PubMed Health⌕ Search

Biomedical subjects

J L Varner

Publications and source records attributed to J L Varner.

8 recordsLinked to original sources
PubMed1992

Stimulus presentation using a programmable signal generator.

Event related potential experiments frequently require the presentation of complicated stimulus patterns to the subject. As the complexity and number of different types of stimuli presented to the subject increase there is a corresponding increase in the amount and complexity of the equipment required to generate the stimuli. Software was developed for complex stimulus presentation which uses a programmable arbitrary/function generator. The various stimulus patterns are stored in digital form in the memory of the signal generator and can be recalled by number. Prior to presentation of a particular stimulus pattern the number of that pattern is sent by the control program to the generator for recall from memory. The signal generator is set to operate in external trigger mode and at the proper time for stimulus presentation the generator is triggered by the control program. The use of the programmable signal generator allows replacement of the typical stimulus presentation equipment such as gates, signal generators, and attenuators by this the programmable generator and provides getter flexibility as to the type and complexity of stimuli which can be generated.

Electric Stimulation↗
PubMed1991

Determination of average heart rate as a sign of the orienting response.

The "O wave" is one component of minute EEG signals recorded from the scalp in response to external stimuli and is one sign of the "orienting response". The orienting response is a fundamental response in which attention is turned to fresh or important sources of information. The orienting response has many signs in the autonomic nervous system including changes in heart rate. Software was developed to track changes in heart rate as an additional sign of the orienting response. Instantaneous heart rate was determined by detecting R waves and calculating the interval between successive R waves. Segments of the instantaneous heart rate, time located to the occurrence of the stimuli, were averaged to produce an average heart rate during the time interval in which the stimuli occurred. Changes in the average heart rate where found to occur in time correspondence to the "O wave" in the EEG signal and thus provide an additional measure of the orienting response.

Attention↗
PubMed1990

Auditory and visual event-related perturbations in the 40 Hz auditory steady-state response.

The effects of salient auditory and visual 'foreground' stimuli on responses to 'background' probe stimuli were investigated. The foreground stimuli were given at long and aperiodic intervals and required a discriminative judgment. Simultaneously, evoked potentials were obtained in response to background probe auditory stimuli presented in a continuous train at about 40/sec. The 40 Hz steady-state rhythm (SSR) evoked under such conditions was extracted using digital averaging and filtering techniques and examined continuously for evidence of change in latency or amplitude during the period surrounding the foreground stimulus. Within the first 200-300 msec after the onset of an acoustic foreground stimulus the latencies of individual peaks in the rhythm were momentarily reduced by a mean of 5.5 msec. A shift in the 40 Hz rhythm was also seen following visual foreground stimuli, although the shift was about one-third that following acoustic stimuli. A latency shift of comparable magnitude was not produced by deliberate manipulation of intensity or signal-to-noise ratio of the stimuli used to evoke the rhythm. The latency shift response is discussed in terms of a transient period of sensory facilitation during orienting or alerting associated with the foreground stimuli.

Acoustic Stimulation↗
PubMed1990

Event-related perturbations in an electrophysiological measure of auditory sensitivity: effects of probability, intensity and repeated sessions.

It is often held that novel or salient stimuli are followed by a brief period of orienting or alerting during which sensory processes are facilitated. Evidence for such a period of facilitation was sought in a paradigm in which evoked responses to weak auditory probe stimuli were examined when given in the presence of salient foreground stimuli, which were varied in probability and intensity, and which were given in two replicate sessions. The background probe stimuli consisted of a continuous train of auditory pip stimuli delivered at a rate of 40 pips per second. Under such conditions of repetitive stimulation a steady-state rhythm (SSR), which is believed to reflect summated early and middle latency evoked responses, is established in the EEG at a corresponding frequency of 40 Hz. The 40 Hz SSR was extracted using a digital averaging and filtering technique and examined continuously for changes in amplitude and latency. The rhythm showed a brief episode during which the latencies of response were decreased. The reduction in latency was greatest at 186 ms after the foreground stimulus, at which time the latencies of individual peaks in the rhythm were reduced by about 3.5 ms. The magnitude of the latency reduction response was larger for intense and for rare stimuli, and showed long-term decrement during the second session. Event-related potential and heart rate responses to the foreground stimulus were also affected by probability, intensity and session, but not in the same pattern. It was hypothesized that the latency shift in the 40 Hz SSR reflects a brief period of sensitization during alerting or orienting responses.

Acoustic Stimulation↗
PubMed1990

A software package for event related potential experiments.

All too frequently, researchers find that experimental designs are dictated by procedures implemented by the available equipment rather than by the questions to be answered. Event related potential experiments are limited by equipment which restricts the type and number of stimuli which can be presented and the length, number, and type of averages which can be computed. A software package was developed for event related potential experiments which provides a high degree of flexibility in designing the stimulus presentation protocol and the averages which can be computed. The package includes a set of user friendly programs for constructing complicated stimulus presentation sequences. In order not to restrict the researcher to a set of predetermined averages computed at the time the experiment is conducted, the EEG data is digitized and stored continuously without regard to times of stimulus presentation. Averages are computed from the continuous EEG data by a separate averaging program which takes as input a table of starting times of the segments to be averaged. The software package is currently in use at Dr. Rohrbaugh's laboratory for a series of complicated event related potential experiments.

Analog-Digital Conversion↗
PubMed1989

Event-related perturbations in an electrophysiological measure of auditory function: a measure of sensitivity during orienting?

The effects of salient foreground stimuli in evoked potentials to weak background probe stimuli were examined in situations requiring passive observation or discriminative judgments of foreground tone stimuli. The background probe stimuli consisted of a continual train of weak acoustic stimuli presented at a rate of about 40 stimuli per second. Under such conditions, a 40-Hz steady-state rhythm (SSR) is established, which has been proposed to consist of the algebraic summation of individual middle-latency components evoked by stimuli in the train. The 40-Hz SSR was averaged over trials and extracted from the composite event-related potential signal using narrow-band digital filtering, for continuous examination of latency and amplitude during the course of the period immediately preceding and following the foreground stimulus. The foreground stimulus was followed by a brief period (peaking at about 200 ms) during which the latency of response to the background probe stimuli was reduced. The extent of this latency reduction was in proportion to the magnitude of the simultaneous slow-wave ERP responses and, to a lesser extent, heart rate responses. It is proposed that the results may reflect a transient period of sensitization during orienting, at a presumably early level in the auditory system, and that the method thus offers a means for determining the extent and temporal course of such effects.

Adult↗
PubMed1978

Interhemispheric synchrony in the EEGs of full-term newborns.

Cross correlations between 4 homologous pairs of ear-reference derivations were calculated for 45-sec digitized samples of typical active and quiet sleep in 19 selected normal newborns at 21--70 h after term birth. Repeat recordings were obtained in 9 subjects 24 h after the original recordings and another set of cross correlations was calculated. For comparison, similar recordings were obtained from 4 asymptomatic young adults and cross correlations were calculated for 45-sec samples of typical stage 4 and stage REM sleep and for stage W both before and after sleep. Mean cross-correlation coefficients at zero time lag were generally higher for the adult sleep data than for the newborn data, but the differences were smaller than had been anticipated. The coefficients for active sleep data in the newborns were higher than for quiet sleep data, while in the adults the coefficients were higher for stage 4 than for stage REM. Coefficients were higher for convexity derivations than for temporal derivations in both newborns and adults. A review of the small available literature suggests that recording and analysis techniques may be significant variables determining the results obtained in such studies. The cross-correlation method of measuring interhemispheric synchrony may prove useful in identifying brain disorders in the postnatal period.

Age Factors↗