Visual selection and posterior parietal cortex: effects of repetitive transcranial magnetic stimulation on partial report analyzed by Bundesen's theory of visual attention.

Posterior parietal cortex (PPC) may contribute to visual selection by exerting top-down influences on visual processing. To seek direct evidence for this, we used 10 Hz repetitive transcranial magnetic stimulation (rTMS) over right or left PPC in nine healthy volunteers during a partial (selective) report task that allows quantitative assessment of top-down control and other parameters. Participants reported digits in a relevant color ("targets") but not those in an irrelevant color ("nontargets") from a brief masked display, in which a target could appear alone or together with an accompanying item (nontarget or target) in the same or opposite hemifield. Generally, a given target is identified better when presented with a nontarget than with another target, indicating top-down selection of task-relevant targets; this applied here with no rTMS or left PPC rTMS. However, rTMS over the right PPC changed the performance pattern. A left target no longer impeded report of a right target more strongly than did a left nontarget, whereas the greater impact of a right target than a right nontarget in disrupting report of a left target was increased. Formal analysis in terms of Bundesen's (1990) theory of visual attention indicated that right PPC rTMS diminished top-down control for the left hemifield while enhancing this for the right hemifield, particularly for bilateral two-item displays. These findings indicate a role for right PPC in top-down spatial selection, which applies even when the target is defined by a nonspatial property (here color).