Introduction
Peripherally-located flickering distractors dilated the perceived time of centrally-located static target (Okajima & Yotsumoto, 2016). We denote the time dilation of this type as the distractor-induced time dilation (DITD), as opposed to the one induced by a dynamic central stimulus. The arguments about the mechanism underlying the DITD remain unsettled. In this study, we investigated the DITD through the manipulation of relative distractor-frequency and the stimulus pattern of different types (flickering or drifting).
Experiment 1
In Experiment 1, we hypothesized that if the distractor-frequency is a critical factor for the DITD, then the high-frequency distractors will lead to more time dilation whereas the low-frequency distractors will lead to less or no time dilation.
Method
Participants Eleven students (three males and eight females) with normal or corrected-to-normal visual acuity took part in Experiment 1.
Stimuli There were four conditions manipulated differently in Experiment 1. Stimuli in the first two conditions (called flickering conditions) were a center-surround pattern, created as follows: a square patch (2º×2º) consisting of 64 random luminance components was used as the central target and eight patches with the same configuration were used as the peripheral distractors (Figure 1). In the drifting conditions, stimuli were the same center-surround pattern except: (1) a patch consisting of four 0.5º×0.5º square components with the contralateral ones remaining identical was used as the central target and eight patches with the same configuration were used as the peripheral distractors; (2) the stimulus pattern drifted leftward or rightward. All distractors were equidistant to the central target at a
retinal eccentricity of 6 degrees. The standard stimulus lasted 3 s with every stimulus component flickering or drifting at 3Hz. The comparison stimuli varied symmetrically with respect to the standard duration in five steps of 0.75 s, from 1.5 s to 4.5 s. In the comparison stimuli, the temporal frequency of the central target was 3Hz, whereas the temporal frequency of the peripheral distractors was 1.5Hz or 6Hz. The number of trials was 280: 40 training trials + temporal frequency (2) × type of dynamic pattern (2) × level of comparison stimuli (5) × repeated times of comparison (6) × sequence of stimulus presentation (2).
Procedure Participants were instructed to fixate their eyes on the fixation cross at the beginning of each trial. After a fixed interval of one second, one stimulus was presented for a designated duration. Participants were asked to pay attention only to the central patches and to avoid observing peripheral distractors. One second after the offset of the first stimulus, the second stimulus was presented. Finally, a response display appeared that allowed participants to make a response. A 2IFC with the method of constant stimuli was used. Participants were asked to compare the sensed duration of the two stimuli by pressing the “left” key if the first one was judged to last longer than the second or the “right” key if the second one was judged as longer. During the stimulus presentation, participants were not allowed to count the time, and all numerical judgments were made without visual aids or timers. An example trial is shown in Figure 2.
Results and Discussion
We fitted the data using a two-parameter cumulative Gaussian function. The PSE value reflects the proportion where participants judged the duration of the standard stimulus as equally as that of the comparison stimulus. The obtained PSEs from the low temporal frequency conditions and the high temporal frequency conditions did not differ in the ANOVA’s result. We attributed the non-significant outcome
Probing time dilation by dynamic peripheral distractors
Keywords: temporal frequency, peripheral distractors, time perception
行動システム専攻心理学コース 姚 啟睿
to the inconsistency of attention allocation across the stimulus patterns. The pattern with the high-frequency distractors might capture attention more often than the pattern with low-frequency distractors. Therefore, even though participants were required to attend only to the central target, we cannot rule out the possibility that attention was averted from the target unintentionally across different conditions.
Experiment 2
In Experiment 2, the allocation of attention to the central target or to the peripheral distractors was
controlled. If the high-frequency peripheral distractors induce time dilation, it would be attributed to that the attention was drawn to the distractors. If the higher-frequency is critical for the improvement of sensitivity, attending to higher-frequency distractors would induce more accurate discrimination performance than attending to the central target.
Method
Participants Seven students (three males and eight females) with normal or corrected-to-normal visual acuity took part in Experiment 2.
Stimuli Stimulus configuration and procedure in Experiment 2 was identical to Experiment 1 with the following
exceptions: (1) only the flickering pattern was employed; (2) two types of chromatic center-surround patterns were used: the central target was green-and-black alternating and the peripheral distractors were red-and-black alternating, and vice versa for the other pattern. The number of
experimental conditions was 280 trials: 40 training trials + temporal frequency (2) × attended components (2) ×level of comparison stimuli (5) ×repeated times of comparison (6) × sequence of stimulus presentation (2).
Procedure The task was identical to Experiment 1 except: Four participants attended to the duration of the red component, while ignoring the duration of the green component. The rest participants completed the reversed procedure.
Results and Discussion
Identical to Experiment 1, we estimated the PSEs and the JNDs (which index the discrimination sensitivity) for each participant separately. There were no significance with regard to the PSEs. For the JNDs, a two-way repeated-measures ANOVA revealed a significant main effect of distractor frequency, F(1, 5) = 7.28, p = 0.04, 𝜂𝑝2= 0.59, and a significant main effect of attention, F(1, 5) = 7.15,
p = 0.04, 𝜂𝑝2= 0.59. The results indicated that attending to the high-frequency distractors significantly improved the discrimination sensitivity. However, the DITD was not observed even after the attention was manipulated, indicating that attention is not a critical factor that accounts for the DITD. We speculated that the relative magnitude between the target frequency and the distractor-frequency in the distractor-frequency spectrum was not large enough so as to trigger a salient DITD.
Experiments 3, 4 and 5
We investigated the probability for a wide range of distractor-frequencies to induce longer time judgments under both the supra-second and sub-second domains, using both flickering and drifting patterns. To this end, the comparison of the time dilation induced by different distractor-frequencies was made possible by introducing a static central target. If the relative frequency difference is a relevant factor, we would expect the time judgments of the central target is dependent on the distractor-frequency. Experiments 3, 4, and 5 served as pilot studies.
Method
Participants Seven students (three males and eight females) with normal or corrected-to-normal visual acuity took part in Experiments 3 and 4. Eight students
participated in Experiment 5.
Apparatus, Stimuli and Procedure Apparatus, Stimuli, and Procedure replicated those in Experiment 1 except the following: Each participant performed 140 trials in total including 20 training trials and 120 experimental trials in Experiments 3 and 4, and performed 280 trials (including 40 training trials) in Experiment 5. In Experiments 3 and 4, the standard stimulus lasted 3 s with all components remained static through the experiment. The comparison stimuli lasted either 2.5 s or 3.5 s and appeared with a static central target and eight peripheral distractors
modulated by one of five frequencies (0.625Hz, 1.25Hz, 2.5Hz, 5Hz, and 10Hz). In Experiment 5, the standard stimulus lasted 600 ms, whereas the comparison stimuli lasted either 450 ms or 750 ms. The stimulus configuration and the modulation of distractor-frequency in Experiment 5 were identical to those in Experiments 3 and 4. We used a flickering pattern in Experiment 3, and a drifting pattern in Experiment 4. In Experiment 5, flickering and drifting conditions were counterbalanced across participants.
Results and Discussion
The dependent variable in Experiments 3, 4, and 5 was the proportion of the comparison stimuli judged as longer than the standard stimulus. All the proportion data were normalized by taking the inverse sine of the square root of the corresponding proportion. In Experiment 3, the ANOVA results revealed a significant main effect of
distractor-frequency, F(4, 24) = 4.51, p = 0.007, 𝜂𝑝2= 0.43. A multiple comparison using Bonferroni’s adjustment showed that the proportions in the 10Hz conditions was
significantly higher than those in the 0.625Hz conditions (p
= 0.02). For the drifting condition in Experiment 5, the main effect of distractor-frequency was significant, F(4, 28) = 3.31, p = 0.02, 𝜂𝑝2= 0.32. A multiple comparison using Bonferroni’s adjustment showed that the proportion in the 0.625Hz conditions was significantly lower than those in the 2.5Hz conditions (p = 0.03) and those in the 5Hz conditions (p = 0.05), respectively. The findings suggested that the amount of distractor-induced time dilation might be an increasing function of distractor-frequency. In Experiments 6 and 7, only 0.625Hz and 5Hz were selected as candidate distractor-frequencies.
Experiments 6 and 7
The following Experiments 6 and 7 served to verify the results from the previous approaches using the method of constant stimuli.
Participants Nine students participated in Experiment 6. Other nine students participated in Experiment 7.
Apparatus, Stimuli and Procedure Apparatus, Stimuli, and Procedure replicated those in Experiment 1 except the following: In Experiment 6, each participant performed 336 trials in total including 48 training trials and 288
experimental trials. In Experiment 7, each participant performed 384 trials in total including 96 training trials and 288 experimental trials. All sessions in Experiment 6 were separated into two days, whereas all sessions in Experiment 7 were divided into three one-day sessions. In both Experiment 6 and 7, participants completed a training block, including trials from all conditions on each day. In Experiment 6, the standard stimulus lasted 600 ms with both the central and the peripheral component static. The duration of comparison stimulus changed in six levels (300, 400, 520, 680, 900 or 1200 ms) with a static central target and flickering or drifting peripheral distractors. In Experiment 6, the standard stimulus lasted 3s and the duration of comparison stimulus in Experiment 7 was determined according to Hashimoto and Yotsumoto’s (2015) study ranging from 1.5 s to 4.5 s in six levels using linear spacing. The flickering and drifting rate was set either at 0.625Hz or 5H.
Results and Discussion
In order to make a direct comparison between the two experiments, all data were converted to the duration distortion ratio (DDR). The resulting DDRs were analyzed via a 2×2×2 mixed design ANOVA with the between-subjects variable specified as “duration (600 ms, 3 s)”, and the two within-subjects variables specified as “distractor-frequency (0.625Hz, 5 Hz)” and “dynamic pattern
(flickering, drifting)”. The results yielded a significant main effect of duration, F(1, 16) = 11.86, p = 0.03, 𝜂𝑝2= 0.43, and a significant main effect of distractor-frequency, F(1, 16) = 9.53, p = 0.007, 𝜂𝑝2= 0.37.
The results showed that the DDRs were higher in the low frequency than those in the high distractor-frequency, indicating that distractor-frequency is a critical factor for the DITD. And the DDRs were higher in the short duration than those in the long duration, indicating that the amount of DITD depends on the stimulus duration (Figure 3).
Conclusions
The present findings regarding the DITD were summarized as follows: (1) both the distractor-frequency and the stimulus duration are a critical factor for the DITD; (2) the amount of DITD might be an increasing function of
both the distractor-frequency and the stimulus duration; (3) the type of dynamic patterns does not affect the DITD.
Reference Okajima, M., & Yotsumoto, Y. (2016).
Flickering task–irrelevant distractors induce dilation of target duration
depending upon cortical distance. Scientific Reports, 6, 32432. doi:10.1038/srep3243
Figure 1. The configuration of the stimulus patterns used in Experiment 1. The flickering stimulus (A) and the drifting stimulus (B).
Figure 2. Illustration of an example trial.
Figure 3. Illustration of the estimated DDRs. The graph shows: (1) the estimated DDRs are higher in 3s conditions than those in the 600 ms conditions; (2) the estimated DDRs
are higher in 5Hz conditions than those in the 0.625Hz conditions. -0.35 -0.25 -0.15 -0.05 0.05 0.15 0.25 0.625Hz 5Hz D u ration Di sto rtion R atio 600ms 3s
