Biology and Philosophy 18: 131–168, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.Neural Plasticity and Consciousness1SUSAN HURLEYPAISUniversity of WarwickCoventry CV4 7ALU.K.E-mail: [email protected] NOËDepartment of PhilosophyUniversity of CaliforniaSanta CruzU.S.A.E-mail: [email protected]. We introduce a distinction between cortical dominance and cortical deference,and apply it to various examples of neural plasticity in which input is rerouted intermodallyor intramodally to nonstandard cortical targets. In some cases but not others, cortical activity‘defers’ to the nonstandard sources of input. We ask why, consider some possible explanations,and propose a dynamic sensorimotor hypothesis. We believe that this distinction is importantand worthy of further study, both philosophical and empirical, whether or not our hypothesisturns out to be correct. In particular, the question of how the distinction should be explainedis linked to explanatory gap issues for consciousness. Comparative and absolute explanatorygaps should be distinguished: why does neural activity in a particular area of cortex have thisqualitative expression rather than that, and why does it have any qualitative expression at all?We use the dominance/deference distinction to address the comparative gaps, both intermodaland intramodal (not the absolute gap). We do so not by inward scrutiny but rather by expandingour gaze to include relations between brain, body and environment.Key words: consciousness, cortical deference, cortical dominance, dynamic sensorimotorcontingencies, explanatory gaps, neural plasticity, perceptual adaptation, phantom limbs,synaesthesiaIntroductionWhy does neural activity in a particular area of cortex give rise to experienceof red, say, rather than green, or to visual experience rather than auditory?Why, for that matter, does it have any conscious qualitative expression at all?These familiar questions point to the explanatory gap between neural activityand qualities of conscious experience.132In fact, these questions indicate that there are three different types ofexplanatory gap for consciousness.2There’s the absolute gap: Why shouldneural processes be ‘accompanied’ by any conscious experience at all? Andthere are two comparative gaps. First, there’s the intermodal comparativegap: Why does certain neural activity give rise to visual rather than auditoryexperience, say? Second, there’s the intramodal comparative gap: Why doescertain neural activity give rise to experience as of red, say, rather thanexperience as of green?It seems natural to adopt an inward focus in response to such explan-atory gap questions, to assume that they must be answered in terms of theintrinsic properties of the neural correlates of consciousness. But there arewell-known grounds for skepticism about this strategy of response. Neuralproperties are qualitatively inscrutable.3If you were to land in the visualsystem as a microscopic alien, you couldn’t tell, by looking around at thelocal fireworks, whether experience was happening, or whether, if it was,it was visual experience, or whether, given that it was visual, it was visualexperience as of something red. Mueller’s theory (1838) of “specific nerveenergies” recognized this. On his view, it is not the intrinsic character ofthe neural activity that makes it visual. Rather, it is the fact that the neuralactivity is set up by stimulation of the retina, and not, say, the cochlea. Butthis view still leaves an explanatory gap unbridged: why should differences inthe peripheral sources of input, leading to differences in the cortical locationsof the neural activity, make for the difference between what it is like to seeand what it is like to hear?We suggest that an inward focus in response to explanatory gap worriescan be misleading. To find explanations of the qualitative character of experi-ence, our gaze should be extended outward, to the dynamic relations betweenbrain, body, and world. In this paper we apply this general strategy to thecomparative explanatory gaps, both intermodal and intramodal. We set asidethe absolute gap, dividing in hopes of conquering. We believe we can makeprogress by concentrating on the comparative gaps; whether our approachwill help to bridge the absolute gap is a further question.We take our start from consideration of neural plasticity. This phenomenondeserves serious attention from philosophers concerned with explanatorygaps, since it reveals that neural activity in a given area can change notjust its function but also its qualitative expression. We introduce a distinc-tion between cortical dominance and cortical deference,4andapplyittovarious examples in which input is rerouted either intermodally or intramod-ally to nonstandard cortical targets. In some cases of rerouting but not others,cortical activity ‘defers’ to the nonstandard sources of input and takes on thequalitative expression typical of the new source.133This distinction is puzzling, and raises closely related empirical andphilosophical issues. What explains why qualitative character defers tononstandard inputs in some cases but not others? How does explanation ofthis difference address the comparative explanatory gaps?5After laying outthe dominance/deference distinction, with both intermodal and intramodalillustrations, we consider and criticize some possible explanations of it. Wethen put forward a dynamic sensorimotor account of the distinction. Thispromising hypothesis has the potential, if correct, to bridge the comparativeexplanatory gaps.Whether or not our dynamic sensorimotor proposal turns out to be correct,our main claim here is that the dominance/deference distinction is importantand worthy of further study, both empirical and philosophical. Explainingthis distinction will help us to understand how qualities of consciousness arerelated to the rest of the natural world.The distinction introduced: cortical dominance vs. cortical deferenceWhat happens when areas of cortex receive input from sensory sources thatwould not normally project to those areas? When an area of cortex is activatedby a new source, what is it like for the subject? Is the qualitative character ofthe subject’s experience determined by the area of cortex that is active, or bythe source of input to it?Empirical work on neural plasticity shows that it can go either way. Incases of cortical dominance, cortical activation from a