DOES EXIST A BRAIN AREA FOR METAPHOR
COMPREHENSION?
Functional magnetic resonance imaging (fMRI) is a good diagnostic tool, operating at time scales and spatial dimension far finer than previous brain-imaging techniques. However, it recently has become a research technique to draw bold conclusions about sexual differences in the brain, people anticipating pain, false memories, word and face recognition, psychotic and normal minds, impassive and empathetic. It is very easy to overextend the value of this technology, but we should consider that fMRI imaging is a very gross technique for research purposes, with real limitations that are too often overlooked. For example, because each voxel encompasses thousands of neurons, thousands or even millions may have to fire to significantly light up a region, it is as an entire section of a stadium had to shout to be heard.
Another big problem is that fMRI
depends on cerebral blood flow (CBF). The fine regulation of CBF is still
poorly understood, and an entire branch of research is devoted to study
problems related to vascular supply and hemodynamic response of the brain. Our
encephalon has a high metabolic activity due in part to the energy requirements
of constant neuronal activity. It demands about 15% of the cardiac output and
utilizes 25% of the total oxygen consumption of the body. Blood and brain work
together, but neuronal firing and synaptic activity takes milliseconds, whereas
the blood supply follows by two to six seconds, therefore a detected increase
in blood flow might be feeding more than one operation. Moreover, the
controversy over how the blood flow energy supply is regulated is far from a
solution, as noted last year (Claire
Peppiatt & David Attwell, Neurobiology: feeding the brain. Nature 431, page 137, Sept
2004).
Our italian counterpart,
BM&L-Italia, is actively studying problems related to this new kind of
localizationism, which is well represented in a recent issue of Scientific
American magazine: “A study of individuals who are unable to make sense of
figures of speech has helped scientists identify a brain region they believe
plays a key role in grasping metaphors” (Brain Region Linked to Metaphor Comprehension, “On the
Web” Sci. Am. 293, (2), 4, 2005).
We
suggest reading David Dobbs article: Fact or
Phrenology? Scientific American Mind 16 (1), 24-31, 2005.