Time after stress: the role of attention and visual perception

Our ability to accurately compute the durations of events in the millisecond and second range underlies our performance in a plethora of key cognitive and sensorimotor tasks which are critical for behavioural regulation and survival, like orienting attention, making predictions, speech recognition, production and motor action. Despite this, our perception of the duration of an event in this range is not always veridical, but can be biased by external or internal factors, such as stimulus features or changes in psycho-physiological arousal (such as those induced by stress). More specifically, previous studies, corroborated by a large body of anecdotical reports, have conveyed that stressful or threatening stimuli or events leading to psycho-physiological (or arousal) changes are associated with the overestimation of time, so that people under stress perceive time as passing slower. It has been proposed that stress-induced temporal biases may be advantageous from an evolutionary perspective in that they allow a better adaptation of the observer to an external threat (favouring a better detection of threats and rapid motor responses). However, it remains largely unknown a) whether the effect of stress is limited to the aversive event which caused it, or lingers further after the event is gone, and b) which is the ultimate cause of perceptual duration overestimation. The research in this thesis exploits evidence form psychophysical research on time perception in the realm of emotionally relevant and irrelevant sensory stimuli, and psychophysiological stress research, as starting points to explore the relationship between stress-related physiology and the perception of time in the short time scale. The goal is to understand whether stress-physiology biases temporal perception by altering high-level cognitive (i.e., attention, Chapter 7 2), low-level perceptual (i.e., contrast perception, Chapter 3) processes, or both. To addressed this complex research question, I conducted three within-subjects studies: two of them (Chapters 2 and 4) where I combined methods from psycho-physiology and psychophysics to investigate the effect of stress on time perception, and a purely psychophysical one (Chapter 3) where I attempted to better characterize the effect of stimulus contrast on duration perception independently from stress. A secondary scope of my PhD project was to explore the phasic (i.e., event-evoked) pupillary dynamics while participants were judging stimulus durations. To this end, for all the studies included in this thesis, I collected pupil size measurements as participants performed behavioural temporal tasks, in a bid to understand which components of a pupillary response may be associated to timing mechanisms. As a whole, the work in this thesis suggests that pupil size may be an important non-invasive measure to gain a better grasp on the processes underlying the encoding of temporal information embedded in sensory stimuli, and it sheds some light on the complex relationship between stress-physiology and the perception of time. In fact, this work suggests that temporal biases in the milliseconds time scale (i) exist in relation to sensory stimuli which are not associated to the source of stress and presented after it has ceased, (ii) that these effects are dose- and time-dependent, and (iii) that they may be associated to modulation of high-level attentional and low-level perceptual processes.