Reading in callosal agenesis

It has been suggested that deficits in explicit phonological processing are causal in developmental dyslexia. Deficits in such skills have been reported in developmental phonological dyslexia, though not in developmental surface dyslexia. The reading performance of two children with callosal agenesis, who have been previously shown to have impairments on rhyming tasks, are reported. Neither child is dyslexic in the traditional sense, since word reading levels are appropriate for age. However, both children have impaired development of the phonological reading route despite normal lexical skills. The pattern of their reading is therefore comparable to developmental phonological dyslexia. Problems in explicit phonological processing may be causal in the failure to establish an efficient phonological reading route but this is insufficient to create difficulty with word recognition itself. The corpus callosum may be essential for the normal development of a phonological reading route.     

A further study of language function in callosal agenesis

A subset of the tests of language ability administered by M. Dennis (1981, Brain and Language, 12, 33-53) was given to two adult acallosal patients. One of the patients studied showed widespread language deficits not restricted to the syntactic-pragmatic core as in Dennis' patient. The other showed a very specific deficit which does not encompass syntactic-pragmatic skills. The data does not support the view that the corpus callosum is necessary for the normal development of specific language functions.     

Paralinguistic processing in children with callosal agenesis: emergence of neurolinguistic deficits

Recent research revealed impaired processing of both nonliteral meaning and affective prosody in adults with agenesis of the corpus callosum (ACC) and normal intelligence. Since normal children have incomplete myelination of the corpus callosum, it was hypothesized that paralanguage deficits in children with ACC would be less apparent relative to their peers. The Familiar and Novel Language Comprehension Test (FANL-C) and Prosody Test were given to 18 children with ACC and normal intelligence and 17 controls matched for age (7-13 years), education, and IQ (83-122). When controlling for age, children with ACC were significantly poorer in comprehension of the precise meaning of both literal and nonliteral items on the FANL-C. Adults with ACC had previously been shown to have difficulty only on nonliteral items. The effect size for nonliteral comprehension in children with ACC was smaller than that seen in adults. There was only a trend for the child ACC group to perform worse on the recognition of affective prosody. Thus, while deficits in paralinguistic processing were apparent, children with ACC were not as clearly different from age peers as adults, and were equally deficient at comprehending literal and nonliteral expressions. The differences in results between adults and children with ACC are thought to reflect incomplete callosal development in normal children, and the importance of the corpus callosum in the early stages of the development of the ability to process literal language.     

Ten pen men: Rhyming skills in two children with callosal agenesis

Cases of callosal agenesis provide unique opportunities to investigate the normal role of the corpus callosum in the development of cognitive functions, including language. The only language impairment which has been consistently observed in three acallosal patients is on the retrieval of words from rhyming cues. Two new cases of callosal agenesis in children of normal intelligence are presented. Their performance on a variety of rhyming tasks involving both production and recognition of rhyme is reported. Both children display deficits and possible explanations are discussed.     

Homotopic callosal inhibition

Previous theories concerning the function of the corpus callosum have included a “topographic” column-to-column excitatory theory and a diffuse “regional” inhibitory theory. Here it is shown that a topographic inhibitory model, in conjunction with a postulate concerning surround inhibition among cortical columns, produces complementary patterns of cortical column firing which, although qualitatively different, do not imply quantitative metabolic asymmetries. The cognitive implications of such homotopic inhibition are discussed.     

Plasticity of the callosal system

A series of experiments examined the potential plasticity of the callosal system in both epileptic patients and in kittens submitted to corpus callosotomy at various ages. The patients were tested for unilateral discrimination and interhemispheric transfer of tactile information. The youngest patient was also required to perform additional inter- and intrahemispheric comparisons of visual and tactile stimuli. The animals were tested for interhemispheric transfer of visual discriminations. The results suggest that in both animals and humans there exists a critical period before which callosal section does not disrupt interhemispheric communication. The results also indicate that the compensatory mechanisms used to achieve interhemispheric transfer in the absence of the corpus callosum may vary according to the sensory modality involved. The possible physiological and/or functional mechanisms responsible for callosal plasticity are discussed.     

Anton's syndrome following callosal disconnection

Anosognosia for cortical blindness, also called Anton's syndrome, is a rare neurological disorder usually following bilateral lesions to occipital cortices. Neuropsychological, morphological and functional neuroimaging (SPECT and fMRI) findings are reported in a patient who incurred Anton's syndrome after an ischaemic lesion confined to the left occipital lobe involving the corpus callosum. The present case study suggests that Anton's syndrome may also follow from lesions disconnecting the occipital cortices.     

Behavioural indexes of callosal functioning in Williams syndrome

Williams syndrome (WS) is a neurogenetic disorder that stems from a microdeletion on chromosome 7. Recent anatomical studies have found evidence for corpus callosum abnormalities in WS. However, to date, the impact of these structural differences on callosal functionality remains unclear. The aim of the present study was to investigate interhemispheric communication and hemispheric asymmetry in individuals with WS relative to mental age‐matched controls. This was assessed using bilateral and unilateral presentations of visual stimuli in a picture‐naming task. Results found both groups to exhibit a bilateral field advantage and a left visual advantage on unilateral presentations. However, while a significant performance increase with age was found for controls, no such correlation was found for individuals with WS. Taken together, these findings suggest that despite some evidence for an atypical developmental pathway in WS, both interhemispheric communication and hemispheric asymmetry are functionally intact in this population.     

The role of callosal connections in speech prosody

A 39-year-old right-handed woman suffered an aneurysmal hemorrhage damaging the anterior four-fifths of the corpus callosum as shown on MRI. Computer-aided acoustical analyses of fundamental frequency (Fo) contours and durational patterns were performed on emotive and nonemotive utterances at 4 weeks, 4 months, and 1 year postsurgery. The patient read sentences in each of five tones (happy, sad, angry, neutral, questioning) or with emphasis on certain words. She showed little Fo distinction with intended mood at 4 weeks, but her performance improved over time. This improvement in speech production was accompanied by an improvement in perceptual judgments of her intended tone by six normal listeners. Fo patterns characteristic of emphatic stress and question forms were found at all test periods, but again improved with time. Durationally, the patient showed appropriate emotive and nonemotive distinctions on most sentences. These results provide acoustic evidence that interhemispheric connections via the corpus callosum are important to proper Fo programming, especially emotive distinctions. The results suggest that the right hemisphere contributes to Fo programming but, following callosal damage, such programming can later be performed by the left hemisphere.     

A choice reaction time index of callosal anatomical homotopy

Tachistoscopically presented bilateral stimulus pairs not parallel to the meridian produced significantly longer RTs on a task requiring discrimination of shapes (Go/no-Go) than pairs emplaced symmetrically on each side of the meridian in Desjardins and Braun [Desjardins, S., & Braun, C. M. J. (2006). Homotopy and heterotopy and the bilateral field advantage in the Dimond paradigm. Acta Psychologica, 121,  125–136]. This was explained by the fact that there are more homotopic than heterotopic fibers in the corpus callosum. However: (1) different parts of the visual field were not equiprobably stimulated, possibly causing subtle biases, (2) the predicted cost of vertical asymmetry was tested only with bilateral stimuli, and (3) interstimulus distance was at the outer limit of callosal midline fusion (10.6°). Here, a tachistoscopic experiment with 24 normal participants replicated the between-field vertical symmetry advantage [Desjardins, S., & Braun, C. M. J. (2006). Homotopy and heterotopy and the bilateral field advantage in the Dimond paradigm. Acta Psychologica, 121, 125–136.], but without irrelevant stimulation conditions and with more proximal stimuli. In addition, a significant specific cost of vertical asymmetry of 7 ms was found for between-field integration over within-field integration. As far as we know, this is the first demonstration of an effect of callosal anatomical homotopy with reaction time.     

Effects of callosal lesions in a model of letter perception

During cognitive tasks, the cerebral hemispheres cooperate, compete, and in general, interact via the corpus callosum. Although behavioral studies in normal and split-brain subjects have revealed a great deal about the transcallosal exchange of information, a fundamental question remains unanswered and controversial: Are transcallosal interhemispheric influences primarily excitatory or inhibitory? In this context, we examined the effects of simulating sectioning of the corpus callosum in a computational model of visual letter recognition. Differences were found, following simulated callosal sectioning, in the performance of each individual hemisphere, in the mean activation levels of hemispheres, and in the specific patterns of activity, depending on the nature of the callosal influences. Together with other recent computational modeling results, the findings are most consistent with the hypothesis that transcallosal influences are predominantly excitatory, and they suggest measures that could be examined in future experimental studies to help resolve this issue.     

Individual differences in callosal efficiency: correlation with attention

Previous studies of clinical populations and normal children have suggested that the efficiency of callosal transfer correlates with the ability to sustain attention. The purpose of the present study was to determine whether the same might be true for normal adults. Subjects were 42 right-handed adults. The efficiency of the transfer via the anterior callosum was assessed on a bimanual coordination task. The efficiency of the posterior callosum was measured on a tachistoscopic task that required subjects to compare two lines when both were presented either to the same visual field or to opposite visual fields. Sustained attention was measured on a vigilance task in which the time between target presentations (ISI) was varied. Performance on the bimanual task correlated with the ability to sustain attention over the entire 20 min of the vigilance task. The efficiency of the posterior callosum was related to the ability to detect targets that occurred after relatively long ISIs.     

Sentence comprehension following agenesis of the corpus callosum

The sentence comprehension skills of a 6-year-old girl with callosal agenesis were compared to skills of three other children matched for age and verbal IQ. Sentence-picture matching and acting out tasks were used with reversible active, passive, and center-embedded relative clause sentences. The acallosal subject showed a deficit in syntactic comprehension. The difficulty was due to a failure to assign correct semantic roles to some sentence forms, not to a lack of ability to discriminate among the sentence forms. The data are consistent with a previous report by M. Dennis (1977, In Topics in child neurology, pp. 189-212) that in acallosal subjects syntactic comprehension can be adversely affected. Because this acallosal subject is only 6 years old, follow-up studies will be needed to determine whether she eventually acquires normal syntactic skills.     

The association between stress, hemispheric specialization, and callosal interactions

This study examines the effects of stress on hemispheric specialization and callosal transfer. Previous research suggests that the two cerebral hemispheres are asymmetric in their regulation of and response to stress. Further, studies have suggested that the activity of the corpus callosum may also be affected by stress. A group of 28 participants completed the Spielberger State-Trait Anxiety Inventory and performed a bilateral Stroop task. The bilateral Stroop task contains separate conditions which measure both hemispheric specialization and callosal transfer. A significant correlation was found between state anxiety and our measure of callosal transfer. Specifically, those individuals with greater state anxiety experience more efficient callosal transfer than did those with lower state anxiety. We conclude that state anxiety enhances callosal transfer without affecting hemispheric specialization.     

A critical review of congenital phantom limb cases and a developmental theory for the basis of body image

Reports of phantom limbs amongst aplasics (i.e., subjects who congenitally lack one or several limbs) have often been presented as evidence that body image is 'hard-wired' in the brain and that neither sensory input nor proprioceptive feedback are essential to its formation. Although attempts have been made to account for these phantoms by other means, these have been on a case by case basis and no satisfactory alternative framework has been proposed. This paper collates the accounts of aplasic phantoms and presents them as compatible with a four-part hypothesis, in which body image is learnt from experience during both pre- and post-natal development, and in which cross-cortical connections and mirror neurons play prominent roles. This model unites several previously disparate theories to offer a viable solution to several longstanding phantom limb mysteries and serves to highlight avenues worthy of further inquiry.     

Initial right hemisphere activation of subordinate word meanings is not due to homotopic callosal inhibition

Previous results (Burgess & Simpson, 1988a) have suggested that subordinate meanings are activated in the right hemisphere only when they have been inhibited in the left hemisphere. Such findings are consistent with a homotopic callosal inhibition view of hemispheric interaction (Cook, 1986). The current study employed prime-target stimulus onset asynchronies intermediate to those used by Burgess and Simpson and obtained equivalent priming of subordinate meanings over visual fields. These data rule out homotopic callosal inhibition as the mechanism responsible for initial activation of subordinate meanings in the right hemisphere and challenge homotopic inhibition as a general mechanism of interhemispheric interaction.     

Interhemispheric cooperation and non-cooperation during word recognition: evidence for callosal transfer dysfunction in dyslexic adults

Participants report briefly-presented words more accurately when two copies are presented, one in the left visual field (LVF) and another in the right visual field (RVF), than when only a single copy is presented. This effect is known as the 'redundant bilateral advantage' and has been interpreted as evidence for interhemispheric cooperation. We investigated the redundant bilateral advantage in dyslexic adults and matched controls as a means of assessing communication between the hemispheres in dyslexia. Consistent with previous research, normal adult readers in Experiment 1 showed significantly higher accuracy on a word report task when identical word stimuli were presented bilaterally, compared to unilateral RVF or LVF presentation. Dyslexics, however, did not show the bilateral advantage. In Experiment 2, words were presented above fixation, below fixation or in both positions. In this experiment both dyslexics and controls benefited from the redundant presentation. Experiment 3 combined whole words in one visual field with word fragments in the other visual field (the initial and final letters separated by spaces). Controls showed a bilateral advantage but dyslexics did not. In Experiments 1 and 3, the dyslexics showed significantly lower accuracy for LVF trials than controls, but the groups did not differ for RVF trials. The findings suggest that dyslexics have a problem of interhemispheric integration and not a general problem of processing two lexical inputs simultaneously.     

Characterizing congenital amusia

The ability to make sense of the music in our environment involves sophisticated cognitive mechanisms that, for most people, are acquired effortlessly and in early life. A special population of individuals, with a disorder termed congenital amusia, report lifelong difficulties in this regard. Exploring the nature of this developmental disorder provides a window onto the cognitive architecture of typical musical processing, as well as allowing a study of the relationship between processing of music and other domains, such as language. The present article considers findings concerning pitch discrimination, pitch memory, contour processing, experiential aspects of music listening in amusia, and emerging evidence concerning the neurobiology of the disorder. A simplified model of melodic processing is outlined, and possible loci of the cognitive deficit are discussed.     

Laminar-dependent dendritic spine alterations in the motor cortex of adult rats following callosal transection and forced forelimb use

Previously, the authors found that partial denervation of the motor cortex in adult animals can enhance this region's neuronal growth response to relevant behavioral change. Rats with partial corpus callosum transections that were forced to rely on one forelimb for 18 days had increased dendritic arborization of layer V pyramidal neurons in the opposite motor cortex compared to controls. This was not found as a result of denervation alone or of forced forelimb use alone. However, it seemed possible that each independent manipulation (i.e., forced forelimb use alone and callosal transections alone) resulted in neural structural alterations that were simply not revealed in measurements of dendritic branch number and/or not inclusive of layer V dendrites. This possibility was assessed in the current study with a reexamination of the Golgi-Cox impregnated tissue generated in the previous study. Tissue was quantified from rats that received either partial transections of the rostral two-thirds of the corpus callosum (CCX) or sham operations (Sham) followed either by 18 days of forced use of one forelimb (Use) or unrestricted use of both forelimbs (Cont). Measurements of apical and basilar dendrites from pyramidal neurons of layer II/III and layer V were performed to detect spine addition resulting from either increased spine density or the addition of dendritic material. As hypothesized, significant spine addition was found following forced forelimb use alone (Sham+Use) and callosal transections alone (CCX+Cont). However, forced use primarily increased spines on layer II/III pyramidal neurons, whereas callosal transections primarily increased dendritic spines on layer V pyramidal neurons in comparison to Sham+Cont. A much more robust increase in layer V dendritic spines was found in animals with the combination of forced forelimb use and denervation (CCX+Use). In contrast to the effects of forced use alone, however, CCX+Use rats failed to show major net increases in spines on layer II/III neurons. These results indicate that while callosal denervation may greatly enhance the neuronal growth and synaptogenic response to behavioral change in layer V, it may also limit spine addition associated with forced forelimb use in layer II/III of the motor cortex.