Neural mechanisms underlying spatial judgements on seen and imagined visual stimuli in the left and right hemifields...

We investigated the neural networks involved in spatial judgements on visual and imagined stimuli in the left and right hemifields. Twenty healthy male right-handers were scanned using fMRI. In an adaptation of the Mental Clock Test, subjects judged whether the angle between visually presented or to be imagined clock hands at a given time (e.g. "12:20") was >90 degrees or <90 degrees . The angle formed by the clock hands was always located either on the left or right hemifield of the clock face, constituting a 2x2 factorial design with factors stimulus type (visual "V", imagined "I") and attention to spatial hemifield (left "L", right "R"). The contrast I>V revealed a bilateral neural network which included the superior parietal, prefrontal and antero-dorsal cingulate cortex. While right superior parietal cortex was activated during both RV and LV, left superior parietal cortex was only active during RV. The contrast RI versus LI (and vice versa) did not reveal significant differences with respect to activation height. However, a masking procedure and the calculation of a laterality index showed that RI recruited larger areas in the left than in the right hemisphere, while LI led to symmetrical activations in both hemispheres. Our data thus confirm that there are hemifield-specific asymmetries in brain activity during spatial processing of both visual and imagined stimuli. Our data indicate, however, that these asymmetries are less clear and of a different nature in the latter. Overall, our findings converge with neuropsychological data showing that representational and visuospatial neglect do not always co-occur.