The main news of this report is that, unlike what is seen in schizophrenia, there appears to be no difference in MRI fusiform gyrus volumes in SPD subjects compared with normal comparison subjects. The small effect size (0.001) suggests that simply increasing the subject number will not affect this result.
However, these findings are interesting as they demonstrate that only selective regions within the temporal lobe seem to be affected in SPD morphometrically (
Dickey et al., 2002a) and functionally (
Buchsbaum et al., 2002), although more diffuse abnormalities are seen in schizophrenia (
McCarley et al., 1999;
Shenton et al., 2001;
Siever et al., 2002). For example, brain abnormalities in SPD have been demonstrated in the left superior temporal gyrus (
Dickey et al., 1999;
Downhill et al., 2001), specifically Heschl's gyrus (
Dickey et al., 2002b), and whole temporal lobe (
Downhill et al., 2001), but not in medial temporal lobe structures specifically (
Dickey et al., 1999), whereas medial temporal lobe structures (
Barta et al., 1990;
Buchanan et al., 1993;
Shenton et al., 1992) and fusiform gyrus (
Lee et al., 2002) have been implicated in schizophrenia. It is conjecture, but perhaps more language-oriented areas of the temporal lobe (left superior temporal gyrus and Heschl's gyrus) are affected in SPD (
Dickey et al., 2002b), whereas areas more involved in emotional processing (amygdala:
Whalen et al., 1998; and fusiform gyrus:
Lang et al., 1998) are spared in SPD.
Whether other areas of visual processing, specifically striate and extrastriate cortex, are implicated in SPD remains unknown. Thalamic abnormalities have been reported in SPD (
Hazlett et al., 1999), but the lateral geniculate nucleus, which is involved in transmission of visual information, has not been separately examined.
Several limitations to this study may attenuate the findings. First, this study does not examine females with SPD. Both male and female schizophrenic subjects postmortem exhibit an abnormality of the fusi-form gyrus (
McDonald et al., 2000), and perhaps inclusion of female SPD subjects would have altered the results. Second, the recruitment strategy selected only for subjects who have not been treated pharmacologically for SPD, and this narrow subject group may effect the generalizability of the finding. Finally, this study examined volume only, and it may be that there are differences in the shape of the fusiform, as others have demonstrated differences in the shape of other brain regions in SPD (
Downhill et al., 2000).
There is a correlation between degree of impairment from magical thinking and illusions with smaller right fusiform gyrus volumes in SPD subjects. The relationship between magical ideation and the fusiform gyrus is not clear, in part as magical ideation specifically has been infrequently studied. Speculatively, however, magical ideation may engage visual imagery. Indeed, half of our subjects reported ‘‘seeing’’ images when asked questions from the SCID II designed to probe for magical ideation. Generating mental images does involve fusiform activation (
D'Esposito et al., 1997). The correlation between illusions and smaller fusiform volume is more clear as direct electrical stimulation of the fusiform gyrus in epilepsy patients produces complex visual illusions such as faces, animals, panoramas, images from memory, and metamorphopsia (
Lee et al., 2000), not unlike what is described by the SPD subjects in this study. Future functional imaging studies of fusiform activity may be important to more fully understand the role of this gyrus in schizophrenia spectrum disorders.
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