The sensorimotor approach holds that the experienced quality of a raw sensory feel is constituted by the quality of the sensorimotor interaction that is being engaged in when one has the feel. If we accept this premiss, then the special quality of "presence" possessed by sensory feels as compared to other mental states must somehow be found in something special about the sensorimotor interactions involved when we have sensory experiences. A working hypothesis which seems to give a reasonable account is the idea that the special "presence" of sensory experience has to do with two aspects of sensorimotor interactions: corporality and alerting capacity.
CorporalityWe define corporality of activation in a neural channel as the extent to which that neural activation systematically depends on movements of the body. 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.
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.
Corporality appears to be an important factor that correlates with 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. Note that we intend corporality to be an objective, physically measurable quantity. It is not a psychological construct. We want a neurophysiologist to be able to put an electrode into a sensory channel and measure whether there are changes as a function of voluntary actions.
Alerting capacityWe 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: "grabbiness", or capacity to provoke exogenous attentional capture.
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 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 in the organism's cognitive processing (interference occurs only indirectly, when, for example, the organism falls to the ground, provoking pain).
Like corporality, we take alerting capacity to be an objectively measurable parameter of the activation in a sensory pathway.
Incomplete controlAnother idea about the peculiar nature of raw sensory feel is that it might have something to do with incomplete voluntary control: What we call sensory experiences are both under our control, and not under our control. They are under our control to the degree that, because of their corporality, we can voluntarily modulate them (generally by moving our bodies and thereby changing the position of the sensors in the environment). They escape our control to the extent that there are cases where sensory input can change of its own accord without our intervention. Indeed not only can sensory input escape our control, it can even take over control of our mental activity. This happens because of the alerting capacity of sensory channels: when there are sudden transitory events in the sensory input which grab our attention incontrovertibly and cause us to think about the source of those events.
Voluntary actionJust as for cognitive access, the definition of "voluntary" depends on deciding on a point of view that one takes in describing a system. For example from the point of view of a neuroscientist presumably the supposed voluntary actions of a person can in fact be shown simply to be the consequence of neural processes, and so not be voluntary in the normal sense of the word. On the other hand the notion of "voluntary" in humans makes sense when one takes a point of view where one considers the human from the outside, and one attempts to account for the human's behavior. When that behavior can best be described by saying that the human had a choice about an action and made the decision to engage in one action and not others, then we use the word "voluntary".
Since here we want the notion of voluntary also to apply to organisms for which the notion of "self" is not necessarily applicable, we will use the notion of cognitive access rather than conscious access in our definition of voluntary. We will say that an action is voluntary when in describing the system, it makes sense to say that it had cognitive access to a number of possible actions, and proceeded to engage upon one action rather than the others. Note that it is not sufficient merely to have cognitive access to the action in question: simply having cognitive access to an action does indeed involve a choice, namely being able to use the action rather than something else in one's rational behavior. But the "something else" can be all the other things that the system could "think about". Here we require a choice between the action and other possible actions. We thereby exclude actions (like an automatic knee jerk for example) that the system might "attend to" without having "decided" to execute them.
We again stress that like for cognitive access, the notion of voluntary is in the "eye of the beholder". It is one way (out of possibly other ways) of describing the behavior of a system, seen from a particular point of view.