Why a Lover’s Touch Feels More Thrilling Than Your Own

Certainty, efficiency, adaptability, self-agency, and even your sense of self, all owe to your ability to predict and attenuate the sensory consequences of your movements

The incredible numbness of being

Your ability to dampen self-caused sensation gives you a sense of self-agency and, by extension, knowledge of yourself as a conscious, separate being, able to act upon your environment and others.

It explains why you can’t tickle yourself (but those with schizophrenia can) and why a lover’s touch is more thrilling than your own. Your ability to mute sensations that are the consequence of your actions is also why the second line in the classic sibling gambit of ‘You can hit me back, just don’t tell Mum, okay?’ is, without fail, ‘No fair, I didn’t hit you as hard!’ (You probably did.)

And, if we could regress even further back into childhood, it illuminates why you glowed with joy upon overturning your bowl of mashed bananas (effecting change on the environment) and were transfixed by a video game when given an unplugged controller (the illusion of same) as a toddler.

Before we discuss what delineates self from “other”, let’s consider what happens when you make a move.

Making a move

If you were to scratch yourself right now — discreetly, of course, as you are a well-mannered reader—it would necessitate the firing of neurons so that impulses travel down axons and the appropriate muscles contract.

Before this occurs, your sense of proprioception would communicate the location of your arm. Additionally, vision, which is more fine-grained than proprioception and thus often overrides it, is used to plan the trajectory your arm would have to take to wherever it is you’re feeling itchy.

The brain is a mighty prediction organ hellbent on refining its model of the external world (and the internal states of your body) through internalising statistical regularities. The more guided by context and past experiences your perceptions are, the more likely they are to be accurate and thus serve you well.

To ensure your survival and the propagation of your genes, you anticipate the likely consequence of your every action. Constantly speculating about everything from what would happen if you made some social gaffe (the rest of your tribe would shun you, and you’d starve to death, naturally) to the sensations you’d feel upon touching some part of your body (probably related to aforementioned faux pas).

That must be the key—we must have some way of telling self-generated sensory consequences apart from those caused by an external agent. We must have some way of distinguishing ourselves as separate from other entities and our surroundings.

Estimating sensory feedback

You generate a copy of every motor command known as an efference copy. This copy or “echo” allows you to anticipate the sensory feedback you will receive upon completing a particular action, such as hearing yourself speak or rubbing your hands together in glee. This estimated feedback is called the corollary discharge.

Below is an illustration of a cognitive model depicting how we generate a prediction and compare it with the actual sensory feedback received a fraction of a moment later. The last stage involves determining if there’s a discrepancy between the two. If there is a match between prediction and observations, it suggests the movement was self-generated.

Internal Forward Model diagram by Michelle Costanzo / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

The best way to predict the future is to invent it

Sensory feedback allows us ‘to learn relationships between motor commands and outcomes to correct movement errors’ by making adjustments as we track a movement visually and kinesthetically. As sensory feedback is perceived at a delay, predicting feedback saves us time and frees us to plan our next move.

Body movements ‘require compensatory eye and head adjustments in order to avoid perturbation of visual information processing’ or the ability to perceive the surrounding environment is compromised. Fortunately for us, we can anticipate and therefore compensate for the shifting of images on the retina after each rapid eye movement (saccade) to ensure visual stability and acuity.

The efference copy supersedes reactive vestibulo-ocular reflexes in driving these adjustments. Ergo, we aren’t merely reactive but proactive, able to predict the future to enhance our motor adaptability and efficiency.

Predictive attenuation is also protective, sparing us a sensory onslaught. For instance, ‘auditory research in humans and animals revealed that phonation or sound-making actions inhibit the auditory system, thus protecting auditory sensitivity’. That explains the lack of flinching from drill instructors, sir, yes, sir.

Our sensorimotor system — and those of many creatures, both vertebrate and invertebrate—can distinguish signals resulting from self-generated voluntary movement (re-afferent sensory information) from sensations caused by something external to us. The comparison of received and anticipated sensory feedback (corollary discharge) facilitates this distinction.

Where expectation matches reality, we can be sure a sensation emanated from us and not from anyone else. Once we’ve made this determination, we can expend fewer cognitive resources on such inputs (by muting them) as they likely cannot alert us to an existential threat or a reproductive opportunity.

Key concepts

Efference copy: A duplicate motor command used to generate the corollary discharge.

Corollary discharge: The estimated sensory feedback available to you before the actual feedback from your movement (re-afference) becomes available.

Re-afference: Sensory signals produced by your motor output. (Afference refers to receiving input in general.)

Predictive attenuation: When predicted (corollary discharged) and actual sensations (re-afferent feedback) match, the intensity of the sensation is reduced as expending cognitive resources on self-generated sensations is not a high priority.

Tactile gating: Distinct from the phenomenon of predictive attenuation, we are inclined to feel sensation more keenly (including externally caused sensation) when at rest (gating) than during a movement. (So next time you’re getting a blood draw, start flailing your arm to take the edge off.)

However, tactile gating (a reduction in sensitivity) does not occur when the somatosensory information is ‘relevant to the to-be-executed movement’, showing that this phenomenon is context and goal-dependent.

Sensory gain: The amplification of a somatosensory, visual, auditory, or olfactory sensory afferent (signal) relevant to attaining a goal given the ‘behavioural context and the state of the body and the environment’.

Habituation: The ability to largely ignore a sensation, such as a scratchy shirt tag, due to its persistence.

Examples of attenuation to tickle your fancy

Image by Trueffelpix

I tickle you with a feather: no efference copy is created, so there is nothing to compare afference with; the sensation is not dampened as this stimulus demands attention by dint of being external and of potential survival relevance.

You tickle yourself with a feather: efference copy is created, and corollary discharge matches re-afference; the sensation is dampened to spare you drowning in a cacophony of irrelevance.

I grasp your hand and make you tickle yourself: no efference copy is generated with passive motion; the perceived-as-external sensation is not dampened.

You latch onto my hand as I’m tickling you: efference copy is created, corollary discharge and re-afference are a match, but there is some discrepancy; the sensation is dampened, but only to the degree that the sensory prediction was accurate.

In a nutshell, sensory consequences can be identical but perceived differently depending on the cause (or what the cause is perceived to be—more on that later).

Social touch versus self-touch

Social touch

Social touch, that is, non-sexual, pleasant, emotionally communicative touch, allows us to convey ‘affective and socially relevant information’ through a consoling back rub or a supportive hand squeeze.

Incidentally, suppose you’re hypersensitive to social touch. In that case, as is common in autism and attention deficit hyperactivity disorder (ADHD), this results from a starker neural distinction between Self and Other through both a ‘stronger deactivation during self-touch’ and an ‘increased activation during other-touch’. (For enhanced social cognition, it’s, unfortunately, better to have a blurrier boundary to enhance visceral empathy.)

Self-touch

Some self-touch is felt very clearly and serves an important function, such as refocusing attention, regulating emotions and self-soothe, inhibiting pain, scratching an itch, or sexual or sensual pleasure. (Both self-scratching and masturbation are ‘potentially not even attenuated’.)

If self-touch, in contrast to social touch, were of no behavioural relevance, it would be cancelled out entirely rather than merely attenuated to varying degrees. If cancelled out, we would no longer possess the certainty and interoceptive (awareness of the state of the body) abilities that result from updating our internal model of the bodily self. Constant feedback, even if muted, is beneficial.

Being in touch with every square inch of their furry little bodies is undoubtedly why felines are far more contented than humans. Not that I suggest you start licking yourself clean. Perhaps—being less limber—you could try body brushing instead? Besides stimulating circulation and sloughing off dead skin, body brushing increases emotional awareness and self-regulation.

In a nutshell, the ability to feel self-touch is essential not only for the relief it provides but also for updating our simulation of the world and everything in it—including our very vessels.

Attenuated sensation: evidence from the lab

Temperature

VanDoorn and colleagues (2006) used a machine to move participants’ fingers across a cold tile. Participants perceived the tile as being colder when their movement was guided rather than when they actively touched it, although the tile was 15°C in both conditions.

When instructed to touch the tile, participants were able to predict and attenuate the sensation of coldness by generating an efference copy of touching the tile. Participants were able to use the “coldness” radiating off the tile coupled with their prior experiences of touching objects of a similar temperature to create a sensory estimate of its likely iciness.

If you recall the ice bucket challenge of a few years ago, you’ll realise you’d have been better off drenching yourself than recruiting a friend to help. (As for the cinnamon challenge, you’d have been better off not doing that. What were you thinking?)

Pain and pressure

Wang and colleagues (2011) tested whether self-generated movement influences pain and pressure perception. The researchers found that self-induced pain is perceived as less painful than externally generated pain.

Further, a distinct activation pattern was observed for both types of pain. Scans showed that activation in pain-related areas was inhibited in the case of self-generated pain. (If captured by the enemy, request to use the pliers on yourself—it’ll hurt less and earn you the grudging respect of their spymaster.)

No need to worry: the pain lab rats experienced was caused by nothing more than a pointy polyhedral bead, while pressure sensations were induced using spherical beads. Participants either squeezed the bead or had their hands squeezed over it and reported muted pressure and pain sensations in the passive rather than active touch scenario.

Lack of intentional movement is why your toe hurts when you stub it, even though you’re to blame. (Well, you and whoever invented table legs.)

Discrepancies in feedback

Introducing a feedback delay or trajectory rotation can diminish our sense of agency over an action. Researchers achieve this effect by getting participants to press down on a force sensor with a widget communicating that same pressure back at a lag or to a slightly incongruent location.

Predicting the sensory consequences of other people’s actions

Mirror-touch “synaesthesia” (MTS) is the ‘conscious experience of tactile sensations induced by seeing someone else touched’. Mirror neurons fire when observing someone perform an action, particularly one of social significance, and serve to broaden one’s motor repertoire and facilitate empathy. Hyperactivity within the mirror system may explain MTS*.

Alternatively, those with MTS may have a fuzzy Self-Other boundary, over-extending their bodily self to encompass the body of others due to a cognitive issue. Lending credence to this is the observation that individuals with MTS show a more profound vicarious reaction to the rubber hand illusion, being more adept at “minding” an unrelated joke shop appendage as though it were their mitt. Paw. Meathook. (There aren’t any good slang terms for ‘hand’.)

Anticipating visual consequences

Your movements are not only associated with specific and predictable sensory consequences but are also concomitant with certain visuals. For example, you could close your eyes in a pitch-black room, wave your hand in front of your face, and “see” it. But don’t get excited; you’re not Batman. You’re just hallucinating. (This is, of course, different to waving your hand in non-absolute darkness and detecting the shifts in brightness that you can make out through your eyelids.)

The better you are at tracking your hand movements in the dark, the better you’ll be in ‘evok[ing] normally concomitant visual perceptions’. You’ll be more adept at generating likely visual feedback for the actions that usually accompany it.

Likewise, these ‘kinesthesis-induced visual sensations’ are more potent in people with synesthesia, whose multisensory connectivity is greater, leading to a blending of the senses.

Combining intent, kinesthesia, and vision to make better prosthetics

When sensory feedback is congruent with intent and execution, we can be fooled into thinking we have control.

Prosthetic devices can be challenging to control without sensation in the replacement limb. Amputees provided with kinesthetic feedback by vibrating the muscles used for prosthetic control can improve movement fluidity through a combination of ‘intent, kinesthesia, and vision’. It also gives them a comforting ‘sense of agency over the robotic movements’.

The authors argue that a feedback-control closed-loop ‘opens a pathway to [the] seamless integration of minds and machines’, which sounds creepy at first blanch, but it’s really no different from forging a connection with a hammer or a vehicle through vibrotactile feedback.

Beliefs about authorship

Suppose there is a causal link between an action and a sensory change in the environment. In that case, belief about authorship can modulate whether a sound or tactile sensation is attenuated, even if you really were the cause but were misled to believe otherwise.

My gaslighting tip of the day is: If accused of being noisy, say, ‘That’s just your eyelids blinking shut and not my antique clock or the clicking of my dentures, sonny Jim’.

The gift of sensory inhibition

Your relative inability to tickle yourself may seem trivial. However, it’s a product of the same mechanism which lets you tell external reality apart from the random neural firing of your brain.

To show you just how vital it is to distinguish the internal and external worlds, let’s explore the nightmare suffered by those with a source monitoring deficit. Imagine feeling as though your thoughts and subvocalisations belonged to an external—and malicious—entity. Imagine believing that inescapable visual, auditory, tactile, olfactory, or gustatory hallucinations were real.

When the subconscious rises to the surface

Ideally, the subconscious should take a backseat. After all, external threats can harm you and even snuff out your existence, while macabre images and unpleasant thoughts pose no physical risk. Nor is any self-touch likely to be unwanted or malicious (alien hands aside).

In psychosis, there is a breakdown of the ability to monitor the source of one’s perceptions. Early researchers into schizophrenia noted that it’s as though ‘the unconscious is at the surface, conscious’, and other variations on that remark. Freud himself observed that these outwardly perceived hallucinations, such as taunting and hostile voices, were manifestations of a patient’s neuroses, rising uninhibited to the surface.

Individuals who are hallucination-prone or experience a sense of passivity, such as those high in schizotypy, can tickle themselves with far less attenuation.

Self and self-agency

Imbued with self-agency, we feel, also a sense of mastery. When a baby knocks food off their highchair and burbles delightedly, they are communicating their joy upon realising they can influence the world that exists beyond their fingertips.

This ability affirms to a child a most thrilling fact: they exist and can alter their environment in the pure expression of their will. Psychoanalyst Francis Broucek, who coined the phrase ‘the pleasure of being the cause’, theorised that the denial of this pleasure, once experienced, is deeply traumatic to a child and causes them to withdraw and disengage from the world around them.

Despairing parents of toddlers take note: very young children knock things over or squeeze the cat despite being chastised, not due to delight over self-agency or a desire to flex their independence, but due to a lack of cognitive control. That is, the ability to knit together knowledge (‘Squeezing the cat is frowned upon.’) with execution (‘What steps do I take to refrain from squeezing the cat?’)

I act, therefore I am

Sea squirt by Panaiotidi

The humble, tubular sea squirt devours its own “brain” (eyes and spine-like notocord) once it no longer needs it to coordinate movement, having found a permanent home on the seafloor. Sessile organisms, it turns out, don’t have much use for a brain, a noodle, or a noggin. The brain didn’t evolve for thinking, good heavens, no. Much as you can philosophise and wax poetic, your brain evolved to guide action. Movement, it seems, is the key to consciousness; I act, therefore I am (a sentient being and not a brainless potato).

It’s no surprise that the cerebellum, which is instrumental in coordinating movement and aids in sensory attenuation, also instructs the frontal systems responsible for cognitive control.

Cognitive control enables you to navigate your environment with intentionality and in the context of social cues. Thus, you’re able to plan and execute a sequence of actions to achieve your goals while cognisant of the various constraints of your environment.

Meanwhile, cognitive flexibility allows you to adjust according to changes in the environment and to relinquish maladaptive strategies. Specifically, it is the ability to attend to relevant rather than irrelevant information in a dynamic fashion.

You feel a sense of agency not merely due to predictive attenuation but because you are aware of your actions and their antecedents and effects. You are aware of your mental processes (meta-cognition) and can self-regulate and plan to pursue a complex goal.

Parting thoughts

Predicting the sensory input of your motor output reduces uncertainty, making you efficient and adaptive. It also reduces cognitive load and imbues you with life and self-affirming self-agency. And yet, you’d trade it all to be able to tickle yourself properly, wouldn’t you?

Footnotes

*I stumbled upon neurologist Joel Salinas who claims to have MTS (and every other actual synaesthesia under the sun). He’s utterly insufferable; I wish he could experience the pain I felt watching his self-absorbed nonsense. That’s likely not the case; in a Reddit AMA, Salinas states the presence of the condition is falsifiable as though this somehow constitutes proof that he has MTS. Colour me—dark indigo and lavender—sceptical.

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