Functional MRI methods and findings in schizophrenia

Functional MRI imaging methods have allowed researchers to combine neurocognitive testing with structural neuroanatomical measures, take into consideration both cognitive and affective paradigms, and subsequently create computer-aided diagnosis techniques and algorithms. Functional MRI has several benefits, such as its non-invasive quality, relatively high spatial resolution, and decent temporal resolution. One particular method used in recent research is resting-state functional magnetic resonance imaging, rs-fMRI. fMRI imaging has been applied to numerous behavioral studies for schizophrenia, the findings of which have hinted toward potential brain regions that govern key characteristics in cognition and affect.

In a 'reformulation' of the binary-risk vulnerability model, researchers have suggested a multiple-hit hypothesis which utilizes several risk factors — some bestowing a greater probability than others — to identify at-risk individuals, often genetically predisposed for schizophrenia. The process of defining clinical criteria of schizophrenia for early diagnosis has posed a great challenge for scientists.

Methodology

A rapid increase of studies in schizophrenia have covered topics such as abnormal activity in "motor tasks, working memory attention, word fluency, emotion processing, and decision making." In contrast to the abundance of research centered on positive symptoms of the disorder, fMRI research for schizophrenia primarily analyzes the 'failures' of the neural system and consequent behavioral deficits. To confirm that a task activates identical regions in schizophrenia patients vs controls, the given task typically begins easy so that both patients and healthy comparison subjects perform close to 100% accuracy; the task is then increased in difficulty to distinguish activation between two groups with varying abilities of individuals.

The 'basic symptoms' approach

The 'basic symptoms' approach for schizophrenia, which emerged from "retrospective descriptions of the prodromal phase," represents a framework for a large portion of fMRI research, which evaluates changes in cognition and sensory perception that may affect higher-level information processes. Some researchers oppose the tendency of researchers to attribute schizophrenia to higher-order processes like working memory, attention, and executive processing, instead choosing to inspect impairments in basic sensory and perceptual functions. Deficits in basic sensory functions influence higher-order processes such as auditory emotion recognition, perceptual closure, object recognition, etc. In the visual system, for example, rudimentary deficits in the function of the magnocellular system results in impairments in higher-order processes like perceptual closure, object recognition, and reading. On the other hand, fMRI data has also suggested the opposite. In one study, researchers found significantly differing activity between healthy and schizophrenic patients in the left dorsal parietal cortex and left ventrolateral prefrontal cortex; as these regions are essential components of a frontal-parietal executive system, hypo-activity in these regions for schizophrenia patients during working memory tasks were theorized to be associated with deficits in executive functioning.

Resting-state fMRI

The 'disconnectivity hypothesis' is a key theory, describing the failure of mechanisms underlying schizophrenia, specifically the failure to integrate information properly. Functional connectivity, which fMRI evaluates, is defined as the coordination of activity between brain regions. It is measured as "temporal correlations of low frequency oscillations in the BOLD signal between anatomically distinct brain areas," and can reveal resting state networks. The cause for the correlations in fMRI measurements is theorized to be due to "correlated firing rates of interconnected neurons." Resting-state functional magnetic resonance imaging (rs-fMRI) has become a powerful tool to examine the functional connectivity of networks throughout the brain, such as the default mode network (DMN).

Abnormal brain connectivity has long been theorized as a fundamental cause of psychosis in schizophrenia. rs-fMRI can help evaluate regional interactions at rest, and whether there are altered, reduced, or hyperactive connections in psychiatric disorders, like schizophrenia. During resting-state fMRI experiments, participants are instructed to relax, stay awake, but not think of anything. It is important to note that resting state networks can change between eyes open and eyes closed conditions. Researchers then measure spontaneous brain activation. There are several advantages to studying the resting-state of brain networks — the primary reason being that spontaneous neural activity accounts for a majority of the brain's activity in contrast to task-based neural activity. Additionally, rs-fMRI eliminates confounding effects such as differing performances between healthy subjects and patients in tasks; rs-fMRI also requires less movement than task-based fMRI studies. Seed-based analysis/ROI approaches to analyzing functional connectivity are common in rs-fMRI for schizophrenia. A seed (region of interest) is first selected, and BOLD time series are then extracted from the seed and all other voxels. After preprocessing, the temporal correlation between the seed and other brain voxels is determined and a functional connectivity map is produced by the software. Seed-based comparisons in rs-fMRI have revealed functional disconnectivity in schizophrenia patients in numerous studies, using different ROIs for their seeds — in general, schizophrenia patients show reduced connectivity. This information is compatible with experiment findings suggesting reduced activation in the amygdala in schizophrenia patients during sadness mood induction, for example.