On the relevance of mental imagery beyond stress-related psychiatric disorders.

If a patient with major depressive disorder (MDD) reported that he is hearing self-derogatory thoughts aloud he would probably be diagnosed with psychotic depression. This might be followed by a change in medication rather than the offer of specific therapeutic strategies that target intrusive mental images. Yet in support of a growing literature of mental imagery in psychiatric disorders (1) recent research by our group shows that about one in two patients with mild to moderate MDD and three in four patients with obsessive-compulsive disorder (OCD) report sensory properties of their cognitions (2;3). In fact, most of us share such experiences: if we remember how we met our loved one, we sometimes see a visual image of how we first met them and this visual image can be accompanied by intense positive emotions. Similarly we may vividly remember how it hurt when we were beat up in the school yard and again this tactile image may come with intense negative emotions. On a more systematic level, mental imagery has been defined as the experience of conscious contents that possesses sensory properties and therefore resembles actual perceptual experience (1;4;5). The perceptual properties can be visual but can also cover other sensory modalities such as tactile, acoustic or somatic experience. In contrast to cognitions, mental images are not purely verbal or abstract (5). If these mental images occur involuntarily they are also referred to as “intrusions” (5). Mental imagery has been recognized in a number of disorders (5). Intrusions are a diagnostic feature of stress-related disorders such as acute stress disorder, posttraumatic stress disorder (PTSD) and dissociative disorder (6). While intrusive mental images are also recognized in obsessive compulsive disorder they are not part of the diagnostic criteria of depressive disorders (6). Intrusive mental images may be regarded to exist on a spectrum where actual psychotic symptoms are at the far end and defined by impaired reality testing (7, p. 371). Interestingly, the definition of hallucinations (6, p. 87) overlaps with that of intrusive mental images. Just like hallucinations, intrusive mental images may at times be vivid and clear, have the full force and impact of normal perceptions and be out of voluntary control (8). To illustrate this point, it has been found by our group that a considerable number of healthy controls and patients with OCD also report hearing voices (8). The same study showed that only about a third of schizophrenics (31.1%) but a full third of healthy “voice hearers” (33.3%)