No Evidence for an Effect of the Distance Between the Hands on Tactile Temporal Order Judgments

Abstract

Localizing somatosensory stimuli is an important process, as it allows us to spatially guide our actions toward the object entering in contact with the body. Accordingly, the positions of tactile inputs are coded according to both somatotopic and spatiotopic representations, the latter one considering the position of the stimulated limbs in external space. The spatiotopic representation has often been evidenced by means of temporal order judgment (TOJ) tasks. Participants’ judgments about the order of appearance of two successive somatosensory stimuli are less accurate when the hands are crossed over the body midline than uncrossed but also when participants’ hands are placed close together when compared with farther away. Moreover, these postural effects might depend on the vision of the stimulated limbs. The aim of this study was to test the influence of seeing the hands, on the modulation of tactile TOJ by the spatial distance between the stimulated limbs. The results showed no influence of the distance between the stimulated hands on TOJ performance and prevent us from concluding whether vision of the hands affects TOJ performance, or whether these variables interact. The reliability of such distance effect to investigate the spatial representations of tactile inputs is questioned.

Keywords

distance effect temporal order judgment spatiotopic representation proprioception vision touch

Mapping somatosensory stimuli on the body surface is an important function as it helps not only to identify which part of our body is in contact with external stimuli but also to locate these stimuli in the surrounding space. Ultimately, somatosensory mapping helps to translate spatially-located perception of bodily contacts into spatially-guided motor behaviors in order to finely tune manipulation with innocuous objects and, also, defensive reaction to noxious external objects (Brozzoli et al., 2014; Graziano & Cooke, 2006).

Somatotopy represents an almost point-by-point readout for the brain of the spatial arrangement of the somatosensory receptors in the skin and in internal organs. More exactly, it corresponds to the ordered projection of the afferent responses from the receptors to spatially segregated groups of neurons in the brain (Penfield & Boldrey, 1937). However, as the body limbs, and consequently the somatosensory receptors, are constantly moving in external space, such a spatial representation might be inefficient to appropriately localize the contacting objects in external space and orient behaviors toward their location. Therefore, the spatiotopic representation uses external space as reference frame and considers the relative position of the body part on which the stimulus is applied (e.g., Azañon & Soto-Faraco, 2008; Driver & Spence, 1998; Graziano et al., 1997; Shore et al., 2002; Smania & Aglioti, 1995; Yamamoto & Kitazawa, 2001). It thereby represents a crucial spatial representation of somatic information as it anchors somatosensory perception in a representation of the body posture and limb movements in external world, with the purpose of interacting with objects in the environment close to the body (Brozzoli et al., 2014).

The ability to code somatosensory (tactile or nociceptive) information according to spatiotopic reference has been assessed, among others, using temporal order judgment (TOJ) tasks (Badde et al., 2014, 2015; Crollen, Albouy, et al., 2017; Crollen, Lazzouni, et al., 2017; Crollen et al., 2019; De Paepe et al., 2015; Heed & Azañón, 2014; Röder et al., 2004; Sambo et al., 2013; Shore et al., 2002; Vanderclausen, Bourgois, et al., 2020; Vanderclausen, Manfron, et al., 2020; Yamamoto & Kitazawa, 2001). In these tasks, participants judge the temporal order of two successive somatosensory stimuli, one applied to each hand, separated by different temporal delays. Participants’ judgments are typically less accurate when their hands are crossed, when compared with an uncrossed posture. Such an effect is accounted for by the fact that the somatotopic representation mismatches the spatiotopic representation (e.g., when crossed, a stimulus applied on the left hand is coming from the right part of space). The fact that TOJ performance depends on the location of the hands in external space suggests that nociception and touch, in addition to the somatotopic coding, are automatically coded according to a spatiotopic reference frame, a process called somatosensory remapping (Driver & Spence, 1998).

Spatiotopic coding of somatosensory information has also been investigated using other experimental procedures (e.g., Azañón Camacho, et al., 2010; Azañón, Longo, et al., 2010; Azañón & Soto-Faraco, 2008; Moayedi et al., 2016; Overvliet et al., 2011; Smania & Aglioti, 1995; Soto-Faraco & Azañón, 2013; Soto-Faraco et al., 2004; Tame et al., 2011) such as manipulating the distance between the limbs to which the somatosensory stimuli were applied. The rationale is that, as the somatotopic representations of the two stimulated skin areas do not change between the different postural conditions, potential differences in participants’ performance between two conditions can only be attributed to the metric distance between the stimulated limbs in external space. Studies have indeed shown that the discrimination of tactile stimuli applied on one hand was more sensitive to distraction induced by tactile stimuli applied on the opposite hand when the two hands were close together, when compared with when they were further apart (Driver & Grossenbacher, 1996; Evans et al., 1992; Lakatos & Shepard, 1997; Soto-Faraco et al., 2004). During tactile TOJ tasks, judgments are more difficult with decreasing the distance between the hands (Roberts et al., 2003; Shore et al., 2005). Such an effect was also shown when distance manipulation was made through mirror-reflected images of the hands while their physical distance was unchanged (Gallace & Spence, 2005).

However, while crossing the hands during tactile and nociceptive TOJ tasks has provided very robust and reliable effects across the different studies, results from the studies having manipulated the distance between the stimulated limbs are very small and less consistent. For instance, Shore et al. (2005) did not replicate the distance effect between arms when different fingers of the hands were stimulated. Similarly, Kuroki et al. (2010) showed that participants’ judgments were more sensitive to the anatomical distance between two stimulated areas on the skin surface than to the distance in external space between the two body parts. As suggested by the study of Gallace and Spence (2005), the postural effect during tactile TOJ could depend on visual feedback about hand positions. The data of that study indeed illustrate the important role that vision of the body plays in somatosensory perception, sometimes taking priority over proprioceptive cues (see also Botvinick & Cohen, 1998; Gallace & Spence, 2005; Pavani et al., 2000; Soto-Faraco et al., 2004; Torta et al., 2015). Accordingly, Cadieux and Shore (2013) showed that preventing vision of the stimulated hands reduced the crossed hands deficit during tactile TOJ (at least when the task procedure minimized the use of spatiotopic coordinates). The aim of the present study was therefore to test the influence of seeing the hands on the modulation of tactile TOJs induced by changing the spatial distance between the two stimulated hands. Participants performed a TOJ task with vibrotactile stimuli delivered to their fingertips with the hands placed according to two postures: close versus far distance from each other. One group of participants could see their hands during the experiment, while another group was blindfolded. It was hypothesized that, because visual information about position of the limbs in external space was prevented, participants would mostly rely on anatomical representations. We therefore expected reduced distance effects in blindfolded participants, when compared with those who could see their hands. In other words, we expected to observe significant differences in TOJ sensitivity between the two hand postures in the group of sighted participants, while such a difference would be reduced in the blindfolded group.

Methods

Participants

Based on the sample size of our previous studies (e.g., Filbrich, Alamia, Blandiaux, et al., 2017; Filbrich, Alamia, Verfaille, et al., 2017; Vanderclausen et al., in press), 30 volunteers participated in the study. Four participants were excluded from the analyses due to unreliable performances during the task (see Data Analyses section). The mean age of the remaining 26 participants (15 women) was 23.88 years old (SD = 3.54, range = 18–36). Inclusion criteria were as follows: normal or corrected-to-normal visual acuity, no prior history of severe neurological, psychiatric, or chronic pain disorder, no traumatic injury of the upper limbs within the last 6 months, no regular use of psychotropic drugs, and no intake of analgesic drugs (e.g., nonsteroidal anti-inflammatory drugs, paracetamol) within the 12 hr preceding the experiment. According to the Flinders Handedness Survey (Flanders) (Nicholls et al., 2013), one participant was left-handed, one was ambidextrous, and all the others were right-handed. The local ethic committee in agreement with the sixth revision (2008) of the Declaration of Helsinki approved the experimental procedure. All volunteers signed an informed consent before starting the experiment and received financial compensation for their participation.

Stimuli and Materials

Vibrotactile stimuli were applied on each hand by means of two vibrotactile transducers (TL-002-14R Haptuator, Tactile Labs, Canada) driven by standard audio amplifiers and lasting 10 ms at 440 Hz (interruption of the last cycle did not affect stimulus perception). Stimuli were remotely controlled using MATLAB 2014. Participants held the vibrotactile transducers at ∼30 cm from the edge of the table, one in each hand between the thumb and the forefinger. The intensities of the vibrotactile stimuli were matched between the two hands, in order for the stimuli to be perceived as equally intense. If necessary, they were adapted before or during the experiment when this criterion was not met anymore. A small strip of tape pasted on the table along the participant’s midsagittal plane at 40 cm from the edge of the table (i.e., at ∼10 cm above the hands) was used as a fixation point for the group of participants who could see the stimulated hands (see Figure 1).

Procedure

The experiment consisted of a TOJ task performed on two tactile stimuli, one applied to each hand. More exactly, for each pair of left and right stimuli, participants were asked to discriminate the order of temporal succession between the two stimuli. They responded verbally by saying aloud “left” or “right”, which of the two tactile stimuli they perceived either as having occurred first (“which is first” response condition), or as having occurred second (“which is second” response condition), depending on the experimental blocks (with the aim of minimizing response biases, see Filbrich et al., 2016; Shore et al., 2001; Spence & Parise, 2010). Verbal responses were encoded by the experimenter (by pressing one of two keys on the computer, which triggered the next trial 2000 ms later). No instruction regarding the speed of their responses or any feedback regarding the accuracy of their performance was given to the participants during the task. The participants were pseudorandomly assigned to one of two groups of equal size. In one group, the participants could see their hands and were asked to maintain their gaze on a fixation point (vision group). In the other group, they wore a blindfold to prevent vision of their hands throughout the experiment (blindfolded group). The blindfolding was manipulated between participants, instead of within, to avoid extending the duration of the experiment that might have impacted their ability to focus on the task. For each group, the task was performed under two posture conditions, either with the hands close to each other or further apart.

The experiment took place in a dimly illuminated testing room. Participants were seated on a chair and rested their arms on a table while holding the transducers in their hands slightly above the table surface in order to avoid noise from the vibrations of the transducers against it. Their head was stabilized with a chin rest to minimize movement during the experiment. Participants’ hands were always equally distant from their midsagittal plane. In the far posture, the hands were placed at a distance of 70 cm from each other, as measured between the two vibrotactile transducers, while in the close posture, the distance was of 2 cm. Before starting the experiment, participants completed a practice session to get familiarized with the vibrotactile stimuli and the experimental setup. This practice session consisted of 4 blocks of 10 trials each, one block for each combination of posture (close vs. far hands) and the response conditions (‘which is first” vs. ‘which is second”). During this practice phase, only two stimulus onset asynchronies (SOAs) were used, that is, ±145, ±200. Participants’ performance was not recorded during practice blocks, and no feedback was given to them.

During the experiment, each participant was presented with 4 blocks of 40 trials each. During two blocks, the participants’ hands were placed according to the close posture condition, while they were placed in the far posture in two other blocks. Each participant started the experiment with two blocks of the same posture condition and then continued with the two blocks of the other condition; the order of the posture condition was counterbalanced across participants of the same group. For each block of the same posture condition, one was performed with the “which is first” instruction, the other with the “which is second” instruction; order of the response conditions was randomized. Noises from experimental devices were covered by means of a white noise played through earphones. Each block lasted approximately 5 min, and the entire experiment lasted approximately 30 min, including instructions and the practice session.

For each pair of stimuli, the two vibrotactile stimuli were separated by 1 out of 22 possible time intervals (SOAs for stimulus onset asynchronies): ±5, ±10, ±15, ±30, ±45, ±60, ±75, ±90, ±145, ±200, and ±400ms (negative values indicate that the vibrotactile stimulus on the left hand was applied first; positive values that the right hand stimulus was applied first). At each trial, the presented SOA was selected using the adaptive psi method (Kingdom & Prins, 2010) based on the participant’s responses in all the previous trials (see Filbrich, Alamia, Verfaille, et al., 2017; Vanderclausen, Bourgois, et al., 2020; Vanderclausen, Manfron, et al., 2020 for details regarding the use of the psi method for TOJ tasks). More specifically, the algorithm adopts a Bayesian framework with the ultimate goal of estimating the posterior probability of the parameters of interest (i.e., the α (point of subjective simultaneity [PSS]) and the β (slope)) without probing extensively all the SOAs. The core idea is to minimize the expected entropy (i.e., uncertainty) of the posterior distribution trial by trial, such as the response of the participant at each trial provides the most information about the distribution of the parameters. In other words, the algorithm, given all the information collected so far in the previous trials, infers which condition (i.e., SOA) is the most informative in the next trial in order to estimate the joint distribution of the parameters.

Measures

The TOJ performance, that is, the probability to perceive one of the two stimuli as occurring first as a function of the SOA, was fitted online for each participant and each condition using the logistic function f(x)=1/(1+exp(-β(x-α))) (Kingdom & Prins, 2010; Kontsevich & Tyler, 1999). Here, the proportion of left first responses was plotted as a function of SOA. Because the adaptive method was used, the α and the β values of the function were estimated at each trial and, at the end of the stimulation block, the last estimates were taken as measures of the participants’ performance. The β parameter characterizes the slope of the psychometric function and describes the noisiness of the results (i.e., entropy). Therefore, this parameter reflects the precision of the participants’ responses during the experiment, that is, their performance at the task (Kingdom & Prins, 2010; Kontsevich & Tyler, 1999). It constitutes the main parameter of interest in this study, as the slope is often used to derive the just noticeable difference (JND) in typical TOJ experiments (Heed & Azañón, 2014; Spence & Parise, 2010) and was shown to be affected by the distance between the hands during tactile TOJ (Roberts et al., 2003; Shore et al., 2005). The α parameter is the threshold of the function and refers to the PSS. Although the contribution of the α parameter to current data was less relevant, this parameter was also measured in order to assess the presence of possible biases that could affect the estimation of the slope (see Filbrich, Alamia, Blandiaux, et al., 2017; Filbrich, Alamia, Burns, et al., 2017; Vanderclausen et al., 2017). A third nonstandard parameter was derived a posteriori to further characterize participants’ performances. Based on all trials presented for each condition, we computed the mode of the presented SOA to index which of the time interval values was the most often presented to a given participant (see Vanderclausen, Bourgois, et al., 2020; Vanderclausen, Manfron, et al., 2020).

Because the adaptive psi method is based on a Bayesian framework, priors had to be postulated (Kingdom & Prins, 2010). We used a prior distribution of 0 ± 20 ms to estimate the α parameter, as no bias toward one of the two hands was expected. Based on previous experiments performed with healthy volunteers, 0.06 ± 0.6 was chosen as a prior distribution to estimate the β parameter (Filbrich, Alamia, Verfaille, et al., 2017; Vanderclausen et al., in press). Two other parameters, the λ and γ, were fixed in advance for all participants and not used for analyses. The λ corresponds to the lapse rate, that is, the probability of giving an incorrect response independently of stimulation variables, and the γ to the guess rate, that is, the probability of giving a correct answer whereas the stimulus has not been detected (Prins, 2012). As suggested by Kingdom and Prins (2010), the λ was set at 0.02 and the γ at 0.

Data Analyses

Data were excluded from further statistical analyses if the slope of the psychometric function could not be reliably estimated during the 40 trials within one condition (i.e., the parameter estimate did not converge on a stable value on the last trials). Before statistical analyses, data from the two response conditions (“which is first” and “which is second”) were averaged together for each group and for each posture condition.

One-sample t tests were used to examine the potential presence of biases toward one of the two hands by comparing the averaged PSS values of each condition to 0. Next, in order to assess the influence of experimental manipulations on participants’ TOJ performances, analyses of variance (ANOVAs) with posture (far vs. close) as within-subject variable and group (vision vs. blindfolded) as between-subject variable were performed separately on the PSS, the slope, and the mode values. Effect sizes were measured by means of partial eta squared for ANOVAs and Cohen’s d for t tests. Significance level was set at p ≤ .05. Finally, classic frequentist statistical analyses were complemented by Bayesian statistics (using Bayesian repeated measures ANOVAs with JASP 0.9.2.0, University of Amsterdam, the Netherlands) performed on the PSS, slope, and mode values, respectively. To this aim, we computed a Bayes factor to quantify the alternative hypothesis (HA) relative to the null hypothesis (H0) (BF₁₀, Cauchy prior = 0.707). Interpretations are based on the classification scheme established by Lee and Wagenmakers (2013); Wagenmakers et al. (2017).

Results

The psychometric curves fitting TOJ performances are illustrated in Figure 2. Individual data and their means are displayed on Figure 3. Analyses of the slope values did not reveal any significant results for the main effect of posture, F(1, 24)=0.788, p = .384, η²_p=0.032, for that of group, F(1, 24)=0.346, p = .562, η²_p=0.014, or for the interaction between the two variables, F(1, 24)=0.062, p = .805, η² _p =0.003. Regarding Bayesian analyses, anecdotal evidences were shown in favor of H0 for the posture (BF₁₀ = 0.38, error = 1.636) and group factors (BF₁₀ = 0.54, error = 1.556) and for their interaction as well (BF₁₀ = 0.36, error = 1.202). This suggests that there is no sufficient evidence allowing us to draw conclusions about the effect of the distance between the stimulated hands or the possibility to see them, on TOJ performances.

Figure 1.

Design of the experiment. Participants performed a temporal order judgment task on pairs of tactile stimuli applied one to each hand by means of vibrotactile transducers (illustrated by the black diamonds) held between the thumb and the forefinger. Hands were placed so that the vibrotactile transducers were held at a distance of either ∼70 cm (hands far; left) or ∼2 cm (hands close; right) from each other. One group of participants could see their hands during the task (vision group; top), while participants of the other group were blindfolded (blindfolded group; bottom). The participants of the vision group were asked to maintain their gaze on a central point at ∼40 cm in front of them (illustrated by the gray circle).

Figure 2.

TOJ curves. The figure illustrates the curves of the psychometric function based on group averaged data, from the fitted responses of the vision group participants (left) and of the blindfolded group participants (right) separately, when performing the task with the hands placed either in a far (solid gray lines) or in a close (dotted black lines) posture. The x axis corresponds to the different possible stimulus onset asynchronies (i.e., SOA). Negative values indicate that the left hand was stimulated first, and the positive values indicate that the right hand was stimulated first. The y axis refers to the proportion of trials in which the tactile stimulus applied on the left hand was reported as being perceived first.

Figure 3.

TOJ parameters. The figure shows scatter plots displaying individual data (black dots) and their means (black strips), respectively, for the slopes and PSS values of the TOJ task as well as the modes of the SOAs used during the adaptive procedure. The data are plotted according to the group (vision group: left; blindfolded group: right) and the experimental condition (hands in the far posture: left inner sections; hands in the close posture: right inner sections). Data of the same participant are linked by the gray lines.

The one-sample t tests comparing the PSS values to 0 did not reveal any significant results (all t(12)≤1.648, all p ≥ 0.125), suggesting that there was no significant bias affecting the perception of the stimuli applied to one of the two hands in none of the conditions and for either group. The ANOVA performed on the PSS values showed no significant effect for the posture, F(1, 24)=0.072, p = .791, η² _p =0.003, for the group variables, F(1, 24)=0.142, p = .709, η² _p =0.060, or for the interaction between the two variables, F(1, 24)=2.182, p = .153, η² _p =0.083. Bayesian ANOVA analyses revealed moderate evidence in favor of H0 related to the effect of posture (BF₁₀ = 0.275, error = 0.819) and anecdotal evidences in favor of H0 for to the main effect of group (BF₁₀ = 0.488, error = 1.465) and the interaction between posture and group as well (BF₁₀ = 0.832, error = 1.126).

The ANOVA analyses about the mode of the SOA did not reveal any significant effect of the posture, F(1, 24)=0.477, p = .496, η²_p =0.019, of the group, F(1, 24)= 0.916, p = .348, η²_p =0.037, nor significant interaction between the two variables, F(1, 24)=0.477, p = .496, η²_p =0.019 (Figure 2C). Finally, Bayesian analyses revealed moderate evidence in favor of H0 for the main effect of posture (BF₁₀ = 0.331, error = 0.950), anecdotal evidences in favor of H0 for the main effect of group (BF₁₀ = 0.57, error = 0.981), and for the interaction between posture and group as well (BF₁₀ = 0.44, error = 2.493).

Overall, it appears that tactile TOJ performances were not affected by the spatial distance between the stimulated hands. However, results from Bayesian statistics prevent us from concluding on the role of vision on these performances or on the interaction between posture and group variables.

Discussion

The present study aimed to test the influence of the distance in external space between two body limbs on the ability to discriminate the temporal order between two tactile stimuli, one applied to each of those limbs. We also investigated whether such an effect could be modulated by the possibility of seeing or not the limbs on which tactile stimuli were applied. Previous experiments reported decreased performances in judging the order of appearance of two tactile stimuli when the hands are positioned close together when compared with when they are placed farther away, as indexed by higher JND values in the close posture (Gallace & Spence, 2005; Roberts et al., 2003; Shore et al., 2005). These studies confirm that spatial processing of somatosensory inputs integrates information about the relative posture of the body limbs in external space, in line with the results from the studies having tested tactile TOJ performance with the hands in a crossed posture (e.g., Crollen, Lazzouni, et al., 2017; Crollen et al., 2019; Heed & Azañón, 2014; Röder et al., 2004; Shore et al., 2002; Yamamoto & Kitazawa, 2001). It was also suggested that these effects might be mostly driven by the actual vision of the limbs position (Cadieux & Shore, 2013; Gallace & Spence, 2005). However, in the present study, we did not succeed to provide reliable evidence in favor of the influence of the distance between the stimulated limbs, and its modulation by actual vision of the limbs, on tactile TOJ performances.

One of the main differences between the present and previous experiments, having tested the influence of hands' distance on tactile TOJ performance, relies on the fact that the different time intervals (i.e., SOAs) between the two consecutive stimuli were administrated by means of an adaptive procedure in the present experiment, while previous studies used methods of constant stimuli (Gallace & Spence, 2005; Roberts et al., 2003; Shore et al., 2005). In these previous studies, each SOA condition was repeated 8 to 15 times in block of 80 to 90 stimuli, whereas the stimulation blocks of the present experiment consisted only of 40 trials selected among 22 possible SOA conditions. It is therefore possible that our experimental procedure did not succeed to fully sample the psychometric function fitting participants’ responses and did not properly measured limb distance effect during somatosensory TOJ tasks. However, this seems unlikely as, on the contrary, one of the main advantages of psychophysics adaptive methods over constant stimuli methods is to allow more reliable estimation of the parameters of the psychometric function even when a limited number of stimuli is used (Filbrich, Alamia, Burns, et al., 2017; Kontsevich & Tyler, 1999). Accordingly, using the same adaptive method as the one used in the present experiment, previous studies succeeded to reliably estimate both the threshold (Filbrich, Alamia, Blandiaux, et al., 2017; Filbrich, Alamia, Burns, et al., 2017; Filbrich, Alamia, Verfaille, et al., 2017; Filbrich et al., 2018; Legrain et al., 2018; Manfron et al., 2019; Torta et al., 2018; Vanderclausen et al., 2017) and the slope parameters (Vanderclausen, Bourgois, et al., 2020; Vanderclausen et al., in press; Vanderclausen, Manfron, et al., 2020) of the TOJ-fitting psychometric function and to significantly measure changes in participants’ judgments according to experimental manipulations such as the posture of the stimulated limbs. For instance, Vanderclausen and colleagues (Vanderclausen, Bourgois, et al., 2020; Vanderclausen et al., in press; Vanderclausen, Manfron, et al., 2020) have shown significant and reliable crossing hands effects during TOJ tasks with somatosensory (vibrotactile or radiant heat) stimuli delivered using the adaptive psi procedure.

Therefore, one of the most probable reasons why we failed to replicate the limb distance effect during somatosensory TOJ tasks could rely on the fact that such an effect is weaker than that observed when the stimulated limbs are crossed over the body midline. Indeed, while the crossing hands effect has been recurrently and robustly demonstrated during somatosensory TOJ tasks, the limb distance effect seems to be of smaller magnitude (Roberts et al., 2003). For example, in Shore et al. (2005), the averaged differences between the JND of the close and far posture conditions range from 9 to 20 ms. Furthermore, the distance effect varies according to the use of TOJ (Kuroki et al., 2010; Shore et al., 2005) or simultaneity judgment tasks (Axelrod et al., 1968; Kuroki et al., 2010). Conversely to the manipulation of crossing the hands over the body midline, when the distance between the stimulated limbs is manipulated, each hand remains in its hemispace, and, therefore, the somatotopic and spatiotopic representations remain aligned with each other, even when the hands are placed close together. Indeed, TOJ studies suggest that spatiotopic representation of tactile inputs can be automatically generated and overlaps in times with that of the somatotopic one, at least at the time scale of TOJ experimental designs (e.g., Azanon, Camacho, et al., 2010; Badde & Heed, 2016; Badde et al., 2016). Therefore, their coactivation generates mismatching responses when the limbs are crossed as the left hand is in the right part of space and vice versa. It has been suggested that decreased TOJ performances during the crossed hands posture might actually reflect the effort that the brain has to make in order to inhibit the response from the irrelevant spatial representation and select the relevant one (Crollen et al., 2019; Vanderclausen, Manfron, et al., 2020). By comparison, although decreasing spatial distance between two stimuli might actually induce more attentional competition between the two stimuli (see, for instance, Mangun & Hillyard, 1987, 1988 for similar effect in the visual domain), such competition might be of lesser magnitude than that induced by crossing the hands. Therefore, it might not be reliably sampled during TOJ tasks manipulating the distance between the stimulated limbs.

As a consequence, the possibility to evidence reliable effects of the spatial positions of the stimulated body parts could depend on the cognitive goals manipulated by the task instruction and on the associated attentional requirements. For instance, experiments involving fine discrimination of tactile stimuli did evidence significant effect of the spatiotopic distance between the stimulated body parts (Driver & Grossenbacher, 1996; Lakatos & Shepard, 1997; Soto-Faraco et al., 2004). Accordingly, it might be instructive to compare the results of the temporal judgment studies based on whether participants are asked to judge the order between the stimuli or whether they are simultaneous or not. It has been indeed suggested that simultaneity judgments are less demanding in terms of attentional resources that TOJs (Fujisaki et al., 2012). However, Kuroki et al. (2010) did not evidence any significant distance effect during neither temporal order nor simultaneity judgment tasks. Further studies will be needed to disclose and describe the conditions under which reliable effect of manipulating the spatial distance between the body parts on which somatosensory stimuli are applied might be observed during perceptual tasks.

In conclusion, it seems that changing the spatial distance between the stimulated limbs is less reliable than foreseen in aiming to highlight the complex cognitive processes used to spatially represent somatosensory stimuli. On the contrary, crossing the hands is a more suitable technique to experimentally dissociate the respective contributions of the somatotopic and spatiotopic reference frames underlying the spatial representations of somatic stimuli.

Footnotes

Authors' Note

The raw data are available at the following link: .

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: C. V. and V. L. are supported by the Funds for Scientific Research of the French-speaking Community of Belgium (F.R.S.-FNRS).

ORCID iD

Louise Manfron

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