Spatially coincident vibrotactile noise improves subthreshold stimulus detection

Abstract

Stochastic Resonance (SR) is a phenomenon, mainly present in nonlinear detection systems, in which the addition of certain amount of noise, called optimal noise, has proven to enhance detection performance of subthreshold stimuli. When added noise is present only during the stimulus, an additional enhancement can be reached. This phenomenon was called time Coincidence Enhanced Stochastic Resonance (CESR). The aim of this study was to study the effect of spatially distributed vibrotactile noise in subthreshold stimuli detection. The correct response rates from two different stimuli conditions were compared, using four tactile stimulator systems to excite four different spatial locations on the fingertip. Under two different conditions, the stimuli were present in only one randomly chosen stimulator. For the first condition, all stimulators contain optimal noise level. In the second condition, the optimal noise was present only at the stimulator with the stimulus. SR threshold principle should not produce different correct response rates between the two conditions, since in both cases the noise enables the subthreshold stimulus to go above threshold. The stimulus signal used was a rectangular displacement controlled pulse that lasted 300ms within a 1.5s attention interval, applied to the exploratory zone of the index finger of 13 human subjects. For all subjects it was found that detection rates were better (p<0.0003) when noise was spatially coincident with the stimulus, compared to the condition in which noise was present simultaneously in all the stimulators. According to our literature review this is the first report of SR being influenced by the spatial location of the noise. These results were not found previously reported, so represent the discovery of a new phenomenon. We call this phenomenon Spatial-Coincidence-Enhanced Stochastic Resonance (SCESR). As results show, the optimal noise level is dependent on the relative position between stimulus and noise.

Fig 1. Photograph that shows a piezoelectric actuator used for mechanical stimulation.

(1) Connection electrodes attached to piezo electric lead terminals, (2) a round ended plastic tip fixed to one end of the piezoelectric stripe. This picture shows only one of the four mechanical stimulators.

Fig 2. Schematic diagram of the mechanical stimulation module.

(1) Plastic tip, (2) piezoelectric stripe actuator, (3) rocker metallic mounting, (4) pivoting axis, (5) counterweight bolt, (6) actuator electrode, (7) upper surface where subjects rest their fingers and hands, (8) round plastic ends in contact with the fingertip, (9) a subject’s index finger. This Fig shows only two of the four mechanical stimulator modules.

Fig 3. Actual four channel mechanical stimulation system.

(1), (2), (3) and (4) mechanical stimulation modules. (5) Plastic clay that aids subjects to place their index finger tips over the stimulators, (6) set of 4 round ended plastic tips that make contact with the index fingertip.

Fig 4. Actual scene from a subject under study.

(1) The hand support, subjects lay their hands on a flat surface so that their index fingers are located directly over the mechanical stimulators, (2) an arm support, a set of foam blocks provides comfortable support during the experiment procedures, (3) headphones through which subjects receive audible cues for the start of the attention interval during which the stimulus may occur.

Fig 5. Two consecutive trials example for single channel stimuli signal used in experiments.

Noise last during all attention interval. Pulse signal has a fixed duration of 300ms. Time to pulse start is randomly selected from trial to trial. A 3s pause between trials with no stimulus is used as response window.

Fig 6. Percentage of correct responses as function of noise level.

Blue diamonds represents P(C) when noise is applied only in the stimulator that contains rectangular pulse signal. Red squares represents P(C) when noise is applied on four stimulators simultaneously.

Fig 7. Actual signals captured under four different stimulus conditions.

(1) C1: pulse in one stimulator; noise in four stimulators, (2) C2: no pulse; noise in four stimulators, (3) C3: pulse in one stimulator; noise only on the stimulator that contains the pulse, (4) C4: no pulse; noise in one stimulator only.

Fig 8. Measured mean values of tactile perception thresholds in micrometers.

Blue bars represent measured thresholds for rectangular pulse stimuli. Red bars represent measured thresholds for noise applied in one stimulator. Green bars represent measured thresholds for noise applied in four stimulators. Error bars represent standard deviations over three different measurements for threshold for each subject on three different days.

Fig 9. Measured mean values of optimal noise level in micrometers for standard deviation of Gaussian noise.

Blue bars represent optimal noise level present only in the same stimulator that contains the rectangular stimuli signal. Red bars represent optimal noise level present in four stimulators simultaneously.

Fig 10. Measured means values for tactile threshold and optimal noise which produce SR, measured as standard deviations.

The blue bars indicate the individual threshold level for each subject expressed in micrometers. The red bars represent corresponding one stimulator noise level that produces SR for each subject; noise is expressed in micrometers of the noise standard deviation. Error bars represent standard deviations over three measurements for threshold.

Fig 11. Percentage of correct responses for each subject.

Blue bars correspond to P(C1)+P(C2) in which noise is present in all four stimulation points. Red bars correspond to P(C3)+P(C4) where noise is present only at the stimulation point that contains a rectangular pulse.

Fig 12. Percentage of correct answers on 4AFC task.

Subjects were asked to identify the position where the stimulus was. Blue bars represent the results for condition C1. Red bars represent the results for condition C2. Error bars represent standard deviation over three measurements.