Altered numerosity perception: A transcranial magnetic stimulation study

In our daily life we constantly use numerosity, i.e. the set size of a number
of items, to guide our behaviour and decisions. Functional magnetic resonance
imaging (fMRI) experiments on numerosity perception in humans have reliably
found that areas in the parietal lobe are activated during numerosity
manipulation. Recently, we (Harvey et al., 2013; 2015; 2017) found a small
region in posterior parietal cortex (PPC) were neural populations respond to
specific numerosities. Although fMRI is a powerful technique for researching
the human brain in vivo, it is still fundamentally correlational in nature.
Using transcranial magnetic stimulation (TMS) we will investigate the causal
link between this topographical map and numerosity perception. We hypothesise
that the numerosity map underlies numerosity perception and consequently in
these experiments we will specifically target the numerosity maps. So far we
have described topographical organisation of the PPC numerosity map but no
studies have causally linked the numerosity map to perception. We intend to
selectively disrupt the numerosity maps to determine whether this map is
causally linked to perception of numerosity. To assess the disruption of the
numerosity maps, we will test performance on an enumeration task and a
numerosity discrimination task, since the disruption of the numerosity maps
might have a different effect on these tasks, given their distinct
characteristics. Participants will also perform an object size discrimination
task, which serves primarily as a control task. Moreover, Harvey et al. (2015)
found that the numerosity maps partially overlap with the object size maps, so
potential disruption of the object size discrimination performance by
stimulating the numerosity maps could further validate the findings of Harvey
et al. (2015) and be an informative addition to existing literature.

The aim of the present study is to causally detect whether disrupting
processing in the numerosity map with TMS alters numerosity perception and/or
object size perception.

Single-blind cross-over intervention with active and control sites.

Participants visit the lab on 5 separate days. Every day consists of a one-hour
session. In the first session their motor threshold value will be measured to
determine the intensity of the TMS. In the subsequent sessions, the
participants will receive TMS, up to a maximum of 10Hz for 0.5s at an intensity
of 120% motor threshold over the right hemisphere in (1) the numerosity map in
the PPC, (2) non-numerosity related area of the PPC, (3) primary visual cortex
(V1), and (4) vertex.

All research procedures are well-validated and with careful screening of
subjects for TMS exclusion criteria represents a very minimal risk. The
stimulation combined with the tasks and measurements consists of a moderate
burden for research participants. Participants are adult, mentally able
healthy volunteers who give informed consent and are compensated for their time
investment.