Research indicates that pheromone signals are detected through an organ 3 inches inside the nose called the Vomeronasal Organ (VNO).

The vomeronasal organ, or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Ludvig Jacobson in 1813. Studies indicate that when the VNO detects the pheromone, it sends a response signal to the brain. This is different than the sense of smell that detects scents. The interpersonal "chemistry" (attraction, arousal, interest, excitement, etc) that people experience in relationships happens on a subconscious level.

Within the subject of pheromones the mention of a human VNO will raise controversy. While there is no question that a VNO exists in lower mammals the existence in humans of a VNO is subject to interpretation by many scientists. The concept may, or may not be critical to the human pheromones perception. While the VNO in lower mammals is essential for sexual signal reception in humans the detection and processing of these signals may be autogenous. Our personal conviction is that humans have an active VNO.

The vomeronasal organ is mainly used to detect pheromones, chemical messengers that carry information between individuals of the same species, hence is sometimes referred to as the "sixth sense." The VNO has two separate types of neuronal receptors. The receptors are distinct from each other and form the large family of receptors in the main olfactory system. Evidence shows that the VNO responds to nonvolatile cues which stimulate the receptor neurons. Information is then transferred to the accessory olfactory bulb as well as other centres of the brain such as the anterior part of the hypothalamus. Scientists are still studying the receptors to determine whether the VNO is devoted to exclusively to sensing pheromones, or whether pheromone receptors are part of a more complex system along with olfactory receptors in the nasal passage.

Its presence in many animals has been widely studied and the importance of the vomeronasal system to the role of reproduction and social behavior (through influence on anterior hypothalamus) has been shown in many studies. Its presence and functionality in humans is widely controversial.

It is key to note that the vomeronasal organ detects other compounds in addition to pheromones and that some pheromones are detected by the main olfactory system. Experimental studies showed potentially functional VNO near the base of nasal septum in adults. In the experiment, applying chemicals derived from adult human skin to the VNO showed changes in the autonomic nervous system (ANS), and in the periodicity of the follicle stimulating hormone (FSH) and Luteinizing (LH) from the pituitary gland, indicating that potentially functional VNO-hypothalamic-pituitary-gonadal axis exists. [3].

3.  Meredith, M Sensory processing in the main and accessory olfactory systems:                                 
     comparison and contrasts. J. Steroid  Biochem. Molec. Biol. 39:601-614, 1991.