Investigating the Impact of Inter-Stimulus Delay on Temporal Processing

Abstract

Time is a universally fundamental concept studied by psychologists, neuroscientists, and philosophers, yet the field of time perception remains not fully understood. In experimental lab settings, Interstimulus Interval (ISI) is a critical factor during the presentation of stimuli. Researchers across various fields, including attention, perception, and memory, typically vary ISI to avoid inherent learning effects that might skew results. However, it remains unclear how variations in ISI affect temporal processing. This raises important questions about the influence of interstimulus delay on temporal judgment and sensitivity. To address these questions, we adopted a temporal discrimination paradigm where a standard stimulus is followed by a comparison stimulus. Instead of keeping ISI constant, we manipulated ISI to two different durations: 500 ms (short) and 1000 ms (long). We conducted two studies, each comprising two experiments. In Study 1, we examined the influence of varied ISI on temporal judgment in visual and auditory discrimination tasks. For the visual discrimination task, data were obtained from 35 participants, each performing 182 trials. The findings revealed significant differences in the Point of Subjective Equality (PSE) between short and long ISIs, suggesting an overestimation for long ISI and underestimation for short ISI. Statistically significant differences in the Difference Limen (DL) were also observed for both ISI conditions. In the auditory discrimination task, data were gathered from 28 participants, each performing 168 trials. The results were consistent with the visual task, showing low PSE (overestimation) for long ISI and high PSE (underestimation) for short ISI. However, no statistically significant differences in DL were found for auditory tasks. In Study 2, we investigated how variations in ISI affect emotional context-dependent temporal processing. Experiment 1 involved 22 participants, each performing 196 trials, with standard and comparison stimuli presented for different durations. The findings mirrored those of Study 1, indicating temporal dilation for long ISI and temporal compression for short ISI, with no significant differences in DL. Experiment 2 involved 38 participants, each performing 196 trials, with both standard and comparison stimuli presented for the same durations. We analyzed the proportion of long responses (p-long) for each duration under two conditions (short vs. long delay). A 2x7 repeated measures ANOVA (Delay: Short ISI vs. Long ISI; Durations: 200 ms to 800 ms) showed significant main effects for both duration and delay, but no interaction effect. Overall, the findings suggest that varying ISI influences temporal judgment. Both short and long ISIs affect perceived duration, supporting aspects of prominent cognitive models such as the Internal Clock Model and the Attentional Gate Model. However, these models only partially explain the timing mechanisms observed, indicating that brain-level processes must also be considered.