An fMRI Study of Response and Semantic Conflict in the Stroop Task

Abstract

An enduring question in selective attention research is whether we can successfully ignore an irrelevant stimulus and at what point in the stream of processing we are able to select the appropriate source of information. Using methods informed by recent research on the varieties of conflict in the Stroop task the present study provides evidence for specialized functions of regions of the frontoparietal network in processing response and semantic conflict during Stroop task performance. Specifically, we used trial types and orthogonal contrasts thought to better independently measure response and semantic conflict and we presented the trial types in pure blocks to maximize response conflict and therefore better distinguish between the conflict types. Our data indicate that the left inferior PFC plays an important role in the processing of both response and semantic (or stimulus) conflict, whilst regions of the left parietal cortex (BA40) play an accompanying role in response, but not semantic, conflict processing. Moreover, our study reports a role for the right mediodorsal thalamus in processing semantic, but not response, conflict. In none of our comparisons did we observe activity in the anterior cingulate cortex (ACC), a finding we ascribe to the use of blocked trial type presentation and one that has implications for theories of ACC function.

Keywords: Stroop; Stroop 2-1 mapping; fMRI; response conflict; selective attention; semantic conflict; task conflict.

FIGURE 1

Trial types employed and comparisons made in the experiment to enable the indexing of the different conflict types in the Stroop task. Having two separate trial types that permit the measurement of semantic conflict without response conflict (Semantic-associative and Non-response set trials) meant that our measurements of response conflict [Incongruent (response set) – Semantic-associative] and semantic conflict (Non-response set and Neutral word trials) were orthogonal to each other.

FIGURE 2

Functional magnetic resonance imaging activation elicited by: (A) semantic conflict indexed using a non-response set – neutral words contrast. (B) Response conflict indexed using an incongruent – semantic associates contrast. (C) Stroop interference indexed using an incongruent – neutral words contrast. Activation color represents t-values.