Perceptual and response bias in visuospatial neglect due to frontal and parietal repetitive transcranial magnetic stimulation in normal subjects

Recently some authors have challenged the conventional association of directional motor neglect with damage of frontal structures, showing that pure sensory perceptual neglect (classically associated with parietal lesion) can follow damage of right frontal cortex. The aim of the present study was to assess the type of defect in visuo-spatial attention consequent upon a virtual frontal or parietal lesion induced by transcranial magnetic stimulation in normal subjects. To this purpose eleven subjects performed a visuospatial task requiring judgement about the length of the two segments of asymmetrically bisected horizontal lines, presented for 50 ms on a computer monitor. After each visual stimulus, subjects made a binary forced choice decision according to two different response conditions: A and B. In condition A, they had to name (right or left) the longer segment and in B the shorter segment of the line. The task was given in baseline condition and during repetitive transcranial magnetic stimulation. Trains of 10 stimuli at 25 Hz of frequency were applied over right frontal premotor and right posterior parietal areas, synchronously with visual stimuli. Parietal and frontal magnetic stimulation gave rise to significant perceptual bias as compared to baseline performance (i.e. subjects made opposite errors in the two response conditions). No significant response bias (i.e. the tendency to name the same side of the line in the two response conditions) was induced by magnetic stimulation on parietal and frontal sites. The present study highlights both the relevant contribution of frontal cortex in the determinism of neglect and the predominant role of sensory perceptual factors in parietal and frontal neglect.