Interoception is the perception of bodily cues, and Shands and Schor (1982) have suggested that panic patients "are interoceptive experts, being able to describe significant changes in almost every organ system and region of the body" (p. 108). This issue has been investigated in a series of studies by Ehlers and her colleagues. In the first two studies, Ehlers, Margraf, Davies, and Roth (1988) and Ehlers, Margraf, Roth, Taylor, and Birbaumer (1988) did not find that panic disorder patients were more accurate in their cardiac perceptual abilities, compared to controls, in a task in which they had to match the frequency of a train of audible pips to their own heart rate. There are a number of problems with this methodology; for example, the participant is required to monitor both an internal and an external signal. That is, the task requires abilities over and above those involved in interoception. This is particularly important because interoception is known to be impaired under conditions of external stimulation (e.g., Pennebaker, 1982). In a further study, therefore, Ehlers and Breuer (1992) employed a revised methodology using patients with panic disorder, infrequent panickers, patients with other anxiety disorders, and normal controls. The participants were required to count their heartbeats silently during signalled intervals without taking their pulse or using any other strategies such as holding their breath. In the second of three experiments, panic disorder patients showed better performance on the heartbeat-tracking task than the other groups. In the third study, patients with panic disorder and patients with generalised anxiety disorder showed better heart rate perception than depressed controls. Ehlers (1995) further showed in a longitudinal design that more accurate heart rate perception predicted subsequent relapse over a 1-year follow-up even though patients were in remission at initial testing. A recent prospective longitudinal study by Godemann, Schabowska, Naetebusch, Heinz, and Strohle (2006) showed that patients who had developed vestibular neuritis were more likely to have developed panic disorder after 6 months if they had excessive and continuing fears of the initial vertigo and nausea experienced as part of the vestibular dysfunction.
The research we have chosen concerning information-processing biases in panic disorder raises a number of points. First, panic patients interpret ambiguous stimuli as threat-related; however, in patients without any agoraphobic symptomatology, this bias is specific to ambiguous bodily sensations. Second, panic patients, along with other anxiety groups, appear to have greater interoceptive acuity relative to normal controls. Both of these sets of studies imply the existence of a dominant schematic model in panic disorder patients. As we have suggested in Chapter 5, such models will serve to facilitate the processing of congruent information and inhibit the processing of incongruent information within the SPAARS system. Consequently, ambiguous material is likely to be interpreted in a panic-related way and attentional resources are likely to be focused on interoceptive changes. In line with this, other information-processing research indicates that, first, there is some evidence that the relationship between representations of bodily sensations and putative catastrophic interpretations of those sensations is a strong one within the memory structures of individuals with panic disorder (e.g., Clark, 1996); second, panic patients exhibit memory biases for panic-related material (e.g., Coles & Heimberg, 2002); and, finally, panic patients exhibit attention biases for panic-related material on a number of attention-related tasks (e.g., MacLeod, Campbell, Rutherford, & Wilson, 2004).
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