Dissociable effects of inter-stimulus interval and presentation duration on rapid face categorization

Fast periodic visual stimulation combined with electroencephalography (FPVS-EEG) has unique sensitivity and objectivity in measuring rapid visual categorization processes. It constrains image processing time by presenting stimuli rapidly through brief stimulus presentation durations and short inter-stimulus intervals. However, the selective impact of these temporal parameters on visual categorization is largely unknown. Here, we presented natural images of objects at a rate of 10 or 20 per second (10 or 20 Hz), with faces appearing once per second (1 Hz), leading to two distinct frequency-tagged EEG responses. Twelve observers were tested with three squarewave image presentation conditions: 1) with an ISI, a traditional 50% duty cycle at 10 Hz (50-ms stimulus duration separated by a 50-ms ISI); 2) removing the ISI and matching the rate, a 100% duty cycle at 10 Hz (100-ms duration with 0-ms ISI); 3) removing the ISI and matching the stimulus presentation duration, a 100% duty cycle at 20 Hz (50-ms duration with 0-ms ISI). The face categorization response was significantly decreased in the 20 Hz 100% condition. The conditions at 10 Hz showed similar face-categorization responses, peaking maximally over the right occipito-temporal (ROT) cortex. However, the onset of the 10 Hz 100% response was delayed by about 20 ms over the ROT region relative to the 10 Hz 50% condition, likely due to immediate forward-masking by preceding images. Taken together, these results help to interpret how the FPVS-EEG paradigm sets temporal constraints on visual image categorization.