The advantage of skills over brain statesThe purpose of the sensorimotor approach is to show how phenomenal consciousness, usually thought to be outside the scope of a functional approach, can usefully be considered to consist in having cognitive access to a particular fact, namely the fact that one is currently engaged in a sensorimotor skill. Why this emphasis on skill rather than simply to brain states?
The answer resides in the fact that the appeal to skill allows the felt characteristics of feel to be accounted for in a natural way: similarities and differences in felt characteristics can be connected with similarities and differences in the sensorimotor laws underlying skills. Making the connection between skill and feel is easy to do, because sensorimotor laws are describable in terms of facts about the perceptible world.
In contrast, if the onus of explanation is put on neural states, as is the case in the classical "neural correlate" approach, there is no natural way of making the connection between similarities and differences of feel and similarities and differences in the neural states. This is because neural states are physical characteristics of the firing patterns of neurons, and have no natural relation to facts of the perceptible world. Additional "linking hypotheses" have to be made to justify, for example, why particular neurons should produce particular sensory qualities or why particular firing patterns should produce particular modulations in feel.
Question 1: the origin of feelThis question has been eluded, by essentially agreeing that if we think properly about what we mean by feel, then we realize that feels are things which cannot be generated. It is logically incorrect to think that feels are states or things that happen to humans. We realize instead that what we mean by a feel occurring is that an agent has cognitive access to the fact that it is interacting with its environment in a particular mode. The quality of the feel then is simply constituted by the characteristics of the mode of interaction.
Question 2: differences in sensory qualityClassically an explanation for the fact that input in different sensory channels provokes different types of sensation is usually sought in the different cortical areas that are involved. However this hypothesis provides no more help than the early idea of Müller's, according to which different nerve channels were equipped with specific nerve qualities. Special cortical wiring, brain mechanisms, cortical areas or nerve qualities do no more than raise the further question of what exactly it is about the cortical areas or the nerve channels that produces the particular sensations.
On the other hand the sensorimotor approach provides a natural way of dealing with the question of why sensations in different sensory modalities have different feels. If feeling involves cognitively accessing the fact that one is currently engaged in a particular mode of sensorimotor interaction with one's environment, then the quality of a feel will be constituted by the particular laws that govern the interaction.
Seeing is a form of interaction in which blinks, movements of the eyes, of the body and of outside objects provoke very particular types of change in sensory input. The laws governing these changes are quite different from the laws governing sensory input in the auditory modality. For example, when one sees, moving forward produces a particular expanding flow-field on the retina, whereas when one hears, the change in sensory input is now mainly an increase in amplitude of the signal. Later sections on particular sense modalities provide more discussion, in particular on an explanation of color quality. The section on sensory substitution discusses an empirical prediction deriving from these ideas.
Question 3: why feels have a quality at allTo answer this question we invoke the notions of corporality and alerting capacity. They will contribute to understanding what provides sensory experiences with their particular sensory quality, and more precisely, what makes for the difference between truly sensory and other experiences.
To see our notions at work, 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. It is clear that these different cases provide different degrees of sensory feel. Our claim is that these different degrees of sensory feel precisely reflect different degrees in corporality and alerting capacity.
Thus, when an object is in full view, it comes with the fullest intensity of sensory feel. But it is here also the case that how the observer moves will immediately affect the incoming sensory stimulation. Also, 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. In terms of the concepts we defined above, this means that an object in full view has both high corporality and high alerting capacity.
Contrast this with just knowing that an object is somewhere, but out of view. While knowledge about an object in another room might certainly be conscious, it lacks real sensory feel. 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.
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 visual presence, because it has a degree of corporality, as we can easily bring it into view by a slight movement.. The ‘boundary extension’ phenomenon 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. "Change blindness" shows that when alerting capacity is interfered with, the experience of perception ceases.
These examples show that the differing degrees of what one might call "sensory feel” (perhaps Hume's "vividness" or Husserl's "Leibhaftigkeit") can be accounted for plausibly in terms of the notions of corporality and alerting capacity.
The "sensory phenomenality plot"The degree to which mental phenomena possess corporality and alerting capacity can be systematized in what one might call a "sensory phenomenality plot". Such a figure reveals that those states that possess both corporality and alerting capacity correspond precisely to cases which provide true sensory experiences.
As follows from the discussion above, vision, touch, hearing, and smell are the prototypical sensory states and indeed have high corporality and high alerting capacity. High corporality derives from the fact that changes in head or limb positions 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). High alerting capacity is provided by the fact that sudden changes in visual, tactile, auditory or olfactory stimulation provoke immediate peremptory orienting behaviors.
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 plotted thinking and recalling from memory at the other extreme, because they have neither corporality or alerting capacity, as we pointed out above.
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 2 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), 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, and interestingly and perhaps justifiably, on the phenomenality plot, ressembles proprioception…
NB: In the above it is important to note that we consider that our plot only charts the degree to which mental phenomena have sensory or perceptual quality, and not consciousness per se. In particular, when we claim that thought has no sensory quality, we are not saying that thought is not conscious.
Question 4: ineffabilityAppealing to skills in a theory of sensory quality provides a natural way of accounting for the fact that sensations are personal and inaccessible to others.
The reason is that cognitive access in the sensorimotor theory is cognitive access to the fact that one is exercising a particular sensorimotor skill, and not to each and every muscular contraction or change in sensory input that occurs. The situation is like tying your shoelaces: you know that you are tying your shoelaces, but you cannot say exactly how you are doing it, exactly what are the positions of your fingers, exactly what are the muscle movements involved. You have cognitive access to certain aspects of the shoelace tying skill, like to differences in the mode of shoelace tying when the shoelaces are of different rigidity, length, thickness, etc., but you cannot describe in detail how these differences modify what you do.
Similar considerations would then apply to feel: the sensorimotor contingencies involved in seeing red are cognitively inaccessible to you. Nevertheless you know that they are different from those involved in seeing other colours, and you have some access to degrees of difference and similarity.
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