Regular ArticleA Parametric Study of Prefrontal Cortex Involvement in Human Working Memory
Abstract
Although recent neuroimaging studies suggest that prefrontal cortex (PFC) is involved in working memory (WM), the relationship between PFC activity and memory load has not yet been well-described in humans. Here we use functional magnetic resonance imaging (fMRI) to probe PFC activity during a sequential letter task in which memory load was varied in an incremental fashion. In all nine subjects studied, dorsolateral and left inferior regions of PFC were identified that exhibited a linear relationship between activity and WM load. Furthermore, these same regions were independently identified through direct correlations of the fMRI signal with a behavioral measure that indexes WM function during task performance. A second experiment, using whole-brain imaging techniques, both replicated these findings and identified additional brain regions showing a linear relationship with load, suggesting a distributed circuit that participates with PFC in subserving WM. Taken together, these results provide a “dose–response curve” describing the involvement of both PFC and related brain regions in WM function, and highlight the benefits of using graded, parametric designs in neuroimaging research.
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Cognitive control processes, including those involving frontoparietal networks, are highly variable between individuals, posing challenges to basic and clinical sciences. While distinct frontoparietal networks have been associated with specific cognitive control functions such as switching, inhibition, and working memory updating functions, there have been few basic tests of the role of these networks at the individual level.
To examine the role of cognitive control at the individual level, we conducted a within-subject excitatory transcranial magnetic stimulation (TMS) study in 19 healthy individuals that targeted intrinsic (“resting”) frontoparietal networks. Person-specific intrinsic networks were identified with resting state functional magnetic resonance imaging scans to determine TMS targets. The participants performed three cognitive control tasks: an adapted Navon figure-ground task (requiring set switching), n-back (working memory), and Stroop color-word (inhibition).
Hypothesis: We predicted that stimulating a network associated with externally oriented control [the “FPCN-B” (fronto-parietal control network)] would improve performance on the set switching and working memory task relative to a network associated with attention (the Dorsal Attention Network, DAN) and cranial vertex in a full within-subjects crossover design.
We found that set switching performance was enhanced by FPCN-B stimulation along with some evidence of enhancement in the higher-demand n-back conditions.
Higher task demands or proactive control might be a distinguishing role of the FPCN-B, and personalized intrinsic network targeting is feasible in TMS designs.
Replay of incidentally encoded episodic memories in the rat
2024, Current BiologyAlthough events are not always known to be important when they occur, people can remember details about such incidentally encoded information using episodic memory. Importantly, when information is explicitly encoded for use in an expected test of retention (as in most assessments in animals), it is possible that it is used to generate a planned action1,2,3; thus, the remembered action can occur without remembering the earlier episode. By contrast, when a test is unexpected, transforming information into an action plan is unlikely because the importance of the information and the nature of the test are not yet known. Thus, accurate performance in an unexpected test after incidental encoding documents episodic memory.1,2,3,4,5,6,7,8 Here, we present evidence that rats replay episodic memories of incidentally encoded information in an unexpected assessment of memory. In one task,9 rats reported the third-last item in an explicitly encoded list of trial-unique odors. In a second task,10 rats foraged in a radial maze in the absence of odors. On a critical test, rats foraged in the radial maze, but scented lids covered the food. Next, memory of the third-last odor was assessed. All participating rats correctly answered the unexpected question. These results suggest that rats encoded multiple pieces of putatively unimportant information, and later they replayed a stream of episodic memories when that information was needed to solve an unexpected problem. We propose that rats replay episodic memories of incidentally encoded information, which documents a critical aspect of human episodic memory in a non-human animal.
Lateral Prefrontal Stimulation of Active Cortex With Theta Burst Transcranial Magnetic Stimulation Affects Subsequent Engagement of the Frontoparietal Network
2024, Biological Psychiatry: Cognitive Neuroscience and NeuroimagingA critical unanswered question about therapeutic transcranial magnetic stimulation is what patients should do during treatment to optimize its effectiveness. Here, we address this lack of knowledge in healthy participants, testing the hypotheses that stimulating the left dorsolateral prefrontal cortex (dlPFC) while participants perform a working memory task will provide stronger effects on subsequent activation, perfusion, connectivity, and performance than stimulating resting dlPFC.
After a baseline functional magnetic resonance imaging session to localize dlPFC activation and the associated frontoparietal network (FPN) engaged by an n-back task, healthy participants (N = 40, 67.5% female) underwent 3 counterbalanced sessions, separated by several weeks, during which they received intermittent theta burst stimulation (iTBS) followed by magnetic resonance imaging scans as follows: 1) iTBS to the dlPFC while resting passively (passive), 2) iTBS to the dlPFC while performing the n-back task (active), and 3) iTBS to a vertex site, while not engaged in the n-back task and resting passively (control).
We found no difference in n-back performance between the 3 conditions. However, FPN activation was reduced while performing the n-back task in the active condition relative to the passive and control conditions. There was no differential activity in the FPN on comparing passive with control conditions, i.e., there was no effect of the site of stimulation. We found no effects of state or site of stimulation on perfusion or connectivity with the dlPFC.
In this study, the state of the brain while receiving iTBS affected FPN activation, possibly reflecting greater efficiency of FPN network activation when participants were stimulated while engaging the FPN.
How a pilot's brain copes with stress and mental load? Insights from the executive control network
2024, Behavioural Brain ResearchIn aviation, mental workload and stress are two major factors that can considerably impact a pilot’s flight performance and decisions. Their consequences can be even more dramatic in single-pilot aircraft or with the forthcoming single-pilot operations where the pilot will fly alone and will not be able to be assisted in case of difficulty. An accurate and automatic monitoring of the pilot’s mental state could help to prevent the potentially dangerous effects of an excess mental workload and stress. For example, some tasks could be allocated to automation or to a ground-based flight crew if a mental overload or significant stress is detected. In the current study, the brain activity of 20 private pilots was recorded with a fNIRS device during two realistic flight simulator scenarios. The mental workload was manipulated with the added difficulty of a secondary task and stress was induced by a social stressor. Our results confirmed the sensitivity of the fNIRS readings to variations in the mental workload, with increased HbO2 concentration in regions of the executive control network (ECN), in particular in the dorsolateral prefrontal cortex and in lateral parietal regions, when the difficulty of the secondary task was high. The social stressor also triggered an HbO2 increase in the ECN, especially when it was combined with high mental workload. This latter result suggests that mental workload and stress together can have cumulative effects, and coping with both factors is possible at the expense of an extra recruitment of the ECN. Finally, results also revealed a time-on-task effect, with a progressive reduction of the HbO2 signal in the ECN during the flight scenario, suggesting that these regions are sensitive to short term habituation to the tasks. Overall, fNIRS efficiently indexed mental load, stress, and practice effects.
Effective connectivity analysis of verbal working memory advantage across materials for pathological smartphone users by fNIRS
2023, Psychiatry Research - NeuroimagingPrevious studies have found working memory (WM) advantages of the pathological smartphone use (PSU) group, but most of which were emphasized in the network-related domain. Whether the advantages can transfer to other domains has yet to be confirmed. In particular, exploring from a brain mechanism perspective is necessary. Using the classical N-back paradigm, this study selected network-related words and neutral words as materials combined with fNIRS to probe the verbal WM characteristics of the PSU group. The results showed that β in channel 3, channel 4, and channel 5 were significantly lower in the PSU group than those in the control group The analysis of the region of interest revealed that the PSU group showed significantly lower β in the l-DLPFC and frontopolar. Granger Causality results showed that functional connectivity between frontopolar and R-DLPFC for the PSU group was significantly higher than for the control group in the network word condition. These results demonstrate that the PSU group has an advantage in WM, transferring from the network-related stimulus to the neutral stimulus. The advantages of network stimulus were related to bidirectional connectivity between frontopolar and R-DLPFC. Also, the l-DLPFC and frontopolar are associated with the cross-material consistency of WM.
Effects of current and past depressive episodes on behavioral performance and subjective experience during an N-back task
2023, Journal of Behavior Therapy and Experimental PsychiatryDepression impairs working memory (WM). And, while many studies have documented impairment in WM during depression remission, those using the N-back task did not find differences between individuals with remitted depression and healthy controls. One reason for these findings may be that certain depression phenotypes, such as the childhood-onset form, which is likely to be associated with persistent WM problems, are underrepresented or unevenly represented in the studies. Because childhood-onset depression (COD) affects individuals while cognitive development is still ongoing, it is more likely to have lasting detrimental effects, as evidenced in residual memory impairment, than depression that onsets later in life. Further, it is unclear if depression episodes have cumulative effects on WM when measured via the N-back.
We examined the effects of depression on WM performance (response time, accuracy, signal detection d’) and subjective experience (difficulty, mental effort required) during a four-level N-back task among 112 adults with COD (42 currently depressed; 70 remitted depressed) and 80 never-depressed controls.
Compared to never-depressed controls, there was minimal evidence of impaired WM performance among participants with remitted or current depression; the groups also reported overall similar subjective experiences during the N-back. Notably, number of lifetime depressive episodes had a detrimental cumulative effect on response accuracy and d’.
WM was assessed only in regard to verbal memory. The sample size of currently depressed cases was smaller than that of the other groups.
WM remains largely intact among adults with remitted COD, but increased number of depression episodes worsens WM performance.