The role of the left head of caudate in suppressing irrelevant words

Abstract

Suppressing irrelevant words is essential to successful speech production and is expected to involve general control mechanisms that reduce interference from task-unrelated processing. To investigate the neural mechanisms that suppress visual word interference, we used fMRI and a Stroop task, using a block design with an event-related analysis. Participants indicated with a finger press whether a visual stimulus was colored pink or blue. The stimulus was either the written word "BLUE," the written word "PINK," or a string of four Xs, with word interference introduced when the meaning of the word and its color were "incongruent" (e.g., BLUE in pink hue) relative to congruent (e.g., BLUE in blue) or neutral (e.g., XXXX in pink). The participants also made color decisions in the presence of spatial interference rather than word interference (i.e., the Simon task). By blocking incongruent, congruent, and neutral trials, we identified activation related to the mechanisms that suppress interference as that which was greater at the end relative to the start of incongruency. This highlighted the role of the left head of caudate in the control of word interference but not spatial interference. The response in the left head of caudate contrasted to bilateral inferior frontal activation that was greater at the start than at the end of incongruency, and to the dorsal anterior cingulate gyrus which responded to a change in the motor response. Our study therefore provides novel insights into the role of the left head of caudate in the mechanisms that suppress word interference.

Figure 1. Predictions for activation related to conflict and its control during incongruent trials

Top shows expectation that activation related to conflict and control will (a) increase at the start of a run of incongruent trials and (b) decrease at the end of a run of incongruent trials. C = run of congruent trials. I = run of incongruent trials. Lower two rows show the differing expectations for conflict and control. The expectation is that conflict will be higher at the start of a run of incongruent trials than the end of a run of congruent trials, i.e., (a) will be greater than (b). In contrast, control is expected to be higher at the end of a run than the beginning of the run, i.e., (b) will be greater than (a). The shape of these effects across an incongruent run could either be gradual (middle row) or primarily occur on the second run of a trial and then plateau for the rest of the run (lower row). The changing effect of control during a congruent or neutral run could also be gradual or stepwise, however, our experimental design and analysis do not allow us to determine how activation related to control changed over trials of the same type.

Figure 2. Examples of stimulus runs, trials and types

This figure needs to be viewed in color to appreciate the congruency and incongruency between the words and their color. The columns provided the following information: Run number with 4 to 6 stimuli; Trial number within the run; Stimulus on the screen; Response = left for blue hue and right for pink hue; Change = change in hue (+= yes there is a change); Type of condition; Switch between conditions: nC = event where trial type switched from neutral (n) to congruent (C). cI = event where trial type switched from congruent (c) to incongruent (I). iN = event where trial type switched from incongruent (i) to neutral (N), nI = event where trial type switched from neutral (n) to incongruent (I), iC = event where trial type switched from incongruent (i) to congruent (C) and cN = event where trial type switched from congruent (c) to neutral (N). Upper case = the event, lower case = the previous trial type. Conflict change occurs at the onset or offset of incongruent trials. At the onset of incongruent tirals, activation related to conflict and control increases. At the offset of incongruent trials, activation related to conflict and control decreases.

Figure 3. Illustration of Stroop and Simon paradigms

The sequence of events for each trial involved fixation for 400ms, followed by a blank screen for 100ms, the stimulus for 630ms, another blank screen for 310ms.

Figure 4. In-scanner behavioral data

Reaction times (RT) in milliseconds (ms) and accuracy (in %) for congruent (open squares) and incongruent (triangles) trials across each of the 6 serial positions in a run of one type of trial (see Figure 2). For further details on means and standard errors, see Appendix.

Figure 5. Activation change for controlling interference

Top (a): The results from fMRI Analysis 2 that compared trials where there was a change in condition. Red and yellow indicate the location of activation related to the control of interference (i.e. higher at the end of a run of Incongruent trials). The red area was involved in the control of interference in both the Stroop and the Simon tasks (p<0.01). The yellow area was involved in the control of interference in the Stroop task more than the Simon task. White indicates the location of activation related to response change during both the Stroop and Simon task ( p<0.001). The axial slices are positioned at z = +6 (left) and z = −2 (right). The sagittal slices are positioned at x= −4 (left side of figure) and x= −16 (right side of figure). Middle (b): Relative parameter estimates for activation change from one condition to another in the left head of caudate (at x=−18; y=+18; z= 8). C = congruent, I = Incongruent, N= Neutral, c = in the context of congruent, i = in the context of incongruent, n = in the context of neutral. R = Response change. Bars on the mean of each estimate represent 90% confidence intervals. The scale is identical for both Stroop and Simon and the effects were extracted from the same analysis (Analysis 2). The feature to note is the decrease in activation for iC, as predicted for control in Figure 1. We propose that the reason that this decrease was not observed for iN is there is no urgency to release word inhibition mechanisms when a neutral trial is presented but there is when a congruent trial is presented because word inhibition is potentially disadvantageous. Lower (c) : The results from fMRI Analysis 3 showing activation (parameter estimates) in the left head of caudate for sustained incongruency relative to sustained neutral trials (black) and sustained congruency relative to sustained neutral trials (MNI co-ordinates: x= −14 y=+8 z=+12) Inc. = parameter estimates for incongruent sustained trials > neutral sustained trials. Con. = parameter estimates for congruent sustained trials > neutral sustained trials. Black bars = Stroop task (first 2 columns), White bars = Simon task (second two columns). The lines on each bar represent the standard error across subjects in the parameter estimate. The right hand image shows the caudate activation on a sagittal slice positioned at x=−14 (in MNI co-ordinates).

Figure 6. Activation change for the onset of interference

Top: Inferior frontal activation (in white) for the onset of incongruency rendered on the SPM canonical brain. Left hemisphere shown on left and right hemisphere shown on right Lower: The relative parameter estimates for each switch condition (Analysis 2) at the location of the effects shown directly above in the top row (i.e., left inferior frontal cortex on left and right inferior frontal cortex on right). C = congruent, I = Incongruent, N= Neutral, c = in the context of congruent, i = in the context of incongruent, n = in the context of neutral. R = Response change. Lines on each bar represent 90% confidence intervals. The scale is identical for both Stroop and Simon tasks.