[Sex specific differences in hemispheric lateralization in schizophrenia? An MEG-MRI study]

A source-imaging (low-resolution electromagnetic tomography) study of the EEGs from unmedicated men with schizophrenia

Imaging studies and quantitative electroencephalography (EEG) have often, but not consistently, implicated the left hemisphere and the prefrontal cortex in schizophrenia. To help clarify this picture, a spatial filter shown to be effective for enhancing differences between EEG populations was combined with low-resolution electromagnetic tomography and used to compare the source-current densities from a group of 57 male subjects with schizophrenia and a group of 65 matched controls. To elicit differences, comparisons were made during resting conditions and during verbal and spatial cognitive challenges to the subjects. Estimates of the source-current density were derived from 43-electrode recordings of the EEG reduced to the delta, alpha and beta frequency bands. The patients were unmedicated and were selected according to DSM-IV criteria. As a group, they were severe, chronic states with both deficit negative and superimposed florid psychotic symptomatology. The results confirm that schizophrenia is a left-hemispheric disorder centered in the temporal and frontal lobes. They also suggest that, in schizophrenia, functions normally performed by these regions in controls are assumed by homologous regions in the opposite hemispheres.

Reduced laterality of the source locations for generators of the auditory steady-state field in schizophrenia

BACKGROUND

Reduced hemispheric asymmetry of evoked field generators in schizophrenia in secondary auditory cortex has been replicated with mixed success. This disparity is possibly due to the complex nature of the 100-msec generators typically investigated. In this study, we evaluated a source located in primary auditory cortex, responsible for the steady-state response, to determine whether similar asymmetries were present.

METHODS

Contralateral evoked fields were recorded with a 37-channel neuromagnetometer in response to 40-Hz pulse trains. Dipole models were used to localize the sources in both hemispheres of schizophrenic subjects (n = 17) and normal control subjects (n = 18).

RESULTS

Control subjects were asymmetrically lateralized, with sources in the right hemisphere approximately 6 mm further anterior than those on the left. In contrast, schizophrenic subjects showed no significant difference between hemispheres.

CONCLUSIONS

The findings support theories linking schizophrenia to disturbed asymmetry of temporal lobe function. The steady-state response, having a more discrete Heschl's gyrus generator location, may be better suited for evaluating auditory asymmetry.

Altered hemispheric asymmetry of auditory magnetic fields to tones and syllables in schizophrenia

BACKGROUND

A growing body of literature suggests that schizophrenic patients often do not show the normal brain hemispheric asymmetry. We have found this for simple tones presented to the right ear in a previous study. In this study we extended this investigation to left ear stimulation and verbal stimuli.

METHODS

With a whole-head neuromagnetometer, contra- and ipsilateral auditory-evoked magnetic fields in response to tones (1000 Hz) and to the syllables ("ba") delivered to the left and right ears in separate runs were compared between schizophrenic patients (n = 17) and healthy control subjects (n = 15).

RESULTS

In response to tones, all control subjects showed the expected asymmetry (contralateral predominance) of the auditory-evoked magnetic N100m (dipole moment). In the patient sample asymmetry was reversed following tones presented to the left ear in 47% and following tones to the right ear in 24%. In response to syllables, the asymmetry was similar between groups. In patients compared with control subjects the N100m was located more anterior without asymmetry between hemispheres.

CONCLUSIONS

Results suggest that deviation from the normal functional lateralization in schizophrenia appears in a proportion of patients at a basic stage of auditory processing, but may be compensated for at higher levels such as the processing of syllables.

Magnetoencephalography and magnetic source imaging: technology overview and applications in psychiatric neuroimaging

Magnetoencephalography (MEG) is an electrophysiologic brain imaging technology that has been applied to the study of mental illness, particularly schizophrenia. Like electroencephalography, it provides excellent temporal resolution, and in combination with magnetic resonance imaging, can also provide good spatial resolution. Studies of the auditory system in schizophrenia using MEG have demonstrated an abnormality in functional cerebral asymmetry, in which persons with schizophrenia typically show reduced, or reversed, cerebral asymmetry compared with normal subjects. This abnormality is sex-specific; it is more pronounced in males with schizophrenia. These findings have not been demonstrated using other neuroimaging strategies. Thus, MEG appears to offer a unique and valuable contribution to psychiatric neuroimaging. Current research and clinical applications of MEG are limited, however, by the high cost of instrumentation. The cost of MEG systems should improve as more applications are developed, in schizophrenia as well as other neuropsychiatric conditions, and hospitals begin to invest in the technology.