Interference control in working memory : neurobehavioral properties and age differences

Abstract

This doctoral thesis aimed to investigate the basic processes of interference control (IC) in working memory (WM). We sought to unravel some of the underlying mechanisms of IC by examining brain correlates, while also trying to understand the differences that arise with advancing adult age. The major findings from these studies are summarized below.

Study I. This study consisted of four separate experiments, with different participants in each. Three different versions of the recent-probes task was used to test the effects of a high- compared to a low-interference context on behavioral and neural measures. A combined analysis of all experiments showed that a high-interference context improved performance, both reaction times and accuracy, on the other trial types within the same task. Neuroimaging results revealed greater engagement of inferior frontal gyrus, striatum, parietal cortex, hippocampus, and midbrain in participants performing the task in the high- than participants in the low-interference context. Study II. This study compared groups of older and younger adults to examine age effects of interference control. A verbal 2-back task including proximal and distant lures revealed that older adults were more affected by interference overall, for both proximal and distant lures. Whereas younger adults overcame and were no longer affected by interference beyond 5-/6-back lures, older adults were still highly affected at the most distant 9-/10-back lures. Study III. This study expands on study II by examining brain activity during successful interference control, using a similar task setup. Proactive interference and brain activity in bilateral inferior frontal gyrus and dorsal anterior cingulate cortex declined with increasing lure distance. This decline in relevant brain regions was coupled with an increase in brain activity in left anterior hippocampus. In addition, each successfully resolved lure trial was divided into two groups, those that influenced performance negatively and those that did not. A whole brain analysis showed greater brain activity in bilateral inferior frontal gyrus, bilateral middle frontal gyrus, and dorsal anterior cingulate gyrus for the lure trials that took longer time to respond to accurately. Study IV. This study used four-year follow-up data from the Betula study to examine whether prefrontal brain volumes can be linked to the ability to control interference. Individuals were separated with respect to their ability to control interference. We observed that participants with superior interference control had larger volume of the ventrolateral area of prefrontal cortex (PFC), regardless of participant demographics. Change in volume over a 4-year period could not be linked to change in interference control.

Conclusions. The data in this doctoral thesis contribute with new insight into IC in WM. Data suggests that positive task-scale adjustments, due to conflict, is beyond trial-to-trial adaptation, affecting both behavioral and biological aspects of cognition. Improved performance on non-conflict trials could be explained by an upregulation of resources to cope with the high interference environment. Moreover, a characteristic of the general age-related IC deficit was uniquely demonstrated in older adult’s inability to cope with some of the temporal properties of PI, being negatively affected by the most distant lure trials. Results suggest two distinct means for resolving interference in WM. One involves top-down controlled processes for actively resolving lure items within the focus of attention, and the other involves hippocampus related retrieval of source information from outside the focus of attention. The latter is arguably more difficult for older adults due to the strain on hippocampus, and links well with our own results. Results also suggests two distinct levels of influence by interference in WM, which is either being unaffected by PI or being affected negatively by PI, suggesting an ability that can completely overcome PI. Lastly, these results provide new evidence that a relative IC score can be related to volume of specific and relevant regions within PFC, and that this relationship is not modulated by age. This supports a view that grey matter volume in PFC regions play a specific role in overcoming interference during a WM task.