Sandwiched visual stimuli are perceived as shorter than the stimulus alone

Acta Psychologica 203 (2020) 102982

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Sandwiched visual stimuli are perceived as shorter than the stimulus alone Riku Asaoka

T

1

Department of Psychology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan

A R T I C LE I N FO

A B S T R A C T

Keywords: Time perception Temporal processing Visual masking Perceptual grouping Time reproduction task

A visual stimulus is perceived as shorter when a short sound is presented immediately before and after the visual target than when the visual target appears alone. It remains unclear whether the time compression occurs in an intramodal condition. Therefore, the present study examined how and when non-target sandwiching stimuli affect the perceived filled duration of target visual stimuli. We further hypothesized that this effect could be modulated by temporal and spatial proximity between the target and non-target stimuli. Experiments 1a, 1b, and 2 showed that non-target stimuli could decrease the perceived duration only when the inter-stimulus interval between these stimuli was 0 ms, using time reproduction and category estimation methods. Experiments 3 revealed that the time compression effect did not occur when both the non-target preceding and trailing stimuli were spatially distinct from the target. Experiment 4 demonstrated that either the preceding or trailing stimulus induced the time compression effect when the non-target stimuli were presented at the same position as the target stimuli. We discuss the implications of the time compression effect induced by non-target sandwiching stimuli with reference to the Scalar Expectancy Theory and the Neural Readout Model. We speculated that the attenuation of neural responses to the target via visual masking or perceptual grouping may be attributable to the time compression effect.

1. Introduction An accurate and stable estimation of time is very important to maintain proper perception, cognition, and behavioral processes in everyday activities that range from daily rhythm to communication with others. Despite its importance, humans do not have a sensory organ dedicated to time perception. Several models of time perception have been proposed. A leading theory is the Scalar Expectancy Theory (SET; Gibbon, 1977; Gibbon, Church, & Meck, 1984). This model assumes that time perception is performed via an internal clock-like mechanism, memory processes, and decision strategies. The internal clock-like mechanism contains a pacemaker, a mode switch, and an accumulator. The pacemaker begins to emit pulses at a certain rate at the onset of a target stimulus. During a target duration or an interval, the mode switch closes and allows the pulses emitted from the pacemaker to be collected in the accumulator. At target offset, the switch opens and prevents further accumulation of pulses. The number of pulses collected in the accumulator is compared with a reference time from memory. This comparison process decides the perceived duration in a linear relationship; a higher number of collected pulses indicate a longer perceived duration.

A recent study has proposed a Neural Readout Model which says that the neural response required to process a target stimulus is proportional to the perceived duration of the target (Eagleman & Pariyadath, 2009; Pariyadath & Eagleman, 2007, 2012). That is, lower neural activity leads to shorter perceived duration, and higher neural activity leads to longer perceived duration. Consistent with this hypothesis, recent event related potential and electroencephalography studies show direct evidence for neural correlates of temporal coding (e.g., Ernst et al., 2017; Horr, Wimber, & Di Luca, 2016; Li, Chen, Xiao, Liu, & Huang, 2017). Subjective duration is known to be distorted by non-temporal factors such as stimulus modality (Wearden, Edwards, Fakhri, & Percival, 1998), stimulus structure (Grondin, 1993; Rammsayer & Lima, 1991), repeated presentation (Matthews, 2011; Pariyadath & Eagleman, 2012), or audio-visual integration (Chen & Yeh, 2009; Morein-Zamir, SotoFaraco, & Kingstone, 2003). For example, filled duration (duration marked by one stimulus) is perceived as longer in duration than empty duration (interval marked by two short stimuli). Moreover, perceived duration can be altered when non-target stimuli are presented separately from the target stimulus. Evidence shows that a preceding visual flicker or sound(s) lengthens the perceived duration of subsequently

E-mail address: [email protected]. Present address: Department of Psychology, Kanazawa University, Kakuma, Kanazawa, Ishikawa, 920-1192 Japan; The address at which the author actually did the work: Department of Psychology, Graduate School of Arts & Letters, Tohoku University, Kawauchi 27-1, Aoba-ku, Sendai, Miyagi 980-8576 Japan 1

https://doi.org/10.1016/j.actpsy.2019.102982 Received 8 June 2019; Received in revised form 26 November 2019; Accepted 5 December 2019 0001-6918/ © 2019 Elsevier B.V. All rights reserved.

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proximity. There were two reasons for this hypothesis. First, spatial and temporal proximity are needed to induce several masking effects of non-target stimulus (stimuli) on time perception. As mentioned above, masking can decrease the perceived duration of the empty interval marked by two flashes (Derichs & Zimmermann, 2016; Zimmermann et al., 2014). This effect was robust in a multisensory setup where a brief tactile and visual stimulus defined one empty interval (Zimmermann, Derichs, & Fink, 2016). The mask did not induce the time compression effect when the position of the tactile and visual markers was spatially separate (Zimmermann et al., 2016) and the ISI between either the marker and the mask stimulus was increased (Derichs & Zimmermann, 2016). Furthermore, for filled time perception, the backward masking effect on perceived duration increased with increasing the ISI between the target and masking stimuli for < 70 ms (Cantor & Thomas, 1976). These findings indicate that the effect of sandwiching stimuli on filled time perception also can be modulated by the temporal and spatial relationship between the non-target and target stimuli. Secondly, several recent studies demonstrated that the effect of neighboring stimuli on time perception is strongly related to perceptual grouping between target and non-target stimuli (e.g., Derichs & Zimmermann, 2016; Klink, Montijn, & van Wezel, 2011; Ortega, Guzman-Martinez, Grabowecky, & Suzuki, 2012; Zhou, Yang, Mao, & Han, 2014). Our previous study has shown that both time dilation and compression effects induced by non-target sandwiching sound are based on audio-visual perceptual grouping (Asaoka & Gyoba, 2016). Perceptual grouping between more than two stimuli has been shown to occur in both temporal and spatial domains (Han, Humphreys, & Chen, 1999; Otto, Ögmen, & Herzog, 2009). Zhou et al. (2014) found that the preceding non-target stimuli compressed the perceived duration of the target (both were Gabor patches). Interestingly, the magnitude of the time compression effect dramatically reduced when the non-target and target had different orientations or were presented at different locations. They considered that a non-target stimulus which is similar to the target in orientation or spatial position rapidly causes strong neural adaptation to the target via perceptual grouping, and thus the decreased neural response leads to the time compression effect. Their interpretation is supported by the finding that the suppression of neural activity is proportional to the difference in spatial proximity or their orientation similarity (Nelson, 1991). Their results are consistent with the hypothesis that lower neural activity leads to a shorter perceived duration (Eagleman & Pariyadath, 2009; Pariyadath & Eagleman, 2007, 2012). In the present study, we examined whether, how, and when sandwiching visual stimuli (non-targets) affect the perceived duration of target visual stimuli. As mentioned above, these findings so far suggest that spatial and temporal proximity should play a dominant role in this effect on visual time perception. In Experiments 1 and 2, we examined the temporal rule for this effect with manipulation of the ISI between the target and non-target stimulus. As a result, the perceived duration was significantly shorter in the ISI 0 ms condition than in target-only or longer ISI conditions. Experiment 3 examined the spatial rule of the time compression effect. In addition, Experiment 4 examined whether either the preceding or trailing non-target stimulus is responsible for inducing the time compression and whether they have an additive effect on the perceived duration.

presented targets (Droit-Volet, 2003; Droit-Volet & Wearden, 2002; Penton-Voak, Edwards, Percival, & Wearden, 1996; Treisman, Faulkner, Naish, & Brogan, 1990; Wearden et al., 1998). Ono and Kitazawa (2010, 2011) showed that the perceived duration of the interval was compressed by repetitive auditory or visual stimuli that followed the test interval. Moreover, when target visual stimuli are sandwiched by non-target auditory stimuli, the perceived duration of the target changes depending on the ISI between these stimuli (Asaoka & Gyoba, 2016). In particular, the perceived duration of the visual stimuli increased when the ISI was 200 ms, while it decreased when the ISI was 0 ms, as compared to the target-only condition. The time dilation induced by sounds could be interpreted by temporal ventriloquism, which refers to a phenomenon in which sounds presented in close temporal proximity of a visual stimulus attract its perceived temporal occurrence (Morein-Zamir et al., 2003; Vroomen & de Gelder, 2004). That is, the first sounds would attract the onset timing of visual stimuli and the second sound would attract the offset timing of visual stimuli, which would lead to a longer perceived duration. However, the mechanisms underlying the time compression effect induced by sandwiching sounds remains unclear. Several previous studies have examined the effect of non-target preceding or trailing stimuli on visual time perception. However, little is known about the effect of sandwiching stimuli on time perception. In the present study, we examined whether, how, and when non-target visual sandwiching stimuli affect the perceived duration of target visual stimuli. We presented either only one target stimulus or one target stimulus sandwiched by non-target visual stimuli. This procedure is similar to a traditional masking paradigm. It has been shown that a visible target stimulus can be incorrectly perceived by the presentation of a non-target stimulus in the same spatial location and temporal proximity, known as visual masking (for reviews; Breitmeyer, 1984, 2007; Enns & Di Lollo, 2000; Kouider & Dehaene, 2007). Moreover, masking can reduce the neural responses to the target stimulus in the early visual area (Lamme, Zipser, & Spekreijse, 2002; Macknik & Livingstone, 1998). Therefore, non-target sandwiching stimuli could degrade the neural responses of the filled target duration in the same way as masking. Therefore, we expect that the perceived duration of visual stimuli would be shorter when the non-target stimuli are presented before and after the target than when the only visual stimulus is presented because the size of the neural responses to the target would be lower in the former condition than in the latter condition. In fact, in the case of empty duration perception, a short interval marked by two flashes is perceived as shorter when a whole-field random texture mask is presented in temporal proximity of either flash than when the mask is not presented (Derichs & Zimmermann, 2016; Zimmermann, Born, Fink, & Cavanagh, 2014). However, less recent studies have consistently reported that a longer inter-stimulus interval (ISI) between the forward or backward masking stimuli and the target stimulus leads to longer perceived duration compared to that during shorter ISI conditions or no-masking condition in both the auditory (Kallman, Beckstead, & Cameron, 1988; Kallman, Hirtle, & Davidson, 1986; Massaro & Idson, 1976, 1978) and visual modalities (Cantor & Thomas, 1976). Cantor and Thomas (1976) examined the backward masking effect on visual time perception and found that the perceived duration increased with an increase in the ISI between the target and masking stimuli for < 70 ms. They proposed that an integration of the actual target duration, ISI, and processing time after the onset of a mask could predict the backward masking effect. Their model predicts that the perceived duration of visual stimuli will be longer when the mask is presented than when it is not presented, and that a longer ISI will generate a longer perceived duration than an ISI of 0 ms. These findings indicate two possibilities: that non-target sandwiching stimuli can compress or dilate the perceived duration of visual stimuli. We hypothesized that the effect of sandwiching stimuli on visual time perception would be strongly related to temporal and spatial

2. Experiment 1a Masking effects have been found to occur when the target and masking stimuli are presented in close temporal proximity (for reviews; Breitmeyer, 1984, 2007; Enns & Di Lollo, 2000). Temporal proximity plays a dominant role in the perceptual grouping in the visual system (Han et al., 1999). Our previous study, although an audio-visual case, found that sandwiching tones could influence the perceived duration of visual stimuli only when the ISI was brief (Asaoka & Gyoba, 2016). 2

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Based on these findings, we can expect that sandwiching stimuli affect the perceived duration of a target stimulus only when these stimuli are presented in close temporal proximity. In Experiment 1a, we manipulated the ISI between the target and non-target stimuli. If the presentation of non-target stimuli can decrease the neural response to the target, it should be perceived as having a shorter duration, in the same way as several previous studies (Zimmermann et al., 2014, 2016). However, several previous studies examining the intramodal masking effect on time perception have reported that longer ISI between the forward or backward masking and target stimuli lead to longer perceived duration compared to that in shorter ISI conditions or nomasking condition (Cantor & Thomas, 1976). We tested these hypotheses in Experiment 1a.

Immediately after the key press, a reproduction stimulus identical to the target stimulus appeared. The participant's task was to press the “0” key again when the time that had elapsed from the beginning of the reproduction stimulus was judged to correspond to the length of the target duration. The trial was complete after the reproduction stimulus disappeared after the second key pressing response. The participants were instructed to ignore the non-target stimuli and were told not to count seconds or tap a rhythm mentally during the presentation of the target and reproduction stimuli. After 16 practice trials, each participant completed 256 trials: 4 (ISI conditions) × 4 (the durations of the target) × 16 (repetitions). The trial order was randomized. 2.1.4. Analysis We used constant error (CE = reproduced duration – target duration) to examine the effect of sandwiching stimuli on visual time perception. CE values quantified the size of error and systematic bias in the reproduced durations. Negative values indicate that the reproduced durations were shorter than the target duration while positive values indicate that the reproduced durations were longer than the target duration. CE had the advantage of allowing direct comparison of the data collected from different target durations (e.g., Gamache & Grondin, 2010; Grondin & Killeen, 2009). At first, we excluded reproduced durations that exceeded a range of mean ± 3 SD as outliers for each target duration. Total valid data comprised 99.03% of the obtained data. Next, we calculated the mean CEs for each ISI condition (see Fig. 2), and we conducted a one-way repeated measure (ISI condition) ANOVA. The Since Mendoza's multisample sphericity test was statistically significant (p < .05), ChiMuller correction was applied. We used the Shaffer's modified sequentially rejective Bonferroni procedure as multiple comparison throughout the present study.

2.1. Materials and methods 2.1.1. Participants Thirteen observers, including the first author (R. A.), participated in this experiment (10 men and 3 women). Their ages ranged from 20 to 26 (mean = 23.31, SD = 2.21) years. All participants reported having normal or corrected-to-normal vision and normal audition. Except for R.A., participants did not know the purpose of the experiment. The participants provided written informed consent prior to the experiment. All experiments reported in this paper were approved by the ethics committee of the Graduate School of Arts and Letters, Tohoku University, and were carried out in accordance with the Declaration of Helsinki. 2.1.2. Apparatus The visual stimuli were generated and controlled by means of a custom-made program that was written using MATLAB (MathWorks, Inc.), Cogent Graphics and 2000 toolboxes (www.vislab.ucl.ac.uk/ cogent.php) and a PC (Dell: XPS720; OS:Vista), and they were displayed on a 19 in CRT-display (SONY, Trinitron GDM-F520; Resolution: 1024 × 768 pixels; Refresh rate: 60 Hz). The experiment was carried out in a dark room without clock-like devices. The participants viewed the monitor binocularly from a distance of about 57.3 cm with their head stabilized on a chinrest.

2.2. Results and discussion An ANOVA on the CE revealed a significant main effect, F (1.72, 20.66) = 19.10, p < .001, η2p = 0.61. Multiple comparisons showed that the CE was smaller in the ISI 0 ms condition than in all other conditions (ps < .05). No other significant differences were found. The result indicated that the perceived duration of visual stimuli decreased only when the ISI was 0 ms, and that perceived durations were almost the same in the ISI 100, 300 ms, and control conditions. Therefore, sandwiching stimuli compressed the perceived duration of visual stimuli only when they were presented before and after the target without temporal intervals (0 ms). Since the mean CEs were 131.42 and 220.85 ms in the ISI 0 ms and control conditions, respectively, the sandwiching stimuli compress the perceived duration of visual stimuli by approximately 89 ms on an average. Experiment 1a examined whether and how sandwiching stimuli alter the perceived duration of the target with manipulation of ISIs using a time reproduction task. Two new findings were obtained. First, presentation of sandwiching visual stimuli could compress, rather than dilate, the perceived duration of visual stimuli, which is consistent with the expectation of the Neural Readout Model (NRM). This result is inconsistent with the previous finding that backward masking led to the time dilation effect (Cantor & Thomas, 1976). We will discuss this difference in the General Discussion (Section 7). Secondly, the time compression effect was observed only in the ISI 0 ms condition. The result shows that temporal proximity is a very important factor for the occurrence of the time compression effect.

2.1.3. Stimuli and procedure The target visual stimuli consisted of black unfilled circles or triangles (2.0° wide, as same as Asaoka & Gyoba, 2016). The non-target visual stimuli were white-black checkerboards (2.0° wide). The reason for using white-black checkerboards was that participants can easily discriminate the target from non-target stimuli. Spatial frequency of the checkerboards was 1.0 cycle/deg. We prepared two types of whiteblack checkerboards by interchanging the positions of the white and black sections. The target and non-target stimuli were presented at the center of the display on a gray background. The duration of the target stimulus was 300, 500, 700, or 900 ms, while that of the non-target stimuli was always 50 ms. We used the time reproduction task, which is a classically used measure of time perception. Fig. 1 illustrates the sequence of events for the trials. The participants pressed the space bar to initiate each trial. The trials began with a black fixation cross presented at the center of the screen, for 500 ms. Then, a blank screen was presented with a randomly chosen duration of 500–1000 ms in 50 ms steps. In the control condition, only the target stimulus (an unfilled circle or triangle) was presented for one of four durations (300, 500, 700, or 900 ms). In the other conditions, the non-target stimuli were presented before and after the presentation of the visual target stimulus in each trial. The ISIs between the target and non-target stimuli were manipulated at 0, 100, or 300 ms. The first (preceding) and second ISIs (trailing) were always of the same duration. After the blank screen was presented with a randomly chosen duration from 300 to 500 ms in 50 ms steps, the participants were asked to press the “0” key.

3. Experiment 1b Experiment 1a clearly showed that a visual stimulus sandwiched by non-target stimuli was perceived as shorter in the ISI 0 ms condition than in the control condition using a time reproduction task. However, 3

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Fig. 1. Schematic representation of the procedure of Experiment 1a. The top and bottom boards represent the stream of the ISI 0 ms condition and ISI 100 or 300 ms conditions, respectively. In the control condition, only the target stimulus appeared. The target was presented for one of four durations: 300, 500, 700, or 900 ms. The participants had to start and end the presentation of the reproduction stimulus by pressing the “0” key when they judged that the reproduction duration equaled the target duration.

3.1.2. Apparatus, stimuli, and procedure The apparatus and stimuli were identical to those used in Experiment 1a. We used the category estimation method. In this method, the participants evaluated the perceived duration of the target stimulus using categories on a Likert scale (e.g., long, moderate, and short). This method can reduce individual differences in perceived duration because the participants need to learn the temporal duration of each category prior to the experiment. Another advantage is that it can suppress the occurrence of temporal-order effects on time perception that have been reported in previous studies (Grondin, 2001; Nakajima, ten Hoopen, & van der Wilk, 1991; Rose & Summers, 1995), because only one stimulus is presented during a trial. The participant's task was to classify the perceived duration of visual stimuli into one of four categories using keys on the computer's keyboard (‘0’ for short, ‘1’ for a bit short, ‘2’ for a bit long, and ‘3’ for long). The target visual stimuli were presented for 300, 500, 700, or 900 ms, which corresponded to the short, a bit short, a bit long, and long categories, respectively. Prior to the test phase, there were three training phases that enabled the participants to make individual subjective standards regarding the duration of the visual stimuli. In the first training phase, the participants were asked to look at the visual stimuli 16 times (there were 4 repetitions per duration condition) and

we can see that the CE increases with increasing ISI (see Fig. 2). The results indicate that the timing of key pressing in reproduction was determined by the timing of the non-target stimuli rather than that of the onset and offset of the target stimulus. That is, shorter and longer ISIs seem to lead to shorter and longer reproduced duration, respectively. Such a response bias may be the reason for the time compression effect. Therefore, Experiment 1b was conducted to examine whether the “response bias” hypothesis is true or not, using the method that does not require the participant to press the key twice.

3.1. Materials and methods 3.1.1. Participants Twelve observers participated in this experiment (7 men and 5 women). Their ages ranged from 20 to 28 (mean = 22.50, SD = 2.81) years. Two of them had participated in Experiment 1a, including the first author (R.A.). All participants reported having normal or corrected-to-normal vision. Except for R.A., participants did not know the purpose of the experiment. The participants provided written informed consent prior to the experiment.

Fig. 2. Results for Experiment 1a. The vertical axis indicates the mean constant errors and the horizontal axis indicates the inter-stimulus interval (ISI) between the non-target and target stimuli. Higher y values represent longer perceived durations. Error bars represent the standard errors of the mean (n = 13), **p < .01. 4

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4.1. Materials and methods

memorize the presentation duration of each category. In the second training phase, the participants were asked to categorize the perceived duration of visual stimuli as accurately as possible. The participants completed 64 trials in a randomized order (i.e., 16 repetitions per duration condition). In the first and second training phase, non-target stimuli were not presented. In the third training phase, the participants received 16 trials in which non-target stimulus was presented before and after the target. The sequence of the third training phase was identical to that of the test phase. The ISIs between the target and nontarget stimuli were randomly chosen among 0, 100, and 300 ms in each trial. No feedback was given in the second and third training phases. After completing training phases, each participant completed 256 trials: 4 (ISI conditions) × 4 (the durations of the target) × 16 (repetitions). The sequence of one trial was almost the same as Experiment 1a. After the stimulus presentation, the participants were asked to classify the duration of visual stimulus into one of four categories by pressing one of four keys (from 0 to 3) on the keyboard according to the individual standards established in the training phases. The trial order was randomized.

4.1.1. Participants Sixteen observers participated in this experiment (6 men and 10 women). Their ages ranged from 19 to 25 (mean = 20.88, SD = 1.36) years. Four of them had participated in Experiment 1b, including the first author (R.A.). All participants reported having normal or corrected-to-normal vision. Except for R.A., participants did not know the purpose of the experiment. The participants provided written informed consent prior to the experiment. 4.1.2. Apparatus, stimuli, and procedure The apparatus, stimuli, and procedure were identical to those used in Experiment 1a, except for ISI. The ISI condition had eight levels (0, 17, 33, 50, 67, 83, 100 ms, and control conditions). After 16 practice trials, each participant completed 448 trials: 8 (ISI conditions) × 4 (the durations of the target) × 14 (repetitions). 4.1.3. Analysis Reproduced durations that exceeded a range of mean ± 3 SD for each target duration were excluded as outliers. The total valid data of 16 participants was 99.17%. Mean CEs were calculated for each ISI condition (see Fig. 4), and we conducted a one-way repeated measure ANOVA on the CE. Since Mendoza's multisample sphericity test was statistically significant (p < .05), Chi-Muller correction was applied.

3.2. Results and discussion We calculate mean scores of categorizations (from 0 to 3) for each ISI condition (see Fig. 3, different target durations were grouped). An ANOVA with ISI condition revealed a significant main effect, F (3, 33) = 25.23, p < .001, η2p = 0.70. Multiple comparisons showed that the mean score was smaller in the ISI 0 ms condition than in the ISI 100, 300 ms and control conditions (ps < .05). No significant difference was found between the 100, 300 ms, and control conditions. These results indicated that the time compression effect occurred only in the ISI 0 ms condition, replicating the results of Experiment 1a. Thus, the time compression effect is not specific to the time reproduction method, suggesting that the “response bias” hypothesis is not true.

4.2. Results and discussion An ANOVA on the CE revealed a significant main effect, F (2.51, 37.68) = 14.26, p < .001, η2p = 0.49. Table 1 shows all the statistically significant differences revealed by multiple comparisons. It was found that CE was smaller in the ISI 0 ms condition than in all other conditions (ps < .05). However, the CE in the 17 ms or more ISI conditions was not significantly different from the control condition. These results indicate that the time compression effect occurs only in the 0 ms condition, which is consistent with the results of Experiments 1a and 1b. Thus, it is plausible that the temporal window for the time compression is very narrow (< 17 ms). The mean CEs were 61.01 ms and 136.56 ms in the 0 ms and control conditions, respectively, suggesting that the average size of the time compression effect was 76 ms. The results suggest the narrow temporal window for this effect. However, Fig. 4 and the results of the multiple comparison analysis clearly showed that CE tended to decrease with a decrease in ISI. For example, multiple comparisons showed that CE was shorter in the 17 ms condition than in the 50, 67, 83, and 100 ms conditions, and that it was shorter in the 33 ms condition than in the 83 and 100 ms conditions. These results suggest that the effect of sandwiching stimuli on visual time perception is gradual with ISIs. To further assess the data quantitatively, we conducted a simple linear regression analysis between the ISI and CE. The obtained formula was y (CE) = 0.78x (ISI) + 69.88 (r2 = 0.95). The result suggested that the perceived duration lengthens by about 0.78 ms when the ISI increases by 1 ms. Furthermore, a higher coefficient of determination means that this data is well fitted by linear regression. Many previous studies on visual masking have shown that the masking effects linearly increase or decrease as a function of the length of ISI between the target and mask (e.g., Spencer & Shuntich, 1970). In summary, the effect of the sandwiching stimuli on visual time perception gradually changed with the length of ISI, although only the specific condition (0 ms condition) was statistically shown to be significantly different from the control condition, suggesting the narrow temporal window for the time compression effect.

4. Experiment 2 Experiments 1a and b clearly showed that the sandwiching stimuli could compress the perceived duration of visual stimuli in the ISI 0 ms condition, but not in the 100 ms condition. Thus, it remains unclear whether the time compression occurs when ISI is between 0 and 100 ms. Experiment 2 manipulated ISI in smaller steps than Experiments 1a and 1b to examine the temporal window for the time compression effect.

Fig. 3. Results for Experiment 1b. The vertical axis indicates the mean score in the category estimation task. The horizontal axis indicates the inter-stimulus interval (ISI) between the target and non-target stimuli. Higher y values represent longer perceived durations. Error bars represent the standard errors of the mean (n = 12). **p < .01.

5. Experiment 3 Experiment 3 examined the spatial rule of the time compression 5

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Fig. 4. Results for Experiment 2. The vertical axis indicates the mean constant error (CE) and the horizontal axis indicates the inter-stimulus interval (ISI) between the target and non-target stimuli. Higher y values represent longer perceived durations. Error bars represent the standard errors of the mean (n = 16). **p < .01. Gray dotted line represents results from a simple linear regression between the ISI and CE.

5.1.2. Apparatus, stimuli, and procedure The apparatus, stimuli, and procedure were identical to those used in Experiment 1a, except for following. In order to examine the effects of spatial overlapping between the target and non-target, we manipulated the position of the non-target stimulus. In the same position condition (SP condition), both target and non-target stimuli appeared in the center of the display. In the different positions condition (DP condition), the target stimuli appeared at the center of the display while the non-target stimuli were presented at the right or left side of the target stimulus. The spatial distance was at a visual angle of 3.0° between the center of the target and non-target stimuli. Since both target and nontarget stimuli had 2.0° in length and breath, there was no spatial overlapping between these stimuli (spatial gap was 1.0 deg). The preceding and trailing stimuli were presented at the right side of the target in half of the trials for the DP condition and at the left side of the target in the other half of the trials. The ISIs between target and non-target stimuli were 0 ms in the SP and DP conditions. In the control condition, only target stimuli were presented at the center of the display. After 16 practice trials, each participant completed 168 trials: 3 (non-target position conditions) × 4 (the durations of the target) × 14 (repetitions). The sequence of one test trial is shown in Fig. 5.

Table 1 Observed statistically significant differences in Experiment 2. Comparison

t value

p value

0 ms < 50 ms 0 ms < 83 ms 0 ms < 100 ms 0 ms < 67 ms 0 ms < 33 ms 17 ms < 100 ms 17 ms < 83 ms 17 ms < 67 ms 17 ms < 50 ms 33 ms < 83 ms 0 ms < control 33 ms < 100 ms 50 ms < 83 ms 50 ms < 100 ms 0 ms < 17 ms

9.35 9.00 8.82 8.35 7.04 6.63 6.49 5.92 5.33 5.03 4.44 4.31 3.96 3.92 3.87

< .001 < .001 < .001 < .001 < .001 < .001 < .001 < .001 < .01 < .01 .01 .01 .02 .02 .02

*Bold means a comparison of the ISI 0 ms condition vs. the other condition.

effect induced by the sandwiching stimulus. Masking effects (either forward or backward masking) were only observed when the target and masking stimuli were projected to the same region on the retina (Felsten & Wasserman, 1980; Turvey, 1973), and they reduced critically as the spatial separation between the target and mask increased (Growney, Weisstein, & Cox, 1977; Schiller, 1966). Recent findings indicated spatial selectivity in the mechanism underlying visual time perception (e.g., Burr, Tozzi, & Morrone, 2007; Johnston, Arnold, & Nishida, 2006; Zhou et al., 2014). For example, Johnston et al. (2006) showed that adapting to local gratings could shorten the perceived duration of testing gratings when the adapting and testing gratings had same spatial position. These findings suggest that spatial overlap between the target and non-target stimuli modulates the effect of sandwiching stimuli on visual time perception. Since sandwiching stimuli that are spatially separate from the target do not affect the neural responses of the target, the time compression effect should not occur. Experiment 3 tested this point.

5.1.3. Analysis Reproduced durations that exceeded a range of mean ± 3 SD in each target duration were excluded as outliers. Data from one participant were excluded from further analysis because < 70% of his data were valid. Next, we re-calculated the mean and SD for each participant, and performed outlier treatment. The total valid data of 15 participants was 99.48%. Mean CEs were calculated for each condition (see Fig. 6), and we conducted a one-way repeated measure ANOVA on the CE. 5.2. Results and discussion There was a significant main effect in the CE, F (2, 28) = 14.43, p < .001, η2p = 0.51. Multiple comparisons showed that the CE was smaller in the SP condition than in the DP and control conditions (ps < .05), and no significant difference was found between the DP and control conditions. These results indicated the time compression occurred in the SP condition, but it did not occur in the DP condition. Therefore, the time compression effect induced by sandwiching stimuli showed spatial selectivity. The results were consistent with the findings that several time distortions showed spatial specificity (Burr et al., 2007; Johnston et al., 2006; Zhou et al., 2014). The mean CEs were 62.47, 116.99, and 115.56 ms, in the SP, DP, and control conditions, respectively, suggesting that presenting both sandwiching stimuli compressed the perceived duration of visual stimuli by approximately 53 ms on an average.

5.1. Materials and methods 5.1.1. Participants Sixteen observers participated in this experiment (9 men and 7 women). Their ages ranged from 20 to 25 (mean = 21.81, SD = 1.56) years. Two of them, including the first author (R.A.), had participated in Experiment 1b. All participants reported having normal or correctedto-normal vision and did not know the purpose of the experiment (except for R.A.). The participants provided written informed consent prior to the experiment. 6

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Same Press the “0” key

Different Press the “0” key Fixation 500 ms

Blank 500–1000 ms

Non-target 50 ms

Target

Non-target 50 ms

Blank 300–500 ms

Response

Reproduction

Time Fig. 5. Schematic representation of the procedure of Experiment 3. The target was presented for one of four durations: 300, 500, 700, or 900 ms. In the same position condition, both target and non-target stimuli appeared in the center of the display. In the different positions condition, the target stimuli appeared at the center of the display while the non-target stimuli were presented at the right or left side of the target stimulus, without spatial overlap. In the control condition, only the target stimulus appeared. The participants had to start and end the presentation of the reproduction stimulus by pressing the “0” key when they judged that the reproduction duration equaled the target duration.

6.1. Materials and methods 6.1.1. Participants Fifteen observers participated in this experiment (9 men and 6 women). Their ages ranged from 21 to 26 (mean = 22.40, SD = 1.72) years. One of them had participated in Experiments 1a and 1b. One, one, and six others had participated in Experiments 1a, 1b, and 3, respectively. Except for the first author, the remaining five were new participants and all participants did not know the purpose of the experiment. The participants provided written informed consent prior to the experiment. 6.1.2. Apparatus, stimuli, and procedure The apparatus, stimuli, and procedure were identical to those used in Experiment 1a, except for the presentation of the non-target stimuli. Both the target and non-target stimuli appeared in the center of the display. In the preceding condition, only the non-target stimulus was presented before the target. In the trailing condition, only the nontarget stimulus was presented after the target. In the sandwiching condition, the non-target was presented before and after the target. In the control condition, only the target visual stimuli appeared. All the ISIs between the target and non-target stimuli were 0 ms. After 16 practice trials, each participant completed 224 trials: 4 (conditions) × 4 (the durations of the target) × 14 (repetitions). The trial order was randomized. The sequence of one test trial is shown in Fig. 7.

Fig. 6. Results for Experiment 3. The vertical axis indicates the mean constant error and the horizontal axis indicates locations of the non-target stimulus. Higher y values represent longer perceived durations. Error bars represent the standard errors of the mean (n = 15). **p < .01.

6. Experiment 4 In Experiments 1a, 1b, 2 and 3, we presented both preceding and trailing non-target stimuli in each trial. This led to the question, which preceding or trailing stimulus is a critical factor, or whether both stimuli are needed for the occurrence of the time compression effect. Traditional visual masking literature showed that forward and backward masking had different effects on participants' performance (Eriksen, 1966; Ogmen, Breitmeyer, & Melvin, 2003; Schiller, 1966). In contrast, no difference was reported between the auditory forward and backward masking effects on auditory empty or filled duration (Kallman et al., 1986). Since it remains unclear whether preceding or trailing stimuli has a different or the same effect on visual time perception, Experiment 4 tested this point. From the NRM viewpoint, preceding or trailing stimuli may degrade neural responses to the target, which may compress perceived duration to the same degree. Moreover, the decrease in neural responses should be larger when the visual target is sandwiched by non-target stimulus than when either preceding or trailing stimulus is presented. Thus, we can expect that the size of the time compression should be larger in the sandwiching condition than in either stimulus condition.

6.1.3. Analysis Reproduced durations that exceeded the range of mean ± 3 SD in each target duration were excluded as outliers. Data from one participant were excluded from further analysis because < 60% of his data were valid. Next, we re-calculated the mean and SD for each participant and performed outlier treatment. The total valid data of 14 participants was 98.9%. Mean CEs were calculated for each condition (see Fig. 8), and we conducted a one-way repeated measure ANOVA on the CE. 6.2. Results and discussion An ANOVA on the CE revealed a significant main effect, F (3, 39) = 15.40, p < .001, η2p = 0.54. Multiple comparisons showed that the CE was smaller in the preceding and trailing conditions than in the control condition (ps < .05). The results indicated that only the preceding or trailing stimulus was sufficient to induce the time compression effect. There was no significant difference between the preceding or trailing conditions. Moreover, the CE was smaller in the sandwiching 7

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Fig. 7. Schematic representation of the procedure of Experiment 4. The target was presented for one of four durations: 300, 500, 700, or 900 ms. In the preceding condition, the non-target stimulus was presented before the target. In the trailing condition, the non-target stimulus was presented after the target. In the sandwich condition, the non-target stimulus was presented before and after the target. In the control condition, only target stimulus was presented. The participants had to start and end the presentation of the reproduction stimulus by pressing the “0” key when they judged that the reproduction duration equaled the target duration.

almost 3.5 times more likely than the alternative hypothesis that the magnitude of the time compression effect differs in these two conditions (BF01 = 3.54). Hence, the result indicated that preceding and trailing stimuli had the same effect on visual time perception. 7. General discussion 7.1. Summary of the results The aim of the present study was to test whether, how, and when sandwiching a visual stimulus alters the perceived duration of the target. Experiments 1a and 1b showed that the sandwiching stimulus could compress the perceived duration of visual stimuli only when ISI was 0 ms using time reproduction and category estimation tasks. Experiment 2 demonstrated that the perceived duration tended to decrease with a decrease in ISI, although the statistical temporal window of the time compression was very narrow (from 0 ms to < 17 ms). Experiment 3 showed that the time compression effect occurred when the target and non-target stimuli were presented at the same positions, but not at the different positions. Experiment 4 demonstrated that the time compression occurred with either the preceding or the trailing stimulus and that the effect of sandwiching a visual stimulus on visual time perception was additive. These results suggested that the temporal and spatial relationship between the target and non-target stimuli play a dominant role in the effect of sandwiching stimuli on visual time perception.

Fig. 8. Results for Experiment 4. The vertical axis indicates the mean constant error and the horizontal axis indicates conditions. Higher y values represent longer perceived durations. Error bars represent the standard errors of the mean (n = 14). **p < .01. *p < .05.

condition than in the other conditions (ps < .05). Thus, the magnitude of the time compression effect was larger in the sandwiching condition than in the preceding or trailing stimulus conditions. The mean CEs were 82.77, 85.87, 52.74, and 119.21 ms, in the preceding, trailing, sandwiching, and control conditions, respectively. The results indicated that the sandwiching stimulus compressed the perceived duration of visual stimuli by about 66 ms on an average. Presenting only the preceding or trailing stimulus compressed the visual subjective duration by approximately 36 and 33 ms on an average, respectively. These results suggest that both presentations produced an additive effect on the time compression effect, which is consistent with the expectation of the Neural Readout Model. A Bayesian paired t-test on CE from the preceding and trailing conditions was conducted to evaluate the null hypothesis that the magnitude of the time compression effect was almost the same in these two conditions. If the Bayesian Factor (BF) is > 3, this can be used as evidence to support a null hypothesis (Rouder, Speckman, Sun, Morey, & Iverson, 2009). This analysis showed that the null hypothesis was

7.2. Neural readout model (NRM) As mentioned above, it has been proposed that perceived duration depends on the magnitude of the neural response to the target stimulus (Pariyadath & Eagleman, 2007, 2012). This model predicts that lower neural activity leads to shorter perceived duration and higher neural activity leads to longer perceived duration. This neural activity explanation can account for the time compression effect induced by sandwiching stimuli. Traditional masking studies have shown that masking reduces the responses of the target stimulus in V1 (Lamme et al., 2002; Macknik & Livingstone, 1998). Recent studies on time 8

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reduced. However, the time compression effect induced by the trailing stimulus cannot be explained by this hypothesis because the sequential order of the presentation is reversed (the target is first and the nontarget is second). Therefore, we cannot conclude whether perceptual grouping is a necessary condition for the time compression effect induced by sandwiching stimuli. Feature similarity is known to be another rule of perceptual grouping. In the present study, we used simple triangular, round, and checkerboards as target and non-target stimuli. Thus, perceptual grouping by similarity might not occur since these stimuli have different features. Several previous studies have shown that the proximity rule operates prior to the similarity rule (Ben-Av & Sagi, 1995; Han & Humphreys, 1999; Quinlan & Wilton, 1998), and not only the proximity rule but also the similarity rule can affect the perceived duration of visual stimuli (Zhou et al., 2014, 2015). Peterson, Gözenman, Arciniega, and Berryhill (2015) have examined the relationship between visual working memory and stimulus similarity using the ERP technique and showed that some items grouped perceptually can reduce the amplitude of a specific ERP component. This result suggests that perceptual grouping by similarity can reduce the total neural amount of the target stimulus. Therefore, further study is needed to examine whether and how feature similarity affects or modulates the degree of the time compression effect induced by sandwiching stimuli. This examination will clarify the relationship between the time compression effect and perceptual grouping.

perception accumulated evidence that activity in the visual cortex is strongly related to the perceived duration of visual stimuli (e.g., Burr et al., 2007; Johnston et al., 2006; Zhou et al., 2014). In fact, the perceived duration of an empty interval decreased when either interval marker was masked (Derichs & Zimmermann, 2016; Zimmermann et al., 2014, 2016). That is, the sandwiching stimuli degraded the neural response to the on-offset of the target stimulus, which resulted in a decrease in the total amount of neural activity required to process the target stimulus. Therefore, the perceived duration of the visual stimulus was shorter when the non-target stimuli were presented immediately before and after the target. The interpretation may explain why the time compression effect occurs only when the non-target stimuli are presented close to the target in time and space. When a certain temporal interval is inserted between the target and non-target stimuli, the transient response to the preceding or trailing stimulus might not affect the neural response to the target. As regarding the space, neurons in the early visual area have been known to have strong spatial selectivity since these neurons have small receptive fields (e.g., Kastner et al., 2001). Thus, sandwiching stimuli that were spatially distant from the target might not have an effect on the neural activity of the target stimulus, leading to no time compression effect. Given that proximity in space and time is needed for masking effects to occur (for reviews; Breitmeyer, 1984, 2007; Enns & Di Lollo, 2000), these explanations seem to support the time compression effect. Furthermore, the additive effect observed in Experiment 4, the magnitude of the time compression effect was larger in the sandwiching condition than in the preceding and trailing stimulus only conditions, which can be well explained by the NRM. The attenuation of neural responses could be larger when the sandwiching stimuli are presented before and after the target than when they were presented before or after the target. Although our results suggest that the neural activity in the early visual cortex is responsible for the time compression effect, a limitation of the present study was that the stimuli used were not optimal to examine V1 responses. That is why we used the same target stimulus as our previous study (Asaoka & Gyoba, 2016). Therefore, it would help to examine the time compression effect with more optimal stimuli, such as sine-wave grating, as the target stimuli. Further, it is not clear whether the magnitude of responses of neurons in the early visual cortex really correlates to the magnitude of the time compression effect induced by the sandwiching stimuli. In the future, further fMRI or ERP studies will be necessary to examine the correlation between neural activity and the effect of sandwiching stimuli on visual time perception.

7.4. Scalar expectancy theory In this section, we discuss the time compression effect induced by sandwiching stimuli with reference to the scalar expectancy theory (SET; Gibbon, 1977; Gibbon et al., 1984). As mentioned above, the SET assumes that time perception is performed via a combination of an internal clock-like mechanism, memory processes, and decision strategies. Several time distortions induced by neighboring stimuli have been interpreted due to changes in the pacemaker rate (Penton-Voak et al., 1996; Wearden et al., 1998), or changes in the closing and opening timings of the mode switch (Asaoka & Gyoba, 2016; Klink et al., 2011). In this regard, if sandwiching stimuli decelerate the pacemaker rate, the magnitude of the time compression effect should be proportional to the length of the target duration. Otherwise, if sandwiching stimuli change the mode-switch timing, the magnitude of the time compression effect should be constant in any target duration. In order to test these possibilities, we conducted a one-way ANOVA with the target duration on the difference in the CE between the ISI 0 ms and control conditions (degree of the time compression effect) from Experiments 1a, 1b, 2, 3, and 4, respectively. If the hypothesis on the change in the pacemaker speed is true, a significant main effect of target duration should be found. If the hypothesis on the change in mode-switch timing is true, a significant main effect will be not found. As a result, we cannot get evidence that the magnitude of the time compression effect increases with the length of target duration.2 These results indicated that sandwiching stimuli with ISI 0 ms had almost the same effect on visual time perception across the different target duration conditions, suggesting that the time compression effect is attributed to the change in the timings of the mode-switch rather than that to the pacemaker speed. This would explain the result that either the preceding or the trailing stimulus could induce the time compression effect and that they have an additive effect. This interpretation explains the narrow temporal

7.3. Perceptual grouping Our results showed that temporal and spatial proximity are very important factors for the occurrence of the time compression effect. This rule is similar to that of perceptual grouping, known as proximity rules. Perceptual grouping is a process involved in chunking or segmentation of information. In the visual system, perceptual grouping can occur in spatial and temporal domains (Han et al., 1999; Otto et al., 2009). Intramodal (vision-vision) and intermodal (vision-audition) perceptual grouping are known to alter the performance in temporal order judgment tasks in some previous studies (Nicol & Shore, 2007; Vatakis & Spence, 2007). Regarding the perceived duration, the effect of neighboring stimuli can be modulated by perceptual grouping between the target and non-target stimuli (e.g., Zhou et al., 2014; Zhou, Yang, Zhang, Zhang, & Mao, 2015). These findings suggest that perceptual grouping plays an important role for in processing of visual time. Nelson (1991) showed that when two stimuli were presented close together in space and time, the neural response to the second stimulus was smaller than that to the first stimulus. This neural suppression by perceptual grouping may be able to explain the time compression effect. That is, since the preceding stimulus and target were perceptually grouped, the neural response to the target was

2 Experiment 1a: F (3, 36) = 0.60, p = .62, η2p = 0.05; Experiment 1b: F (3, 33) = 2.56, p = .07, η2p = 0.19; Experiment 2: F (3, 45) = 10.21, p < .01, η2p = 0.40. Multiple comparison showed that the time compression was larger in the 300 ms condition than in the other conditions; Experiment 3: F (3, 42) = 0.22, p = .99, η2p = 0.01; Experiment 4: F (3, 39) = 0.20, p = .90, η2p = 0.02.

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window of the time compression effect. When there is a certain time interval between the target and non-target stimuli, the non-target stimuli may not change the mode-switch timing, thus resulting in no changes in the perceived duration. This idea also seems to be consistent with the NRM framework. The mode-switch timing in the SET could be strongly related to the starting or end point of processing for the target stimulus. If the starting point of the processing is delayed and/or end point is hastened, the total amount of neural response should decrease. However, it is relatively difficult to explain the spatial selectivity of the time compression effect induced by sandwiching stimuli. Recently, it has been proposed that there are sensory-specific pacemaker systems (e.g., Gamache & Grondin, 2010; Kanai, Lloyd, Bueti, & Walsh, 2011). Several studies pointed out the importance of low-level visual information processing, such as V1, for time perception (Burr et al., 2007; Johnston et al., 2006; Zhou et al., 2014). These results suggest that the visual specific pacemaker is located in V1, and that transient responses of neurons in V1 are assumed to be involved in the mode-switch timing. If so, it is possible that the mode-switch timing is sensitive to spatial distance between the target and non-target stimuli. Therefore, spatial selectivity of the time compression effect can be interpreted by a combination of the SET and NRM.

threshold, and that the time compression does not occur when both interval markers are masked, since neural attraction should not occur. However, our data showed that the presentation of both preceding and trailing stimuli generated the strongest time compression effect. Zimmermann's studies (2014, 2016) used empty target intervals marked by two flashes, while the present study used filled target duration defined by one stimulus. If the difference is interpreted from the perspective of neural mechanisms, it is possible that the perceived duration of an empty interval stems from two neural responses (on to each of the starting and ending markers), while the perceived duration of filled intervals arises from a single neural response. In the case of the filled duration perception, it is more plausible that the perceived duration is determined by an amount of neural responses during the processing of the target stimulus, rather than by the temporal difference between onset and offset of neural responses. Therefore, in filled time perception, the presentation of stimuli both before and after the target decreases or interferes with the onset and offset responses of neurons in the early visual cortex, leading to the strongest time compression effect. In contrast, in empty time perception, the presentation of stimuli both before and after the target does not induce neural attraction, and thus does not lead to a time compression effect.

7.5. Differences in the results between the present and previous studies

8. Conclusions

As mentioned in the introduction, Cantor and Thomas (1976) proposed that an integration of the actual target duration, ISI, and processing time after the onset of the mask could predict the backward masking effect on visual subjective duration. Based on this model, the perceived duration of visual stimuli will be longer in the ISI 0 ms condition than in the control condition, and that longer ISI will generate a longer perceived duration. However, the perceived duration was almost the same in the ISI 100 ms, 300 ms, and control conditions in Experiments 1a and 1b. Moreover, Experiment 4 showed that the trailing stimulus also induced the time compression, rather than the time dilation, as well as the preceding and both presentation conditions. Therefore, Cantor and Thomas' (1976) model could not explain the present results. This inconsistency in the results may be attributable to the difference in length of the target duration. Cantor and Thomas (1976) employed a target duration of 20 or 50 ms, while the present study used the visual target for at least 300 ms. Thus, since target visibility was poor due to shorter duration in Cantor and Thomas's study, their participants might have had to use contextual information to perform temporal discrimination, such as ISI or processing time after onset of masking. Since the target stimulus used in the present study had high visibility, the sandwiching stimuli may not have induced time dilation. The perceived duration of an empty interval decreased when either interval marker was masked (Derichs & Zimmermann, 2016; Zimmermann et al., 2014, 2016). This result was interpreted as follows: the perceived duration of the empty interval depends on a temporal difference between peaks of neural responses to the start and end markers of the interval. If no mask was presented, interval starting and ending markers have strong onset signals and thus induce neural responses with a sharp peak and a narrow spread. It is therefore easy to estimate the interval from the peak of the starting marker to the peak of the ending one. However, the presentation of masking strongly degraded the onset strength of the starting or ending marker and induced neural responses with a lower peak and a broad spread. Then, two different neural responses can be established, which can induce more variable perceived duration. In order to minimize this variability, the perceived duration can be determined so that the neural peaks to the masked interval marker is attracted towards that of the no-masked interval marker, consequently compressing the empty interval. Consistent with this hypothesis, Derichs and Zimmermann (2016) have found that presenting the mask close to the interval start or end marker decreases not only the perceived empty duration but also the perceptual

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