Biomedical subjects

L K McEvoy

Publications and source records attributed to L K McEvoy.

10 recordsLinked to original sources

PubMed2001

Neurophysiological signals of working memory in normal aging.

To examine how neurophysiological signals of working memory (WM) change with normal aging, we recorded EEGs from healthy groups (n=10 each) of young (mean age=21 years), middle-aged (mean=47 years), and older (mean=69 years) adults. EEGs were recorded while subjects performed easy and difficult versions of a spatial WM task. Groups were matched for IQ (mean=123; WAIS-R) and practiced in task performance. Responses slowed with age, particularly in the more difficult task. Advanced age was associated with decreased amplitude and increased latency of the parietal P300 component of the event-related potential and an increase in the amplitude of a frontal P200 component. Spectral features of the EEG also differed between groups. Younger subjects displayed an increase in the frontal midline θ rhythm with increased task difficulty, a result not observed in older subjects. Age-related changes were also observed in the task-related alpha signal, the amplitude of which decreases as more neurons become involved in task-related processing. Young adults showed a decrease in alpha power with increased task difficulty over parietal regions but not over frontal regions. Middle-aged and older adults showed decreased alpha power with increased task difficulty over both frontal and parietal regions. This suggests that normal aging may be associated with changes in the fronto-parietal networks involved with spatial WM processes. Younger subjects appear to use a strategy that relies on parietal areas involved with spatial processing, whereas older subjects appear to use a strategy that relies more on frontal areas.

Adult↗

PubMed2000

Test-retest reliability of cognitive EEG.

OBJECTIVE: Task-related EEG is sensitive to changes in cognitive state produced by increased task difficulty and by transient impairment. If task-related EEG has high test-retest reliability, it could be used as part of a clinical test to assess changes in cognitive function. The aim of this study was to determine the reliability of the EEG recorded during the performance of a working memory (WM) task and a psychomotor vigilance task (PVT). METHODS: EEG was recorded while subjects rested quietly and while they performed the tasks. Within session (test-retest interval of approximately 1 h) and between session (test-retest interval of approximately 7 days) reliability was calculated for four EEG components: frontal midline theta at Fz, posterior theta at Pz, and slow and fast alpha at Pz. RESULTS: Task-related EEG was highly reliable within and between sessions (r0.9 for all components in WM task, and r0.8 for all components in the PVT). Resting EEG also showed high reliability, although the magnitude of the correlation was somewhat smaller than that of the task-related EEG (r0.7 for all 4 components). CONCLUSIONS: These results suggest that under appropriate conditions, task-related EEG has sufficient retest reliability for use in assessing clinical changes in cognitive status.

Adolescent↗

PubMed1999

Electroencephalographic imaging of higher brain function.

High temporal resolution is necessary to resolve the rapidly changing patterns of brain activity that underlie mental function. Electroencephalography (EEG) provides temporal resolution in the millisecond range. However, traditional EEG technology and practice provide insufficient spatial detail to identify relationships between brain electrical events and structures and functions visualized by magnetic resonance imaging or positron emission tomography. Recent advances help to overcome this problem by recording EEGs from more electrodes, by registering EEG data with anatomical images, and by correcting the distortion caused by volume conduction of EEG signals through the skull and scalp. In addition, statistical measurements of sub-second interdependences between EEG time-series recorded from different locations can help to generate hypotheses about the instantaneous functional networks that form between different cortical regions during perception, thought and action. Example applications are presented from studies of language, attention and working memory. Along with its unique ability to monitor brain function as people perform everyday activities in the real world, these advances make modern EEG an invaluable complement to other functional neuroimaging modalities.

Brain↗

PubMed1999

Neurophysiological indices of strategy development and skill acquisition.

In order to examine neurophysiological changes associated with the development of cognitive and visuomotor strategies and skills, spectral features of the EEG were measured as participants learned to perform new tasks. In one experiment eight individuals practiced working memory tasks that required development of either spatial or verbal rehearsal and updating strategies. In a second experiment six individuals practiced a video game with a difficult visuomotor tracking component. The alpha rhythm, which is attenuated by functional cortical activation, was affected by task practice. In both experiments, a lower-frequency, centrally distributed alpha component increased between practice sessions in a task-independent fashion, reflecting an overall decrease in the extent of cortical activation after practice. A second, higher-frequency, posterior component of the alpha rhythm displayed task-specific practice effects. Practice in the verbal working memory task resulted in an increase of this signal over right posterior regions, an effect not seen after practice with the spatial working memory task or with the video game. This between-task difference presumably reflects a continued involvement of the posterior region of the right hemisphere in tasks that invoke visuospatial processes. This finding thus provides neurophysiological evidence for the formation of a task-specific neurocognitive strategy. In the second experiment a third component of the alpha rhythm, localized over somatomotor cortex, was enhanced in conjunction with acquisition of tracking skill. These alpha band results suggest that cortical regions not necessary for task performance become less active as skills develop. In both experiments the frontal midline (Fm) theta rhythm also displayed increases over the course of test sessions. This signal is associated with states of focused concentration, and its enhancement might reflect the conscious control over attention associated with maintenance of a task-appropriate mental set. Overall, the results suggest that the EEG can be used to monitor practice-related changes in the patterns of cortical activity that are associated with task processing. Additionally, these results highlight the importance of ensuring that subjects have developed stable strategies for performance before drawing inferences about the functional architecture underlying specific cognitive processes.

Adult↗

PubMed1999

Deblurring.

In most instances, traditional EEG methodology provides insufficient spatial detail to identify relationships between brain electrical events and structures and functions visualized by magnetic resonance imaging or positron emission tomography. This article describes a method called Deblurring for increasing the spatial detail of the EEG and for fusing neurophysiologic and neuroanatomic data. Deblurring estimates potentials near the outer convexity of the cortex using a realistic finite element model of the structure of a subject's head determined from their magnetic resonance images. Deblurring is not a source localization technique and thus makes no assumptions about the number or type of generator sources. The validity of Deblurring has been initially tested by comparing deblurred data with potentials measured with subdural grid recordings. Results suggest that deblurred topographic maps, registered with a subject's magnetic resonance imaging and rendered in three dimensions, provide better spatial detail than has heretofore been obtained with scalp EEG recordings. Example results are presented from research studies of somatosensory stimulation, movement, language, attention and working memory. Deblurred ictal EEG data are also presented, indicating that this technique may have future clinical application as an aid to seizure localization and surgical planning.

Brain↗

PubMed1991

The timing of the processes underlying lateralization: psychophysical and evoked potential measures.

This article describes a technique to measure binaural integration time. A binaural noise with an interaural time difference of 0.8 msec was presented in three conditions: alone, with intervening noise that was identical between the two ears, or with uncorrelated intervening noise. Both behavioral responses and evoked potentials were recorded. When the stimulus was presented in a quiet background, it was accurately detected and lateralized with durations as short as 2 msec. The N1 peak of the evoked potential occurred at approximately 90 msec. When the stimulus occurred as a brief change in an ongoing correlated binaural noise, a duration of 10 msec was necessary before the sound could be accurately lateralized or an evoked potential elicited. The N1 peak occurred at approximately 120 msec. When the stimulus occurred as a change in an ongoing uncorrelated binaural noise, a duration of 60 msec was necessary for the subject to lateralize the stimulus and for an evoked potential to be elicited. The N1 peak occurred at about 130 msec. These results suggest that a period of approximately 60 msec is required to detect the correlation of an ongoing binaural noise and that a somewhat shorter period is necessary to track changes in a sound source that has already been lateralized. The simplicity of this technique makes it an attractive tool for assessing central auditory function.

Acoustic Stimulation↗

PubMed1991

Human evoked potentials and the lateralization of a sound.

If an identical noise is presented to each ear with one ear receiving the noise slightly earlier than the other, the listener perceives the sound as originating from the side of the leading ear. If the interaural time-difference reverses, the subject perceives a shift in the lateralization of the sound to the other ear. This shift in lateralization evokes a late auditory potential with a negative wave at 135 ms and positive waves at 75 and 220 ms. This evoked potential specifically indexes central auditory processing since information about the timing of the auditory stimuli must be compared between the two ears. The response increases in amplitude with increasing interaural time-difference reaching maximum values between 0.3 and 1.5 ms. The response is evoked through acoustic frequencies below 2,000 Hz. In patients with multiple sclerosis the response is often abnormally delayed or small. The response may therefore be helpful in the clinical evaluation of patients with central auditory dysfunction.

Acoustic Stimulation↗

PubMed1991

Effects of stimulus parameters on human evoked potentials to shifts in the lateralization of a noise.

Changing the interaural time difference (ITD) of a continuous binaural noise causes a shift in the perceived lateralization of the noise and evokes a late auditory evoked potential with negative peak at 130 ms and a positive peak at 220 ms. The response is mainly evoked by stimulus frequencies below 2,000 Hz and is mediated through the middle and apical regions of the cochlea. The threshold for perceiving the lateralization reversal and for eliciting a clear evoked potential is approximately 15 dB higher than the intensity required to perceive the onset of the noise. Increasing the ITD up to 1.0 ms increases the amplitude of the evoked potential and the perceived lateralization of the noise. Further increases in the ITD decrease the amplitude of the evoked potential and make the perception of the sound less 'compact'. Decreasing the intensity of the sound in one ear decreases the response to a change in ITD, but recognizable responses occur with interaural intensity differences up to 30 dB.

Acoustic Stimulation↗

PubMed1990

Human evoked potentials to shifts in the lateralization of a noise.

A continuous noise was generated by running a sequence of random numbers through a digital-analog converter and connecting the output through an amplifier and filter to an earphone. Two channels were programmed to generate identical noise stimuli with one channel delayed relative to the other. When these stimuli were presented through earphones, the subject lateralized the noise to the side receiving the leading stimulus. Changes in the relative timing of the two stimuli caused the noise to shift its lateralization. Since these shifts occurred without any detectable change in the ongoing monaural noise, any potentials they evoked were specifically related to binaural interaction. The response recorded from the vertex contained a positive-negative-positive complex with peak latencies of 75, 136 and 220 ms. This response was similar to that evoked by the onset of a monaural stimulus although it was slightly smaller and significantly later. Despite several attempts, we were unable to record any definite earlier evoked potentials.

Adolescent↗

PubMedDate not supplied

Dynamic cortical networks of verbal and spatial working memory: effects of memory load and task practice.

Working memory (WM), the ability to briefly retain and manipulate information in mind, is central to intelligent behavior. Here we take advantage of the high temporal resolution of electrophysiological measures to obtain a millisecond timescale view of the activity induced in distributed cortical networks by tasks that impose significant WM demands. We examined how these networks are affected by the type and amount of information to be remembered, and by the amount of task practice. Evoked potentials (EPs) were obtained from eight subjects performing spatial and verbal versions of a visual n-back WM task (n = 1, 2, 3) on each of three testing days. In well-trained subjects, WM tasks elicited transient responses reflecting different subcomponents of task processing, including transient (lasting 0.02-0.3 s) task-sensitive and load-sensitive EPs, as well as sustained responses (lasting 1-1.5 s), including the prestimulus Contingent Negative Variation (CNV), and post-stimulus frontal and parietal Slow Waves. The transient responses, with the exception of the P300, differed between the verbal and spatial task versions, and between trials with different response requirements. The P300 and the Slow Waves were not affected by task version but were affected by increased WM load. These results suggest that WM emerges from the formation of a dynamic cortical network linking task-specific processes with non-specific, capacity-limited, higher-order attentional processes. Practice effects on the EPs suggested that practice led to the development of a more effective cognitive strategy for dealing with lower-order aspects of task processing, but did not diminish demands made on higher order processes. Thus a simple WM task is shown to be composed of numerous elementary subsecond neural processes whose characteristics vary with type and amount of information being remembered, and amount of practice.

Adult↗