Activation and habituation in olfaction - An fMRI study

This study investigated human BOLD responses in primary and higher order ol factory cortices following presentation of short- and long duration odorant stimuli using a 3-T MR scanner. The goal was to identify temporal differen ces in the course of the response that might underlie habituation. A short- duration stimulus (9 s) consistently activated the primary olfactory cortex (POC). After a long stimulus (60 s), the temporal form of the response dif fered in different parts of the olfactory network: (1) The POC (piriform, e ntorhinal cortex, amygdala) and, interestingly, the hippocampus and, to a c ertain degree, the anterior insula show a short, phasic increase in the sig nal, followed by a prolonged decrease below baseline. (2) In the orbitofron tal cortex a sustained increase in activation was seen, This increase laste d approximately as long as the duration of odorant presentation (similar to 60 s). (3) The mediodorsal nucleus of the thalamus and the caudate nucleus responded with an increase in signal which returned to baseline after appr oximately 15 to 30 s. The correlated biphasic hemodynamic response in the P OC, hippocampus, and anterior insula during prolonged olfactory stimulation suggests that these three areas may interact closely with each other in th e control of habituation. These results extend recent data which showed hab ituation of the rat piriform cortex and dissociation between the POC and th e orbitofrontal cortex. (C) 2001 Academic Press.