The ability of patients with brain damage to generate mental visual images

The ability to form mental visual images was investigated in normal controls, patients with localized brain damage, and patients with Parkinson's disease. Verbal and visuospatial imagery tasks were compared with tasks in the same modality that did not demand the formation of visual images. In the verbal part, memorizing of word pairs was tested for abstract words, concrete words without imagery instruction, and concrete words with imagery instruction. In the visuospatial part, three tests required the evaluation of spatial features of imagined letters. In one of them mental rotations had to be performed in the exploration of the imagined stimulus. The visuospatial control tasks tested the ability to judge the position of perceived dots and mentally to rotate a seen mannekin. The pattern of results was determined mainly by the differential sensitivity of visuospatial and verbal tasks to right and left brain damage, respectively. Multidimensional scaling of correlations between test results did not show a close relationship between the results of verbal and visuospatial imagery tasks, but suggested that brain damage affected visuospatial imagery tasks differentially from visuospatial control tasks. Analysis of single cases showed that patients with lesions of the left temporo-occipital region failed to benefit from the imagery instruction in the memorizing of concrete words. These patients could nonetheless solve correctly the visuospatial imagery tasks. Possibly this dissociation is due to differences in the amount or the quality of information conveyed by the visual images rather than to the verbal or visuospatial mode of processing to which the images are subjected.