Brief communication Contralateral visual search deficits following TMS

Transcranial magnetic stimulation of posterior parietal vs. superior temporal sites cause differential effects on conventional conjunction vs. feature search tasks, respectively. We now report that when a decision has to be made on the target’s left/right location, different lateralized deficits emerge in these two cases. With full-field arrays, we found a specific PPC search deficit for contralateral space. With smaller, structured arrays presented in left or right hemispace, we found a specific STG deficit for contralateral parts of the array. Transcranial magnetic stimulation (TMS) of the right posterior parietal cortex (PPC) impairs difficult (serial) search for visual conjunctions, but not difficult feature search. In contrast, TMS of right superior temporal gyrus (STG) impairs difficult feature search, but not conjunction search (Ellison, Schindler, Pattison, & Milner, 2004). A similar deficit has been found to follow electrical stimulation of the STG (Gharabaghi, Fruhmann Berger, Tatagiba, & Karnath, 2006). These findings showed that disruption of either area can cause deficits in visual exploration, but that these deficits are task dependent. Our work had been stimulated by the current debate as to which of these two regions forms the core substrate for visual neglect symptoms (Karnath, Ferber, & Himmelbach, 2001; Karnath, Fruhmann Berger, Kuker, & Rorden, 2004; Mort et al., 2003, Vallar & Perani, 1986), given that a fundamental aspect of neglect is a lateralized failure in exploratory search (Karnath, Milner, & Vallar, 2002). While not directly bearing on this debate, our data did provide independent evidence that both of these areas are involved, in different ways, in guiding visual search in healthy participants. In contrast to clinical neglect, however, the observed search deficits were observed uniformly across the search display. Although there are many general reasons why TMS might fail to mimic the behavioural changes caused by major brain lesions, this difference remains challenging, since TMS studies of line bisection have shown asymmetrical disruption (Bjoertomt