276 VOLUME 8 | NUMBER 3 | MARCH 2005 NATURE NEUROSCIENCE BRIEF COMMUNICATIONS Attending to local form while ignoring global aspects depends on handedness: evidence from TMS Carmel Mevorach 1 , Glyn W Humphreys 1 & Lilach Shalev 2 Our perceptions of the whole and of the parts of a visual stimulus are mediated by different brain regions. We used low- frequency transcranial magnetic stimulation (TMS) to show for the first time that opposite, homologous regions in the two hemispheres are involved in attending to local parts for left- and right-handed individuals. The brain regions that focus on the ‘trees’ while ignoring the ‘forest’ are switched as a function of handedness. The ability to attend to and identify global (the ‘forest’) or local (the ‘trees’) aspects of a hierarchical object 1 is lateralized between the two hemispheres in the human brain; the left hemisphere is biased toward the local level, whereas the right hemisphere is biased toward the global level 2 . Hence, patients with unilateral damage may have difficulty in identifying local or global information according to the site of lesion 3 . However, this assignment is not absolute, as damage to the right hemisphere can disrupt global 4 as well as local 5 identification. No previous studies have assessed whether the brain regions that focus on parts and wholes differ in left- and right-handers, though handedness is known to influence language lateralization 6 and some aspects of spatial perception 7 . Here we show differential lateralization of function for attention to local aspects of form in homologous areas in the right and left cerebral hemispheres. Transcranial magnetic stimulation (TMS) is a technique in which transient disruption of normal brain activity is induced by the application of focal magnetic pulses to specific regions on the scalp 8 . We used TMS to assess the role of the left and right posterior parietal lobes (PPL; electrode sites P3 and P4) for right- (n = 11, mean age = 29.18) and left-handed participants (n = 11, mean age = 25.36; handedness was assessed using the Edinburgh Handedness Inventory) when they were required to identify global and local forms in two stimulus categories (letters and shapes) under conditions of focal attention (see Fig. 1). Statistical analysis showed contrasting effects of TMS for right- and left-handed participants specifically when identifying local congruent displays as compared with incongruent displays (the global interference effect; see Fig. 2). This effect did not interact with stimulus category (letters or shapes). For right-handers, TMS over the left PPL increased global interference as compared with TMS over the right PPL (Fig. 2a,c). For left-handers, TMS over the right PPL resulted in larger global-to-local interference than did TMS over the left PPL (Fig. 2b,c). There were no effects on the global identification task. Notably, TMS affected performance primarily when attentional con- trol was required in order to ignore an irrelevant level (on incongruent relative to congruent trials). These findings fit with previous studies pointing to an attentional involvement in global and local identification and in the apparent lat- eralization of these processes in right-handed individuals 4,9,10 . More than this, though, our data show that opposite, homologous regions in the left and right hemispheres control attention to local form in left- and right-handed individuals. In right-handers, the left PPL controls 1 Behavioural Brain Sciences Centre, School of Psychology, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. 2 Department of Education and Psychology, The Open University Israel, 108 Ravutski St., Ra'anana, Israel. Correspondence should be addressed to C.M. (c.mevorach@bham.ac.uk). Published online 6 February 2005; doi:10.1038/nn1400 Figure 1 Typical display sequence showing congruent and incongruent compound stimuli. A red compound figure was presented either to the left, center or right of fixation and was composed of either ‘H’ and ‘D’ (letter condition) or ‘crosses’ and ‘boxes’ (shape condition). On different blocks of trials, participants were asked to identify the global or the local elements while ignoring information on the other level. In half of the trials, the compound figures consisted of the same global and local elements (congruent trials), and in the other half, of different global and local elements (incongruent trials). For TMS, a 70-mm figure-eight coil connected to a MagStim Rapid stimulator (MagStim) was positioned over the left or right posterior parietal lobe (P3 or P4 on the 10–20 EEG coordinate system). TMS trains consisted of 600 1-Hz pulses with intensity set to 90% of the participants’ motor threshold (this can produce prolonged inhibition of the stimulated cortical site 11 including P3 and P4; ref. 12). Each block consisted of 96 trials and was given six times to each participant (three blocks per category: letters or shapes). In a first session, a pre-TMS run was followed by one of the two TMS trains, which was immediately followed by another run of the task; in a second session there was only the TMS train followed by the task. The order of both the stimulus category and the TMS site was counterbalanced across participants. As participants initially performed the pre-TMS condition, differences between pre- and post-TMS could be masked by practice. Consequently our focus was on the two post-TMS conditions. The experiment was approved by the Ethics Committee of the School of Psychology, Birmingham University. Written informed consent was obtained from all participants. © 2005 Nature Publishing Group http://www.nature.com/natureneuroscience