Tuesday, February 17, 2009
Brain imaging can reflect expected, rather than actual, nerve activity
Work by Sirotin and Das illustrates how the brain thinks ahead. Electrical signalling among brain cells summons the local delivery of extra blood — the basis of functional brain imaging. And the usual assumption is that an increase in blood flow means an increase in electrical activity. The experiments by Sirotin and Das show that blood can be sent to the brain's visual cortex in the absence of any stimulus, priming the neural tissue in apparent anticipation of future events. (They observed this mismatch in alert rhesus monkeys by simultaneously measuring vascular and neural responses in the same region of the visual cortex. Changes in the blood supply were monitored by a sensitive video camera peering at the surface of the brain through a transparent window in the animal's skull, and local electrical responses of neurons were measured with a microelectrode.) Their results show that cortical blood flow can depart wildly from what is expected on the basis of local neural activity. Blood can be sent in anticipation of neural events that never take place.