NB: this paper is a shortened version of a working paper with the same title co-authored by O’Regan, Myin & Noë, which can be found on http://nivea.psycho.univ-paris5.fr

Why does seeing provide us with a qualitatively different sensory experience than hearing, taste or touch? Indeed, why does sensory input provoke a perceived sensory experience at all, when the majority of other neural activity in the brain is not associated with any phenomenal experience?

It might be thought that the answers to these two questions lies in the neural mechanisms involved. But though knowledge is rapidly accumulating concerning the neurobiological mechanisms involved in consciousness(cf. Rees, Kreiman & Koch, 2002 for an overview), there still remains the problem of how to capture the 'qualitative' aspects of experience with a scientific approach. There would seem to be an unbridgeable 'explanatory gap' (Levine, 1983) between what it is like to have a sensory experience, and the neural correlates or physical mechanisms involved.

The present paper shows how a step can be made towards bridging this gap. The approach involves taking an at first sight counterintuitive stance, namely that sensation consists in exercising an exploratory skill (cf. O'Regan & Noë, 2001a;  Myin & O'Regan, 2002; see Torrance, 2002, for further references to skill theories).

Taking the skill approach allows the first question about the experiential quality of sensation to be addressed, namely why the experienced qualities of different sensations differ the way they do.

Second, when skill theories are supplemented by two concepts which we refer to as "corporality" and "alerting capacity", then the second question about the experienced quality of sensations can be addressed, namely why they have an experienced sensory quality at all.

Sensation as a skill: Explaining within-modal sensory differences

The basic tenet of the skill theory is that having a sensation is a matter of the perceiver knowing that he is currently exercising his implicit knowledge of the way his bodily actions influence incoming sensory information (O'Regan & Noë, 2001a).

An illustration is provided by the sensation of softness that one might experience in holding a sponge (Myin, in press). Having the sensation of softness consists in being aware that one can exercise certain practical skills with respect to the sponge: one can for example press it, and it will yield under the pressure. The experience of softness of the sponge is characterized by a variety of such possible patterns of interaction with the sponge. Following MacKay (1962) we call the laws that describe these sensorimotor interactions “sensorimotor contingencies” (O'Regan & Noë, 2001a).When a perceiver knows, in an implicit, practical way, that at a given moment he is exercising the sensorimotor contingencies associated with softness, then he is in the process of experiencing the sensation of softness.

This skill-based approach to sensation has a tremendous advantage. It 'fits' the experience of softness in a way which a description in terms of a correlated neural process cannot. Thus, the experience of softness is easily characterized by the skill-based approach because it resides in, is constituted  by, the exploratory skill involved: In particular it is impossible to imagine one is going through all the exploratory patterns of softness, yet experiencing hardness.

On the other hand if we postulate that the experience of softness is generated by a neural process, then we have to explain why and how the particular neural process involved gives precisely the sensation of softness, and not some other sensation like the sensation of hardness. Neural explanations will always ultimately require some additional hypothesis to be made about how particular neural processes are linked to particular sensations.

The alternative, skill-based, or “sensorimotor” approach to understanding the experience of softness thus appears to provide a way of obviating the difficulty encountered by an approach which appeals to neural correlates to explain the quality of experience, namely the problem of the link between particular neural processes and particular sensations. If we could apply the skill idea to all sensations in general, then we would be on the way to a scientific approach to the problem of explaining phenomenal quality.

However at first sight it seems difficult to conceive of such a skill-based approach to sensations, since it would appear that most sensations can be perceived without any exploratory skill being engaged. For example, having the sensation of red or of a bell ringing does not seem to involve the exercising of skills. On the other hand, given the advantage we would gain from adopting a skill-based approach to sensations, it is worth considering such an approach more seriously.

Indeed it turns out that such an approach is possible after all (cf. O'Regan & Noë, 2001a,b,c; Myin & O'Regan, 2002; Noë, 2002a,b; Noë, forthcoming). It suffices to realize that sensations are never instantaneous, but are always extended over time, and that at least potentially, they always involve some form of activity.

Consider for example the sensation of red. The incoming sensory data concerning a red patch of color actually depends on eye position. Because retinal cones are distributed in a strongly non-uniform way on the retina, when the eye moves over a red patch, depending on eye position, the quality of the incoming information will be very different. The color red is thus associated with a particular law, or sensorimotor contingency, linking incoming stimulation to eye position. This sensorimotor contingency is part of what constitutes the sensation of red. The sensation of green is associated with, is constituted by, a different sensorimotor contingency, namely the one which is characteristic of green.

Another type of sensorimotor contingency associated with red depends on body motions. Consider the light reflected from a red piece of paper. Depending on where the observer is positioned with respect to ambient illumination, the paper can, for example, reflect more bluish sky light, more yellowish sunlight, or more reddish lamplight. Such laws of change constitute another type of sensorimotor contingency that constitute the sensation of red, and make it different from the sensation of green. The fact that color sensation can indeed depend on body motions has been suggested by Broackes (1992) and further work on color from a related perspective is reported in Myin (2001; cf. also Pettit, in press). Experimental tests of this hypothesis have been done by Kohler (1951), and McCollough (1965) and are in progress (O’Regan et al., 2001; Bompas & O’Regan, 2003).

In the preceding paragraphs we have shown how to apply a skill-based approach to something which a priori seems not to involve a skill, namely the sensation of color. We therefore have the potential to construct a scientific theory of the nature of color experience which explains the differences between different color sensations in terms of the different skills that are engaged when one looks at colors. Similar arguments can be constructed for explaining the different experiences within other sensory modalities.

Sensation as a skill: Explaining between-modal sensory differences

While the differences described above all concern sensory differences within  a given sensory modality, a second important aspect of sensory experience concerns differences between different modalities: The fact that for example, hearing involves altogether a different quality as compared to seeing, which has a different quality as compared to tactile sensation.

We propose to again apply the idea that sensation involves the exercising of sensorimotor contingencies : differences between modalities come from the different skills that are exercised. The difference between hearing and seeing amounts to the fact that among other things, one is seeing if, when one blinks, there is a large change in sensory input; one is hearing if nothing happens when one blinks, but, there is a left/right difference when one turns one's head, etc. Some other modality specific sensorimotor contingencies are specified in Table 1.

Table 1. Some sensorimotor contingencies associated with seeing and hearing

Action:	SEEING	HEARING
Blink	Big change	No change
Move eyes	Translating flowfield	No change
Turn head	Some changes in flow	Left/right ear phase and amplitude difference
Move forward	Expanding  flowfield.	Increased amplitude in both ears

The sensorimotor theory and its explanation of intra and intermodal sensory differences, as just reviewed, has previously been treated in a number of papers (O'Regan & Noë, 2001a,b,c; Myin & O'Regan, 2002; Noë, 2002a,b; Noë, forthcoming). As said, compared to a neural correlate approach the approach has the tremendous advantage of providing a principled account of within- and between-modal differences in sensory quality, and of escaping the problem of explaining why and how particular neural processes give rise to particular sensations.

In addition the sensorimotor approach leads to an interesting prediction. According to this approach, the quality of a sensory modality does not derive from the particular sensory input channel or neural circuitry involved in that modality, but from the laws of sensorimotor contingency that are implicated. It should therefore be possible to obtain a visual experience from auditory or tactile input, for example, provided the sensorimotor laws that are being obeyed are the laws of vision (and provided the brain has the computing resources to extract those laws).

The phenomenon of sensory substitution is coherent with this view. Sensory substitution has been experimented with since Bach-y-Rita constructed a device to allow blind people to "see" via tactile stimulation provided by a matrix of vibrators connected to a video camera (Bach-y-Rita, 1967) . Today, a number of new devices are being tested with the purpose of substituting different senses: visual-to-tactile (Sampaio, Maris & Bach-y-Rita, 2001; visual-to-auditory: Veraart, Cremieux & Wanet-Defalque, 1992;  auditory to visual: Meijer, 1992; auditory-to-tactile: Richardson & Frost, 1977). One particularly interesting finding consistent with the sensorimotor theory is that users of such devices sometimes testify that a transfer of modalities indeed takes place. For example, a woman wearing a visual-to-auditory substitution device will explicitly describe herself as seeing with it (cf. the presentation by Pat Fletcher at the Tucson 2002 Consciousness conference, available on http://www.seeingwithsound.com/tucson2002f.ram).

Sensory substitution devices are still in their infancy. In particular, no systematic effort has been undertaken up to now to analyze the laws of sensorimotor contingency that they provide. From the sensorimotor approach, it will be the similarity in the sensorimotor laws that such devices recreate which determines the degree to which users will really feel they are having sensations in the modality being substituted.

Related phenomena which also support the idea that the experience associated with a sensory modality is not wired into the neural hardware, but is rather a question of sensorimotor contingencies, comes from the experiment of Botvinick & Cohen (1998), where the “feel” of being touched can be transfered from one's own body to a rubber replica lying on the table in front of one (see also related work on the body image in tool use: Yamamoto, & Kitazawa, 2001; Iriki, Tanaka & Iwamura,1996; Farne & Ladavas, 2000). The finding of the Sur group (Roe, Pallas, Hahm & Sur, 1990), according to which ferrets can see with their auditory cortex can also be interpreted within the context of the present theory (Hurley & Noë, in press).

Corporality and alerting capacity: Explaining sensory presence

We now come to the second purpose of the present article, which is to address the question of why sensations have a sensory experiential quality at all.

Consider the difference between seeing an object in full view, seeing an object partially hidden by an occluding object, being aware of an object behind one's back, and thinking, remembering or knowing about an object.

Theorists have tried to describe and capture such differences in various ways. Hume, for example opposed (perceptual) sensations and 'ideas' (recollections of sensations and thoughts), in terms of ‘vivacity'  and 'force' (Hume, 1777/1975).  Husserl proposed the notion of an object being experienced as 'being present in the flesh' (having 'Leibhaftigkeit') as an essential ingredient for truly perceptual experience (Husserl, 1907/1973; cf. Pacherie, 1999; c.f.Merleau-Ponty, 1945, for similar use of the notion ‘presence’). In contemporary descriptions of perceptual consciousness, such a distinction is often made in terms of 'qualia', those special qualitative or phenomenal properties that characterize sensory states, but not cognitive states (Levine, 1983; Dennett, 1988).

While these notions seem descriptively adequate, we propose they should and can be complemented with an explanatory story that accounts for why sensory experience differs in these respects from other conscious mental phenomena. Our claim is that, within a skill-based, sensorimotor theory, the notions of “corporality” and “alerting capacity” provide precisely this missing explanatory addition.

Corporality

We define corporality as the extent to which activation in a neural channel systematically depends on movements of the body (in previous publications we used the term "bodiliness": O'Regan & Noë, 2001b; Myin & O'Regan, 2002; O'Regan, Myin & Noë, in press). Sensory input from sensory receptors like the retina, the cochlea, and mechanoreceptors in the skin possesses corporality, because any body motion will generally create changes in the way sensory organs are positioned in space, thereby causing changes in the incoming sensory signals. Proprioceptive input from muscles also possesses corporality, because there is proprioceptive input when muscle movements produce body movements. Corporality is an objectively measurable parameter of neural activation.

Note that we intend the term corporality to apply to any neural channels in the brain whatsoever, but because of the way it is defined, with the exception of muscle commands themselves and proprioception, only neural activation which corresponds to sensory input from the outside environment will generally have  corporality. For example neural channels in the autonomic nervous system that measure parameters such as the heartbeat or digestive functions, because they are not systematically affected by movements, will have no corporality even though they may carry sensory information. Note also that memory processes or thinking have no corporality, because body movements do not affect them in any systematic way.

 We claim that corporality is one important factor that explains the extent to which a sensory experience will appear to an observer as being truly sensory, rather than non-sensory, like a thought, or a memory. In Philipona, et al. (2003a, b) it is shown mathematically how this notion can be used by an organism to determine the extent of its own body and the fact that it is embedded in a three dimensional physical world in which the group-theoretic laws of euclidean translations and rotations apply.

Alerting capacity

We define the alerting capacity of sensory input as the extent to which that input can cause automatic orienting behaviors that peremptorily capture the organism's cognitive processing ressources. Alerting capacity could also be called: capacity to provoke exogenous attentional capture, but this would be more cumbersome. In previous papers we have also used the term "grabbiness" (O'Regan & Noë, 2001b; Myin & O'Regan, 2002; O'Regan, Myin & Noë, in press).

Pain channels for example have alerting capacity, because not only can they cause immediate, automatic and uncontrollable withdrawal reactions, but they also can cause cognitive processing to be modified and attentional ressources to be attributed to the source of the pain. Retinal, cochlear and tactile sensory channels have alerting capacity, since not only can abrupt changes in incoming signals cause orienting reflexes, but the organism's normal cognitive functioning will be modified so as to be centered upon the sudden events. For example a sudden noise not only can cause the organism to turn towards the source of the noise, but the noise will additionally, peremptorily, modify the course of the organism's cognitive activity so that, for example if it is human, it now takes account of the noise in current judgments, planning, and  linguistic utterances. Autonomic pathways do not have alerting capacity, because sudden changes in their activation do not affect cognitive processing. For example, while sudden changes in vestibular signals cause the organism to adjust its posture and blood pressure automatically, these adjustments themselves do not generally interfere with the organism's cognitive processing (interference occurs only indirectly, when, for example, the organism falls to the ground and must interact in a new way with its environment).

Like corporality, alerting capacity is an objectively measurable parameter of the activation in a sensory pathway.

Using corporality and alerting capacity to explain sensory presence

Equipped with the notions of corporality and alerting capacity, let us consider again the difference in degrees of sensory presence associated with seeing an object in full view, seeing an object partially hidden by an occluding object, being aware of an object behind one's back, or thinking of an object in another room. Our claim is that these differences precisely reflect different degrees in corporality and alerting capacity.

When an object is in full view, observer motion will immediately affect the incoming sensory stimulation. There is therefore corporality.

There is also alerting capacity: Any change that occurs in the object, such as a movement, a shape, color, or lightness change, will immediately summon the observer's attention. This is because low-level transient-detection mechanisms exist in the visual system that peremptorily cause an attention shift to a sudden stimulus change.

Thus, having both corporality and alerting capacity, an object in full view should be associated with an experience of strong phenomenal presence. This is indeed the case.

Contrast this situation with just knowing that an object is somewhere, but out of view. Clearly, in this case, there is no corporality, since the stimulus changes caused by bodily movements do not concern that object. Similarly, there is no alerting capacity, as the changes that the object might undergo do not immediately summon the perceiver’s attention.

Our claim is that having little corporality and little alerting capacity, an object out of view should not be associated with an experience of phenomenal presence. This is indeed the case.

An object only partially in view because of an occluding object or an object known to be behind one's back provide borderline cases. For example, the occluded part might be said to still have some phenomenal presence (Gregory, 1990; Merleau-Ponty, 1945; O'Regan & Noë, 2001a; Noë & O'Regan, 2002; Noë, 2002b) because it has a degree of corporality, as we can easily bring it into view by a slight movement..

As empirical confirmation of these ideas we may cite the ‘boundary extension’ phenomenon of Intraub & Richardson (1989) according to which observers overestimate what can be seen of a partially occluded object is coherent with this view. Amodal completion may be an example where one has an intermediate kind of “almost-visual” feeling of presence of a shape behind an occluder. The phenomenon of "change blindness", also shows that when alerting capacity is interfered with, the experience of perception ceases (O’Regan, Rensink & Clark, 1999;  Rensink, O’Regan & Clark, 1997; for a review see Simons, 2000; see also the “slow change” phenomenon: Simons et al. 2000; Auvray & O’Regan, 2003).

These examples show that the differing degrees of what one might call "sensory presence” (perhaps Hume's "vividness" or Husserl's "Leibhaftigkeit") can be accounted for plausibly in terms of the physically measurable notions of corporality and alerting capacity.

The "sensory phenomenality plot"

Figure 1 plots the degree of corporality and alerting capacity for a number of different mental phenomena. It is immediately apparent that those mental phenomena with both high corporality and alerting capacity are precisely those which are generally considered to have strong phenomenal “presence”.

Fig. 1. A sensory phenomenality plot

Thus: vision, touch, hearing, and smell have high corporality and high alerting capacity. Their high corporality derives from the fact that changes in head or limb position have an immediate effect on visual, auditory or tactile sensory input (smell is less clear, but sniffing, blocking the nose, moving the head, do affect olfactory stimulation -- Steriade, 2001). High alerting capacity is provided by the fact that sudden changes in visual, tactile, auditory or olfactory stimulation provoke immediate orienting behaviors that peremptorily modify cognitive processing. With high corporality and high alerting capacity, vision, touch, hearing and smell are plotted in the top right hand region of the sensory “phenomenality plot”, and are, according to our hypothesis expected to have strong phenomenal presence. This is of course true, because they are after all the prototypical sensory modalities.

What characterizes pain is its particularly large amount of alerting capacity. Here it is virtually impossible to prevent oneself from attentively focussing on the noxious stimulation. Pain also has corporality, but to a lesser extent. Moving one's body can generally modify the pain (one can remove one's finger from the fire; rub the aching limb and change the incoming sensations), but there are cases like headaches or toothaches which are more problematic. Headaches and toothaches are characterized by the fact that associated sensory input changes only moderately as a function of things that one can do such as press on the head, chew with one's teeth. This lack of an ability to easily modulate the sensory stimulation by body motions, that is, a reduced corporality, could possibly correspond to a particular aspect of pains such as headaches which distinguishes them from vison, touch hearing and smell, namely that they have an interior quality, often not clearly localized.

We have previously mentioned thinking about an object in another room, and in general thinking and recalling from memory have neither corporality or alerting capacity. They are plotted near the origin of our phenomenality plot, and have little phenomenal presence, as expected.

Proprioception is the neural input that signals mechanical displacements of the muscles and joints. Motor commands which give rise to movements necessarily produce proprioceptive input, so proprioception has a high degree of corporality. On the other hand, proprioception has no alerting capacity: body position changes do not peremptorily cause attentional ressources to be diverted to them. We therefore expect that proprioception should not appear to have an experienced sensory quality. Indeed it is true that though we generally know where our limbs are, this position sense does not have a sensory nature.

The vestibular system detects the position and motion of the head, and so vestibular inputs have corporality. However they have no alerting capacity. This is because although sudden changes in body orientation immediately result in re-adjusting reactions, these do not per se interfere with current cognitive processing. Coherent with our expectations therefore, the vestibular sense is not perceived as corresponding to an experience. We know we are standing vertical, but we do not have the experience of this in the same sense as we have the experience of hearing a bell or seeing a red patch.

Speculatively we suggest our plot also can track phenomena intermediate between sensory and mental states. One of several examples very tentatively included as points in Figure 1 is richness. The feeling of being rich is a case where there is a limited form of corporality (there are things one can do when one is rich like get the money from the bank teller, buy an expensive car, but this is nothing like the immediate and intimate link that action has on visual perception, for example), and little alerting capacity (there is no warning signal when one’s bank account goes empty). As a consequence, the feeling of being rich is somewhat, though not entirely sensory.

Consciousness

The argument made in this paper has concerned the nature of sensation: What gives sensation its “experienced” quality, what makes sensory qualities the way they are. But note: we have purposefully not touched upon the question of why and when sensations are conscious.

Our claim would now be that a sensation is conscious when a person is poised to cognitively make use of the sensation in their judgments, decisions, and rational behavior: that is, when the person has cognitive access to the sensation.

Now note that having cognitive access to a fact is something that can support scientific description and explanation (see Dennett,1978, Baars, 1988). It amounts to what Block (1995) has called Access Consciousness, and is something which, though it may constitute a difficult thing to implement in a machine, is nevertheless describable in functionalist terms. There is no a priori logical difficulty in using scientific methods to understand cognitive access or Access Consciousness (although there may be practical difficulties).

We have also defined sensation in a way which is not problematic from a scientific point of view, namely in terms of sensorimotor skills. The different types of sensations and their experienced characteristics -- their similarities and differences, their experienced "presence", can all be accounted for in terms of the differences between the skills, and in terms of the way the neural channels are tuned to the environment, namely by the properties of corporality and alerting capacity.

If having a conscious sensory experience amounts to having cognitive access to sensations, then what has previously been considered mysterious, namely what Block has called Phenomenal Consciousness, can now be decomposed into two scientifically tractable components: conscious sensory experience would in our approach consist in having Access Consciousness of sensations. Since Access Consciousness is amenable to scientific methods, and since sensations, being sensorimotor skills, are also amenable to scientific methods, under our approach Phenomenal Consciousness now also comes within the domain of science.

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