Dyslexic adolescents: evidence of impaired visual and auditory language processing associated with normal lateralization and visual responsivity.

The questions of whether chronically dyslexic adolescent suffer any deficits of simple language stimulus processing or are less left hemisphere dominant than normal reading controls were addressed. The dyslexics were chosen for clarity of their specific reading problem and were older than dyslexics previously studied with lateralizing tests. Tasks administered in Experiment I were unilateral and bilateral tachistoscopic work recognitions and a tachistoscopic recognition report-time task for single lateralized letter stimuli. Experiment II, conducted a year later, readministered these tasks with modifications, and added dichotic digits and motor reaction time-stimulus detection tasks. It was concluded that right handed, chronic dyslexics: (1) possess left hemisphere language specialization; (2) show normal interhemispheric processing delays for single letter stimuli; (3) are, unlike nondyslexis but equally poor-reading Ss, clearly impaired in their efficiency of visual and auditory processing of simple language stimuli; (4) possess clear auditory memory deficits for verbal material; and (5) may possess an additional deficit of left hemisphere visual association area function.     

Interhemispheric transmission time in persons with Down syndrome

Background The study of cerebral specialization in persons with Down syndrome (DS) has revealed an anomalous pattern of organization. Specifically, persons with DS elicit a right cerebral hemisphere lateralization for receptive language and a left cerebral hemisphere lateralization for the production of simple and complex movements: a pattern quite different from the left hemisphere lateralization typically characterizing the aforementioned processes in the non‐DS population. It is thought that the putative separation between speech perception and movement planning systems as well as the cost of interhemispheric integration impedes verbal‐motor behaviours in persons with DS. Moreover, morphological anomalies of callosal structure may further amplify between‐hemisphere communication difficulties in the DS population. In the present investigation, we employed a behavioural technique (i.e. the Poffenberger paradigm; Poffenberger) to determine whether global anomalies of callosal structure further amplify deficits in interhemispheric communication. Methods Fourteen individuals with DS and 25 chronological age‐matched and gender‐equated participants without intellectual disability performed a visuomotor reaction time (RT) test with their left or right hand to visual stimuli appearing left or right of visual fixation. Typically it is reported that responses to visual stimuli appearing ipsilateral to the responding hand (i.e. the uncrossed condition) are faster than responses wherein visual stimuli and responding hand are contralaterally mapped (i.e. the crossed condition). The increased RT associated with the crossed condition is reported on the order of 4 ms and has been interpreted to reflect the physiological result of interhemispheric transmission. Results Not surprisingly persons with DS exhibited slower and more variable RTs relative to control counterparts. In addition, a reliable RT advantage favouring the uncrossed conditions was observed among control participants but not persons with DS. Conclusions In keeping with the extant literature, RT performance of the DS group was slower and more variable than control counterparts. This finding has been interpreted to reflect an ‘adaptive reaction’ wherein the perceptual‐motor abilities of persons with DS are not optimized to respond to externally paced stimuli. In terms of evaluating interhemispheric transmission via the Poffenberger paradigm, our results show the finite measures of explicit brain–behaviour relations characterizing so‐called healthy controls are not always tenable in the DS population. Indeed, we believe such a finding underpins the aforementioned ‘adaptive reaction’ exemplifying preferred movement control in persons with DS.     

Differences in reaction times and average evoked potentials as a function of direct and indirect neural pathways

Average evoked potentials and manula response latencies were collected during a simple detection task in which brief visual stimuli were presented to the left and right visual fields. Latencies generated by the ipsilateral stimulus-hand combinations were shorter than contralateral combinations only under certain conditions, impugning the hypothesis that the reaction time difference reflects interhemispheric transfer time. Certain evoked potential components recorded contralateral to the stimulus occurred earlier than their ipsilateral counterparts, but whether this difference can be interpreted as representing interhemispheric transfer time is also questioned.     

Memory functions in aphasic and non-aphasic stroke patients

Abstract

We studied selected memory functions in 15 left-hemisphere stroke patients with aphasia, five non-aphasic left-hemisphere stroke patients, 16 right-hemisphere stroke patients and 10 normal controls. Memory was tested systematically for immediate recall of digits and letters, short-term (15 s) recognition of auditory and visual words and of non-verbal designs, and long-term (15 s) recognition of auditory and visual words. CT scan lesion localizations were recorded for all stroke patients. The aphasic group was deficient on immediate and short-term auditory and visual verbal memory tasks, while the right hemisphere group was inferior on both visual non-verbal patterns. Of non-aphasic left-hemisphere stroke patients, only a single patient with a left temporo-occipital lesion showed immediate and short-term verbal memory deficits. Right and left hemisphere aphasic stroke patients with isolated subcortical lesions demonstrated memory impairment, though to a lesser extent than patients with cortical lesions. All groups of stroke patients were deficient on long-term auditory and visual verbal memory. The results are discussed in terms of cerebral localization of memory functions and of the relationships of aphasia to memory dysfunction.     

Evidence of a possible isolation of left hemisphere visual and motor areas in sinistrals employing an inverted handwriting posture

Levy's hypothesis of ipsilateral motor control of the lateral distal musculature in left handers employing an inverted handwriting posture was tested in a true simple reaction time paradigm. Left-handers employing a non-inverted handwriting posture (NHP) showed performances consistent with normal (contralateral) motor control organization. Inverted handwriting posture (IHP) sinistrals showed performances that were not consistent with Levy's hypothesis nor with the simple hypothesis of normal (contralateral) control. The results for IHP sinistrals were compatible with a model of stimulus-response mediation which posits contralateral motor control and an intrahemispheric “disconnection” of left hemisphere visual areas from left hemisphere manual motor areas. The hypothesis that IHP sinistrals must utilize interhemispheric pathways for motor responses to stimuli channeled to, or formulated in, the left hemisphere was discussed in relation to the hypothesis of topographic inversion consequent to transcallosal relay. The proposed model suggests that hand inversion in writing is an accommodation to the consequences of interhemispheric relay and not to ipsilateral motor control.     

Is interhemispheric transfer of visuomotor information asymmetric? Evidence from a meta-analysis

Using a meta-analytic procedure we have analysed 16 studies employing a simple unimanual reaction time (RT) paradigm and lateralized visual stimuli to provide an estimate of interhemispheric transfer time in normal right-handed subjects. We found a significant overall RT advantage of the left visual field over the right and of the right hand over the left. These asymmetries can be explained by a superiority of the right hemisphere for the detection of simple visual stimuli and by a corresponding superiority of the left hemisphere for the execution of the manual response, respectively. Alternatively, they may be interpreted as related to an asymmetry of interhemispheric transmission of visuomotor information, with transfer from the right hemisphere (side of stimulus entry) to the left (side of response generation) faster than in the reverse direction. Although a direct test of these hypotheses is still lacking, we think that the evidence available is more in keeping with the latter possibility.     

Ipsilateral auditory extinction following frontal and basal ganglia lesions of the left hemisphere

When tested on a dichotic digits listening task, patients with unilateral lesions of the left frontal lobe and left anterior basal ganglia exhibited ipsilateral auditory extinction. This phenomenon has been documented before, but only in patients with lesions involving the region posterior and lateral to the posterior aspect of the left lateral ventricle. As patients with lesions in the equivalent region of the right hemisphere also exhibit extinction of verbal material arriving in their left ears it has been hypothesized that these posterior lesions disrupt an interhemispheric pathway connecting the two temporal lobes. As it appears that the ipsilateral auditory pathways are suppressed under conditions of dichotic stimulation, such an interhemispheric pathway would be the means whereby left-ear verbal stimuli reached the left (speech) hemisphere. Therefore a disruption of this pathway could result in left-ear extinction for verbal material. The finding that left anterior lesions also result in left-ear extinction poses problems for this hypothesis and in particular for the suggested posterior route of the interhimispheric pathway.     

Right hemispheric dysfunction in schizophrenia

This study uses the Poffenberger (1912) paradigm, which compares the difference between "crossed" (stimuli and motor response areas are contralateral) and "uncrossed" (stimuli and motor response areas are ipsilateral) conditions to estimate interhemispheric transfer time. Simple reaction time (RT) was recorded to stimuli presented to the left visual field (LVF), right visual field (RVF), or bilaterally (BVF) in individuals with schizophrenia (n = 10) and controls (n = 14), who responded using either the left or right hand. While the results provide no evidence for differences between the groups in information transfer between the hemispheres, the schizophrenia group were significantly slower to respond to LVF stimuli, suggesting right hemisphere dysfunction.     

Variability of right hemisphere activation during semantic word processing in aphasic patients: an electrophysiologic study in three patients

After stroke, the interhemispheric reorganisation of the neural network implicated in language is hypothesized to be a function not only of the site of lesion but also of the residual impairment. With a multiple case approach, we tested this hypothesis in three chronic aphasic patients. Two patients, GE (capsulo-lenticular stroke) and JHN (fronto-temporal stroke) showed formal residual semantic difficulties, while the third patient (EG, large sylvian lesion) did not. Brain electric activity was analysed during a categorisation task of tachistoscopically presented words in the left and the right visual field. The temporal analysis of brain activity showed that both patients with semantic residual difficulties activated the right hemisphere (RH) during some steps of word processing. In the third patient, without semantic impairment, the RH was activated only during a short time period. Further more, RH activation was shown to be dependent on the visual field of word presentation. Phonological impairment was not predictive of RH activation. These results suggest that RH activation, particularly anterior regions, can occur during semantic processing of words as a function of semantic residual impairment.     

Effects of passive tactile and auditory stimuli on left visual neglect

Patients with left-sided visual neglect fail to copy the left part of drawings or the drawings on the left side of a sheet of paper. Our aim was to study the variations in copying drawings induced by passive stimulation in patients with left-sided visual neglect. No stimulation at all, tactile unilateral and bilateral, binaural auditory verbal, and nonverbal stimuli were randomly applied to 14 patients with right-hemisphere strokes. Only nonverbal stimuli decreased the neglect. As nonverbal stimuli mainly activate the right hemisphere, the decrease in neglect suggests right-hemispheric hypoactivity at rest in these patients. The absence of modification of neglect during verbal stimulation suggests a bilateral hemispheric activation and the persistence of interhemispheric imbalance. Our results showed that auditory pathways take part in the network involved with neglect. Passive nonverbal auditory stimuli may be of interest in the rehabilitation of patients with left visual neglect.     

Symmetry of callosal information transfer in schizophrenia: a preliminary study

OBJECTIVE: While there is much evidence to suggest left hemisphere dysfunction and interhemispheric transfer deficits in schizophrenia, the right hemisphere is rarely implicated. This study uses 128-channel EEG to assess whether asymmetry of interhemispheric transfer found in normal individuals is present in those with schizophrenia, and whether this might point to a right-hemisphere dysfunction. METHODS: Simple reaction time (RT) was recorded to stimuli presented to the left visual field (LVF), right visual field (RVF) or bilaterally (BVF) in 13 males with schizophrenia and 13 controls. 128-Channel EEG was simultaneously recorded. Interhemispheric transfer time (IHTT) in each direction was calculated by comparing the latencies of N160 EP components in the hemispheres contralateral and ipsilateral to stimulation. RESULTS: While controls showed faster information transfer from the right-to-left hemisphere, this asymmetry was not present in the schizophrenia group who also exhibited a concomitant decrease in the amplitude of the N160 in the right hemisphere. CONCLUSIONS: Results are interpreted with reference to a loss of rapidly conducting myelinated axons in the right hemisphere in schizophrenia.     

Interhemispheric visual interaction in a patient with posterior callosectomy

The role of anterior commissure (AC) and anterior parts of corpus callosum in visual interactions was investigated in a partial split-brain patient whose posterior and middle parts of the corpus callosum were resected surgically leaving intact only a thin portion of anterior corpus callosum. Although the primary visual areas of the two hemispheres are disconnected in the patient, we found that visual distracters presented to one hemisphere (in a crowding paradigm) impaired recognition of the target stimulus presented to the other hemisphere. The normal control group showed the same result. To rule out the possible contribution of subcortical areas to this interaction, we repeated the same crowding task with texture-defined stimuli. The patient again showed an interhemispheric interaction, even though subcortical structures respond poorly or do not respond at all to texture defined shapes. Despite the evidence for interhemispheric interaction, a classic match-to-sample task confirmed that the patient was unable to explicitly report when stimuli in left and right hemifields were the same or different. Similarly, in a search task, the patient's reaction time was unaffected by distracters in the hemifield opposite the target whereas normals' response time was affected. Considering the dissociation between these two tasks, we conclude that the anterior commissure and/or the anterior corpus callosum contribute to interhemispheric interactions in the attentional selection of location.     

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.     

The effects of multimodality stimulation on hemispheric alpha asymmetries of aphasic and normal subjects

Abstract

Hemispheric alpha asymmetries of normal males, normal females, and male aphasics were observed from anterior and posterior placement sites for recall and recognition of multimodality (auditory and visual) presented high and low imagery words and during resting conditions. No significant differences were observed during the resting condition between groups. Males were not found to have hemispheric asymmetries for any of the stimulus or task conditions. Females showed lower alpha power for the low imagery words compared to the high imagery words. Aphasic males were found to have significantly less alpha power in the right compared to the left hemisphere across conditions and tasks. A significant group × hemisphere × imagery interaction revealed significantly less alpha power in the left hemisphere (although not as low as the right) for low imagery words compared to the high imagery words for the aphasic subjects. This finding suggests greater involvement of the left hemisphere in processing multimodality information in aphasics that has not been reported for unimodality information (Moore 1989a, Moore and Lux 1987). Behavioural responses were found to be effected by group classification, task condition, and imagery value.     

Transcranial direct current stimulation improves word retrieval in healthy and nonfluent aphasic subjects

A number of studies have shown that modulating cortical activity by means of transcranial direct current stimulation (tDCS) affects performances of both healthy and brain-damaged subjects. In this study, we investigated the potential of tDCS to enhance associative verbal learning in 10 healthy individuals and to improve word retrieval deficits in three patients with stroke-induced aphasia. In healthy individuals, tDCS (20 min, 1 mA) was applied over Wernicke's area (position CP5 of the International 10-20 EEG System) while they learned 20 new "words" (legal nonwords arbitrarily assigned to 20 different pictures). The healthy subjects participated in a randomized counterbalanced double-blind procedure in which they were subjected to one session of anodic tDCS over left Wernicke's area, one sham session over this location and one session of anodic tDCS stimulating the right occipito-parietal area. Each experimental session was performed during a different week (over three consecutive weeks) with 6 days of intersession interval. Over 2 weeks, three aphasic subjects participated in a randomized double-blind experiment involving intensive language training for their anomic difficulties in two tDCS conditions. Each subject participated in five consecutive daily sessions of anodic tDCS (20 min, 1 mA) and sham stimulation over Wernicke's area while they performed a picture-naming task. By the end of each week, anodic tDCS had significantly improved their accuracy on the picture-naming task. Both normal subjects and aphasic patients also had shorter naming latencies during anodic tDCS than during sham condition. At two follow-ups (1 and 3 weeks after the end of treatment), performed only in two aphasic subjects, response accuracy and reaction times were still significantly better in the anodic than in the sham condition, suggesting a long-term effect on recovery of their anomic disturbances.     

Inter- and intra-hemispheric processing of visual event-related potentials in the absence of the corpus callosum

Interhemispheric differences of the N100 latency in visual evoked potentials have been used to estimate interhemispheric transfer time (e.g., Saron & Davidson, 1989). Recent work has also suggested that the P300 component could reflect the efficacy of interhemispheric transmission (Polich & Hoffman, 1998). The purpose of the present study was to study the differential role of the corpus callosum (CC) and anterior commissure (AC) in the interhemispheric propagation of these two electrophysiological components. Thus, the amplitude and latency distribution of the N100 and P300 components were analyzed using high-density electrical mapping in a subject with agenesis of CC but preservation of AC, a subject with agenesis of both CC and AC, and 10 neurologically intact control subjects. The task consisted of a modified visual oddball paradigm comprising one frequent and two rare stimuli, one presented on the same and the other on the opposite side of the frequent stimulus. Interhemispheric differences in latency were found for the N100 component in controls. However, in the acallosal subjects, this component was not identifiable in the indirectly stimulated hemisphere. In controls, no interhemispheric differences were observed in the distribution of the P300 latency and amplitude to rare and frequent stimuli. The distribution of the P300 amplitude in the acallosal subject with an AC was identical to that of the controls, whereas in the acallosal subject lacking the AC, the amplitude was greater in the hemisphere receiving the frequent stimuli, regardless of the visual hemifield in which the rare stimuli were presented. In both acallosal subjects, hemispheric differences in the P300 latency were observed, the latencies being shorter in the hemisphere directly stimulated for all categories of stimuli. These results suggest that the interhemispheric transfer of both the N100 and P300 components relies on the integrity of cortical commissures. Possible P300 generator sources are discussed.     

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.     

Left hemisphere superiority for visuospatial functions in left-handers

Male and female left- and right-handers have been tested with a divided visual field technique on a visuospatial (discrimination of angle width) and on a verbal task (vowel-consonant discrimination) using either a choice or a Go-No-go manual reaction time paradigm. Right-handers showed the expected pattern of hemispheric asymmetries with an advantage of the right hemisphere in the visuospatial task and an advantage of the left hemisphere in the verbal task. Such effects were statistically reliable only in male subjects. Left-handers, on the contrary, showed a different pattern of asymmetries. In the visuospatial task there was an overall superiority of the left hemisphere, while no hemispheric asymmetry was found in the verbal task.     

A tapping test in apraxia.

An enigma in understanding the mechanism of motor apraxia is why patients with disease restricted to the left hemisphere cannot imitate motions with the left hand despite intact visual motor apparatus. Liepmann suggested that motor memories are stored in the left hemisphere and that in apraxia these engrams are either destroyed or separated from the right hemisphere. If Liepmann's hypothesis is correct, then, in addition to poor performance on commands and imitation, there should be poor performance in a motor task. To ascertain if there is a motor defect of the non-paretic hand of apraxic patients, twenty apraxic patients were given a rapid finger tapping test and their performance was compared with the left hand performance of aphasic right hemiparetic non-apraxic controls. The aptaxic group performed this task significantly slower than the control group, thereby giving support to Liepmann's hypothesis of apraxia.     

Hemispheric asymmetry and interhemispheric transfer in reaching programming.

The purpose of this study was to explore the intrahemispheric processes and the interhemispheric transfer that occur during the programming of a pointing movement. Twenty five subjects participated in this experiment: 12 were right-handed (Rhr), 12 left-handed (Lhr), and 1 was left-handed with a posterior callosal lesion. The task consisted in producing an open loop pointing response toward a visual target appearing briefly on the right or the left of a central fixation point. Reaction times (RTs) were shorter for the Rhrs when reaching with the left hand than with the right hand. No such hand-related difference was observed in the Lhrs. The left hand advantage indicates that one process was faster in the right hemisphere of Rhrs. This faster process appears not to be visual but motor or visuomotor. For either hand, responses were faster when the target appeared in the visual field homolateral to the pointing hand (uncrossed condition) than when it appeared contralaterally to the hand (crossed condition). The crossed vs uncrossed difference did not vary between Rhrs and Lhrs or between the hands. The transfer time between the hemispheres was symmetrical whatever its direction. The partially callosotomized left-handed subject was two-fold slower than the control Lhrs. His uncrossed responses were faster than the crossed ones, but his interhemispheric transfer time was very asymmetrical: it was normal from right to left hemisphere but was highly increased in the opposite direction. An attempt at modelling the RT data is proposed and the possibility of different callosal locations for the interhemispheric transfer is discussed.