Visual Selection and Posterior Parietal Cortex: Effects of Repetitive Transcranial Magnetic Stimulation on Partial Report Analyzed by Bundesen's Theory of Visual Attention

Abstract

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).

Visual Selection and the Human Frontal Eye Fields: Effects of Frontal Transcranial Magnetic Stimulation on Partial Report Analyzed by Bundesen's Theory of Visual Attention

“…This allowed separate parameters concerning top-down control, sensory effectiveness, and attentional weight to be measured within a single visual task (see Materials and Methods, Data analysis, below). We have previously examined the impact of parietal TMS on Bundesen parameters in this task (Hung et al, 2005). Applying TMS to the left or right human FEF permitted a new comparison between the frontal effects studied here and the parietal effects reported previously (Hung et al, 2005) to further specify the distinct roles of different nodes in the network modulating visual processing.…”

“…For blocks with TMS, magnetic stimulation was delivered synchronously with the onset of the stimulus array to be reported (during the partial-report or the color-report control task) in a five-burst train at 10 Hz for 500 ms, with a fixed intensity of 60% of maximum stimulator output (corresponding to 86 -115% of the intensity of the resting motor threshold for participants in this experiment, when the same TMS stimulator was applied over M1 outside the experiment). Previous studies have shown that, with these stimulation parameters, such bursts of TMS are able to induce transient disruptions of neural processing in FEF (Muggleton et al, 2003;O'Shea et al, 2004; for effects of the same TMS protocol in the current paradigm when applied over PPC instead of FEF, see Hung et al, 2005).…”

“…The general procedures were modified from a previous study (Duncan et al, 1999) that applied Bundesen's (1990) TVA approach to study effects of permanent brain damage in patients. The exact procedures adhered to those used in our previous parietal TMS study of healthy volunteers (Hung et al, 2005) for the selective partialreport paradigm, being identical to that study in all respects except for the new TMS sites used here [now over left or right FEF, rather than over left or right posterior parietal cortex (PPC)]. Our use of equivalent methods, including identical TMS parameters except for the changed TMS sites, allowed a direct comparison of the present new FEF effects to the PPC effects found previously.…”

“…Our use of equivalent methods, including identical TMS parameters except for the changed TMS sites, allowed a direct comparison of the present new FEF effects to the PPC effects found previously. Given the similarity in methods, we summarize the protocol only briefly here; see Hung et al (2005) for further details.…”

“…To ensure that any TMS effects on selection by task-relevant color in the above partial-report task could not be due to any effects on gross color perception, we also tested a control task requiring report of stimulus color rather than digit identity (Duncan et al, 1999;Habekost and Bundesen, 2003;Hung et al, 2005). The stimulus array now comprised a single digit in red or green at one of the four possible positions (hence eight display types); these positions were identical to those used in the partial-report task.…”