potentials from 0.3 to 1700 μV in amplitude. The ERP in response to white flashes begins with a cornea positive component (R1) followed by a larger negative wave (R2). On repetitive stimulation the ERP amplitude is progressively reduced. From the decrement of R2 to the three first flashes it can be estimated that the destruction of 10% of the visual pigment of the dark-adapted eye produces an ERP of about 65 πV. The relation between the flash intensity and the amplitude of the early potential is linear from 0.3 (measured by averaging) to 140 μV. For higher values its amplitude is progressively saturated. An FPV of more than 100 μV amplitude leaves after bleaching an appreciable residual potential, which can be considered as a melanin FPV.

Flash bleaching of visual pigments in man investigated by early receptor potential recording

Abstract The amplitude of the ERP produced by a given flash in the human eye provides a measure of the amount of visual pigments present in the eye. After a flash of very high intensity and short duration (1 msec) the ERP to a weaker second flash delivered 2 sec later is not reduced in amplitude below 15–24 per cent of the ERP produced by the same flash in the dark-adapted eye. This suggests a proportional reduction of the amount of visual pigments still present in the eye after the bleaching flash, which is in agreement with fundus reflectometry results of Weale for the human retina.

A convenient stimulus and situation coding system for the tape recording of event related potentials

Abstract A low-cost versatile electronic coding apparatus is described for use in tape-recorded psychophysiological experiments. The first circuit serves to encode, to record in AM mode on tape and to retrieve the occurrence in time of four independent sensory stimuli, using only one tape track. The second is used to write in FM mode on another tape track any figure from 1 to 9 characterizing as many experimental situations. This apparatus is reliable and helpful for the automatic averaging of event-related cerebral potentials in complex psychological situations.