Sources of bias and uncertainty in a visual temporal individuation task

Despite a clear ability to detect temporal modulations of visual stimuli in excess of 50 Hz, temporal individuation and serial order judgment tasks can be performed only when stimuli alternate at much slower rates, and the nature of such sluggishness remains unclear. One example of a task with a slow temporal limit is the individuation of a cued letter in a rapid serial visual presentation (RSVP) stream. The present study investigates the nature of the code used to perform such a slow temporal individuation task and the sources of uncertainty involved. The results demonstrate that temporal, rather than ordinal, position in the RSVP stream is critical in serial order estimation, suggesting the involvement of a noisy temporal code. In addition to variability in temporal coding, observers' choices are also limited by a number of other factors, such as categorical errors and biases related to the position of the cue in the letters' stream. Attentional filtering improves categorization, but crucially, it does not seem to increase the temporal precision of judgment. Generalizing the present results, I suggest that perception of order is limited by an internal temporal sampling instability that is distinct and independent from attention and that, similarly to temporal jitter in a clock, acts as a low-pass filter that hinders the judgment of the order of events that unfold too quickly.