Haptics: Understanding the Senses of Touch

I’ve been working on converting the following text I have written into video:

Intro to Haptics: youtu.be/9FifA6rSkyg
Thermoception: youtu.be/pi3jgGcFD24
Mechanoreception: https://www.youtube.com/watch?v=nj0XMJAGs_U
Nocioception: Coming soon

From mobile phones, video game consoles and personal computers, users are interacting with a multitude of interactive technologies on a daily basis. Interaction through touch is what we call haptics.

Most of us likely have used haptic technology through vibrotactile feedback. The vibrations you feel when your phone alerts you of a message you just received or a game controller makes you aware of damage your character is receiving.

As we head towards the future we are likely to see haptic technologies in consumer devices expand to benefit from our full range of touch. Your watch may heat up to discreetly alert you of an important message, or your gamepad might cool down to set the mood as you enter arctic territory. To better understand what is possible, we should better understand how our sense of touch works.




What is touch?

In primary school you may have learned about the five traditional senses – vision, taste, smell, hearing, and touch. This is a bit of a simplification, and a narrow scope of what exists.

The depth and complexity of what we consider the single sense of touch often indicates that it can be divided further into different senses in their own right. We call these pain, pressure, temperature and proprioception (sense of one’s self). This gets interesting as having a better understanding of these separate senses give us a better understanding of what we can do with them.

For this article, proprioception will not be a great area of interest to us as the majority of touch perception is at the skin level, if not slightly beneath.

The combination of perception of these senses, as well as with the more traditional senses (e.g. sight and hearing) provides a channel and bandwidth in which to deliver information. This allows for a combination of perception of the different dimensions of roughness, warmth, cold, pressure, size, location and weight.

For example, touching something wet may be a perception of both pressure and temperature. So if we are to mimic the feeling of wet, we can do so by emulating both its temperature and pressure.

It is not farfetched to think that soon enough we could create virtual environments where users use haptic gloves to grab objects in the virtual environment that are nearly indistinguishable from the real thing.

Temperature

Let’s take a look at the sense of temperature. Temperature is of particular interest as it can be used in order to intensify perception of other variables of touch. For example, cooling may make a heavy object be perceived as heavier.

Temperature receptors are distributed throughout the skin, different areas having different densities of receptors. The hand, face (particularly the tongue and lips), and fingertips have the highest densities of these temperature receptors, while for example the sole of the foot has the least.

The information can be organized visually as sensory spots. The main idea is that the smaller the spots (higher density of receptors) the higher temperature sensing resolution exists on the skin. It is important to note that warmth sensory spots (areas that detect warmth) are relatively rare; our skin primarily consists of cold sensory spots. For this reason, a person is able to detect changes in cold temperature far more accurately than hot.

Thresholds for warm receptors are twice those of cold. Cold receptors have a resolution of roughly 0.02 to 0.07 degrees Celsius, more accurate then heat receptors, which tend to have a resolution of 0.03 to 0.09 degrees Celsius. Resolution in this case is the accuracy at which temperature change can be detected; the skin is quite sensitive to very small changes in temperature.




However, this does not take into account the speed of the temperature change. Fast or extreme changes (dangerous) are detected far sooner than slow changes. As such, a change of 5 degrees Celsius may go unnoticed as long as it is within the neutral area (an area of temperature that is not detected as too cold or too hot). This is because of adaption, which can happen anywhere between 17 and 40 degrees Celsius.

In order to determine what is perceived as hot or cold, a neutral or null point needs to be defined. Again, the problem is that the physiological zero is never always the same. This is because a subject’s perception will eventually adapt to their surrounding environment.

A simple experiment is to put one hand in cold water, and another in warm water. After five minutes, dipping one’s hand in water that was previously at the physiological zero will feel hot to the hand that was laying in cold water, and cold to the hand that had been in warm water.

Another aspect to consider is spatial summation; the larger the area of skin that is stimulated, the greater magnitude of response will exist, which in turn reduces the reaction time to a stimulus.

Temperature receptors are poor at spatial recognition. The only exception is as temperature nears the pain threshold, higher intensities are spatially easier to detect. It is interesting to note that thresholds diminish if applied simultaneously at symmetric locations of the body. This is not the case asymmetrically. Consequently, temperature spatial recognition is mediated centrally.

Pressure

Pressure as a tactile response offers a fast and highly accurate interface of information transmission, particularly in the hand, which is highly sensitive in this area.

It has been shown that the blind and deaf can hear simply by placing their hand over a person’s mouth. The already fast reaction to pressure further increases if the stimulus is expected.

With the simple use of vibration, devices should be able to transmit plenty of information to a subject. As age progresses sensitivity to pressure is reduced, but as measured relatively, thresholds do not tend to change.

It is also important to note that spatial awareness as it relates to the sense of pressure is incredibly high. Pressure is a very accurate form of haptic stimulation.

Shall we use Pain?

Pain is a multidimensional sense (intensity and unpleasantness) that is unique in that it can be initiated at any given part of the body including directly in the central nervous system.

Heat, cold, pressure, electricity and chemical irritation all have pain thresholds (point where the sense turns to pain) that can be used to induce pain. On the extreme end there is what is known as the supratheshold, which is incredibly difficult to measure.

For one it may often be beyond a subject’s tolerance level, but also as pain reaches this level of intensity sensors can begin to suppress information, which in turn alters existing thresholds.

As far as measuring pain goes, pain is typically measured in a single dimension. This may be done on a simple scale of 1-10, however it is measured, there should be some range that depicts the scale from no pain to severe pain (possibly with mild and moderate somewhere in between).

Furthermore, when experimentally measuring levels of pain there are many psychological errors to be aware of. The anxiety and anticipation of pain itself may trigger a faster or higher level of reaction than expected. Culturally, ideas of being tough or correct may inhibit a person from giving accurate feedback from painful stimulus. Alteration of existing thresholds also exists and can be seen in the staircase of pain.

As pain increases, so does the pain threshold. This means that a given level of painful stimulus that may have been sensed as pain the first time around, may not be felt as painful later on. Another problem with measuring data is how significant the differences in the levels are. Should the difference in reducing severe pain down to mild pain be the same as reducing mild pain down to no pain? Probably not, but it is hard to design a proper and informative scale.

Above is a table with a quick breakdown of each sense of touch. This may be useful when considering what technologies to use or implement.




The Future!

During my Master’s thesis work I had focused on determining how effectively heat can be used as useful feedback. With little question it was found that users reacted fairly quickly to what would be considered small and gradual amounts of heat.

The ultimate idea and hope is that by using more channels of communication to a user, the further you enhance the user experience.

What is the next evolution in gadgets and mobile devices? As we move towards more natural interfaces, it is not unreasonable to imagine that we may begin to see more natural forms of communication through our devices.

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