The Oxford Handbook of Functional Brain Imaging in Neuropsychology and Cognitive Neurosciences by Andrew C. Papanicolaou

Author:Andrew C. Papanicolaou
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2017-03-15T00:00:00+00:00

Notes

1.The illusion that the brain is more active during the performance of experimenter-defined tasks than during experimenter-defined rest periods is reinforced by the outcome of the signal averaging procedure used in MEG and in electrophysiology, more generally. There, the prestimulus record represents activity during rest, whereas activation is represented by whatever is recorded following exposure to the stimulus to be processed. And, in order to visualize the brain’s output that reflects that processing, instead of subtracting the first part of the record from the second, we average successive pre- and poststimulus records. This operation reduces or flattens the prestimulus part (the prestimulus baseline) and reveals the brain’s output that is specific to the stimulus and its processing, against that flattened baseline, in the form of the familiar evoked or event-related response waveforms.

But the leveled-off prestimulus averaged activity is not a sign that the brain is inactive before it is prompted by the stimulus to engage in processing. Rather, it is a sign that all sorts of activations corresponding to the execution of all sorts of other functions along with those involved in processing the stimulus, both before and after stimulus presentation, have been removed through averaging (see Chapter 2).

2.It is worth commenting at this juncture on a curious and a hopefully instructive fact: What may be obvious to common sense is often hailed as a scientific discovery when assumptions, contrary to common sense, but held tacitly by scientists, are contradicted by laboratory observations. The discovery that our brain does not become inactive or necessarily less active when we are not performing a particular task belongs to that category of curious fact. There is hardly a person that ever believed that when we rest and relax, our mind (therefore our brain as well) shuts down. Rather, everyone knows that when we close our eyes and relax we begin to think and to imagine all sorts of things albeit not necessarily in a systematic fashion. Yet Hans Berger is credited with the discovery that the brain is always active, having shown that the EEG activity does persists not only when we are not performing specific tasks but even when we are in the state of deep sleep. Similarly, Ingvar is credited with the same insight having documented the existence of the hyperfrontal pattern, as are others who discovered the DMN (see Buckner et al., 2008). But genuine discovery has nothing to do with realizing what should be and is, in fact, obvious to those unencumbered by counter-intuitive assumptions; in this case, with the assumption that the brain shuts down or slows down when we are not involved in particular tasks. However, the specification of the particular brain structures that comprise DMN, much like the specification of the hyperfrontal pattern or of the pattern of EEG during rest, sleep, or during intense mental activity, is not at all obvious, no matter what commonly held assumptions one may entertain. And it is these specifications that constitute genuine, even if less than revolutionary, discoveries.

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