Central to the concept of attention is the fact that identical stimuli can be processed in different ways. In olfaction, attention may designate the identical flow of air through the nose as either respiration or olfactory exploration. Here we have used functional magnetic resonance imaging (fMRI) to probe this attentional mechanism in primary olfactory cortex (POC). We report a dissociation in POC that revealed attention-dependent and attention-independent subregions. Whereas a temporal subregion comprising temporal piriform cortex (PirT) responded equally across conditions, a frontal subregion comprising frontal piriform cortex (PirF) and the olfactory tubercle responded preferentially to attended sniffs as opposed to unattended sniffs. In addition, a task-specific anticipatory response occurred in the attention-dependent region only. This dissociation was consistent across two experimental designs: one focusing on sniffs of clean air, the other focusing on odor-laden sniffs. Our findings highlight the role of attention at the earliest cortical levels of olfactory processing.

Neural representations created in the absence of external sensory stimuli are referred to as imagery, and such representations may be augmented by reenactment of sensorimotor processes. We measured nasal airflow in human subjects while they imagined sights, sounds and smells, and only during olfactory imagery did subjects spontaneously enact the motor component of olfaction--that is, they sniffed. Moreover, as in perception, imagery of pleasant odors involved larger sniffs than imagery of unpleasant odors, suggesting that the act of sniffing has a functional role in creating of olfactory percepts.

Before exposure, the experimental group (Fig. 1a) showed 25% accuracy (s.d. 11%), which is not significantly different from 33% chance (binomial from chance: Pǃ0.9). Exposure for 21 days doubled the androstenone-detection accuracy in both the exposed nostril (from 25% to 55%, change from baseline: t(11)ǃ3.3, P<0.007; binomial from chance: P<0.002) and the unexposed nostril (from 25% to 49%, change from baseline: t(11)ǃ2.3, P<0.04; binomial from chance: P<0.02). There was no significant difference in the extent of improvement between the exposed and unexposed nostrils (t(11)ǃ0.4, Pǃ0.7).We screened a further 50 subjects to obtain 12 more non-detectors (9 female) for a control study (Fig. 1b) in which we tested for confounding possibilities: first, subjects were initially selected for nondetection, and thus their performance could either remain the same or improve, but not deteriorate (a 'floor effect'); second, even a minute leak in the block may have enabled the blocked nostril to learn; third, participation in olfactory testing may improve performance over time. The control study was identical to the experimental, except that both nostrils were blocked.Control subjects (Fig. 1b) did not differ from the experimental group at baseline (mean accuracy at baseline: experimental group, 25%, s.d. 11; control group, 27%, s.d. 11, t(22)ǃ0.44, Pǃ0.7), and remained at chance detection after 21 days of exposure (change from baseline: t(11)ǃ1.6, Pǃ0.14; binomial from chance: Pǃ0.4). The difference between the two groups after exposure (mean accuracy after 21 days, average for both nostrils) was 52% (s.d. 20) in the experimental group and 35% in the control group (s.d. 14; t(22)ǃ2.3, P<0.03), negating possible confounding factors.We exploited the paired anatomy of the olfactory system 5,6 to demonstrate that the plasticity that underpins the emergence of androstenone detection originates in the central components of the olfactory system. These components may be likened to pattern recognition, which occurs at the olfactory bulbs or in primary olfactory cortex -a substrate that shares information from both nostrils 7 and is optimized for olfactory learning 8-10 . We do not rule out a contribution to plasticity from the peripheral components of the olfactory system 11,12 -peripheral receptors may be induced in the unexposed nostril in response to a central signal (direct or hormonal, for example). It remains to be determined how central and peripheral mechanisms could interact to maximize plasticity in the olfactory system.

The physiological and psychological effects of 2 human sex-steroid derived compounds, 4.16-androstadien-3-one (AND) and l,3,5(10),16-estratetraen-3-ol(EST) were measured in 24 subjects who participated in a within-subjects, double-blind experiment. A dissociation was evident in the physiological effects of AND, in that it increased physiological arousal in women but decreased it in men. EST did not significantly affect physiological arousal in women or men. Neither compound significantly affected mood. AND is an androgen derivative that is the most prevalent androstene in human male sweat, male axillary hair, and on the male axillary skin surface. The authors argue that AND's opposite effects on physiology in men and women further implicate this compound in chemical communication between humans.

It has been estimated that approximately 30% of the population is unable to detect the odor of androstenone. These estimates, however, were made using tests and criteria optimized for identifying detection. Such criteria favor Type II over Type I errors--that is, they are excellent at identifying true detectors at the cost of erroneously labeling some detectors as non-detectors. Because these criteria were used to identify non-detectors, it is possible that the rate of non-detection may have been overestimated. To test this we screened 55 subjects for non-detection employing previously used methods. This screen yielded nine putative non-detectors, a 16.3% putative non-detection rate. We then retested these putative non-detectors using a forced choice (yes-no) paradigm to obtain a precise measure of their sensitivity. We found that this group of putative non-detectors was significantly above chance at detecting androstenone (P < 0.001), despite very low self-confidence in their performance. Based on the results of the signal detection analysis in this sample, we estimate the rate of actual androstenone non-detection in young healthy adults is between 1.8 and 5.96%, which is significantly lower than previously estimated. This finding is significant considering the implications of specific anosmias on the understanding of odor discrimination.