Abstract
Functional localization has historically been one of the primary goals of neuroscience. There is still debate, however, about whether it is possible, and if so what kind of theories succeed at localization. I argue for a contextualist approach to localization. Most theorists assume that widespread contextual variability in function is fundamentally incompatible with functional decomposition in the brain, because contextualist accounts will fail to be generalizable and projectable. I argue that this assumption is misplaced. A properly articulated contextualism can ground successful theories of localization even without positing completely generalizable accounts. Via a case study from perceptual neuroscience, I suggest that there is strong evidence for contextual variation in the function of perceptual brain areas. I then outline a version of contextualism that is empirically adequate with respect to this data, and claim that it can still distinguish brain areas from each other according to their functional properties. Finally, I claim that the view does not fail the norms for good theory in the way that anticontextualists suppose. It is true that, on a contextualist view, we will not have theories that are completely generalizable and predictive. We can, however, have successful partial generalizations that structure ongoing investigation and lead to novel functional insight, and this success is sufficient to ground the project of functional localization.
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Notes
Since I only discuss one brain area, it is of course possible that other parts of the brain will be less amenable to contextualist analysis. McCaffrey (2015) suggests such a view in his “functional heterogeneity hypothesis.” My goal here is to articulate contextualism for perceptual brain areas, which are important test cases for functional localization. How far the view extends is an empirical question, as McCaffrey notes.
Out of the extant philosophical options, this characterization most resembles “systems” accounts of functions (Cummins 1975). As Craver (2007) notes, systems accounts normally must be supplemented with an account of causal relevance, to show which properties of the parts are actual components of the causal organization that produces the explanandum. In systems neuroscience, it is assumed that the electrical signaling activity of a part of the brain is its relevant causal contribution, and I will adopt this assumption.
Functional decomposition is not the only explanatory goal for which we might discuss the parts of a system. We might, for instance, want to explain the evolutionary history of a brain area, or its morphological properties. But this kind of explanation is at least a primary goal of neuroscientists involved in pursuing localization.
I take it that these are three core desiderata for theories of functional localization. I don’t claim that they are exhaustive, but they seem to capture relatively accurately the concerns that theorists have expressed.
To be fair, Rathkopf is concerned with a particular form of functional explanation, namely descriptions that explain why a certain anatomical structure is present. He admits that context-sensitive descriptions might be useful for other purposes.
Establishing a privileged correlation is by no means an easy matter. Perceptual neuroscientists have developed a variety of sophisticated strategies for arguing that one unique type of information is represented by a brain area in all cases, often despite seeming influence of other types on its responses. I present below the data for contextual variation that I take to best avoid the strategies, but I will not make the full argument here. I discuss the strategies in detail, and give arguments against them for these cases, in (Burnston 2015).
Some areas are sub-divided into functionally specified parts. For instance, V1 is a single anatomical area, but comprises different parts specialized for representing wavelength, displacement, and oriented edges. V2 has a parallel internal organization. The hierarchy proposed by the MFH theory is posited to then separate these features into distinct areas corresponding to our standard perceptual attributives of motion, color, shape, etc. This sort of subdivision is entirely compatible with absolutism, so long as the distinct parts within the areas represent only one feature. Below I will explain that subdivision is not appropriate in the case of MT.
For a review of these results, see (Dobkins and Albright 2004).
This is based on the attendant assumption that motion perception follows the border with the smallest displacement at each time step, which is independently well-established (Dobkins and Albright 1994).
One can compare this with non-categorical shifts, such as those posited by “gain-increase” mechanisms of attention, which suggest that top-down attention does not change what is represented, but simply heightens the response to a particular represented property (Treue and Trujillo 1999).
This would occur in a circumstance where the moving stimulus was precisely at the plane of fixation. See Fig. 3.
There have yet to be detailed studies, along the lines of Britten et al. (1996) or Dodd et al. (2001), establishing the functional use of this information. Given the progression of the field, however, and MT’s detailed responses to tilt and slant, it is reasonable to expect that such studies will discover the functional usefulness of these responses.
Thanks to Nancy Cartwright for pushing me on the disposition objection. There is in fact, a third strategy, which attempts to save absolutism by searching for a deeper functional principle that explains all of the specific functions that an area like MT performs. Several absolutists (including some of those cited in the “Introduction” section) have argued that, rather than representing a specific type of information or contributing to a particular type of task, each neural area performs a particular type of computation in any context in which it functions. These views are still absolutist, in that they still posit univocal functions that are intended to account for all cases. I argue against this alternative form of absolutism in (Burnston 2016).
To actually establish that V4 is not a feature-specific area would require a thorough analysis of the evidence, along the lines of what I gave above for MT. I won’t do so here; while I believe such an argument can be made, my primary interest in this section is to analyze the consequences for decomposition if both MT and V4 are genuinely context sensitive in their functioning.
Importantly, this does not mean that contextualism is “unfalsifiable” in any interesting sense. Posits about particular conjuncts and how they differentiate between areas are highly falsifiable. See the discussion below.
Strictly speaking, a contextualist view needn’t be unbounded. Functional properties could be context-sensitive and conjunctive even if it were possible to give a complete list of the conjuncts. There are two reasons for embracing unboundedness. First, since it is the strongest form of contextualism, avoiding the epistemic worries evinced by absolutists for this kind of view should heavily lessen the motivations for denying contextualism writ large. Second, I agree with Rathkopf that once we open our function ascriptions to contextual variation, it will be hard to know that we have a complete theory. At several times during investigation of MT, consensus views of its function have been overturned, and it is part of the appeal of contextualism that it has the epistemic modesty not to rule out this possibility a priori. Thanks to Gary Ebbs for pushing me to clarify this point.
Thanks to Olivier Morin and Ben Sheredos for pressing the ecological validity objection.
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Acknowledgments
I would like to thank William Bechtel, Jonathan Cohen, Rick Grush, Thomas Albright, and John Serences for invaluable advice during the development of this project. Thanks also to Ann-Sophie Barwich, Olivier Morin, Isabella Sarto-Jackson, and Ben Sheredos for extremely helpful comments on an earlier version of the manuscript. This material was presented to colloquia audiences at Tulane University and Indiana University, Bloomington, and I am grateful to both for helpful discussion.
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Burnston, D.C. A contextualist approach to functional localization in the brain. Biol Philos 31, 527–550 (2016). https://doi.org/10.1007/s10539-016-9526-2
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DOI: https://doi.org/10.1007/s10539-016-9526-2