Hi everyone and welcome to my CHI presentation on the vibrotactile funneling illusion and localization performance on the head.
My name is Oliver Beren Kaul - everyone just calls me Beren -
and together with my Professor Michael Rohs and three undergrad students: Benjamin, Kerem, and Kamillo,
I've been working on this topic for quite a while.
So, if you are unfamilliar with the vibrotactile Funneling Illusion,
it's the phenomena, where if you take two vibrotactile actuators,
and you put them at a certain distance from each other that cannot reach over a certain threshold,
then you will basically just feel one actuation in between of them.
On the forearm for example, this is about 8 cm maximum distance
So if you take two actuators, one here and here,
You'll only feel one central actuation if you turn both of them on at the same time.
So, let me first give you a brief introduction about my previous work.
I've been working on a concept I called HapticHead in my CHI 2016 and 2017 publications,
and it is basically a bathing cap with a chin strap attached to it and a total of 24 vibrotactile actuators.
You can use this concept in a variety of use cases in Virtual and Augmented Reality,
such as virtual snow raining on your head, or to find objects at arbitrary positions in 3D space around you,
or to guide blind people around without any additional tools.
When I first started researching tactile feedback and tactile perception across the human body, I stumbled across a variety of different works.
They were able to show that the tactile perception varies widely across the human body.
For example, on the fingertip, your tactile perception is much more accurate than on the forearm.
Obviously, tactile interface design has to be informed by tactile perception research.
When I initially designed the HapticHead interface, I had a look at a variety of different works
and I did not find that many works in the domain of localization performance or the funneling illusion on the head.
So, while I did actually find some works which covered part, but not all regions of the head in terms of localization performance and the occurrence of the funneling illusion,
these works actually had quite serious study design issues which we are also looking to fix with our current work.
One of the works we are covering in our paper is Diener et al. from 2017.
They introduced VibrationCap, which is quite similar to HapticHead but without the chin strap.
They also researched localization accuarcies on the human head but their user study had two major issues:
One of them was that the distances between actuators on their prototype is not equal.
and the other is that participants used a user interface to enter positions for their user study
and this obviously has quite a large influence on results, especially since the user interface also showed varying distances between actuators.
The other work we are covering is Kerdegari et al. from 2014.
They also researched the centralizing bias and vibrotactile funneling illusion and also the localization performance and midline bias, but only on the forehead.
Now, they did have some serious study design issues.
First, they used a measuring tape ranging from 0 to 17 centimeters with actuators placed from 0 to 15 centimeters
and participants used pens to point to positions in front of a mirror.
So obviously, participants wouldn't point to positions below 0 centimeters or above 15 centimeters, because they knew, there were no actuators there.
In conclusion, they found a significant midline bias on the forehead.
The midline bias is a phenomena where people tend to feel vibrations as closer to the saggital midline of the human body, than they actually are.
We were quite sure that this significant midline bias was caused by their study design and that all other results were also heavily biased by their study design.
So, in order to get a better overview of the localization performance and funneling illusion all around the head,
we built a prototype quite similar to Kerdegari et al. but with a few specific fixes.
First, we used a measuring tape ranging from 6 to 31 centimeters,
With actuators at 11 to 26 centimeters, also spaced 2.5 centimeters apart and with tolerances of less than 1 millimeter.
We then chose a total of six head regions to get a more complete overview of tactile perception on the human head.
On this slide you can see the prototype applied to the six head regions.
Our participants obviously also wore noise cancelling headphones which played white noise at quite a high volume
in order to mask noises caused by the vibration actuators.
Contrary to Kerdegari et al., we actually did not put our participants in front of a mirror to point to locations
but they had to point to locations blindly using their index fingers and small metal pins mounted to them
which should be much more accurate than using pens in this situation.
As we were looking to disprove other studies, we recruited quite a lot of participants for reliable data.
However, only one of our participants was left-handed and we also had one ambidextrous participant,
so we cannot draw reliable conclusions on the left versus right-handed participants.
In total, we conducted 42 trials per head region and per participant.
This included three repetitions of the seven single actuator trials and all 21 possible combinations of two actuators at distances 2.5 to 15 centimeters.
The study was fully counterbalanced and in total we had more than 3000 single actuator and more than 3000 multi actuator trials.
On the lower part of the slide you can actually see the experimenter view where our experimenters used to enter positions for the participants which they uttered verbally.
These sliders can be clicked on and dragged along the measuring tape.
So, let's take a look at the results of the single actuator trials first.
The forehead performed best in terms of localization performance, followed by headTop, chin, headBackBottom, headBackTop and lastly headSides.
Interestingly, Diener et al. found that their headTop region performed worst compared to all other head regions that they investigated.
This was very likely caused by their study design where they placed three actuators on their headTop region, compared to two actuators on the forehead and two actuators on their headBack for example.
Next up is the midline bias evaluation.
Contrary to Kerdegari et al, we actually did not find a significant midline bias for the forehead.
We even found a very slight outward bias to the right side for the forehead but this was very far from being significant at 0.3 millimeters.
As mentioned earlier, this differing result was very likely caused by the questionable prototype and study design by Kerdegari et al.
Furthermore, we also did not find significant midline biases for the headTop, headBackBottom and merged headSides regions
However, we did find quite a significant midline bias for the chin.
We assume that the rather significant midline bias for the chin region was caused by the structure of the jaw bone which actually transmits vibration from one place to the other
and also because the radius - if you imagine the head as a sphere - is smaller for the chin region compared to the other parts of the head.
Now, for the headBackTop region we actually encountered a rather unexpected outward bias, but only on the left side of the head.
We imagine this is because most of our participants were right-handers and when they felt only one location, they were trying to point to the left side of the head with their right hand
and as you can see here, this is quite difficult and also you point a bit flat like this
so you could imagine it like this, pointing like this. So, the actual position of the pin is a bit more in front than the point of vibration that they felt.
We assume that this is only a very localized phenomena which does not appear in any other head region in our data.
So, in order to get a more complete overview of the mean absolute localization error across the entire human head,
we calculated this measure for every single actuator and then merged the data for the symmetric head regions to get this interesting heat map of localization accuracy across the entire human head.
Interestingly, the localization performance is by far best in the center of the forehead and then gradually decreases when coming closer to the ear openings of the human head.
Our participants actually verbally complained about the region close to the ear openings because it was much harder for them to determine where exactly the vibration was coming from.
Furthermore, while the headTop region scored second to best in terms of absolute localization error across all positions, the central position actually scored quite a bit worse than the other positions.
And this repeats itself for the headBackTop position where the center also scored worse than the other positions and this gradually became better towards the sides.
So, let's take a look at the results from the multi actuator trials now. First, occurrence of the funneling illusion.
In these diagrams you can see how many participants pointed to one location or to two locations based on the distance between the actuators.
So, at 2.5 centimeters on the forehead, almost every participant pointed to one location, so the funneling illusion occurred for almost all of them at the distance of 2.5 centimeters.
When we look at larger distances, obviously more participants will point to two locations, the further they are apart.
So the diagram for the forehead can actually be directly compared to a similar diagram in Kerdegari et al.'s work
Where they found similar results except for the 5 centimeter distance, where they found that almost 80 percent of their participants pointed to two locations instead of one location.
We expect this is again due to their different prototype design and also due to small differences in insulation between the actuators.
Overall, you can see that the head regions are quite different in terms of the occurrence of the funneling illusion,
especially the headSides regions had massive differences to the other head regions,
where almost 20 percent thought that only one actuator was active even at 15 centimeters distance between the actuators.
We also found quite interesting results in terms of centralizing bias
And the forehead region can actually be compared again directly to Kerdegari et al.'s work
who in this case found very similar results to our results in the domain of 2.5 to 10 centimeters (they actually didn't check the other distances).
And we can see that the centralizing bias is around 2.5 to 3 centimeters in this region
but the further the distance, the less of a centralizing bias occurs on the forehead.
This is obviously also a result of the funneling illusion occurring less at larger distances, because in case the funneling illusion occured, we assume a distance of zero for the centralizing bias.
So in case of the other head regions, we can see that the centralizing bias for the headTop, chin, and headSides region
actually all feature quite large centralizing biases, especially also in the larger distances.
However, for the headBackTop and headBackBottom areas, we actually see a lot smaller centralizing biases,
especially in case of the headBackBottom, starting at 7.5 centimeters distance, we see almost no centralizing bias at all.
So in conclusion, we found a strongly varying vibration localization performance across the entire human head.
Contrary to Kerdegari et al., we did not find a significant midline bias on the forehead,
but we did find a significant midline bias on the chin.
We also did not find a significant overall dominant hand effect in our data,
but there might still be some local effects such as for the headBackTop region that weird outward bias on the left side of the head.
Furthermore, we found strongly varying funneling illusion and centralizing bias occurrences all across the human head.
Our results inform the design of any kind of tactile interface on the head, including those integrated into Virtual Reality or Augmeted Reality headsets.
For example, you might take an existing interface such as HapticHead and optimize it in a way in terms of actuator count and positioning,
so that more than 50 percent of its users experience the funneling illusion effect.
This in turn might lead to better guidance performance for example.
One of the limitations of our work is that we did not do an investigation on the effect of hair density on localization performance.
While Diener et al. did not find such an effect, we assume that there might still be an effect that they missed and we did not investigate yet.
Furthermore, future work might also decide that the gaps left in the data of our study are still too large and then rebuild our prototype and do an even larger study on localization performance and funneling illusion on the head.
So in conclusion, there are obviously a lot more details in the paper, but for now I will leave you here with the references of my talk
and if you have any further questions or comments, please feel free to contact me on my email address or on social media.
Thanks for your attention and bye bye! :)