rTMS over the intraparietal sulcus disrupts numerosity processing

Abstract

It has been widely argued that the intraparietal sulcus (IPS) is involved in tasks that evoke representations of numerical magnitude, among other cognitive functions. However, the causal role of this parietal region in processing symbolic and non-symbolic numerosity has not been established. The current study used repetitive Transcranial Magnetic Stimulation (rTMS) to the left and right IPS to investigate the effects of temporary deactivations of these regions on the capacity to represent symbolic (Arabic numbers) and non-symbolic (arrays of dots) numerosities. We found that comparisons of both symbolic and non-symbolic numerosities were impaired after rTMS to the left IPS but enhanced by rTMS to the right IPS. A signature effect of numerical distance was also found: greater impairment (or lesser facilitation) when comparing numerosities of similar magnitude. The reverse pattern of impairment and enhancement was found in a control task that required judging an analogue stimulus property (ellipse orientation) but no numerosity judgements. No rTMS effects for the numerosity tasks were found when stimulating an area adjacent but distinct from the IPS, the left and right angular gyrus. These data suggest that left IPS is critical for processing symbolic and non-symbolic numerosity; this processing may thus depend on common neural mechanisms, which are distinct from mechanisms supporting the processing of analogue stimulus properties.

Fig. 1

The comparison tasks. a The experimental design was the same for all the experiments. b In each display one of the following stimuli was presented: Arabic numbers close and far from the reference number 65 (Experiment 1), dots close and far in number from a reference display containing 65 dots (Experiment 2), or ellipses close or far from a circle with vertical or horizontal elongation (Experiment 3). The top row of the figure shows an example of stimuli far from the reference, the bottom row stimuli close to the reference (pictured stimulus sizes are not to scale). c In addition to baseline (no TMS), each experiment was performed after real or sham TMS; these conditions were counterbalanced between subjects and each followed by a 30-minute-rest period

Fig. 2

Sites and targeting of rTMS and sham stimulation. In Experiments 1 and 2, all sites in (a) were tested over 2 days; in Experiment 3, the left and right IPS sites and one of the sham sites were tested in one day. The order of stimulation conditions was fully counterbalanced across subjects. In (b) is the illustration produced by the stereotactic system (Briansight) of the application of rTMS in a representative subject. Yellow lines illustrate an example of where each stimulus was applied during an entire 10 min rTMS train for each of the four stimulation positions. Green dots correspond to the point on the scalp where each TMS pulse was delivered. The stimulation coil rested tangentially on the subject's scalp and the handle pointing posteriorly parallel to the subject's midsagittal plane as calculated by the frameless stereotactic

Fig. 3

Performance impairments following rTMS. Bars indicate the mean of difference in response time, and numbers in parentheses indicate the mean of differences in error rates after rTMS relative to sham stimulation for close versus far comparisons for the experiments with (a) Arabic numbers, (b) dots, and (c) ellipses. rTMS conditions in which response times significantly differed from sham stimulation are indicated by asterisks inside the bars (*P < 0.05, **P < 0.01)