Neural Plasticity and Consciousness: A Dynamic Sensorimotor Approach

Susan Hurley and Alva No

 

Distinguish comparative and absolute explanatory gaps: why does neural activity in a particular area of cortex have this qualitative expression rather than that, and why does it have any qualitative expression at all? Our general strategy: focus on comparative gaps, both intermodal and intramodal; set absolute gap aside. Address comparative gaps not by inward scrutiny but rather by expanding our gaze to include relations between brain, body and environment.

We introduce a distinction between cortical dominance and cortical deference, and apply it to various examples of neural plasticity in which input is rerouted intermodally or 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 the distinction is important and worthy of further study, both philosophical and empirical, whether or not our hypothesis turns out to be correct. In particular, the question of how the distinction should be explained is linked to intermodal and intramodal comparative gaps.

 

1.                  The distinction introduced: cortical dominance vs. cortical deference

When inputs are neurally rerouted, does the area of cortex activated by a nonstandard source of input dominate the nonstandard input source and retain its normal qualitative expression or does it defer to the nonstandard source of input? Dominance may be assumed as the norm. But: 1) deference occurs also; 2) neural supervenience does not entail dominance.

 

2.                  The distinction schematized.

A & B: peripheral sources of input

1 & 2: cortical areas normally activated by inputs from A and B, respectively

Rerouting: A instead of B projects to 2

When 2 is activated by A, is its qualitative expression the B feeling (dominance, illusory) or the A feeling (deference, veridical)?

A/B comparison can be intermodal or intramodal


 

Qualitative A-feeling ?? B-feeling: dominance

Expression: A-feeling: deference

 

 

Of

Cortical 1 2

Area:

 

 

 


 

 

 

 

 

 

 

Input A B

Source:

 

 

3.      The distinction at work.

Intramodal dominance: referral of touch to face to phantom arm

Intermodal dominance: color-graphemic synaesthesia (under one interpretation)

Intermodal deference: visual to auditory rerouting in ferrets; early blind Braille readers

(Intramodal deference: see section 7 below)

 

4.      How can the distinction be explained?

Intermodal deference/intramodal dominance? Not satisfactory: there seems to be intermodal dominance, and even if intermodal rerouting is only necessary for deference, wed want to know why.

Early deference/late dominance? Is there early dominance in congenital phantoms? Again, even if early rerouting only necessary for deference, we want to know why.

Both these suggestions explanatorily shallow.

 

  1. Intermodal plasticity without neural rerouting: adaptation to TVSS

Extend deference to rerouting between distal and peripheral sources of input, not between peripheral source and cortical target.

TVSS: Distal source of visual input rerouted to peripheral source of tactile input, and on to somatosensory cortex, with arguably visual qualitative expression. This only occurs if subject controls movement of camera, not if someone else does or if he is moved passively. Switch from tactile pattern of dynamic sensorimotor (DSM) contingencies to visual pattern explains intermodal deference in TVSS; know-how governed by characteristically visual patterns of DSM contingency makes TVSS-perception qualitatively visual.

 

  1. A dynamic sensorimotor hypothesis.

Not remapping from input sources but changes in DSM contingencies drives changes in qualitative expression. Distinct patterns of DSM contingencies characterize different modalities and different qualitative characters within modalities.

Intermodal changes in qualitative expression driven higher-order changes in relations between DSM mappings. Difference modalities governed by different characteristic patterns of DSM contingency.

The link between such DSM patterns and qualitative character is intelligible, not brute. Relevance to intermodal comparative explanatory gaps. Does the point extend to intramodal comparative gaps? Yes.

 

  1. Intramodal plasticity without neural rerouting: adaptation to reversing goggles.

Sources of visual and proprioceptive inputs: left arm and right arm

Cortical target areas: left and right visual cortex and left and right proprioceptive cortex

Goggles produce external remapping between sources of input and visual cortex by left-right reversing goggles: conflict between co-stimulated areas of visual and proprioceptive cortex. Old DSM contingencies no longer hold. Taylor: veridical visual adaptation with practice. Deferential intramodal change in qualitative expression of visual cortex. DSM hypothesis: deference driven by loss and regaining of DSM know-how.

 

  1. A challenge.

Harris: nonveridical proprioceptive adaptation to reversing goggles, not veridical visual adaptation. Also, familiarization makes vision seem normal. Still a kind of intramodal deference, driven by change in DSM contingencies. But which kind of deference does DSM view predict?

Regaining know-how when DSM contingencies change requires intermodal harmony and ability to negotiate environment. Veridical visual adaptation la Taylor does both. If adaptation instead involves illusory proprioception, la Harris, then a canceling secondary adaptation or illusion is also required to restore full know-how: your wedding ring on your right hand REALLY looks leftward to you, but comes to seem to look rightward. This secondary adaptation would restore your know-how, but at cost of denying self-evidence of the qualities of your experience. DSM view predicts the two illusions cancel out qualitatively as well as practically.

 

 

 

  1. The dominance/deference distinction explained.

Qualitative expression cannot be explained just in terms of area of cortex active, nor additionally in terms of character of peripheral input sources. Rerouting, internal or external, changes pattern of DSM contingencies in which given areas of cortex participate. Deference reflects agents know-how in relation to patterns of DSM contingency characteristic of specific modalities or qualities but using nonstandard neural paths.

DSM approach regards deference as the norm. It predicts dominance where agent is passive or where rerouted input generates dangling cortical activity, not tied into patterns of DSM contingency.

DSM approach predicts deference for ferret and Braille cases, dominance for phantom case. Further prediction: if phantom patient stroked own face, while watching in mirror, would get deference instead. Explanation of dominance in synaesthesia: clue from abnormal covert priming?

DSM hypothesis promising but not proven, worth further attention.

 

  1. Conclusion.

Main aims: draw dominance/deference distinction, relate it to comparative explanatory gaps, raise question of how to explain distinction. Have also proposed an expanded gaze strategy and a DSM hypothesis.

An empirical account can scratch explanatory gap itches. DSM approach compatible with neural supervenience, but more explanatory. Characteristic patterns of DSM contingency are more qualitatively scrutable than NCCs (taken out of context of DSM patterns in which they function). Neural supervenience may be true, but inward scrutiny still be the wrong way to address explanatory gaps.

 

 


Very preliminary notes for second short talk on work in progress:

How the DSM hypothesis is compatible with neural supervenience but more explanatory.

 

11.  Neural supervenience of qualitative character as orthodoxy.

General form: no mental difference without a nonmental difference. To be fully specified supervenience claims require some parameter tweaking: are duplicates covered in same, all, or nearby worlds? What is relevant boundary? Which types of mental properties covered: content vs. qualitative character?

Here: neural supervenience of qualitative character across near worlds. Nonmental neural duplicate in same qualitative states as twins in near worlds.

 

  1. Neural supervenience and the dominance/deference distinction.

Neural supervenience not disputed, but it no more explains our distinction than it addresses comparative explanatory gaps. Neural supervenience does not predict cortical dominance or rule out deference reflecting extraneural sources of input.

 

  1. Extended neural supervenience and induced neural changes.

Distinguish two reasons for claiming that supervenience is compatible with cortical deference.

If we extend the boundary within which qualitative character supervenes on intrinsic properties, we can preserve supervenience. Not our reason: neural supervenience is compatible with deference to extraneural sources of input.

Our reason: Rerouting can induce changes in intrinsic neural properties of cortical activity even at the level of single cells. This can happen in cases of deference just as much as in cases of dominance. Thus deference reflecting even extraneural sources does not challenge neural supervenience.

 

14.  Why the distinction needs explanation whether or not qualitative expression supervenes on neural properties.

Neural supervenience, even neural correlates of consciousness, dont explain why we get deference in some cases, dominance in others, any more than they address the comparative gap question: why do these neural properties have this qualitative expression rather than that?

Objection: Just use the table of neural correlates of consciousness (once we have it) to look up which neural properties have which qualitative expressions. There may be no further explanation.

 

  1. The qualitative inscrutability of neural properties and the expansionist strategy.

Neural supervenience though true may mislead us by directing our attention inwardly to intrinsic neural properties, which are qualitatively inscrutable, thus leading straight to the comparative explanatory gaps. But we may be looking in the wrong place. How hard it is to bridge explanatory gap may depend on how the nonphenomenal side of the gap is conceived.

Some invoke protopsychons or fundamental psychophysical laws at this point.

Our strategy: expand our gaze to include DSM contingencies, complex and active relations of embodied organisms to their environments, as well as neural activity. Our DSM hypothesis has the potential, if correct, to scratch explanatory itches in a way that neural supervenience, though correct, does not.