Demonstrating habituation of a startle response to loud noise.

While classical conditioning is no doubt a standard part of most if not all introductory psychology courses, habituation is less prevalent as a topic. A search of Teaching of Psychology and others sources uncovered classroom demonstrations of classical conditioning processes, (Gibb, 1983; Kohn and Kalat, 1992; Sparrow & Fernald, 1989; Vernoy, 1987) while no examples of demonstrations of habituation were found. Most psychology students are familiar with the startle reflex, which is "a universal and phylogenetically ancient reflex," (Howard & Ford, 1992, p. 695); it is one of the innate, defensive reflexes of humans and other vertebrates that occur in response to unconditioned eliciting stimuli such as an unexpected loud noise. A startle reflex that is elicited by an unpredicted noise is referred to as an acoustic or audiogenic startle reflex, (Wilkens, Hallett, & Wess, 1986; Brown, Rothwell, Thompson, Britton, Day, & Marsden, 1991; Gluck, Mercado, & Myers, 2008). Habituation is the process by which such reflexes are modified by experience; hence, it is a basic form of learning (Bouton, 2007; Gluck et al., 2008). A normal audiogenic startle reflex was first quantifiably studied by Jacobsen (1926), with further study by Landis and Hunt, (1936, 1937, 1939). The acoustic startle reflex consists of eye movements, especially blinking, facial grimacing, contractions of flexor muscles in the neck, trunk and elbows, with extensor muscles being activated much less often. Movements of the head and the remainder of the body are less reliably involved (Brown et al., 1991; Wilkens et al., 1986). Habituation occurred for all of these components with the exception of blinking (Wilkens et al., 1986). Howard and Ford (1992) observed considerable variation between individuals in the topography and duration of the startle reflex. As noted by Wilkens et al., (1986) the startle reflex is subject to habituation. Typically, an unconditioned response or reflex that is elicited by an unconditioned stimulus will show habituation if the stimulus is repeated (Gluck et al., 2008). That is, measures of the response will show orderly decreases in magnitude as the stimulus is repeatedly applied. More simply put, habituation is the process by which a stimulus loses its attention-grabbing properties, i.e., its "novelty," and decreases in responsiveness to the stimulus are seen when the stimulus is presented repeatedly or for an extended period. Through the process of habituation, an organism ceases to engage in attending or other responses to the stimulus; the stimulus comes to be ignored (Bouton, 2007; Gluck et al., 2008). To illustrate habituation, the contingencies needed to elicit an acoustic startle reflex were arranged and repeated to record the habituation of the various responses. * METHODOLOGY Due to difficulties with an audience observing a person undergo a startle reflex, the demonstration to be described was conducted in another setting and digitally videotaped for later use in the classroom. Prior to any observations, protocol approval was obtained from the South Dakota State University Institutional Review Board. Four research assistants were involved, one of whom had volunteered to be videotaped and exposed to an eliciting stimulus; we initially needed to determine an effective unconditioned stimulus (US) with which to elicit the startle reflex, with the designated volunteer out of the room. We initially tested balloons being popped as suggested by Vernoy (1987), but found this event was not effective at eliciting a significant response. After some trial and error, we decided to strike a metal table with a standard weight claw hammer, which at an approximate distance of 1 m elicited a clearly observable startle reflex. A sound decibel meter allowed us to quantify the decibel level, approximately 80 dB. To produce rapid habituation, an inter-stimulus interval (ISI) of 15 s was employed. …