Pupil dilation in time perception

Time perception corresponds to how we perceive and understand the duration, sequence, and pacing of incidents in our surroundings and is the subjective expe- rience of the course of time. The integration of data from various sensory inputs as well as higher-order cognitive processes like attention, recall, and decision-making are all part of this intricate and multidimensional process. Pupil size is known to increase in response to all kinds of demands like attention, working memory, decision-making, difficult perception, memory recall, etc. In fact, studies show increased pupil size to any kind of task demands. A recent study suggested a dichotomy of task demands based on their distinct neural responses (Farooqui and Manly, 2018). The commonly recognized type of task demand (i.e., type I demand) pertains to issues like increased attention, working memory, the complexity of decisions, etc. For what is being done at that moment. This kind of demand increases activity in the control-related fronto- parietal regions and is well-known to increase pupil size. Farooqui and Manly suggested a different kind of demand (or type II demand) that is related to the length and complexity of the episode of cognition being organized as one unit. They argued that this demand comes about because we execute long tasks as one entity. We, e.g., write emails, shop and not individually execute their very- many component acts. Longer tasks require a longer period of cognition to be organized and controlled as one unit. They evidenced that this demand leads to a deactivation of the control-related fronto-parietal regions and suggested that this demand may psychophysiologically be the opposite of type I demands. The current study investigates this thesis. We used a time-interval replication task. Time-interval replication involves cre- ating an extended cognitive episode. Longer intervals will require creating longer episodes and hence involve higher type II cognitive demands. If type II demands indeed generate the opposite psychophysiological response compared to type I de- mands, we can expect longer time-interval replications to lead to a decreased pupil size. Participants were given various time intervals, e.g., 11 seconds, to replicate by pressing a button at the beginning and at the end of the interval. We used a set of short (8-12 seconds) intervals and a set of long (14-18 seconds) intervals. Long intervals were expectedly more demanding and led to a more erroneous performance. We tested 30 participants (22 females, ages 18-27). If, as is popularly thought, pupil size increases during any kind of demand, then it should be larger when participants replicated longer durations. In contrast, if type II demands of creating longer cognitive episodes were psychophysiologically opposite, then pupil size may be smaller during longer durations. We found this was indeed the case. We show that task demands related to organizing longer cognitive episodes may indeed be distinct from the commonly recognized task demands related to increased attention and working memory.