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.

Wednesday, June 27, 2007

The Four Ages of Functional Neuroimaging: Part 2

A brief look at the evolution of functional neuroimaging over the last 50 years may offer some insight as to why we are just now grappling with deepest philosophical issues surrounding functional neuroimaging and its relation to its sister disciplines. The history of functional neuroimaging can be roughly divided into to four ages or epochs, which we might call: the age of iron, the age of bronze, the age of silver, and the age that we are currently in, or at least on threshold of entering, the golden age. In the iron age, which lasted from approximately 1955 to 1975, scientists such as Seymour Kety, Lou Sokaloff, David Ingvar and others were the first to measure the brain cerebral blood flow while a subject engaged in what they called “mental activity”. Subjects were asked to perform mental calculations, read silently, read aloud, count backwards and forwards, and this mental effort was revealed in the metabolic changes that were being observed in the brain. The Iron Age established that human thought had metabolic consequences that could be measured and localized to regions in the brain. The sophistication of these early studies was entirely on the physiological side, and certainly at this point the technology was sufficiently crude and unwieldy that it was not viewed as appropriate for the examination of the brain’s information processing capabilities.

At about 1982, human neuroimaging entered the Bronze Age, which heralded both technological and methodological advances in the field. Positron emission tomography (PET) with the O-15 tracer combined with the logic of cognitive subtraction, opened up entirely new vistas in the potential for functional localization in the brain. Michael Posner and Marcus Raichle, moreover, showed that a collaboration between neuroscience and cognitive psychology was essential to studying the brain basis of cognition, and that the same tools and methods employed in experimental psychology – the reaction time subtraction logic of Donders and the additive factors methodology of Sternberg – were reinvented in the context of a this new multi-pixel dependent variable – brain activation. Attention, memory, language, perception, and mental imagery were all studied in the PET scanner, and new ideas relating these traditional concepts to activity in the brain were formed.

The Silver Age (approximately 1993-2000) brought with it an unprecedented expansion in neuroimaging research, vastly improved statistical methodology (an acronym, SPM or “statistical parametric mapping”) and with the emergence of fMRI, the field was no longer just for the tiny minority with access to an expensive PET scanner. Most would admit that it was during this period that the sheer number of “activation” studies, the proliferation of what has been derisively termed “blobology”, or “technicolor phrenology”, cast a certain pall over what was otherwise an extraordinary era of scientific advance. The problem was that many neuroimaging studies carried out during this era were of the “lets just do it and see what lights up” variety, while theory-driven research and hypotheses were not uncommonly eschewed. For any ad-hoc “psychological process” that one could invent, a researcher could be sure to find its “neural correlate” – in brilliant hues – somewhere in the cingulate gyrus, the insula or another numbered Brodmann area. Indeed, in the Silver Age of neuroimaging, there were no “failed studies”. It was probably during this period, however, that many cognitive psychologists and neuroscientists, watching interestedly from the sidelines, decided that functional neuroimaging was not worth the effort. Indeed, even from within the neuroimaging ranks, it was clear, as one toured the poster section at the annual Human Brain Mapping conference, that the only thing that outnumbered the colored blobs was the number explanations for them.

Every new scientific field or endeavor experiences growing pains. The period of the late 1990’s in neuroimaging was both a necessary and inevitable step in the evolution of the field at large. An analogy might be made between this process and a similar one that occurs in human development, for example. A child in infancy learns the relations between the movements of the oral articulators and the sounds that such movements produce through a process known as “babbling”, an auditory-motor tuning process that proceeds through a kind of random exercise of the speech muscles. It is perhaps not too much of a stretch to say that during the Silver Age of neuroimaging research, a similar kind of “tuning” process was occurring whereby certain systematic relations between experimental manipulations or contexts on the one hand, and regions of brain activation on the other hand, were being worked out. The accumulation of studies pointing to some systematic relationship between a “cognitive process” and a corresponding brain region, forges a link between a hypothesized function, on the one hand, and an anatomical location, on the other. As the number of studies pointing at a neural correlate of this or that cognitive function begin to mount, some brave and ambitious researcher decides to christen the anatomical area for its functional properties. Suddenly, the fusiform gyrus is not merely a bump on the ventral surface of the human brain but is the “fusiform face area”: structure and function are merged into a single moniker; or the anterior cingulate gyrus, it is no longer merely a name for a particular cerebral convolution, but has come to refer to function as well: “conflict resolution”. The renaming of parts of the brain to incorporate their specific function is triumph of the silver age of neuroimaging. It is in principle no different than the labeling of the back part of the occipital lobe as “visual cortex”, on the basis of neurophysiology and lesion work. Indeed, during the Silver Age many provisional labels were affixed to diverse structures of the brain, but only a very few of these “cognitropes”, if I may coin a phrase, managed to stick. But the ones that did stick are the labels that have mattered, that were reliable, and have been both the cornerstones and the targets of current hypotheses and theories in cognitive neuroscience.

Tuesday, June 26, 2007

The Four Ages of Functional Neuroimaging: Part 1

In the increasingly interdisciplinary world of the science of human behavior, a certain conversation can be heard again and again in the gathering places where the practitioners of neuroscience and cognitive psychology are occasionally found together. The neuroscientist, having patiently listened to a psychologist present his latest theoretical model – resplendent with the boxes and arrows constituting the “mental modules” of some particular piece of the cognitive system – shakes his head, wondering aloud what possible relevance these chalkboard chimera might have to someone who studies the live matter of the brain. The cognitive psychologist of a certain stripe takes a similar view of the neuroscientists’ efforts, which he or she maintains sheds very little light on the functional properties of the mind. Both camps argue for epistemological supremacy: cognitive psychology for the ghost in the machine, and neuroscience for the machine in the ghost. Standing somewhere in the middle, amid the crosstalk, straining to be heard above the din of argument, stands the cognitive neuroscientist, unsure of which camp he is addressing (or belongs to), but nevertheless confident that he holds the key, the answer to the debate. “We must study both”, he asserts. “We must study the ghost in the machine and machine in the ghost!”

Unfortunately, the cognitive neuroscientist is not unlike the spurned and neglected offspring of two parents that despise each other and consider their child a wastrel and, ultimately, a mistake. Indeed, it is only after the cognitive neuroscientist succeeds in gaining the attention of his audience (in this hypothetical gathering) that psychology and neuroscience turn towards their wayward child and nod their heads and point their fingers in disdainful unison. If there is one thing they can agree on, it is that cognitive neuroscience and its favored technological toy -- functional magnetic resonance imaging (fMRI) -- has nothing to offer them. By the same token, functional neuroimaging and cognitive neuroscience, generally, taking the remarkable success of the movement as a self-evident mark of its scientific worth and validity, has never made a particularly sustained or rigorous effort to make the case for why functional neuroimaging matters to neuroscience and cognitive psychology.

In the last year, however, this has begun to change. Advocates of functional neuroimaging in a number of review papers have laid out a formal case for the legitimacy and relevance of the field. Most of the recent discussion, particularly articles by Henson (2005), Poldrack (2006) and De Zubicaray (2006) has been aimed at cognitive psychology, and has argued for both the relevance and theoretical benefits that functional neuroimaging can bring to the field of cognitive psychology. In a recent issue of the journal Cortex, however, Coltheart (2006) and Page (2006) forcefully argued the position that, essentially, functional neuroimaging has so far contributed nothing to cognitive psychology. A number of examples suggesting how neuroimaging had indeed informed cognitive psychology were then proffered in response, but Coltheart remained unconvinced. It is the purpose of the present article, not to rehash the debate or contribute a technical innovation with regard to the inference in functional neuroimaging, but rather to provide a slightly different perspective on the debate, and to show that, perhaps, this recent flare up arises out of a basic disagreement (or confusion) over what such terms as ”mind”, “mental”, “cognitive process”, and “cognitive theory” really mean.

An essay entitled: The Four Ages of Functional Neuoimaging

I am going to inaugurate this blog with a essay in (four parts) entitled: "The Four Ages of Functional Neuroimaging". The essay is about, among other things, the evolution of the science of functional neuroimaging imaging, how neuroscience and cognitive pyschology view congitive neuroscience, and a bit about why functional neuroimaging is getting better all the time. I had considered sending this essay out for publication, but, really (as you will find out) the style is a bit much at times. Or perhaps not -- you be the judge.