Once the optimally made image is viewed under optimal conditions by the trained and experienced clinician, that image should be optimally interpreted. Unfortunately, physiological phenomena within each clinician may exert their influence.¹ As an educator and consultant in oral and maxillofacial radiology I have observed two such phenomena that are particularly important, certainly with regard to student and general clinicians. These are the reversible (or ambiguous) figures and the Mach band effect.

Perhaps the best known of such reversible (or ambiguous) figures is the “two faces or vase” figure. When such a figure is viewed. only one orientation can be perceived. This phenomenon is believed to arise from the transmission of sensory data to the visual cortex by way of at least two alternate pathways. The clinical importance of this phenomenon is that the clinician’s perception of a particular image may change due to fatigue that occurs in one pathway compelling the image data to be transmitted by the other pathway. Figure 3.1 shows this phenomenon with some of my undergraduate students in formal examinations. Although it is expected that the clinician’s experience of this phenomenon will decline with further training and practice, fatigue will always be a problem when the workload increases. Examination-induced fatigue may have contributed to these students’ response to this type of image.