Activation and habituation in olfaction--an fMRI study.

This study investigated human BOLD responses in primary and higher order olfactory cortices following presentation of short- and long-duration odorant stimuli using a 3-T MR scanner. The goal was to identify temporal differences 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 differed in different parts of the olfactory network: (1) The POC (piriform, entorhinal cortex, amygdala) and, interestingly, the hippocampus and, to a certain degree, the anterior insula show a short, phasic increase in the signal, followed by a prolonged decrease below baseline. (2) In the orbitofrontal cortex a sustained increase in activation was seen. This increase lasted approximately as long as the duration of odorant presentation ( approximately 60 s). (3) The mediodorsal nucleus of the thalamus and the caudate nucleus responded with an increase in signal which returned to baseline after approximately 15 to 30 s. The correlated biphasic hemodynamic response in the POC, hippocampus, and anterior insula during prolonged olfactory stimulation suggests that these three areas may interact closely with each other in the control of habituation. These results extend recent data which showed habituation of the rat piriform cortex and dissociation between the POC and the orbitofrontal cortex.