Affective modulation of post-auricular reflexes to serial probes : a test of the nursing hypothesis

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The post-auricular reflex (PAR) is a vestigial microreflex which is thought to be a component of startle. However, unlike prototypical components of startle (e.g. acoustic blink reflex), the PAR is potentiated during appetitive motivational states and suppressed during defensive ones. An electromyographic (EMG) experiment with 37 men and 47 women tested if the Nursing Hypothesis could satisfactorily explain this anomalous PAR response. The Nursing Hypothesis proposes that infants of an ancestral mammalian species instinctively retracted their pinnae to signal their intention to nurse. Accordingly, appetitive emotions would still prime the ear-retraction musculature of adult humans, thereby causing facilitation of the PAR if a startling stimulus occurred. The present study failed to support the Nursing Hypothesis. Participants viewed one of 14 categories of slides while a series of startle probes was presented. Male participants showed the expected enhancement of PAR amplitude as nursing-related stimuli such as photographs of delicious food were viewed. Female participants by contrast had suppression of PAR amplitude when photographs of healthy breasts or of faces of beautiful women were viewed. Alternate hypotheses of PAR modulation are discussed.