Neural Mechanisms of Transsaccadic Integration of Visual Features

This thesis explores the neural mechanisms of transsaccadic integration of visual features. In the study, I investigated the cortical correlates of transsaccadic integration of object orientation in multiple reference frames. In a functional MRI adaptation (fMRIa) paradigm, participants viewed sets of two orientation stimuli in each trial and were asked to indicate if the orientations were the same (Repeat condition) or different (Novel condition). Stimuli were presented in one of three spatial conditions: 1) space-fixed, 2) retina-fixed and 3) frame-independent. Results indicate that, in addition to common activation in frontal motor cortical regions in all three spatial conditions, parietal and occipitotemporal regions are active in the space-fixed condition, parietofrontal regions are active in the retina-fixed condition, and parietofrontal and occipitotemporal regions are active in the frame-independent condition. In conclusion, these results indicate that transsaccadic integration involves differential activation of cortical areas, depending on the frame of reference

Cortical Mechanisms for Transsaccadic Perception of Visual Object Features

The cortical correlates for transsaccadic perception (i.e., the ability to perceive, maintain, and update information across rapid eye movements, or saccades; Irwin, 1991) have been little investigated. Previously, Dunkley et al. (2016) found evidence of transsaccadic updating of object orientation in specific intraparietal (i.e., supramarginal gyrus, SMG) and extrastriate occipital (putative V4) regions. Based on these findings, I hypothesized that transsaccadic perception may rely on a single cortical mechanism. In this dissertation, I first investigated whether activation in the previous regions would generalize to another modality (i.e., motor/grasping) for the same feature (orientation) change, using a functional magnetic resonance imaging (fMRI) event-related paradigm that involved participants grasping a three-dimensional rotatable object for either fixations or saccades. The findings from this experiment further support the role of SMG in transsaccadic updating of object orientation, and provide a novel view of traditional reach/grasp-related regions in their ability to update grasp-related signals across saccades. In the second experiment, I investigated whether parietal cortex (e.g., SMG) plays a general role in the transsaccadic perception of other low-level object features, such as spatial frequency. The results point to the engagement of a different, posteromedial extrastriate (i.e., cuneus) region for transsaccadic perception of spatial frequency changes. This indirect assessment of transsaccadic interactions for different object features suggests that feature sensitive mechanisms may exist. In the third experiment, I tested the cortical correlates directly for two object features: orientation and shape. In this experiment, only posteromedial extrastriate cortex was associated with transsaccadic feature updating in the feature discrimination task, as it showed both saccade and feature modulations. Overall, the results of these three neuroimaging studies suggest that transsaccadic perception may be brought about by more than a single, general mechanism and, instead, through multiple, feature-dependent cortical mechanisms. Specifically, the saccade system communicates with inferior parietal cortex for transsaccadic judgements of orientation in an identified object, whereas as a medial occipital system is engaged for feature judgements related to object identity