fMRI form adaptation and size repetition enhancement in different subdivisions of the lateral occipital complex.

Research findings on which brain areas demonstrate fMRI adaptation to the form, orientation, and size of visual stimuli has been mixed. Studies demonstrate effects in various subdivisions of the lateral occipital complex (LOC), including retinotopically tuned areas LO-1 and LO-2, and dorsal stream areas in the intraparietal sulcus (IPS). Therefore, we aimed to examine fMRI adaptation in four subdivisions of the LOC (LO-proper, posterior fusiform sulcus, LO-1, and LO-2) and three discrete regions in the IPS (caudal IPS, IPS-proper, and anterior IPS) to clarify the role that these structures play in form, orientation, and size processing. Participants performed three tasks which involved judging whether two serially presented novel objects shared the same form, orientation, or size. On each version of the task, one feature varied from trial-to-trial (e.g., form) while the other two features (e.g., orientation and size) were held constant. In this way, we were able to examine the unique fMRI signal changes in response to changes in form, orientation, and size in isolation. Form adaptation - a decrease in fMRI signal following repeated presentation of the same stimulus - was present in LO-proper and pFS, highlighting the role of LOC in form processing. Size repetition enhancement - an increase in fMRI signal following repeated presentations of the same stimulus - was observed in pFs. We propose that the latter result demonstrates the effects that top-down factors can have on visual areas, specifically when there is stimulus uncertainty. There was no evidence of orientation processing in any of the regions examined. Neither form adaptation nor size repetition enhancement was present in the three IPS regions-of-interest. Last, retinotopically defined LO1 and LO2 could not be reliably identified in participants and therefore we were unable to examine adaptation in these areas as we originally intended to do.