Cortical Mechanisms of Visual Target Memory and Movement Planning and Execution for Reaches and Saccades in Humans

The cortical mechanisms for reach have been studied extensively, but directionally selective mechanisms for visuospatial target memory, movement planning, and movement execution have not been clearly differentiated in the human. It is also unclear how effector-specificity evolves in the human brain across these three phases for reaches and saccades. To study these phenomenon, an event-related fMRI design with three key phases was used to break apart a movement into target memory, movement planning and movement execution phases. In the first experimental chapter (chapter 2) directionally selective mechanisms were studied in a memory-guided reach task that informed the subject to perform a pro- or anti-reach after the target memory phase. Using the pro/anti instruction to differentiate visual and motor directional selectivity during planning, we found that one occipital area showed contralateral visual selectivity, whereas a broad constellation of left hemisphere occipital, parietal, and frontal areas showed contralateral movement selectivity. Temporal analysis of these areas through the entire memory-planning sequence revealed early visual selectivity in most areas, followed by movement selectivity in most areas, with all areas showing a stereotypical visuo-movement transition. Cross-correlation of these spatial parameters through time revealed separate spatiotemporally correlated modules for visual input, motor output, and visuo-movement transformations that spanned occipital, parietal, and frontal cortex. In the second experimental chapter (Chapter 3), effector-specific activation for reaches and saccades was studied using a similar design that informed subjects of the effector after the target memory phase. Our analysis revealed more medial (pIPS, mIPS, M1, and PMd) activity during both reach planning and execution, and more lateral (mIPS, AG, and FEF) activity only during saccade execution. These motor activations were bilateral, with a left (contralateral) preference for reach. Apart from right FEF, effector-specific contrasts comparing reach and saccade activation revealed significantly more parietofrontal activation for reaches than saccades during both planning and execution. Cross-correlation of reach, saccade, and reach-saccade activation through time revealed spatiotemporally correlated activation both within and across effectors in each hemisphere, but with higher correlations in the right hemisphere. Taken together, these results demonstrate highly distributed, coordinated occipital-parietal-frontal networks for both reach and saccade, with effector-specific activation.