The results of this study showed that schizophrenia patients and controls do not differ in the magnitude of activation, or in their degree of overlap, in LO, retinotopically organized areas, or hMT+ when processing basic visual stimuli. However, the topography for LO was more wide-spread in patients compared to controls, while the topography in retinotopically organized areas and hMT+ were comparable between groups. The finding of wide-spread topography in LO was unexpected, and the results should be interpreted cautiously until the effect is replicated in further studies.
The relative broad topography of LO in schizophrenia patients could possibly be explained by less specialization of cortex or compensatory activation within LO. It may be possible that neurons in LO are are less specialized, so more of them respond to any given localizer stimulus (but not at a high level, which is why we did not see amplitude differences). Alternatively, responding neurons are just as specilaized, but not as segregated spatially, so that the same number are responding, but they are spread out more. Future studies will need to be conducted to fully explore these possibilities.
Another possible explanation for the broad topography in LO in schizophrenia patients may be our use of spatial smoothing. Spatial smoothing may result in an artifically high volume of activation in areas of high magnitude of activation. However, given that hemodynamic blurring of magnitude and volume are never truly independent and we used a relatively small spatial smoothing filter (only 5 mm FWHM), we do not believe that these results are do to our spatial smoothing.
The finding of no significant differences in activation in retinotopically organized areas is fairly consistent with the literature (Barch et al., 2002; Braus et al., 2002
), though some studies have found functional deficits in retinotopically organized areas (e.g., (Dakin et al., 2005
; Schechter et al., 2005a
). The differences may be due to the differences in stimuli and task used in our study vs. other studies. Furthermore, other studies have found structural deficits in retinotopic regions. For example, Dorph-Petersen et al. (2007)
in a post-morten study found reduced volume and neuron number in primary visual cortex in schizophrenia patients. Butler et al. (2006)
, using diffusion tensor imaging, found reduced white matter integrity in optic radiations. While our study found normal activation
in primary visual areas we cannot conclude that structural
deficits do not exist. It is possible that structural deficits result in abnormal functioning, though structural deficits were not assessed in the present study.
The lack of activation magnitude differences in hMT+ was contrary to our expectations, as reports in the functional imaging literature (Lencer et al., 2005
) as well as psychophysical literature (e.g., Chen et al., 2004
; Chen et al., 2005
) implicate hMT+ deficits in schizophrenia. The use of a simple moving ring pattern with little attentional demand in our study may explain this discrepancy. The use of more demanding tasks that engage attentional resources to a greater degree may better detect patient-control differences (e.g., Curtis et al., 1999
; Ross et al., 2000
The finding of broader topographical organization in LO in schizophrenia patients may help explain their poorer performance on tasks that involve object recognition, such as visual backward masking tasks. We have shown in normal controls that area LO may be a key region mediating backward masking (Green et al., 2005
) as LO was more sensitive to longer intervals between the target and mask (i.e., weaker masking). If activity in LO is diffuse and not well-organized in schizophrenia, schizophrenia patients may require longer intervals between the target and mask, compared to normal controls, to fully process the target. Alternatively, neurons in LO may be less selective and respond equally to both the target and the mask, thus resulting in increased masking effects. We are currently examining activity in LO during a backward masking task in schizophrenia patients which might be able to futher elucidate the impact of diffuse or less-specialize neurons in LO on object recognition tasks.
The goal of this study was to define key regions of interest (retinotopically organized areas, hMT+ and LO) in early and middle stages of visual processing in schizophrenia to basic visual stimuli and explore regional brain activation. No previous study to our knowledge examined the functional integrity of the all three visual processing areas in schizophrenia patients. This study provides evidence that two key regions, retinotopically organized areas and hMT+, have normal magnitude of activation and topography in schizophrenia, at least with the basic visual stimuli used in the current study. The finding of diffuse topography in area LO in schizophrenia patients is convergent with poorer performance on specialized object recognition tasks. These findings can help guide fMRI studies in visual processing in schizophrenia.