At EVER2017, we reported that visual detection of a temporal gap becomes difficult when spatial frequency separation is introduced between markers delimiting the gap. In that report, however, gap thresholds were measured separately, in different runs, for within-frequency and across-frequency gap detection. Therefore, our previous findings may be simply explained by the effect of spatial-frequency specific adaptation on visible persistence. Here, we reassessed the across-frequency interference in gap detection using a procedure uncontaminated by spatial-frequency specific adaptation. Gaussian-windowed sinusoidal gratings were used for gap markers. Spatial frequencies of the leading and trailing markers were manipulated to be either 2 or 4 cpd. Gap thresholds were measured using a two-interval, two-alternative forced choice method with a two-down/one-up interleaved adaptive staircase procedure targeting 70.7% correct detection: Four staircases corresponding to the four combinations of marker frequencies were interleaved in a single run and randomly selected in each trial. Sixteen observers with normal vision completed two runs, and the mean of estimated thresholds between two runs for each condition were subject to analysis. Gap thresholds for across-frequency markers (2-4 and 4-2 conditions) were significantly higher than those for within-frequency markers (2-2 and 4-4 conditions). For within-frequency gap detection, gap thresholds were significantly higher for 4 cpd than for 2 cpd markers, which was consistent with the well-known decreased temporal resolution with increasing spatial frequency. We confirmed our previous findings that temporal gaps bounded by different frequency markers are more difficult to detect than those bounded by identical frequency markers. We propose that perceptual discontinuity signaling the frequency separation interferes with gap representation and longer gaps are required for correct gap detection.
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