Hemispheric preference in visuospatial processing: a complementary approach with fMRI and lesion studies

Abstract

Historically, the left cerebral hemisphere has been considered specialized for language, whereas the right cerebral hemisphere is aligned with spatial processes. However, studies have called into question adherence to this model and suggested that both hemispheres participate in language and spatial cognition. Using functional Magnetic Resonance Imaging (fMRI) and human brain lesion studies, we determined whether these complementary techniques could clarify issues of hemispheric dominance. Using a modified Benton Judgement of Line Orientation (JLO) test, considered a relatively pure spatial processing task, we found robust and significant (p < 0.0005) bilateral superior parietal lobe activation on fMRI in ten right-handed male adult volunteers. This was corroborated by lesion data in a cohort of 17 patients who showed significant JLO impairments after either right or left parietal lobe damage, with right parietal damage associated with somewhat more severe deficit. Detailed wavelet analysis of the fMRI time-series did, however, reveal a more dominant role of the right parietal lobe in "kick-starting" the task. To our knowledge, this is a novel way of using fMRI to address functional hemispheric differences in a cognitive task that is known to have bilateral representation.

Figure 1

a, b: a pictorial representation of Benton's judgment of line orientation task.

Figure 2

Voxels coloured red represent brain areas activated by the “ON” condition, and those coloured yellow by the “OFF” condition. These generic brain activation maps (GBAM) are superimposed on a conventional 3D volumetric spoiled GRASS MR image—the distances in mm below (−) and above (+) the intercommisural (AC–PC) plane are indicated by the numbers beneath each image. Radiological convention is adopted, where the left side of the image corresponds to the right side of the brain.

Figure 3

Superior parietal cortical activation is shown on a three‐dimensional surface rendered brain.

Figure 4

Parietal activation time series plotted against normalised image intensity at the frequency of maximum response (i.e., at four times the fundamental frequency). The temporal variation in response was evaluated over a range of frequencies from 8 times to one‐half the frequencies of alternation at the ON/OFF conditions, reducing the sampling frequencies by ⁴√2 at each step. In the bottom half of the picture, orange corresponds to the right side, and purple to the left side. The difference between the right and left is shown in purple in the upper half of the picture.