S2C10: Touch & Interoception

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a) The sense of touch

We have nearly completed our exploration of the human sensory systems and it is now the turn of the sense of touch. In many ways that are similar to proprioception, a topic we spent some time on in S2 Section 9.e, this sense is partly informed by mechanoreceptors, meaning the receptors involved are sensitive to mechanical forces such as pressure or stretch. However, in addition to mechanoreceptors, light receptors and chemoreceptors, our organism and in particular our skin is also endowed with other types of receptors providing signals with respect to temperature and potentially damaging stimuli to our central nervous system; we’ll cover these two in the next section.

A particularity of the sense of touch is the presence of receptors across our entire body, for this reason it is categorized under the term of somatosensory system, alongside the proprioceptors and the receptors for pain and temperature I just mentioned. The touch receptors embedded in our skin are called cutaneous receptors but this indicates their location rather than function. As relates to touch, there are five types of cutaneous mechanoreceptors:

Tactile corpuscles are extremely sensitive to pressure and therefore provide information relating to textures and shapes. They are present in high concentration in the hairless parts of our body such as the fingertips.
Merkel nerve endings have small receptive fields and record light touch stimuli, including prolonged ones. They seem to be particularly involved in the detection of shapes and as usual they communicate data in the form of neuron firing patterns.
Lamellar corpuscles complement the two previous types of receptors by being sensitive to deeper pressure, including higher frequency vibrations. They also have much larger receptive fields.
Ruffini corpuscles detect stretch of the skin and assist in detecting slippage and ensuring proper grip. As one would expect, based on this functionalities, their highest concentration is in our hands and especially around the fingertips.
Krause corpuscles could be described as a variant of the tactile ones but tend to be located within mucous membranes rather than the skin.

These corpuscles follow the traditional route of mechanoreceptors for purpose of signal transduction: pressure creates an opening of ion channels with sodium cations depolarizing the afferent neurons and generating action potentials travelling up to the central nervous system.

This time around I started with a description of receptors and transduction instead of the usual “what functions does this sense serve?”; but I have not forgotten and it is time to ask ourselves the question. The intuitive inclination would be to answer that touch enables us to detect the contact of a foreign item, be it a rock or another organism, thus providing information about the contours of our environment and potential predator threats. Except that in the predator case it is likely to be too late. This leaves us with shape and texture detection, which suggests an active exploration rather than a passive detection. In fact, touch is a very active sense because as we have seen with Ruffini corpuscles, it also makes skills such as holding objects possible. This concept is encapsulated under the name of haptic perception and it extends to the exploration of the world around us through touch, including by using intermediary objects such as a wooden stick. It does seem so natural and yet it is far from obvious we should be able to sense the ground, trees and other features of our environment with a tool as we would directly with our hand and fingers.

b) Nociception

This is not to say the role skin plays in detecting danger isn’t central to our survival, it just means this isn’t primarily mediated by touch receptors. Instead, the detection of potentially harmful or noxious stimuli falls under nociception and is permitted by three categories of receptors providing the central nervous system with information regarding temperature, pressure or deformation, and contact with certain chemical substances. We’ll concentrate on the first two here and just note that we saw an example of the last one in the previous chapter on taste since the pungency of some foods such as chilli is actually experienced as pain rather than flavour.

Thermal nociceptors, also called thermoreceptors, come in different varieties and specialize in either informing about cooling or heating, that is changes in temperatures above or below what the organism considers standard or safe, as well as about the crossing of specific temperature thresholds. In the case of changes, these would result in an increase in neuron firing whereas for binary thresholds it would be a neuron resting state versus a firing state. Providing indications about perceived temperature is important in several aspects, not simply removing your hands or feet from a hot surface. Other use cases include the cooling effect of evaporation which triggers the blinking of our eyes so there are cold receptors in our cornea, heat receptors on our tongue and heat receptors within our body to monitor the temperature within our organism, an interoceptive kind of signal we’ll look into later in this chapter. In short, thermoreceptors are scattered throughout our body, not just on our skin.

The signal transduction of thermoreceptors follows the usual chemical cascade of opening ion channels, membrane depolarization and action potentials, however the way temperature directly triggers the rate of opening of the ion channels isn’t fully understood yet – it might be a function of charge differentials between two sides of the membranes or rates of diffusion of those ions. In any case, with this type of sensitivity available in the tool kit, it is possible for evolution to fashion receptors answering different thermal stimuli by activating either in a binary fashion at certain temperature points or in a gradual way across certain temperature ranges.

Unlike heat and cold signals, pain signals are originated by mechanical nociceptors such as the free nerve endings embedded in our skin which respond to deformation or pressure in excess of a particular threshold. These signals are effectively a warning that the organism is experiencing a potentially dangerous situation and will activate a response coordinated by our central nervous system such as withdrawing our hand when a finger touches a sharp thorn to avoid a skin injury. Unfortunately, the signal doesn’t get muted even when we can’t undo the damage, as with a broken arm.

Being mechanical in nature, these nociceptors are activated through pressure or stretch that open ion channels, etc. until the data reaches the central nervous system where it is interpreted and then experienced as either pain or pleasure, depending on both context and intensity, courtesy of qualia – if you are unfamiliar with the concept, you may want to refer to S2 Section 7.e or to my book Higher Orders (Chapters 11 & 12).

This ability for an organism to enjoy certain stimuli, or to the contrary dislike others, really is a tremendously powerful tool to regulate our behaviour and nature or evolution has gone all in when it comes to pain or pleasure, in particular in the context of sexual intercourse where really the end goal is plain to see, namely reproduction of the genes. Of course, this is a figure of speech since there is no such entity as Nature or Evolution and there is nothing conscious or voluntary in the evolutionary process of biological organisms. As is often said: “evolution is blind”.

c) Integrating somatosensory signals

There is one piece of the puzzle missing between the recording of a stimulus and the reaction to either prolong the experience of this stimulus or avoid it after the signal has been processed by the central nervous system. This missing link is the ability to localize the origin of the information within the organism and by symmetry to direct muscle instructions to the proper parts of our body. This is an aspect I alluded to at the outset of S2 Section 9.e on proprioception and it calls for a little elaboration to understand how it is accomplished.

The way brains of animals with central nervous systems have developed to bridge this information gap is by making spatial representations of the relevant receptors. This sensory mapping as we call it isn’t quite like drawing a map on a sheet of paper but probably more like the code that would embed all the information required to display such a map on a screen, including the nature and coordinates of receptors with coordinates potentially taking the form of distance and angles between sets of receptors. Furthermore, for increased efficiency, there seems to be several maps of the various sensory systems available, which doesn’t prevent the integration of information across areas like proprioception, touch, pain and temperature – what we call altogether the somatosensory system. Indeed, integration is indispensable in developing complex skills such as balance, hand-eye coordination and removing a thorn from our skin with our fingers.

Based on observations and experiments, this sensory mapping sometimes dubbed sensory homunculus is based on the innervation and density of receptors. As such, this internal representation of our body would look quite different from what we see in a mirror and would be endowed with massive hands, lips and tongue, and quite large sex organs and eyes. The main pathway for somatosensory signal transmission and processing includes the spine, the brainstem and the primary somatosensory cortex located within the parietal lobe of our brain.

d) Interoception

This ability to sense stimuli across our various sensory receptors and pinpoint their origin within our nearly-fixed shaped organism is called interoception while exteroception refers to the sensory information originating from outside our body, such as hearing and vision. This clustering under one umbrella may not reflect the specialized aspects of some of the sensory systems and their neural pathways but it highlights how interconnected these systems are at the level of the central nervous system so that signals relating to a particular system will often times be acted upon by upregulating or downregulating a different one.

And “often times” can mean several times per seconds because the entire purpose of the integration of signals and this whole inter-connectedness is to monitor the internal state of our body, leveraging techniques such as its spatial representation, in order to maintain homeostatic conditions and keep on living. When it comes to regulating the cardiovascular system for example, the job of accelerating or decreasing the heartbeat rate and ensuring oxygen and carbon dioxide pressure, pH level, and temperature all stay within the appropriate bands is unceasing.

To recap, here is a comprehensive but non-exhaustive list of the various systems, according to the prevalent scientific terminology, playing a role in interoception:

The circulatory system, which includes the cardiovascular system, was covered in Chapter 1 of this second series with a particular focus on the capillary network and the role and content of the blood
This tied in closely with the respiratory system permitting the exchange of gases between the external atmosphere and the blood vessels. The respiratory system was also discussed in Chapter 1, including a description of the respiratory tract and an explanation of the concept of passive gas diffusion.
The digestive system was tackled in Chapter 2 with a look at the mechanisms of nutrients absorption, the role of enzyme and an overview of the human gastrointestinal tract.
We then learned in the same Chapter 2 about the complementary renal system and the role it plays in regulating fluid content and volume, a job that encompasses the filtering of our bloodstream and the maintenance of the appropriate level of electrolytes, key ingredients for all those ion channels in our body.
The endocrine system described in Chapter 4 was our first look at a pan-organism system and provided the first inkling about the need to synchronise and manage other systems so they don’t overshoot one way or another and are fine-tuned for both homeostasis and success in the outside world, judged in terms of survival and reproduction.
The immune system, explained in Chapter 5, showed how tricky and technical it can be to identify external pathogens from the organism’s own cells. Detection of such pathogens can trigger a complex set of adaptative responses from other systems such as increasing our body temperature, all with the end goal of returning to proper homeostatic conditions.
The ability to feel changes in temperatures or certain temperature thresholds and experiencing pain or pleasure thanks to nociceptors happens both on the outside and in the inside of our body. This was the topic of the previous sections of this chapter and, as is the case for the immune system, this information will often result in responses from our central nervous system executed through one or several of the systems mentioned earlier in this list.
Likewise, as we saw in Chapter 9, the signals stemming from our proprioceptors will be integrated with those from other systems and acted upon by our musculoskeletal system.

To re-iterate, this classification into systems is an artefact of our scientific knowledge, a type of heuristics to help us understand functions with well-defined sets of inputs, processing and outputs. However, there is no pilot in evolution and pathways often mix. For example, the vagus nerve takes care of circulatory and digestive systems and the processing within our brain splits along different networks to specialized areas that can be called upon to treat several types of sensory data. Biology is complexity, squared.

e) Time perception and circadian rhythm

I expect most readers would have been surprised to discover there really isn’t just five senses and ultimately there is no hard line dividing what makes a sensory system from another stream of data based on internal or external stimuli. In that respect, there is a strong case for proposing that our ability to perceive the passage of time, also called chronoception, qualifies as our eight sense. The seven others being vision, olfaction, taste, hearing, the vestibular sense, proprioception and touch, whereas interoception is more like a meta-sense.

When we talk about time perception we should distinguish two aspects: the one attached to our subjective experience, including the reliving of memories, and the regulatory aspect underpinned by an ability of our body to measure and estimate the passage of time.

Examples of the first aspect would be how one can perceive time as slowing down, or maybe the movement of objects and other entities slowing down when experiencing “flow” or being “in the zone”, during physical activities in particular. Of course, this strong internal focus and coordination of our organism doesn’t alter time itself, it distorts our perception of it compared to our standard non-flow experience – it is about relative perceptions. This may have to do with the intensity and synchronicity of neurons firing and probably also involve quite a bit of neuromodulators and hormones flowing in our brain and body. Likewise, when we do exciting things, this increased chemical flow will dial up the frame rate of memory formation, resulting in a higher per-second density of memories which, when recalled, suggests some kind of time dilation compared to mundane baseline experience such as brushing one’s teeth or going for a standard run, as opposed to participating in a running race which comes with some adrenaline release.

As interesting as this might be, it is nonetheless more of a by-product or side-effect of the second aspect – the real sensory aspect, in my opinion – which consists in our ability to estimate how much time has lapsed without external clues. According to the latest scientific data available, humans seem to have the ability to evaluate how much time has lapsed across ranges with different order of magnitudes, spanning the sub-second, up to a few minutes and fractions or multiples of days. The last one is called circadian rhythm and allows us to time our waking up and sleeping time; it is calibrated based on mainly visual clues such as light intensity and its mix of frequencies, thereby reflecting the range of typical light conditions and spectrum from morning to evening and night. Even without those visual clues, this internal stopwatch will allow us to estimate the passage of days and when we travel across time zones the strong inertia of our body clock creates the jetlag syndrome.

The etymology of circadian can be translated as “approximately a day”, and this type of physiological time evaluation and time-based regulation is not only ubiquitous in the animal kingdom but also exists among plants and fungi. The physiology of the circadian rhythm is often explained through the triggers of sunlight inhibiting the release of melatonin and other activation-inhibition dominoes between various parts of our nervous systems. However there also must be some kind of default threshold values at play since we can maintain a regular sleep pattern and even estimate time quite accurately over the course of several days without light exposure, although a drift will eventually occur. Nothing mysterious here, just an incomplete mapping at this point. Indeed, if we imagine a certain neuromodulator is released at a constant rate throughout the day, just like an hourglass it will eventually act as a stop watch to tell the time when a certain concentration of this chemical substance has been reached and a properly tuned chemoreceptor will be able to start another hour glass, corresponding to the theoretical sleep period possibly, whilst the chemicals from the first hourglass are being cleared and then re-synthesized, ready to start another trip in a few hours’ time. Note that this is just a personal theoretical explanation meant to serve as an abstract proof of concept.

f) Trivia – What it is like to be an octopus

In a now widely-cited paper called “What Is It Like to Be a Bat?” published in 1974, Thomas Nagel discussed the idea of consciousness with the emphasis being on the fact that there is something it feels like to be a conscious organism. In Higher Orders, I take a different approach and split the concepts of consciousness and phenomenology or qualia. And here, I would like to concentrate on the experiencing aspect, the qualia element, by asking ourselves the question of whether we can imagine what it is like to be a bat.

The answer, to me, is “sort of”. I can in my mind’s eye imagine some kind of dim night vision where I distinguish shapes but not their minute details, shadows and texture. And no colour you may say. To which I would answer, why not? Maybe a bat’s brain paint sound wave pressures in colours, who knows? So we can only half-guess by extrapolating from our existing senses.

However, if I now ask the question of whether we can imagine what it’s like to be an octopus, our answer ought to be a resounding “no”. Why is that and why the different answer? Because this mollusc species has a partially-centralized and partially-distributed nervous system and it is very likely some degree of mental representation takes place based on sensory signals in each of its eight arms. Our intuitive approach is to think this should feel like multiplexing, with 8 or 9 representations coming up at the same time except this would still assume a centralized process where the representations are integrated and experienced as one. Instead, it is much more likely there is a hybrid type of central representations existing in addition to 8 independent ones the central brain has no experience of. This doesn’t imply a total of nine conscious entities though, only 9 areas of experience within the organism. And clearly, this is not something we can remotely fathom.

Again, why can’t we? Simply because our imagination is not a magic ethereal ability to dream new experiences. It is restricted to variations and combinations of existing concepts in our mind that found their way in our central nervous system through prior experience or genetic inheritance. And so, we can’t imagine what an extra dimension would look like and we can’t grasp and express the true nature of the fundamental interactions, light and matter because we have never perceived anything like them before directly. We do perceive their effects on our various sensory receptors but this experience is always mediated and most likely incomplete.

I hope you enjoyed this second series.