Evolution of neuropsychological dysfunction during the course of schizophrenia and bipolar disorder

BACKGROUND

Neurocognitive dysfunction in schizophrenia (SZ), bipolar (BD) and related disorders represents a core feature of these illnesses, possibly a marker of underlying pathophysiology. Substantial overlap in domains of neuropsychological deficits has been reported among these disorders after illness onset. However, it is unclear whether deficits follow the same longitudinal pre- and post-morbid course across diagnoses. We examine evidence for neurocognitive dysfunction as a core feature of all idiopathic psychotic illnesses, and trace its evolution from pre-morbid and prodromal states through the emergence of overt psychosis and into chronic illness in patients with SZ, BD and related disorders.

METHOD

Articles reporting on neuropsychological functioning in patients with SZ, BD and related disorders before and after illness onset were reviewed. Given the vast literature on these topics and the present focus on cross-diagnostic comparisons, priority was given to primary data papers that assessed cross-diagnostic samples and recent meta-analyses.

RESULTS

Patients with SZ exhibit dysfunction preceding the onset of illness, which becomes more pronounced in the prodrome and early years following diagnosis, then settles into a stable pattern. Patients with BD generally exhibit typical cognitive development pre-morbidly, but demonstrate deficits by first episode that are amplified with worsening symptoms and exacerbations.

CONCLUSIONS

Neuropsychological deficits represent a core feature of SZ and BD; however, their onset and progression differ between diagnostic groups. A lifetime perspective on the evolution of neurocognitive deficits in SZ and BD reveals distinct patterns, and may provide a useful guide to the examination of the pathophysiological processes underpinning these functions across disorders.

Characterizing cognitive heterogeneity on the schizophrenia-bipolar disorder spectrum

BACKGROUND

Current group-average analysis suggests quantitative but not qualitative cognitive differences between schizophrenia (SZ) and bipolar disorder (BD). There is increasing recognition that cognitive within-group heterogeneity exists in both disorders, but it remains unclear as to whether between-group comparisons of performance in cognitive subgroups emerging from within each of these nosological categories uphold group-average findings. We addressed this by identifying cognitive subgroups in large samples of SZ and BD patients independently, and comparing their cognitive profiles. The utility of a cross-diagnostic clustering approach to understanding cognitive heterogeneity in these patients was also explored.

METHOD

Hierarchical clustering analyses were conducted using cognitive data from 1541 participants (SZ n = 564, BD n = 402, healthy control n = 575).

RESULTS

Three qualitatively and quantitatively similar clusters emerged within each clinical group: a severely impaired cluster, a mild-moderately impaired cluster and a relatively intact cognitive cluster. A cross-diagnostic clustering solution also resulted in three subgroups and was superior in reducing cognitive heterogeneity compared with disorder clustering independently.

CONCLUSIONS

Quantitative SZ-BD cognitive differences commonly seen using group averages did not hold when cognitive heterogeneity was factored into our sample. Members of each corresponding subgroup, irrespective of diagnosis, might be manifesting the outcome of differences in shared cognitive risk factors.

Testing models of thalamic dysfunction in schizophrenia using neuroimaging

Neural models of schizophrenia have implicated the thalamus in deficits of early sensory processing and multimodal integration. We have reviewed the existing neuroimaging literature for evidence in support of models that propose abnormalities of thalamic relay nuclei, the mediodorsal thalamic nucleus, and large-scale cortico-thalamic networks. Thalamic volume reduction was found in some but not all studies. Studies of the early stages of schizophrenia suggest that thalamic volume reduction is present early in the course of the illness. Functional imaging studies have revealed task related abnormalities in several cortical and subcortical areas including the thalamus, suggesting a disruption of distributed thalamocortical networks. Chemical imaging studies have provided evidence for a loss of thalamic neuronal integrity in schizophrenia. There is, at present, inadequate data to support the hypothesis that schizophrenia is associated with abnormalities of sensory relay or association nuclei. There is evidence for a perturbation of cortico-thalamic networks, but further research is needed to elucidate the underlying mechanisms at the cellular and systems levels. The challenges ahead include better delineation of thalamic structure and function in vivo, the combination of genetic and imaging techniques to elucidate the genetic contributions to a thalamic phenotype of schizophrenia, and longitudinal studies of thalamic structure and function.

Functional disconnection between the visual cortex and right fusiform face area in schizophrenia

Patients with schizophrenia show impairment in processing faces, including facial affect and face detection, but the underlying mechanisms are unknown. We used functional magnetic resonance imaging (fMRI) to characterize resting state functional connectivity between an independent component analysis (ICA)-defined early visual cortical network (corresponding to regions in V1, V2, V3) and a priori defined face-processing regions (fusiform face area [FFA], occipital face area [OFA], superior temporal sulcus [STS] and amygdala) using dual regression in 20 schizophrenia patients and 26 healthy controls. We also investigated the association between resting functional connectivity and neural responses (fMRI) elicited by a face detection paradigm in a partially overlapping sample (Maher et al., 2016) that used stimuli equated for lower-level perceptual abilities. Group differences in functional connectivity were found in right FFA only; controls showed significantly stronger functional connectivity to an early visual cortical network. Functional connectivity in right FFA was associated with (a) neural responses during face detection in controls only, and (b) perceptual detection thresholds for faces in patients only. The finding of impaired functional connectivity for right FFA (but not other queried domain-specific regions) converges with findings investigating face detection in an overlapping sample in which dysfunction was found exclusively for right FFA in schizophrenia during face detection.