# Feeling presence in the dark

Angèle Pillot and Frédérique de Vignemont

## Abstract (154 words)

In this paper, we show that there is a distinctive mode of spatial awareness in blind individuals, which we call sense of volume, that is not to be confused with echolocation based on self-generated sounds. It is based on the analysis of variations in the ambient sound field and it provides locational information about objects in one's surroundings. We propose that the sense of volume offers a primitive contact with the outside world. It does not give access to perceptual objects as such, but it should rather be understood as a rudimentary sense of presence that expresses that there is something out there at a certain distance and in a certain direction. We conclude by highlighting its tactile quality. More specifically, we argue that the sense of volume consists in a form of *tactile anticipation*: when one experiences something as being out there, one tactually expects that one could come in contact with it.
Poor young fellow! How on earth did he know that van was there? Must have felt it. See things in their forehead perhaps: kind of *sense of volume*. Weight or size of it, something blacker than the dark. Wonder would he feel it if something was removed. Feel a gap. (Joyce, 1922, p. 173, our underline)

In the rich opus of *Ulysses*, James Joyce tries at one point to capture the spatial phenomenology of a blind man walking in Dublin. The man has a long cane that gives him information about the surface that he is in contact with, but his sense of space is not exhausted by the tactile sensations that he experiences at the tip of his cane. In addition, he has what Joyce calls a ‘sense of volume’, which involves no intermediary tool. He knows that there is something in front of him without being in contact with it. How? Joyce offers no definite reply. The blind man may possibly ‘feel’ it or ‘see it in his forehead’. This form of spatial awareness is not the mere product of the writer’s imagination. It has been described as early as Diderot (1749) in his *Letter on the blind for the use of those who see*. For instance, Diderot reports the case of the blind mathematician, Saunderson, who could recognise places he had once visited “by the sound the walls and pavement reflected” (p. 116). This was the result of a unique ability to detect material objects, such as walls and buildings, to localize them, and even to discriminate what they are made of. This may sound like a cheap trick or some kind of extrasensory ability but this special form of awareness of the outside world is well known to professionals working with visually impaired individuals, though largely neglected in the experimental literature. The objective of this paper is to offer the first philosophical attempt to analyze its nature, determining what information it carries and what sensory modality it involves.

Our paper will remain largely exploratory, based on subjective reports from visually impaired subjects – including from the first author of the present paper (Pillot et al., 2023) – and on a relatively small experimental literature (e.g., Supa et al., 1944; Seki et al., 1994; Ashmead and Wall, 1999; Seki and Nakamura, 2000; Seki and Ito, 2022; Kritly et al., 2021).
In the first section, we will briefly review what is known of echolocation in humans and show
that the sense of volume shares with it some features, but still constitutes a distinctive mode of
access to spatial properties that does not depend on the active production of sounds. We then
propose that it offers a primitive contact with the outside world. It does not give access to
perceptual objects as such, but it should rather be understood as a rudimentary sense of
presence. We shall conclude by highlighting its tactile quality.

**1. Human echolocation**

Thomas Nagel (1974) famously asked what it is like to be a bat. He chose this specific animal because of its distinctive mode of perceiving the world thanks to echolocation, which Nagel assumed to be alien to human beings:

> Bat sonar, though clearly a form of perception, is not similar in its operation to any
> sense that we possess, and there is no reason to suppose that it is subjectively like
> anything we can experience or imagine. (Nagel, 1974, p. 438)

Contrary to Nagel's assumption, we may be able to have some bat-like experiences in what is
known as 'human echolocation'. The term echolocation was first used by Griffin (1944) to refer
to the ability to detect and localize objects or surfaces on the basis of the acoustic properties of
the echo of self-generated sounds (acoustic shadow). Bats echolocate using biosonar: the
emitted signals are mainly in the ultrasonic range (approximately 20,000 Hz). Humans
echolocate using clicks generated with their mouth and tongue. Sound reflection causes mono-
and binaural variations in echo timing, spectrum (pitch) and intensity (loudness), from which
one can learn to extract different types of spatial information. For instance, one study compared
the performance of blind participants before and after learning and practicing echolocation for
10 weeks (Norman et al., 2021). It was found that at the beginning they were at chance level in
the size estimation of disks placed in front of their face but after 20 sessions, they were above 80% accuracy. Likewise, they were able to judge the orientation of a large rectangle (horizontal, vertical, right side up, left side up) and to navigate in different mazes (T-maze, U-maze, Z-maze) with significantly less collisions and more speed.<fnref n="1" /> One of the participants described her new life after learning echolocation as follows:

> For example, inside I now walk without trailing walls or furniture. I can walk with things in my hand and I use echoes (i.e. click-based echolocation) to locate doorways and also if for example cupboard doors are open or shut. When I hoover, I use it (i.e. click-based echolocation) to find the hoover again; taking rubbish out around the corner and locating the bins etc. (Norman et al., 2021, p. 26)

More generally, it has been found that thanks to human echolocation, one can make judgments about location (Thaler et al., 2011; Teng et al., 2012), motion (Thaler et al., 2014a), shape (Milne et al., 2014), size (Teng & Whitney, 2011), and texture (DeLong et al., 2007). Some expert echolocators are even able to perform accurate object identification (Thaler et al. 2011).

Spatial awareness in visually impaired individuals can be based on the reflection not only of self-generated sounds but also of ambient sounds produced by construction work, cars, footsteps, people speaking, music, and so forth (Ashmead et al., 1998). This spatial awareness is based on the analysis of variations in the ambient sound field, which provide locational information about objects in their surroundings, especially in closed environments. For instance, they can detect walls, buildings, vehicles, poles, bus stops, trees, and hedges (Ashmead and Wall, 1999; Carlson-Smith and Wiener, 1996). They can also detect reliefs on a surface such as a balcony on a façade. Two main characteristics have been ascribed to this type

<footnote n="1">¹ Interestingly, sighted individuals who participated in this study also showed improved performance.</footnote>
of spatial awareness: (i) the absence of sound production by the perceiver and (ii) the experience of tactile feelings when approaching a material object (Seki and Nakamura, 2000).

This form of spatial awareness has been relatively neglected and it is not even clear how to refer to it. Some have named it passive echolocation (Kritly and al., 2021). However, the metaphor of the sonar implied by the term “echolocation” may not be appropriate to qualify this ability that does not involve self-generated sounds (Ashmead and Wall, 1999; Schenkman and Nilsson, 2010). Others have called it “facial vision” (Dresslar, 1893; Supa et al., 1944; Ammons et al., 1953) because the phenomenology primarily consists in sensations on the face. However, this term assumes that it is visual and for now we want to stay neutral on this question. In this respect, the terms ‘sense of mass’ used in French (*le sens des masses*) and ‘sense of volume’ used by Joyce (1922) are more neutral. Both refer to architectural notions. Though there are many ways to interpret the distinction between the two, one may say that mass refers to the amount of matter that occupies space. It often implies bulk, density, and weight. By contrast, volume can also refer to empty areas contained by planes. Interestingly, blind individuals describe being able to detect holes and voids. For instance, Diderot noted that Saunderson could distinguish between a open street and a closed alley. Likewise, blind individuals report that they can detect the hole created by an entrance in a wall or sense an arch that creates a space of resonance. We can thus answer Joyce’s question: the blind man could in principle feel a gap. However, he might not be able to feel the presence of another person. Visually impaired individuals are not very sensitive to objects in the environment other than large urban elements. In particular, other people’s bodies are too small and the flesh is too soft, so to speak, to reverberate the sound in a distinct manner.

Here we shall tentatively suggest that the sense of volume consists in a specific form of sense of presence. The sense of presence has been originally described to refer to the kind of visual awareness that one has in real life seeing, when one is ‘face to face’ with an object, as opposed
to the kind of visual awareness that one has when seeing an object that is depicted in a picture (e.g., Noë, 2005; Matthen, 2010; Dokic, 2010). Compare the following two visual experiences. One sees a tree; one sees the painting of a tree. Only in the former case does one experience the tree as being present. The sense of presence makes one aware that one is located in the same space as the tree (sense of *spatial* presence). The sense of presence can also make one aware that the tree actually exists (sense of *real* presence): what one perceives exists independently of one's perceptual experience of it. We shall argue that the sense of volume is a *special* form of presence, a special way of being aware of mass and void, which differs from other sensory forms of spatial awareness.<fnref n="2" /> Our objective in this paper is to explore this special way to experience space. It is characterized by two main features. First, one does not experience the presence of a *perceptual object*; one only experiences the presence *of something*. Its intentional object is rudimentary and primitive. Secondly, its phenomenology is tactile. The sense of presence has been mainly described in the visual modality but arguably it is common to all the senses. For instance, compare the experience of listening to Wagner's opera of Siegfried in streaming and the experience of facing the orchestra playing in Bayreuth. In the latter case, you feel the musicians as being present, even with your eyes closed. One can then talk of auditory presence. Likewise, one may talk of tactile presence (Vignemont, 2021). Here we propose that thanks to the sense of volume, blind individuals are *tactually aware of the presence of something out there*.

## 2. A raw access to the world

Our first claim is that the sense of volume gives only a relatively raw access to the world, providing information about mass and void, but not about what some call perceptual objects.

<footnote n="2">² It has been primarily described in visually impaired individuals but preliminary work has been found that sighted participants can also experience it (Pillot et al. 2024).</footnote>
To perceive objects, one needs to perceive the entity as united and bounded, to expect it to persist through motion and changes, and to be able to compare it to other objects and decide whether it is similar or different (Spelke, 1985). As we shall now argue, it does not seem that the sense of volume affords such a perception because its content is too indeterminate to support the relevant forms of binding and type-identification.

Let us briefly return to echolocation. In her attempt to reply to Nagel's question, Akins (1993, p. 147) argues that bats do not have access to perceptual objects: "in all likelihood it does not use its sonar system to construct representations of the objects around it". On her view, echolocation is not to be compared to a diminished version of vision, with similar spatial content but only less fine-grained or with some peculiarities. Her conclusion is based on a principle of parsimony. She assumes that there is no need to posit internal models of objects to explain the relationship between the bat's sensory detectors and its behavior. To illustrate this, she imagines the case of what she calls a wingflapper, which utilizes relevant information supplied by various "dials" and "indicators" (corresponding to specific auditory information processing in the bat) in direct relation with the motor cortex. In her description, the "instruments" suffice to explain the way the bat catches a fly. It need not represent the insect as a 3-dimensional individual.

However, recent evidence on motion extrapolation in bats challenges Akins's radical conclusion (Salles et al. 2020). There is an acoustic delay inherent to echolocation, which results from the time it takes for a sonar call to travel through the air to the insect and the returning echo to arrive at the bat. In addition to the neural cost of processing the information, this can lead to over 100ms for each sonar snapshot. In the meantime, the insect location has likely changed in a more or less erratic way and it can possibly be hidden behind other objects. Yet, bats are extremely efficient at capturing insects. It is hard to see how Akins's alleged direct connection between the sonar system and the motor system can suffice to explain their success.
Instead, Salles and coll. (2020) argue that bats build up internal models of the trajectory of the insect based on the accumulation of acoustic snapshots through time, which are used to predict the on-going motion of the insect between each sonar call. In addition, when the insect is momentarily occluded, the bat can still use the internal models and find the insect back when it reappears. It thus seems that what is accessed through echolocation displays key features of perceptual objects, including object persistence, object tracking, and motion completion.

If we now consider echolocation in humans, visually impaired individuals face the same challenges as bats due to the acoustic and neural delays, and thus similarly need to build up internal models. Lopes (2000) further argues that thanks to human echolocation, visually impaired individuals have perceptual experiences whose content is partially similar to visual experiences, representing the location, shape, and size of objects in the way vision does. Nonetheless, there are differences between echolocation in bats and in humans, and in particular the fact that humans have to learn and practice before being able to use reflected sounds to get information about the environment. One may thus conceive human echolocation as a form of spatial *skill*, instead of spatial perception (Norman et al., 2021).

When it comes to the sense of volume, which also requires learning and practice, it is more difficult to determine whether or not it can represent perceptual objects. The sense of volume is not used to track small insects in motion; it is primarily about large static elements in the landscape. As a consequence, the sense of volume often gives access only to a small fraction of these large elements. For instance, while walking along a wall, one does not have access to the wall in its total height, though the whole wall affects the ambient sound. Blind individuals report that they are typically aware of the presence of only the portion of wall that corresponds to the area between their chest and the top of their head. They can move to extract more information but the range of exploratory movements that they can perform is limited. They cannot simply move away from the wall to have a larger grasp of it the way one would do for
vision, because if they go too far, they risk losing access to it. Even if they could explore the volume by moving around it, the question would remain: would that suffice to produce the experience of the *whole three-dimensional bounded and united object*? In order to reply, one needs to analyze in details what information the sense of volume carries.

The sense of volume informs one that there is something out there. It performs two main functions: detection and localization. However, its detection range is limited and varies depending on the richness of the surrounding environment and on the subject's activity. In non-ecological settings such as testing rooms, participants can detect volumes within a radius from one to two meters (Seki and Nakamura, 2000) but this can be explained by the relatively poor number of reflecting surfaces (Schenkman & Nilsson, 2010; Tonelli et al., 2016). By contrast, in complex urban environment, blind individuals can report the presence of volumes located up to 10 meters away. However, if they are walking while focusing on the nearby obstacles, the detection range shrinks to their immediate surroundings, up to two meters. Furthermore, the sense of volume does not cover what is behind them. For instance, if a silent object approaches behind their back, they do not sense it. It is actually on the sides that the sense of volume has been found to be the most sensitive (Seki, 1997).

Not only can blind individuals detect the presence of a volume, such as a large obstacle for instance, but they can also localize it and experience whether it is looming or receding (Seki and Ito, 2022). As a consequence, they can point in its direction and adjust their bodily movements to avoid it. Typically, they can walk towards a wall, stop before contact, and then move around it or along it. They can also assess whether an opening is wide enough to pass through (Gordon and Rosenblum, 2004). Localization can be both in egocentric frame of reference (relative to the subject) and allocentric frame (relative to other volumes). For instance, they can accurately sense that the material object is within reach, thus being able to judge its
distance from their body. They can also judge the relative position between two material objects, determining which one is in front of the other or to its left.

Detection and localization are basic sensory abilities but they do not suffice for *perceptual experiences of voluminous persistent three-dimensional objects*. One might claim, for instance, that a sunflower can detect and localize the sun. It should be noted, however, that blind individuals can have access to further properties of the volumes. For instance, they report that they can discriminate what the volume is made of, such as wood, concrete, stone, glass, metal, and vegetation. To each of these materials correspond different feelings. It can be a matter of intensity: a wall made of concrete is associated with a more intense feeling. The variation can also be qualitative, comparable to different timbres of musical instruments. Just as timbre describes the particular grain of an instrument, regardless of its pitch or loudness, there is a grain to the sense of volume that varies according to the material. In their study using echolocation, Delong and coll. (2007) asked their participants about the cues that enabled them to discriminate between different materials. Participants gave the following descriptions: the aluminum sphere was “percussive” or “flat”, the brass sphere was “hollow and distant” or “ringy;” the nylon sphere was “fuzzy” or “damp;” and the glass sphere was “sharp” or “crisp” (Delong et al., p. 613). In another study on the sense of volume this time, Kritly and coll. (2021) investigated wall texture discrimination using sound field variations. The impulse response of six different wall textures was recorded and then played back to participants in a listening test. The participants described the flat surface as “basic, short, natural”, the circular surface as "short, neutral", the parabolical surface as “full, rich, strong”, the crenelated obstacle as “soft, longer”, the wall with an aperture as “hollow, empty, weak” and the staircase as “bird, arrow” (Kritly et al, 2021, p.6).

We further know that blind individuals can successfully avoid obstacles that they detect through their sense of volume. This indicates that they have some rough estimation of their size.
However, the degree of precision of their metric judgments has never been experimentally investigated. The content may be relatively sketchy and rough-grained. There need not be a representation of sharp contours. One may further argue that there need not be a representation of a *persistent* object. A distinctive criterion for the ability to perceive objects as such is sensitivity to their permanence. For instance, it has been found that infants as early as four-months old are able to visually track an object when they see it passing behind another, the motion being partially occluded, and correctly anticipate its reappearance on the other side on the basis of its speed and path (Rosander and von Hofsten, 2004). However, one may doubt that the sense of volume affords motion extrapolation to the same extent. We saw earlier that thanks to the sense of volume blind participants could detect whether a wall was looming toward them or receding. They were thus sensitive to motion. However, what would happen if their ears were covered for a few seconds, thus blocking the acoustic signals? Would they expect the looming wall to be closer, thus predicting its trajectory on the basis of a mental model of its motion? Would they even be able to recognize that it was the same wall looming toward them? It seems unlikely.

To recapitulate, thanks to the sense of volume one can detect and localize mass and void. One can further feel when they are in motion and judge their direction. Finally, one can estimate their approximate size and discriminate what they are made of. These conclusions are still preliminary and relatively sketchy, primarily based on introspective reports since very few experimental studies have been done to assess the precise sensitivity of the sense of volume. Further empirical work would be needed to reach a more systematic analysis. For now, however, it seems that the sense of volume has a relatively indeterminate content.

### 3. What sensory mode?
Now that we have a better understanding of the type of information the sense of volume gives access to, we can ask: is the sense of volume a sensory or a cognitive phenomenon? And if it is a sensory phenomenon, what sensory modality best characterizes it?

One can think of two types of cognitive interpretations, either in terms of non-observational knowledge or in terms of indirect perception. The first hypothesis is that blind individuals only *believe* that there is an obstacle in front of them but they do not feel it. On this view, there is no phenomenological counterpart to the sense of volume. Their spatial belief about their surroundings is based on cues that are detected unconsciously. If this is a correct description of the sense of volume, then it qualifies as being *non-observational* in Anscombe's (1957) sense because it is not grounded in sensations. She argues, for instance, that knowledge of one's bodily position is without observation: one knows the one's legs are crossed, but one barely feels sensations that could explain one's knowledge. Whether or not this is true of bodily awareness (see Vignemont, 2018 for discussion), one may reject this interpretation for the sense of volume. It actually seems to get the phenomenon as the exact opposite of the way it has been described. Blind individuals do not report a clear knowledge associated to weak or even absent phenomenology. It is rather the reverse: sketchy knowledge associated to present – though possibly fuzzy – sensory phenomenology.

Alternatively, one may acknowledge the phenomenological basis but argue that the sense of volume is only a specific type of indirect perception. According to Dretske (2006), I can *indirectly* hear that the postman is coming because I *directly* hear that the dog is barking, and because I know that the dog barks each time that the postman comes. Likewise, one might just claim that one directly hears ambient sound on the basis of which one indirectly perceives that there is a wall. Alternatively, indirect perception could be based on tactile experiences. What matters is that in both cases, the sense of volume is not based on a direct access to the world. It requires additional knowledge and inferences. This interpretation may sound more promising
and at some level, it is true but it is not the level that we are interested in. Imagine the following
situation. A visually impaired person walking in a city knows that she must turn to the next
street on the right to arrive home. At one point she detects a modification in her experiences.
She becomes aware of the presence of a void here. She then senses that there is the street at
which she should turn. Her street awareness may qualify as indirect. She comes to this
awareness on the basis of her sensation of the void and of her knowledge that she feels that way
each time that there is a street opening. Her sense of volume does not suffice to provide her
information about items such as streets or other urban elements. Her awareness of these
elements is indirect. By contrast, her awareness of volumes and void, we argue, is direct. No
further knowledge is required.

If we now grant that the sense of volume is truly a sensory phenomenon, we need to
determine which sensory modality it involves. A similar question has been raised in the case of
sensory substitution devices. In tactile vision substitution systems (TVSS), a TV camera is
connected via a computer to an array of stimulators in contact with the skin of one part of the
body (e.g. abdomen, back, thigh, forehead, fingertip, or tongue). Visual information is then
converted into vibrotactile or electrotactile stimulations. It has been found that blind
participants can learn to successfully use this type of system to detect objects looming toward
them and to react appropriately (Bach-y-Rita, 2004). TVSS has given rise to a large debate as
to whether the participants have visual or tactile phenomenology associated with their new
spatial skill (e.g. Block, 2003; Hurley and Noë, 2003). There is no easy answer and some have
argued that it should be rather conceived in multisensory terms (Auvray, 2019). Could the sense
of volume, which is also hard to classify among the senses, be multisensory as well?

The sense of volume does not involve any substitution device but it seems to involve several
types of sensory cues like TVSS. The fact is that in ecological settings, one always has a
plurality of sensory experiences that are integrated together. For instance, Ammons and
colleagues (1953) ran one of the first studies to compare the sense of volume and echolocation using an outdoor obstacle detection task with blind participants. They found that although echo analysis was the most reliable and systematically used cue, other signals contributed to obstacle detection, such as skin pressure caused by the deflection of wind and the participants' own breath, solar radiation reflected by the obstacle or the obstacle's shadow, the smell of the surrounding objects, and so forth. One may be tempted to defend a strong interpretation of this finding, according to which the sense of volume is constitutively multimodal. However, the fact that multiple sources of information (acoustic, thermic, olfactory, etc) are combined to improve the reliability and efficiency of information processing does not suffice to prove the multimodal nature of the sense of volume. Although it can be hard to isolate what specifically comes from the sense of volume compared to other sources of information, one can still argue that there is a distinctive sense of volume endowed with a distinctive phenomenology, which cannot be reduced to a compound of multiple signals across various sensory modalities. We shall now analyse whether the sense of volume should be primarily understood in auditory, visual or tactile terms. To do so, we will use Grice's (1962) criteria originally proposed to individuate the classic five senses. He analyses each sense at four levels: (i) physical stimulation, (ii) sensory organ, (iii) properties, and (iv) phenomenal character.

In the 1940s, many agreed that the sense of volume was an auditory ability (e.g. Cotzin and Dallenbach, 1950; Supa et al., 1944). One can naturally reach this conclusion if one focuses on Grice's first two criteria. Though objects in the environment, such as walls, do not produce sounds, they still cause variations in the sound field (sound reflection, insulation, etc.) and the sense of volume is based on the analysis of these acoustic signals. As a consequence, if the environment is noisy, one can completely lose the sense of volume and be disoriented. The sense of volume thus responds to the same physical properties as audition. It uses also the same sensory organ. It is through the ears that the acoustic analysis is done. If the ears are plugged
or if one wears a hood or a hat, one can lose one's sense of volume (Supa et al., 1944). More generally, all phenomena that modify the diffusion of sound or saturate hearing can disrupt or prevent the sense of volume. For instance, it does not work in an aquatic medium, and it works only in an aerial medium. As already noted by Diderot, the sense of volume is affected by the smallest atmospheric change. In particular, if the weather is windy, one cannot localize volumes correctly. If it snows, the sensations are dimmer. The sense of volume thus shares the same enabling conditions as audition.

However, if we use Grice's other criteria, then it becomes less clear how to individuate the sense of volume. Consider the properties that it gives access to. Some of these properties, such as the relative distance to the volume, are typically accessible through vision and audition. However, others, such as texture, are primarily available to touch. Finally, there is the phenomenological criterion, which has played a central role in the discussion of sensory substitution. Visually impaired individuals do not merely *know* that there is a building in front of them. It does *feel like* something specific, though the distinctive phenomenology associated to the sense of volume is hard to capture. What is it like? Diderot described Saunderson's ability as follows: he “saw” by means of his *skin*” (p. 107, our underline). Does it mean that the sense of volume is experienced under the visual mode, possibly with the help of visual imagery, or under the tactile mode, many blind individuals reporting feeling something closely related to skin pressure (Arias, 1996)?

Let us briefly return to sensory substitution devices. One of them, known as “The vOICe”, converts visual images to auditory signals. It was found that after months of regular use, blind participants could report experiencing visual phenomenology (Ward and Meijer, 2010). If we now turn to echolocation, a study showed that it was correlated with the vividness of visual imagery in non-congenitally blind individuals (Thaler et al., 2014). In blind experts, another study found significantly higher activity in the visual cortex during the processing of echo
sounds as compared to the processing of echo-less control sounds (Thaler et al. 2011).
Interestingly, the same study showed that there was no differential activity in auditory cortex.
In the case of the sense of volume, however, no systematic study has been done and the precise
role of visual imagery remains unclear. Arguably, if it does contribute, it must be only in non-
congenitally blind individuals. Even then, it is hard to conceive how visualizing something in
one's mind could make one feel that this thing is real. It rather seems that it goes the other way
around. One feels that there is something out there, which can in turn produce the corresponding
visual imagery.

Here, we defend a non-visual interpretation of the sense of volume. More specifically, we
propose that the sense of volume is primarily a *tactile* sense of presence. This may seem
counterintuitive insofar as touch typically gives information about what is in contact with one's
body, whereas the sense of volume is about what is at a distance. However, as we shall now
see, one can anticipate in the tactile mode what is still at distance from the body.

4. Tactile presence

It is first interesting to note that the sense of volume can be disrupted if one wears something
that covers one's face, such as masks during the Covid-19 pandemic. Yet feelings of pressure
on the face are rarely discussed in the literature because they have been conceived as mere
synesthetic side effects. However, at the phenomenological level, these tactile sensations may
be the most salient ones, and the closer one is to a volume, the more vivid the sensations are.
These feelings are not intransitive, like tickles or itches. They are sensations of something out
there that causes them. They indicate a presence. We propose that they can have two origins,
namely, tactile stimulation and tactile anticipation.
Soundwaves can physically affect skin tissue, being then translated into tactile stimulations.
This can be compared to the bodily experience that one has in a concert hall. One may then feel
enveloped, immersed in an acoustic medium that creates the sensation of being in vibration
with the surrounding air. Another comparison can be made with the transmission of sound by
bone conduction when wearing headphones. One feels it on one's face, though one is aware
that it is caused by the sound. Likewise, the sensations are concentrated in the face area for the
sense of volume. Accordingly, the sense of volume is the most sensitive for objects located in
the azimuth plane at ear level. Those that are less high than the hip are difficult to detect, unless
by moving one's head and body so that one's ears face downward. By contrast, those that are
several meters above one's head, such as billboards, are beyond the reach of the sense of
volume, unless it produces a resonance, as in the case of a balcony or a roof.

The sense of volume can thus be associated to physical tactile stimulations but it has also
been found that anesthetizing the facial skin has no effect on the perception of obstacles
(Köhler, 1964). Hence, tactile stimulations are not necessary to the sense of volume. However,
there is another tactile dimension to the sense of volume, an anticipatory one, which we believe
to be central to the phenomenon and which is independent of tactile inputs. In the same way
that the brain can generate visual anticipation (for instance when an object momentarily
disappears behind an occluder), it can also generate tactile anticipation. Though one is not yet
in contact with an object, one can expect that one will soon be. There are no actual tactile
stimulations, and yet one experiences something like *pre-contact*, an anticipatory awareness of
possible contact.

The existence of such tactile anticipation has been empirically confirmed by studies on what
is known as *peripersonal space*. The term peripersonal space refers to the immediate
surroundings of one's body. It has been shown to be processed in a specific manner both at the
sensory and at the motor levels, by contrast with far space (for review, see Vignemont et al.,
2021). The term ‘peripersonal space’ was introduced by Rizzolatti and his colleagues in 1981 after discovering in monkeys bimodal neurons that fire both when a tactile stimulation was applied and when a visual stimulus close to the body was presented. In humans, it has been found that patients with tactile extinction after brain lesion are no longer able to detect tactile stimulation on one side of the body when tactile stimuli are concurrently applied on the other side, but also when auditory stimuli are concurrently presented in the immediate surroundings of the body (Farnè and Làdavas, 2002). One way to interpret this effect is that the brain anticipates impending contact on the basis of the auditory information, resulting into a competition with the actual tactile stimulation, and the subsequent extinction in these patients.

In healthy participants, it has also been shown that sounds in proximity can influence tactile processing (Serino, 2019). In one study, participants received tactile stimulation either on their left or right hand while simultaneously hearing a sound approaching either from the left or from the right (Canzoneri et al., 2012). It was found that they were faster at detecting tactile stimuli when the moving sound was coming closer to the body on the same side. It can be explained by the fact that what is close can be soon in contact with the body. The brain thus selected a specific predictive mechanism to anticipate the potential forthcoming contact in order to prepare in advance how to react. In brief, one does not wait until stumbling into obstacles, one can simply anticipate to avoid them.

Evidence shows that the boundaries of peripersonal space are malleable. For instance, it has been found that they expand either when one's bodily capacities are increased thanks to a tool (Farnè and Làdavas, 2000) or when one is anxious (Sambo and Iannetti, 2013). One can speculate that the absence of vision, which is the spatial sense par excellence, would also lead to the extension of peripersonal space. Roughly speaking, one needs more time to be prepared since one can no longer see the obstacle from afar. Consequently, we suggest that tactile anticipatory awareness is heightened in visually impaired individuals. It does not wait for the
last minute to occur, when objects are already within less than 1 meter from their body. In addition, whereas tactile anticipation normally remains below the threshold of consciousness in sighted individuals, it is plausible that blind individuals have conscious access to it, thus experiencing sensations on the skin, even though they are relatively dim. This might result into tactile imagery but there is no evidence so far. To be clear, those tactile sensations are not caused by physical tactile stimulations, but by the anticipation of possible contact with a material object in the immediate environment. Many have described how memory can contribute to perceptual experiences but the same story can be true also of anticipation. Munton (2022), for instance, compares the relation between perception and memory to a luggage carrousel, the two being so intertwined that it is sometimes difficult to take them apart. Here we argue that anticipation can also be in the carrousel. In peripersonal perception, the contact that one anticipates is absorbed in a multimodal experience in which one can no longer distinguish what is and what may soon be:

> predictions that are so tightly woven into one's current experience that unless something goes very wrong (as it does in the Helmholtz/Mach phenomenon), they are not experienced as separate predictions at all, but become silently absorbed into the perceptual event along with incoming sensory information, in a way that erases all record of their status as anticipations. (Grush, 2009, p. 198)

What is especially interesting in the case of peripersonal space is that the anticipation is on the tactual mode. Many have claimed in the history of philosophy that touch is the primary sensory modality because it is the only one to give us a sense of the reality of what we perceive (Armstrong, 1997; Condillac, 1754; Maine de Biran, 1812; Massin and Vignemont, 2020). For instance, it is said in the Bible that St Thomas would not believe that Jesus had resurrected unless he was able to put his finger in the holes left by the nails. In other words, he would not believe that Jesus was present and real unless he could touch him. In more technical terms,
"resistance to our muscular effort is the only sense which makes us aware of a reality independent from ourselves" (James, 1880). When we actively touch an object, we exert a force on it and feel not only our effort, but also the resistance of the object to our effort. Interestingly, the sense of volume is often described as an obstacle sense. An obstacle is something that resists, something that can even hurt. Sensing that something could block one's path is thus sensing that it is real. Resistance can also be understood in acoustic terms: the surface has a causal effect on the sound that it sends back. The sense of volume thus expresses that there is something beyond the perceiver, something that acts on the soundwaves, but that could also act on the perceiver. We purposefully use the term 'something', thus leaving relatively indeterminate what one experiences as being out there.

The sense of volume may be the most rudimentary tactile sense of presence. It is not yet about the presence of objects as such. It differs from the way presence has been analysed in the visual domain. According to Dokic and Martin (2015), the sense of presence consists in an affective experience distinct from the visual experience but still bound to it at the phenomenological level. For instance, when one is visually aware of a tree as being present, one visually experiences the shape, size, orientation, and colour of the tree, and additionally, one experiences the tree as being present. In principle, one could experience presence independently of any sensory experience. This characterization, however, does not seem to apply well to the sense of volume. It seems indeed difficult to clearly take apart within the sense of volume a full-blown sensory experience and a distinctive sense of presence. It rather seems that there is a unique experience, with a sensory content that is relatively poor, carrying only a limited amount of information, but presented under a special mode that has a strong assertoric force, to borrow Matthen's (2005) phrase. It expresses a brute, or a raw, presence. Thanks to the sense of volume, one feels spatially connected to a volume (sense of spatial presence). One also feels it as being real, independent from the sensations that it causes (sense of real presence).
As a consequence, it can have an effect on oneself, by blocking one's path for instance, and one must act in consequence.

## 5. A sense of absence

Before concluding, one should note that the sense of volume allows for the detection of mass, but also of void. If there is no mass in a specific region of space, blind individuals then describe experiencing a feeling of emptiness, whether it should be characterized in terms of presence of void, or absence of mass. Much has been said on the metaphysical and perceptual status of holes (Casati and Varzi, 1994), and we shall not venture in this issue here. We will simply analyse how one experiences voids in vision and touch before returning to the sense of volume.

Martin (1992) 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. Arguably, the sense of volume does not work on the visual model. Compare now with the tactile sense of absence. At first sight, it might seem that there is no such thing. When I feel an empty glass between my fingers, I am tactually aware of its circular shape, but not of the hole inside. The glass could be completely full without modifying my tactile experience because it is introspectively impossible to distinguish between the tactile experience of an empty space and the absence of tactile experiences. Still, there are situations in which one can tactually feel absence and it is when one expects something to be there and there is nothing (MacCumhaill, 2017; Farennikova, 2013). Imagine that you put your hand in a bag looking for the chocolate bar that you have just bought and you find that the bag is empty. You then experience the lack of resistance under your fingers. There may be other views but on one interpretation at least, you feel the absence because of some kind of error signal indicating the falsity of your tactile
anticipation that there should be something blocking your exploration. We propose that something similar can happen in the sense of volume. One experiences voids when one tactually anticipates the presence of a mass and one experiences the lack of resistance.

## Conclusion

Our first objective here was to show that there is a distinctive mode of spatial awareness in blind individuals that is not to be confused with echolocation based on self-generated sounds. We argued that this sense of volume consists in a rudimentary form of presence that expresses that there is something out there at a certain distance and in a certain direction. We further explored its sensory nature. Depending on the criterion that one uses, the sense of volume falls into one sensory domain or another but if we focus on its phenomenology, then it appeared that it is primarily characterized by its tactile dimension. More specifically, we propose that the sense of volume consists in a form of *tactile anticipation*. When one experiences something as being out there, one tactually expects that one could come in contact with it.

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