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.