Opposite patterns of change in perception of imagined and physically induced pain over the course of repeated thermal stimulations.

BACKGROUND: Individuals frequently show habituation to repeated noxious heat. However, given the defensive function of human pain processing, it is reasonable to assume that individuals anticipate that they would become increasingly sensitive to repeated thermal pain stimuli. No previous studies have, however, been addressed to this assumption. Therefore, in the current study, we investigated how healthy human individuals imagine the intensity of repeated thermal pain stimulations, and compared this with the intensity ratings given after physically induced thermal pain trials. METHODS: Healthy participants (N = 20) gave pain intensity ratings in two conditions: imagined and real thermal pain. In the real pain condition, thermal pain stimuli of two intensities (minimal and moderate pain) were delivered in four consecutive trials. The duration of the peak temperature was 20 s, and stimulation was always delivered to the same location. In each trial, participants rated the pain intensity twice, 5 and 15 s after the onset of the peak temperature. In the imagined pain condition, participants were subjected to a reference pain stimulus and then asked to imagine and rate the same sequence of stimulations as in the induced pain condition. RESULTS: Ratings of imagined pain and physically induced pain followed opposite courses over repeated stimulations: Ratings of imagined pain indicated sensitization, whereas ratings for physically induced pain indicated habituation. The findings were similar for minimal and moderate pain intensities. CONCLUSIONS: The findings suggest that, rather than habituating to pain, healthy individuals imagine that they would become increasingly sensitive to repeated thermal pain stimuli. SIGNIFICANCE: This study identified opposite patterns of change in perception of imagined pain (sensitization) and physically induced pain (habituation). The findings show that individuals anticipate that they would become increasingly sensitive to repeated pain stimuli, which might also have clinical implications.

Natural scene recognition with increasing time-on-task: the role of typicality and global image properties.

Human observers can recognize natural images very effectively. Yet, in the literature there is a debate about the extent to which the recognition of natural images requires controlled attentional processing. In the present study we address this topic by testing whether natural scene recognition is affected by mental fatigue. Mental fatigue is known to particularly compromise high-level, controlled attentional processing of local features. Effortless, automatic processing of more global features of an image stays relatively intact, however. We conducted a natural image categorization experiment (N = 20) in which mental fatigue was induced by time-on-task (ToT). Stimuli were images from 5 natural scene categories. Semantic typicality (high or low) and the magnitude of 7 global image properties were determined for each image in separate rating experiments. Significant performance effects of typicality and global properties on scene recognition were found, but, despite a general decline in performance, these effects remained unchanged with increasing ToT. The findings support the importance of the global property processing in natural scene recognition and suggest that this process is insensitive to mental fatigue.