Individual threshold adjustment for each finger

[kuff]

Even with each pad on each finger being correctly calibrated, there is still a noticeable difference between the degree to which they trigger, even when they are perceived to be grabbing the ball with equal force.

For example, if grabbing the ball with all five fingers, and doing so in a way that seems to be even amongst all fingers, the ring finger responds with a far greater pressure value, and thus a larger amplitude, than the rest.

This is caused by how pressure is being calculated (probably). When the non-physicalized fingers protrude the surface of the sphere, the pressure is deemed to be the distance between the end finger bone and the center of the sphere (with some additional considerations). However, as a bi-product of how one would naturally grab a sphere, even when one perceives their grip force to be uniformly distributed, some fingers will be able to protrude further than others, just by the nature of the structure of the human hand.

To mitigate this, individual pressure thresholds for each finger must be defined. These can probably be hardcoded as "set-and-forget" thresholds that scale with the size of the grabbable object (sphere).

[MartinGarenfeld]

Which type of force modulation are you applying here? Is it spatial, amplitude or a mix? In any case, it is likely better for the protocol that the force levels are increasing/decreasing simultaneously for all fingers, since the task for the subject will be to reach a target force level common for all fingers. If the force levels are modulated independently for each finger depending on how much "force" each virtual finger applies, this task will very likely be way too difficult, as the target is only reached if each finger applies exactly the correct force. I think we need to have some criteria for the force levels, e.g, contact/force level 1 stimulation is activated when two/three fingers are contacting the ball, and force level 2 stimulation is then activated when two/three fingers are applying that "force". Or something like this. Whatever seems more natural in the scene.

[kuff]

That is a very good point. What I'm discussing in this issue is what I've observed during testing with the current amplitude modulation, which for the time being is continuous with no explicit force levels. This will of course change in the final implementation, but in that case we would still need to ensure that each finger is being considered more equally.

For example, if grabbing the ball with all five fingers, and doing so in a way that seems to be even amongst all fingers, the ring finger responds with a far greater pressure value, and thus a larger amplitude, than the rest.

Staying with this example, the ring finger would contribute an outsized amount to the perceived force being applied to the system, and the proposed individual thresholds would remedy this.

Based on the point you raise, maybe we need one variable that is some sort of average of all the pressures applied by each finger, to limit the chance of any confusion as you mention. In your example, that would mean that the different force levels would be regulated by an average force-applied value from x number of fingers. This may possibly be something we can investigate in a small pilot test.

[kuff]

Alternatively to an average value of x number of fingers, we can simply consider the maximum pressure value to be the force being applied, meaning the finger which is protruding the sphere the most. This is the logic currently used to discolor the sphere in shades of red in correspondence to pressure being applied, and, although this feedback is only visual, it seems largely coherent with the pressure one perceives to be applying from my experience.

[MartinGarenfeld]

That is a very good point. What I'm discussing in this issue is what I've observed during testing with the current amplitude modulation, which for the time being is continuous with no explicit force levels. This will of course change in the final implementation, but in that case we would still need to ensure that each finger is being considered more equally.

How the stimulation is perceived in each finger is very subjective. The fact that you sense a stronger sensation in the ring finger during continuous increasing amplitude compared to the other fingers might not be the case for another individual. Also, the relation between increasing amplitude values and the perceived sensation intensity is not necessarily linear. To estimate the precise amplitude value increment for which a change in sensation intensity is perceived you need to determine the just noticeable difference. However, this is a very tedious process. So a compromising approach to the amplitude modulation could be to measure the sensation/localization threshold (LT) and the discomfort threshold (DT - the stimulation intensity just before it becomes unpleasant) for each pad. Those two levels would then the extreme levels, e.g level 1 and 4 is we decide on 4 levels. The intermittent level would then be in between those extremes separating the levels equidistantly, e.g. Level 2 = LT + (DT-LT) 1/3 and Level 3 = LT + (DT-LT) 2/3. This is what we did in the attached study: "First, the sensation thresholds were determined for all pads and then the amplitude values were fine-tuned by comparing the sensation intensity in neighboring pads. The goal was to achieve similar sensations across pads. The resulting amplitude was adopted as the first stimulation level. The same was then done for the discomfort threshold, and the resulting amplitude was adopted as the fourth stimulation level. The second and third stimulation levels were determined as the values that equidistantly divide the range between the first and fourth level." (https://github.com/med-material/vrtactility/files/10390233/Garenfeld_2020_J._Neural_Eng._17_046034.pdf)

Alternatively to an average value of x number of fingers, we can simply consider the maximum pressure value to be the force being applied, meaning the finger which is protruding the sphere the most. This is the logic currently used to discolor the sphere in shades of red in correspondence to pressure being applied, and, although this feedback is only visual, it seems largely coherent with the pressure one perceives to be applying from my experience.

I think this sounds great. If this what feel most synchronized between visual and tactile feedback, it is the way to go. But it needs to be tested.

[kuff]

This issue thread will most likely grow to encompass the entire pressure level feature with all test cases (frequency, spatial, mixed, none). Will update here with progress on that...