Understanding the neural basis of hemispatial neglect symptoms: an investigation in normal subjects

Abstract

This thesis sought to investigate the recent claim by Karnath et al. (2001) that the crucial locus of neurological damage in neglect patients Vies in the right superior temporal gyrus (STG), and not in the right posterior parietal cortex (PPC), as conventionally thought. In chapter 2, using Transcranial Magnetic Stimulation (TMS), we first tested the involvement of the right STG in a task commonly used in the diagnosis of neglect, the landmark task. No evidence was found for a critical involvement of the right STG in the processing of this task, though evidence was found for the involvement of the right PPC. In contrast, however, when we examined the effects of TMS on exploratory search, a double dissociation between right STG and right PPC was found. When the processing of conjunction items was required, involvement of the right PPC (and not STG) was found, in accordance with previous research (Ellison et al, 2003). When difficult exploratory search through feature items was required however, the right STG (not PPC) was found to be involved In both cases, however, the deficit was present bilaterally across the search array, in contrast to the search deficits seen in patients with visual neglect, which are typically greater on the contralesional side. In chapter 3 we sought to determine whether it might be possible to induce such contralateral search deficits, using variations on the previous tasks. Experiment 1 required subjects to report the location of the search target (left or right) on each trial. We now found a significant deficit in contralateral search when TMS was applied to the PPC region. Experiment 2 used smaller, structured stimulus arrays, presented briefly to eliminate eye movements and to direct the target item to either the left or right hemisphere. This task was unaffected by TMS applied to either of the two sites. Experiment 3 presented similar small arrays for a longer duration, in either left, central, or right visual space. We found a contralateral within-array ('object-based') deficit when TMS was applied to the STG, whether the array was presented in the left or right hemispace, when a left/right response was required. There was, however, no relative 'hemispatial' deficit when the array was located on the contralateral left side. These results may indicate separate specializations for 'spatial' versus 'within-object’ search in the two brain regions studied and are discussed in terms of the neglect and visual search literatures.