Interference resolution in the left inferior frontal gyrus.

Abstract

Prior functional neuroimaging work has suggested a posterior portion of the left inferior frontal gyrus (IFG) is related to interference resolution across multiple task domains. This thesis explores this discrete region within the left IFG as a possible candidate for a fundamental processing component in cognitive control. Brain activity was examined using functional magnetic resonance imaging on a verbal working memory task. On some trials, interference arose because of a non-matching probe having a high familiarity. On other trials, additional response-related conflict arose because of a non-matching probe having been a matching probe on the immediately preceding trial. Consistent with previous work, interference due to high familiarity was associated with left inferior prefrontal activation. The response-conflict condition, when compared with high-familiarity trials, was associated with anterior cingulate cortex activation, but with no additional left prefrontal activation. This dissociation points to differing contributions of specific cortical areas to cognitive control, which are based on the source of conflict. Subjects who completed both the verbal working memory task and a verb generation task designed to engage selection mechanisms showed left inferior frontal activity on high interference trials on both tasks. The patterns of left frontal activity in each task indicate that the recruited area of left IFG was in fact the same across task domains. Finally, senior subjects showed a pattern of frontal bilaterality in response to interference compared to young subjects. Senior subjects who had greater difficulty with interference showed increased activation related to interference, while better performing seniors showed reduced activation related to interference. However, analysis of the time course of activations revealed that the reduced activations observed in good seniors were primarily due to increased activation on non-interference trials, not because of decreased activity in trials with familiar probes. The work presented in this thesis provides evidence for the neural mapping of a fundamental component of cognitive control. The process appears to work to select or bias conceptual representations in response to interference, but activity in this region is also proactively modulated by more general cognitive factors.