Snorri Steinn Stefánsson Thors

When two brief vibrotactile stimulations are sequentially applied to observers’ lower back, there is systematic mislocalization of the stimulation: if the second stimulation is of higher intensity than the first one, observers tend to respond that the second stimulation was above the first one, and vice versa when weak intensity stimulation follows a strong one. This haptic mislocalization effect has been called the intensity order illusion . In the original demonstration of the illusion, frequency and amplitude of the stimulation were inextricably linked so that changes in amplitude also resulted in changes in frequency. It is therefore unknown whether the illusion is caused by changes in frequency, amplitude or both. To test this, we performed a multifactorial experiment, where we used L5 actuators that allow independent manipulation of frequency and amplitude. This approach enabled us to investigate the effects of stimulus amplitude, frequency and location, and to assess any potential interactions among these factors. We report four main findings: (1) we were able to replicate the intensity order illusion with the L5 tactors; (2) the illusion mainly occurred in the upwards direction, or in other words, when strong stimulation following a weaker one occurred above or in the same location as the first stimulation; (3) the illusion did not occur when similar stimulation patterns were applied in the horizontal direction; and (4) the illusion was solely due to changes in amplitude, whereas changes in frequency (100 Hz vs 200 Hz) had no effect.

While vibrotactile stimulation shows promise for sensory substitution devices, a crucial question concerns vibrotactile spatial resolution. We examined the optimum distance between three voice coil actuators (model: lofeltL5) on the forearm. Three actuators were embedded in a fabric-based vibrotactile sleeve where the actuators were placed in enclosures 3D-printed on the fabric. We used the relative point localization method where observers must discriminate whether two successive stimulations are in the same location or not. The resolution was measured for five vibrotactile sleeves, each with different distances between the actuators on the longitudinal axis of the forearm. The various distances were tested in a random order. In experiment one, pairs of stimuli were delivered sequentially in a random order to two adjacent actuators of the tactile sleeve on the upper side of the forearm. The task was to identify the perceived direction of the second stimulation (up, down, or the same) relative to the first one. Experiment two involved the same procedure but for the underside of the forearm. Taking the restrictions of the physical dimensions of the forearm and the design considerations into account, our results suggest that 20 mm is the optimum distance between the voice coil actuators (Model: Lofelt L5) for successful discrimination with high accuracy between the two stimulus locations on the forearm. There were no significant differences between the upper and undersides of the forearm.