Changes in Evoked Potentials Depending on Attention Level in Performance of Visual Tasks

Evoked potentials (EPs) in the parietal and temporal leads were recorded in 14 young subjects in response to successively administered right- and left-side simple visual symbols, squares and circles, during passive viewing and reactions to randomly presented target stimuli. Depending on task conditions and context, the stimuli were divided into four groups: (1) passively perceived stimuli, (2) irrelevant stimuli administered on the side opposite to the target, (3) irrelevant stimuli on the side of the target, and (4) target stimuli. The EPs were averaged over the groups. With an increase in the demands of attention from the first to the fourth groups of stimuli, a linear increase in activation, estimated by the total amplitude of the N1–P3 component, was observed in the parietal leads. The P3b component was mainly responsible for the growth of the EP amplitude. In the temporal leads, the activation was substantially weaker than in the parietal leads and displayed lower between-group differences. The results support the idea that the parietal cortex in humans is of primary importance in tasks involving visual attention and stimuli selection.     

Hemispheric Asymmetry in the Auditory Facilitation Effect in Dual-Stream Rapid Serial Visual Presentation Tasks

Even though auditory stimuli do not directly convey information related to visual stimuli, they often improve visual detection and identification performance. Auditory stimuli often alter visual perception depending on the reliability of the sensory input, with visual and auditory information reciprocally compensating for ambiguity in the other sensory domain. Perceptual processing is characterized by hemispheric asymmetry. While the left hemisphere is more involved in linguistic processing, the right hemisphere dominates spatial processing. In this context, we hypothesized that an auditory facilitation effect in the right visual field for the target identification task, and a similar effect would be observed in the left visual field for the target localization task. In the present study, we conducted target identification and localization tasks using a dual-stream rapid serial visual presentation. When two targets are embedded in a rapid serial visual presentation stream, the target detection or discrimination performance for the second target is generally lower than for the first target; this deficit is well known as attentional blink. Our results indicate that auditory stimuli improved target identification performance for the second target within the stream when visual stimuli were presented in the right, but not the left visual field. In contrast, auditory stimuli improved second target localization performance when visual stimuli were presented in the left visual field. An auditory facilitation effect was observed in perceptual processing, depending on the hemispheric specialization. Our results demonstrate a dissociation between the lateral visual hemifield in which a stimulus is projected and the kind of visual judgment that may benefit from the presentation of an auditory cue.     

Visuospatial Tasks Affect Locomotor Control More than Nonspatial Tasks in Older People

Background

Previous research has shown that visuospatial processing requiring working memory is particularly important for balance control during standing and stepping, and that limited spatial encoding contributes to increased interference in postural control dual tasks. However, visuospatial involvement during locomotion has not been directly determined. This study examined the effects of a visuospatial cognitive task versus a nonspatial cognitive task on gait speed, smoothness and variability in older people, while controlling for task difficulty.

Methods

Thirty-six people aged ≥75 years performed three walking trials along a 20 m walkway under the following conditions: (i) an easy nonspatial task; (ii) a difficult nonspatial task; (iii) an easy visuospatial task; and (iv) a difficult visuospatial task. Gait parameters were computed from a tri-axial accelerometer attached to the sacrum. The cognitive task response times and percentage of correct answers during walking and seated trials were also computed.

Results

No significant differences in either cognitive task type error rates or response times were evident in the seated conditions, indicating equivalent task difficulty. In the walking trials, participants responded faster to the visuospatial tasks than the nonspatial tasks but at the cost of making significantly more cognitive task errors. Participants also walked slower, took shorter steps, had greater step time variability and less smooth pelvis accelerations when concurrently performing the visuospatial tasks compared with the nonspatial tasks and when performing the difficult compared with the easy cognitive tasks.

Conclusions

Compared with nonspatial cognitive tasks, visuospatial cognitive tasks led to a slower, more variable and less smooth gait pattern. These findings suggest that visuospatial processing might share common networks with locomotor control, further supporting the hypothesis that gait changes during dual task paradigms are not simply due to limited attentional resources but to competition for common networks for spatial information encoding.     

Cerebral Evoked Potential Correlates in Forced-paced Tasks

CEREBRAL evoked potentials can be monitored by averaging scalp records in subjects performing psychological tasks. The waveform is affected not only by the physical features of the sensory stimulus but also by cognitive and motivational parameters^1–7. These can be manipulated through verbal instructions given to the subject and several studies have shown that a slow positive component of about 300 ms peak latency (P₃₀₀) can be elicited by stimuli to which perceptual significance is attached^2,5–8. We have examined the evoked potential correlates of sensory overload in forced-paced auditory tasks and find that the cerebral “decision” potential can reveal intermittency in the perceptual channel.     

Cervicothoracic Multisegmental Transpinal Evoked Potentials in Humans

The objectives of this study were to establish the neurophysiological properties of the transpinal evoked potentials (TEPs) following transcutaneous electric stimulation of the spine (tsESS) over the cervicothoracic region, changes in the amplitude of the TEPs preceded by median nerve stimulation at group I threshold, and the effects of tsESS on the flexor carpi radialis (FCR) H-reflex in thirteen healthy human subjects while seated. Two re-usable self-adhering electrodes, connected to function as one electrode (cathode), were placed bilaterally on the clavicles. A re-usable electrode (anode) was placed on the cervicothoracic region covering from Cervical 4 – Thoracic 2 and held under constant pressure throughout the experiment. TEPs were recorded bilaterally from major arm muscles with subjects seated at stimulation frequencies of 1.0, 0.5, 0.33, 0.2, 0.125, and 0.1 Hz, and upon double tsESS pulses delivered at an inter-stimulus interval of 40 ms. TEPs from the arm muscles were also recorded following median nerve stimulation at the conditioning-test (C-T) intervals of 2, 3, 5, 8, and 10 ms. The FCR H-reflex was evoked and recorded according to conventional methods following double median nerve pulses at 40 ms, and was also conditioned by tsESS at C-T intervals that ranged from −10 to +50 ms. The arm TEPs amplitude was not decreased at low-stimulation frequencies and upon double tsESS pulses in all but one subject. Ipsilateral and contralateral arm TEPs were facilitated following ipsilateral median nerve stimulation, while the FCR H-reflex was depressed by double pulses and following tsESS at short and long C-T intervals. Non-invasive transpinal stimulation can be used as a therapeutic modality to decrease spinal reflex hyper-excitability in neurological disorders and when combined with peripheral nerve stimulation to potentiate spinal output.     

Specificity of Auditory Cognitive Evoked Potentials in Musicians

We measured characteristics of evoked potentials, EPs, developing after presentation of significant tonal acoustic stimuli in subjects systematically engaged in music training (n = 7) and those having no corresponding experience (n = 10). The peak latencies of the P3 component in the left hemisphere of musicians were significantly shorter than those in non-musicians (on average, 279.9 and 310.2 msec, respectively). Musicians demonstrated no interhemisphere differences of the latencies of components N2, P3, and N3, while a trend toward asymmetry was obvious in non-musicians (the above components were generated somewhat later in the left hemisphere). The amplitudes of EP components demonstrated no significant intergroup differences, but the amplitude of the P3 wave was higher in the left hemisphere of non-musicians than that in the right hemisphere. Possible neurophysiological correlates of the observed specificity of EPs in the examined groups are discussed.     

Modifications of Visual Evoked Potentials in Patients with Glaucoma

We compared the visual evoked EEG potentials (VEP) elicited by presentation of a reversal chess pattern in patients with glaucoma and in the control group. Amplitudes, peak latencies of the main VEP components (N75, P100, and N145), interpeak intervals, and interpeak magnitudes were measured, and a spectral analysis of the averaged VEP was performed. In patients suffering from glaucoma, the latencies of the N75 and P100 components were greater, while the interpeak intervals P100-N145 and N75-N145 were shorter, than those in the control group. Glaucoma-related changes in the VEP spectral characteristics, in particular a drop in the spectral power of oscillations corresponding to the alpha rhythm, were observed.     

Hemispheric Asymmetry in Prosody Perception by Healthy Subjects and Schizophrenic Patients

The interhemispheric interactions in perception of Russian prosody were studied in the norm and in schizophrenia as a clinical model of impaired hemispheric interactions. Monaural presentation of stimuli and binaural presentation in a free acoustical field were used. Sentences with main variants of Russian prosodic intonations were used as stimuli. The response time and the number of erroneous responses were recorded. In binaural listening without headphones, no significant difference in the percent of errors in identifying the emotional prosody was found between healthy subjects and schizophrenics. Compared with the healthy subjects, the patients made more errors in understanding the logical stress and fewer errors in understanding the syntagmatic segmentation. By response time, a significant dominance of the left ear was revealed in the healthy subjects during monaural listening to sentences with emotional prosody and complete or incomplete sentences, whereas no significant ear dominance was found in the schizophrenics. During monaural listening to sentences with logical stress, the response time was shorter when stimuli were presented to the right ear both in the healthy subjects and in the schizophrenics. The results testified that the functional brain asymmetry in schizophrenics is flattened. The flattening was less evident in the perception of a logical stress in a sentence and did not significantly affect the efficiency of identification of emotional prosody and syntagmatic segmentation of a sentence.     

Spectral Characteristics of Short-Latency Auditory Evoked Potentials in Neurological Diseases

A comparative study of the power spectra of short-latency auditory evoked potentials (SLAEPs) was made in norm and pathological states (neurinoma of the acoustic nerve, concussion of the brain, and epilepsy). Rather specific pattern of changes in the power of the medium- and high-frequency components along with the appearance of subcomponents was revealed. It is supposed that spectral analysis of SLAEP is highly informative and may be useful for neurophysiological clinical diagnostics.     

Evoked Potentials of the Brain and Errors of Perception

Study of the relationships between the parameters of a stimulus acting on the sense organs and the perception of that stimulus is one of the most urgent problems of psychology and physiology. In psychophysics it has been demonstrated that the subjective image of a stimulus is not a mirror reflection of the acting stimulus, but is the result of the interaction of a large number of both sensory and extrasensory variables [1-5, 27].     

Effects of Blood Glutamate Scavenging on Cortical Evoked Potentials

It is well known that traumatic or ischemic brain injury is followed by acute excitotoxicity caused by the presence of abnormally high glutamate (Glu) in brain fluids. It has recently been demonstrated that excess Glu can be eliminated from brain into blood following the intravenous administration of oxaloacetate (OxAc), which, by scavenging blood Glu, induces an enhanced and neuroprotective brain-to-blood Glu efflux. In this study, we subjected rats to intravenous OxAc administration (i.v., 12.5, 25, and 50 mg/kg, respectively), and studied its effects on somatosensory evoked cortical potentials (EPs). Against our expectation, the amplitudes of EPs did not decrease but increased in a dose- and time-dependent manner after OxAc administration. Similar effects were observed when blood Glu scavenging was enhanced by combining OxAc (12.5 mg/kgbw) with recombinant glutamate–oxaloacetate transaminase (GOT, 0.14 nmol/100 g rat). On the basis of these results, we suggest that the changes of amplitudes of the EPs involve not only a glutamatergic but also the weakening of a GABAergic component. We cannot rule out the possibility that OxAc penetrates into the brain and improves mitochondrial functions.     

Expression of Hemispheric Asymmetry and Psychological Type in the EEG Traveling Wave

Spontaneous EEG patterns were recorded from 16 derivations in the parieto-occipital area over 2 min for subjects in the resting state with the eyes closed. Further, computer analysis of the current pattern of EEG phase relationships between all derivations was conducted, followed by visualization in real time of the trajectory and velocity of the traveling EEG wave as a computerized animation over the contour of the head. On the basis of visual observations and objective statistical analysis, we found consistent individual characteristics of the time course and trajectory of the traveling wave, which were compared to the results of psychological testing of the subjects. Most characteristic were modulations of the electric activity (traveling) in the transverse direction (from left to right and from right to left) and along the diagonal from the left anterior to the right posterior areas. Distinct groups of subjects were found with the predominance of one or the other trajectory type. The specific direction of the diagonal traveling was significantly correlated with the level of extroversion of the subject: extroverts were characterized by traveling of electric waves from the occiput forward to the vertex along the diagonal indicated, whereas for introverts, traveling from the vertex to the occiput was typical.     

Action Potentials Evoked by Light in Traps of Dionaea muscipula Ellis

At low light intensities (less than 50 µmol m^–2s^–1) illumination evokes transient depolarization of membranepotential in mesophyll cells of the leaf-trap of Dionaea muscipulaEllis. Darkening causes hyperpolarization approximately symmetricto the response to illumination. The amplitude as well as therate of potential changes depend on light intensity. After exceedinga definite threshold (usually between 50 and 80 µmol m^–2s^–1)the depolarization plays the role of a generator potential andan all-or-none action potential (AP) is released. Switchinglight off in a depolarization phase of an AP does not changeits shape and the amplitude. When the light intensity is increasedto 80–150 µmol m^–2 s^–1 a single lightstimulus triggers two successive APs. The time interval betweenthe two APs decreases with increasing stimulus strength andreaches the minimum between 300 and 400 µmol m^–2s^–1. At higher light intensities the interval increasesagain, and finally only a single AP is triggered. It was shownthat the effect was evoked by light but not by temperature changeaccompanying illumination. An inhibitor of the photosyn-theticelectron transport chain, DCMU, blocked the generator potentialsmediating between light absorption and APs. Residual responsesto light stimuli in plants treated with DCMU had reverse polarityand strongly reduced the amplitudes. (Received September 16, 1997; Accepted January 16, 1998)     

Clinical Correlations of Motor and Somatosensory Evoked Potentials in Neuromyelitis Optica

Background

Motor and somatosensory evoked potentials (MEPs and SSEPs) are sensitive tools for detecting subclinical lesions, assessing disease severity, and determining the prognosis for outcomes of patients with inflammatory neurological diseases such as multiple sclerosis. However, their roles in neuromyelitis optica (NMO), a severe inflammatory neurological disease that predominantly involves optic nerves and spinal cord, have not yet been clarified.

Methods and Findings

Clinical symptoms and examination findings at relapses of 30 NMO patients were retrospectively reviewed. Abnormal MEPs were observed in 69.2% of patients. Patients with abnormal motor central conduction time (CCT) of the lower limbs had higher Kurtzke Expanded Disability Status Scale (EDSS) scores than those with normal responses (P = 0.027). Abnormal SSEPs were found in 69.0% of patients. Patients with abnormal lower limb sensory CCT had higher EDSS scores than those with normal responses (P = 0.019). In 28 patients followed up more than 6 months, only one of 11 patients (9.1%) with normal SSEPs of the lower limbs had new relapses within 6 months, whereas 8 of 17 patients (47.1%, P = 0.049) with abnormal SSEPs of the lower limbs had new relapses.

Conclusions

These results indicate MEPs and SSEPs of the lower limbs are good indicators for the disability status at relapses of NMO. Lower limb SSEPs may be a good tool for reflecting the frequency of relapses of NMO.     

Sensory Attenuation Assessed by Sensory Evoked Potentials in Functional Movement Disorders

Background

Functional (psychogenic) movement disorders (FMD) have features associated with voluntary movement (e.g. distractibility) but patients report movements to be out of their control. One explanation for this phenomenon is that sense of agency for movement is impaired. The phenomenon of reduction in the intensity of sensory experience when movement is self-generated and a reduction in sensory evoked potentials (SEPs) amplitude at the onset of self-paced movement (sensory attenuation) have been linked to sense of agency for movement.

Methods

We compared amplitude of SEPs from median nerve stimulation at rest and at the onset of a self-paced movement of the thumb in 17 patients with FMD and 17 healthy controls.

Results

Patients showed lack of attenuation of SEPs at the onset of movement compared to reduction in amplitude of SEPs in controls. FMD patients had significantly different ratios of movement onset to rest SEPs than did healthy controls at each electrode: 0.79 in healthy controls and 1.35 in patients at F3 (t = -4.22, p<0.001), 0.78 in healthy controls and 1.12 at patients C3 (t = -3.15, p = 0.004) and 0.77 in healthy controls and 1.05 at patients P3 (t = -2.88, p = 0.007).

Conclusions

Patients with FMD have reduced sensory attenuation as measured by SEPs at onset of self-paced movement. This finding can be plausibly linked to impairment of sense of agency for movement in these patients.     

Clinical Utility and Limitations of Intraoperative Monitoring of Visual Evoked Potentials

Objectives

During surgeries that put the visual pathway at risk of injury, continuous monitoring of the visual function is desirable. However, the intraoperative monitoring of the visual evoked potential (VEP) is not yet widely used. We evaluate here the clinical utility of intraoperative VEP monitoring.

Methods

We analyzed retrospectively 46 consecutive surgeries in 2011-2013. High luminance stimulating devices delivered flash stimuli on the closed eyelid during intravenous anesthesia. We monitored VEP features N75 and P100 and took patients'' preoperative and postoperative visual function from patient charts. Postoperative ophthalmologic workup was performed in 25 (54%) patients and preoperatively in 28 (61%) patients.

Results

VEP recordings were feasible in 62 of 85 eyes (73%) in 46 patients. All 23 eyes without VEP had impaired vision. During surgery, VEPs remained stable throughout surgery in 50 eyes. In 44 of these, visual function did not deteriorate and three patients (6 eyes) developed hemianopia. VEP decreased transiently in 10 eyes and visual function of all was preserved. VEPs were lost permanently in 2 eyes in two patients without new postoperative visual impairment.

Conclusions

Satisfactory intraoperative VEP monitoring was feasible in all patients except in those with severe visual impairment. Preservation of VEPs predicted preserved visual function. During resection of lesions in the visual cortex, VEP monitoring could not detect new major visual field defects due to injury in the posterior visual pathway. Intraoperative VEPs were sensitive enough to detect vascular damage during aneurysm clipping and mechanical manipulation of the anterior visual pathway in an early reversible stage. Intraoperative VEP monitoring influenced surgical decisions in selected patients and proved to be a useful supplement to the toolbox of intraoperative neurophysiological monitoring.