Measuring the sensitivity of tactile temporal order judgments in sighted and blind participants using the adaptive psi method.

Spatial locations of somatosensory stimuli are coded according to somatotopic (anatomical distribution of the sensory receptors on the skin surface) and spatiotopic (position of the body parts in external space) reference frames. This was mostly evidenced by means of temporal order judgment (TOJ) tasks in which participants discriminate the temporal order of two tactile stimuli, one applied on each hand. Because crossing the hands generates a conflict between anatomical and spatial responses, TOJ performance is decreased in such posture, except for congenitally blind people, suggesting a role of visual experience in somatosensory perception. In previous TOJ studies, stimuli were generally presented using the method of constant stimuli-that is, the repetition of a predefined sample of stimulus-onset asynchronies (SOA) separating the two stimuli. This method has the disadvantage that a large number of trials is needed to obtain reliable data when aiming at dissociating performances of groups characterized by different cognitive abilities. Indeed, each SOA among a large variety of different SOAs should be presented the same number of times irrespective of the participant's performance. This study aimed to replicate previous tactile TOJ data in sighted and blind participants with the adaptive psi method in order to validate a novel method that adapts the presented SOA according to the participant's performance. This allows to precisely estimate the temporal sensitivity of each participant while the presented stimuli are adapted to the participant's individual discrimination threshold. We successfully replicated previous findings in both sighted and blind participants, corroborating previous data using a more suitable psychophysical tool.

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

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.

Testing the exteroceptive function of nociception: The role of visual experience in shaping the spatial representations of nociceptive inputs.

Adequately localizing pain is crucial to protect the body against physical damage and react to the stimulus in external space having caused such damage. Accordingly, it is hypothesized that nociceptive inputs are remapped from a somatotopic reference frame, representing the skin surface, towards a spatiotopic frame, representing the body parts in external space. This ability is thought to be developed and shaped by early visual experience. To test this hypothesis, normally sighted and early blind participants performed temporal order judgment tasks during which they judged which of two nociceptive stimuli applied on each hand's dorsum was perceived as first delivered. Crucially, tasks were performed with the hands either in an uncrossed posture or crossed over body midline. While early blinds were not affected by the posture, performances of the normally sighted participants decreased in the crossed condition relative to the uncrossed condition. This indicates that nociceptive stimuli were automatically remapped into a spatiotopic representation that interfered with somatotopy in normally sighted individuals, whereas early blinds seemed to mostly rely on a somatotopic representation to localize nociceptive inputs. Accordingly, the plasticity of the nociceptive system would not purely depend on bodily experiences but also on crossmodal interactions between nociception and vision during early sensory experience.

The influence of visual experience and cognitive goals on the spatial representations of nociceptive stimuli.

Localizing pain is crucial because it allows for detecting which part of the body is being hurt and identifying in its surrounding which stimulus is producing the damage. Nociceptive inputs should therefore be mapped according to somatotopic ("which limb is stimulated?") and spatiotopic representations ("where is the stimulated limb?"). Because the body posture constantly changes, the brain has to realign the different spatial representations, for instance when the arms are crossed with the left hand in the right space and vice versa, to adequately guide actions towards the threatening object. Such ability is thought to be dependent on past sensory experience and contextual factors. We compared performances of early blind and normally sighted participants during temporal order judgement tasks. Two nociceptive stimuli were applied, one on each hand, with the hands either uncrossed or crossed. Participants reported which stimulus they perceived as first presented, according to either its location on the body or the position of the stimulated hand, respectively, prioritizing anatomy or external space as task-relevant reference frame. Relative to the uncrossed posture, sighted participants' performances were decreased when the hands were crossed, whatever the instruction be. Early blind participants' performances were affected by crossing the hands during spatial instruction, but not during anatomical instruction. These results indicate that nociceptive stimuli are automatically coded according to both somatotopic and spatiotopic representations, but the integration of the different spatial reference frames depends on early visual experience and ongoing cognitive goals, illustrating the plasticity and the flexibility of the nociceptive system.

Investigating peri-limb interaction between nociception and vision using spatial depth.

In order to adapt behavior to a potentially body damaging threat, it is crucial to coordinate the perception of the location of the threat in external space and that of the location of the potential damage on the body surface. Such ability presupposes interactions between nociceptive stimuli and visual stimuli occurring close to the body. We hypothesized that these interactions would rely on multisensory representations of each limb itself, extending its boundaries slightly into external space, with less influence of the global representation of the body as a whole. In most studies investigating such representations, spatial organization of the somatic and extra-somatic stimuli is made according to an egocentric frame of reference centered on the main axes of the participant's whole body (e.g. the trunk), dissociating the left vs. the right side of space. Here, the contribution of such a body-centered frame was minimized, by placing participants' hands in front of them along their anteroposterior body axis, one at a proximal, the other at a distal location. They performed a temporal order judgment task on pairs of visual stimuli, one delivered close to each hand. Visual stimuli were preceded by nociceptive stimuli applied either on one hand, or on both hands simultaneously (control condition). Results showed that, as compared to the control condition, participants' judgements were biased in favor of the visual stimuli the closest to the stimulated hand, irrespective of their distance from the trunk. This finding supports the idea that the mechanisms underlying nociceptive-visual interactions are based on spatial representations that primarily use the stimulated limb rather than the whole body as a coordinate system.

Using temporal order judgments to investigate attention bias toward pain and threat-related information. Methodological and theoretical issues.

Recently, Vanden Bulcke, Crombez, Durnez, and Van Damme (2015) investigated whether the attentional prioritization of a specific location due to the anticipation of pain is modality specific or multisensory. They used a temporal order judgment task in which participants judged the order of either two tactile or two visual stimuli, one presented on each hand. Additionally, participants either expected the occurrence of a painful stimulus on one hand or the absence of any pain. Results showed that participants' judgments were biased to the advantage of the stimuli, tactile or visual, presented at the location where pain was expected. The authors concluded that the anticipation of pain leads to a multisensory prioritization of information presented at the threatened spatial location. Here, we would like to question their conclusion in terms of a genuine attentional modulation of multisensory nature, based on methodological and theoretical grounds.

Spatial and numerical processing in children with non-verbal learning disabilities.

Consistently with the idea that numbers and space interact with each other, the present paper aimed to investigate the impact of non-verbal learning disabilities (NVLD) on spatial and numerical processing. In order to do so, 15 NVLD and 15 control children were required to perform different spatial (the line bisection and Simon tasks) and numerical tasks (the number bisection, number-to-position and numerical comparison tasks). In every task, NVLD children presented lower accuracy scores than the control group. While both groups manifested similar pseudo-neglect and Simon effects, they however differed in the numerical comparison task: while control children presented the standard SNARC effect in the uncrossed and crossed postures, no SNARC effect was observed in the NVLD group. Our results therefore suggest that NVLD affects the accuracy and the nature of the mental number line by decreasing its precision and the saliency of its left-to-right orientation.