An odor, scent, or fragrance involves one or more chemical compounds that become volatized, typically at a very low concentration, and that humans perceive by the sense of olfaction (or smell). Over time, different words have come to be associated with negative or positive scents. For example, in many parts of the world the word odor typically has a negative connotation, even indicating that something stinks or reeks. Scents or aromas typically refer to something pleasant. The term smell, when used as a noun, is used for both unpleasant as well as pleasant odors. Memories related to odors, scents, or fragrances can be very powerful. Olfactory receptors in the brain link directly with the limbic system or mammalian layer, the part of the brain where emotional impulses arise. Emotional memories that are connected with smells, therefore, can be very powerful—positively or negatively, depending on the situation and what you smelled at the time. A study led by C. Bushdid estimated that humans can discriminate among more than one trillion olfactory stimuli—nothing like the sensitivity exhibited by dogs such as bloodhounds and beagles, but pretty impressive nevertheless.