The surface area of the cortex is theoretically important but more complicated to measure than cortical volume. Its theoretical importance is primarily due to a close relationship between the surface area (not the volume) of the cortex at any region and the number of neurons in that region. The surface of the cortex in humans is highly convoluted and this creates difficulties in measuring its surface area. We report here on a straightforward extension of stereologic methods to magnetic resonance images that is simple, efficient and elegant. We studied the method's reliability and the relationship of measurement error to biological variation and suggest a simple method, using variance components analysis, for quantifying all the relevant parameters for efficient design of experiments. We applied this method to a small sample of 15 healthy young male subjects, matched individually on age and parental socioeconomic status to 15 healthy female subjects and measured the surface areas of both cerebral hemispheres. We found no evidence for gender differences or asymmetry in this small study, unlike a previous report using post-mortem material.
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