Functional anatomy of spatial mental imagery generated from verbal instructions

Abstract

Positron emission tomography (PET) was used to monitor regional cerebral blood flow variations while subjects were constructing mental images of objects made of three-dimensional cube assemblies from auditorily presented instructions. This spatial mental imagery task was contrasted with both passive listening (LIST) of phonetically matched nonspatial word lists and a silent rest (REST) condition. All three tasks were performed in total darkness. Mental construction (CONS) specifically activated a bilateral occipitoparietal-frontal network, including the superior occipital cortex, the inferior parietal cortex, and the premotor cortex. The right inferior temporal cortex also was activated specifically during this condition, and no activation of the primary visual areas was observed. Bilateral superior and middle temporal cortex activations were common to CONS and LIST tasks when both were compared with the REST condition. These results provide evidence that the so-called dorsal route known to process visuospatial features can be recruited by auditory verbal stimuli. They also confirm previous reports indicating that some mental imagery tasks may not involve any significant participation of early visual areas.

Fig. 1.

Example of one of the series of four three-dimensional cube assemblies (adapted from Shepard and Metzler, 1971) that the subjects had to build mentally during the PET acquisition in the CONS condition. Thirty seconds before the¹⁵O-labeled water injection, the subjects were asked to visualize a starting cube at the center of their field of view (gray on the figure) and to add cubes according to a list of 11 directional words binaurally delivered by earphones at 0.5 Hz. For example, the first cube assembly of the figure corresponded to the list: right, down, down, back, back, back, up, up, back, back, right.

Fig. 2.

Post-PET session matching task. Although the subjects were not explicitly instructed to memorize the mental objects they had just built, they were invited in the minutes after the CONS task to identify, among four different series, the one they had just built (series 2 in this figure). The strategy used by the subjects to perform this matching task (verbal description or matching a mental image of the object) was investigated systematically and taken as an indirect indication that mental objects had actually been created during the task.

Fig. 3.

SPMs corresponding to the CONS versus REST condition comparison (top), to the CONS versus LIST condition comparison (center), and to the LIST versus REST condition comparison (bottom). Z volumes are projected in three orthogonal directions, sagittal, coronal, and transverse, and reach threshold at Z₀ = 3.1 (p < 0.001; not corrected for multiple comparisons). Stereotactic coordinates of local maxima within the activated areas are given in Tables 1–3.

Fig. 4.

Reanalysis with SPMs of a previous PET activation study on mental spatial exploration (Mellet et al., 1995), as compared with the REST condition. The Z volume is projected in three orthogonal directions, sagittal, coronal, and transverse, and reaches threshold at Z₀ = 3.1 (p < 0.001; not corrected for multiple comparisons). Stereotactic coordinates of local maxima of the activated areas are given in Table 5.