Multisensory coding of peripersonal space

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Behavioral consequences of multisensory integration processing

Multisensory integration enhances neural response to multisensory events compared to unimodal ones, but it also impacts behavioral reactions to them. Multisensory coding gives multiple advantages in terms of behavioral responses. First of all, bimodal events are detected faster than unimodal ones [Hershenson, 1962, Spence et al., 1998, Suied et al., 2009]. This effect is usually named the Redundant Signal Effect (RSE) [Kinchla, 1974]. Multisensory events also increase detection accuracy. In noisy situations for example, having a visual access to a speaker’s face increases speech comprehension [Sumby and Pollack, 1954]. Different modalities can bring complementary information, thus the number of different sensory information available usually improves event comprehension.
Furthermore, multisensory integration increases detection sensitivity. Sensory stimulations at the detection threshold are detected more often when they are combined with another modality signal. This phenomenon has been described for audiovisual integration, with bimodal events increasing detection of sub-threshold visual [Lovelace et al., 2003] and auditory events [Bolognini et al., 2005]. The sensitivity gain can depend on stimuli movements. Studies found that visual stimuli approaching a part of the body increase tactile sensitivity only on this part of the body [Kandula et al., 2015, Van der Biest et al., 2016]. More precisely, Cléry and colleagues showed that the sensitivity increase only at the time and place of the expected collision of the object with the body [Cléry et al., 2015a].

Peripersonal space definition based on multisensory neurons activity

Specific neurons of the pre-motor ventral cortex [Rizzolatti et al., 1981], parietal cortex [Colby et al., 1993] and putamen [Graziano and Gross, 1993] discharge only in response to sensory events near the body. Looking at the places where those neurons discharge describes a space around the body: PPS. An important aspect of this space is that it is coded by multisensory neurons. Mmultisensoryultisensory neurons discharge in the presence of two or three different types of sensory stimulation.
These neurons are mainly audiotactile, visuotactile and audiovisual neurons. For example, visuotactile neurons coding for the space around the face discharge when the macaque’s face is touched, but also when visual stimulations are near the face (for a review [Holmes and Spence, 2004]).

Two distinct fronto-parietal networks for peripersonal space representation?

PPS representation is based on fronto-parietal networks. In macaque brains, PPS is coded in the areas AIP (anterior parietal area), 7b and VIP (ventral intraparietal area) of the parietal cortex, and F4 and F5 of the area 6 of the pre-frontal cortex (see figure 2.3). A recent review looking at the anatomical connections and functional similarities of those areas claims that they constitute two separated fronto-parietal sub-networks [Cléry et al., 2015b].

The VIP-F4 parieto-frontal network: coding a defensive space?

The VIP – F4 network is thought to have a defensive function as it is linked to the implementation of protective behaviors for the body with an over-representation of the face and hands areas. Micro-stimulations of those two regions provoke stereotypical defensive behaviors such as eye blinking and squinting, retraction of the head, withdrawal of the hand or blocking arm movements [Graziano and Cooke, 2006].

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Peripersonal space as a defensive margin

Another attributed function of PPS is to code a safety margin around the self. The following experimental methods to study PPS are based on behavioral reactions to threat.

The Hand-Blink Reflex

A recent method was developed by Sambo and Iannetti to measure the defensive space around the eyes [Sambo et al., 2012b, Sambo et al., 2012a]. The technique relies on the blink reflex elicited when a potential danger is moving near the eyes. It is a prototypical defensive reflex that may be elicited by abrupt and intensive stimuli in various sensory modalities (visual, auditory and somatosensory). The authors record the electromyographic activity of the orbicularis oculi muscle bilaterally, which is involved in this blink reflex [Berardelli et al., 1999]. To elicit a blink reflex, authors send electrical stimulations on the median nerve on participants wrists [Alvarez-Blanco et al., 2009]. They manipulate the position of the hand in space, to measure the blink reflex when the potential danger is located at different positions in space (see figure 3.3). The intensity of the hand blink reflex (HBR) is not modulated in a linear way by the distance of the hand. There is a critical distance at which the proximity of the hand to the eye starts increasing the HBR: this distance is considered as the limit of the defensive PPS around the eyes (DPPS).

Table of contents :

I Introduction
1 Space management 
1.1 Animal spatial behaviors
1.2 Human spatial behaviors
2 Multisensory coding of peripersonal space 
2.1 Multisensory integration
2.1.1 Multisensory integration processes
2.2 Multisensory coding of near space in primates
2.2.1 Peripersonal space definition based on multisensory neurons activity
2.2.2 Two distinct fronto-parietal networks for peripersonal space representation?
2.2.3 Flexibility of peripersonal space
3 Peripersonal space in humans 
3.1 Behavioral measuring methods of peripersonal space
3.1.1 Peripersonal space as a multisensory integration area
3.1.2 Peripersonal space as a reaching area
3.1.3 Peripersonal space as a defensive margin
3.1.4 Attentional bias
3.2 Modulating factors of peripersonal space
3.2.1 Peripersonal space extent is body-centered
3.2.2 Emotional influence on peripersonal space
3.3 Peripersonal space as safety zone or space of voluntary motor actions
4 Peripersonal space in social contexts 
4.1 Space perception and social cognition
4.2 Social modulation of peripersonal space
4.3 Peripersonal space and personal space
II Experimental contribution 
5 General Methodology 
5.1 Experimental methodology
5.2 The auditory space
5.3 Multisensory integration processes involved in the audiotactile method
6 Handedness and peripersonal space 
6.1 Description of the study and main findings
6.2 Anisotropy of lateral peripersonal space is linked to handedness
6.3 Additional analysis
6.3.1 Supplementary Information
7 Peripersonal space in social contexts 
7.1 Study description and main findings
7.2 Social coding of the multisensory space around us
8 Measuring PPS in human: a methodological investigation 
8.1 Introduction
8.1.1 Limits of the actual method
8.1.2 Paths to overcome the limitations?
8.2 PPS measured with detection rates
8.2.1 Method
8.2.2 Results
8.2.3 Discussion
8.3 PPS measured using shorts sounds
8.3.1 Description of the study
8.3.2 Method
8.3.3 Results
8.3.4 Discussion
8.4 Towards a new protocole for PPS measurement?
9 Discussion 


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