Bilateral visual field processing and evoked potential interhemispheric transmission time

The relationship between the efficiency of interhemispheric interactions via the corpus callosum and the speed and accuracy in making comparisons of information simultaneously presented to the right and left visual fields was studied by comparing bilateral (vs unilateral) advantages in matching letters, with evoked potential measures of interhemispheric transmission time (EP-IHTT). The primary findings was a strong correlation suggesting that larger bilateral field advantages in reaction time are associated with faster EP-IHTT. However, the association between EP-IHTT and bilateral advantage was strong only for transmission speed from left hemisphere to right hemisphere, but not for speed of transmission in the opposite direction. The data are consistent with a hypothesis of asymmetric homologue enhancement, i.e. a directionally asymmetric callosal influence which facilities processing of letter stimuli in the right hemisphere allowing for increased response speed and accuracy of bilateral visual field comparisons.     

Somatosensory evoked potential activity: a measure of interhemispheric transfer in schizophrenia?

Jones and Miller (1981) reported that schizophrenics could be distinguished from healthy controls by latency differences in their ipsilateral and contralateral somatosensory evoked potentials (SEPs), considered a measure of corpus callosum conduction time. Subsequent replication studies failed to follow the method of the original study. The present study represents a precise replication. An additional control for contamination of SEPs by an auditory stimulus generated by the vibrotactile stimulus was used. Ipsilateral SEP activity was identified but poor definition of a positive peak in the range of interest prevented measurement of peak latency. Evidence was advanced to suggest that such ipsilateral potentials result from activity at the vertex. It was concluded that this method is inappropriate for the investigation of interhemispheric transmission.     

Visual evoked potentials to lateralized visual stimuli and the measurement of interhemispheric transmission time

Visual evoked potentials (VEPs) to lateralized light flashes were recorded from the parietal midline, and from homologous occipital and central sites, in a GO/NOGO reaction time task. The N160 component of the VEP was found to be larger over the hemisphere contralateral to the visual field of stimulus exposure at all pairs of lateral electrodes. At the occipital sites only, N160 latency was also shorter from the contralateral hemisphere, by an average of approximately 14msec. This was not so centrally, where a non-significant value of approximately 4 msec was obtained. These data are considered to be consistent with Milner and Lines' hypothesis that callosal transmission occurs at different rates in different functional regions of the corpus callosum.     

Visual evoked potential

Under the pathological conditions, the VEP may show changes in amplitude, latency, or waveform in one or more of its components. The major advantage of the pattern reversal VEP over the flash VEP lies in smaller variability in the waveform and latency of its components in the healthy population. The flash VEP is, however, particularly useful to infants and newborns. The variation in the waveform of the flash VEP was evaluated, and Type V was abnormal waveform. Subsequently, patients with various CNS diseases were examined. Acute hemiplegic patients showed high amplitude or increased latency patterns. In 45 percent of the West syndrome cases, VEP showed abnormal responses. The patients with asphyxia, respiratory distress syndrome and mental retardation showed increased latency. Latency was decreased in hypoglycemia and hypocalcemia of newborns, and it was increased or decreased in autism and epilepsy. In our study, it was concluded that the rhythmic after-discharge is a true response. This assumption is supported by the following observations: (1) the after-discharge of VEP appeared before the occurrence of the alpha-wave in EEG; (2) the frequency of the after-discharge was generally higher than that in EEG; and (3) the frequency of the after-discharge did not change between the conditions of the resting state and hyperventilation. There was no after-discharge in VEP in patients with a history of encephalitis even when their IQs were normal, and the latency was increased in pattern reversal VEP.     

Visual evoked potential abnormalities in dyslexic children.

Developmental reading disability (dyslexia) has traditionally been attributed to impaired linguistic skills. Recent psychophysical data suggest that dyslexia may be related to a visual perceptual deficit. A few visual evoked potential (VEP) studies have addressed this hypothesis, but their results are far from consistent. We submitted 9 dyslexic subjects and 9 age- and sex-matched normal controls to checkerboard pattern reversal VEPs. The main experimental variables were: large (0.5 cycles per degree; cpd) and small (2 cpd) checks and two reversal frequencies (2.1 Hz and 8 Hz); mean luminance and contrast (60 cd/m2 and 50%, respectively) were kept constant in all four conditions. Transient VEP (2.1 Hz) parameters did not differ between controls and dyslexics at 2 cpd. At 0.5 cpd, N70 amplitude was significantly smaller and N70 latency significantly shorter in dyslexics. Amplitudes for the fundamental frequency (8 Hz), as well as for the second and third harmonics of the steady-state VEPs were smaller in dyslexics for both stimulus sizes. A discriminant analysis correctly classified each subject. Our data confirm the hypothesis of a perceptual deficit in dyslexic subjects. The abnormalities are related to spatial and temporal stimulus frequencies: they appear when large stimuli are presented, or when the stimulation frequency is high. These data support the hypothesis of selective magnocellular dysfunction in dyslexia.     

Bilateral field advantage and evoked potential interhemispheric transmission in commissurotomy and callosal agenesis.

The role of the corpus callosum versus other cerebral commissures in the interhemispheric integration of visual information was studied in four individuals with complete agenesis of the corpus callosum, two individuals with partial agenesis, one total commissurotomy patient, and normal individuals. Evoked potential (EP) indices of interhemispheric transmission of visual sensory responses were observed during matching of unilateral and bilateral visual field letters and patterns. Neither the commissurotomy nor any of the acallosal patients had ipsilateral hemisphere visual EPs (P1 and N1), demonstrating that the posterior callosum is necessary for interhemispheric transmission of these components of visual evoked potentials. While the commissurotomy patient could not compare bilaterally presented letters, the anterior commissure of the acallosal patients appeared to be sufficient for interhemispheric comparison of single letters. However, bilateral comparison of more complex visual patterns resulted in considerable difficulty for complete agenesis patients, while comparison of patterns was more nearly normal when anterior callosal fibers were present (partial agenesis).     

Visual evoked potential in man: early oscillatory potentials.

Short latency visual evoked oscillatory potentials to bright light stimulation were recorded from the scalp of 15 normal human adult subjects. The onset latencies of these potentials recorded over anterior frontal and posterior scalp regions were 9--17 msec and 13--24 msec, respectively. The frequency of the oscillations was about 100 c/sec. These potentials were widespread in their distribution over the scalp but were most prominent at midline and parasagittal recording locations. Like similar potentials recorded in animals, it seems that these potentials arise in both subcortical and cortical visual structures. The mechanism underlying the generation of these potentials and their possible functional significance are discussed.     

Visual evoked potential habituation in mental deficiency

Habituation of visual evoked potentials (VEPs) may be a helpful tool in the comparative investigation of mental and neural physiologic processes. Through-train and across-train procedures of VEP averaging were performed during habituation in three groups of male children exhibiting mental retardation (idiopathic, exogenous, chromosomal) and in normal children. The children with idiopathic and exogenous mental retardation reached the same VEP habituation level as the normal children (amplitude reduction of 20-60% of early and late VEP components). The across-train averaging showed however a slower than normal development of habituation in these mentally retarded subjects. Habituation failed to occur in the low-grade mentally retarded children, who displayed a reverse tendency of increasing VEP amplitudes and of VEP variability during repetitive flash applications.     

Reaction time measures of interhemispheric transfer time in reading disabled and normal children

This experiment was designed to test whether reading disabled boys differ from matched controls on behavioral measures of interhemispheric transfer time (IHTT). Specifically, we proposed that language-disordered reading disabled children who had deficits in naming would show either faster or slower IHTTs compared with controls. From an initial group of 118 right-handed males, we selected a group of 25 disabled and 25 normal readers, matched on age. All subjects had to obtain a full scale IQ of 90 or above, a PIQ score of 85 or above, and a scaled score of 7 or above on the Block Design Subtest of the WISC-R. After meeting additional criteria for group assignment, manual reaction time (RT) measures of IHTT were obtained in response to simple visual and tactile stimuli during two laboratory testing sessions. Half the trials were conducted with the hands in an uncrossed orientation and half with the hands crossed in order to examine the effects of spatial compatibility on estimates of IHTT. The results revealed no overall group differences in IHTT for any of the conditions. However, correlations between IHTT measures and indices of cognitive performance indicated that faster IHTTs were significantly correlated with poorer performance on measures of reading and language function in the dyslexic group. These data are discussed within the context of a model of interhemispheric transfer deficits in disabled readers.     

Visual evoked potential in patients with cerebral asthenopia

Cerebral asthenopia is often overlooked as a symptom in diffuse brain lesion. An objective correlate of this symptom has so far never been demonstrated. Averaged visual evoked potential (VEP) in 10 patients with asthenopia was compared with 20 normal subjects. Both eyes and each eye alone were stimulated using bipolar recording in the midline and over each of the occipital lobes. There was no difference of latency of the VEPs in the two groups, but the amplitude of the most prominent component was significantly reduced in the patients. There was also a difference in the two groups regarding habituation and lateralisation. No amplitude difference could be found in the somatosensory evoked potential. The amplitude difference in VEP, as an objective correlate of asthenopia, is probably modal specific and suggests involvement of the visual cortex. VEP is unsuitable as a diagnostic tool due to the great overlap between amplitudes in asthenopic patients and control subjects.     

Visual evoked potential changes following renal transplantation

We have followed a group of 18 uremic patients through living-related donor renal transplantation (RTX) using pattern-reversal VEPs. Recordings were made prior to and 10 weeks after surgery at high, medium and low spatial frequencies. Prior to RTX, mean latency of the P100 component of the VEP was 107 msec. Individual values did not correlate with blood urea nitrogen or creatinine. Patients requiring hemodialysis did not differ from non-dialyzed patients. Ten weeks after RTX P100 latencies were significantly shortened while N75 latencies were unchanged. Several diabetic patients exhibited the appearance of previously unrecorded wave forms. P100 latency increased significantly with increasing spatial frequency before and after transplantation. Diabetic patients demonstrated a consistent increase in P100 amplitude while non-diabetic patients demonstrated a consistent decrease in P100 amplitude after RTX. The data indicate that renal transplantation has beneficial effects on the central nervous system of uremic patients not seen with chronic hemodialysis and that these effects may be quantitatively measured using the VEP. The data further suggest that electrophysiological effects of uremia and diabetes may be additive, but reversible after RTX. Alterations in the uremic and diabetic VEP may be related to retinal or more proximal central nervous system structures.     

Handedness effects on interhemispheric transfer time: a TMS study

The crossed-uncrossed difference (CUD) estimates the interhemispheric transfer time (ITT) through the corpus callosum. Previous research has shown that transcranial magnetic stimulation (TMS) to the occipital cortex determines an increased CUD during cognitive tasks. The aim of the present study was to investigate whether TMS stimulation applied at a motor stage can interfere with the ITT, comparing the performance of left- and right-handed people. Results showed a significant TMS effect, i.e. increasing reaction times were reported when stimulation was delivered on the left primary motor area. Effects were more evident when information was primarily perceived through the dominant hemisphere. Both left and right stimulations increased CUD times in right-handed subjects; however, left-handed subjects showed significant effects associated with left stimulation only. Furthermore, in both groups, TMS produced larger effects in the crossed than in the uncrossed condition. TMS stimulation increased reaction times, thus supporting the idea that the interhemispheric transfer of visuo-motor information occurs at a motor processing stage. The dominant hemisphere seems to play a major role within this process: our data indicates that left- and right-handed people have different ITT latencies associated with the transfer of information to the contralateral hemisphere.     

Visual interhemispheric transfer to areas 17 and 18 in cats with convergent strabismus

Commissural connections between primary visual cortical maps of the two hemispheres are essential to unify the split representation of the visual field. In normal adult cats, callosal connections are essentially restricted to the border between areas A17 and A18, where the central vertical meridian is projected. In contrast, early convergent strabismus leads to an expanded callosal-receiving zone, as repeatedly indicated by anatomical experiments. We investigated here the functional correlates of this widespread distribution of callosal terminals by analysing transcallosal visual responses in five anaesthetized and paralysed 4-10-month-old cats whose right eye had been surgically deviated on postnatal day 6. After acute section of the optic chiasm, single-unit activity was recorded from A17 and A18 of the right hemisphere while the left eye was visually stimulated. A total of 108/406 units were transcallosally activated. While they were more frequent at the 17/18 border (46% of the units recorded within this region), numerous transcallosally activated units were located throughout A17 (16%), A18 (27%) or within the white matter (17%). In all regions, transcallosally driven units displayed functional deficits usually associated with strabismus, such as decreased binocularity and ability to respond to fast-moving stimuli, and increased receptive field size. Many units also displayed reduced orientation selectivity and increased position disparity. In addition, transcallosal receptive fields were manifestly located within the hemifield ipsilateral to the explored cortex, with almost no contact with the central vertical meridian. Comparison with data from normal adults revealed that strabismus induced a considerable expansion of the callosal receiving zone, both in terms of the cortical region and of the extent of the visual field involved in interhemispheric transfer, with implications in the integration of visual information across the hemispheres.     

Visual evoked potential monitoring of optic nerve function during surgery.

A study was made with intra-operative flash--visual evoked potentials (VEP) monitored using a fibre-optic/contact lens photo stimulator in 57 patients undergoing intra-orbital surgical procedures with potential risk to the optic nerve. The VEPs recorded under enflurane and nitrous oxide anaesthesia did not differ significantly in latency or amplitude from the pre-operative recordings. Transient abolition of the VEP was seen under many circumstances and did not correlate with the outcome of surgery, but absence of a previously normal VEP for more than four minutes during surgical manipulation within the orbit did show a correlation with post operative impairment of vision. The technique provides early warning to the surgeon of threats to the integrity of the optic nerve.     

Visual evoked potential abnormalities in dementia with Lewy bodies

ObjectivesVisuo-perceptual deficits and visual hallucinations (VHs) are common disturbances in patients with dementia with Lewy bodies (DLB) and those with Parkinson’s disease (PD). In particular, delays in visual evoked potential (VEP), reversed by l-dopa administration, have previously been observed in PD patients. Impairment in metabolic functions of dopaminergic amacrine cells within the inner plexiform layer of the retina has been largely documented and has been posited as the underlying cause of visual and retinal alterations in PD. The aims of the present study were to investigate the presence of VEP abnormalities in DLB patients, as compared to a PD control group, and to assess the presence of significant correlations between neurophysiological measures and clinical symptoms (i.e., presence of visuospatial deficits and/or visual hallucinations).MethodsFifteen DLB patients and fifteen matched PD patients underwent pattern reversal before and after l-dopa administration, and a short neuropsychological assessment.ResultsIn DLB patients, we observed delay of the P100 latency to foveal stimuli in both eyes compared to normative values. Compared to PD, DLB patients showed higher values of the P100 latency for foveal stimulation from the right eye prior to l-dopa administration (p = 0.018). No correlations between VEP alterations, visuo-spatial deficit and visual hallucinations were found.DiscussionOur findings demonstrated a longer P100 delay in DLB than in PD patients, especially along the right visual pathway. In contrast to previous studies, which focused on a dopaminergic pre-geniculate impairment of visual pathways, our evidence suggests that other mechanisms, possibly relying on thalamic involvement, which is known to be dysfunctional in DLB, can interfere with VEP abnormalities.     

Visual evoked potential changes in migraine. Influence of migraine attack and aura

OBJECTIVE: To assess the visual evoked potential (VEP) changes in migraines with and without aura. STUDY DESIGN: A clinical study in which the VEP results of 45 migraineurs (study group) and 22 healthy volunteers (control group) were compared. Of 45 migraineurs, 29 had migraine with aura (MA) and 16 had migraine without aura (MOA), and they were examined both during and between the migraine attacks. METHODS: The patients and healthy controls underwent VEP assessment. On VEP recording, mono-ocular stimulation was performed by means of the pattern reversal check board. The latencies of N1, P1 and N2, and the N1--P1 amplitude were noted. The following comparisons were made between NI, P1 and N2 latencies and N1--P1 amplitudes of the migraine and control groups; during and between attack the VEP results of the patients with MA and MOA. RESULTS: The VEP results of the migraineurs and healthy controls were similar (P>0.05). The during attack results of MA, during and between attack results of MOA, and the results of the control group were also similar (P>0.05). N2 latency significantly elongated in patients with MA in the attack free period than it was during the attack (P=0.01), and was also longer than it was in the control group (P=0.01). CONCLUSIONS: There is involvement of the visual pathway in MA rather than MOA, and differentiation between these subtypes of the migraine disease may be performed on the basis of VEP findings manifesting by the prolongation of the N2 wave latency. This contention should be confirmed by further studies.