Dreaming: A neuropsychological view.

Abstract

Sleep and dream research has recently been invigorated by convergent data from complementary neuropsychological sources such as brain-imaging studies of normal sleep, clinical investigations of dream disorders in brain-damaged patients and analyses of dream reports from healthy individuals. Here we show that by adopting a neuropsychological approach to dream phenomenology we can gain important insights into the processing of information within specialised brain regions during sleep. We first review clinical studies revealing that dreaming can cease or be reduced after lesions within parieto-occipital and deep bifrontal regions, but that dreaming remains largely unaffected by dorsolateral prefrontal lesions. Damages in occipito-temporal visual regions can selectively affect the visual quality of the dreams and often co-occur with similar visual impairments while awake. Based on these clinical observations, it is thus possible to infer which brain structures might underlie normal dreaming. We then describe some cardinal phenomenological features of normal dream experience. Using multidimensional statistical methods, we identified recurrent cognitive dimensions in large samples of dream reports without any a priori coding of their content. Our analyses revealed well-segregated and consistent cognitive dimensions in the dreams from different individuals, including verbal and motor activities, visual processing such as colour vision and motion processing, and strong emotions such as fearful experiences. These findings suggest that dreaming might implicate distinct but reproducible patterns of activation across the motor, visual and limbic systems. In the last part, we introduce a neuropsychological approach to normal dreaming (i.e. dreams from neurologically healthy individuals). In particular, we show that some bizarre but common features in normal dreams present striking similarities with neuropsychological symptoms observed in brain-damaged patients while being awake, suggesting specific commonalities in brain organisation. For example, dreams present many instances of delusional misidentification for people or places characterised by a mismatch between the identity and the physical appearance of a character or a place. Dreams contain many other visual distortions such as the multiplication of a visual percept in time or space, deficits in size perception, reduced colour vision, etc. These bizarre features of visual processing during sleep suggest reduced neural activity within specialised visual areas and/or functional disconnections within the visual system as well as from other higher-level brain regions. In conclusion, by providing specific predictions about likely patterns of cerebral activity during sleep, dream studies open up a new road for the interpretation of future brain maps of human sleep while shedding light on the varieties of conscious brain states.