Prefrontal activity and impaired memory encoding strategies in schizophrenia

Modulation of brain complexity in schizophrenia patients with auditory verbal hallucinations by low-frequency rTMS stimulation

BACKGROUND

Entropy is a critical measure for assessing the complexity and irregularity of brain signals. Understanding how brain entropy can be influenced by non-invasive neurostimulation in psychiatric patients remains a clinically relevant issue.

OBJECTIVE

This study aims to explore whether low-frequency repetitive transcranial magnetic stimulation (rTMS) can modulate brain entropy in schizophrenia patient with auditory verbal hallucinations (AVH).

METHODS

A case-control design was employed in this study. Low-frequency (1 Hz) rTMS targeting at left temporoparietal junction was administered to schizophrenia patients with AVH. Brain entropy (sample entropy) was calculated from resting-state functional magnetic resonance imaging (fMRI) data. Comparisons of sample entropy were made between the schizophrenia patients and healthy controls, as well as within the patient group pre- and post-rTMS.

RESULTS

Following rTMS treatment, patients showed a reduction in clinical symptoms, including positive symptoms and AVH. Neurocognitive improvements were also observed in domains such as verbal and visual memory. Furthermore, patients exhibited increased sample entropy in regions including the prefrontal cortices and temporal lobes compared to healthy controls. However, this elevated entropy was reduced post-rTMS, particularly in areas associated with AVH. The language network and default model network, initially showing high mean sample entropy, demonstrated a significant decrease after rTMS treatment. These changes in brain entropy were correlated with clinical improvements.

CONCLUSION

This modulation of neural activity complexity induced by the low-frequency rTMS may underlie the observed clinical and cognitive improvement in schizophrenia.

Neuroimaging features of cognitive impairments in schizophrenia and major depressive disorder

Cognitive dysfunctions are one of the key symptoms of schizophrenia (SZ) and major depressive disorder (MDD), which exist not only during the onset of diseases but also before the onset, even after the remission of psychiatric symptoms. With the development of neuroimaging techniques, these non-invasive approaches provide valuable insights into the underlying pathogenesis of psychiatric disorders and information of cognitive remediation interventions. This review synthesizes existing neuroimaging studies to examine domains of cognitive impairment, particularly processing speed, memory, attention, and executive function in SZ and MDD patients. First, white matter (WM) abnormalities are observed in processing speed deficits in both SZ and MDD, with distinct neuroimaging findings highlighting WM connectivity abnormalities in SZ and WM hyperintensity caused by small vessel disease in MDD. Additionally, the abnormal functions of prefrontal cortex and medial temporal lobe are found in both SZ and MDD patients during various memory tasks, while aberrant amygdala activity potentially contributes to a preference to negative memories in MDD. Furthermore, impaired large-scale networks including frontoparietal network, dorsal attention network, and ventral attention network are related to attention deficits, both in SZ and MDD patients. Finally, abnormal activity and volume of the dorsolateral prefrontal cortex (DLPFC) and abnormal functional connections between the DLPFC and the cerebellum are associated with executive dysfunction in both SZ and MDD. Despite these insights, longitudinal neuroimaging studies are lacking, impeding a comprehensive understanding of cognitive changes and the development of early intervention strategies for SZ and MDD. Addressing this gap is critical for advancing our knowledge and improving patient prognosis.

The role of semantic clustering in the relationship between verbal memory and psychosocial functioning in schizophrenia and bipolar disorder: Possible distinct cognitive pathway compared to healthy controls

BACKGROUND

Verbal memory (VM) is impaired in schizophrenia (SZ) and bipolar disorder (BD), and predicts psychosocial functioning. However, there is a lack of research exploring the role of VM component processes, including semantic clustering, in these disorders. Semantic clustering might impact this association, as effective semantic memory strategies may reflect unimpaired executive control, leading to an adequate functioning. We aimed to investigate VM components in SZ and BD, and the role of semantic clustering in the relationship between VM and functioning.

METHODS

We included 495 participants (156 SZ, 172 BD, and 167 healthy controls (HC)) that underwent an assessment using the Hopkins Verbal Learning Test - Revised for VM and the Functioning Assessment Short Test for psychosocial functioning. We compared groups through ANOVAs and investigated the effect of semantic clustering in the relationship between VM total immediate free recall and functioning through linear regression models.

RESULTS

SZ had worse overall VM performance compared to BD, which performed worse than HCs. HCs used more semantic clustering than SZ and BD, but there were no differences between the two clinical groups. In HCs, semantic clustering impacted the relationship between VM performance and functioning, while no interaction was observed in SZ or BD.

LIMITATIONS

Cross-sectional design; no medication effects or other cognitive functions were assessed.

CONCLUSIONS

SZ and BD may use an alternative cognitive pathway in which the relationship between VM and functioning is independent of complex cognitive processes such as semantic clustering, supporting the cognitive remediation targeting of VM in these disorders.

Reduced Functional Connectivity in Brain Networks Underlying Paired Associates Memory Encoding in Schizophrenia

BACKGROUND

Deficits in relational episodic memory encoding are characteristic of schizophrenia (SZ), but whole-brain multivariate analyses of these deficits have been lacking. Open science has provided task-based functional magnetic resonance imaging (fMRI) data investigating paired associate encoding in SZ, but it has not yet been mobilized to address this gap in the literature. Therefore, in this study, we use previously unpublished task fMRI data to conduct the first network-level investigation of impaired relational episodic encoding in SZ.

METHODS

Using fMRI data acquired from 40 healthy control participants and 40 age- and sex-matched persons with SZ, we examined the networks involved in successful versus unsuccessful encoding of verbal paired associates using an associative semantic strategy.

RESULTS

Constrained principal component analysis for fMRI revealed 3 distinct functional networks recruited during encoding: a responding network, a linguistic processing/attention network, and the default mode network. Relative to the healthy control group, the SZ group exhibited aberrant activity in all 3 networks during successful encoding; namely, hypoactivation in the linguistic processing/attention network, lower peak activation in the responding network, and weaker suppression in the default mode network. Independent of group effects, a pattern of stronger anticorrelating linguistic processing/attention-default mode network activity during successful encoding significantly predicted subsequent retrieval of paired associates.

CONCLUSIONS

Together with previous observations of language network hypoactivation during controlled semantic processes, these results suggest that abnormalities in networks representing language and meaning may contribute to difficulties employing deep semantic strategies during relational episodic encoding in SZ.

Greater individual variability in functional brain activity during working memory performance in Schizophrenia Spectrum Disorders (SSD)

Heterogeneity has been a persistent challenge in understanding Schizophrenia Spectrum Disorders (SSD). Traditional case-control comparisons often show variable results, and may not map well onto individuals. To better understand heterogeneity and group differences in SSD compared to typically developing controls (TDC), we examined variability in functional brain activity during a working memory (WM) task with known deficits in SSD. Neuroimaging and behavioural data were extracted from two datasets collectively providing 34 TDC and 56 individuals with SSD (n = 90). Functional activity in response to an N-Back WM task (3-Back vs 1-Back) was examined between and within groups. Individual variability was calculated via the correlational distance of fMRI activity maps between participants; mean correlational distance from one participant to all others was defined as a 'variability score'. Greater individual variability in functional activity was found in SSD compared to TDC (p = 0.00090). At the group level, a case-control comparison suggested SSD had reduced activity in task positive and task negative networks. However, when SSD were divided into high and low variability subgroups, the low variability groups showed no differences relative to TDC while the high variability group showed little activity at the group level. Our results imply prior case-control differences may be driven by a subgroup of SSD who do not show specific impairments but instead show more 'idiosyncratic' activity patterns. In SSD but not TDC, variability was also related to cognitive performance and age. This novel approach focusing on individual variability has important implications for understanding the neurobiology of SSD.

Episodic Memory and Schizophrenia: From Characterization of Relational Memory Impairments to Neuroimaging Biomarkers

Episodic memory research in schizophrenia has a long history already which has clearly established significant impairments and strong associations with brain measures and functional outcome. The purpose of this chapter is not to make an exhaustive review of the recent literature but to highlight some relatively recent developments in the cognitive neuroscience field of episodic memory and schizophrenia. Hence, we present a contemporary view focusing specifically of relational memory which represents a form of episodic memory that refers to associations or binding among items or elements presented together. We describe the major tasks used and illustrate how their combination with brain imaging has: (1) favored the use of experimental memory tasks to isolate specific processes with specific neural correlates, (2) led to a distributed view of the neural correlates of memory impairments in schizophrenia where multiple regions are contributing, and (3) made possible the identification of fMRI biomarkers specific to episodic memory. We then briefly propose what we see as the next steps for memory research in schizophrenia so that the impact of this work can be maximized.

Polygenic Risk for Schizophrenia Has Sex-Specific Effects on Brain Activity during Memory Processing in Healthy Individuals

Genetic risk for schizophrenia has a negative impact on memory and other cognitive abilities in unaffected individuals, and it was recently shown that this effect is specific to males. Using functional MRI, we investigated the effect of a polygenic risk score (PRS) for schizophrenia on brain activation during working memory and episodic memory in 351 unaffected participants (167 males and 184 females, 25–95 years), and specifically tested if any effect of PRS on brain activation is sex-specific. Schizophrenia PRS was significantly associated with decreased brain activation in the left dorsolateral prefrontal cortex (DLPFC) during working-memory manipulation and in the bilateral superior parietal lobule (SPL) during episodic-memory encoding and retrieval. A significant interaction effect between sex and PRS was seen in the bilateral SPL during episodic-memory encoding and retrieval, and sex-stratified analyses showed that the effect of PRS on SPL activation was male-specific. These results confirm previous findings of DLPFC inefficiency in schizophrenia, and highlight the SPL as another important genetic intermediate phenotype of the disease. The observed sex differences suggest that the previously shown male-specific effect of schizophrenia PRS on cognition translates into an additional corresponding effect on brain functioning.

Altered functional connectivity of the dentate nuclei in patients with schizophrenia

Dentate nuclei (DN) are vital structures in the anatomical circuits that link the cerebellum to the cerebrum. However, the characteristics of DN functional connectivity (FC) in schizophrenia remain largely unknown. In this study, we investigated the FC of the DN in patients with schizophrenia and examined their possible clinical correlates using resting-state functional magnetic imaging data. We found that the patient group had greater DN FC with the parietal lobe (e.g., postcentral gyrus and superior parietal lobule) and less DN FC with the prefrontal cortex (e.g., superior frontal gyrus), posterior cingulate cortex, and regional cerebellum (e.g., vermis 4-5 and crus I) than did the control group. Furthermore, some abnormal connectivities of the DN with these regions significantly correlated with psychiatric symptoms. These results suggest that the DN circuits are disturbed and may participate in the pathophysiology of schizophrenia.

Moving beyond the mean: Subgroups and dimensions of brain activity and cognitive performance across domains

Human neuroimaging during cognitive tasks has provided unique and important insights into the neurobiology of cognition. However, the vast majority of research relies on group aggregate or average statistical maps of activity, which do not fully capture the rich intersubject variability in brain function. In order to fully understand the neurobiology of cognitive processes, it is necessary to explore the range of variability in activation patterns across individuals. To better characterize individual variability, hierarchical clustering was performed separately on six fMRI tasks in 822 participants from the Human Connectome Project. Across all tasks, clusters ranged from a predominantly 'deactivating' pattern towards a more 'activating' pattern of brain activity, with significant differences in out-of-scanner cognitive test scores between clusters. Cluster stability was assessed via a resampling approach; a cluster probability matrix was generated, as the probability of any pair of participants clustering together when both were present in a random subsample. Rather than forming distinct clusters, participants fell along a spectrum or into pseudo-clusters without clear boundaries. A principal components analysis of the cluster probability matrix revealed three components explaining over 90% of the variance in clustering. Plotting participants in this lower-dimensional 'similarity space' revealed manifolds of variations along an S 'snake' shaped spectrum or a folded circle or 'tortilla' shape. The 'snake' shape was present in tasks where individual variability related to activity along covarying networks, while the 'tortilla' shape represented multiple networks which varied independently.

Correlations between age, biomedical variables, and cognition in patients with schizophrenia

Objective

To illustrate the influence of clinical variables on cognition performance in patients with schizophrenia (SCZ).

Methods

Using the 66nao Brain Training device (a novel measurement tool), the cognitive performance of 99 patients with SCZ was evaluated. Patients were diagnosed by the ICD-10 diagnostic criteria for SCZ, and their age were 16–68 years old. Furthermore, we explored the relationship between age, biomedical variables and specific cognitive domains in patients with SCZ. Patients were divided into two groups: various of cognitive domains impairment group and non-impairment group according to the norm scores. All data were analyzed using RStudio Version 1.0.44 (RStudio, Inc.)

Results

Patients with SCZ had obvious cognitive impairment in total and five subdomains of cognitive function. We found that 1) SCZ patients with impaired cognitive total score experienced significant older age and longer illness duration compared with those with normal cognitive total score. 2) SCZ patients with impaired memory experienced significant older age compared with those with normal memory. 3) SCZ patients with impaired attention showed significant lower serum triglyceride (TG) level compared with those with normal attention. 4) SCZ patients with impaired flexibility performed significant longer illness duration compared with those with normal flexibility. 5) SCZ patients with impaired cognitive agility performed significant older age, longer duration, and higher systolic blood pressure (SBP) compared with those with normal cognitive agility. 6) The age, illness duration and SBP in patients with impaired time perception were marginally different from those of subjects with normal time perception.

Conclusion

There are five dimensions (memory, attention, flexibility, cognitive agility, and time perception) of cognitive dysfunction in SCZ patients. Age, illness duration, TG, and SBP might play vital roles in various subdomains of the cognitive deficits respectively in patients with SCZ.

MRI-Based Multimodal Approach to the Assessment of Clinical Symptom Severity of Obsessive-Compulsive Disorder

OBJECTIVE

This study assessed the associations of the abnormal brain activation and functional connectivity (FC) during memory processing and brain volume alteration in conjunction with psychiatric symptom severity in patients with obsessive-compulsive disorder (OCD).

METHODS

Twenty-OCD patients and 20-healthy controls (HC) underwent T1-weighted and functional imaging underlying explicit memory task.

RESULTS

In memory encoding, OCD patients showed higher activities in right/left (Rt./Lt.) inferior temporal gyrus (ITG), medial prefrontal cortex (MPFC), dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC), compared with HC. In task-based FC, caudate (Cd) was positively connected with DLPFC and ITG in OCD, while HC showed different connectivities of Cd-ACC and Rt.-Lt. ITG. In memory retrieval, only Cd was activated in OCD patients. Cd was positively connected with DLPFC and vmPFC in OCD, but negatively connected between same brain areas in HC. OCD patients showed increased gray matter (GM) volumes of cerebellum, DLPFC, orbitofrontal cortex (OFC), hippocampus, Cd and ITG, and concurrently, increased white matter volumes of DLPFC. In OCD patients, GM volumes of Cd and OFC were positively correlated with HAMA and Y-BOCS. Functional activity changes of Cd in OCD were positively correlated with Y-BOCS.

CONCLUSION

Our findings support to accessing clinical symptom and its severity linked by brain structural deformation and functional abnormality in OCD patients.

Greater Individual Variability in Functional Brain Activity during Working Memory Performance in young people with Autism and Executive Function Impairment

BACKGROUND

Individuals with autism spectrum disorder (ASD) often present with executive functioning (EF) deficits, including spatial working memory (SWM) impairment, which impedes real-world functioning. The present study examined task-related brain activity, connectivity and individual variability in fMRI-measured neural response during an SWM task in older youth and young adults with autism and clinically significant EF impairment.

METHODS

Neuroimaging was analyzed in 29 individuals with ASD without intellectual disability who had clinically significant EF impairment on the Behavior Rating Inventory of Executive Function, and 20 typically developing controls (participant age range=16-34). An SWM N-Back task was performed during fMRI. SWM activity (2-Back vs. 0-Back) and task-related dorsolateral prefrontal cortex (DLPFC) connectivity was examined within and between groups. Variability of neural response during SWM was also examined.

RESULTS

During SWM performance both groups activated the expected networks, and no group differences in network activation or task-related DLPFC-connectivity were found. However, greater individual variability in the pattern of SWM activity was found in the ASD versus the typically developing control group.

CONCLUSIONS

While there were no group differences in SWM task-evoked activity or connectivity, fronto-parietal network engagement was found to be more variable/idiosyncratic in ASD. Our results suggest that the fronto-parietal network may be shifted or sub-optimally engaged during SWM performance in participants with ASD with clinically significant EF impairment, with implications for developing targeted interventions for this subgroup.

Neural correlates of cognitive deficits across developmental phases of schizophrenia

Schizophrenia is associated with cognitive deficits across all stages of the illness (i.e., high risk, first episode, early and chronic phases). Identifying the underlying neurobiological mechanisms of these deficits is an important area of scientific inquiry. Here, we selectively review evidence regarding the pattern of deficits across the developmental trajectory of schizophrenia using the five cognitive domains identified by the Research Domain Criteria (RDoC) initiative. We also report associated findings from neuroimaging studies. We suggest that most cognitive domains are affected across the developmental trajectory, with corresponding brain structural and/or functional differences. The idea of a common mechanism driving these deficits is discussed, along with implications for cognitive treatment in schizophrenia.

Separable and Replicable Neural Strategies During Social Brain Function in People With and Without Severe Mental Illness

OBJECTIVE

Case-control study design and disease heterogeneity may impede biomarker discovery in brain disorders, including serious mental illnesses. To identify biologically and/or behaviorally driven as opposed to diagnostically driven subgroups of individuals, the authors used hierarchical clustering to identify individuals with similar patterns of brain activity during a facial imitate/observe functional MRI task.

METHODS

Participants in the Social Processes Initiative in Neurobiology of the Schizophrenia(s) study (N=179; 109 with a schizophrenia spectrum disorder and 70 healthy control participants) underwent MRI scanning at three sites. Hierarchical clustering was used to identify new data-driven groups of participants; differences on social and neurocognitive tests completed outside the scanner were compared among the new groups.

RESULTS

Three clusters with distinct patterns of neural activity were found. Cluster membership was not related to diagnosis or scan site. The largest cluster consisted of "typical activators," with activity in the canonical "simulation" circuit. The other clusters represented a "hyperactivating" group and a "deactivating" group. Between-participants Euclidean distances were smaller within clusters than within site or diagnostics groups. The deactivating group had the highest social cognitive and neurocognitive test scores. The hierarchical clustering analysis was repeated on a replication sample (N=108; 32 schizophrenia spectrum disorder, 37 euthymic bipolar disorder, and 39 healthy control participants), which exhibited the same three cluster patterns.

CONCLUSIONS

The study findings demonstrate replicable differing patterns of neural activity among individuals during a socio-emotional task, independent of DSM diagnosis or scan site. The findings may provide objective neuroimaging endpoints (biomarkers) for subgroups of individuals in target engagement research aimed at enhancing cognitive performance independent of diagnostic category.

Memory and cognition in schizophrenia

Episodic memory deficits are consistently documented as a core aspect of cognitive dysfunction in schizophrenia patients, present from the onset of the illness and strongly associated with functional disability. Over the past decade, research using approaches from experimental cognitive neuroscience revealed disproportionate episodic memory impairments in schizophrenia (Sz) under high cognitive demand relational encoding conditions and relatively unimpaired performance under item-specific encoding conditions. These specific deficits in component processes of episodic memory reflect impaired activation and connectivity within specific elements of frontal-medial temporal lobe circuits, with a central role for the dorsolateral prefrontal cortex (DLPFC), relatively intact function of ventrolateral prefrontal cortex and variable results in the hippocampus. We propose that memory deficits can be understood within the broader context of cognitive deficits in Sz, where impaired DLPFC-related cognitive control has a broad impact across multiple cognitive domains. The therapeutic implications of these findings are discussed.

Neurocognitive and Perceptual Processing in Genetic Mouse Models of Schizophrenia: Emerging Lessons

During the past two decades, the number of animal models of psychiatric disorders has grown exponentially. Of these, genetic animal models that are modeled after rare but highly penetrant mutations hold great promise for deciphering critical molecular, synaptic, and neurocircuitry deficits of major psychiatric disorders, such as schizophrenia. Animal models should aim to focus on core aspects rather than capture the entire human disease. In this context, animal models with strong etiological validity, where behavioral and neurophysiological phenotypes and the features of the disease being modeled are in unambiguous homology, are being used to dissect both elementary and complex cognitive and perceptual processing deficits present in psychiatric disorders at the level of neurocircuitry, shedding new light on critical disease mechanisms. Recent progress in neuroscience along with large-scale initiatives that propose a consistent approach in characterizing these deficits across different laboratories will further enhance the efficacy of these studies that will ultimately lead to identifying new biological targets for drug development.

Brain Responses at Encoding Predict Limited Verbal Memory Retrieval by Persons with Schizophrenia

Objective

Special attention has been given to verbal memory deficits in schizophrenia because they are apparent in healthy biological relatives of affected individuals, indicating a link to genetic risk for the disorder. Despite a growing consensus that encoding abnormalities contribute to poor verbal memory in the disorder, few studies have directly examined how neural responses during encoding contribute to later memory performance.

Method

We evaluated event-related potentials (ERPs) during encoding of verbal material by patients with schizophrenia, healthy first-degree biological relatives of patients, and healthy controls. The extent to which N1, N400, and anterior and parietal Late Positive Components (LPCs) explained encoding accuracy and later memory of material was investigated.

Results

Encoding accuracy was associated with asymmetry in anterior LPCs toward right frontal brain regions and was most evident in relatives. N1 was abnormal at encoding in schizophrenia and differentially accounted for later memory performance. In controls better recall of verbal material was predicted by a larger early occipital (N1) encoding response; however, in patients with schizophrenia smaller N1 encoding responses were related to better recall. Interestingly, better recognition of verbal material across groups was also predicted by smaller N1 amplitudes during encoding of word stimuli.

Conclusion

Separable patterns of electrophysiological response during encoding appear to differentially support recall and recognition of material from memory. Similar patterns of electrophysiological response across patient and relative groups suggest that those who carry genetic liability for schizophrenia share deviations in the neural activity related to encoding of material into episodic memory.

Differential patterns of contextual organization of memory in first-episode psychosis

Contextual information is used to support and organize episodic memory. Prior research has reliably shown memory deficits in psychosis; however, little research has characterized how this population uses contextual information during memory recall. We employed an approach founded in a computational framework of free recall to quantify how individuals with first episode of psychosis (FEP, N = 97) and controls (CON, N = 55) use temporal and semantic context to organize memory recall. Free recall was characterized using the Hopkins Verbal Learning Test-Revised (HVLT-R). We compared FEP and CON on three measures of free recall: proportion recalled, temporal clustering, and semantic clustering. Measures of temporal/semantic clustering quantified how individuals use contextual information to organize memory recall. We also assessed to what extent these measures relate to antipsychotic use and differentiated between different types of psychosis. We also explored the relationship between these measures and intelligence. In comparison to CON, FEP had reduced recall and less temporal clustering during free recall (p < 0.05, Bonferroni-corrected), and showed a trend towards greater semantic clustering (p = 0.10, Bonferroni-corrected). Within FEP, antipsychotic use and diagnoses did not differentiate between free recall accuracy or contextual organization of memory. IQ was related to free recall accuracy, but not the use of contextual information during recall in either group (p < 0.05, Bonferroni-corrected). These results show that in addition to deficits in memory recall, FEP differed in how they organize memories compared to CON.

First-episode psychosis patients exhibit impaired memory recall and deviation in how context is used to support recall ability. A US team of researchers led by the University of Pittsburgh’s Vishnu Murty examined how FEP affects an individual’s ability to organize memory based on context, by noting how well patients could recall words from a spoken list. Alongside recollection accuracy, Murty’s team assesed participant ability to recall words said proximally in sequence, and the ability to recall words from the same category—measuring ‘temporal clustering’ and ‘semantic clustering.’ The researchers found that patients with FEP had reduced recall ability and less temporal clustering. Recall accuracy and IQ were also found to be related. This study increases knowledge of FEP-related cognitive changes and could help to target specific therapies.

Cognitive Functions and Neurodevelopmental Disorders Involving the Prefrontal Cortex and Mediodorsal Thalamus

The mediodorsal nucleus of the thalamus (MD) has been implicated in executive functions (such as planning, cognitive control, working memory, and decision-making) because of its significant interconnectivity with the prefrontal cortex (PFC). Yet, whilst the roles of the PFC have been extensively studied, how the MD contributes to these cognitive functions remains relatively unclear. Recently, causal evidence in monkeys has demonstrated that in everyday tasks involving rapid updating (e.g., while learning something new, making decisions, or planning the next move), the MD and frontal cortex are working in close partnership. Furthermore, researchers studying the MD in rodents have been able to probe the underlying mechanisms of this relationship to give greater insights into how the frontal cortex and MD might interact during the performance of these essential tasks. This review summarizes the circuitry and known neuromodulators of the MD, and considers the most recent behavioral, cognitive, and neurophysiological studies conducted in monkeys and rodents; in total, this evidence demonstrates that MD makes a critical contribution to cognitive functions. We propose that communication occurs between the MD and the frontal cortex in an ongoing, fluid manner during rapid cognitive operations, via the means of efference copies of messages passed through transthalamic routes; the conductance of these messages may be modulated by other brain structures interconnected to the MD. This is similar to the way in which other thalamic structures have been suggested to carry out forward modeling associated with rapid motor responding and visual processing. Given this, and the marked thalamic pathophysiology now identified in many neuropsychiatric disorders, we suggest that changes in the different subdivisions of the MD and their interconnections with the cortex could plausibly give rise to a number of the otherwise disparate symptoms (including changes to olfaction and cognitive functioning) that are associated with many different neuropsychiatric disorders. In particular, we will focus here on the cognitive symptoms of schizophrenia and suggest testable hypotheses about how changes to MD-frontal cortex interactions may affect cognitive processes in this disorder.

Strategy for Semantic Association Memory (SESAME) training: Effects on brain functioning in schizophrenia

Self-initiation of semantic encoding strategies is impoverished in schizophrenia and contributes to memory impairments. Recently, we observed that following a brief training, schizophrenia patients had the potential to increase the self-initiation of these strategies. In this study, we investigated the neural correlates underlying such memory improvements. Fifteen schizophrenia patients with deficits in self-initiation of semantic encoding strategies were enrolled in a Strategy for Semantic Association Memory (SESAME) training. Patients underwent a memory task in an fMRI scanner. Memory performance and brain activity during the task were measured pre- and post- training, and changes following training were assessed. We also investigated if structural preservation measured by the cortical thickness of the left dorsolateral prefrontal cortex (DLPFC) predicted memory improvement post-training. Memory training led to significant improvements in memory performance that were associated with increased activity in the left DLPFC, during a task in which patients needed to self-initiate semantic encoding strategies. Furthermore, patients with more cortical reserve in their left DLPFC showed greater memory improvement. Our findings provide evidence of neural malleability in the left DLPFC in schizophrenia using cognitive strategies training. Moreover, the brain-behavioural changes observed in schizophrenia provide hope that memory performance can be improved with a brief intervention.

Differing Time of Onset of Concurrent TMS-fMRI during Associative Memory Encoding: A Measure of Dynamic Connectivity

There has been a distinct shift in neuroimaging from localization of function into a more network based approach focused on connectivity. While fMRI has proven very fruitful for this, the hemodynamic signal is inherently slow which limits the temporal resolution of fMRI-only connectivity measures. The brain, however, works on a time scale of milliseconds. This study utilized concurrent transcranial magnetic stimulation (TMS)-fMRI in a novel way to obtain measures of dynamic connectivity by measuring changes in fMRI signal amplitude in regions distal to the site of stimulation following differing TMS onset times. Seventeen healthy subjects completed an associative memory encoding task known to involve the DLPFC, viewing pairs of objects which could be semantically related or unrelated. Three pulses of 10 Hz repetitive TMS were applied over the left DLPFC starting either at 200, 600, or 1000 ms after stimulus onset. Associations for related pairs were better remembered than unrelated pairs in a post-scan cued recall test. Differences in neural activity were assessed across different TMS onsets, separately for related and unrelated pairs. Time specific TMS effects were observed in several regions, including those associated with higher-level processing (lateral frontal, anterior cingulate), visual areas (occipital), and regions involved in semantic processing (e.g., left mid-temporal and medial frontal). Activity in the frontal cortex was decreased at 200 ms post-stimulus for unrelated pairs, and 1000 ms post-stimulus for related pairs. This suggests differences in the timing across conditions in which the DLFPC interacts with other PFC regions, consistent with the notion that the DLPFC is facilitating extended semantic processing for related items. This study demonstrates that time-varying TMS onset inside the MRI can be used to reliably measure fast dynamic connectivity with a temporal resolution in the hundreds of milliseconds.