Cognitive Science of Philosophy Symposium: Philosophy of Touch in the Laboratory

Welcome to the Brains Blog’s Symposium series on the Cognitive Science of Philosophy. The aim of the series is to examine the use of diverse methods to generate philosophical insight. Each symposium is comprised of two parts. In the target post, a practitioner describes their use of the method under discussion and explains why they find it philosophically fruitful. A commentator then responds to the target post and discusses the strengths and limitations of the method.

In this symposium, Tony Cheng (NCCU) surveys empirical work aiming to establish the existence of tactile fields and provides several conceptual and theoretical clarifications. Commentator Frédérique de Vignemont (Institut Jean Nicod) then considers whether we can tactually feel emptiness — and what that might mean for tactile fields.

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Philosophy of Touch in the Laboratory

Tony Cheng

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Philosophical Scepticism Concerning Tactile Fields

In his post “Philosophy of Perception in the Laboratory” (The Brains Blog, April 13^th 2021), Jorge Morales illustrated how age-old philosophical questions, in his case whether we literally see elliptical shapes when we see round coins from an angle, can possibly be answered with some helps from empirical methods. In this post, I will explain how a similar methodology, broadly construed, has been invoked to answer some specific questions in philosophy of the senses, notably spatial touch. Let’s begin with some historical background.

In his Individuals (1959), where a descriptive metaphysics programme is carried out, P. F. Strawson holds this view concerning spatial touch:

Evidently the visual field is necessarily extended at any moment… The case of touch is less obvious: it is not, e.g., clear what one would mean by a “tactual field.”

(Strawson, 1959, p. 65, emphasis added)

One can cast doubt on how Strawson thinks of the visual field, but for our purposes we shall focus on tactual or tactile fields. What Strawson says about it is actually quite weak: it is unclear to him what one would mean by that term. One natural reaction is that perhaps a “tactile field” means something similar to a “visual field.” This has been picked up by later philosophers, and they make stronger points based on this Strawsonian observation. Here are some prominent examples:

There is in touch no analogue of the visual field of visual sensations.

(O’Shaughnessy, 1989, p. 38, emphasis added)

[T]he visual field plays a role in sight which is not played by any sense field in touch. Touch is dependent on bodily awareness and if, or where, that involves a sense field, it does so in a strikingly different way from that in which visual experience involves the visual field.

(Martin, 1992/2011, p. 202, emphasis added)

[T]he structural feature of normal visual experience that accounts for the existence of its spatial sensory field is lacking in the form of bodily awareness involved when one feels a located bodily sensation.

(Soteriou, 2013, p. 120, emphasis added)

These passages all exemplify the Strawsonian observation, albeit in slightly different ways: while O’Shaughnessy commits to visual sensations, arguably Martin and Soteriou do not, for example. Also, Martin actually allows for certain notion of tactile fields; his view is only that if there can be said to be a tactile field at all, it works in a strikingly different way from the visual field. The trouble is that what counts as “strikingly different” can be up for grabs. In the 1992 paper, Martin concedes that there might be an “etiolated notion” of a tactile field: as long as there are tactile experiences that go beyond point sensations, it counts as having tactile fields. But of course that would be theoretically uninteresting. Inspired by this line of thought, cognitive neuroscientist Patrick Haggard and various team members at the UCL Action & Body Lab have designed and conducted a series of experiments that seek to examine such hypotheses concerning tactile fields. We will look into some basic outlines of such studies. In what follows I will refer to them as “the Strawson team” and “the Haggard team,” respectively.

Empirical Vindications of Tactile Fields

The first such study can be found in Serino, Giovagnoli, de Vignemont, and Haggard (2008). In this early study, the authors investigated if tactile stimuli on the palm can be perceived as complex stimulus patterns in accordance with basic spatial principles. In each trial, participants were asked to judge the intensity of a target stimulus to the palm, ignoring two brief preceding touches at nearby flanker locations. The basic finding is that the judgements of the target intensity were boosted by flankers when the target lay on the line joining the flankers in comparison to when the target lay away from this line. They therefore inferred that these tactile spatial pattern perceptions are sustained by tactile spatial organisations sustained by the skin, i.e., tactile fields. Before looking at their follow-up studies, one caveat is in order: it is possible that what the Haggard team means by a “tactile field” is different from what the Strawson team means by such term. Let’s set this aside for the moment, but we will come back to this crucial question in section 3.

The Haggard team soon did a follow-up study, mainly because the 2008 one did not reveal the spatial properties of tactile fields other than showing the importance of collinearity. In Haggard and Giovagnoli (2011), they further investigated the spatial properties of tactile fields by studying perception of large-scale tactile spatial patterns on the hand, arm, and back. This time they conducted a series of four experiments to look in more detail, and readers are encouraged check out the paper for details. For summaries, please see Cheng (2019) and Cheng (2020), though in those two papers I already embed the experimental details into various theoretical contexts. The Haggard team in this 2011 study drew the conclusion that tactile fields support computations of spatial relations between individual stimulus locations, and thus underlie tactile pattern perceptions.

I was involved in the third empirical attempt. In Fardo, Beck, Cheng, and Haggard (2018), we found certain path integration bias in tactile pattern perceptions. Path integration was originally a phenomenon found in animal navigation: when returning from the ending point to the starting point, many animals tend to exemplify certain inward bias. A common idea is that the systematic biases indicate genuine spatial representations. Now, the stimuli in this new set of experiments were continuous tactile motion stimuli consisting in curvilinear paths defined by two concatenated sine waves with varying width, height, and opposite phases, forming an asymmetric S-shape. Participants’ task was to bisect the S-shape. We found that participants consistently exhibited inward path integration bias, which is comparable with the bias exhibited in animal navigation. In the crucial control condition, where only the starting point and the ending point were touched, they exhibited no such bias. From the fact that one key spatial marker robustly obtained on parts of the human skin, we inferred that tactile fields also sustain such tactile pattern perceptions. We also have a theoretical paper that situates this study into wider theoretical contexts (Haggard, Cheng, Beck, and Fardo, 2017).

After that, we actually attempted a further study, and obtained some preliminary results, but for practical reasons it has not yet been finished. Cataldo, Cheng, Schwenkler, and Haggard (in preparation), which was initiated thanks to the SSNaP project at Duke University, was designed to specifically test whether the tactile system exhibits a preference for spatially structured tactile stimuli. We invoked simultaneous multiple stimuli constituting complex structures to test this hypothesis. It is reminiscent of the distinction between serial spatial representation and simultaneous spatial representation (Evans, 1980). Since the study has not been fully carried out, I do not review the results here. One notable point is that we used EEG measurements, unlike the previous studies, which were only behavioural. In Cheng (2019), I provide more details of this so far unfinished study. This following figure illustrates the basic idea:

This completes my all-too-sketchy introduction to the series of empirical studies on tactile fields. In the remainder of this post, I will discuss some clarifications of the issues, theoretical ramifications, and clues for future directions.

Clarifications and Ramifications

The first clarification concerns the very idea of tactile fields. One key is that whatever they are, they are not receptive fields: “receptive field” is a well-established physiological notion, but it is not the target of the current debate. In the 2008 study, the Haggard team says that in this context, a field “refers to a representation in which perceptual contents have spatial properties and relations which derive from the spatial properties and relations of corresponding stimuli” (Serino, Giovagnoli, de Vignemont, and Haggard, 2008, p. 355). This does not solve the problem, as “representation” is a technical term that tends to generate more problems than it solves. Even more confusingly, earlier the Haggard team published a paper on tactile receptive fields (Haggard, Christakou, and Serino, 2007). Now this is how I understand tactile fields: they are psychological constructs postulated to explain behavioural results, just like attention and working memory. These constructs are psychologically real, and they have physiological underpinnings. In the case of tactile fields, their physiological underpinnings are “skin space,” understood as a flattened receptor surface or sheet (Haggard, Cheng, Beck, and Fardo, 2017). Whether this understanding of tactile fields is the same as the Strawson team’s understanding needs further investigations.

The next clarification concerns the controversial notion(s) of Gestalt. Tactile pattern perceptions naturally lead to discussions of Gestalt. In their 2008 study, the Haggard team seemed to endorse the idea that tactile Gestalts are involved in these perceptions (p. 358-359), but in the 2011 paper, they seemed to take it back:

This finding goes against the view that tactile spatial patterns are based on grouping of individual stimuli, and come to be perceived as a gestalt. Gestalt perception would imply that the pattern is perceived as a single perceptual unit, over and above the sum of individual stimulations. We found no evidence a tactile gestalt in this sense, although our data do not rule out the possibility.

(Haggard and Giovagnoli, 2011, p. 68)

However, in the fourth, unfinished study, at least in the manuscript, we are playing with the idea that the data should be explained by tactile Gestalts. I have not made up my mind here; this needs further conceptual clarifications.

There are two theoretical ramifications that I will only mention briefly. One is about bodily self-awareness: “In touch… the linkage between primary experience and self-consciousness seems stronger than in vision. The linkage shows that the body is a physical as well as a psychological object. In this sense, tactile pattern perception presupposes a self that is an object embedded in the world, rather than simply a view point on the world” (Haggard and Giovagnoli, 2011, p. 74). We further develop this line of thought, together with some discussions of body ownership, in Cheng and Haggard (2018). Also concerning Molyneux’s question, I argue that the similarities between visual fields and tactile fields support the positive answer of certain version of the question (Cheng, 2020). For recent discussions of these issues, see Cheng (2022a), Green (2021a, 2021b), and Skrzypulec (2021, 2022).

Roads Ahead

In closing, I provide two clues for future directions, one philosophical and one empirical. On the one hand, as we have seen in the previous section, there have been some further discussions of issues concerning tactile fields, including those about bodily self-awareness and Molyneux’s question. However, the Strawson team, notably Martin and Soteriou, has not attempted to respond to the Haggard team. Perhaps they feel there is no need, because the two sides have different theoretical concerns. But it would be helpful if the conversation can be bi-directional. In particular, it would be helpful if they could point out whether there are subtle differences in our understandings of tactile fields, and if not, how they would respond to the empirical challenges.

On the other hand, this line of tactile fields studies, though designed to respond to the Strawsonian observation in philosophy, has attracted attention in the empirical literature too. In their monograph In Touch with the Future: The Sense of Touch from Cognitive Neuroscience to Virtual Reality (2014), Alberto Gallace and Charles Spence notice the above 2008 and 2011 studies, and after summarising the results, they have some interesting suggestions:

Given the suggestion that both the visual and tactile field might be based on “spatial” Euclidean rules (Serino et al.), one might wonder whether or not there are different “fields” for each sensory modality, or whether instead a unified multisensory/amodal field might exist (cf. Blumenfeld, 1936). That is, would two visual stimuli and one tactile stimulus automatically be organized into a triangle as a function certain conditions of stimulus presentation? And, if the answer of this question should be found to be in the affirmative, would the constraints affecting the organization of visuotactile stimuli act at a low or high level of information processing?

(Gallace & Spence, 2014, p. 61)

This is reassuring in that even if the attempted interdisciplinary conversation with the Strawson team is not entirely successful, at least the empirical studies have had some impacts in cognitive psychology and neuroscience themselves. Together with the ensuing philosophical works on bodily self-awareness, Molyneux’s question, and other sensory fields (Wilson, forthcoming), we can be confident that this series of empirical studies on tactile fields has provided rich ground for further investigation, both in philosophy and in the relevant empirical sciences.

Postscript

I thank Frédérique de Vignemont, who has taught me much about bodily awareness and somatosensory senses via talks and writings, for being willing to comment on this post. My IEP entry on bodily awareness (2022b), for example, emulates her excellent one for SEP, though I have tried my best to avoid repetitions.

One specific question I have in mind for Frédérique relates to her involvement in the 2008 tactile field study, which she has not since followed up on. From some of her writings, I suspect that she has turned her back on the earlier view. It would be extremely valuable to have her insights on this. I will respond to her commentary in the comments section below.

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Commentary: Tactual field and tactile void

Frédérique de Vignemont

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These last few years, there has been a new surge of interest for the notion of tactile field (e.g. MacCumhaill, 2017; Richardson, 2011; Skrzypulec, 2022; Munro, 2021; Serrahima, 2022; Haggard et al., 2017; Cheng, 2019). Tony Cheng brings an empirical perspective to it, describing the many interesting results collected over fifteen years by Patrick Haggard’s team, making the issue hopefully more tractable, but possibly more complicated too. Indeed, we now need to answer three related questions: 

1. Is there a tactile field?
2. If so, to what extent does it have the same properties as the visual field?
3. Does what Haggard calls the skin field correspond to the philosophical notion of tactile field? 

Here I shall focus on the last question brought to us by Tony. Since he did a perfect job describing experimental findings on the skin field showing spatial pattern recognition and path integration biases in tactile perception, I shall not recapitulate them. Instead, I shall simply analyse how they answer Martin’s (1992) original worry about the notion of tactile field. In his seminal paper “Sight and Touch”, he compares seeing a ring-shaped object, as a Polo mint, and grasping a glass in one’s hand.

In both cases, one is aware of the circular shape but the way the shape is presented differs. I am visually aware of the ring of the Polo mint, but also of the hole inside, on the background given by my visual field, defined as the spatial array of visual impressions. By contrast, when I feel the glass between my fingers, I am tactually aware of its circular shape, but not of the hole inside. My awareness of the hole depends on the awareness of the relative position of my fingers. Martin concludes that bodily awareness plays the role of field for touch.

What is at stake here is not whether tactile experiences are spatially organized — Haggard has convincingly shown that they are — but rather how we tactually experience regions that are not tactually stimulated. Furthermore, the question is not whether we can feel tactile presence in the absence of tactile stimulation by some form of tactile (or amodal) completion. Again, we know that this is possible, as illustrated by the cutaneous rabbit illusion: repeated rapid tactile stimulations at the wrist, then near the elbow, can create the illusion of touches at intervening locations along the arm, as if a rabbit were hopping along it. The question is whether we can tactually feel emptiness. I shall call Martin’s argument the tactual void objection.

This, I believe, is not what has motivated Haggard’s research so far. Still, could it be empirically investigated? It is important to distinguish three possible types of awareness of tactual void.

• Proprioception-based awareness of tactual void: In Martin’s glass example, different non-continuous body parts are tactually stimulated together. Another example is the climber case: can she tactually feel the space between her stretched hands and feet? Martin (1992) replies negatively, arguing that the awareness of the emptiness is mediated by the proprioceptive awareness of the relative locations of the body parts.
• Skin-based awareness of tactual void: In Haggard’s experiments, tactile stimuli are presented within a continuous skin region, such as the palm or the back. Even though they could be across a joint like the wrist, the surface of skin that is stimulated remains a continuous sheet. Imagine now that instead of seeing the polo mint, you feel it being pressured on your palm.[1] What do you feel? Arguably, you feel the ring. You are tactually aware of its circular shape. But do you feel the hole inside? If by hole, one means its linings, what delineates it, then yes, you do feel the hole. But if by hole, one means the emptiness inside the ring, then it is question-begging. One can distinguish the ring from a solid full circle, but this discrimination can be based only on the differences in the respective borders of these two shapes. It does not show that one feels the tactual void. Likewise, being able to discriminate two tactile stimulations as being distinct is not the same as to experience the emptiness between them as such. The problem here is that it is introspectively difficult to distinguish between tactile experiences of an empty space and the absence of tactile experiences. Could we experimentally take the two apart? This is a real methodological challenge, but I do not doubt that if someone can do it, it will be Patrick Haggard and his team. As far as I know, this hasn’t been done yet.
• Expectation-based awareness of tactual void: Imagine now that while blindfolded you walk in a room, exploring it with a cane. At last, it seems that we have here a clear case in which you do tactually feel absence, by experiencing the lack of resistance (Martin, 1996; MacCumhaill, 2017; Farennikova, 2013). But is this awareness given by touch itself? On one interpretation, you feel the absence because of some kind of error signal indicating the falsity of your tactile prediction that there may be something blocking your way. Arguably, if you did not anticipate the possibility of obstacles, you would not have such an experience. Nor would you have it if you were not moving. Would that reply to Martin’s original objection? I doubt it because it involves much more than touch itself.  

To conclude, it seems to me that if Tony wants to claim that Haggard’s findings are relevant for the philosophical debate on the tactile field, he has to show that the capacity to perceive emptiness is not a necessary requirement for the notion of sensory field. This discussion, however, can only be conceptual, and not empirical.  

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[1] One might wonder why Martin did not use this example, which allows for a more direct comparison with vision.

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References

Cataldo, A., Cheng, T., Schwenkler, J., & Haggard, P. (in preparation). Constructing the tactile field: Somatosensory gearing for spatial structures.

Cheng, T. (2019). On the very idea of a tactile field, or: A plea for skin space. In T. Cheng, O. Deroy, & C. Spence (Eds.), Spatial senses: Philosophy of perception in an age of science. New York: Routledge.

Cheng, T. (2020). Molyneux’s question and somatosensory spaces. In G. Ferretti & B. Glenney (Eds.), Molyneux’s question and the history of philosophy. New York: Routledge.

Cheng, T. (2022a). Spatial representations in sensory modalities. Mind and Language, 37(3), 485-500.

Cheng, T. (2022b). Bodily awareness. The Internet Encyclopedia of Philosophy. Retrieved from: https://iep.utm.edu/bodily-awareness/

Cheng, T., & Haggard, P. (2018). The recurrent model of bodily spatial phenomenology. Journal of Consciousness Studies, 25(3-4), 55-70.

Evans, G. (1980). Things without the mind. In Z. van Straaten (Ed.), Philosophical subjects: Essays presented to P. F. Strawson. Oxford: Clarendon Press.

Fardo, F., Beck, B., Cheng, T., & Haggard, P. (2018). A mechanism for spatial perception on human skin. Cognition, 178, 236-243.

Farennikova, A. (2013). Seeing absence. Philosophical studies, 166(3), 429-454.

Gallace, A., & Spence, C. (2014). In touch with the future: The sense of touch from cognitive neurosciences to virtual reality. Oxford: Oxford University Press.

Green, E. J. (2021a). The puzzle of cross-modal shape experience. Nous, online first.

Green, E. J. (2021b). Representing shapes in sight and touch. Mind and Language, online first.

Haggard, P., Cheng, T., Beck, B., & Fardo, F. (2017). Spatial perception and the sense of touch. In F. de Vignemont & A. J. T. Alsmith (Eds.), The subject’s matter: Self-consciousness and the body. Cambridge, MA: MIT Press.

Haggard, P., Christakou, A., & Serino, A. (2007). Viewing the body modulates tactile receptive fields. Experimental Brain Research, 180(1), 187-193.

Haggard, P., & Giovagnoli, G. (2011). Spatial patterns in tactile perception: Is there a tactile field? Acta Psychologica, 137(1), 65-75.

Mac Cumhaill, C. (2017) ‘The tactual ground, immersion and “the space between”.’, Southern journal of philosophy., 55 (1). pp. 5-31.

Martin, C.B. (1996) How it is: Entities, Absences and Voids’. Australasian Journal of Philosophy, 74(1), pp. 57-65.

Martin, M. G. F. (1992). Sight and touch. In T. Crane (Ed.), The contents of experience. Cambridge: Cambridge University Press. Reprinted in F. Macpherson (Ed.), The senses: Classic and contemporary philosophical perspectives. Oxford: Oxford University Press.

Morales, J. (2021). Philosophy of perception in the laboratory. The Brains Blog, https://philosophyofbrains.com/2021/04/13/cognitive-science-of-philosophy-symposium-philosophy-of-perception-in-the-laboratory.aspx

Munro, D. (2021). Visual and Bodily Sensational Perception: an Epistemic Asymmetry. Synthese, 198, 3651–3674.

O’Shaughnessy, B. (1989). The sense of touch. Australasian Journal of Philosophy, 67(1), 37-58.

Richardson, L. (2011). Bodily Sensation and Tactile Sensation. Philosophy and Phenomenological Research, 86(1), 134–154.

Serino, A, Giovagnoli, G., de Vignemont, F., & Haggard, P. (2008). Spatial organisation in passive tactile perception: Is there a tactile field? Acta Psychologica, 128(2), 355-360.

Serrahima, C. (2022). Sensory fields: the visual and the bodily. Philosophical Studies, 1-22.

Skrzypulec, B. (2021). Spatial content of painful sensations. Mind and Language, 36(4), 554-569.