The anatomical pathways for processing of odorous stimuli include the olfactory nerve projection to the olfactory bulb, the trigeminal nerve projection to somatosensory and insular cortex, and the projection from the accessory olfactory bulb to the hypothalamus. In the majority of tetrapods, the sex-specific effects of pheromones on reproductive behavior is mediated via the hypothalamic projection. However, the existence of this projection in humans has been regarded as improbable because humans lack a discernable accessory olfactory bulb. Here, we show that women smelling an androgen-like compound activate the hypothalamus, with the center of gravity in the preoptic and ventromedial nuclei. Men, in contrast, activate the hypothalamus (center of gravity in paraventricular and dorsomedial nuclei) when smelling an estrogen-like substance. This sex-dissociated hypothalamic activation suggests a potential physiological substrate for a sex-differentiated behavioral response in humans.

The cerebral activations were studied by measurements of regional cerebral blood flow (rCBF) with positron emission tomography (PET), during passive and birhinal smelling of AND, EST, and odorless air (AIR). Twenty-four healthy subjects (12 females) participated. Smelling of AND and EST was assumed to cause cerebral activation if rCBF was higher than during smelling AIR (see Experimental Procedures).

In the present study, we therefore investigated possible sex differences in cerebral activation, using two different sex hormone-resembling substances—first in females, then in males. These compounds were 4,16-androstadien-3-one (AND), a derivative of testosterone produced in human axillary secrete in concentrations which are up to twenty times higher in men compared to women; and oestra-1,3,5(10),16-tetraen-3-ol (EST), a substance resembling naturally occurring oestrogenes (). Three specific issues were addressed. (1) Do AND and EST activate the human brain? (2) Are the activations sex specific, i.e., do AND and EST activate different regions in males and female? (3) Are the activations located in regions mediating reproductive behavior?

Olfaction in humans with special reference to odorous 16-androstenes their occurrence, perception, and possible social, psychological, and sexual impact.

Female axillary extract applied to the upper lip can alter the timing of ovulation and menstruation of the recipient (). This phenomenon is suggested to underlie the menstrual synchrony among roommates and is assumed to be mediated by the hypothalamus (). Exposure to male axillary secretion is reported to give more regular menstrual cycles (). The axillary and skin secretions contain compounds resembling sex hormones (). Monti-Bloch et al. found that such compounds induce changes in body temperature, skin conductance, respiration, and heart rate and produce surface electrical potential recorded from the nasal epithelium of the vomeronasal pit in a sex-related way. The authors, therefore, suggested that the steroids they used had pheromone-like characteristics (). One of them, oestra-1,3,5(10),16-tetraen-3yl acetate, was shown to elicit cerebral activation in males in nonodorous, undetectable concentrations (). This finding could reflect a cerebral effect which is unrelated to odor. Also, it has recently been reported that a putative pheromone receptor gene is expressed in human olfactory mucosa (). These data raise the question whether there are compounds that via the nasal mucosa activate the human hypothalamus in a sex-specific mode. If so, such compounds would fulfill one important criterium to qualify as candidates for putative pheromones in humans. Theoretically, the signals from such compounds could in humans, as in pigs and ferrets, be mediated by other sensory pathways than VNO.

Olfaction in humans with special reference to odorous 16-androstenes their occurrence, perception, and possible social, psychological, and sexual impact.

The pheromones are, according to the original definition, volatile compounds secreted into the environment (in sweat, urine) by one individual of a species, perceived by another individual of the same species, in whom they trigger a behavioral response or physiological change (). Pheromones are, in the majority of mammals, transduced in the vomeronasal organ (VNO) (situated in the nasal cavity) to signals which, via the accessory olfactory bulb, the medial amygdala, and stria terminalis, reach the anterior hypothalamus (). In ferrets and pigs, an alternative pathway via the main olfactory bulb has been suggested (). Via the hypothalamus and its connections, the pheromones influence sexual behavior and reproductive functions () in a sex-specific way (). Although VNO is reported to exist in human adults (), it is uncertain whether and how possible pheromone signals may be mediated, as the accessory olfactory bulb regresses after the fetal period (), and alternative neuronal connections from VNO to the brain have not been convincingly demonstrated (). Nevertheless, the possibility of pheromone-like effects in humans is discussed in the literature.

Comparative morphology and histochemistry of glands associated with the vomeronasal organ in humans, mouse lemurs, and voles.

Sensitivity and behavioral responses to the pheromone androstenone are not mediated by the vomeronasal organ in domestic pigs.

Women and men rated AND and EST similarly with respect to odor pleasantness, familiarity, irritability, and intensity ( Figure 3 ). There were no differences in the adaptation rate to the odor of the respective compound, i.e., the time from the presentation to subjective loss of perception of odor (in females, 104 ± 24 s and 104 ± 30 s for AND and EST, respectively, versus 96 ± 36 s and 108 ± 36 s in males). No significant sex- or compound-related difference was found in breathing frequency or breathing amplitude ( Figure 4 ). The olfactory thresholds for AND and EST were within the normal range (10M to 3 × 10M). Thus, all the measured psychophysical parameters were similar in males and females.

The vertical axis shows percent change from baseline. The baseline values were measured during the 120 s immediately preceding the respective scan. Data are expressed as mean and SEM

To evaluate whether AND and EST differently activated other regions than the hypothalamus, AND and EST were compared to each other with explorative statistics (see Experimental Procedures). In females, AND activated right fusiform and lingual gyrus compared to EST, whereas, in the males, EST activated these regions in relation to AND ( Table 1 ). These areas have been attributed to visual imagery of faces (), and, although the activation in the sex opposite to the given compound is of note, the functional significance of this finding is presently unclear. No other significant activations were observed, but, in females, a cluster emerged in the anterior hypothalamus during the AND versus EST contrast when lowering the threshold (p < 0.1).

Comparisons between the two sexes were finally conducted with the random effect analysis (SPM 99, Wellcome Foundation, London), using a rectangular mask delineated on the standard brain MR image, covering the entire hypothalamus. Also, this calculation showed that the hypothalamic activation with AND was significantly higher in females (corrected p = 0.001; peak coordinate: 6, −6, 2), whereas the activation with EST was significantly more pronounced in males (corrected p = 0.002; peak coordinate: −8, 6, −12).

Next, we conducted a volume of interest (VOI) analysis () to examine whether the observed hypothalamic activations were truly sexually dimorphic. The respective hypothalamic clusters (obtained during AND versus AIR in females and EST versus AIR in males) were used as VOIs. The mean normalized rCBF was calculated in each subject during smelling of AND and EST. The values from the two conditions (six values per subject) were then compared within and between the two groups of subjects. In the between sexes comparisons, the difference between the respective compound and AIR in the same VOI were entered in the ANOVA model. The comparisons between AND and EST in the hypothalamic VOIs showed the following. When females smelled AND, the rCBF in the hypothalamic VOI (covering the preoptic and the ventromedial nucleus) was higher than when they smelled EST (57 ± 2 ml/min/100 g versus 55 ± 3 ml/min/100 g; F = 9.3, p = 0.01, one-way repeated measure ANOVA). The corresponding value in males smelling AND was 53 ± 4 ml/min/100 g. The calculated sex difference was significant: F = 10.5, p = 0.004, two-way repeated measure ANOVA. Conversely, when men smelled EST, the rCBF in the hypothalamic VOI (covering the dorsomedial and paraventricular nucleus) was higher than when they smelled AND (55 ± 2 ml/min/100 g versus 53 ± 4 ml/min/100 g; F = 5.5, p = 0.04, one-way repeated measure ANOVA). The corresponding value in females smelling EST was 53 ± 4 ml/min/100 g. The calculated sex difference was significant: F = 15.5, p < 0.001, two-way repeated measure ANOVA. There was a significant sex × VOI interaction of AND and EST activation in AND- and EST-defined VOIs (F = 28.7, p < 0.001, two-way repeated measure ANOVA).

We used the atlas ofto specifically localize the hypothalamic nuclei (see Experimental Procedures). According to Schaltenbrand, the hypothalamic activation in females covered the preoptic area and ventromedial nucleus with a center of gravity corresponding to the preoptic nucleus. The hypothalamic activation by EST in males covered the dorsomedial and paraventricular nucleus extending to lower fornix, with a center of gravity corresponding to the dorsomedial hypothalamic nucleus (). Coregistration and superpositioning of the two hypothalamic clusters on individual MR images showed a minor overlapping, with centers of gravity distanced by about 10 mm.

At p < 0.1, the AND versus AIR contrast in females showed clusters not only in the hypothalamus but also in right amygdala + piriform cortex, anterior cingulate, and right lingual gyrus. Accordingly, in the EST versus AIR contrast in males, at p < 0.1, clusters appeared in right and left amygdala + piriform + insular cortex and in anterior cingulate in addition to the hypothalamus. No other clusters were observed in relation to AIR in males or females ( Table 1 ). Thus, the pattern of activation with AND and EST clearly showed reciprocal features in the two sexes.

In females, AND activated the anterior-ventral hypothalamus ( Figure 1 Figure 2 ) but not the olfactory regions (amygdala, piriform, orbitofrontal, and insular cortex) (). These olfactory regions were activated when females smelled EST ( Table 1 ). To the contrary, males activated the hypothalamus but not the olfactory regions when smelling EST ( Figure 1 Table 1 ). When males smelled AND, no activations were found at a probability of p < 0.05. When lowering the threshold (p < 0.1), clusters appeared, however, in right amygdala + piriform cortex, right cerebellum, and right postcentral gyrus ( Table 1 ).

(A) Normalized rCBF values in the hypothalamic VOI during smelling of AND and EST in females and males. Comparisons between AND and EST were carried out within each sex group. (B) Comparisons between males and females, based on the difference between the tested compound and AIR. There was a significant sex × VOI interaction (F = 27.7, p < 0.001). The AND versus AIR difference in males was zero. *p < 0.05, **p < 0.01, ***p < 0.001. For further details, please see the Experimental Procedures and Results sections

(A) Normalized rCBF values in the hypothalamic VOI during smelling of AND and EST in females and males. Comparisons between AND and EST were carried out within each sex group. (B) Comparisons between males and females, based on the difference between the tested compound and AIR. There was a significant sex × VOI interaction (F = 27.7, p < 0.001). The AND versus AIR difference in males was zero. *p < 0.05, **p < 0.01, ***p < 0.001. For further details, please see the Experimental Procedures and Results sections

The Sokoloff color scale illustrates z values (0.0–4.5). The clusters were thresholded at 3.1; thus, only regions with z > 3.1 and cluster size > 0.8 cm 3 are shown. (A) AND versus AIR in females. (B) EST versus AIR in females. (C) EST versus AIR in males. Subject's right side is to the left. The Talairach coordinates are given. The same brain sections are shown for the two contrasts within each sex group to illustrate the lack of hypothalamic activation with AND in males and EST in females. Only the significant clusters (p < 0.05) are shown. The AND versus AIR in males showed no clusters at p < 0.05 and is therefore not illustrated

The Sokoloff color scale illustrates z values (0.0–4.5). The clusters were thresholded at 3.1; thus, only regions with z > 3.1 and cluster size > 0.8 cm 3 are shown. (A) AND versus AIR in females. (B) EST versus AIR in females. (C) EST versus AIR in males. Subject's right side is to the left. The Talairach coordinates are given. The same brain sections are shown for the two contrasts within each sex group to illustrate the lack of hypothalamic activation with AND in males and EST in females. Only the significant clusters (p < 0.05) are shown. The AND versus AIR in males showed no clusters at p < 0.05 and is therefore not illustrated

The Sokoloff color scale illustrates z values (0.0–4.5). The clusters were thresholded at 3.1; thus, only regions with z > 3.1 and cluster size > 0.8 cm 3 are shown. (A) AND versus AIR in females. (B) EST versus AIR in females. (C) EST versus AIR in males. Subject's right side is to the left. The Talairach coordinates are given. The same brain sections are shown for the two contrasts within each sex group to illustrate the lack of hypothalamic activation with AND in males and EST in females. Only the significant clusters (p < 0.05) are shown. The AND versus AIR in males showed no clusters at p < 0.05 and is therefore not illustrated

Discussion

Gower et al., 1988 Gower D.B.

Nixon A.

Mallet A.I. The significance of odorous steroids in axillary odour. Monti-Bloch and Grosser, 1991 Monti-Bloch L.

Grosser B. Effect of putative pheromones on the electrical activity of the human vomeronasal organ and olfactory epithelium. Monti-Bloch and Grosser 1991 Monti-Bloch L.

Grosser B. Effect of putative pheromones on the electrical activity of the human vomeronasal organ and olfactory epithelium. Jacob and McClintock 2000 Jacob S.

McClintock M.K. Psychological state and mood effects of steroid chemosignals in women and men. Sobel et al. (1999) Sobel N.

Prabhakaran V.

Hartley C.A.

Desmond J.E.

Glover G.H.

Sullivan E.V.

Gabrieli J.D. Blind smell brain activation induced by an undetected air-borne chemical. Royet et al., 1999 Royet J.P.

Koenig O.

Gregoire M.C.

Cinotti L.

Lavenne F.

Le Bars D.

Costes N.

Vigouroux M.

Farget V.

Sicard G.

et al. Functional anatomy of perceptual and semantic processing for odors. The purpose of the present study was to investigate whether there are chemical compounds which when smelled produce sex-differentiated activation in the human brain similar to the brain activation associated with pheromones in other species. Two substances related to male and female sex hormones were used: AND, a compound suggested to have some pheromone-like properties, repeatedly found in human axillary sweat and in urine, with concentrations that are up to twenty times higher in men than women (); and EST, which resembles naturally occurring estrogens (). EST is less documented than AND. Nevertheless, it was found to best fulfill the inclusion criteria, as its odor characteristics have in our previous studies as in the present ( Figure 3 ) been rated similarly by males and females, which was the prerequisite to allow any conclusions about possible sex differences not related to the mere odor stimulus. Also, like AND, EST is reported to cause change in mood and in skin conductance and heart and respiratory frequency in a sexually differentiated manner (). Furthermore,reported in a functional MR study that oestra-1,3,5(10),16-tetraen-3yl acetate, a compound very similar to EST, activates the brain in nonodorous concentrations, suggesting a stimulus other than the odor. Despite the fact that functional MR in limbic regions due to susceptibility artifacts may give slightly different results than PET, activation was found in anterior mesial thalamus-fornix-hypothalamus, thus, an area covering portions of our EST versus AIR cluster in males. At variance from our results, they also found a cluster in the right inferior frontal and cingulate cortex. This difference may, however, be attributed to different experimental procedures. The subjects in Sobel's study were instructed to judge whether an odor or nonodor was presented, whereas our subjects had to avoid all judgements of the presented items. The conscious judgement of odors is known to engage frontal lobes, including the anterior cingulate and inferior frontal gyrus ().

Wersinger and Baum 1997 Wersinger S.R.

Baum M. Sexually dimorphic processing of somatosensory and chemosensory inputs to forebrain luteinizing hormone-releasing hormone neurons in mated ferrets. Yokosuka et al. 1999 Yokosuka M.

Matsuoka M.

Ohtani-Kaneko R.

Iigo M.

Hara M.

Hirata K.

Ichikawa M. Female-soiled bedding induced fos immunoreactivity in the ventral part of the premamillary nucleus (PMv) of the male mouse. Oomura et al., 1988 Oomura, Y., Aou, S., Koyama, Y., Fujita, I., and Yoshimatsu, H. (1988). Central control of sexual behavior. Brain Res. Bull. 20, 683–670. The hypothalamic activation seems, however, congruent with pheromone effects as they are described in animals. Pheromone stimuli from male ferrets augment Fos immunoreactivity (IR) in the main olfactory bulb, preoptic, and ventromedial nuclei of female ferrets. Soiled bedding from female mouse induces IR in the ventral premamillary nucleus in their male conspecifics (). That these findings are relevant for the changes in sexual behavior is suggested by electrophysiological experiments: the electric stimulation of the ventromedial nucleus in female monkey is shown to elicit the copulatory act, whereas the stimulation of the dorsomedial hypothalamic nucleus in male monkey elicits penile erection and yawning (). Notwithstanding that spatial restrictions by the resolution of PET scanner and the 10 mm filtering hamper a separation between our two hypothalamic activations, it is of note that the respective center of gravity in our male and female subjects was remarkably congruent with the cited animal data.

Stefanczyk-Krzymowska et al., 2000 Stefanczyk-Krzymowska S.

Krzymowski T.

Grzegorzewski W.

Wasowska B.

Skipor J. Humoral pathway for local transfer of the priming pheromone androstenol from the nasal cavity to the brain and hypophysis in anaesthetized gilts. Anand Kumar et al., 1970 Anand Kumar T.C.

David G.F.X.

Umberkoman B.

Saint K.D. Uptake of radioactivity by body fluids and tissues in Rhesus monkeys after intravenous injection or intranasal spray of tritium-labelled oestradiol and progesterone. Hurtig et al., 1994 Hurtig R.R.

Hichwa R.D.

O'Leary D.S.

Boles Ponto L.L.

Narayana S.

Watkins G.L.

Andreasen N.C. Effects of timing and duration of cognitive activation in [15O]water PET studies. The routes by which AND and EST reached hypothalamus are not apparent from the present experiments, nor has the study been designed to address this issue. Theoretically, the signals could have been transduced via the nasal mucosa or via absorption of the steroids into the nasal vasculature to have direct effects upon the central nervous system. Humoral transport of the bore pheromone androstenol has been shown in an in vivo study in gilts () and of 3H-oestradiol in monkeys (). Androstenol was measured in the carotid rete after 2 min, with a maximum between 40 and 50 min after application, and 3H-oestradiol was measured in cerebral spinal fluid 1 min after application, with a maximum after 3 min. However, because the blood flow information is obtained during the first 30 s of the PET scan, it is unlikely that the steroid distribution to the vasculature, CSF and brain, is rapid enough to allow recording of its possible accumulation in the hypothalamus and the presumptive subsequent effect on rCBF (). Second, both steroids were detected in the hypothalamus after 60 min, which, if the same route of distribution was operating in our study, implies that the steroid effect on rCBF could persist throughout several scans (10 min elapsed between the different scans) and thus include the baseline condition. Third, the measured concentration of oestradiol in hypothalamus was in the cited studies not higher than in the cortex, suggesting a regional distribution incompatible with the present observations. Such activation would be expected if the observed effects were due to a humoral transport to the respective hypothalamic nuclei. However, in the absence of specific binding data from humans, the possibility of such a pathway cannot be definitively excluded. It is important to emphasize that even if the rCBF effects of AND and EST were mediated humorally, the observations still provide support for the principal hypothesis, namely, that these compounds would activate the sexually dirmorphic hypothalamic areas in a sex-related way.

Swaab and Fliers 1985 Swaab D.F.

Fliers E. A sexually dimorphic nucleus in the human brain. Byne et al. 2000 Byne W.

Lasco M.S.

Kemether E.

Shinwari A.

Edgar M.A.

Morgello S.

Jones L.B.

Tobet S. The interstitial nuclei of the human anterior hypothalamus an investigation in volume, cell size, number and density. Scott et al., 2000 Scott C.J.

Tilbrook A.J.

Simmons D.M.

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Ing N.H.

Clarke I.J. The distribution of cells containing estrogen receptor-alpha (ERalpha) and ERbeta messenger ribonucleic acid in the preoptic area and hypothalamus of the sheep comparison of males and females. Fernandez-Gausti et al., 2000 Fernandez-Gausti A.

Kruijver F.P.M.

Fodor M.

Swaab D.F. Sex differences in the distribution of androgen receptors in the human hypothalamus. As for the alternative explanation of a neuronal response, one must assume that AND and EST activate different peripheral chemosensory sites within the nasal mucosa and that, in females, the hypothalamus primarily receives signals from the AND-responding chemosensors and, in males, those from EST-responding chemosensors. Simply that the chemosensory sites and therefore the routes to the hypothalamus are different is, however, not sufficient to explain the observed sex-different activations. Neither can the different targets in the two sexes be explained from differences in odor perception, adaptation, respiratory pattern, or behavior ( Figure 3 Figure 4 ). Thus, there must be a sex factor/s that influences either the neuronal network transferring the signals to the appropriate targets in the hypothalamus or sex factors which are expressed in the anterior hypothalamus or both. One factor could be the male and female sex hormones, which can influence the possible peripheral chemosensors, the pathways to hypothalamus, as well as the hypothalamic target nuclei in a sex-differentiated manner. Another is the inherent sex difference in morphology and/or neurochemistry of the hypothalamic nuclei. For example, the interstitial nucleus 3 of the anterior hypothalamus (INAH3) is larger in men (). The preoptic and ventromedial nuclei contain higher concentrations of estrogen receptors in females (), whereas, in the dorsomedial nucleus, the androgen receptors are more abundant in males (). Notably, both types of sex hormones and receptors are found in males as well as in females, and the sex differences regard the respective concentrations. This may explain why the sex difference in hypothalamic rCBF during AND and EST was significant only when the more sensitive VOI analysis was applied but subsignificant with the GLM analysis (Experimental Procedures).

Cain and Murphy 1980 Cain W.S.

Murphy C.L. Interaction between chemoreceptive modalities of odour and irritation. Yousem et al. 1997 Yousem D.M.

Williams S.C.

Howard R.O.

Andrew C.

Simmons A.

Allin M.

Geckle R.J.

Suskind D.

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Brammer M.J.

Doty R.L. Functional MR imaging during odor stimulation preliminary data. Savic et al., 2000a Savic I.

Gulyas B.

Berglund H. Odorous stimuli are processed differently depending on the cranial nerves involved. Cain and Murphy, 1980 Cain W.S.

Murphy C.L. Interaction between chemoreceptive modalities of odour and irritation. Small et al., 1997 Small D.M.

Jones-Gotman M.

Zatorre R.J.

Petrides M.

Evans A.C. Flavor processing more than the sum of its parts. Takagi, 1984 Takagi S.F. The olfactory nervous sysstem of the old world monkey. Somewhat unexpectedly, during the conditions activating the hypothalamus, i.e., during smelling of AND in females and EST in males, clusters appeared in olfactory regions only when lowering the threshold for significance (p < 0.1). This was despite indifferent ratings of subjective perception of both compounds' odor component by both sexes ( Figure 3 ). A similar phenomenon has, however, been reported with the combined trigeminal and olfactory odorants, which activate the olfactory areas to a minor extent even when the odor component is strong (). We recently found, for example, that the odorant acetone strongly activates cortical regions related to the trigeminal stimulus but only to a minor extent the amygdala and no other classical olfactory regions (). A similar phenomenon was also detected with the combined trigeminal and olfactory odorant butanol (I.S., unpublished data). The underlying mechanism could be an inhibition of the olfactory stimulus by the trigeminal stimulus (). It is possible that such an interaction represents a more general phenomenon when a volatile odorous compound elicits two different chemosensory stimuli (pheromone and odorous, trigeminal and odorous). It is worth mentioning that a similar bimodal interaction has been reported during the perception of flavor (). We are, therefore, putting forward the hypothesis that, depending on the sex of the recipient, AND and EST are transmitted primarily as pheromone-like signals (to anterior hypothalamus) or odor signals (to the olfactory brain). When the hypothalamic pathway dominates (i.e., in females smelling AND and males smelling EST), the olfactory signals may be suppressed but still perceived and the olfactory regions engaged only to a lesser degree. The hypothalamic pathway may be direct from the olfactory bulb and separate from the bulbo-olfactory pathway, as shown in the old world monkey, which, like humans, lack the accessory olfactory nerve ().

Gower and Ruparelia 1993 Gower D.B.

Ruparelia B.A. Olfaction in humans with special reference to odorous 16-androstenes their occurrence, perception, and possible social, psychological, and sexual impact. Gower et al. 1988 Gower D.B.

Nixon A.

Mallet A.I. The significance of odorous steroids in axillary odour. Smals and Weusten 1991 Smals A.G.

Weusten J.J. 16-ene-steroids in the human testis. Gower et al., 1988 Gower D.B.

Nixon A.

Mallet A.I. The significance of odorous steroids in axillary odour. Leinders-Zufall et al., 2000 Leinders-Zufall T.

Lane A.P.

Puche A.C.

Ma W.

Novotny M.V.

Shipley M.T.

Zufall F. Ultrasensitive pheromone detection by mammalian vomeronasal neurons. Pheromones are (1) bodily secreted, (2) detected by another individual, (3) affect the neuroendocrine brain, and/or (4) behavior. According to previous studies, the first criterion is definitively fulfilled by AND (). The present study shows that both AND and EST sex-specifically activate relevant hypothalamic regions. We have, however, not examined whether the short exposure to AND and EST altered the subjects reproductive/behavior functions and thus have not tested the last criterion. Also, to avoid activations due to a dissolvent or its interaction with the respective steroid (), both AND (200 mg) and EST (200 mg) were used in pure, crystalline form. The air concentration reaching the subjects in the scanner (which had a continuos air suction device) was considerably lower but could still be higher than the physiological. However, it has recently been shown that the magnitude of neuronal response to pheromones does not change with pheromone concentration (). These findings indicate that the present data may well be relevant also for the physiological conditions.

Thus, notwithstanding that the existence of human pheromones is still an open question, we suggest that the present observations of a sex-differentiated hypothalamic activation in humans provide a fundament for further, extensive research of chemosensory signals in humans.