Hemispheric differences for visual evoked potentials from checkerboard stimuli

White matter and latency of visual evoked potentials during maturation: A miniature pig model of adolescent development

BACKGROUND

Continuous myelination of cerebral white matter (WM) during adolescence overlaps with the formation of higher cognitive skills and the onset of many neuropsychiatric disorders. We developed a miniature-pig model of adolescent brain development for neuroimaging and neurophysiological assessment during this critical period. Minipigs have gyroencephalic brains with a large cerebral WM compartment and a well-defined adolescence period.

METHODS

Eight Sinclair™ minipigs (Sus scrofa domestica) were evaluated four times during weeks 14-28 (40, 28 and 28 days apart) of adolescence using monocular visual stimulation (1 Hz)-evoked potentials and diffusion MRI (dMRI) of WM. The latency for the pre-positive 30 ms (PP30), positive 30 ms (P30) and negative 50 ms (N50) components of the flash visual evoked potentials (fVEPs) and their interhemispheric latency (IL) were recorded in the frontal, central and occipital areas during ten 60-second stimulations for each eye. The dMRI imaging protocol consisted of fifteen b-shells (b = 0-3500 s/mm2) with 32 directions/shell, providing measurements that included fractional anisotropy (FA), radial kurtosis, kurtosis anisotropy (KA), axonal water fraction (AWF), and the permeability-diffusivity index (PDI).

RESULTS

Significant reductions (p < 0.05) in the latency and IL of fVEP measurements paralleled significant rises in FA, KA, AWF and PDI over the same period. The longitudinal latency changes in fVEPs were primarily associated with whole-brain changes in diffusion parameters, while fVEP IL changes were related to maturation of the corpus callosum.

CONCLUSIONS

Good agreement between reduction in the latency of fVEPs and maturation of cerebral WM was interpreted as evidence for ongoing myelination and confirmation of the minipig as a viable research platform. Adolescent development in minipigs can be studied using human neuroimaging and neurophysiological protocols and followed up with more invasive assays to investigate key neurodevelopmental hypotheses in psychiatry.

The electrophysiological assessment of visual function in Multiple Sclerosis

•

VEPs have largely been replaced by MRI in modern MS diagnosis and management.

•

Multifocal VEPs are superior to traditional VEPs in evaluating the integrity of the visual system.

•

Physiological asymmetry limits interpretation of small VEP differences.

The assessment of vision is integral to the diagnosis and monitoring of patients with multiple sclerosis (MS). Visual electrophysiology, previously a critical investigation in patients with suspected MS, has in large part been supplanted by magnetic resonance imaging in clinical routine. However, the development of multi-focal visual evoked potentials and the advent of putative re-myelinating therapies that can be monitored with these techniques has led to a resurgence of interest in the field. Here, we review the clinical applications, technical considerations and limitations of visual evoked potentials in the management of patients with MS.

Does Interhemispheric Competition Mediate Motion-Induced Blindness? A Transcranial Magnetic Stimulation Study

Motion-induced blindness (MIB) is a phenomenon, perhaps related to perceptual rivalry, where stationary targets disappear and reappear in a cyclic mode when viewed against a background (mask) of coherent, apparent 3-D motion. Since MIB has recently been shown to share similar temporal properties with binocular rivalry, we probed the appearance–disappearance cycle of MIB using unilateral, single-pulse transcranial magnetic stimulation (TMS)—a manipulation that has previously been shown to influence binocular rivalry. Effects were seen for both hemispheres when the timing of TMS was determined prospectively on the basis of a given subject's appearance–disappearance cycle, so that it occurred on average around 300 ms before the time of perceptual switch. Magnetic stimulation of either hemisphere shortened the time to switch from appearance to disappearance and vice versa. However, TMS of left posterior parietal cortex more selectively shortened the disappearance time of the targets if delivered in phase with the disappearance cycle, but lengthened it if TMS was delivered in the appearance phase after the perceptual switch. Opposite effects were seen in the right hemisphere, although less marked than the left-hemisphere effects. As well as sharing temporal characteristics with binocular rivalry, MIB therefore seems to share a similar underlying mechanism of interhemispheric modulation. Interhemispheric switching may thus provide a common temporal framework for uniting the diverse, multilevel phenomena of perceptual rivalry.

Face and Word Recognition in Patients with Left and Right Hemispheric Lesions: Evidence from Reaction Times and ERPs

Abstract: We recorded reaction times (RTs) and event-related potentials (ERPs) in patients with unilateral lesions during a memory search task. Participants memorized faces or abstract words, which were then recognized among new ones. The RT deficit found in patients with left brain damage (LBD) for words increased with memory set size, suggesting that their problem relates to memory search. In contrast, the RT deficit found in patients with RBD for faces was apparently related to perceptual encoding, a conclusion also supported by their reduced P100 ERP component. A late slow wave (720-1720 ms) was enhanced in patients, particularly to words in patients with LBD, and to faces in patients with RBD. Thus, the slow wave was largest in the conditions with most pronounced performance deficits, suggesting that it reflects deficit-related resource recruitment.

Hemispheric differences for visual matrix processing: stimulus size and spatial frequency effects

Hemispheric processing differences were assessed by presenting square matrices that varied in size and the number of filled-in cells. Subjects judged whether the matrix contained an even or odd number of filled cells. Experiment 1 employed relatively small matrix sizes (2 x 2, 3 x 3, and 4 x 4), and Experiment 2 employed relatively large matrix sizes (4 x 4, 6 x 6, and 8 x 8). Response time was shorter and error rates lower for left visual field/right hemisphere (LVF/RH) presentations compared to right visual field/left hemisphere (RVF/LH) presentations, with the larger matrices demonstrating the strongest visual field/hemispheric effects. Increases in the number of filled cells contributed to increases for the LVF/RH response time advantage only for the larger arrays. Analysis of the data from both studies collapsed across the number of filled cells produced highly consistent LVF/RH advantages for both response time and error rate, with stronger LVF/RH advantages found for the larger matrix sizes of both studies. The findings suggest that visual stimulus spatial frequency is a key determinant of hemispheric processing advantages, but that this factor is constrained by stimulus size variation. Theoretical implications with respect to the hemispheric processing double filtering by frequency model are discussed.

Hemispheric asymmetries for spatial frequency discrimination in a selective attention task

Hemispheric specialization for spatial frequency processing was investigated by measuring reaction times to sinusoidal gratings in 12 healthy subjects. Stimuli of 1.5, 3, and 6 c/deg were randomly presented at two peripheral locations in the left (LVF) and right (RVF) upper visual hemifields during a selective attention task. Subjects were instructed to pay covert attention and to respond to a frequency in a given hemifield ignoring all other stimuli. Results showed that RTs were significantly faster at LVF than RVF for low frequency gratings, and at RVF than LVF for high frequency gratings. Furthermore, RTs were faster to 6 than 1.5 c/deg at the RVF, while there was not a significant difference at the LVF. In our view, these findings in a task requiring fast and accurate spatial frequency discriminations may be interpreted in terms of a hemispheric asymmetry for spatial frequency processing.

Event-related potentials and models of performance asymmetries in face and word recognition

Performance asymmetries in divided visual field studies may be ascribed either to hemispheric differences in processing efficiency or to the costs of interhemispheric transfer towards the superior hemisphere. In order to distinguish between these alternatives, event-related potentials (ERPs) were recorded while subjects had to recognize laterally presented faces or words. As expected, behavioural left- and right-field advantages were observed for faces and words, respectively. Regardless of stimulus type, the ERPs displayed a sustained temporo-parietal negativity over the hemisphere stimulated directly via the contralateral hemifield. Both this hemifield-dependent negativity (HDN) and the performance asymmetries diminished to insignificance when the same stimuli were presented but subjects simply made a left-right decision about stimulus location. We conclude that the HDN is no obligatory, stimulus-bound ERP component but depends on lateralized cognitive processing. The stimulus-unspecific and time-invariant topography of the HDN might indicate that it relates to the allocation of processing resources to the directly stimulated hemisphere. The findings suggest that both faces and words were processed predominantly in the directly stimulated hemisphere, supporting an efficiency explanation of the performance asymmetries.

Functional interhemispheric asymmetries at birth as demonstrated by somatosensory evoked potentials

Functional cerebral hemispheric asymmetries detectable at birth have been suggested by a number of neuroanatomic, neuroradiologic, and clinical neurophysiologic modalities. The aim of this study was to determine whether functional interhemispheric asymmetries can be identified using electrophysiologic measures. As part of a prospective study, somatosensory evoked potentials following median nerve stimulation were recorded in nine healthy full-term newborns on day 2 or 3 of life, and somatosensory evoked potentials were repeated at 2 and 6 months of age. These children were subsequently examined at 1 and 3 years of age by a pediatric neurologist and all had normal examinations. Handedness was determined at 3 years by questioning the parent and by clinical observation. Three of nine were left-handers. All three left-handers had clearly identifiable neonatal N19 parietal potentials over the right hemisphere. Following right median nerve stimulation, contralateral parietal potentials were absent on two of the three and questionable in the third. Asymmetries were not clearly present in right-handers although only one showed an increased maturation of the right hemisphere relative to the left. At 2 months of age, interhemispheric differences were no longer clearly evident. This data suggests that preferential hemispheric asymmetries are masked by 2 months of age. This differential rate of development demonstrated by neonatal somatosensory evoked potentials may be an early indicator of ultimate handedness.

Contributions of stimulus encoding and memory search to right hemisphere superiority in face recognition: behavioural and electrophysiological evidence

The relative contributions of stimulus encoding and memory search to right hemisphere (RH) superiority in face recognition were investigated in a memory search task with lateralized probe stimulus presentation. RH superiority in reaction time and latency of a late positive ERP component was independent of stimulus degradations, but increased as a function of memory set size. These results suggest equal efficiency of the hemispheres in stimulus encoding, but RH superiority in memory search. In addition, a long-lasting ERP negativity contralateral to the stimulated hemifield is discussed with reference to models of hemispheric processing in hemifield studies.

The effects of hemispheric differences on feature perturbations

Hemispheric differences for feature perturbations were investigated in two experiments. Stimulus displays consisting of five small squares arranged in a single row were presented tachistoscopically, with the subject instructed to state in which square a horizontal tick mark was located. Ticks could occur in any of the three middle squares, with half of the ticks presented on the inside and half presented on the outside of the square in relation to the fovea. Experiment 1 presented each array of five squares to the right or left of fixation at one of three distances from the fovea. Experiment 2 manipulated the distance between the squares and kept foveal distance constant. In each experiment, fewer errors were made when stimuli were presented to the left visual field/right hemisphere than when they were presented to the right visual field/left hemisphere, when ticks migrated toward the fovea. Experiment 1 found that increasing the distance from the fovea increased the error rate, but did not change the hemispheric differences. Experiment 2 found that increasing the distance between the squares did not change hemispheric effects reliably. The data imply that hemispheric differences for perceptual processing begin very early during sensory analysis.

Hemispheric local/global processing revisited

Hemispheric differences for local and global processing were assessed with square and rectangular stimulus shapes which contained either squares or rectangles within them. Experiment 1 manipulated the overall size and area of the stimulus items, while experiment 2 kept stimulus area constant and manipulated the number of shapes inside the stimulus. In different conditions subjects judged whether the inside shapes (local processing) or whether the outside shapes (global processing) were squares or rectangles. Reaction time measures revealed that hemispheric differences were unrelated to the local or global processing requirements as well as the spatial frequency of the stimulus texture. A left hemisphere advantage generally was obtained across stimulus and processing conditions, with the pattern of task effects dependent on the specific stimuli employed. The results suggest that the overall stimulus shape rather than processing mode or specific spatial frequency is a primary determinant of hemispheric differences for visual information.

Hemispheric differences in visual search of simple line arrays

The effects of perceptual organization on hemispheric visual-information processing were assessed with stimulus arrays composed of short lines arranged in columns. A visual-search task was employed in which subjects judged whether all the lines were vertical (same) or whether a single horizontal line was present (different). Stimulus-display organization was manipulated in two experiments by variation of line density, linear organization, and array size. In general, left-visual-field/right-hemisphere presentations demonstrated more rapid and accurate responses when the display was perceived as a whole. Right-visual-field/left-hemisphere superiorities were observed when the display organization coerced assessment of individual array elements because the physical qualities of the stimulus did not effect a gestalt whole. Response times increased somewhat with increases in array size, although these effects interacted with other stimulus variables. Error rates tended to follow the reaction-time patterns. The results suggest that laterality differences in visual search are governed by stimulus properties which contribute to, or inhibit, the perception of a display as a gestalt. The implications of these findings for theoretical interpretations of hemispheric specialization are discussed.