Age-related reorganization of functional networks for successful conflict resolution: a combined functional and structural MRI study

Abstract

Aging has readily observable effects on the ability to resolve conflict between competing stimulus attributes that are likely related to selective structural and functional brain changes. To identify age-related differences in neural circuits subserving conflict processing, we combined structural and functional MRI and a Stroop Match-to-Sample task involving perceptual cueing and repetition to modulate resources in healthy young and older adults. In our Stroop Match-to-Sample task, older adults handled conflict by activating a frontoparietal attention system more than young adults and engaged a visuomotor network more than young adults when processing repetitive conflict and when processing conflict following valid perceptual cueing. By contrast, young adults activated frontal regions more than older adults when processing conflict with perceptual cueing. These differential activation patterns were not correlated with regional gray matter volume despite smaller volumes in older than young adults. Given comparable performance in speed and accuracy of responding between both groups, these data suggest that successful aging is associated with functional reorganization of neural systems to accommodate functionally increasing task demands on perceptual and attentional operations.

Figure 1

Top: Stroop Match-to-Sample design, illustrating 4 conditions: incongruent-match, congruent-match, incongruent-nonmatch, and congruent-nonmatch. A color cue (XXXX) presented for 450ms was followed by an incongruent or congruent Stroop target stimulus that appeared for 1100ms after an inter-stimulus interval (ISI) of 300ms. The inter-trial interval (ITI) was 1450 ms. Subjects matched the color (red, green or blue) of the cue to the ink color of the Stroop stimulus. Bottom: fMRI block design illustrated for 8 blocks. Each block consisted of 6 trials (9 TRs). Each block lasted for of 19.8 sec. Stroop stimuli in each block were either congruent (word BLUE written in blue font) or incongruent (word BLUE written in red font). In half of the blocks cue-target color either matched or did not match, in the other half of the block match and nonmatch trials were mixed. In total 36 blocks were presented in pseudo-random order ensuring that each condition was equally often represented.

Figure 2. Stroop interference and condition repetition

Brain regions demonstrating age differences in Stroop-related activation in mixed response blocks (upper panel) and same response blocks (lower panel). Regions in red reflect greater blood oxygen level-dependent (BOLD) response in older than young adults, and regions in green reflect greater BOLD response in young than older adults. The threshold has been lowered to P < 0.005 uncorrected for display purpose. Bar graphs on the right are showing mean BOLD signal intensity differences extracted using MarsBaR (http://marsbar.sourceforge.net/) during Stroop (incongruent (INC) minus congruent (CON)) performance for older and young adults for regions showing group differences significant at P < 0.001 uncorrected (Table 2).

Figure 3. Stroop interference and perceptual cueing

Brain regions demonstrating age differences in Stroop-related activation with matching color cues. The threshold has been lowered to P < 0.005 uncorrected for display purpose. Regions in red reflect greater BOLD response in older than young adults, and regions in green reflect greater BOLD response in young than older adults (left image). Bar graphs are showing mean BOLD signal intensity differences extracted using MarsBaR (http://marsbar.sourceforge.net/) during Stroop-match (INC-CON) performance for older and young adults for regions showing group differences significant at P < 0.001 uncorrected (Table 3).

Figure 4. Response conflict, incongruent and congruent information

Brain regions demonstrating age differences in response conflict-related activation (incongruent nonmatch – match) (upper panel) and non-conflict matching-related activation (congruent nonmatch – match) (lower panel). Regions in red reflect greater BOLD response in older than young adults, and regions in green reflect greater BOLD response in young than older adults. The threshold has been lowered to P < 0.005 uncorrected for display purpose. Bar graphs on the right are showing mean BOLD signal intensity differences extracted using MarsBaR (http://marsbar.sourceforge.net/) during cue-target color matching (nonmatch – match) for incongruent (upper panel) and congruent Stroop target stimuli (lower panel) in older and young adults for regions showing group differences significant at P < 0.001 uncorrected (Table 4).

Figure 5

VBM-detected gray matter volume differences (older < young adults) superimposed on the study-specific normalized mean group image from all subjects and presented in the sagittal, axial and coronal plane (T = 6.68, P_{FWE corrected} = 0.025) (Good et al., 2001). The color bar represents the t scores.