Deficit in switching between functional brain networks underlies the impact of multitasking on working memory in older adults

Abstract

Multitasking negatively influences the retention of information over brief periods of time. This impact of interference on working memory is exacerbated with normal aging. We used functional MRI to investigate the neural basis by which an interruption is more disruptive to working memory performance in older individuals. Younger and older adults engaged in delayed recognition tasks both with and without interruption by a secondary task. Behavioral analysis revealed that working memory performance was more impaired by interruptions in older compared with younger adults. Functional connectivity analyses showed that when interrupted, older adults disengaged from a memory maintenance network and reallocated attentional resources toward the interrupting stimulus in a manner consistent with younger adults. However, unlike younger individuals, older adults failed to both disengage from the interruption and reestablish functional connections associated with the disrupted memory network. These results suggest that multitasking leads to more significant working memory disruption in older adults because of an interruption recovery failure, manifest as a deficient ability to dynamically switch between functional brain networks.

Fig. 1.

Experimental paradigm. Participants performed four blocked and counterbalanced tasks. For interrupting stimuli (IS), distracting stimuli (DS), and no interference (NI), an encode scene was followed by a delay and a probe scene at the end of each trial. For IS and DS, an interfering face stimulus appeared middelay, requiring attention only in IS. For NI, an equal-length delay replaced the interfering face. For passively viewed (PV), participants were told to passively view the scene and face, and the probe consisted of responding to the direction of an arrow.

Fig. 2.

Visual association cortex activity. BOLD response in the FFA shows modulation based on task instructions; that is, activity is highest in the interruptor condition (IS vs. PV and IS vs. DS, P < 0.05). The amount of attention that participants allocate toward an interruptor (IS, enhancement) correlates negatively with their WM performance (younger: R = −0.54, P < 0.05; older: R = −0.6, P < 0.05).

Fig. 3.

MFG functional connectivity with PPA and FFA during WM maintenance. (Upper) In both younger and older adults, PPA connectivity with the MFG is maintained in all tasks in delay 1 (D1). In both NI and DS, connectivity is maintained in the interference period, whereas in IS connectivity declines significantly during interruption. Of note, during the second delay (D2), younger participants exhibit a return of significant connectivity between the MFG and the PPA, whereas the older participants do not. (Lower) FFA connectivity with this MFG region occurs only during the interruption period in both age groups, and remains elevated in the D2 period only in older adults. PPA and FFA regions in the axial slice are from a representative participant. Asterisks signify a significant difference between IS and NI (or DS).