Introducing a New Haptic Illusion to Increase the Perceived

Resolution of Tactile Displays

Rebekka Hoffmann

1,2

, Manje A. B. Brinkhuis

, Árni Kristjánsson

and Runar Unnthorsson

Faculty of Psychology, University of Iceland, Sæmundargata 2, Reykjavik, Iceland

Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, University of Iceland,

Sæmundargata 2, Reykjavik, Iceland

Keywords: Haptic Illusion, Error of Localization, Tactile Spatial Acuity, Vibrotactile Resolution, Tactile Displays.

Abstract: Tactile high-resolution displays gained importance during the last decade due to their wide range of

application areas. To maximize the throughput of information developers can be tempted to mount as many

tactile actuators (tactors) as possible on a haptic device, thereby risking to overexert the user’s sense of

touch, and to critically decrease its usability. Studies therefore explore ways of increasing the perceived

resolution of tactile displays by exploiting haptic illusions. We demonstrate a new spatiotemporal haptic

illusion that has not been described in literature yet. We conducted an experiment, in which we manipulated

the vibration intensity of two successive tactor activations, the direction of consecutive tactor activations

(up, down) and inter-tactor distance (40, 20, or zero mm). Fourteen naive participants judged whether the

second tactor activation was above or below the first activation. Our results suggest that varying the

sequence of activations with different intensities leads to an error of localization. High intensity activations

followed by low intensity activations resulted in an illusory downward movement, and vice versa. The

haptic intensity-movement illusion provides a promising possibility to enhance the information conveyed in

tactile displays, without increasing the tactor density at the cost of the product’s usability, comfort and

ergonomy.

1 INTRODUCTION

Tactile displays using vibrating motors (tactors)

accompanied by effective haptic languages, are

particularly powerful tools providing a great variety

of application areas. Tactile high-resolution displays

are an especially auspicious choice as hardware

components for Sensory Substitution devices

(SSDs), non-invasive human-machine interfaces that

draw on the central nervous system by bypassing the

impaired peripheral components (Kristjánsson et al.,

2016; Reich et al., 2012). It is an exciting possibility

to augment the sensory experiences of people, who

have deficiencies in one sensory dimension, by

conveying the missing information through an intact

sense, like touch (Sorgini et al., 2018). For the 253

million people living with vision impairment (WHO,

2018), transforming visual into haptic information

may constitute a vital alternative to costly invasive

technologies (Striem-Amit et al., 2012), and research

in the field of tactile displays to support visually

impaired in orientation and mobility has gained

impetus (Bach-y-Rita and Kercel, 2003; Cosgun et

al., 2014; Kristjánsson et al., 2016). Within the

Sound of Vision project, a multisensory SSD was

developed for the blind that provides continuous,

real-time haptic representations of the environment

by means of a high-resolution tactile display

(Hoffmann et al., 2018). Additionally to serving as

visual aids, the development of devices transforming

auditory information into tactile information aims to

improve the life quality of hearing impaired by

extracting features of speech in a sound-to-touch

system (Novich and Eagleman, 2015), or enhancing

experience of music perception (Jack et al., 2015;

Nanayakkara et al., 2013). Moreover, beyond the use

of tactile displays as SSDs, there are plenty of

further areas of applications. For instance,

vibrotactile arrays can support people with balance

impairments by providing vibratory feedback (Wall

and Weinberg, 2003), they can enhance the

immersive experience in virtual environments for

entertainment, or professional training (Faroque et

al., 2015), and provide crucial means of

Hoffmann R., Brinkhuis M., KristjÃ ˛ansson Ã ˛A. and Unnthorsson R.

Introducing a New Haptic Illusion to Increase the Perceived Resolution of Tactile Displays.

DOI: 10.5220/0006899700450053

In Proceedings of the 2nd International Conference on Computer-Human Interaction Research and Applications (CHIRA 2018), pages 45-53

ISBN: 978-989-758-328-5

communication and guidance for firemen, police and

rescue teams in difficult environments, e.g. when

navigating in dense smoke (Carton and Dunne,

2013).

Since, in all application contexts, highly complex

information must be conveyed, a key consideration

in the development of tactile displays is to find ways

of maximizing the throughput of information.

Regarding SSDs, the bypassed non-functional sense

usually has a much higher resolution than the sense

of touch: In order to convey speech information,

bandwidths between 64 and 110 bits per second are

required - a range of bandwidths that is not

supported by the skin (Novich and Eagleman, 2015).

Furthermore, music consists of highly complex

compositions of auditory elements like rhythm,

timbre and harmony, and when trying to transform

these elements to tactile information, the original

auditory information may extend beyond the tactile

spectrum (Karam et al., 2009). Whereas audible

vibrations span frequencies from 20 Hz up to 20

kHz, our tactile system can only detect vibrations

ranging in frequency from 10 Hz and 1000 Hz

(Nanayakkara et al., 2013). To convey the full visual

experience would rely, similarly, on being able to

convey information at a high resolution to encode

the characteristics of a visual object (e.g. depth,

brightness, colour, distance, direction, quantity, size

and elevation), through tactile displays. Attempts to

mimic the functioning of the human visual system

through tactile stimulation, would be thwarted by the

skin’s limited processing capacity.

In the attempt to convey as much information as

possible, developers may be tempted to mount as

many tactors as possible on a tactile device, which

may overexert their user’s sense of touch. Therefore,

an important body of research focusses on

determining vibrotactile spatial acuity, meaning the

minimal possible spacing between tactors on a given

body site before their loci become indistinguishable

(Jóhannesson et al., 2017; Jones and Sofia, 2012;

Van Erp, 2005). However, to simply mount the

highest density of tactors on a device, even if

correctly considering the tactile spatial acuity, still

involves practical drawbacks from a usability point

of view. If users are to be expected to use a

particular device, a plethora of requirements need to

be considered, e.g. portability, comfort, accessibility

of interface, and appearance (Kristjánsson et al.,

2016). Every additional tactor entails the use of

more cables, bigger control units, increased weight,

size and bulkiness of the device, and increased heat

and noise emissions, as well as increased production

costs. These implications critically decrease

practical and ergonomic functioning (Dakopoulos

and Bourbakis, 2010) of the device, leading to a

trade-off between accuracy and usability, varying

the number of tactors.

Another important body of research therefore

explores innovative ways of increasing the amount

of information that can be conveyed through

vibrotactile signals, e.g. by exploiting haptic

illusions. In the past decade, the rate of discovery of

new tactile and haptic illusions has increased

greatly, indicating growing interest in the subject

(Hayward, 2015; Lederman and Jones, 2011). Haptic

illusions arise when the interpretation of an object

through the sense of touch does not correspond to

the physical stimulus. Beyond well-known tactile

illusions with optical geometrical counterparts

(Gentaz and Hatwell, 2004; Ziat et al., 2014), tactile

perception is subject to characteristic spatiotemporal

illusions related to the underestimation of inter-

stimulus distance and overestimation of inter-

stimulus time, e.g. the tau and kappa effect, or

apparent movement illusion (Carter et al. 2008;

Lechelt and Borchert, 1977). Once understood,

especially the spatiotemporal illusions that involve

an error of localization, like the funneling illusion

and the sensory saltation illusion, could be exploited

to increase the perceived resolution of tactile

displays. The funneling illusion (Gardner and

Spencer, 1972) occurs when two adjacently located

vibratory stimuli are presented simultaneously and,

instead of being perceived separately, the associated

sensation is perceived to originate from between the

two tactors. Barghout and colleagues (2009) applied

multiple “funneling” stimuli on the forearm and

were able to create a continuous touch sensation by

manipulating the tactor intensities. They thereby

established an additional information channel and

increased the spatial resolution of vibrotactile

perception. Further, Cholewiak and Collins (2000)

explored the sensory saltation illusion (also called

the cutaneous rabbit, Geldard and Sherrick, 1972),

which is evoked by tapping two or more separate

regions of the skin in rapid succession. Tapping, for

instance, wrist and elbow creates a sensation of

sequential taps hopping up the arm from wrist to

elbow, although there is no physical stimulus

between the two actual stimulus locations.

Cholewiak and Collins (2000) placed a row of seven

tactors at three body sites and activated them

sequentially to draw a line on the skin, comparing

two presentation modes: veridical, where each tactor

was activated, and saltatory mode, where only the

first, fourth, and seventh tactor were activated. They

found that that the saltatory mode produced

CHIRA 2018 - 2nd International Conference on Computer-Human Interaction Research and Applications

equivalent sensations to the veridical mode, and that

both resulted in clearly perceived lines at each body

site. These results promote the possibility of

omitting tactors by exploiting haptic illusions like

sensory saltation.

In the current paper, we introduce a new haptic

illusion, for which manipulation of vibratory

intensity (frequency, acceleration) of a sequence of

vibrotactile stimuli leads to an error of location. The

illusion was discovered during the Sound of Vision

project (Hoffmann et al., 2018), and, to the best of

our knowledge, has not been described in the

literature yet. When a vibratory stimulus of high

intensity is followed by a second vibratory stimulus

of lower intensity at the same location, the location

of the 2

stimulus is erroneously perceived to be

below the 1

stimulation location (in vertical plane).

The same seems to apply vice versa, with a more

intense 2

vibratory stimulation being erroneously

perceived to be located above the first stimulation.

Importantly, the described haptic intensity-

movement illusion could be exploited to increase the

perceived resolution of haptic products. This is a

promising option to increase the amount of

information that can be conveyed in high-resolution

tactile displays, instead of increasing the density of

tactors and thereby lowering the product’s usability,

comfortability and ergonomy. In what follows we

describe an experiment that we conducted to

demonstrate illusory movement caused by vibratory

intensity-changes, by assessing the participant’s

responses while systematically manipulating the

intensity of vibratory tactor stimulations.

2 METHOD

2.1 Participants

Fourteen students at the University of Iceland (7

females, 21-28 years, M = 23.79, SD = 1.89),

participated. They were naive about the purpose of

the study, and gave written informed consent before

the experiment started. The experiments were

approved by the appropriate ethics committee and

conformed to the Declaration of Helsinki.

2.2 Apparatus

An array of 4 x 4 tactors was glued to a 15 cm thick

layer of foam, as shown in Figure 1, whereby the

central part of the device consisted of 4 additional

cm of foam to ensure good fit in the spine area. The

foam was mounted on a plastic frame containing the

electronics board, battery and charger circuit, and

was equipped with straps to be fastened around the

participant’s waist, with the tactor array placed on

the lower back. We created custom software, written

in Python, and making use of the Psychopy library

(Pierce, 2009) for stimulus presentation. The tactors

were Eccentric Rotating Mass (ERM) motors,

covered in a plastic shell (diameter: 9 mm, length:

25 mm, Precision Microdrives, 2018; model #307-

103), and were controlled with a simple Darlington

driver. Based on previous work determining

vibrotactile spatial acuity (Jóhannesson et al., 2017),

the ERMs were placed as close as physically

possible, at an inter-tactor distance of 20 mm,

measured from center-to-center (c/c). The tactors

were run at two intensities, either at high intensity (4

V), with 230 Hz vibration frequency (100 mA, 7 G),

or at low intensity (1 V), with 90 Hz vibration

frequency (30 mA, 1G).

Figure 1: Array of 4 x 4 Eccentric Rotating Mass motors

mounted 20 mm (centre-to-centre) apart from each other

on a frame with foam, to be placed centrally at the

participant’s lower backs.

2.3 Stimulus

Each trial consisted of a pair of successive tactor

activations. The tactors were activated along the

vertical axis, meaning that the 2

tactor activation

within each trial was either above, below or the at

the same location as the 1

tactor activation. Within

each trial, the intensity of tactor activation was

systematically varied in two conditions, with either

the 1

activation being strong (230 Hz, 7 G) and the

being weak (90 Hz, 1G), aiming to induce a

perceived shift of localization downwards, or vice

versa, with the 1

activation being weak and the 2

being strong, aiming to induce a perceived shift of

localization upwards. Additionally, the inter-tactor

distance was systematically varied across trials, in

Introducing a New Haptic Illusion to Increase the Perceived Resolution of Tactile Displays

five conditions: The 2

tactor activation was either

40 mm (c/c) below the 1

(with one inactive tactor

in between) or the 2

tactor activation was 20 mm

(c/c) below the 1

(adjacent tactors). The same

approach was applied for the upwards direction,

with 40 and 20 mm (c/c) inter-tactor distance.

Finally, the fifth condition consisted of the same

tactor being activated twice. On each trial (in all

conditions), the tactors were turned on for 200 ms

with an inter-stimulus interval of 50 ms, and an

inter-trial interval of 1500 ms.

Overall, the experiment consisted of 400 trials,

and every condition was assessed by 40 trials in

random order. The experiment was divided into four

blocks of 100 trials each, whereby the location of the

tactor, the distance to the 2

tactor, the direction

of intensity-change, and whether the 2

tactor was

above, below or the same as the 1

tactor, were

randomized and equally balanced within one block.

2.4 Procedure

Participants were requested to wear a sheer shirt to

avoid the vibrations of the tactors being absorbed by

thick fabric. After signing the informed consent,

participants took seat in front of a computer screen

in a quiet room. They were outfitted with

headphones and the tactor array, which was placed

centrally in the participant’s lower back.

Subsequently, participants received task instructions.

Importantly, all participants were naive about the

full set of inter-tactor-distance conditions and not

informed about the possibility that the same tactor

might be activated twice. In a two-alternative forced

choice task (2AFC), participants were asked to judge

whether they perceived the 2

tactor activation to be

located above or below the 1

activation by pressing

the up and down arrow keys on a standard keyboard,

respectively. Further, participants were encouraged

to close their eyes to avoid visual distraction, and

they wore headphones playing white noise to mask

the sound of the tactors to avoid that it would serve

as an auditory cue. After a short training session of

100 trials, the experiment started and took about 15

minutes to complete. There was a break after each

completed block, where participants could decide

individually when to proceed. All in all, the

procedure took about half an hour.

2.5 Statistical Analysis

In order to demonstrate the proposed haptic illusion

of an apparent shift of location induced by an

intensity-change of tactor activation (downwards in

case of strong-to-weak, and upwards in case of

weak-to-strong), the trials in which the same tactor

was activated twice without the participants being

aware of it, were of particular interest for the

analysis. If the responses were independent of the

intensity-change, in both conditions, the

participant’s responses would be random and the

proportion of up/down responses would therefore be

equally distributed at chance level (0.5). If the

intensity-change, however, induced an apparent

movement, the probability of participants

responding with up in the strong-weak condition

may be attenuated and, likewise, the probability of

responding up in the weak-strong condition may

increase. Furthermore, the effect of illusionary

movement caused by the intensity-change may be

most pronounced when the inter-tactor distance is

zero, whereas increasing inter-tactor distance could

increase discriminability when judging the direction

of activations, minimizing the illusory effect.

One sample, one sided t-tests were conducted in

R (R Project for statistical computing) to assess if

the participant’s responses for the same tactor

activation differed significantly from chance level

(0.5). Further, a repeated measures ANOVA was

conducted to assess the main overall effects of inter-

tactor distance, presentation direction, intensity-

change direction (strong-to-weak, weak-to-strong),

as well as possible interactions. Generalized eta

squared statistics (η

) were calculated with R

ezANOVA function and are reported as

recommended for within-subject designs (Olejnik

and Algina, 2003). Pairwise post-hoc comparisons

were conducted (Tukey's HSD adjusted) to assess

whether the two conditions of intensity-change

(strong-to-weak vs. weak-to-strong) differed for the

same tactor activation, the close inter-tactor

distances (20 mm c/c) as well as for the far inter-

tactor distances (40 mm c/c), for up and down

presentation direction, respectively.

3 RESULTS

Figure 2 reports the ratio of up-responses as a

function of intensity-change condition, and distance

between the two stimuli. While Panel A shows the

two distance conditions, where the 2

tactor

activation was below the first, Panel B shows the

condition, in which the same tactor was activated

twice, and Panel C plots the two distance conditions,

in which the 2

activation was above the first.

CHIRA 2018 - 2nd International Conference on Computer-Human Interaction Research and Applications

Figure 2: Panel A shows the ratio of up-responses as a function of intensity-change, and inter-tactor distance between the

two successively activated tactors for the condition, in which the second tactor activation was below the first. Panel B

shows the ratio of up-responses for the condition, in which the same tactor was activated twice, and Panel C shows the same

for the two distance conditions, in which the second tactor activation was above the 1st. Error bars represent the standard

error of the mean (SEM).

Figure 2 indicates that the ratio of up-responses

within each presentation condition is substantially

influenced by the sequence of intensity changes of

the vibratory stimuli, which was confirmed by the

statistical analysis. Besides the significant main

effect of presentation direction and distance (F(4,52)

= 100.10, p < .001, η

= .72), we found a highly

significant main effect of intensity-change direction

on the up-response ratio with a large effect size

(F(1,13) = 13.35, p < .003, η

= .31).

In detail, for the trials in which the same tactor

was activated twice, participants did not respond

randomly but were influenced by the intensity-

change direction. If the first stimulation was of high

vibratory intensity followed by a low intensity

stimulation, participants appeared to perceive the 2

stimulus to be below the first and responded up

significantly below chance level (t(13) = -3.95, p <

.001). Likewise, in case the first stimulus was of low

intensity followed by a high intensity stimulus,

participants tended to erroneously perceive the 2

stimulus above the first, and responded up

significantly above chance level (t(13) = 2.81, p =

.007).

Furthermore, there was a significant interaction

between the two main effects of presentation

direction/distance and the intensity-change direction

(F(4, 52) = 8.26, p < .001, η

= .05), meaning that

varying the direction of intensity-change within each

stimulus pair affected the ratio of up-responses

differently, depending on the presentation direction

and distance. The pairwise comparisons indicate that

the influence of intensity-change was strongest when

the same tactor was activated twice, with a

difference in the up-response ratios of -0.34 (95%

CI: -.55 to -.12) between the strong-to-weak and

weak-to-strong condition, which was significant (p <

.001). The influence of intensity-change was lower

but significant when the 2

tactor activation was

close (20 mm c/c) to the 1

, for both up (-0.27, 95%

CI: -.48 to .06, p = .003) and down (-0.24, 95% CI: -

.46 to -.03, p = .016) presentation directions. At the

furthest inter-tactor distance of 40 mm (c/c), the

influence of intensity-change on the up-response

ratio was, even though still noticeable as a trend, not

significant anymore, neither for the up (-0.15, 95%:

CI: -.37 - .07, p = .44) nor for the down (-0.13, 95%

CI: -.35 - .08, p = .56) presentation directions.

Introducing a New Haptic Illusion to Increase the Perceived Resolution of Tactile Displays

4 DISCUSSION

To demonstrate a spatiotemporal haptic illusion that

could be exploited for increasing the perceived

spatial acuity of tactile displays, we systematically

manipulated the sequence of vibration intensity of

two successive tactor activations. Our results

indicate that varying the sequence of stimuli with

different vibratory intensities (frequency and

acceleration) causes an error of localization. When

the same tactor was activated twice without the

participants being aware of it, they did not respond

randomly. Our results show that when a vibratory

stimulus of high intensity was followed by a

stimulus of low intensity, participants appeared to

perceive the 2

vibration to be located below the

first, even though there was no actual change of

location. Further, our results suggest the same effect

for the opposite direction: presenting a vibratory

stimulus of low intensity followed by high intensity

at the same location provokes a perceived upwards

movement. The effect of illusionary movement was

still apparent in ambivalent situations, when two

adjacent tactors of close inter-tactor distance were

successively activated (20 mm c/c), but decreased

with increasing inter-tactor distance (40 mm c/c).

Even though, to the best of our knowledge, this

haptic illusion has not been described in the

literature yet, there is a body of research on

multisensory correspondences that provides a

theoretical foundation for explaining possible

underlying mechanisms. In general, cross-modal

correspondences refer to universally experienced

associations between apparently haphazard stimuli

across different senses (Spence, 2011). In the

intensity-movement illusion, high vibratory

frequency (and acceleration) in tactile stimulation

appears to be associated with elevation, whereby

low vibratory frequency (and acceleration) appear to

be associated with being located below.

Interestingly, there is a commonly studied and

robust audio-visual cross-modal correspondence

between high and low pitch (auditory frequencies) as

being associated with high and low visual

elevations, respectively (e.g. Jamal et al., 2017).

Additionally, a large amount of research suggests

similarity correspondences between audio

frequencies (high pitch vs. low pitch) and the visual

features of size, colour, brightness and form (e.g.

Gallace and Spence, 2006; Melara, 1989). Further,

the occurrence of synesthetic visuo-haptic

interactions have been documented, with

participants preferentially matching black and white

squares with low-frequency and high-frequency

vibrotactile stimuli, respectively (Martino and

Marks, 2000). Besides these intensely studied vision

based correspondences, research further suggests a

strong cross-modal audio-haptic connection (Nava et

al., 2016; Wilson et al., 2010). It is therefore not

surprising that Occelli et al. (2009) demonstrated a

multisensory correspondence between the pitch of a

tone and the elevation of tactually stimulated

locations. Summing up, these cross-modal

associations found for all senses (visuo-auditory,

visuo-haptic, audio-haptic) linking high frequencies

(of haptic vibration or auditory pitch) to elevation

(spatially on skin, or visually) and opposite, could be

the underlying mechanism for the described haptic

illusion of perceiving an illusory downward

movement when vibratory frequency changes from

high to low (and vice versa). In general, such haptic

illusions are worth studying, since they provide a

powerful tool to gain insight into the limits of haptic

perception (Lederman and Jones, 2011). Tactile

representation of our physical environment relies on

the acuity of the tactile sensory system. Due to the

low receptor density, tactile perception is prone to

spatial imprecision (Jóhannesson et al., 2017), and

the tactile sensory system builds on prior knowledge

and multisensory correspondences to enhance

perceptual resolution beyond the limits set by

sensorineural imprecision (Adams et al., 2004). Such

correspondences usually occur between stimulus

properties that are correlated in nature, and therefore

serve to increase the efficiency of information

processing and support the integration of sensory

data into meaningful representations (Spence, 2011).

Relying on these heuristics, however, comes at the

cost that rare physical events violating the

expectation, as artificially recreated in our

experiment, are misperceived.

Besides serving as a tool for studying cognitive

processes, the haptic intensity-movement illusion

could be applied practically to increase the perceived

resolution of haptic products, as has been shown for

other haptic illusions (Barghout et al., 2009;

Cholewiak and Collins, 2000). This is a promising

option to increase the amount of information that

can be conveyed in high-resolution tactile displays,

while avoiding the trade-off of lowering the

product’s usability, comfort and ergonomy by

increasing the density of tactors. Furthermore, to

exploit tactile illusions may yield benefits beyond

usability related aspects, by possibly bypassing the

anatomical and morphological changes of the skin in

old age. This aspect is especially relevant since the

majority of the target group for SSDs, a key

application area of high-resolution haptic displays, is

CHIRA 2018 - 2nd International Conference on Computer-Human Interaction Research and Applications

of older age. According to the WHO (2018), for

instance, 81% of people who suffer from vision

impairment are older than 50 years. It has been

shown that spatial tactile acuity substantially

decreases with age (Stevens and Patterson, 1995),

resulting in less accurate tactile information

conveyance when using tactile devices. Although a

causal link between impaired tactile acuity in old

age and receptor loss remains controversial (Dinse et

al., 2006), a lower density of mechanoreceptors in

the skin (Bruce, 1980), and slower conduction

velocities of peripheral nerves (Peters, 2002) have

been documented. The intensity-movement illusion

demonstrated in this study might help circumventing

these morphological limitations by increasing

perceived tactile spatial acuity. Moreover, while

haptic illusions can be affected by aging, as shown

by Ballesteros et al. (2012), the current perceptual

effects may rely on neurological mechanisms that

are unrelated to morphological limitations. However,

since the result of our study bases on a sample of

adolescents, with ages ranging from 21 to 28 (M =

23.79), we are cautious to generalize to other ages.

Further studies should therefore explore the effect of

age on the intensity-movement illusion, to

investigate its applicability in SSDs for different age

groups.

It is important to note that the described

intensity-movement illusion needs further empirical

exploration in order to investigate how robustly it

can be replicated across various conditions and

experimental setups, and which factors act in a

facilitating or inhibiting way.

Since the current results are limited to the back

area, future studies should examine if the haptic

illusion extends to other body sites, like abdomen,

arms, legs, or face. It is of particular interest to

explore how the direction of “upward/downward” in

the vertical plane relates to other body parts that

require a more specific definition of direction (e.g.

arms and legs), as the understanding of up or down

depends on their position. Specifically, following the

neurologic classification system for body directions

(proximal vs. distal, medial vs. lateral), “upwards”

relates to “proximal” (towards the brain, e.g. from

wrist towards elbow), and “downward” relates to

“distal” (away from the brain). Such an approach

may yield valuable information on how the haptic

illusion relates to body position.

Finally, future studies should further assess the

effects of using different tactor types, and

spatiotemporal parameters, like inter-stimulus-

interval, stimulus duration and inter-stimulus

distance, on the occurrence of the illusion. We will

conduct follow up experiments to specify the

frequency range in which the illusion can be reliably

demonstrated, and at which frequency differences

the illusion is most likely to occur.

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