Nader Rajaei

Brain networks underlying conscious tactile perception of textures as revealed using the velvet hand illusion

Humans are adept at perceiving textures through touch. Previous neuroimaging studies have identified a distributed network of brain regions involved in the tactile perception of texture. However, it remains unclear how nodes in this network contribute to the tactile awareness of texture. To examine the hypothesis that such awareness involves the interaction of the primary somatosensory cortex with higher order cortices, we conducted a functional magnetic resonance imaging (fMRI) study utilizing the velvet hand illusion, in which an illusory velvet‐like surface is perceived between the hands. Healthy participants were subjected to a strong illusion, a weak illusion, and tactile perception of real velvet. The strong illusion induced greater activation in the primary somatosensory cortex (S1) than the weak illusion, and increases in such activation were positively correlated with the strength of the illusion. Furthermore, both actual and illusory perception of velvet induced common activation in S1. Psychophysiological interaction (PPI) analysis revealed that the strength of the illusion modulated the functional connectivity of S1 with each of the following regions: the parietal operculum, superior parietal lobule, precentral gyrus, insula, and cerebellum. The present results indicate that S1 is associated with the conscious tactile perception of textures, which may be achieved via interactions with higher order somatosensory areas.

Using psychophysical experiment to investigate Velvet Hand Illusion (VHI) characteristics for new tactile display

Tactile illusions as a practical method in virtual reality allow us to induce a real feeling of an objects properties such as shape or texture although the object not physically exists. The moving wires between two hands induce a velvet-like sensation to hands called Velvet Hand Illusion (VHI). We have proposed that VHI can be used in a tactile display in virtual reality. In order to examine this, first, it was mandatory to be revealed physical parameters which might be the effect on the strength of VHI. We investigated effects of distance (D), stroke (r) and velocity (V) of wires on the intensity of VHI by conducting the psychophysical experiments. In accordance with the experimental results, we could propose a psychophysiological model based on the mechanoreceptors afferent units for VHI including type one of slow and fast adaptive mechanoreceptive units (SAI and FAI). In the end, we examined quality and intensity of VHI to be produced by a tactile display in compared with VHI-based two wires. This result indicated that VHI can produce by tactile display so that a person to interpret velvet-like texture in his/her hand.

An investigation of tactile illusion toward tactile displays using psychophysical experiments

In this paper, a basic study for breakthrough in tactile presentation technology is described to progress tactile displays using velvet hand illusion (VHI). In the VHI, a person rubs his/her hands together on either side of wires strung through a frame, producing the sensation of rubbing a very smooth and soft surface like velvet. Since in VHI human subjects feel the surface like velvet that does not actually exist, we might develop a new tactile display capable of generating feeling of material based on the mechanism of VHI. The objective of this study is to elucidate this mechanism. We investigate the VHI using frames equipped with two parallel wires and a series of psychophysical experiments. According to the experiments using Thurstones Paired Comparison, the strength of VHI depends on the wire distance and movement stroke, and VHI caused by passive touch is considerably stronger than that caused by active touch. This result suggests that VHI is controlled by mechanical external stimulation using tactile displays. Furthermore, we found that the strongest VHI is obtained at movement stroke r being approximately equal to wire distance D. According to the abovementioned results, the mechanism of VHI is assumed as follows: although the area bounded by two wires moves relative to the hands, tangential force does not occur on the hand surface except for wire-passing portion, causing operators to experience the illusion of touching a smooth virtual film with a zero coefficient of friction. Since VHI becomes weaker with a small distance between two adjacent wires or a large movement stroke, excessive tangential stimulation prevents the occurrence of VHI.

Investigation on influence of tangential stimulation on velvet hand illusion using psychophysical experiment

In the velvet hand illusion (VHI), a person rubs his/her hands together on either side of wires strung through a frame, producing the sensation of rubbing a very smooth and soft surface like velvet. We focus on VHI to determine the specifications of an actuator for a tactile display enhanced by VHI. We investigate the relationship between wire distance and intensity of illusionary sensation using a series of psychophysical experiments under active and passive touch. According to the experiments using Thurstones Paired Comparison, the strength of VHI depends on the distance between two adjacent wires, and VHI caused by passive touch is considerably stronger than that caused by active touch. This result suggests that VHI is controlled by mechanical external stimulation using tactile displays. We think the mechanism of VHI is as follows: although the area bounded by two wires moves relative to the hands, tangential force does not occur on the hand surface except for wire-passing portion, causing operators to experience the illusion of touching a smooth virtual film with a zero coefficient of friction. Since VHI becomes weaker with a small distance between two adjacent wires, excessive tangential stimulation prevents the occurrence of VHI. Therefore, VHI control requires two actuations: one is for normal actuation on the operators finger surface to generate a touch feeling; the other is appropriate tangential actuation to generate a moving-edge feeling on the operators finger surface.