Marie de Montalembert

Are impairments of time perception in schizophrenia a neglected phenomenon

Based on clinical, phenomenological and neurobiological observations, psychiatrists often report a deficit in time estimation in patients with schizophrenia. Cognitive models of time estimation in healthy subjects have been proposed and developed for approximately 30 years. The current theory in the field of time perception, which is supported by a connectionist model, postulates that temporal judgement is based upon a pacemaker-counter device that depends mostly upon memory and attentional resources. The pacemaker emits pulses that are accumulated in a counter, and the number of pulses determines the perceived length of an interval. Patients with schizophrenia are known to display attentional and memory dysfunctions. Moreover, dopamine regulation mechanisms are involved in both the temporal perception processes and schizophrenia. Thus, it is still unclear if temporal impairments in schizophrenia are related to a specific disturbance in central temporal processes or are due to certain cognitive problems, such as attentional and memory dysfunctions, or biological abnormalities. The authors present a critical literature review on time perception in schizophrenia that covers topics from psychopathology to neuroscience. Temporal perception appears to play a key role in schizophrenia and to be partially neglected in the current literature. Future research is required to better ascertain the underlying mechanisms of time perception impairments in schizophrenia.

Shape from shading: new perspectives from the Polo Mint stimulus.

The visual system exploits prior knowledge on the world to disambiguate a scene. In particular, the inference of shape-fromshading relies on the fact that light comes from above our heads. Recent studies have helped make that assumption more precise and suggested that the preferred light source position was further biased on the left of the vertical. We investigated the generality of this result in two experiments with a different stimulus and a novel task. The stimulus was shaped like a ring (Polo Mint) divided in eight equal sectors. Depending on the assumed light source position, all sectors but one could be perceived as either convex or concave. In the first experiment, observers had to report the different shaped sector position (left or right side of the stimulus). In the second experiment, they had to classify the shape of the odd sector (convex or concave). Various amounts of blur were also applied on each stimulus. Results in both experiments confirm previous studies that observers prefer stimuli lit from the left rather than the right. We also demonstrate that left-lit stimuli give the observers a more sensitive perception of the shape of the objects. Finally, the second experiment confirms a preference for globally convex shapes, especially when the stimulus is severely blurred.