MRI Volume Comparisons
For comparative purposes, provides volumes for all parietal regions studied. As predicted, no group volume differences were found for the total parietal lobe, superior parietal gyrus, or the postcentral gyrus. However, volume differences were noted between the two groups for the inferior parietal lobule, specifically the angular gyrus.
Parietal Lobe Region Relative Volumes in Male Patients With Schizophrenia and Healthy Male Comparison Subjects
A repeated measures ANOVA showed no overall group volume differences or an overall laterality effect (). However, there was a significant laterality-by-group interaction that indicated a difference in asymmetry between the two groups. As shown in , the comparison subjects had a leftward asymmetry (left inferior parietal lobule volume 7.0% larger than the right), and the schizophrenic patients showed a reversed asymmetry (left inferior parietal lobule volume 6.3% smaller than the right).
Inferior Parietal Lobule Volume Differences Between Male Patients With Schizophrenia and Healthy Male Comparison Subjects
Differential Left-Versus-Right Asymmetry in Inferior Parietal Lobule Volume in Male Patients With Schizophrenia and Healthy Male Comparison Subjects
The volumes of the angular gyrus and the supramarginal gyrus for the two groups were significantly different (). There was also a significant region-by-group interaction. Post hoc tests indicated that schizophrenic patients had a larger right angular gyrus than the normal comparison group (). In the comparison subjects, the left angular gyrus volume was considerably larger (18.4%) than the right (paired t=2.71, df=14, p=0.02), whereas in the patients, the left angular gyrus was not significantly different (4.7% less volume) from the right (t=0.7, df=14, p>0.05).
Left-Right MRI Volume Asymmetries
To evaluate further the aforementioned laterality-by-group interaction of the inferior parietal lobule as well as the laterality of the other parietal regions, we computed asymmetry coefficients by using the formula (left–right)/(left+right) × 100 (note that the asymmetry coefficient is dimensionless). A negative value indicates a larger right than left side volume. Student’s t tests were used to compare asymmetry coefficients between the two groups for all of the regions.
There was a significant difference between the two groups in the asymmetry coefficient for the total parietal lobe (). The comparison group exhibited a leftward asymmetry, with the left parietal lobe 6.0% larger than right (paired t=3.18, df=14, p=0.007), while the schizophrenic group exhibited virtually no total parietal asymmetry (t=−1.1, df=14, p>0.30).
Parietal Region Asymmetry in Male Patients With Schizophrenia and Healthy Male Comparison Subjects
Neither group exhibited significant asymmetry of the superior parietal gyrus, and both left and right postcentral gyrus volumes were similar in the schizophrenic and comparison groups. Both groups showed a significant leftward asymmetry, with the left postcentral gyrus being 14.8% larger than the right in the comparison subjects (paired t=2.76, df=14, p=0.02), and 13.4% larger in the schizophrenic patients (paired t=3.00, df=14, p=0.01).
A significant group difference in asymmetry coefficient was evident for the inferior parietal lobule (), which was accounted for by a group difference in the angular gyrus. Note that all of the schizophrenic subjects except two had asymmetry coefficients below the mean (and median) of the subjects in the normal comparison group (). The supramarginal gyrus did not show a significant asymmetry for either group. Thus, although the post hoc tests based on the ANOVA group-by-region interaction did not show statistically significant group differences between left and right angular gyrus in the schizophrenic patients, the differences between groups in the asymmetry coefficient suggests that there is a reversal of the normal asymmetry in the schizophrenic patients for the angular gyrus.
Asymmetry Coefficients for the Inferior Parietal Lobule in Male Patients With Schizophrenia and Healthy Male Comparison Subjects
Correlations Between Parietal Gray Matter Volumes and Anatomically Connected Regions in Prefrontal and Temporal Cortex
For all correlations, significance levels were set at p≤0.05 (two-tailed), which corresponded to r>0.51 for the 15 schizophrenic subjects and r>0.53 for 14 comparison subjects (one comparison subject was dropped because artifact in the prefrontal cortex made this region too difficult to assess the volume accurately). In addition, the values reported here are for absolute volumes, but the correlations were considered significant only if they reached p≤0.05 for both relative and absolute volumes. For all of these correlations, parietal cortex regions included the right and left inferior parietal lobules, superior parietal gyrus, and postcentral gyrus; prefrontal cortex regions included right and left superior frontal, middle frontal, inferior frontal, and orbital frontal gyri; and temporal cortex regions included right and left anterior superior temporal gyrus, posterior superior temporal gyrus, parahippocampal gyrus, and amygdala-hippocampal complex.
High correlations were found between left and right inferior parietal lobule volumes in both schizophrenic (r=0.68, p<0.005) and comparison subjects (r=0.54, p<0.04), as well as between the left and right superior parietal gyrus (schizophrenic subjects: r=0.81, p<0.001; comparison subjects: r=0.75, p<0.001). Additionally, the comparison subjects showed a significant correlation between the left and right postcentral gyrus (r=0.63, p<0.01).
As hypothesized, the schizophrenic group showed several high correlations between gray matter volumes of the inferior parietal lobule and regions of the prefrontal cortex (). provides an illustrative summary of these correlations, with the bottom arrows depicting prefrontal correlations with the left inferior parietal lobule, and the top arrows depicting prefrontal correlations with the right inferior parietal lobule (note: arrows do not imply direction). In addition to these correlations, the left postcentral gyrus correlated significantly with the left superior frontal gyrus (r=0.72, p<0.003) and with the right inferior frontal gyrus (r=0.70, p<0.005). In contrast, the comparison subjects showed no volumetric correlations between inferior parietal lobule volumes and prefrontal volumes at p<0.05.
Volume Correlations Between the Left and Right Inferior Parietal Lobules and Frontal Lobe Structures in 15 Male Patients With Schizophrenia
Gray Matter Volume Correlations Between the Inferior Parietal Lobule and Regions of the Prefrontal Cortex in 14 Male Schizophrenic Subjectsa
The differences between groups in the correlations for respective brain areas were tested by using a Fisher’s z transformation. Of note, significant group differences in correlations emerged for the left inferior parietal lobule and prefrontal structures even though the correlations between left and right inferior parietal lobules and the prefrontal lobe structures were comparably high in the schizophrenic group. This result highlights the salience of the left inferior parietal lobule correlations with prefrontal measures in the schizophrenic group.
In the comparison subjects, but not in the schizophrenic subjects, the inferior parietal lobule asymmetry coefficient correlated inversely with all of the prefrontal structures (r=−0.50 to −0.76, p=0.03 to ≤0.001). In addition, the left postcentral gyrus correlated significantly with several prefrontal structures: the left superior frontal gyrus (r=0.83, p<0.001), left orbital gyrus (r=0.70, p<0.004), right superior frontal gyrus (r=0.64, p<0.01), and the right orbital gyrus (r=0.64, p<0.01).
In the schizophrenic group, both the left and right inferior parietal lobules correlated significantly with the left amygdala (r=0.67, p<0.007) and the left anterior portion of the superior temporal gyrus (r=0.66, p<0.008 ( and ). In the normal comparison group, the one significant correlation was between left inferior parietal lobule and the left amygdala (r=0.53, p=0.04). The group differences in the respective, pairwise correlations, as tested with Fisher’s formula, did not reach statistical significance.
Gray Matter Volume Correlations Between the Inferior Parietal Lobule and Regions of the Temporal Lobe in 15 Male Schizophrenic Subjectsa
Volume Correlations Between the Left and Right Inferior Parietal Lobules and Temporal Lobe Structures in 14 Male Patients With Schizophrenia
Correlations With Clinical Status and Clinical Measures and With Neuropsychological Data
Because of the exploratory nature of the correlations and the issue of multiple tests (i.e., some of the significant results might be due to chance), we report only correlations where p was below 0.01.
No correlations between regions of interest and clinical measures in schizophrenic patients reached the significance level of p<0.01. Exploratory analyses of correlations between neuropsychological measures and brain volume in schizophrenia patients indicated that reduced right inferior parietal lobule volume correlated with lower scores on tests of visual attention and visual memory (Trails B: rs=−0.76, p<0.002; visual reproduction I: rs=0.71, p<0.003; visual reproduction II: rs=0.84, p<0.001). Additionally, reduced inferior parietal lobule asymmetry was related to poor performance on Trails A (rs=−0.70, p<0.005) and Trails B (rs=-0.69, N=14, p<0.004).