The running joke is that Cory cannot get to praxis fast enough, so this episode makes her wait: a proper deep dive into somatosensory processing before praxis is allowed in the room. Somato means body, proprio means oneself, and Tracy widens the lens to show how much lives under this umbrella, tactile, proprioceptive, even interoceptive, and how the same receptors run two pathways, one for discrimination, one for the valence of comfort and threat. It builds, finally, to the praxis clusters that fall out of it: somatodyspraxia, praxis on verbal command, and bilateral integration and sequencing.
Michelle leads a deliberately slow build, hold off on praxis for 40 minutes, and the group starts where it should: what actually is the somatosensory system? Cory traces somato to the Latin for body, and Tracy adds that proprio means oneself, then opens the umbrella wider than most clinicians hold it. The proprioceptors are not just muscle, joint and fascial receptors; the interoceptors of the gut, heart and lungs are somatosensory too, and even vision and listening are mediated by the somatosensory receptors of the eye and ear muscles. Interoception, they note, is not a 1990s discovery but part of the same theory Ayres wrote about, just re-categorised, with today’s emphasis on interoceptive awareness blending discrimination and modulation.
The neuro gets clear and useful: somatosensory information ascends two ways, the dorsal column medial lemniscus system carries fine discrimination, while the anterolateral pathways carry the valence of comfort versus discomfort, the modulation side, and that is true for tactile, proprioceptive and interoceptive input alike. They unpack proprioceptive modulation (why a child may resist a weight shift, where vestibular insecurity translates into proprioceptive discomfort), the tonic and phasic distinction, and how tactile and proprioception work as a tandem you can still tease apart in clinical observations (grading of force reads proprioceptive; fine discrimination and hand-shaping read tactile). A lovely developmental thread, affordances, a metal water bottle a child refused, and Cory’s baby Audrey learning to drink without spilling, shows how the system refines through feedback. Then, finally, praxis: the clusters Ayres’ research revealed, somatodyspraxia (a true somatosensory-based coordination problem), praxis on verbal command, and bilateral integration and sequencing, each identifiable through the EASI or SIPT, each mapping onto a precise treatment plan.
Lightly edited for readability. Speaker labels and chapter markers match the published episode.
Cory: Hello to all you amazing Spirited Conversations podcast listeners. It has been a while since we posted, we have been adding video to our podcasts, and it has created some holdup in the editing process. Today’s podcast has a little snippet of video that we will post to our YouTube channel, so if you do want to watch Tracy explaining one section, where she uses some visual demonstrations to help elicit a point, feel free to jump on and take a look. Otherwise, we hope you enjoy episode 20.
Michelle: Hello my friends. I cannot wait to have another deep dive with you today, we are talking about somatosensory processing. We are going to try not to move too fast into praxis, so, Cory, do not ask a praxis question for at least 40 minutes. Obviously there are huge inputs from somatosensory processing that influence and contribute to praxis, so that is a natural route to go, but we are going to try to hang out in just defining what it is, why it matters, and how it contributes to function for the kids we work with, and possibly how it might influence other systems like the vestibular. So Cory, how do you put it together, what is the somatosensory system to you?
Cory: For the three of us, it automatically brings up tactile input and proprioceptive input. Over time you hear the term enough and you ask, what do you mean, what is that, and people say, oh, it is tactile and prop. So those two together become somatosensory processing. I ended up looking up the Latin root of somato, and it means body, so it makes sense, body processing. What other senses contribute to me processing my body? Pretty significantly the tactile and proprioceptive systems give me information about my body, so that helped me conceptualise it. But when you were talking in the intro, Michelle, I had a thought, and I promise it is not a praxis thing, it is a modulation and discrimination question about somatosensory input. With the other systems, it feels clearer where you get a modulation issue. In the tactile system I can really discern a big response to a small input, and tell that the child’s nervous system is finding that input overwhelming. But in the proprioceptive system I do not really understand how I discern modulation functions. I have never really associated proprioception with modulation, to me it is not part of that primitive evaluative processing, but maybe it is, maybe I am wrong. Let’s clarify.
Michelle: Come on in, Tracy the clarifier.
Tracy: I get to be the clarifier. First of all, I love that you looked up that somato means body, and interestingly proprio means oneself, or from oneself. Before we jump into modulation and discrimination, which we will get to quickly, there is a funny thing that happens when we talk about somatosensory processing, and that is that it is taking information from the body itself and giving information to the nervous system about the self systems. We think of the proprioceptors as the muscle, joint and fascial receptors, but there is a broader category, the interoceptors. Sometimes we use interoception to think just about the deeper receptors in the gut or the viscera, the lungs, the heart, the respiration, the suck-swallow-breathe function, but those are all actually somatosensory receptors. When we talk about interoception, muscle receptors, skin receptors, they are all parts of the somatosensory system. You can carve the world into different categories, interoceptors versus exteroceptors, but what is interesting is that our ear muscles are muscles, and there are somatosensory receptors that guide our listening; our eye muscles take in external visual information, but the information is gathered by the proprioceptors of the eyes, so even our vision is mediated by somatosensation. Listening, swallowing, all of those functions are based in somatosensation. So in some ways the vestibular system is this grand system we talked about last episode, but the somatosensory system is mind-blowingly connected to everything.
Cory: If I am being particular about somato meaning body, all of those things relate to the body, so it makes sense that interoception could also be part of somatosensory processing. So is it just historically that we pulled that out, or Ayres pulled it out, or was that where the thinking was at the time, to separate it rather than have it sit under interoception?
Michelle: I know there is some discussion around the vestibular labyrinth being a proprioceptor, but we shy away from that. In the latest SI book they say, for the purposes of this book we will not go there.
Cory: Do you think it is to do with the fact that the interoceptive processing part of somatosensation was not really recognised earlier on, so maybe it was pulled out to make it a thing, it flew under the radar, and then people went, wait a second, this is really a thing and we should be paying more attention to it? Is that why it is a re-categorisation rather than a new discovery?
Tracy: That is exactly what it is. It is not a concept that was talked about when Dr Ayres was writing, even though Sherrington and people way back in the 1800s, and even before, would talk about it. Now there are people who claim it was discovered in the 1990s, which I am poking fun at, I should regard all scientists fully, and I do, but there is history, you can carve up the world in lots of different ways. Maybe this is the least important thing for us to be talking about, but it confuses people, because they think, oh, interoception, that is so earth-shattering, when it is all under the same theory Ayres wrote about, just different concepts and words applied to different parts of the body and different ways the receptors work. What is interesting in the interoceptive work now is a lot of focus on interoceptive awareness, how being aware of a feeling arising in our body can lead us to a different set of solutions for coping, regulation, or taking care of our physiology, I am hungry, I am thirsty, I am needing something. But awareness is a skill set related to precision of processing, so it is a blending of sensory discrimination and sensory modulation. So let’s talk about how there really are structural and functional neural pathways that support modulation and discrimination, giving rise to different functional capacities, and that is what we have to tune into as clinicians, is the thing troubling this individual based in processing difficulties in one system or the other, or how they interact? Sensory discrimination gives rise to perception-action processing, the capacity to become skilful and use the awareness of our body, everything from throwing an axe in a game, to a neurosurgeon finessing the most intricate manipulation of their fingers, to an artist or musician using fine, nuanced action patterns. The system can be so finely tuned it allows a super high degree of skill, but also the everyday, zipping your coat without looking, opening your water bottle while you open the car door, finessing multiple motor plans at once and being aware of where your elbow is pushing the door and how your hand is turning the jar, all available simultaneously because of the exquisite processing of the somatosensory receptors.
Cory: So can you help me with the modulation component of somatosensation? It is tricky because there are so many different receptors involved, tactile, proprioceptive, and now interoceptive. Is that why it is hard to pull apart the modulation piece, and how should I be thinking about it?
Tracy: The somatosensory receptors, whether tactile or proprioceptive, and whether from the limbs, the core, or the interoceptors, send information ascending through the detail system that goes through a particular set of pathways called the dorsal column medial lemniscus system. A tiny bit of detail is also carried by the partner system, the anterolateral pathways, but most of the anterolateral pathways carry information into lower-level brainstem and limbic processing, and are involved in detecting whether information is comfortable or not. So it is the valence-based processing that is informed by modulation, and that is true of tactile, proprioceptors and interoceptors.
Cory: I had a thought. Is it at the point where, I am thinking of jiu-jitsu, when you get your joints stretched to the point where you think, I might hurt myself here, is that the modulation function of the proprioceptive system coming in, because it is uncomfortable and you are getting a firing?
Tracy: Yes. It is always on the valence of comfort to discomfort, pain versus safety. It sets up that valence and detects when shifts are moving you out of comfort and into discomfort, so the proprioceptors are a really critical part of our pain registration and circuitry. Once I understood this, I could see little ones I was working with who had differences in the way they weight-shifted, often really uncomfortable with it, because the proprioceptors register, wait, you want me to weight-shift, and some of that is going to pull me off my midline. So the relationship of vestibular and proprioceptive processing can set up where the proprioceptors start to be restricted in what is comfortable, a defensive, over-negative response, which is a modulation difficulty, related to poor processing in general but in that case to a proprioceptive over-response.
Michelle: It could be related to vestibular processing as well, in that avoidance, I do not want to move off this space, so I will stay here, but I am starting to feel some pain because I have been in this position a long time. So that is another profile, where the vestibular system says stay put, stay put, but the proprioceptors are going, I want to move, I want to weight-shift, I am feeling this deep pressure and I want to shift it to the other side now.
Cory: So with Michelle’s example, the kiddo is kneeling on one knee and that knee starts to register pain from kneeling so long, weight-shift, so the body wants to shift the weight. But with your example, Tracy, you were talking about kids who do not want to shift the weight due to a midline, body-in-space issue, and this is where the vestibular and proprioceptive integrate. Because of that unsureness about managing their body in space, you get that signal primarily from the vestibular system, but it translates into discomfort or resistance to weight-shifting in the proprioceptive system, because there is a vestibular-based challenge around midline and control, and the weight shift is perceived as threatening to my stability and my sense of myself in space.
Tracy: Maybe, and then it becomes phasic, because it is like, ooh, I will just go right over to the other side really fast. So yes, in the proprioceptive system you can see modulation-based problems with movement patterns, but it is based more in phasic movement fuelling the higher-level proprioceptive pathways, while the tonic holding patterns fire more into the anterolateral pathway, so you see a resistance to maintaining tonic holding patterns in the proprioceptors.
Cory: It is surprising how the vestibular input disrupts the proprioceptive modulation function, because you get a perception of unsureness, or weakened vestibular processing, and that influences weight-shifting, which is a proprioceptive, tactile, somatosensory function. So if I rapidly move my body in space, I fire more into the dorsal column medial lemniscus pathway, which is more discriminative, so I give myself more body awareness. But if I tonically hold my body in a position, I am not getting as much discriminative input, is that right?
Tracy: Yes. If you really tonically hold and you do not have the little phasic adjustments, the tonic can habituate quickly so you lose the source of information about where you are, and the tonic proprioceptors help activate the inhibitory, GABA-based functions that hold deep down-regulation, so they set up habituation cycles and are involved in a lot of the modulatory functions. There are different gating mechanisms, and gating can happen in lots of ways. If you stub your toe, you shake your foot to activate phasic gating, but you could also hold pressure and it might down-regulate rather rapidly, so we have different ways of shifting the intensity of response, related more to the modulation functions, organised by shifts in relative states of comfort or discomfort. If you are upright and activating tonic activation to hold yourself, and then you get engaged in something, you stop maintaining that activation and slide out of it, and then you start to feel that little discomfort Michelle described, I need to weight-shift because I am sagging on my bones, in my joints, and that discomfort is the signal that says, activate again, because the tonic receptors do not stay active without activation and tend to mobilise inhibition. The phasic receptors are quickfire, you cannot maintain quickfire, you fire and recover and fire and recover. But the detail, I quickfire because I reach out in space and touch the object I need, then use it, that is the perception-action cycle, and it sets up the action pattern that changes the firing, the feedback, the sensory signal, so you can have another phasic action in response to the next thing. People very skilful at motor action are continually responding to the update of the phasic firing pattern, and the proprioceptors in the dorsal column keep giving slightly different information, generating a fine-tuning of the motor response. So there are two different pathways, two different systems we always need to talk about, because it is complicated to translate into what we see in kids, is that modulation, is that discrimination? That is why we need good tests and strong clinical reasoning to sort it.
Cory: In the ideal situation it would be a blending of tonic and phasic, right, because maybe I need to tonically hold my core and phasically move my arm.
Michelle: It always gets complicated. That comment you made about discerning and being really specific in our observation and testing, for me that is why I do not bundle them together a lot in my thinking, and probably when I am mentoring staff I pull it out, so tactile is separated from proprioceptive. When I put it together it gets a bit enmeshed. In those examples you provided, Tracy, you do well to pull tactile out and pull proprioception out, as we would with vestibular and vision, knowing that they work together. In my mind I was thinking about holding a posture, the muscle spindles really activated to hold me up, and as I get engrossed in what you are saying, I collapse down, the muscles are stretched, my skin feels stretched so I get tactile input, but then I get to end range in my joints, so the proprioceptors say, bone on bone, end range, perk back up, activate muscles, sit back up. So they work together, but in the clinic I want to watch the child in terms of tactile and in terms of proprioception. When Cory and I did the SOSI with Erna Blanche, she was pulling out what some of the traditional clinical observations merge, tactile, proprioception, praxis, there is too much in that action to say it is purely a test of tactile or proprioceptive function. I came back from a block of intensives and everyone was saying somatosensory processing, and it was like, whoa, who has been here, Tracy? Cory introduced the term. I know it works together, but it helps me to be really specific by not always, in the initial stages, considering it as a unit.
Cory: What do you look at, then, Michelle, if you are thinking specifically about tactile, because you said in the clinic you want to look at tactile processing and proprioceptive processing? Are there distinct things you watch out for that might signal a tactile component is being disrupted, or something is going on with the proprioceptive system?
Michelle: I cannot really think of it off the top of my head. For tactile, I did not have a specific example, other than an unrefined movement in terms of interacting with objects makes me wonder about the tactile system, although that could be proprioception too. For proprioception I always wonder about it in the grading of force and how much pressure they are using, that seems more significantly proprioceptive than tactile. They both always play a role, because of the homunculus they both feed in. Tactile is easy in that if their clothes are bothering them, that is the modulation part, and the discrimination part is, can they discern in a little bunch of beads, can they pick out the thing that is not the bead. So tactile, in sequential finger touching, I look at that, but I really look at the force they use to bump their index finger to their thumb, and the location, which is informed by tactile, so that speaks to me as more proprioceptive than tactile. I try to pull some of those out and then blend it together. It does not always help me, because even when you get into supine or prone positions, some of what we do in the clinical observations has such a praxis and following-verbal-command element to it.
Tracy: It is so interesting to think about where the division is between tactile and proprioception, and sometimes it is hard to separate, partly because once they start getting processed they are really partners. Let’s think about a quintessential activity, like playing in the beans and trying to find something. As the child explores, exploration promotes the discriminative system to engage, to tune in and be motivated to find the thing, to make the discernment between the little spider or rabbit I am looking for versus the rest of the field I am feeling, a figure-ground tactile discrimination. And the proprioceptors are grading, the grading and timing are a bit more proprioceptive in nature, while the fine-tuned response of using the distal part of my finger to create a really fine pincer grasp to find that thing is more tactile. When kids have tactile difficulties, their whole hand operates almost like it has a mitten on it, it does not have that fine quality of making little discernments, so you see more ungraded motion, but also an inability to get the quality from the tactile system that would help the proprioceptors refine. So they work as a tandem. I also think about, as you reach out in space, can you maintain the trajectory of the limb, the directionality, the bilaterality, those are more proprioceptive, but once you get to the object, does the hand configure into a shape to pick it up automatically and quickly, that is more a combination of tactile and proprioceptive guiding it, and when you go to pick it up, it is the tactile system helping guide the finger action. It is hard to say all this in words, this is one of those things where a podcast could be a video, so let me show it. If I am picking this up, my hand should be in the shape of the vessel way back in the reach, and that is proprioceptive, but once I get there, the fine-tuning of how hard, the heaviness, is proprioceptive, while my fingers touching it and conforming to the shape is more tactile. So it depends on the function within the function.
Cory: In my development, when I interacted with objects such as your water bottle and tactilely received information about that object, has that informed my ability to preemptively use my proprioceptive system to shape my fingers, knowing the water bottle is round? I know what it feels like in its sensory elements, it is embodied in me, and now I use that to guide my proprioceptive system to anticipate that I need to shape my hand in a rounded shape to pick it up. Is that how we develop this?
Tracy: Yes, it is how we develop it, through this process of affordances, that is the word we use. The affordances of what we have capacity for, what we understand, and what is available out there in the world shape how we build our schemas and capacities. But say this is a metal water bottle. I remember working with a kid years and years ago, I had a metal water bottle, and he would never touch it, because to him metal did not go with a drinkable container. In the way he had coded the world, a drinkable container had to never be metal, and metal was clearly not a safe thing to drink from. So the affordances were telling us about information and meaning on levels that interact with all of this. It is a tangential story, but it tells you how affordances bring meaning, and meaning comes from the sensory-motor experience.
Cory: I am thinking of a water bottle that is soft plastic with a bit more resistance, not as slippery, versus a metal bottle, particularly in cooler climates, where it has the temperature input and feels more slippery. So proprioception comes in to shape the hand to grab the bottle, tactile says, whoa, it is slippery, put some more pressure on, and then tactile says, got it now, thanks, and helps grab a bit of weight, that is a precise amount of weight, team, we got the drink bottle to the mouth. I guess this gets disrupted when you do not have good quality information, or maybe it is not the receptor itself but the quality being lost in the integration. I do not understand all of it, but the outcome is that it is hard to shape my hand in fine, nuanced ways or adapt to the feedback as I get it. The reason it is so fascinating is that right now Audrey, my baby, is doing all of this, missing her mouth, tipping things too far, and it is so fun to watch, because it happens, and then it refines. Every time she is curious about the way it did not happen the way she thought, there is a tuning in, and sometimes it is fun that it did not happen that way, now it is a new thing. There was clearly an intent and a plan, to bring the cup to her mouth and drink, and sometimes the top of the cup hits her mouth and she tips the water onto her shirt, just coordinating all the actions, but there is ongoing feedback between what happened, what did I get, how do I respond, will I do it differently this time? If there were not adequate integration, or potential for it, it would not continue to refine over time.
Tracy: Some of the individual differences we have not begun to study enough. Some of it is that different receptors have different speeds of processing, which creates this micro-sequence in the way it gets integrated, just as you described, proprioception tells tactile, tactile tells proprioception, the tactile of getting the vessel onto your lip triggers the proprioceptive, then the tactile, interacting in a sequential way, and the different receptors actually fire at different speeds that have to stay organised in sequence. One of the primary problems with integrative processing is that sequences get out of whack, so you see miscoordination, and we just refine it, oh, I need a little more tactile, or a little more proprioception to sort it out. So it drives almost micro-moments of sensory seeking, I need a little more tactile to find where the bottle is on my lip, I need to squeeze it a little harder to control it in my hand, a continual feedback back and forth that smooths out the coordination. We have little micro-moments of miscoordination all the time that get refined, and as we practise, rehearse and repeat, the chances of miscoordination diminish, because the nervous system says, I know how to do that pattern, I know what it means to pick a vessel up from the table and bring it to my mouth. So the toddler learns to drink and then quits spilling, because now they know how much force to put the cup against their lips so they do not dribble, all based on the sensory-motor experience. It is very cool how it refines.
Cory: And it is cool how toys for babies are naturally graded, not all of them, but a lot. They are bigger, or have bigger handles, puzzle pieces, the object that goes onto something else, a lid. A classically graded baby toy is bigger and easier to hit the target, so they get the idea of how to do the action, and then it grades down so they can do it in a finer capacity. It is what we do as OTs in grading the task, grading it to the appropriate capacity for success.
Michelle: Praxis first, then. Let’s talk about the praxis categories, perhaps let’s start there.
Tracy: So the clusters. Ayres did this research to determine the relationships between the different kinds of basic, especially discriminative, processing, and how they gave rise to different functional capacities, working in reverse, by identifying the problem area she identified the cluster. Somatodyspraxia is a body-based coordination problem, the most classic dyspraxia, where you have a hard time knowing where your body is in space, how your limbs move, how the coordination works. You end up seeing a reduction in tactile discrimination, localisation of touch, identifying moving touch, can I tell if you touched me on the right or left side of my hand, closer to my elbow or my shoulder. Most of us take body schema for granted because we are refined at it, so when you meet a child who literally cannot tell if they are supposed to move their foot or their hand in order to climb, because their body map is so unrefined that their hand and foot are almost the same thing, those are the kids who respond beautifully to our interventions, because we help them have the tactile, proprioceptive, vestibular, social, playful, exploratory experiences that fine-tune the homunculus, this body map in the somatosensory cortex, the thalamus, the cerebellum, that allows skilful actions. When that map is not fine-tuned, figuring out how to kick a soccer ball, cast a fishing reel, zip a zipper or pick up a cup of water without spilling is super hard. Somatodyspraxia is a true disruption, more than a coordination disorder, a somatosensory-based difficulty with coordination, a specific, real entity we can identify, and through the EASI tests, or the old SIPT, we can get really precise information and target treatment much more precisely.
Cory: So the EASI and the SIPT would help you discern the clusters out for treatment planning, the pinnacle of tests. But there are other published resources?
Tracy: Yes, you have to understand what you are looking for. You can test tactile discrimination from other tests, and link it to coordination function through other tests, and in the Sensory Integration: Theory and Practice textbooks there are descriptions of how you might use other tests if you do not have those at your fingertips or have not been trained in them. We do not want to limit practice by not having that training, and we want to encourage the highest level of training, so it is both. The next cluster is a really interesting one, a praxis on verbal command problem. You see the same body-based awareness difficulties, but it shows up when you use language to guide the child to use their body in space. You say, put your right hand on top of your head and your left hand on your tummy, and it is not a language-processing or cognitive issue, but they cannot figure it out, they start groping, wait, which hand where, and maybe they put their hand on their face instead of the top of their head. So location of the action is confusing, a really particular cluster, and they can do it readily on imitation, but if you say the words, it is, what are you talking about. Then Ayres identified a couple of other clusters. One is more vestibular-based, but you do see proprioceptive difficulties in it, a problem of bilateral motor coordination, where you also see a lot of oral-tactile weakness, and the midline of the body is organised by the vestibular system, so the speculation is that anything more midline-oriented is harder for the bilateral integration and sequencing problem.
Cory: The acronyms I see are BIS, bilateral integration and sequencing, and BMC, bilateral motor coordination, but they refer to the same thing?
Tracy: Same thing, yes. It is balance-based, you see really classic vestibular-based processing issues, and the bilateral coordination, often the right-left coordination, is strikingly difficult, but bilateral coordination could also be moving from the front of space to the back, or top to bottom, anywhere there is a midline you see a breakdown in the smoothness and accuracy of coordination in those planes of movement.
Michelle: So how would you discriminate this in the testing profile, the vestibular loading versus the tactile?
Tracy: That is exactly right, that is how you would discriminate it: the vestibular loading, and the functional capacity for bilaterality and balance, are the hallmarks of that, and there are partners to the vestibular system, so they may be weak as well. But the hallmark issue would be vestibular, versus somatodyspraxia, where the hallmark issue would be proprioception and tactile, and the vestibular system may be a bit weak but you see more loading of difficulties in the somatic sensory processing. And then you can have a generalised global dyspraxia, where all of those issues show up simultaneously. But these are different clusters, you see groups of children or adults who fall out in a really clean way into them, so they really do exist, they really are difficulties we can describe and identify, and then we can have precise treatment plans that map directly onto those profiles. That is what is so powerful about knowing the profiles.
And that’s a wrap on today’s episode of Spirited Conversations. We hope this sparks something for you, whether it’s a new clinical idea, a fresh perspective, or just the reminder that you are definitely not alone in this work. If this conversation resonated, we would love for you to share it with anyone on their own learning journey. You can find information about the podcast on our website, and you can join us in the courses and communities the Developmental FX team have put together at developmentalfx.org. And if you’re enjoying listening, please subscribe or leave a review, it genuinely helps more people find us. Until next time, keep the conversations spirited!