Functional neuroimaging studies of prospective memory: what have we learnt so far?

Encoding of prospective tasks in the human prefrontal cortex under varying task loads

Successful realization of planned actions requires the brain to encode intentions over delays. Previous research has indicated that several regions in the rostral or anterior prefrontal cortex (PFC) encode delayed intentions. However, different processes may encode the same future task depending on task load during the delay. This difference may depend on the computational resources available when the delay is occupied with an ongoing task and when it is task-free. Here we directly investigated and compared the representation of delayed intentions in the human brain in the presence and absence of ongoing task load during the delay. We acquired fMRI data in combination with an event-based prospective memory design where human subjects remembered to perform the same future tasks over occupied and task-free delays. We used time-resolved multivoxel pattern classification and found that: (1) rostrolateral PFC (BA 46) encoded the delayed intention during both delay types; (2) rostromedial PFC (BA 10) encoded the intentions during occupied delays; whereas (3) a variety of more posterior regions, including the anterior cingulate cortex (BA 24), the supplementary motor area (BA 6), and the precuneus, encoded intentions during task-free delays. Overall, the medial PFC encoded delayed intentions more rostrally in the presence of an ongoing delay task and more caudally in its absence. Thus, rostromedial PFC may play a specialized role in the encoding of prospective memory that depends on higher computational demands (e.g., given higher task load during the delay). In contrast, the rostrolateral PFC is a more general area that encodes future intentions regardless of task load.

Episodic future thinking as a predictor of children's prospective memory

The primary goal of this study was to investigate the relationship among retrospective memory, episodic future thinking, and event-based prospective memory performance in preschool, first-grade, and second-grade children. A total of 160 children took part in the experiment. The study included participants from four age groups: 4-year-olds, 5-year-olds, 6-year-olds, and 7-year-olds. Participants were administered a recognition memory task, a task to test the ability to pre-experience future events, and an event-based prospective memory task. Data were submitted to correlational analyses, analyses of variance (ANOVAs), and logistic regression analyses. Results showed that, overall, all of these abilities improve with age and are significantly correlated with one another. However, when partialling out age and retrospective memory, episodic future thinking and prospective memory performance remained correlated. Logistic regression further showed that age and episodic future thinking abilities were significant predictors of prospective memory performance independent of retrospective memory abilities.

Prospective memory impairment and its implications for community living skills in bipolar disorder

OBJECTIVE

Prospective memory (PM) refers to the ability to undertake intended actions in the future. The functional significance of PM in bipolar disorder (BD) has not yet been investigated. This study examined PM impairment and its role in community living skills in clinically stable individuals with BD.

METHODS

Seventy-six individuals with BD and 44 healthy individuals were assessed with the Chinese version of the Cambridge Prospective Memory Test. Socio-demographic characteristics, retrospective memory (RM; the ability to recall or recognize past information), and intelligence were also measured in all participants. The clinical condition and community living skills of patients with BD were rated independently with standardized instruments.

RESULTS

Patients with BD showed PM impairment and their PM total scores independently predicted the level of community living skills in regression analyses. In follow-up analyses, the contribution was found to be accounted for by the time-based PM scores.

CONCLUSIONS

Given the expanding body of research on the utility of PM in predicting functioning, these findings further support the role of PM in community living skills in individuals with BD. The results suggest that PM training might be an integral part of clinical rehabilitation devised for individuals with BD.

How do we process event-based and time-based intentions in the brain? an fMRI study of prospective memory in healthy individuals

Prospective memory (PM) refers to the ability to remember to do something in the future, either in response to an event (event-based) or after a certain amount of time has elapsed (time-based). While the distinction between event- and time-based PM is widely acknowledged in the literature, little is known about the processes they share and those they do not. This is particularly true concerning their brain substrates, as almost all neuroimaging studies so far have focused on event-based PM. We proposed a functional magnetic resonance imaging paradigm assessing both event-based and time-based PM to 20 healthy young individuals. Analyses revealed that event- and time-based PM both induced activation in the posterior frontal and parietal cortices, and deactivation in the medial rostral prefrontal cortex. In addition, activation more specific to each condition, which may underlie differences in strategic monitoring, was highlighted. Thus, occipital areas were more activated during event-based PM, probably reflecting target-checking, while a network comprising the dorsolateral prefrontal cortex, the cuneus/precuneus and, to a lesser extent, the inferior parietal lobule, superior temporal gyrus, and the cerebellum, was more activated in time-based PM, which may reflect the involvement of time-estimation processes. These results confirm the allocation of attentional resources to the maintenance of intention for event-based and time-based PM, as well as the engagement of distinct mechanisms reflecting the monitoring strategies specific to each condition.

Prospective memory in Parkinson's disease: a meta-analysis

Prospective memory (PM) refers to the ability to remember to carry out an intended action in the future and it is pervasive in our daily living. A failure to execute an intended action (e.g., take medication) at the appropriate juncture in future (e.g., after dinner) can negatively affect our daily functioning and at times, may have devastating effects (e.g., forgetting to turn off the gas stove before leaving the house). Patients with Parkinson's disease (PD) exhibit widespread cognitive deficits including deficits in PM. The present study provides a meta-analytic review of PM in PD. Results across nine studies indicated time and event-based PM to be similarly impaired in PD, with time-based PM compromised to a slightly larger extent (Hedges' g = -0.71) as compared to event-based PM (Hedges' g = -0.55). The impairment in PM is more likely due to failure in self-initiated retrieval of intention to be executed, rather than forgetting the content of the intention itself. Furthermore, factors such as intervening task complexity and the mediating role of other executive functions have also been proposed to be responsible for impaired PM in PD.

Prospective memory in Alzheimer-type dementia: exploring prospective memory performance in an age-stratified sample

Prospective memory (PM), the memory for future intentions, is an essential component of many day-to-day activities. PM accuracy has consistently been found to decline as a function of age and is further impaired in Alzheimer's disease (AD). In the present study, subjective PM failures, PM accuracy, and cost of carrying a PM were recorded in 42 healthy elderly, 34 younger old, and 45 older old AD participants. It was found that PM deficits across the AD cohort did not change as a function of age. In addition, while PM accuracy was impaired compared to age-matched controls, cost of carrying a PM intention did not differ across the three groups. Evidence that AD participants show a reaction time (RT) cost of PM intention alongside an impaired PM accuracy indicates that the PM intention is held, but is not implemented effectively. The fact that the cost is independent of age and dementia suggests that it may not index working memory (WM) resource. At a practical level, the study suggests that for a comprehensive evaluation of PM competence cost of carrying a PM intention should be measured alongside PM accuracy.

Diminished time-based, but undiminished event-based, prospective memory among intellectually high-functioning adults with autism spectrum disorder: relation to working memory ability

Objective: Prospective memory (PM) is the ability to remember to carry out an intended action. Working memory is the ability to store information in mind while processing potentially distracting information. The few previous studies of PM in autism spectrum disorder (ASD) have yielded inconsistent findings. Studies of working memory ability in ASD have suggested a selective impairment of “visual working memory.” However, it remains unclear whether any such impairment is the result of diminished (domain-specific; visual/verbal) storage capacity or diminished (domain-general) processing capacity. We aim to clarify these issues and explore the relation between PM and working memory in ASD. Method: Seventeen adults with ASD and 17 age- and IQ-matched comparison participants completed experimental measures of both event-based (perform action x when event y occurs) and time-based (perform action a at time b) PM, plus a self-report measure of PM skills. Participants also completed a working memory test battery. Results: Participants with ASD self-reported diminished PM skill, and showed diminished performance on the time-based, but not event-based, PM task. On the working memory test battery, visual but not verbal storage capacity was diminished among participants with ASD, as was processing ability. Whereas visual storage was associated with event-based PM task performance among comparison participants, verbal storage was associated among ASD participants. Conclusions: ASD appears to involve a selective deficit in time-based PM and a selective difficulty with aspects of working memory that depend on the storage of visual information. However, event-based PM may be achieved through compensatory strategies in ASD.

Prospective memory in Parkinson disease during a virtual week: effects of both prospective and retrospective demands

OBJECTIVE

This study investigated the effect of Parkinson's disease (PD) on event-based prospective memory tasks with varying demand on (1) the amount of strategic attentional monitoring required for intention retrieval (prospective component), and (2) the retrospective memory processes required to remember the contents of the intention or the entire constellation of prospective memory tasks.

METHOD

Twenty-four older adults with PD and 28 healthy older adults performed the computerized Virtual Week task, a multi-intention prospective memory paradigm that simulates everyday prospective memory tasks. The Virtual Week included regular (low retrospective memory demand) and irregular (high retrospective memory demand) prospective memory tasks with cues that were focal (low strategic monitoring demand) or less focal (high strategic monitoring demand) to the ongoing activity.

RESULTS

For the regular prospective memory tasks, PD participants were impaired when the prospective memory cues were less focal. For the irregular prospective memory tasks, PD participants were impaired regardless of prospective memory cue type. PD participants also had impaired retrospective memory for irregular tasks, which was associated with worse prospective memory for these tasks during the Virtual Week.

CONCLUSIONS

When retrospective memory demands are minimized, prospective memory in PD can be supported by cues that reduce the executive control demands of intention retrieval. However, PD-related deficits in self-initiated encoding or planning processes have strong negative effects on the performance of prospective memory tasks, with increased retrospective memory demand.

Rostral and caudal prefrontal contribution to creativity: a meta-analysis of functional imaging data

Creativity is of central importance for human civilization, yet its neurocognitive bases are poorly understood. The aim of the present study was to integrate existing functional imaging data by using the meta-analysis approach. We reviewed 34 functional imaging studies that reported activation foci during tasks assumed to engage creative thinking in healthy adults. A coordinate-based meta-analysis using Activation Likelihood Estimation (ALE) first showed a set of predominantly left-hemispheric regions shared by the various creativity tasks examined. These regions included the caudal lateral prefrontal cortex (PFC), the medial and lateral rostral PFC, and the inferior parietal and posterior temporal cortices. Further analyses showed that tasks involving the combination of remote information (combination tasks) activated more anterior areas of the lateral PFC than tasks involving the free generation of unusual responses (unusual generation tasks), although both types of tasks shared caudal prefrontal areas. In addition, verbal and non-verbal tasks involved the same regions in the left caudal prefrontal, temporal, and parietal areas, but also distinct domain-oriented areas. Taken together, these findings suggest that several frontal and parieto-temporal regions may support cognitive processes shared by diverse creativity tasks, and that some regions may be specialized for distinct types of processes. In particular, the lateral PFC appeared to be organized along a rostro-caudal axis, with rostral regions involved in combining ideas creatively and more posterior regions involved in freely generating novel ideas.

Dissociable neural routes to successful prospective memory

Identifying the processes by which people remember to execute an intention at an appropriate moment (prospective memory) remains a fundamental theoretical challenge. According to one account, top-down attentional control is required to maintain activation of the intention, initiate intention retrieval, or support monitoring. A diverging account suggests that bottom-up, spontaneous retrieval can be triggered by cues that have been associated with the intention and that sustained attentional processes are not required. We used a specialized experimental design and functional MRI methods to selectively marshal and identify each process. Results revealed a clear dissociation. One prospective-memory task recruited sustained activity in attentional-control areas, such as the anterior prefrontal cortex; the other engaged purely transient activity in parietal and ventral brain regions associated with attentional capture, target detection, and episodic retrieval. These patterns provide critical evidence that there are two neural routes to prospective memory, with each route emerging under different circumstances.

Working Memory, Mental Prospection, Time Orientation, and Cognitive Insight

This research is new in its attempt to take future time orientation, morningness orientation, and prospective memory as measures of mental prospection, and to examine a three-factor model that assumes working memory, mental prospection, and cognitive insight are independent but related higher-order cognitive constructs by using confirmatory factor analysis (CFA). The three-factor model produced a good fit to the data. An alternative one-factor model was tested and rejected. The results suggest that working memory and cognitive insight are distinguishable, related constructs, and that both are distinct from, but negatively associated with, mental prospection. In addition, structural equation modeling (SEM) showed that working memory had a strong positive effect on cognitive insight and a moderate negative effect on mental prospection.

Prospective memory in children and chimpanzees

Prospective memory (PM) involves remembering to do something at a specific time in the future. Here, we investigate the beginnings of this ability in young children (3-year-olds; Homo sapiens) and chimpanzees (Pan troglodytes) using an analogous task. Subjects were given a choice between two toys (children) or two food items (chimpanzees). The selected item was delivered immediately, whereas the unselected item was hidden in an opaque container. After completing an ongoing quantity discrimination task, subjects could request the hidden item by asking for it (children) or by pointing to the container and identifying the item on a symbol board (chimpanzees). Children and chimpanzees showed evidence of prospective-like memory in this task, as evidenced by successful retrieval of the item at the end of the task, sometimes spontaneously with no prompting from the experimenter. These findings contribute to our understanding of PM from an ontogenetic and comparative perspective.

Connectivity-based parcellation of the thalamus explains specific cognitive and behavioural symptoms in patients with bilateral thalamic infarct

A novel approach based on diffusion tractography was used here to characterise the cortico-thalamic connectivity in two patients, both presenting with an isolated bilateral infarct in the thalamus, but exhibiting partially different cognitive and behavioural profiles. Both patients (G.P. and R.F.) had a pervasive deficit in episodic memory, but only one of them (R.F.) suffered also from a dysexecutive syndrome. Both patients had an MRI scan at 3T, including a T1-weighted volume. Their lesions were manually segmented. T1-volumes were normalised to standard space, and the same transformations were applied to the lesion masks. Nineteen healthy controls underwent a diffusion-tensor imaging (DTI) scan. Their DTI data were normalised to standard space and averaged. An atlas of Brodmann areas was used to parcellate the prefrontal cortex. Probabilistic tractography was used to assess the probability of connection between each voxel of the thalamus and a set of prefrontal areas. The resulting map of corticothalamic connections was superimposed onto the patients’ lesion masks, to assess whether the location of the thalamic lesions in R.F. (but not in G. P.) implied connections with prefrontal areas involved in dysexecutive syndromes. In G.P., the lesion fell within areas of the thalamus poorly connected with prefrontal areas, showing only a modest probability of connection with the anterior cingulate cortex (ACC). Conversely, R.F.’s lesion fell within thalamic areas extensively connected with the ACC bilaterally, with the right dorsolateral prefrontal cortex, and with the left supplementary motor area. Despite a similar, bilateral involvement of the thalamus, the use of connectivity-based segmentation clarified that R.F.’s lesions only were located within nuclei highly connected with the prefrontal cortical areas, thus explaining the patient’s frontal syndrome. This study confirms that DTI tractography is a useful tool to examine in vivo the effect of focal lesions on interconnectivity brain patterns.

Evidence for a detrimental relationship between hypertension history, prospective memory, and prefrontal cortex white matter in cognitively normal older adults

Hypertension affects many older adults and is associated with impaired neural and cognitive functioning. We investigated whether a history of hypertension was associated with impairments to prospective memory, which refers to the ability to remember to perform delayed intentions, such as remembering to take medication. Thirty-two cognitively normal older adult participants with or without a history of hypertension (self-reported) performed two laboratory prospective memory tasks, one that relied more strongly on executive control (nonfocal prospective memory) and one that relied more strongly on spontaneous memory retrieval processes (focal prospective memory). We observed hypertension-related impairments for nonfocal, but not focal, prospective memory. To complement our behavioral approach, we conducted a retrospective analysis of available structural magnetic resonance imaging data. Lower white matter volume estimates in the anterior prefrontal cortex were associated with lower nonfocal prospective memory and with a history of hypertension. A history of hypertension may be associated with worsened executive control and lower prefrontal white matter volume. The translational implication is that individuals who must remember to take antihypertensive medications and to monitor their blood pressure at home may be impaired in the executive control process that helps to support these prospective memory behaviors.

Nicotine derived from the electronic cigarette improves time-based prospective memory in abstinent smokers

RATIONALE

It is well established that nicotine improves, and deprivation impairs, cognitive performance and mood in smokers. Prospective memory (PM), remembering to execute a delayed intention at a given time point, is under-explored in smokers. Whilst a handful of studies have shown improved PM with nicotine, the effects of nicotine delivered via the electronic cigarette (e-cigarette) have not been investigated.

OBJECTIVE

This study explores whether, by comparison with placebo, nicotine delivered via the e-cigarette can improve PM, tobacco withdrawal symptoms and desire to smoke in abstinent smokers.

METHODS

Twenty smokers, abstinent for 8-10 h, each completed two experimental sessions under nicotine (18 mg) and placebo (0 mg) e-cigarette conditions. Participants completed a single-item desire-to-smoke scale and the Mood and Physical Symptoms Scale. PM was measured using the Cambridge Prospective Memory Test.

RESULTS

Compared with placebo, the nicotine e-cigarette reduced the desire to smoke and tobacco withdrawal symptoms, and improved time-based but not event-based PM. There was a moderate, marginally significant negative correlation between PM performance during abstinence and nicotine dependence.

CONCLUSIONS

This is the first study to show that nicotine derived via e-cigarette can improve PM in abstinent smokers, suggesting efficient nicotine delivery. The finding that the effect of nicotine was restricted to time-based rather than event-based PM is consistent with the view that nicotine acts to improve performance on strategic (effortful) rather than automatic processing. These findings add to the growing body of evidence that the e-cigarette can replace some of the effects of nicotine derived from tobacco smoking, thus highlighting its potential for smoking cessation.

The effects of focal and nonfocal cues on the neural correlates of prospective memory: insights from ERPs

The present study is the first designed to investigate behavioral and event-related potential (ERP) correlates of the processes involved in focal and nonfocal prospective memory (PM) tasks. Focal tasks are those in which the features of the PM cue are easily extracted from the ongoing activity, whereas the process is more indirect in nonfocal tasks. Strategic monitoring was associated with a slowing of reaction times in ongoing trials and with frontal and parietal ERP modulations. These effects were greater in the nonfocal task, whereas they were smaller, or even absent for some individuals, in the focal task. This indicates that strategic monitoring is engaged to a greater extent in nonfocal tasks, whereas it is less extensively recruited, or not recruited at all by some individuals, in focal tasks. Indeed, the recognition of the PM cue might also occur automatically in focal tasks, as suggested by the FN400 increase in focal PM trials. Nonfocal tasks are supported by more controlled resources not only in retrieval, but also in postretrieval monitoring and coordinating processes. This was reflected in the enhancement of the prospective positivity and frontal slow wave observed in nonfocal PM trials. We interpreted these results as supporting the multiprocess view of PM.

Do adults with autism spectrum disorders compensate in naturalistic prospective memory tasks?

The present study is the first to directly compare event- and time-based prospective memory in Autism Spectrum Disorders (ASD) using a contextual task mirroring real life demands of prospective memory. Twenty-five individuals with ASD and 25 age- and ability-matched controls completed the Dresden Breakfast task which required participants to prepare breakfast following a set of rules and time restrictions. Overall, adults with ASD had less correct time- and event-based prospective memory responses in comparison to controls, which is consistent with previous research in children with ASD. Moreover, ASD participants completed fewer tasks, followed rules less closely, and monitored the elapsing time less closely than controls. Individuals with ASD seem not to be compensating in naturalistic prospective memory tasks.

Impulsivity and self-control during intertemporal decision making linked to the neural dynamics of reward value representation

A characteristic marker of impulsive decision making is the discounting of delayed rewards, demonstrated via choice preferences and choice-related brain activity. However, delay discounting may also arise from how subjective reward value is dynamically represented in the brain when anticipating an upcoming chosen reward. In the current study, brain activity was continuously monitored as human participants freely selected an immediate or delayed primary liquid reward and then waited for the specified delay before consuming it. The ventromedial prefrontal cortex (vmPFC) exhibited a characteristic pattern of activity dynamics during the delay period, as well as modulation during choice, that is consistent with the time-discounted coding of subjective value. The ventral striatum (VS) exhibited a similar activity pattern, but preferentially in impulsive individuals. A contrasting profile of delay-related and choice activation was observed in the anterior PFC (aPFC), but selectively in patient individuals. Functional connectivity analyses indicated that both vmPFC and aPFC exerted modulatory, but opposite, influences on VS activation. These results link behavioral impulsivity and self-control to dynamically evolving neural representations of future reward value, not just during choice, but also during postchoice delay periods.

Time-based prospective memory predicts engagement in risk behaviors among substance users: results from clinical and nonclinical samples

Deficits in prospective memory (PM; i.e., enacting previously learned actions at the right occasion) and risky decision-making (i.e., making choices with a high chance of undesirable/dangerous outcomes) are both common among individuals with substance use disorders (SUD). Previous research has raised the possibility of a specific relationship between PM and risk-taking, and the present study aimed to systematically study if PM provides unique variance in the prediction of risky decision-making. Two samples were included: (1) a group of 45 individuals with SUD currently in treatment, and (2) a nonclinical group of 59 university students with high-risk drinking and/or substance use. Regression analyses indicated that time-based, but not event-based, PM predicted increased risky behavior (e.g., risky sexual practices and criminal behaviors) in both groups after controlling for demographic, psychiatric, and substance use variables, as well as other neuropsychological functions. The current findings contribute to the growing literature supporting the role of PM as a predictor of everyday functioning, and suggest that cognitive rehabilitation may be an important avenue of research as an adjunct to traditional substance use treatment, particularly in addressing the potential adverse effects of PM deficits in the implementation of treatment-related homework activities and risk management strategies.

APOE E4 Carriers show prospective memory enhancement under nicotine, and evidence for specialisation within medial BA10

There is evidence to suggest that the APOE ɛ4 allele (which confers an increased risk of developing dementia) might be associated with cognitive advantages earlier in life. Further, nicotine might selectively benefit ɛ4 carriers. We used fMRI to explore performance on a prospective memory (PM) task in young adults (age 18-30) with and without nicotine using a within-subjects design. Participants performed an ongoing task while retaining a PM instruction to respond to specific stimuli embedded in the task. Nicotine effects varied according to APOE status. Reaction times to the PM cue were improved under nicotine in ɛ4 carriers, but not in ɛ3 carriers. In an event-related analysis, extrastriate responses to PM trials were enhanced by nicotine only in ɛ4 carriers. These differences in early visual processing may contribute to the behavioral findings. Activity in medial BA10 (previously implicated in PM) differentiated ɛ4 from ɛ3 carriers. One BA10 subregion showed greater deactivation in ɛ4 carriers during PM trials. Activity in other BA10 subregions was modulated by PM reaction time, pointing to region-specific effects within medial BA10. In addition, activity in right hippocampal formation was only seen in ɛ4 carriers receiving nicotine. These results demonstrate that cognitive enhancement by nicotine can selectively benefit APOE ɛ4 carriers, and point to genotype-specific differences in neural activity during PM. In addition, these results show that the role of medial BA10 in PM likely involves varying contributions from functionally specific subregions.

The right frontopolar cortex is involved in visual-spatial prospective memory

The involvement of frontopolar cortex in mediating prospective memory processes has been evidenced by various studies, mainly by means of neuroimaging techniques. Recently, one transcranial magnetic stimulation study documented that transient inhibition of left Brodmann Area (BA) 10 impaired verbal prospective memory. This result raises the issue of whether the BA 10 involvement in prospective memory functioning may be modulated by the physical characteristics of the stimuli used. The present study aimed to investigate the role of the frontopolar cortex in visual-spatial PM by means of the application of inhibitory theta-burst stimulation. Twelve volunteers were evaluated after inhibitory theta-burst stimulation over left BA 10, right BA10 and CZ (control condition). In the prospective memory procedure, sequences of four spatial positions (black squares) each were presented. During the inter-sequence delay, subjects had to reproduce the sequence in the observed order (ongoing task forward) or the reverse order (backward). At the occurrence of a target position, subjects had to press a key on the keyboard (prospective memory score). Recall and recognition of the target positions were also tested. We found that prospective memory accuracy was lower after theta-burst stimulation over right BA10 than CZ (p<0.01), whereas it was comparable in left BA10 and CZ conditions. No significant difference was found among the three conditions on recall and recognition of target positions and on ongoing task performance. Our findings provide a novel strong evidence for a specific involvement of right frontopolar cortex in visual-spatial prospective memory. In the context of previous data providing evidence for left BA 10 involvement in verbal prospective memory, our results also suggest material-specific lateralization of prospective memory processes in BA 10.

Anterior prefrontal hemodynamic connectivity in conscious 3- to 7-year-old children with typical development and autism spectrum disorder

Socio-communicative impairments are salient features of autism spectrum disorder (ASD) from a young age. The anterior prefrontal cortex (aPFC), or Brodmann area 10, is a key processing area for social function, and atypical development of this area is thought to play a role in the social deficits in ASD. It is important to understand these brain functions in developing children with ASD. However, these brain functions have not yet been well described under conscious conditions in young children with ASD. In the present study, we focused on the brain hemodynamic functional connectivity between the right and the left aPFC in children with ASD and typically developing (TD) children and investigated whether there was a correlation between this connectivity and social ability. Brain hemodynamic fluctuations were measured non-invasively by near-infrared spectroscopy (NIRS) in 3- to 7-year-old children with ASD (n = 15) and gender- and age-matched TD children (n = 15). The functional connectivity between the right and the left aPFC was assessed by measuring the coherence for low-frequency spontaneous fluctuations (0.01-0.10 Hz) during a narrated picture-card show. Coherence analysis demonstrated that children with ASD had a significantly higher inter-hemispheric connectivity with 0.02-Hz fluctuations, whereas a power analysis did not demonstrate significant differences between the two groups in terms of low frequency fluctuations (0.01-0.10 Hz). This aberrant higher connectivity in children with ASD was positively correlated with the severity of social deficit, as scored with the Autism Diagnostic Observation Schedule. This is the first study to demonstrate aberrant brain functional connectivity between the right and the left aPFC under conscious conditions in young children with ASD.

Learning and memory

Learning and memory functions are crucial in the interaction of an individual with the environment and involve the interplay of large, distributed brain networks. Recent advances in technologies to explore neurobiological correlates of neuropsychological paradigms have increased our knowledge about human learning and memory. In this chapter we first review and define memory and learning processes from a neuropsychological perspective. Then we provide some illustrations of how noninvasive brain stimulation can play a major role in the investigation of memory functions, as it can be used to identify cause-effect relationships and chronometric properties of neural processes underlying cognitive steps. In clinical medicine, transcranial magnetic stimulation may be used as a diagnostic tool to understand memory and learning deficits in various patient populations. Furthermore, noninvasive brain stimulation is also being applied to enhance cognitive functions, offering exciting translational therapeutic opportunities in neurology and psychiatry.

Speech entrainment enables patients with Broca's aphasia to produce fluent speech

A distinguishing feature of Broca's aphasia is non-fluent halting speech typically involving one to three words per utterance. Yet, despite such profound impairments, some patients can mimic audio-visual speech stimuli enabling them to produce fluent speech in real time. We call this effect 'speech entrainment' and reveal its neural mechanism as well as explore its usefulness as a treatment for speech production in Broca's aphasia. In Experiment 1, 13 patients with Broca's aphasia were tested in three conditions: (i) speech entrainment with audio-visual feedback where they attempted to mimic a speaker whose mouth was seen on an iPod screen; (ii) speech entrainment with audio-only feedback where patients mimicked heard speech; and (iii) spontaneous speech where patients spoke freely about assigned topics. The patients produced a greater variety of words using audio-visual feedback compared with audio-only feedback and spontaneous speech. No difference was found between audio-only feedback and spontaneous speech. In Experiment 2, 10 of the 13 patients included in Experiment 1 and 20 control subjects underwent functional magnetic resonance imaging to determine the neural mechanism that supports speech entrainment. Group results with patients and controls revealed greater bilateral cortical activation for speech produced during speech entrainment compared with spontaneous speech at the junction of the anterior insula and Brodmann area 47, in Brodmann area 37, and unilaterally in the left middle temporal gyrus and the dorsal portion of Broca's area. Probabilistic white matter tracts constructed for these regions in the normal subjects revealed a structural network connected via the corpus callosum and ventral fibres through the extreme capsule. Unilateral areas were connected via the arcuate fasciculus. In Experiment 3, all patients included in Experiment 1 participated in a 6-week treatment phase using speech entrainment to improve speech production. Behavioural and functional magnetic resonance imaging data were collected before and after the treatment phase. Patients were able to produce a greater variety of words with and without speech entrainment at 1 and 6 weeks after training. Treatment-related decrease in cortical activation associated with speech entrainment was found in areas of the left posterior-inferior parietal lobe. We conclude that speech entrainment allows patients with Broca's aphasia to double their speech output compared with spontaneous speech. Neuroimaging results suggest that speech entrainment allows patients to produce fluent speech by providing an external gating mechanism that yokes a ventral language network that encodes conceptual aspects of speech. Preliminary results suggest that training with speech entrainment improves speech production in Broca's aphasia providing a potential therapeutic method for a disorder that has been shown to be particularly resistant to treatment.

Prospective memory tasks: a more sensitive method for screening cognitive impairment in ALS?

BACKGROUND

Cognitive change is prevalent in patients with amyotrophic lateral sclerosis (ALS), but still lack a widely accepted and sensitive screening method. In this study, we try to find a sensitive screening battery for detecting subtle cognitive deficits in patients with ALS.

METHODS

Eighty consecutive ALS patients and 57 matched normal controls underwent the Mini-Mental Status Examination (MMSE), the verbal fluency test (VFT), the Stroop Color Word Interference Test (CWT), and the prospective memory (PM) tests, including event-based (EBPM) and time-based (TBPM).

RESULTS

The patients did not differ from the controls in the MMSE, the VFT and the CWT. By contrast, statistically significant differences were found in the PM tests (EBPM: P=0.043; TBPM: P<0.001). More interestingly, TBPM was more sensitive than EBPM in the early-phase patients.

CONCLUSIONS

Prefrontal lobar dysfunction does exist among ALS patients and may spread from the medial to the lateral region. The PM tests seem more sensitive in ALS patients with frontotemporal dysfunction than are the classical cognitive measures.

Preparatory distributed cortical synchronization determines execution of some, but not all, future intentions

Associating intentions to events that cue future behaviors is a central aspect of human cognition. There is limited understanding of the neural dynamics supporting recognition of intention-related events, with little known about how pre-event brain state varies as a function of intention specificity. Prior to recognized events (that cued planned behavior) occurring during an unrelated activity, we found increased steady-state visual evoked potential (ssVEP) and intrinsic gamma synchronization for ill- compared to well-specified events, as measured by EEG. Enhanced fronto-temporo-parietal ssVEP synchrony emerged preceding ill compared to well-specified events, and the degree of synchrony predicted the completion of ill-specified intentions but predicted failure to complete well-specified intentions. Distinct executive processing and neural states are therefore optimal for anticipating and fulfilling future intentions varying in event specificity.

Insights from neuropsychology: pinpointing the role of the posterior parietal cortex in episodic and working memory

The role of posterior parietal cortex (PPC) in various forms of memory is a current topic of interest in the broader field of cognitive neuroscience. This large cortical region has been linked with a wide range of mnemonic functions affecting each stage of memory processing: encoding, maintenance, and retrieval. Yet, the precise role of the PPC in memory remains mysterious and controversial. Progress in understanding PPC function will require researchers to incorporate findings in a convergent manner from multiple experimental techniques rather than emphasizing a particular type of data. To facilitate this process, here, we review findings from the human neuropsychological research and examine the consequences to memory following PPC damage. Recent patient-based research findings have investigated two typically disconnected fields: working memory (WM) and episodic memory. The findings from patient participants with unilateral and bilateral PPC lesions performing diverse experimental paradigms are summarized. These findings are then related to findings from other techniques including neurostimulation (TMS and tDCS) and the influential and more abundant functional neuroimaging literature. We then review the strengths and weaknesses of hypotheses proposed to account for PPC function in these forms of memory. Finally, we address what missing evidence is needed to clarify the role(s) of the PPC in memory.

Electrophysiological correlates of strategic monitoring in event-based and time-based prospective memory

Prospective memory (PM) is the ability to remember to accomplish an action when a particular event occurs (i.e., event-based PM), or at a specific time (i.e., time-based PM) while performing an ongoing activity. Strategic Monitoring is one of the basic cognitive functions supporting PM tasks, and involves two mechanisms: a retrieval mode, which consists of maintaining active the intention in memory; and target checking, engaged for verifying the presence of the PM cue in the environment. The present study is aimed at providing the first evidence of event-related potentials (ERPs) associated with time-based PM, and at examining differences and commonalities in the ERPs related to Strategic Monitoring mechanisms between event- and time-based PM tasks.

The addition of an event-based or a time-based PM task to an ongoing activity led to a similar sustained positive modulation of the ERPs in the ongoing trials, mainly expressed over prefrontal and frontal regions. This modulation might index the retrieval mode mechanism, similarly engaged in the two PM tasks. On the other hand, two further ERP modulations were shown specifically in an event-based PM task. An increased positivity was shown at 400–600 ms post-stimulus over occipital and parietal regions, and might be related to target checking. Moreover, an early modulation at 130–180 ms post-stimulus seems to reflect the recruitment of attentional resources for being ready to respond to the event-based PM cue. This latter modulation suggests the existence of a third mechanism specific for the event-based PM; that is, the “readiness mode”.

Longer ongoing task delay intervals exacerbate prospective memory deficits in HIV-associated neurocognitive disorders (HAND)

The delay interval between encoding a future intention and detection of the retrieval cue is an essential feature of prospective memory (PM). McDaniel and Einstein's ( 2000 ) multiprocess theory posits that greater demands are placed on strategic monitoring processes as the delay interval lengthens. This hypothesis was examined in HIV-associated neurocognitive disorders (HAND), which are associated with strategic dyscontrol of PM, likely secondary to prefrontostriatal circuit pathology. Seventy-eight seronegative adults and 49 individuals with HAND comprised the study groups, which were comparable with regard to demographic, psychiatric, and substance use factors. As part of a comprehensive neuropsychological evaluation, participants were administered a well-validated PM measure that included short (2-min) and long (15-min) task delay interval scales that utilized a standardized word search as the ongoing task. Results revealed a significant interaction of group and delay interval, with significant effects of HAND on PM at long, but not short, delay. The long-delay PM effect in HAND was driven primarily by deficits in time-based PM and was most strongly associated with markers of executive dysfunction. In concordance with the multiprocess theory, individuals with HAND were disproportionately vulnerable to PM deficits at longer ongoing task delay intervals, which appear to be driven by strategic dyscontrol of PM that is consistent with the preferential disruption of prefrontal systems in neuroAIDS (neurological complications of AIDS). Difficulty with successfully completing PM tasks following a longer delay could manifest in real-world problems, such as medication nonadherence and unemployment, and characterizing this specific deficit may inform remediation strategies.

Prospective memory deficits in Ecstasy users: effects of longer ongoing task delay interval

Ecstasy use has been associated with neurotoxicity and neurocognitive impairment in a variety of domains, including prospective memory (ProM), which involves the delayed execution of a previously encoded intention in response to a specific cue. The present study adopted the multiprocess theory of ProM to evaluate the hypothesis that Ecstasy users would evidence differentially impaired ProM on longer versus shorter ongoing task delays. Ecstasy (n = 31) users, high-risk alcohol users (n = 21), and healthy nonusers (n = 31) completed the short (2-min) and long (15-min) delay ProM scales of the Memory for Intentions Screening Test. Results showed a significant group by ProM delay interaction, such that Ecstasy users performed comparably to the comparison groups on short-delay trials, but were impaired on long-delay ProM, particularly for time-based cues. Among the Ecstasy users, long-delay ProM was positively associated with risky decision making, but not with retrospective memory or other aspects of executive functions. These findings suggest that Ecstasy users may be particularly susceptible to deficits in strategic target monitoring and maintenance of cue-intention pairings over longer ProM delays. Findings are discussed in the context of their potential everyday functioning (e.g., academic, vocational) and treatment implications for Ecstasy users.

Structural correlates of prospective memory

Prospective memory (PM) includes the encoding and maintenance of an intention, and the retrieval and execution of this intention at the proper moment in the future. The present study expands upon previous behavioral, electrophysiological, and functional work by examining the association between grey matter volume and PM. Estimates of grey matter volume in theoretically relevant regions of interest (prefrontal, parietal, and medial temporal) were obtained in conjunction with performance on two PM tasks in a sample of 39 cognitively normal and very mildly demented older adults. The first PM task, termed focal in the literature, is supported by spontaneous retrieval of the PM intention whereas the second, termed non-focal, relies on strategic monitoring processes for successful intention retrieval. A positive relationship was observed between medial temporal volume and accuracy on the focal PM task. An examination of medial temporal lobe subregions revealed that this relationship was strongest for the hippocampus, which is considered to support spontaneous memory retrieval. There were no significant structure-behavior associations for the non-focal PM task. These novel results confirm a relationship between behavior and underlying brain structure proposed by the multiprocess theory of PM, and extend findings on cognitive correlates of medial temporal lobe integrity.

The contribution of retrospective memory, attention and executive functions to the prospective and retrospective components of prospective memory following TBI

PRIMARY OBJECTIVE

Despite the prevalence of prospective memory (PM) problems, relatively little is known about the processes underlying impairment following TBI. This study sought to examine PM performance, using a multiple-task, multiple-response video-based paradigm in which initial encoding of the cue-action associations was ensured (Video-Assessment of Prospective Memory; VAPM).

RESEARCH DESIGN

VAPM was designed to allow easy identification of reasons for failure (i.e. cue detection and/or specific action retrieval). Patients with moderate/severe TBI (n = 32) and matched controls (n = 16) also completed standardized neuropsychological assessment including evaluation of episodic retrospective memory (RM), attention, information processing, executive functions and mood.

MAIN OUTCOMES AND RESULTS

As a group, those with TBI were impaired on PM tasks with 50% failing to complete at least 2/6 required tasks despite near perfect performance by controls. Individual profile analyses revealed different reasons for impairment, with RM contributing significantly to both the prospective and retrospective components. This was supported by correlational analyses illustrating a significant relationship between cue detection and RM measures, in addition to measures of executive functions and attention.

CONCLUSIONS

The contribution of RM to both components of PM, along with the finding of heterogeneity in performance among participants have important implications for theoretical understanding and clinical practice.

Modulation of a fronto-parietal network in event-based prospective memory: an rTMS study

Event-based prospective memory (PM) is a multi-component process that requires remembering the delayed execution of an intended action in response to a pre-specified PM cue, while being actively engaged in an ongoing task. Some neuroimaging studies have suggested that both prefrontal and parietal areas are involved in the maintenance and realization of delayed intentions. In the present study, transcranial magnetic stimulation (TMS) was used to investigate the causal involvement of frontal and parietal areas in different stages of the PM process (in particular, target checking and intention retrieval), and to determine the specific contribution of these regions to PM performance. Our results demonstrate that repetitive TMS (rTMS) interferes with prospective memory performance when applied at 150-350 ms to the right dorsolateral prefrontal cortex (DLPFC), and at 400-600 ms when applied to the left posterior parietal cortex (PPC). The present study provides clear evidence that the right DLPFC plays a crucial role in early components of the PM process (target checking), while the left PPC seems to be mainly involved in later processes, such as the retrieval of the intended action.