[Effect of task difficulty on hemispheric differences in visual image processing].

Hemispheric specialization of visual imagery processes is a topic of recurrent debate. Recent theories have argued for a left postcentral center for the generation of visual images. Because of contradictory results the process of imaging has been divided into a left hemisphere component (visual imagery) and a right hemisphere component (supramodal spatial mental imagery). An alternative and more parsimonious account would be that the degree of difficulty of visual imagery tasks is responsible for hemispheric differences: easy tasks are solved faster within the left hemisphere because of a direct access to action control, whereas more difficult tasks rely on a right hemisphere specialization for spatial processing. In our experiment we varied the degree of difficulty by introducing concurrent tasks which were interspersed at random into the series of critical stimuli. Interspersing concurrent tasks prevents subjects from automatizing of critical processes and delays reaction times. For the critical visual imagery tasks we presented either upper or lower case letters to the different visual fields and asked for figural aspects of the corresponding lower or upper case letters, respectively. The interspersed stimuli concerned an estimation of the distance of the stimuli or the localization of the stimuli in the upper or lower part of the screen. We also presented upper and lower case letters in random order, thus forcing the subjects to shift between two different visual imagery tasks. In order to look for the effect of position-repetition priming upon visual field advantages we also analyzed the interaction of the position on the screen and its repetition with hemispheric advantages. In one of the visual imagery tasks, the experiments without concurrent tasks yielded a right visual field/left hemisphere advantage. On the other hand, interspersing other tasks resulted in a left visual field/right hemisphere advantage for visual imagery tasks, though none of the concurrent tasks showed a significant visual field advantage in itself. The same holds for combined and random presentation of the two different visual imagery tasks. The results therefore argue for a right hemisphere advantage for difficult visual imagery tasks, whereas easier tasks (monotonous presentation of only one visual imagery tasks) are processed faster in the left hemisphere. The random repetition of positions on the screen yielded a significant effect but had no influence on the visual field advantages found.