Behavioural and neurophysiological correlates of dyslexia in the continuous performance task.

OBJECTIVE: To investigate whether attentional difficulties are a "core" feature of developmental Dyslexia. METHODS: Behavioural indices and event related potentials (ERPs) were recorded from 10 dyslexic participants (ages 15.5-17.4) and 10 control participants (ages 14.4-18.3) in the Continuous Performance Task (CPT), an established test of attentional performance. Participants were screened to ensure that none was diagnosable as attention deficit (ADHD). RESULTS: There were no significant differences in mean reaction time, error rate or sustained attention between the groups. By contrast, the P3 amplitude was significantly smaller and its latency significantly longer for the dyslexic group. This component was significantly lateralised in controls, whereas in dyslexics it was symmetrical. CONCLUSIONS: Under the relatively light workload conditions of the CPT, "pure" dyslexic participants showed no behavioural signs of attentional difficulties. The attenuated, delayed and symmetrical ERPs in our dyslexic group may reflect abnormal information processing in the right parietal lobe and abnormal interhemispheric asymmetry in Dyslexia. SIGNIFICANCE: The behavioural data suggest that abnormal attentional performance is not a "core" feature of developmental Dyslexia, and highlight the importance of distinguishing between dyslexic participants with and without ADHD symptoms. The presence of electrophysiological markers of Dyslexia in CPT revealed the atypical brain organisation that characterises "pure" Dyslexia.

Effect of signal length on the performance of independent component analysis when extracting the lambda wave.

The aim of the study was to investigate the effect of signal length on the performance of a signal source separation method, independent component analysis (ICA), when extracting the visual evoked potential (EP) lambda wave from saccade-related electro-encephalogram (EEG) waveforms. A method was devised that enabled the effective length of the recorded EEG traces to be increased prior to processing by ICA. This involved abutting EEG traces from an appropriate number of successive trials (a trial was a set of waveforms recorded from 64 electrode locations in a study investigating saccade performance). ICA was applied to the saccade-related EEG and electro-oculogram (EOG) waveforms recorded from the electrode locations. One spatial and five temporal features of the lambda wave were monitored to assess the performance of ICA applied to both abutted and non-abutted waveforms. ICA applied to abutted trials managed to extract all six features across all seven subjects included in the study. This was not the case when ICA was applied to the non-abutted trials. It was quantitatively demonstrated that the process of abutting EEG waveforms was useful for ICA preprocessing when extracting lambda waves.

Exploring sequential stereopsis for co-planarity tasks.

We used the sequential stereopsis paradigm and apparatus described by Enright (Vision Research, 36, (1996) 307-312). The observer's task was to set targets to equidistance in Experiments 1-3, and to make them co-planar in Experiment 4. However, it is argued that in all experiments observers exploited a co-planarity setting strategy. Sequential stereopsis produced good performance throughout in terms of low disparity thresholds when head position was varied by rotations around three axes: vertical (azimuth condition); horizontal (elevation); and midline (tilt). It also produced good performance when the targets were shifted in position so that they both lay on one side of the median plane of the head. These results cannot be accounted for by Enright's isovergence hypothesis unless it is extended to incorporate other information about eye positions. Performance was better but not greatly so in control simultaneous stereopsis conditions, nor did it deteriorate much when the observer's view was restricted solely to the targets by removing visibility of the room in which the apparatus was located. Target settings were typically located on a concave arc centred on the median plane. This effect was quantitatively modelled using disparity correction for a relief task of co-planarity (Gârding, Porrill, Mayhew, & Frisby. Vision Research, 35 (1995) 703-722). This modelling indicated over-estimations of c.10-20 cm in fixation distance for target distances in the range 71.5-112.5 cm.