Salillas manuscript, Number processing
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Motion on numbers: Transcranial Magnetic Stimulation on VIPS
alters both numerical and motion processes.
Elena Salillas
1
, Demis Basso
2
, Carlo Semenza
1
and Tomaso Vecchi
2
1.
Dipartimento di Psicologia. University of Trieste, Italy.
2.
Dipartimento di Psicologia. University of Pavia, Italy.
Correspondence:
Dr. Elena Salillas
Department of Psychology st 445
University of Trieste
Via S. Anastasio 12
34100 Trieste – ITALY
Phone: +39 040 558 2725
Fax: +39 040 558 8022
Email:
salillas@psico.units.it
SUMMARY
In this paper single pulse transcranial magnetic stimulation was applied
to the Ventral Intraparietal Sulcus (VIPS), this showed to alter the performance in both
motion detection and number comparison tasks. In the motion detection task, random
dot kinematograms were presented with different levels of coherence: reaction times
and the threshold for the perception of coherent motion increased when VIPS was
stimulated with respect to the stimulation of the central sulcus (control site). In the
number comparison task, numbers from 1 to 9 had to be compared with the reference
number 5: again, reaction times slowed down when the stimulation was applied to VIPS
with respect to the control site. These data reveal that TMS stimulation in an area
implied in motion perception alters the number comparison process thus showing that
motion and the processing of numbers in a comparison task are associated. If a process
devoted to motion is what was altered by TMS stimulation, then a mechanism implying
motion may – in turn
– be operating on the mental number line.
INTRODUCTION
Different studies hypothesized a relationship between number representation and space
(for a review see: Hubbard et al., 2005) and there is increasing evidence regarding the
connection between these two domains. Indeed, previous data had shown interference
between tasks known to be processed in the dorsal pathway, such as line orientation and
the irrelevant presence of numbers (Fias et al., 2001). However, besides object location,
the dorsal pathway is also implied in motion perception (Ungerleider et al., 1998).
Although a predominance of attention to location processing over attention to other
features such as color or motion has been shown (eg. Hillyard and Anllo-Vento, 1998),
motion perception leads to the emergence of independent motion-related
electrophysiological processes (Anllo-Vento and Hillyard, 1996). The independence
between motion and location perception makes the independent study of motion
possible. In this work we will focus on the relationship between motion and calculation
processes. Our starting point is that a motion component is present in the process of
number comparison and, likely, in other numerical processes that imply core number
activation. If this is the case, areas known as processing motion should show to be
implied in numerical processes. We thus tested for the existence of interference on both
motion and number processing in a specific area of the Intraparietal Sulcus (IPS).
The parietal cortex has been shown (Culham and Kanwisher, 2001) to be
involved in many processes, mainly performing spatial analysis (such as attention,
working memory, representation) and other high cognitive processes (calculation and –
to some extent – language related processes). Besides the human homologue to the
macaque´s MT/V5, some parietal areas have been identified as motion sensitive (for a
recent review see Orban et al., 2005). Four areas have been identified in the Posterior
 Parietal Cortex (PPC; see anatomical distribution in Orban et al., 2003, and Sunaert et
al., 1999): the Ventral Intraparietal Sulcus (VIPS) is the most posterior one, in the
occipital part of the IPS; the second area, the parieto-occipital IPS, resides at the
confluence between the parietal and occipital parts of the IPS; the other two areas are
located in the parietal or horizontal segment of IPS and they are referred as dorsal IPS:
medial and anterior. VIPS corresponds to human vIPS (Shulman et al. 1999) and to the
junction of
intraparietal and transverse occipital sulci (IPTO) (Wojciulik and
Kanwisher, 1999), but it is different from the multimodal area responsive to motion in
the IPS (Bremmer et al., 2001), referred as VIP area (Hubbard et al., 2005). The highly
motion-sensitive area V3a - whose BOLD activations in front of motion stimuli
normally follow in magnitude the activations of MT - is more posterior following VIPS.
In humans, it has been hypothesised that motion information may reach IPS through
projections from V3a (Orban et al., 2005; Vanduffel et al., 2002). On the other hand, in
general, human IPS is more involved in the perception of motion than its monkey
counterpart. This distribution of motion areas in human IPS and its proximity to other
areas related to numbers, lead us to select VIPS as the focus area in our study. We
hypothesized that an area implicated in motion processing could also overlap with the
network processing numbers (Nieder, 2004), and would thus function as a neural locus
of convergence of these two processes.
In the Triple Code Model of number processing (Dehaene and Cohen, 1995),
recently revised by Dehaene and collaborators (2003), the authors proposed that
numbers are coded in three representations, each one located in a different cerebral area,
differentially activated depending on task requirements. The horizontal segment of the
intraparietal sulcus (hIPS) would implement the core representation of quantity, in the
likely form of a mental number line, and it is suggested to be domain-specific. This
representation would be mostly activated in magnitude comparison and approximation
tasks. The posterior superior parietal lobule would support the hIPS via attentional
orientation on the mental number line, like on any other spatial dimension. Finally, the
left angular gyrus would complement the hIPS in language mediated numerical tasks.
The hypothesis that VIP area could be associated to the number processing system,
which has also
been proposed as a multimodal motion responsive region (Hubbard et
al., 2005; Bremmer et al., 2001) is relevant for this study. Certainly, the VIP area has an
anatomical localization similar to the hIPS of Dehaene et al (2003); nonetheless, neither
evidence of a possible relationship between numbers and motion, nor any neural
substrate have been previously shown. On the basis of these considerations, we selected
VIPS (Orban et al 2005) as our area of interest for the following reasons: a) it is not
considered an area related to number processing by Dehaene et. al. (2003) or Hubbard
et. al. (2005); b) it is close to these areas, yet far enough to allow its independent
stimulation; c) it is indeed an area sensitive to motion.
The transcranial magnetic stimulation (TMS) technique allows to modify brain
functions inducing focal electrical fields in the cortical neurons (Walsh and Cowey,
2000), while inhibiting or facilitating behavioural performance. It is non-invasive and
leads to an experimental manipulation of cortical information processing while treating
the stimulated areas as an independent variable. TMS has been used so far in a few
studies aimed at evaluating the neural correlates of number comparison. The angular
gyrus has been shown to be involved with two digit number comparison, bilaterally.
Stimulation on left angular gyrus altered the comparison times of numbers greater than
the reference; in contrast, the right angular gyrus rTMS stimulation altered both higher
and lower number comparison times (Göbel et al., 2001). Repetitive TMS stimulation of
the left inferior parietal lobule disrupted processing of number comparison in Sandrini
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