A study of three cases of familial related agenesis of the corpus callosum

Nonverbal learning disability

Nonverbal learning disability (NLD) is described as a subtype of specific learning disability where the source of the disability is a difficulty in processing nonverbal information. The child with NLD presents with problems in visual, spatial, and tactile perception but with strengths in rote verbal skills. Traditionally, these children were recognized by their difficulties in arithmetic which presented a stark contrast with their strengths in spelling and decoding text. They also exhibited a split between their verbal IQ (VIQ) and performance IQ (PIQ) scores with the VIQ being significantly higher than PIQ. Over time, however, diagnostic criteria have evolved and the broadened definition of the NLD syndrome has led many to question the utility and uniqueness of the NLD diagnosis. In addition, shifting diagnostic standards have made research results difficult to replicate. In short, the research to date leaves many unanswered questions about (1) the definition of the NLD syndrome, (2) the pervasiveness of the academic, social and psychopathological difficulties, (3) the source of the NLD syndrome, and (4) the degree to which it overlaps with other conditions. This chapter outlines a brief history of the NLD syndrome, how it is currently conceptualized, and some of the current debate about the unanswered questions above.

Short-term and long-term memory deficits in handedness learning in mice with absent corpus callosum and reduced hippocampal commissure

The corpus callosum (CC) and hippocampal commissure (HC) are major interhemispheric connections whose role in brain function and behaviors is fascinating and contentious. Paw preference of laboratory mice is a genetically regulated, adaptive behavior, continuously shaped by training and learning. We studied variation with training in paw-preference in mice of the 9XCA/WahBid ('9XCA') recombinant inbred strain, selected for complete absence of the CC and severely reduced HC. We measured sequences of paw choices in 9XCA mice in two training sessions in unbiased test chambers, separated by one-week. We compared them with sequences of paw choices in model non-learner mice that have random unbiased paw choices and with those of C57BL/6JBid ('C57BL/6J') mice that have normal interhemispheric connections and learn a paw preference. Positive autocorrelation between successive paw choices during each session and change in paw-preference bias between sessions indicate that 9XCA mice have weak, but not null, learning skills. We tested the effect of the forebrain commissural defect on paw-preference learning with the independent BTBR T+ tf/J ('BTBR') mouse strain that has a genetically identical, non-complementing commissural trait. BTBR has weak short-term and long-term memory skills, identical to 9XCA. The results provide strong evidence that CC and HC contribute in memory function and formation of paw-preference biases.

Impaired fear memory, altered object memory and modified hippocampal synaptic plasticity in split-brain mice

The hippocampus is critical for memory formation. However, the contributions of the hippocampal commissure (HC) and the corpus callosum (CC) are less clear. To elucidate the role of the forebrain commissures in learning and memory, we performed a behavioural and electrophysiological characterization of an inbred mouse strain that displays agenesis of the CC and congenitally reduced HC (BTBR T+ tf/J; 'BTBR'). Compared to a control strain, BTBR mice have severely impaired contextual fear memory, with normal object recognition memory. Interestingly, continuous environmental "enrichment" significantly increased object recognition in BTBR, but not in control C57BL/6 ('BL/6') mice. In area CA1 of hippocampal slices, BTBR displayed intact expression of long-term potentiation (LTP), paired-pulse facilitation (PPF) and basal synaptic transmission, compared to BL/6 mice. However, BTBR hippocampal slices show an increased susceptibility to depotentiation (DPT), an activity-induced reversal of LTP. We conclude that the HC and CC are critical for some forms of hippocampal memory and for synaptic resistance to DPT. Agenesis of the CC and HC may unmask some latent ability to encode, store or retrieve certain forms of recognition memory. We suggest that the increased susceptibility to DPT in BTBR may underlie the memory phenotype reported here.

Early Visual Evoked Potentials in Callosal Agenesis

Three participants with callosal agenesis and 12 neurologically normal participants were tested on a simple reaction time task, with visual evoked potentials collected using a high-density 128-channel system. Independent-components analyses were performed on the averaged visual evoked potentials to isolate the components of interest. Contrary to previous research with acallosals, evidence of ipsilateral activation was present in all 3 participants. Although ipsilateral visual components were present in all 4 unilateral conditions in the 2 related acallosal participants, in the 3rd, these were present only in the crossed visual field-hand conditions and not in the uncrossed conditions. Suggestions are made as to why these results differ from earlier findings and as to the neural mechanisms facilitating this ipsilateral activation.

Nonverbal learning disability: a tutorial for speech-language pathologists

Nonverbal learning disability (NLD) is a diagnostic category that is unfamiliar to most speech-language pathologists. This brief tutorial describes NLD's characteristics, a theoretical model proposed to explain its source, and areas of overlap between NLD and similar diagnostic categories. The communicative profile, made up of difficulties in pragmatic and semantic language in the presence of relatively preserved syntactic skill, is also discussed. Empirical evidence relevant to NLD is also evaluated. Many questions remain unresolved, but until systematic research provides definitive answers, speech-language pathologists are encouraged to rely on careful description of the individual child's communicative strengths and weaknesses to identify appropriate targets and to focus intervention on improving the child's ability to communicate effectively in everyday contexts.

Hemispheric integration and differences in perception of a line-motion illusion in the divided brain

Five people lacking the corpus callosum (two callosotomized, three with agenesis of the corpus callosum) and neurologically normal subjects were shown vertical lines that appeared instantaneously between pairs of rectangles in one or other visual field. When one of the rectangles flashed prior to the presentation of the line, and the line was in the same visual field, all subjects perceived the line as spreading from the flashed rectangle to the other. Normal subjects and one of the callosotomized subjects showed a slight but significant right visual-field advantage, perhaps reflecting a left-hemispheric superiority in processing rapid temporal events. The illusion was also induced when the line and the flash were in opposite visual fields in one of the callosotomized, one of the acallosal subjects, and about half of the normal subjects, implying interhemispheric integration even in the absence of the corpus callosum.

Permanent or transitory effects on neurocognitive components of the CNV complex induced by brain dysfunctions, lesions and ablations in humans

Since the mid-1960s, essentially using electrophysiological methods, our research group has examined the effects of different brain diseases in humans, both on first- and second-order conditioned responses and on some types of neurocognitive potentials of the CNV complex. This didactic lecture will focus on our various attempts to identify and understand the neuroanatomical and neurophysiological substrates involved in cognitive information processing followed by the conception and execution of sensory-motor and behavioural responses evoked by significant acoustic stimuli, in both pathological situations and normal control subjects. Great interest was, e.g. aroused in the early 1970s by the rare, fortunately unrepeatable, opportunity of examining the CNV patterns in various psychiatric patients treated with psychosurgical Freeman-Watts bilateral prefrontal 'radical' lobotomy, also with repeated recordings (The Responsive Brain (1976) 158; Multidisciplinary Perspectives in Event-Related Brain-Potentials Research (1978) 376) or bimedial bifrontal cingulotomy (Multidisciplinary Perspectives in Event-Related Brain Potential Research (1978) 383). In the same period, investigations into CNV activity recorded in patients submitted to complete callosotomy ('split brain': Attention and Performance, vol. IV (1972) 221; Electroenceph. Clin. Neurophysiol. Suppl. 33 (1973) 161) were also begun and were continued into the 1980s, also with regard to other types of ERP (Brain 111 (1988) 553; J. Cog. Neurosci. 2 (1990) 258). All these data furnished unique information about the sub-second dynamics of unilateral or bihemispheric cortico-cortical and cortico-subcortical interconnections in humans. In recent years, with a classic method of analysis based on sequential scalp-topographic bidimensional neuroelectric mapping and 21/19 electrodes connected to three different references, and binaural/monaural clicks as warning signals (S1), we have repeatedly examined the CNV activity of 11 selected patients submitted to complete ablation of the damaged cortical areas, with uni- or bilateral lesions restricted to the prefrontal or associative parieto-temporal areas. We have always used the standard CNV paradigm (S1-S2 motor-response) which evokes a complex of neurocognitive potentials, including the P300 from S1, which are well-known, since they are certainly among the most studied ERPs in the various ages and races of normal subjects, psychiatric patients and subjects with different brain diseases. The most important results have been, (1) In normal subjects the MRI and the latency differences of CNV component measurements along the bidirectional pathways functionally interconnecting ipsilateral distant associative cortical areas (e.g. the arcuate-superior longitudinal complex bundle) were accounted for by the transcortical conduction time, which varies in our scalp recordings from 1 cm/0.74 to 1.28 ms ( approximately 9.8 m/s). (2) Constantly, no true auditory S1-elicited N1a, b, c, P2, N2, P300 components or CNV slow waves (O- and E-wave) were recordable over the whole of the ablated cortical areas, but only clearly identifiable volume-conducted EP/ERPs generated in other hemispheric structures. (3) The post-S1 ERP/CNV complexes on the intact hemisphere were found to be within the normal limits. (4) Effects of severe disruption on the S1 ERP/CNV complexes evocable on the site and on remote ipsilateral apparently normal anatomo-functionally interconnected brain regions were observed in 5 patients, 4 of whom had extensive frontocortical ablations. In two of the latter the distant disruptive action on the CNV components over the neuroradiologically normal ipsilateral two-way connected post-rolandic sensory and association areas was seen to be partially reversible, showing aspects of a probable slowly evolving diaschisis-like effect. Similar deactivation of some ERP components was observed in reverse on the ipsilateral dorsolateral frontocortical region in the fifth patient with a large parieto-temporal cortex ablation. These data require confirmahese data require confirmation, and when this phenomenon is observable, it must be appropriately monitored with different methods of functional neuroimaging. This will serve not only for medical and neuropsychophysiological diagnosis purposes, but also particularly for a correct and really useful planning of neuro-rehabilitation activities in selected cases.

Redundancy gain in the acallosal brain

The authors measured simple reaction time (RT) to visual stimuli, presented either singly to 1 or the other visual field or in bilaterally presented pairs, to 2 women with callosal agenesis. The stimuli were either white against a black background or gray against an equiluminant yellow background. RTs to bilateral pairs were decreased beyond predictions based on a simple race between independent unilateral processes, implying interhemispheric neural summation. This effect was enhanced under equiluminance in the participant M.M. whose anterior commissure was within normal limits, but not in the participant J.P. whose anterior commissure was enlarged. The anterior commissure may act, relative to its size, to inhibit cortical activation to bilateral pairs, which then acts to decrease subcortical neural summation.

Selective modification of short-term hippocampal synaptic plasticity and impaired memory extinction in mice with a congenitally reduced hippocampal commissure

The hippocampus is critical for forming new long-term memories, but the contributions of the hippocampal commissure (HC) to memory function and hippocampal synaptic plasticity are unclear. To shed light on this issue, we characterized behavioral memory and hippocampal synaptic plasticity in two inbred mouse strains. BALB/cWah1 mice display a range of corpus callosal defects and an intact HC, whereas 9XCA/Wah mice exhibit a complete absence of corpus callosum and a greatly reduced HC. No differences between strains were found in long-term potentiation (LTP) within two synaptic pathways in hippocampal slices. However, paired-pulse facilitation was deficient in area CA1 of slices from 9XCA/Wah, and it was rescued by decreasing extracellular [Ca2+], suggesting that presynaptic calcium dynamics may be altered in this strain. In addition, contextual fear extinction was impaired in 9XCA/Wah mice, but performance on cued fear extinction and on 24 hr memory tests for cued and contextual fear conditioning were not significantly different between strains. Thus, an intact HC is critical for normal extinction of contextual fear. Intact interhemispheric connectivity is not required for acquisition or expression of cued and contextual fear conditioning. LTP was normal in slices from mice that lacked an intact HC, and this was correlated with normal performance on fear conditioning tests. In contrast, impaired short-term synaptic plasticity was correlated with defective contextual memory extinction in mice lacking an intact HC. Thus, the HC in mice is vital for particular aspects of memory function and for short-term synaptic modification in specific hippocampal circuits.