QST was traditionally used to quantify sensory function in healthy people and in patients at risk for neurological impairment [1]. It also serves as a tool to investigate factors that affect pain perception [2–4]. In QST, a measurable stimulus is applied to the skin and the subject or patient reports his or her perception of it. The method employs calibrated instruments to deliver known intensities of physical stimuli, for example, mild electric current (by means of surface electrodes), temperature (via electric thermodes with controlled surface temperatures), touch (using filaments whose bending force depends on diameter and length), pressure (exerted by spring-loaded devices), and vibration (using tuning forks or vibrators that deliver sinusoidal stimuli at a given frequency). Stimulus of a given intensity is applied, and the subject reports whether or not the stimulus is perceived (or, in pain studies, whether or not the stimulus elicits pain). The lowest intensity that is perceptible (or, if pertinent, painful) is the detection threshold.

Two general methods are employed to determine these thresholds: the method of limits and the method of levels. With the method of limits, the stimulus is progressively increased and the subject declares when it first becomes perceptible. With the method of levels, stimulus of a defined intensity is applied and then increased or decreased by specific increments depending on whether or not the subject perceives it (protocols may differ in terms of the number of consistent responses required to progress upward or downward in stimulus intensity).

With the method of limits, sensory information is processed neurologically at the same time as the stimulus intensity is being changed. The inherent response lag leads to a small error in threshold measurement; consequently, thresholds measured with the level of limits skew higher than those measured with the method of levels [5, 6]. Moreover, the rate of change of the stimulus affects thresholds obtained by the method of limits.

The method of levels is known as “forced-choice,” as the subject must declare or “choose” whether or not the stimulus is perceived. Because this method takes longer and is more repetitive, error can result if the subjects become fatigued or distracted as the test proceeds.

Experimental variables such as the application site, the surface area of contact, the frequency of the stimulus (in the case of vibration), and the rate of change of stimulus intensity will affect the absolute value of thresholds measured. Consequently, the absolute values measured are a function of the experimental conditions employed and the lack of standardization complicates comparisons between experiments. Although this chapter reviews a number of published studies, it focuses on the relative thresholds assessed within experiments to draw conclusions about variables that affect sensory perception.

Research on extragenital sites provides some general perspective on sensory perception thresholds. One general finding is that sensitivity to touch, vibration, and thermal stimuli varies by anatomical site. For example, the hands appear to be more sensitive to touch, and especially to vibration, than the feet [1, 7].

Besides the anatomical location itself, advancing age appears a most significant effect on perception of mechanical stimuli. For example, the sensitivity of the hands and feet to touch, skin indentation, and vibration declines with age. This decline becomes apparent by the fifth decade, progresses exponentially after age 65 or 70 [8, 9], and is more severe for the lower than the upper extremities [9, 10, 5]; the latter may reflect the longer distance of the neural pathway that the sensory input must travel.

By contrast, evidence for an age-related decline in the perception of thermal stimuli is inconsistent [5, 7, 8, 11]; some studies found an age-related decline, but several show no change. The disparate results may relate to the experimental conditions employed in each study: thermal stimuli are perceived more easily when a larger skin surface area is stimulated [11].

Gender differences in sensory perception have been found inconsistently. With respect to mechanical stimuli, several studies found no gender differences in perception thresholds on either the forehead [1], the hand and forearm [11], or the foot [10], regardless of age. However, a US study of 350 people found that, specifically among those aged over 50, women were more sensitive to vibration on the dorsum of the hands and feet than men [9]. A Taiwanese study among 484 subjects found vibratory thresholds to be lower in women than in men on the dorsum of foot, with no difference on the thenar eminence of the hand [5].

With respect to thermal stimuli, women exhibited higher sensitivity in a subset of studies. For example, a Dutch study found women to be more sensitive to thermal stimuli on the foot [7]; a Taiwanese study found women to be more sensitive to warm thresholds on both the hand and the dorsal surface of the foot [5]; and a British study found women to be more sensitive than men to heat and cold stimuli on the thenar eminence of the hand, the distal phalanx of the middle finger, and the dorsal surface of the forearm [11]. However, a North American study of 48 people found young men aged 19–31 to be more sensitive than women in the same age group to warm stimuli on the plantar surface of the feet, but found no difference on the thenar eminence of the hand [10]. The variability in results among different studies may relate to the range of the age groupings analyzed or to differences in the types of thermal probe employed.

In summary, anatomical sites vary in their sensitivity to sensory stimuli. Advancing age has been shown to reduce sensory perception of mechanical stimuli on extragenital sites, but limited evidence exists for age-related reductions in the perception of thermal stimuli. Some evidence exists that women are more sensitive than men to mechanical and thermal stimuli, but this may depend on the body site stimulated, the exposure conditions, and the age range of the subjects studied.