Sources of learning and their role in the experience of placebo nausea

Abstract

The purpose of the project was to better understand sources of learning that lead to placebo nausea, with the aim of developing interventions with clinical utility. After introducing theory regarding placebo effects and nausea in Chapter 1, Chapter 2 systematically reviewed studies that had previously attempted to manipulate nausea via the placebo effect. This review revealed that there were some features of interventions associated with success, but that results were often conflicting and a better paradigm to model placebo nausea in healthy participants was required. Chapter 3 introduced such a paradigm, illustrating how Galvanic Vestibular Stimulation (GVS) could be used to administer both placebo and nauseating stimulation under the guise of an experiment looking at the effects of GVS on spatial ability. Chapter 4 presented a study that used this paradigm to explore the development of placebo-induced relief from nausea through instruction, conditioning, and their combination. Both sources of learning reduced nausea on test relative to controls. Chapter 5 refocused on the problem of nausea that is incidentally conditioned in the treatment context. A model of conditioned nausea was tested, and it appeared that conditioning had developed and had also generalized perfectly well to a new test context. This finding was taken as evidence that the GVS device had overshadowed any context-nausea learning, and was used to inform the development of a new latent inhibition paradigm tested in Chapters 6 and 7. Chapter 6 pre-exposed these GVS-related reactive cues in the form of placebo stimulation in an attempt to retard conditioning through latent inhibition. Here, pre-exposure reduced conditioned nausea to the placebo on test relative to a group who did not receive pre-exposure, and had reduced nausea to the level of a control group who received no conditioning. Given that the applicability of any such intervention to applied settings rests on its ethicality, in a novel extension, Chapter 7 tested whether this latent inhibition effect required deception. This study replicated the effect of pre-exposure observed in Chapter 6, as well as finding that latent inhibition occurred to the same extent in a group fully informed as to the nature of the pre-exposure phase. Together, these findings suggest that placebo effects can occur after both instruction and conditioning, and that taking advantage of placebo effects in the clinic could offer an important method of intervening to reduce nausea.