Ackerman points to research showing that different animal species communicate within their own specific lanes of sound, called “auditory niches,” to which their ears are most sensitive. This sensory specialization allows a species’ signals—such as a warning of a predator—to be heard by its own kind without being lost in the din of other creatures.

Countershading is a form of natural camouflage in which an animal’s body is most brightly colored on the parts least exposed to sunlight and darkest on the parts most exposed. A penguin, for example, is white on its belly to blend in with the sky when seen from below and black on its back to blend in with the ocean depths when seen from above.

Ackerman describes the human skin as a “feeling bubble,” a living, breathing “space suit” that defines our individuality. This “bubble” actively protects us, regulates our temperature, and serves as the primary frame for our sense of touch.

Ackerman uses the chemical term hypergolic metaphorically to describe the potent effect of language. In chemistry, two hypergolic substances ignite spontaneously on contact, producing a reaction more powerful than either component alone. The author extends this principle to words and images: Combined deliberately, they can capture emotions and sensations that neither conveys independently, producing meaning that exceeds the sum of their parts.

Lateral inhibition is a form of pain relief based on the principle of sensory confusion. Rubbing the area around an injury causes a “mob of neurons” to try to respond at once (105), creating a mass of signals that can block the original pain message from being clearly perceived by the brain.

The McClintock Effect is named for researcher Martha McClintock, who first described it in 1971. It is a hypothetical phenomenon in which women who live in close proximity, such as in dormitories or as roommates or romantic partners, begin to menstruate at the same time. The book explains that this might be caused by a pheromone present in sweat that affects and synchronizes menstrual cycles, though McClintock’s original findings have never been replicated, and scientists today believe that the phenomenon likely does not exist.

Ackerman uses the term “mute sense” for the sense of smell. She notes that smell alone has no dedicated vocabulary, leaving speakers to rely on comparisons (“smoky,” “floral”) or describe the emotions an odor evokes rather than the odor itself, so that we are often “tongue-tied” when trying to name what we are experiencing (6).

The olfactory bulb is the smell center of the brain, which receives impulses directly from receptor cells in the nose. Unlike other senses, smell signals are sent straight into the limbic system (an evolutionarily older brain region associated with emotion and memory), making an odor’s effect immediate and capable of retrieving memories before conscious thought intervenes.

Pheromones are chemical signals used by animals that trigger specific behaviors and physiological responses in one another. Described as the “pack animals of desire” (26), these scents are used to mark territory, establish hierarchies of power, and signal sexual receptivity. The book also explores the scientific search for their existence and influence in humans.

Proprioceptors are a category of internal sense receptors that inform us about our body’s condition and position in space. From the Latin for “one’s own” (95), they are thought to be largely responsible for physical self-awareness. They monitor everything from limb placement and movement to the status of internal organs.

Ackerman uses the term “sensory misers” for people who are unappreciative or stingy with their sensory experiences. The author argues that people who ignore sensory detail forfeit the accumulated pleasure that makes daily life meaningful, to the point where life becomes, in her phrase, “not worth living” (256).

“Sensuist” is a term coined by the author to describe “someone who rejoices in sensory experience” (xviii). This is distinguished from a “sensualist,” which has come to imply someone primarily concerned with gratifying sexual appetites, though the term “sensualist” originally meant roughly what Ackerman means by “sensuist.” The book itself is presented as an act of celebration for fellow sensuists.

A concept drawn from Aboriginal Australian traditions, songlines are associated with groups such as the Arrernte, describing a labyrinth of invisible roads that crisscross the continent. These paths are encoded in ancient songs whose melodic contours describe the physical features of the land, allowing an expert to navigate vast distances by singing the landscape.

The stereochemical theory of smell posits a direct link between the geometric shapes of molecules and the odor sensations they produce. In this “lock-and-key” model (14), a molecule of the right shape fits into a corresponding neuron niche, which then triggers a specific nerve impulse to the brain.

Synesthesia is a phenomenon in which the stimulation of one sense simultaneously triggers a sensation in another (from the Greek for “perceive together”). A person with this trait might see colors when hearing sounds or taste a flavor when hearing a word, offering a glimpse into a less differentiated mode of perception in which sensory channels overlap.