Functional magnetic resonance imaging of visual object construction and shape discrimination: Relations among task, hemispheric lateralization, and gender

We studied the brain activation patterns in two visual image processing tas ks requiring judgements on object construction (FIT task) or object samenes s (SAME task). Fight right-handed healthy human subjects (four women and lo ur men) performed the two tasks in a randomized block design while 5-mm, mu ltislice functional images of the whole brain were acquired using a 4-tesla system using blood oxygenation dependent (BOLD) activation. Pairs of objec ts were picked randomly from a set of 25 oriented fragments of a square and presented to the subjects approximately every 5 sec. In the FIT task, subj ects had to indicate, by pushing one of two buttons, whether the two fragme nts could match to form a perfect square, whereas in the SAME task they had to decide whether they were the same or not. In a control task. preceding and following each of the two tasks above. a single square was presented at the same rate and subjects pushed any of the two keys at random. Functiona l activation maps were constructed based on a combination of conservative c riteria. The areas with activated pixels were identified using Talairach co ordinates and anatomical landmarks? and the number of activated pixels was determined for each area. Altogether, 379 pixels were activated. The counts of activated pixels did not differ significantly between the two tasks or between the two genders. However, there were significantly more activated p ixels in the left (n = 218) than the right side of the brain (n = 161). Of the 379 activated pixels, 371 were located in the cerebral cortex. The T alairach coordinates of these pixels were analyzed with respect to their ov erall distribution in the two tasks. These distributions differed significa ntly between the two tasks. With respect to individual dimensions. the two tasks differed significantly in the anterior-posterior and superior-inferio r distributions but not in the left-right (including mediolateral, within t he left or right side) distribution. Specifically, the FIT distribution was , overall, more anterior and inferior than that of the SAME task. A detailed analysis of the counts and spatial distributions of activated pi xels was carried out for 15 brain areas (all in the cerebral cortex) in whi ch a consistent activation (in greater than or equal to 3 subjects) was obs erved (n = 323 activated pixels). We found the following. Except for the in ferior temporal gyrus, which was activated exclusively in the FIT task, all other areas showed activation in both tasks but to different extents, Base d on the extent of activation, areas fell within two distinct groups (FIT o r SAME) depending on which pixel count (i.e., FIT or SAME) was greater. The FIT group consisted of the following areas, in decreasing FIT SAME order ( brackets indicate ties): GTi, GTs, GC, GFi, GFd, \GTm, GF\, GO. The SAME gr oup consisted of the following areas, in decreasing SAME/FIT order: GOi, LP s, Sea, GPrC, GPoC, \GFs, GFm\. These results indicate that there are distr ibuted, graded. and partially overlapping patterns of activation during per formance of the two tasks. We attribute these overlapping patterns of activ ation to the engagement of partially shared processes. Activated pixels clustered to three types of clusters: FIT-only (111 pixels ,. SAME-only (97 pixels), and FIT + SAME (115 pixels). Pixels contained in FIT-only and SAME-only clusters n ere distributed approximately equally bet ween the left and right hemispheres, whereas pixels in the SAME + FIT clust ers were located mostly in the left hemisphere. With respect to gender, the left-right distribution of activated pixels was ver) similar in women and men for the SAME-only and FIT + SAME clusters but differed for the FIT-only case in which there was a prominent left side preponderance for women, in contrast to a rig[lt aide preponderance for men, We conclude that (a) corti cal mechanisms common For processing visual object construction anti discri mination involve mostly the left hemisphere, (b) cortical mechanisms specif ic for these tasks engage both hemispheres, and (c) in object construction only, men engage predominantly the right hemisphere whereas women show a le ft-hemisphere preponderance.