Robin Laycock

Using transcranial magnetic stimulation to investigate the cortical origins of motor overflow: a study in schizophrenia and healthy controls

BACKGROUND Previous research has confirmed the presence of increased motor overflow in schizophrenia. There are essentially two theories regarding the cortical origins of overflow. Recent research suggests that both may be correct, and that the cortical origin of overflow is highly dependent upon the population in which it presents. Motor overflow, due to an abnormally active ipsilateral corticospinal tract, may indicate a potentially severe brain abnormality arising in early development. In contrast, bilaterally active corticospinal tracts accounting for overflow probably represent a naturally occurring response during fatiguing contractions. METHOD The cortical origins of motor overflow in 20 participants with schizophrenia and 20 normal controls were investigated through the use of a number of transcranial magnetic stimulation (TMS) protocols. RESULTS Each of the experimental protocols employed independently supported the contention that overflow was originating in the hemisphere contralateral to the involuntary movement. CONCLUSIONS Results indicated that the origins of overflow in schizophrenia are the same as those seen in the normal control group, i.e. motor overflow seems to be due to the presence of bilaterally active corticospinal tracts. Potential explanations for greater motor overflow seen in schizophrenia are discussed.

A comparative study of the effects of repetitive paired transcranial magnetic stimulation on motor cortical excitability

OBJECTIVES Various methods of application of repetitive transcranial magnetic stimulation (TMS) have been evaluated for their potential capacity to alter motor cortical excitability. Initial research suggests that the repetitive application of paired TMS pulses (repetitive paired pulse TMS (rppTMS)) may have greater effects on cortical excitability, perhaps through the facilitation of I-wave interaction. We aimed to compare the post-train effects of 15 min trains of rppTMS to investigate the potential therapeutic application of this technique as well as to compare it to a standard high frequency repetitive TMS paradigm. METHODS Ten normal subjects received three 15 min sessions of rppTMS, 5 Hz high frequency rTMS and sham TMS in randomised order. rppTMS consisted of a single train of 180 pulse pairs (0.2 Hz, 1.5 ms inter-stimulus interval, supra-threshold intensity) administered over 15 min. The rTMS condition involved 750 pulses provided in 5s 5 Hz trains with a 25s inter-train interval at 90% of the RMT. Motor evoked potential size and cortical silent period duration were assessed before and after each session. RESULTS There were no significant changes in cortical excitability produced by any of the stimulation conditions. Five hertz rTMS produced an increase in cortical silent period duration (p=0.004) which was not affected by rppTMS. CONCLUSIONS Fifteen minutes trains of 1.5ms rppTMS do not substantially increase post train cortical excitability. Repetitive brief trains of 5Hz rTMS also do not alter excitability but appear to effect cortical inhibition.

Parietal function in good and poor readers

BackgroundWhile there are many psychophysical reports of impaired magnocellular pathway function in developmental dyslexia (DD), few have investigated parietal function, the major projection of this pathway, in good and poor readers closely matched for nonverbal intelligence. In view of new feedforward-feedback theories of visual processing, impaired magnocellular function raises the question of whether all visually-driven functions or only those associated with parietal cortex functions are equally impaired and if so, whether parietal performance is more closely related to general ability levels than reading ability.MethodsReading accuracy and performance on psychophysical tasks purported to selectively activate parietal cortex such as motion sensitivity, attentional tracking, and spatial localization was compared in 17 children with DD, 16 younger reading-age matched (RA) control children, and 46 good readers of similar chronological-age (CA) divided into CA-HighIQ and a CA-LowIQ matched to DD group nonverbal IQ.ResultsIn the age-matched groups no significant differences were found between DD and CA controls on any of the tasks relating to parietal function, although performance of the DD group and their nonverbal IQ scores was always lower. As expected, CA and RA group comparisons indicated purported parietal functioning improves with age. No difference in performance was seen on any of the parietally driven tasks between the DD and age-nonverbal IQ matched groups, whereas performance differentiated the DD group from the age-matched, higher nonverbal IQ group on several such tasks. An unexpected statistical difference in performance between lower reading age (DD and RA children) and all higher reading age (CA) children was seen on a test of chromatic sensitivity, whereas when high and low nonverbal IQ normal readers were compared performance was not differentConclusionThe results indicate that performance on purported parietal functions improves with age and may be more associated with nonverbal mentation than reading accuracy. Performance on a cognitively demanding task, traditionally considered to rely on ventral stream functions, was more related to reading accuracy.

A transcranial magnetic stimulation study of transcallosal inhibition and facilitation in schizophrenia.

Deficits in cortical inhibition (CI) have been implicated in the pathophysiology of schizophrenia, including decreased transcallosal inhibition (TCI). A closely related phenomenon, which has not yet been studied in schizophrenia, is transcallosal facilitation (TCF). TCI and TCF are thought to maintain a complimentary existence, allowing for the performance of tasks such as unilateral voluntary movement. Therefore, deficient TCI may lead to abnormal expression of TCF. This study aims to confirm the presence of TCI deficits in schizophrenia, and to examine TCF. Thirty consenting participants took part in the study (15 with schizophrenia and 15 healthy controls), although not all were able to complete all aspects of the study. TCI and TCF were measured using dual-pulse transcranial magnetic stimulation methodologies. Patients with schizophrenia exhibited significantly less TCI than controls; there was no difference in TCF, however. The lack of significant TCF findings is discussed in light of the methodological limitations, while the theoretical significance of deficient TCI to the pathophysiology of schizophrenia is considered.

Dorsal stream involvement in recognition of objects with transient onset but not with ramped onset

BackgroundAlthough the ventral visual stream is understood to be responsible for object recognition, it has been proposed that the dorsal stream may contribute to object recognition by rapidly activating parietal attention mechanisms, prior to ventral stream object processing.MethodsTo investigate the relative contribution of the dorsal visual stream to object recognition a group of tertiary students were divided into good and poor motion coherence groups and assessed on tasks classically assumed to rely on ventral stream processing. Participants were required to identify simple line drawings in two tasks, one where objects were presented abruptly for 50 ms followed by a white-noise mask, the other where contrast was linearly ramped on and off over 325 ms and replaced with a mask.ResultsAlthough both groups only differed in motion coherence performance (a dorsal stream measure), the good motion coherence group showed superior contrast sensitivity for object recognition on the abrupt, but not the ramped presentation tasks.ConclusionsWe propose that abrupt presentation of objects activated attention mechanisms fed by the dorsal stream, whereas the ramped presentation had reduced transience and thus did not activate dorsal attention mechanisms as well. The results suggest that rapid dorsal stream activation may be required to assist with ventral stream object processing.

Abrupt and ramped flicker-defined form shows evidence for a large magnocellular impairment in dyslexia

Controversy still exists over whether there is a magnocellular deficit associated with developmental dyslexia. Here we utilised a magnocellular system-biased phantom contour form discrimination task defined by high temporal frequency contrast reversals to compare contrast sensitivity in a group of children with dyslexia and an age- and nonverbal intelligence-matched control group (9-14 years). Stimuli were either abruptly presented for 4 refresh frames (34 ms), or in two reduced transience conditions had contrast progressively ramped on and off over either 4 frames or 10 frames (86 ms). Children in the dyslexia group showed increased contrast thresholds compared with the control group in all three conditions, and thus strong evidence for a magnocellular deficit. Although the absolute size of the differences in threshold scores between control and dyslexic groups increased dramatically between the abrupt and the 4 and 10 frame ramped onset stimuli, the similar effect size across all tasks, and also the similar range of contrast change at the first frame of stimulus presentation across all tasks between groups suggests that a similar neural mechanism could provide the locus of the apparent magnocellular deficit in children with dyslexia for all tasks tested. These results suggest that threshold discrimination of stimuli with low contrast and high temporal frequencies designed to target the magnocellular system, and has great potential for early screening for children at risk of visually derived reading difficulties.

A Combined rTMS and ERP Investigation of Dorsolateral Prefrontal Cortex Involvement in Response Inhibition

The stop signal task is used to investigate inhibition of an initiated response. Converging evidence suggests that right inferior prefrontal cortex is involved in this behavior, although other regions in the prefrontal cortex have also been implicated. One technique used to determine the contribution of specific cortical regions to behavior is repetitive transcranial magnetic stimulation (rTMS). In the present study, fourteen subjects performed the stop signal task before and after receiving a train of rTMS to the left and right dorsolateral prefrontal cortex (DLPFC). The effects of rTMS were determined using event-related potential (ERP) measures that have been associated with response inhibition in previous studies. Stimulation of left and right DLPFC did not affect ERP measures of response inhibition. This negative finding is interpreted with caution, but is consistent with a recent study which found that stimulation of the same region had no effect on a behavioral measure of response inhibition.

Mapping of the Underlying Neural Mechanisms of Maintenance and Manipulation in Visuo-Spatial Working Memory Using An n-back Mental Rotation Task: A Functional Magnetic Resonance Imaging Study

Mapping of the underlying neural mechanisms of visuo-spatial working memory (WM) has been shown to consistently elicit activity in right hemisphere dominant fronto-parietal networks. However to date, the bulk of neuroimaging literature has focused largely on the maintenance aspect of visuo-spatial WM, with a scarcity of research into the aspects of WM involving manipulation of information. Thus, this study aimed to compare maintenance-only with maintenance and manipulation of visuo-spatial stimuli (3D cube shapes) utilizing a 1-back task while functional magnetic resonance imaging (fMRI) scans were acquired. Sixteen healthy participants (9 women, M = 23.94 years, SD = 2.49) were required to perform the 1-back task with or without mentally rotating the shapes 90° on a vertical axis. When no rotation was required (maintenance-only condition), a right hemispheric lateralization was revealed across fronto-parietal areas. However, when the task involved maintaining and manipulating the same stimuli through 90° rotation, activation was primarily seen in the bilateral parietal lobe and left fusiform gyrus. The findings confirm that the well-established right lateralized fronto-parietal networks are likely to underlie simple maintenance of visuo-spatial stimuli. The results also suggest that the added demand of manipulation of information maintained online appears to require further neural recruitment of functionally related areas. In particular mental rotation of visuospatial stimuli required bilateral parietal areas, and the left fusiform gyrus potentially to maintain a categorical or object representation. It can be concluded that WM is a complex neural process involving the interaction of an increasingly large network.

Self-Rated Social Skills Predict Visual Perception: Impairments in Object Discrimination Requiring Transient Attention Associated with High Autistic Tendency

Autism is usually defined by impairments in the social domain but has also been linked to deficient dorsal visual stream processing. However, inconsistent findings make the nature of this relationship unclear and thus, we examined the role of stimulus‐driven transient attention, presumably activated by the dorsal stream in autistic tendency. Contrast thresholds for object discrimination were compared between groups with high and low self‐rated autistic tendency utilizing the socially based Autism Spectrum Quotient (AQ). Visual stimuli were presented with either abrupt or with ramped contrast onsets/offsets in order to manipulate the demands of transient attention. Larger impairments in performance of abrupt compared with ramped object presentation were established in the high AQ group. Furthermore, self‐reported social skills predicted abrupt task performance, suggesting an important visual perception deficiency in autism‐related traits. Autism spectrum disorder may be associated with reduced utilization of the dorsal stream to rapidly activate attention prior to ventral stream processing when stimuli are transient. Autism Res 2014, 7: 104‐111.

Neural Markers Associated with the Temporal Deployment of Attention: A Systematic Review of Non-motor Psychophysical Measures Post-stroke

In recent years, evidence has emerged to suggest abnormal temporal dynamics of attentional processing in stroke patients, especially those presenting with neglect symptoms. However, there has been little profiling of the nature and extent of such temporal anomalies. In addition, many paradigms currently used to measure the time required to deploy visual attention in stroke require a psychomotor response, and may therefore confound performance outcomes. Thus, the aim of this systematic review was to identify and evaluate studies that have employed non-motor psychophysical paradigms to characterize the temporal deployment of visual attention in space. A total of 13 non-motor psychophysical studies were identified, in which stimulus exposure times were manipulated to measure the time course of attentional deployment. Findings suggest that prolonged attentional deployment thresholds are more likely to occur with lesions within more ventral areas of the fronto-parietal network, irrespective of whether patients presented with neglect. Furthermore, this deficit was greater following right-hemispheric lesions, suggesting a dominant role for the right-hemisphere in facilitating efficient deployment of attention. These findings indicate that area and hemisphere of lesion may serve as putative markers of attentional deployment efficiency. In addition, findings also provide support for using non-motor psychophysical paradigms as a more rigorous approach to measuring and understanding the temporal dynamics of attention.