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Dream Mentation Production and Narcolepsy: A Critique

Studies of the mechanisms of dream mentation production continue to stimulate interest among cognitive neuroscientists. The two dream mentation models assessed in Fosse’s study (this issue), the ‘‘activation-only’’ model (e.g., Antrobus, 1991; Foulkes, 1993, 1985) and the Activation, Input source, Modulation (AIM) model (Hobson et al., 2000), are presently the two predominant approaches to the problem of dream production. My recent review of these approaches (Nielsen, 1998, 2000) concluded that several types of evidence for which REM and NREM cognition can be directly compared tend to favor the AIM type of model. However, differences between the theoretical tenets and predictions of the two models are complex, and direct comparisons of them in experimental contexts are relatively rare. Fosse’s work therefore represents an important advance in the development of methodologies for testing the two types of model comparatively. His approach is to examine several distinct measures of sleep mentation in combination to determine how the profile of measures varies across different sleep conditions. In this case, he examined dreams drawn from REM sleep that occurred both at night and during afternoon naps, and dreams from persons with (n 5 15) and without (n 5 9) narcolepsy. Antrobus (2000) proposed that such an approach could possibly be used to resolve the long-standing problem of conducting fair comparisons of dreams from REM and NREM sleep for evidence of qualitative differences. The profile of mentation characteristics studied by Fosse included visual vividness, report length, bizarreness and reflective consciousness. These characteristics are, in theory, isomorphically related to physiological features of REM sleep and should, in principle, either covary in unison with global fluctuations in brain activation or covary differentially with the three components of the AIM model. Findings supported both the activation-only and the AIM models to different degrees. Specifically, within subjects most variables fluctuated with circadian activation level and thus supported the activation-only model. However, between subjects the variables fluctuated differentially—a finding consistent with the AIM-derived notion that narcolepsy patients may have higher than normal levels of aminergic modulation during REM sleep. Presumably, the higher levels of reflective consciousness seen in the dreams of the narcolepsy group are due to higher aminergic reactivation of prefrontal cortical areas during REM sleep. On the other hand, the lower levels of (bi-