Monday, August 13, 2007

The Four Ages of Functional Neuroimaging: Part 3

Rather than treat “cognition” as a separate realm where functions are described and diagrammed on sheets of paper, functional neuroimaging seeks to eliminate the mind-brain barrier, to deny that venerable dichotomy, and to shift the terrain from the ether of psychological abstraction to the material folds of the brain. It does not matter in the long run that the fusiform gyrus might not truly act as a unitary and modular processor of faces that the name (“FFA”) implies; rather, what is important is that this particular assertion about face processing is committed to a neural state of affairs, which is open both to empirical support or falsification. It is a hypothesis which bears itself to all, declining to hide from the protective shade provided by the term “neural correlate”. The FFA is not the neural correlate of the face processor – it is the face processor, that is its function. Thus, one might say that the end of the Silver Age of neuroimaging was characterized by an increasing willingness, bolstered by an accumulation of empirical support, to propose hypotheses about brain function that treated “cognition” as a thing to be described in neural terms, howsoever simplistic and inchoate, and to be informed by data derived from cognitive psychology, neuroscience, and functional neuroimaging itself.

The use of functional neuroimaging techniques in the study of the biological basis of human cognition and behavior is now entering a Golden Age. The necessary, but often atheoretical, project of “mapping” hypothesized cognitive functions onto discrete pieces of cortex is coming to an end. Rather than stating hypotheses in terms of models of cognition and then in effect “searching” for brain correlates (or proxies of cognitive components) many researchers are now taking an integrated approach, where hypotheses about functional anatomy are stated a priori, and imaging results are taken as evidence for or against a stated hypothesis. One no longer asks where a function is located but rather whether an hypothesized functional-anatomical correspondence provides an accurate picture of biological reality. In the areas where functional neuroimaging is having its greatest impact, it has managed to engage the interest of cognitive psychology and neuroscience. For instance, in long-term memory research, behavioral neuroscience, traditional cognitive psychology, and cognitive neuroscience researchers are increasingly involved in a unified pursuit, a joint conversation, centering on the role of the medial temporal lobe in memory. For example, the idea from psychology of a dichotomy between recollection and familiarity in long-term memory is being studied at all levels: in rats with implanted electrodes, with behavioral measures in psychological laboratories, and with human neuroimaging studies. Whereas in prior years, neuroscience and psychology would be carrying out studies in isolation, each with its own idiosyncratic paradigms and nomenclature, the arrival of cognitive neuroscience and human neuroimaging, is increasingly providing the bridge between psychological and brain research, and contributing to the emergence of a unified approach to a particular problem domain.

To give a more specific example of how the field is advancing consider the hypothesis that the function of the inferior frontal gyrus is for the “selection of competing alternatives” in the context of word retrieval (Thompson Schill et al. 1997). This is a classic “Silver Age” function-structure proposition. It identifies a fairly large region of cortex with a particular function, without an overarching model of word retrieval or a specification contextual factors or neural interactions. On the other hand, this hypothesis makes a rather stark and forthright claim about the function of a particular brain region, and has led to a small industry on the role of the inferior frontal gyrus in semantic retrieval. Competing hypotheses have been adduced which have highlighted the distinction between “controlled retrieval” and “selection from competing alternatives”, and has led the development of paradigms specifically designed to arbitrate between these two notions. The neuroanatomical specificity of these hypotheses has also vastly improved with newer theories proposing functional subdivisions within the inferior frontal gyrus that has led to a more nuanced understanding of the function of the region in word retriaval. Interactions with posterior temporal cortex have also been explored and a links between ideas deriving from models of word retrieval such as that of Levelt, have prominently entered the discussion. Thus, what began as an assertion about the functional role of a brain region has led by degrees to a far more sophisticated appreciation of the role of ventrolateral frontal lobe structures in word retrieval.

One might ask what possible bearing does any of this have on cognitive psychology, traditionally conceived as the study of the function of the mind? The answer to this question is, to paraphrase Coltheart, “none”. Physics has nothing to say about meta-physics. Likewise, the discussion of the functions of brain regions and their interactions tells us nothing about the mind – so long as one insists that the mind exists in a realm apart from the mundane exertions of the brain. Thus, the ongoing debates about the hippocampus and the inferior frontal gyrus are not about “mapping” from mind to brain, as it used to be. Rather, structure and function are inseparably linked, and the common practice now is to examine these two facets of brain organization as an integrated whole, just as the mechanic considers the radiator as a thing that serves a particular purpose, without bothering with intermediary “car-minds” and “car-mind processes” and other such ornaments of functionalism.

1 comment:

  1. very much enjoyed reading this essay Brad, I wish I had read it during my degree - it would have given a bit of epistemological context to my studies.

    All the best,