Repetitive transcranial magnetic stimulation over the left parietal cortex facilitates visual search for a letter among its mirror images

Interference by task irrelevant information is seen in visual search paradigms using letters. Thus, it is harder to find the letter 'N' among its mirror reversals 'Icyrillic' than vice versa. This observation, termed the reversed letter effect, involves both a linguistic association and an interference of task irrelevant information - the shape of 'N' or 'Icyrillic' is irrelevant, the search requires merely distinguishing the tilts of oblique bars. We adapted the repetitive transcranial magnetic stimulation (rTMS) methods that we previously used, and conducted three rTMS experiments using healthy subjects. The first experiment investigated the effects of rTMS on the left and right posterior parietal cortex (PPC) on the search performance. The second experiment focused on the role of the left PPC. The third experiment explored whether another left posterior region, known to be involved in word reading (ventral occipito-temporal cortex, vOTC), plays a role. We found that rTMS on right PPC and left VOTC had no effect on the speed and accuracy of the visual search regardless of whether the target is 'N' or its mirror reversal. In contrast, rTMS on the left PPC speeded up the search on finding target 'N' among its mirror images. We suggest that left PPC is involved in letter recognition, and that rTMS on left PPC facilitated our visual search task by reducing task interference triggered by task irrelevant letter recognition.