Intact relational memory and normal hippocampal structure in the early stage of psychosis

Cross-modal associative memory impairment in schizophrenia

Impaired associative memory function in patients with schizophrenia has received considerable attention. However, previous studies have primarily concentrated on unisensory materials, which limits our understanding of the broader implications of this impairment. In this study, we sought to expand on this knowledge by examining two types of associative memory domains in individuals with schizophrenia, leveraging both visual (Vis) and auditory (Aud) materials. A total of 32 patients with schizophrenia and 29 healthy controls were recruited to participate in the study. Each participant participated in an experiment composed of three paradigms in which different abstract materials (Aud-Aud, Aud-Vis, and Vis-Vis) were presented. Subsequently, the discriminability scores of the two groups were calculated and compared in different modal tasks. Results from the study indicated that individuals with schizophrenia demonstrated varying degrees of associative memory dysfunction in both the same and cross-modalities, with the latter having a significantly lower score than healthy controls (t = 4.120, p < 0.001). Additionally, the cross-modal associative memory function was significantly and negatively correlated with the severity of negative symptoms among individuals diagnosed with schizophrenia (r = -0.362, p = 0.042). This study provides evidence of abnormalities in the processing and memorization of information that integrates multiple sensory modalities in individuals with schizophrenia. This is of great significance for further understanding the cognitive symptoms and pathological mechanisms of schizophrenia, potentially guiding the development of relevant interventions and treatment methods.

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.

Neural correlates of transitive inference: An SDM meta-analysis on 32 fMRI studies

Transitive inference (TI) is a critical capacity involving the integration of relevant information into prior knowledge structure for drawing novel inferences on unobserved relationships. To date, the neural correlates of TI remain unclear due to the small sample size and heterogeneity of various experimental tasks from individual studies. Here, the meta-analysis on 32 fMRI studies was performed to detect brain activation patterns of TI and its three paradigms (spatial inference, hierarchical inference, and associative inference). We found the hippocampus, prefrontal cortex (PFC), putamen, posterior parietal cortex (PPC), retrosplenial cortex (RSC), supplementary motor area (SMA), precentral gyrus (PreCG), and median cingulate cortex (MCC) were engaged in TI. Specifically, the RSC was implicated in the associative inference, whereas PPC, SMA, PreCG, and MCC were implicated in the hierarchical inference. In addition, the hierarchical inference and associative inference both evoked activation in the hippocampus, medial PFC, and PCC. Although the meta-analysis on spatial inference did not generate a reliable result due to insufficient amount of investigations, the present work still offers a new insight for better understanding the neural basis underlying TI.

Impaired theta phase coupling underlies frontotemporal dysconnectivity in schizophrenia

Frontotemporal dysconnectivity is a key pathology in schizophrenia. The specific nature of this dysconnectivity is unknown, but animal models imply dysfunctional theta phase coupling between hippocampus and medial prefrontal cortex (mPFC). We tested this hypothesis by examining neural dynamics in 18 participants with a schizophrenia diagnosis, both medicated and unmedicated; and 26 age, sex and IQ matched control subjects. All participants completed two tasks known to elicit hippocampal-prefrontal theta coupling: a spatial memory task (during magnetoencephalography) and a memory integration task. In addition, an overlapping group of 33 schizophrenia and 29 control subjects underwent PET to measure the availability of GABAARs expressing the α5 subunit (concentrated on hippocampal somatostatin interneurons). We demonstrate-in the spatial memory task, during memory recall-that theta power increases in left medial temporal lobe (mTL) are impaired in schizophrenia, as is theta phase coupling between mPFC and mTL. Importantly, the latter cannot be explained by theta power changes, head movement, antipsychotics, cannabis use, or IQ, and is not found in other frequency bands. Moreover, mPFC-mTL theta coupling correlated strongly with performance in controls, but not in subjects with schizophrenia, who were mildly impaired at the spatial memory task and no better than chance on the memory integration task. Finally, mTL regions showing reduced phase coupling in schizophrenia magnetoencephalography participants overlapped substantially with areas of diminished α5-GABAAR availability in the wider schizophrenia PET sample. These results indicate that mPFC-mTL dysconnectivity in schizophrenia is due to a loss of theta phase coupling, and imply α5-GABAARs (and the cells that express them) have a role in this process.

Visual exploration differences during relational memory encoding in early psychosis

Relational memory, or the ability to form contextual associations among items encountered closely in time, is impaired in schizophrenia. The ability to bind items into a relational memory is dependent on the hippocampus, a region that is abnormal in schizophrenia. However, the hippocampus is also involved in exploratory behavior, leaving open the question whether relational memory deficits in schizophrenia are due to failure of relational binding or diminished visual exploration of individual items during encoding. We studied visual exploration patterns during the encoding of face-scene pairs in 66 healthy control subjects and 69 early psychosis patients, to test the hypothesis that differences in visual exploration during the encoding phase can explain task accuracy differences between the two groups. Psychosis patients had lower explicit test accuracy and were less likely to transition from mouth to eyes during encoding. The visual exploration pattern differences between groups did not mediate the relationship between group and explicit test accuracy. We conclude that early psychosis patients have an abnormal pattern of binding items together during encoding that warrants further research.

Cortico-thalamic dysconnection in early-stage schizophrenia: a functional connectivity magnetic resonance imaging study

Studies have indicated thalamus-related network dysfunction in schizophrenia and psychotic disorders. However, whether thalamus-related functional connectivity (FC) contributes to the psychopathology and cognitive deficits of early-stage schizophrenia requires further investigation. A total of 34 patients with early-stage schizophrenia (illness duration = 1.62 ± 1.16 years; age = 26.00 ± 6.34 years) and 34 age- and sex-matched healthy controls were enrolled in our study and underwent comprehensive assessments of the clinical symptoms of schizophrenia, working memory tasks, and resting-state FC magnetic resonance imaging. The patients with early-stage schizophrenia had increased FC of the thalamus with the bilateral postcentral and temporal gyri, inferior occipital cortex, and temporal pole and decreased FC of the thalamus with the vestibulocerebellum and frontal pole compared with the controls. Furthermore, increased FC between the thalamus and temporal pole was positively correlated with positive scores of the Positive and Negative Syndrome Scale for Schizophrenia (PANSS) and negatively correlated with performance on working memory tasks in early-stage schizophrenia. Increased FC of the thalamus with the inferior occipital cortex was positively associated with negative PANSS scores and negatively correlated with Personal and Social Performance Scale scores in early-stage schizophrenia. Our results supported the vital role of thalamus-related network dysfunction in the psychopathology and cognitive deficits of early-stage schizophrenia.

Impaired relational memory in the early stage of psychosis

BACKGROUND

Humans constantly take in vast amounts of information, which must be filtered, flexibly manipulated, and integrated into cohesive relational memories in order to choose relevant behaviors. Relational memory is impaired in chronic schizophrenia, which has been linked to hippocampal dysfunction. It is unclear whether relational memory is impaired in the early stage of psychosis.

METHODS

We studied eye movements during a face-scene pairs task as an indirect measure of relational memory in 89 patients in the early stage of psychosis and 84 healthy control participants. During testing, scenes were overlaid with three equally-familiar faces and participants were asked to recall the matching (i.e. previously-paired) face. During Match trials, one face had been previously paired with the scene. During Non-Match trials, no faces matched the scene. Forced-choice explicit recognition was recorded as a direct measure of relational memory.

RESULTS

Healthy control subjects rapidly (within 250-500 ms) showed preferential viewing of the matching face during Match trials. In contrast, preferential viewing was delayed in patients in the early stage of psychosis. Explicit recognition of the matching face was also impaired in the patient group.

CONCLUSIONS

This study provides novel evidence for a relational memory deficit in the early stage of psychosis. Patients showed deficits in both explicit recognition as well as abnormal eye-movement patterns during memory recall. Eye movements provide a promising avenue for the study of relational memory in psychosis, as they allow for the assessment of rapid, nonverbal memory processes.

Disrupted Habituation in the Early Stage of Psychosis

BACKGROUND

Learning and memory are impaired in schizophrenia. Some theories have proposed that one form of memory, habituation, is particularly impaired. Preliminary evidence suggests that memory impairment is associated with failed hippocampal habituation in patients with chronic schizophrenia. We studied how abnormal habituation of the hippocampus is related to relational memory deficits in the early stage of psychosis.

METHODS

We measured hippocampal activity in 62 patients with early psychosis and 70 healthy individuals using functional magnetic resonance imaging. Habituation was defined as the slope of functional magnetic resonance imaging signal change to repeated presentations of faces and objects. Relational memory ability was measured as the slope of preferential viewing during a face-scene pair eye movement task outside the scanner.

RESULTS

Patients with early psychosis showed impaired relational memory (p < .001) and less hippocampal habituation to objects (p = .01) than healthy control subjects. In the healthy control group, better relational memory was associated with faster anterior hippocampal habituation (faces, r = -.28, p = .03). In contrast, patients with early psychosis showed no brain-behavior relationship (r = .12, p = .40).

CONCLUSIONS

We found evidence for disrupted hippocampal habituation in the early stage of psychosis along with an altered association between hippocampal habituation and relational memory ability. These results suggest that neural habituation may provide a novel target for early cognitive interventions in psychosis.

Impaired associative inference in the early stage of psychosis

Relational memory is impaired in chronic schizophrenia. It is unclear if similar deficits are already present in the early stage of psychosis. We used the Associative Inference Paradigm to test relational memory ability in the early stage of a non-affective psychotic disorder. Eighty-two early stage psychosis patients and 67 healthy control subjects were trained on 3 sets of 30 paired associates: H-F1 (house paired with face), H-F2 (same house paired with new face), F3-F4 (two new faces). Subjects who reached 80% recall accuracy of the paired associates during training were then tested for their ability to recall the previously seen pairs and solve a novel, inferential pairing F1-F2 (faces linked through association to same house). Sixty early psychosis patients (73%) and 67 healthy control subjects (100%) successfully reached the accuracy threshold (80%) during training and were included in the analysis of relational memory. The early stage psychosis patients showed less of an associative inference effect than the healthy controls (pair type by group interaction: F (1,125) = 5.04, p < 0.05). However, the majority of early psychosis patients (52%) displayed intact inferential memory, compared to our prior study which revealed just 16% of chronic schizophrenia patients had intact inferential memory. Patients in the early stage of psychosis show a relational memory deficit, although less pronounced than in chronic schizophrenia. Longitudinal studies are needed to examine the progression of relational memory deficits in schizophrenia and its associations with clinical, functional, and biological measures.

Regionally specific volume deficits along the hippocampal long axis in early and chronic psychosis

Previous studies in psychosis patients have shown hippocampal volume deficits across anterior and posterior regions or across subfields, but subfield specific changes in volume along the hippocampal long axis have not been examined. Here, we tested the hypothesis that volume changes exist across the hippocampus in chronic psychosis but only the anterior CA region is affected in early psychosis patients. We analyzed structural MRI data from 179 patients with a non-affective psychotic disorder (94 chronic psychosis; 85 early psychosis) and 167 heathy individuals demographically matched to the chronic and early psychosis samples respectively (82 matched to chronic patients; 85 matched to early patients). We measured hippocampal volumes using Freesurfer 6-derived automated segmentation of both anterior and posterior regions and the CA, dentate gyrus, and subiculum subfields. We found a hippocampal volume deficit in both anterior and posterior regions in chronic psychosis, but this deficit was limited to the anterior hippocampus in early psychosis patients. This volume change was more pronounced in the anterior CA subfield of early psychosis patients than in the dentate gyrus or subiculum. Our findings support existing models of psychosis implicating initial CA dysfunction with later progression to other hippocampal regions and suggest that the anterior hippocampus may be an important target for early interventions.

Volume alteration of hippocampal subfields in first-episode antipsychotic-naïve schizophrenia patients before and after acute antipsychotic treatment

The nature of hippocampal changes in schizophrenia before first treatment, and whether hippocampal subfields are affected by antipsychotic treatment are important questions for schizophrenia research. Forty-one first-episode antipsychotic-naïve acutely ill schizophrenia inpatients had MRI scans before and six weeks after antipsychotic treatment. Thirty-nine matched healthy controls were also scanned, twenty-two of which were scanned a second time six weeks later. Volumes of hippocampal subfields were measured via FreeSurfer v6.0 using a longitudinal analysis pipeline. Before treatment, schizophrenia patients had no significant changes in total hippocampal volume but exhibited significantly greater subfield volumes than controls in bilateral molecular layers of the hippocampus (ML), bilateral granular cell layers of the dentate gyrus (GC-DG), and bilateral cornu ammonis area 4 (CA4). After six weeks of antipsychotic treatment, patients showed volume reductions compared with pretreatment scans in total hippocampus bilaterally, with subfield volume reduction noted in previously enlarged subfields (i.e., bilateral ML, GC-DG and CA4) and in bilateral hippocampal tails, left CA1, CA3, and fimbria. Subfields with volume increases before treatment were reduced to the level of healthy controls (bilateral ML and GC-DG) or near to it (bilateral CA4) after treatment. These results indicate subfield-specific hippocampal hypertrophy prior to treatment, and that these abnormalities were reduced after acute antipsychotic therapy in a dose-related manner together with volume reductions in other areas that were not hypertrophic before treatment.

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Specific hippocampal subfields were enlarged in patients before treatment.

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Volume decrease in regions with dense D2 receptors (CA3–4 and DG) after treatment.

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Most enlarged subfields pretreatment were reduced to normal level after treatment.

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Dosage of antipsychotics was associated with the degree of volume reduction.

Deterioration of visuospatial associative memory following a first psychotic episode: a long-term follow-up study

BACKGROUND

Cognitive deficits are a core feature of schizophrenia, and impairments in most domains are thought to be stable over the course of the illness. However, cross-sectional evidence indicates that some areas of cognition, such as visuospatial associative memory, may be preserved in the early stages of psychosis, but become impaired in later established illness stages. This longitudinal study investigated change in visuospatial and verbal associative memory following psychosis onset.

METHODS

In total 95 first-episode psychosis (FEP) patients and 63 healthy controls (HC) were assessed on neuropsychological tests at baseline, with 38 FEP and 22 HCs returning for follow-up assessment at 5-11 years. Visuospatial associative memory was assessed using the Cambridge Neuropsychological Test Automated Battery Visuospatial Paired-Associate Learning task, and verbal associative memory was assessed using Verbal Paired Associates subtest of the Wechsler Memory Scale - Revised.

RESULTS

Visuospatial and verbal associative memory at baseline did not differ significantly between FEP patients and HCs. However, over follow-up, visuospatial associative memory deteriorated significantly for the FEP group, relative to healthy individuals. Conversely, verbal associative memory improved to a similar degree observed in HCs. In the FEP cohort, visuospatial (but not verbal) associative memory ability at baseline was associated with functional outcome at follow-up.

CONCLUSIONS

Areas of cognition that develop prior to psychosis onset, such as visuospatial and verbal associative memory, may be preserved early in the illness. Later deterioration in visuospatial memory ability may relate to progressive structural and functional brain abnormalities that occurs following psychosis onset.

Transitive inference deficits in unaffected biological relatives of schizophrenia patients

Currently available treatments have limited efficacy in remediating cognitive impairment in schizophrenia. Efforts to facilitate cognition-enhancing drug discovery recommend the use of varied experimental cognitive paradigms (including relational memory) as assessment tools in clinical drug trials. Although relational memory deficits are increasingly being recognized as a reliable cognitive marker of schizophrenia, relational memory performance among unaffected biological relatives remains unknown. Therefore, we evaluated 73 adolescents or young adults (22 first- and 26 second-degree relatives of schizophrenia patients and 25 healthy controls (HC)) using a well-validated transitive inference (TI) experimental paradigm previously used to demonstrate relational memory impairment in schizophrenia. We found that TI deficits were associated with schizophrenia risk with first-degree relatives showing greater impairment than second-degree relatives. First-degree relatives had poorer TI performance with significantly lower accuracy and longer response times than HC when responding to TI probe pairs. Second-degree relatives had significantly quicker response times than first-degree relatives and were more similar to HC in TI performance. We further explored the relationships between TI performance and neurocognitive domains implicated in schizophrenia. Among HC, response times were inversely correlated with FSIQ, verbal learning, processing speed, linguistic abilities and working memory. In contrast, relatives (first-degree in particular) had a differing pattern of TI-neurocognition relationships, which suggest that different brain circuits may be used when relatives encode and retrieve relational memory. Our finding that unaffected biological relatives of schizophrenia patients have TI deficits lends further support for the use of relational memory construct in future pro-cognition drug studies.

Mapping Thalamocortical Functional Connectivity in Chronic and Early Stages of Psychotic Disorders

BACKGROUND

There is considerable evidence that the thalamus is abnormal in psychotic disorders. Resting-state functional magnetic resonance imaging has revealed an intriguing pattern of thalamic dysconnectivity in psychosis characterized by reduced prefrontal cortex (PFC) connectivity and increased somatomotor-thalamic connectivity. However, critical knowledge gaps remain with respect to the onset, anatomical specificity, and clinical correlates of thalamic dysconnectivity in psychosis.

METHODS

Resting-state functional magnetic resonance imaging was collected on 105 healthy subjects and 148 individuals with psychosis, including 53 early-stage psychosis patients. Using all 253 subjects, the thalamus was parceled into functional regions of interest (ROIs) on the basis of connectivity with six a priori defined cortical ROIs covering most of the cortical mantle. Functional connectivity between each cortical ROI and its corresponding thalamic ROI was quantified and compared across groups. Significant differences in the ROI-to-ROI analysis were followed up with voxelwise seed-based analyses to further localize thalamic dysconnectivity.

RESULTS

ROI analysis revealed reduced PFC-thalamic connectivity and increased somatomotor-thalamic connectivity in both chronic and early-stage psychosis patients. PFC hypoconnectivity and motor cortex hyperconnectivity correlated in patients, suggesting that they result from a common pathophysiological mechanism. Seed-based analyses revealed thalamic hypoconnectivity in psychosis localized to dorsolateral PFC, medial PFC, and cerebellar areas of the well-described executive control network. Across all subjects, thalamic connectivity with areas of the fronto-parietal network correlated with cognitive functioning, including verbal learning and memory.

CONCLUSIONS

Thalamocortical dysconnectivity is present in both chronic and early stages of psychosis, includes reduced thalamic connectivity with the executive control network, and is related to cognitive impairment.

Cognitive impairments in psychotic disorders: common mechanisms and measurement

Decades of research have provided robust evidence of cognitive impairments in psychotic disorders. Individuals with schizophrenia appear to be impaired on the majority of neuropsychological tasks, leading some researchers to argue for a "generalized deficit", in which the multitude of cognitive impairments are the result of a common neurobiological source. One such common mechanism may be an inability to actively represent goal information in working memory as a means to guide behavior, with the associated neurobiological impairment being a disturbance in the function of the dorsolateral prefrontal cortex. Here, we provide a discussion of the evidence for such impairment in schizophrenia, and how it manifests in domains typically referred to as cognitive control, working memory and episodic memory. We also briefly discuss cognitive impairment in affective psychoses, reporting that the degree of impairment is worse in schizophrenia than in bipolar disorder and psychotic major depression, but the profile of impairment is similar, possibly reflecting common mechanisms at the neural level. Given the recent release of the DSM-5, we end with a brief discussion on assessing cognition in the context of diagnosis and treatment planning in psychotic disorders.

Relational memory generalization and integration in a transitive inference task with and without instructed awareness

Two experiments investigated the potential facilitative effects of prior instructed awareness and predetermined learning criteria on humans' ability to make transitive inference (TI) judgments. Participants were first exposed to a learning phase and required to learn five premise pairs (A+B-, B+C-, C+D-, D+E-, E+F-). Testing followed, where participants made judgments on novel non-endpoint (BD, BE and CE) and endpoint inferential pairs (AC, AD, AE, AF, BF, CF and DF), as well as learned premise pairs. Across both experiments, one group were made aware that the stimuli could be arranged in a hierarchy, while another group were not given this instruction. Results demonstrated that prior instructional task awareness led to a minor performance advantage, but that this difference was not significant. Furthermore, in Experiment 2, inferential test trial accuracy was not correlated with a post-experimental measure of awareness. Thus, the current findings suggest that successful TI task performance may occur in the absence of awareness, and that repeated exposure to learning and test phases may allow weak inferential performances to emerge gradually. Further research and alternative methods of measuring awareness and its role in TI are needed.

Relational memory and hippocampal function in psychotic bipolar disorder

Recent cognitive, genetic, and histological studies have highlighted significant overlap between psychotic bipolar disorder and schizophrenia. Specifically, both bipolar disorder and schizophrenia are characterized by interneuron dysfunction within the hippocampus, an essential structure for relational memory. Relational memory impairments are a common feature of schizophrenia, but have yet to be investigated in psychotic bipolar disorder. Here, we tested the hypothesis that psychotic bipolar disorder is characterized by relational memory deficits. We used a transitive inference (TI) paradigm, previously employed to quantify relational memory deficits in schizophrenia, to assess relational memory performance in 17 patients with psychotic bipolar disorder and 22 demographically matched control participants. Functional magnetic resonance imaging was used to examine hippocampal activity during recognition memory in patients and controls. Hippocampal volumes were assessed by manual segmentation. In contrast to our hypothesis, we found similar TI performance, hippocampal volume, and hippocampal recruitment during recognition memory in both groups. Both psychotic bipolar disorder patients and controls exhibited a positive correlation between hippocampal volume and relational memory performance. These data indicate that relational memory impairments are not a shared feature of non-affective and affective psychosis.