Left and right occipital cortices differ in their response to spatial cueing

Abstract We investigated cue and target-related laterality effects with event-related fMRI. Both left and right occipital areas responded maximally when both cue and target were presented in the contralateral visual hemifield (VF), and minimally when cue and target were presented in the ipsilateral VF. However, whereas signal increases in right ventromedial and lateral occipital cortex were intermediate in those trials in which the cue appeared in the VF contralateral to the target (invalid cue trials), signal strength in left occipital cortex was almost identical for valid and invalid cues, i.e., high for RVF cues, and low for LVF cues, independent of the VF of the target. These data support theories which postulate a greater ability of the right hemisphere for bilateral processing. However, these laterality effects were observed earlier in the visual pathway than previously thought, leading to the question whether the hemispheric differences observed in occipital cortex are generated in the activated areas or are the effect of reentrant processes from more anterior areas, potentially in parietal cortex

How the brain controls hand actions: TMS, fMRI and behavioural studies

This thesis focused on testing the predictions made in Milner and Goodale’s model and reports finding from experiments investigating how inputs from both the dorsal and the ventral streams are required when we perform hand actions with objects (Chapter 2) and tools (Chapter 3 & 4) using different paradigms such as real and pantomimed grasping and techniques such as transcranial magnetic stimulation, motion-tracking of hand movements and cutting-edge fMRI multivoxel pattern analysis. The primary aim was to gain a new insight on the role of the dorsal and the ventral visual streams in real grasping and pantomiming and to understand what specific aspects of objects and movements associated with them are represented within the two streams. The first experiment (Chapter 2) examined the causal role of the anterior intraparietal and the lateral occipital in object’s real and pantomimed grasping using TMS. The results showed that real object grasping and pantomime actions without the objects in hand require the left dorsal stream but that information from the ventral stream is additionally required for pantomiming. The experiments in Chapter 3 and 4 investigated how tools and tool related actions are represented within the dorsal and the ventral stream (Chapter 3) and whether different action end-goals affected early grasping kinematics (Chapter 4). Using MVPA we showed that both dorsal and ventral stream regions represent information about functional and structural manipulation knowledge of tools. Moreover, we showed that both streams represent tool identity, which seems in line with our behavioural findings that tool identity affects grasping kinematics. The current work provided a detailed understanding of how the dorsal and the ventral streams interact in tool processing and propose a more sophisticated view of the distributed representations across the two streams. These findings open up a number of research avenues as well as help understanding how actions are disrupted in brain-damaged patients and advance the development of neural prosthetics