What is Touch? A beginner’s guide to our most complicated “sense”.

In this recurring series – Principles and Rules of Haptic Design – I share my design philosophies for the successful implementation of haptics, giving examples and explaining some of the science that underpins the rule. The series is based on my lectures, research, and work to help novice practitioners avoid the pitfalls and mistakes I made when I started in haptics.

Introduction

Every March, for the last three years, I’ve traveled to Salzburg to give a lecture on the basics of touch for Human-Computer Interaction. This pilgrimage does three things for me: I get Mozart out of my system for the year; I’m reminded that, no, walking up a mountain is not as fun as I think; and I do the yearly update of my Haptics 101 lecture, incorporating the little bits of nuance I’ve learned in the past year. In today’s post, rather than start directly on my Principles and Rules of Haptic Design, I’m going to take a step back and start at the fundamentals of touch, covering the biological basics we’ll build from over the rest of this series.

Fundamentals - What is Touch?

Touch is often viewed as a single comprehensive sense, much like sight, or smell. However, look closer, and you quickly learn the truth – touch is a complex sense, made up of not one, but multiple different perceptual pathways, each transmitting distinct information to the brain. Think of the tactile information you receive first thing in the morning when you have a cup of coffee. You feel the smooth hardness of the ceramic, the warm and gentle roundness of the mug, and the heat, and weight of the coffee in your hand. It’s a basket full of different sensations, and our brain’s constantly doing its I love Lucy best to keep up.

When we think of touch, we think of skin. Human skin is a fascinating sensory connundrum. With a surface area roughly equal to nine pizza boxes, our skin is our single largest organ. It’s woven through with nerves that capture pressure, pain (nociception), temperature, itch, stretch, movement, and vibration. These data, collectively form Cutaneous sensations. But, if we only look at the skin, we miss a great deal of touch. Our muscles play a supporting role, helping the brain interpret position, movement, and weight – Kinesthetic or Proprioceptive sensation. Collectively, this coupled system of human skin, sinew, and muscles is called our somatosensory system, and it’s beloved by two kinds of crazies – cannibals, and hapticians. So touch, is our interpretation of information provided by our skin, muscles, and joints. I imagine that definition’s given some of my readerships an aneurysm, but hey, I’ve got to start somewhere.

“Touch, is our interpretation of information provided by our skin, muscles, and joints. ”

Tacticion – The Historical Core of Haptics

So let’s start breaking into the nuance of touch, and try and redeem myself for the oversimplification above. For most hapticians, we start with tactition, or tactile perception. Tacticion is the perception of direct contact and relative motion between the skin and the object(s) of interest. Tactition can further be broken down into tactile information from skin pressure and skin deformation and vibrotactile information from vibration waves propagating through the skin. The nerve endings responsible for tactile information – the mechanoreceptors – are each specialized for a particular role in sensing tactile information.

The tactile receptors of the skin. — *The tactile mechanoreceptors are distributed at different depths and respond to unique types of stimulation.*
Provided by: Wikipedia

Pacinian corpuscles are large, onion-shaped structures located in the deep layers of the skin and connective tissues of the hand. They are highly sensitive to vibration and pressure and respond to changes in pressure over a wide range of frequencies. When a Pacinian corpuscle is stimulated by a mechanical force, it generates an action potential that travels along the nerve fibers to the brain, where the information is interpreted as a sensation of vibration or pressure.

Meissner's corpuscles are smaller, elongated structures located in the upper layers of the skin. They are highly sensitive to light touch and low-frequency vibration and respond to changes in skin indentation. Meissner's corpuscles are densely distributed at the fingertips, where they play an important role in the sense of touch and the discrimination of textures.

Merkel cells are specialized cells located in the epidermis of the skin. They are highly sensitive to pressure and touch and respond to sustained pressure and skin indentation. Merkel cells are found in high density in the fingertips, where they contribute to the perception of fine details and textures.

Ruffini endings are elongated structures located in the deep layers of the skin and connective tissues of the hand. They are highly sensitive to skin stretch and deformation and respond to changes in skin shape and position. Ruffini endings are involved in the perception of hand position and movement, as well as the perception of object shape and texture.

Traditionally, haptics has been dominated by a focus on the tactile mechanoreceptors, driven by the discipline of psychophysics. Although they sound like mad scientists, psychophysicists, are not physicists with psychopathy, but instead people who study the relationship between physical stimuli and the subjective “psychological” sensations they produce in human observers. Tacticion is sensed across the skin, but it’s primarily in the hands, lips, and genetalia where we are the most sensitive. When it comes to the rest of the body, other systems help do the heavy lifting.

Propreioception - Good for more than doing the Robot Dance

Human and robots understand their position in the world similarly. Proprioception is really just our innate understanding of the lengths, forces, and angles applied to each of the joints within our bodies.
Photo by Mathew Schwartz on Unsplash

Proprioception — one’s own conscious and unconscious perception of the forces, movements, relative positions, and angles of the body – helps us understand the weight and shape of objects. Propreioceptive sensing is distributed across the body, if it moves, we have proprioception. Proprioception helps us pop, lock, and drop it, but it also enables us to pick up small delicate objects or hold onto things without dropping them (if you’re not clumsy like me).

Proprioception is a challenge to replicate, (ever see a robot try to pick up an egg?) and also incredibly responsive – our body adjusts to that cup of coffee slipping from our hands so quickly that the signals from the nerves don’t have time to make it all the way to the brain.

The most important proprioceptive receptors are muscle spindles, which are specialized sensory organs located within skeletal muscles and responsible for much of our sense of movement – Kinesthesia . Muscle spindles consist of small bundles of muscle fibers called intrafusal fibers, which are surrounded by sensory nerve fibers. When a muscle is stretched, the muscle spindle is also stretched, and the sensory nerve fibers within the spindle are activated, sending signals to the spinal cord and brain. Our body primarily understands kinesthesia as two properties – the magnitude of movement, and direction. Signals from the muscles, tendons, and joints travel to the spinal cord, where they are integrated with other sensory and motor signals to generate motor commands that control movement and posture.

Other Cuteneous Sensations - What makes us Human

Although historically less loved in haptic academic publication output, I’ve always been partial to what I call secondary cutaneous sensations – temperature, pain, itch, and pleasure. Much of these sensations are responsible for the “baser” needs in humans – social connection, and defense.

*Even the unpleasant parts of touch are fascinating. Our skin can make us feel in so many different ways.*
Photo by Diana Polekhina on Unsplash

Thermoreception is our sense of hot, and our sense of cold. Expectations drive thermoreception -- we expect a shaded stone to be cold, and metal near a fire to be hot. We pull from our memories and other sensory cues to determine our expectations of temperature. Thermal sensing allows us to discriminate and identify material properties of objects, from which we can tune manipulation behaviors accordingly. Additionally, of all our haptic senses, thermoreceptors have some of the strongest hysteresis -- they remember their former state.

Nociception relates to the encoding and processing of damage. It relies on nociceptors (pain receptors) that can detect mechanical, thermal, or chemical damage through pain. Nociceptive signals drive our reflex reactions and are a crucial part of the haptic system's primary purpose -- to protect the body. Nociception drives our natural bias towards rejection – we tend to recoil from unknown tactile sensations subconsciously.

Pruriception is our sense of itch. The process of itch sensation begins with the activation of specialized sensory receptors called itch receptors or pruriceptors. These receptors are located on the surface of the skin and respond to specific types of irritants or allergens. The activation of these prurireceptors can also trigger the activation of nearby mechanoreceptors and nocireceptors, which may contribute to the overall sensation of itch. For example, scratching or rubbing the skin can activate nearby mechanoreceptors, which may temporarily alleviate the sensation of itch by providing a counter-stimulus.

The existence of a separate pathway for tactile pleasure is still actively being debated, but pleasurable touch can be perceived through a variety of tactile stimuli, such as gentle stroking or rubbing of the skin, and is thought to be mediated by a specialized type of nerve fiber called the C-tactile (CT) afferent. When CT afferents are stimulated by caress-like touch, the brain processes this sensory information and activates reward centers in the brain, suggesting that they play a key role in the neural mechanisms underlying pleasure and reward, and explaining social and grooming behaviors.

Touch is A Complex Sense

Touch is hardly simple. While we’re taught that it’s just one of “five” senses, we can see it’s more akin to six different child-senses – tacticion, kinesthetic perception, nociception, thermoreception, pruriception, and pleasure – hiding in a trenchcoat and pretending to be one adult sense. Yet, we don’t experience these sub-modalities independently from one another. When we interact with the world, we take perceptions of each of these different sensory channels and combine them to produce a cohesive tactile picture of the interaction. In trying to understand haptics, we isolate each channel and track the effects of a stimulus on a single channel. When we need to design a product that utilizes touch, we, however, need to ensure the opposite, designing for the holistic experience, targeting the correct sensation to each of the channels in order to produce a clean, clear, comprehensive experience. In our next episode, I’ll discuss how to deconstruct an interaction and successfully incorporate the right sensations into your haptic experiences.