Left, but not right, rostrolateral prefrontal cortex meets a stringent test of the relational integration hypothesis

Abstract

Much of what is known about the function of human rostrolateral prefrontal cortex (RLPFC; lateral Brodmann area 10) has been pieced together from functional magnetic resonance imaging (fMRI) studies over the past decade. Christoff and colleagues previously reported on an fMRI localizer task involving relational integration that reliably engages RLPFC in individual participants (Smith, R., Keramatian, K., and Christoff, K. (2007). Localizing the rostrolateral prefrontal cortex at the individual level. NeuroImage, 36(4), 1387-1396). Here, we report on a modified version of this task that better controls for lower-level processing demands in the relational integration condition. Using identical stimulus arrays for our experimental and control conditions, we find that right RLPFC is sensitive to increasing relational processing demands, without being engaged specifically during relational integration. By contrast, left RLPFC is engaged only when participants must consider the higher-order relationship between two individual relations. We argue that the integration of disparate mental relations by left RLPFC is a fundamental process that supports higher-level cognition in humans.

Figure 1

In our version of the relational matching task, each condition involved a yes/no judgment based on a stimulus array including four shapes with varying textures. On Shape trials, participants indicated whether there was at least one shape match present in any of the stimuli pairs. On Texture trials, participants indicated whether there was at least one texture match present in any of the stimuli pairs. On Dimension trials, participants indicated whether the stimuli in the bottom pair matched along the same dimension as the stimuli in the top pair. Shape and Texture trials required low-level relational processing, whereas Dimension trials required relational integration.

Figure 2

fMRI results for our relational matching task (N=15). A. A whole-brain comparison of Dimension versus Feature tasks (p < .005 uncorrected, >10 voxels) revealed that left RLPFC was relatively more engaged by Dimension trials, whereas rostromedial PFC was relatively more engaged by Feature trials. At a liberal statistical threshold (p < .01 uncorrected), right RLPFC was also engaged by Dimension relative to Feature trials. Shown in B. and C. are plots of RLPFC parameter estimates for each condition relative to the resting baseline. The x-axis features the mean RT for each condition; Shape RTs were faster than Texture RTs, which in turn were numerically but not statistically faster than Dimension RTs. Left RLPFC was specifically engaged when relational integration was required, whereas right RLPFC was more generally engaged as a function of relational task demands.