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PubMed2006

Do the hemispheres differ in their preparation for global/local processing?

It has recently been suggested that hemispheric differences for global and local processing already occur in response to visual pre-cues that direct attention to a specific level. However, in the supporting studies cue information was confounded with the form of the cues. In order to dissolve the confound, we compared event-related brain potentials towards cues differing in form with those towards identically formed color cues. As a result, hemispheric differences were found only for the former cue type. The data thus show that the mere cue information does not produce hemispheric asymmetries associated with global/local target stimulus processing.

Adult↗
PubMedDate not supplied

Multifocal magnetoencephalogram applied to objective visual field analysis.

PURPOSE: To establish an objective visual field analysis by visual evoked magnetic fields. METHODS: Forty-eight focal areas of the visual field were stimulated by the visual evoked response imaging system (VERIS). The multifocal visual evoked magnetic fields (mfVEFs) of 11 healthy subjects were recorded. The output signals were recorded with VERIS, and the second-order kernel was calculated. The equivalent current dipoles (ECDs) were estimated, and the relative positioning of ECDs was determined by a magnetoencephalography (MEG) system. RESULTS: The mfVEFs consisted of either two- or three-peak waves. Large amplitude mfVEFs were elicited when the stimulus was confined to 6 degrees of the central visual field, but a strong response could not always be obtained between 6 degrees and 12 degrees. All ECDs were estimated to originate in the occipital striate cortex. The ECDs for the upper (lower) field stimulations were estimated to be on the lower (upper) cortex, while those for right (left) field stimulations were on the left (right) cortex. CONCLUSIONS: The locations of mfVEF peak ECDs were correlated with the stimulated visual field and generally matched the cruciform model. In combination with the multifocal technique, MEG can be used for objective visual field analysis.

Adult↗
PubMed1991

Saccade onset and offset lambda waves: relation to pattern movement visually evoked potentials.

The lambda (lambda) wave is an occipital EEG potential which occurs when saccadic eye movements are made against an illuminated contrast background. There is some disagreement concerning the presence of sub-components to the lambda-wave, and its relationship to visually evoked potentials. In the present study, lambda-waves were recorded with saccades of different durations (30-110 ms) and compared to VEPs associated with pattern movements of similar durations and velocity. It was found that the lambda-wave consisted of a saccade onset component with positive sub-components at 59 and 100 ms after saccade onset, and a saccade offset component with a positive potential at 74 ms after saccade offset. With small saccades of 30 ms duration or less, these components superimposed to form a single lambda-wave. In the case of pattern movement VEPs, a movement onset component of latency 110 ms following movement onset, and a movement offset component at 89 ms after movement offset, were identified. The similar behaviour of the lambda-wave and VEP under these conditions supports the view that the lambda-wave is a visually evoked potential resulting from movement of the visual field across the retina during a saccadic eye movement.

Adult↗
PubMed1993

Suppression of visual responses of neurons in inferior temporal cortex of the awake macaque by addition of a second stimulus.

The responses of neurons, in inferior temporal cortex of the awake macaque, to single stimuli and pairs of stimuli were examined. The responses of most neurons were weaker to pairs of stimuli than to the best single stimulus of that pair presented alone. This 'suppression by a second stimulus' did not appear to be stimulus-selective and the suppression was greater when the second stimulus appeared in receptive field locations that exhibited weaker responses. This phenomenon suggests competitive interactions between IT neurons that may be involved in visual attention or learning or both.

Animals↗
PubMed1982

The early wave of the visual evoked potential to sinusoidal gratings: responses to quadrant stimulation as a function of spatial frequency.

VEPs were elicited by sinusoidal grating patterns of differing spatial frequencies presented in each of the 4 quadrants of the visual field. Analysis of the early wave of the VEP indicated a pattern of polarity inversions when wave forms were compared across the horizontal meridian of the visual field. With the exception of low spatial frequencies, clear polarity inversions also occurred across the vertical meridian of the visual field in the case of laterally placed electrodes. Comparison of the clearly identifiable early peak with records for the same subject using central field stimulation showed the complexity of the latter records and the inherent difficulty of isolating and interpreting specific component waves of the VEP without adequate topographical data from quadrant stimulation.

Brain Mapping↗
PubMed1982

Pattern-reversal visual evoked potentials and retinal eccentricity.

The effect of stimulation of discrete areas of the retina on visual evoked potentials (VEP) was studied in 16 normal volunteers. The stimulus consisted of a constant luminance 2 degrees 18' field containing checks of 34'30' reversing at a frequency of 500 msec. The amplitude of the VEP was highest at the fixation point and inside the 2 degrees isopter. Rapid amplitude decrement was noted with stimuli located within the 2-4 degrees isopters. No identifiable response was obtained outside the 4 degrees or 6 degrees isopter with a 2 degrees 18' stimulus. VEP could, however, be elicited by increasing the size of the stimulus. The smallest size of a field required to evoke a detectable response also varied in relations to retinal eccentricity. Stimulation at 0 degree, 8 degrees and 14 degrees horizontal eccentricities with fields of a size estimated to activate an equivalent amount visual cortex produced VEPs of similar amplitude.

Electroencephalography↗
PubMed1983

Transient visually evoked potentials to the pattern reversal and onset of sinusoidal gratings.

Transient visually evoked potentials (VEPs) recorded in response to the contrast reversal and onset of spatially sinusoidal gratings have been investigated. Previous reports of an increase in the latency of the response at higher spatial frequencies have been confirmed but only for spatial frequencies higher than 2 c/deg. At lower spatial frequencies it is suggested that two positive components interact resulting in a departure from a monotonic relationship between latency and spatial frequency. Below 1 c/deg pattern reversal and pattern onset modes of stimulation produced VEPs of similar amplitude and wave form. Above 1 c/deg the amplitude of the pattern onset response peaked at a higher spatial frequency than the response to pattern reversal, and the response was dominated by a negative (N1) rather than a positive component (P1). This distinction has been corroborated by investigating the effect of field size variation. The peak amplitudes shifted to a lower spatial frequency with increase in field size but at all field sizes the N1 component of the pattern onset response peaked at a higher spatial frequency than the other components measured. It is attempted to relate these findings to previous studies of both grating and checkerboard VEPs and to psychophysical studies of contrast sensitivity.

Electroencephalography↗
PubMed1983

The recovery cycle of the pattern visual evoked potential in normal subjects and patients with multiple sclerosis.

The recovery cycle of amplitude and latency of the P100 of the chequerboard pattern reversal visual evoked response was studied in 16 normal subjects and 20 patients with multiple sclerosis. There was no clear-cut pattern of recovery with respect to amplitude but at interstimulus intervals of less than 40 msec the latency of the test response P100 tended to be significantly delayed, the magnitude of this delay being virtually constant at intervals of 20 msec or less. Only 8 of 40 eyes from the 20 multiple sclerosis patients showed a pattern of recovery of latency significantly different from the normal subjects. Possible mechanisms for these observations are discussed with particular reference to current concepts of the pathophysiology of multiple sclerosis.

Electroencephalography↗
PubMed1984

Scalp potential fields evoked by grating stimuli: effects of spatial frequency and orientation.

Visually evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 subjects. Grating stimuli of different spatial frequencies were presented on a circular test field, and potential fields evoked by vertical, horizontal and oblique stimuli were compared. Component latencies were determined by computing a reference-independent measure of field power. Scalp field maps were constructed at component latencies and compared over all conditions. Significant influences of both spatial frequency and orientation on component latencies were found. Component latencies were shortest for 2.3 c/deg gratings, and for vertically oriented stimuli. Latencies increased for horizontal, and further for oblique, gratings. The effect of spatial frequency or orientation on the location of the major components on the scalp was less pronounced. On the other hand, there were significant differences both in component latency and scalp distribution when upper and lower hemiretinal stimuli were compared. Upper hemiretinal stimulation yielded shorter latencies and a more anteriorly located potential peak on the scalp.

Adult↗
PubMed1984

Hemispheric asymmetry of visual evoked potentials in patients with well-defined occipital lesions.

Visual evoked potentials were examined in 3 patients who had well defined unilateral occipital lobectomy. In an attempt to resolve discrepancies in the interpretation of hemispheric asymmetries in evoked potentials, recordings were obtained from a wide array of electrodes with multiple montages during full-field and hemi-field stimulation with checkerboard pattern reversal and during full-field flash stimulation. The discrepancies appear to be due primarily to differences in electrode montages used in recording. The use of a midfrontal reference electrode reliably produces a paradoxical EP lateralization: abnormal EPs are recorded over the intact occipital lobe and normal EPs are recorded over the ablated occipital lobe. These findings are fully consistent with the interpretation of Barrett et al. (1976) that the generator of the P100 has an equivalent dipole located in the calcarine cortex on the posteromedial surface of the contralateral hemisphere. Our findings support the use of the Queen Square montage (Halliday et al. 1977). The use of electrodes at O1 and O2 referred to the ipsilateral earlobe yields inconsistent lateralization and should be discouraged. The lateralization of EP abnormalities did not differ for flash and pattern reversal, although the asymmetries were sometimes more prominent with pattern reversal.

Brain↗
PubMed1985

Neurophysiological signs of brain damage due to glue sniffing.

Recordings of the brain electrical responses to pattern reversal (checkerboard) visual stimuli showed significantly increased latencies in a group of 12 young people who had been sniffing glue over a prolonged period. Only small improvements were observed in two people who had repeat recordings after 6 months' abstinence. These results show that chronic glue sniffing can lead to long lasting brain damage.

Adhesives↗
PubMed1982

The effect of change in a stimulus sequence on P300.

In past studies, the P300 amplitude of the ERP has been found to be enhanced for infrequent, unpredictable and/or task-relevant eliciting events. A fourth factor, namely the physical change between two consecutive stimuli in event sequences used, has often been confounded with frequency, predictability and relevance. The current study examined whether change also increases the P300 amplitude. Fourteen adults viewed sequences of slides in which predictability, task (counting) and change varied within subject. ERP was recorded at Cz, Pz, Oz and Fz. P300 amplitudes were significantly larger: (1) for unpredictable than for predictable events, (2) for counted than for non-counted stimuli, and (3) for stimulus change rather than no stimulus change. The change effect interacting with counting suggested that P300 amplitude may be particularly increased by stimulus changes imbedded within only certain cognitive tasks.

Adult↗
PubMed1993

Electrophysiological correlates of hierarchical stimulus processing: dissociation between onset and later stages of global and local target processing.

In the discussion of whether the processing of hierarchically structured stimuli proceeds from the more global to the more local level or vice versa, it is frequently assumed that the relative speed of global/local target identification (response time (RT) advantage) and the direction of interference from local/global distractors reflect the order of processing. Studies both in brain-injured patients and in normals, however, have demonstrated that RT advantage and interference are dissociable, leading to the conclusion that they do not provide a valid index of the order of global/local processing. The aim of the present event-related brain potential (ERP) study was to assess electrophysiological correlates of global/local processing and to determine how the relative speed of responding to global and local targets is related to these ERP measurements. In a divided-attention paradigm, subjects were asked to respond to hierarchically structured letter stimuli that contained a target letter either at the global or at the local level. The behavioral results confirmed a dissociation between RT advantage and interference. ERP analysis revealed an early posterior negative component (denoted as N250) as a sign of early global/local target perception. It was found that RT advantage is not a valid measure of the onset nor of the time course of this component. Furthermore, the N250 components to global and local targets exhibited a different time course and a different topographical distribution, suggesting that they are determined by separate processing structures. Together, the behavioral and electrophysiological results support the view that global and local target perception may be mediated by separate brain systems acting, at least initially, in parallel.

Adolescent↗
PubMed1982

Pattern-evoked potentials and Bloch's law.

Experiments are described which show that the latencies, unlike the amplitudes, of the C1 and C2 components of the human pattern-onset VEP do not conform to the contrast equivalent of Bloch's law. Whereas each component's amplitude is linearly related to the log of the (contrast X duration) product below a critical duration of 50 msec, its latency is independent of the stimulus pattern's duration and determined by its contrast alone; increasing by 30-35 msec for a 1.4 log unit contrast decrease. The relationship between these results and corresponding single-unit findings in the cat is discussed.

Evoked Potentials, Visual↗
PubMed1983

Visual evoked potentials to double-pulse pattern presentation.

The temporal resolution and summation characteristics of human cortical processes were investigated by recording an individual VEP component, C1, to the double-pulse presentation of pattern pairs of both the same (+ve/+ve; -ve/-ve) and of opposite (+ve/-ve; -ve/+ve) contrast polarity at varying onset-to-onset intervals (SOA). The results show that comparable limiting SOA values of 40-50 msec are needed for the C1 components to the two patterns to separate out to form a double-peaked response and for the stimulus to be seen as two distinct events. Also, the amplitude variations of the single-peaked response obtained at shorter SOAs show evidence of complete precortical response integration for SOA values below 5-10 msec and partial integration for values up to 30-40 msec, which again correspond to the results of related psychophysical studies. These VEPs show no reflection, however, of the inhibition/summation effects reported for subjective responses to pattern pairs of the same/opposite contrast polarity at SOA values between 40 and 70 msec. The implications of these findings are discussed.

Evoked Potentials, Visual↗