To highlight differences in the perceptual mechanisms used to achieve SJs and TOJs, the way in which the same group of participants performed on each task was compared for a variety of different stimulus types. To compare the tasks three hypotheses were explored. First, we expected less sensitivity to COA in TOJs than SJs; second, we expected TOJ PSSs to be audio-leading and SJ PSSs to be video-leading and; finally, we expected no correlation between TOJs and SJs on the two most widely used performance measures in synchrony perception research: PSS and TIW. In line with previous work 
, the psychophysical evidence provided strongly indicates that TOJs and SJs are indeed supported by different perceptual mechanisms for all stimulus types tested.
The first hypothesis of a lower sensitivity to COAs in TOJs than SJs was explored using three measures: a subjective appreciation of task difficulty, data exclusion rates, and the TIW. Interestingly, these three measures provided mixed results. First, for all stimulus types the majority of participants reported the TOJ task to be subjectively the most difficult. Second, exclusion rates were considerably larger for TOJs than SJs for BFD, PLD and FV, but not the other stimulus types. Third, amongst the four stimulus types for which we could compare the TIW across tasks, only for FV did we find larger TIWs for TOJs and, interestingly, participants actually had significantly narrower TIWs for TOJ/BF than for SJ/BF, i.e., they were more sensitive to COA for TOJs than SJs.
Using the same stimuli and participants, our results confirm what previous studies have found using different experiments and participants, showing that TIWs for speech are wider during TOJs than SJs 
. Also comparing TIWs, Maier and colleagues 
found no main effect of task but did find an interaction between task and stimulus type: the TOJ TIW was wider than that of SJ when an unmodified natural speech stimulus was presented but not for other types of modified speech. Despite the TIW being one of the most widely used performance measures in synchrony perception research, studies directly comparing the two tasks have rarely mentioned TIW differences 
. It is not clear whether this is because no differences were found or whether they were not explored. Our data indicates that, at least for some stimulus types, this may be due to there being no significant difference in TIWs across the tasks. However, even if comparing TIWs fails to find differences in sensitivity, such differences may still exist, and other outcome measures may capture them. Here, we demonstrated that most participants found the TOJ task to be subjectively more difficult than the SJ task for all the stimulus types presented, plus considerably more participants were unable to successfully achieve TOJs for two of our stimulus types (BFD and PLD), which they could achieve SJs for.
One possible reason why many more TOJ data sets had to be excluded for BFD and PLD than for BF, BFV and to a lesser extent FV is related to the difference in how asynchrony was created for these stimulus types. Due to the overall longer duration of the original PLD stimulus 
, asynchronous conditions could be created for both PLD and BFD by cutting the stimuli from a larger movie sequence, after separating the audio and visual timelines to produce cue asynchrony (middle panel of ). This ensured that there was audio and visual information at both the beginning and end of the stimulus and that duration remained a constant 3 seconds for all COA levels 
. In contrast, BF, BFV, and FV had more finite audio and video sequences, i.e., shorter sequences could not be cut from larger sequences, and separating them to produce asynchrony created gaps between the audio and visual cues at both the beginning and end of the audiovisual stimulus; consequently, stimulus duration increased with increasing COA level (left and right panels of ). The information provided by the creation of such gaps may be more salient to making TOJs than SJs; therefore, the lack of such gaps in BFD and PLD may have made it more difficult for participants to successfully make TOJs about those stimuli. In line with this speculation, Maier and colleagues 
recently found that for speech stimuli participants mainly used the information present at the beginning and end of a sentence to make TOJs, while the full stimulus appeared to be used to make SJs. However, the current data also highlight that factors other than timing characteristics play an important role in TOJs: PLD and BFD had the same onset, offset and duration yet no participants could successfully make TOJs for BFD, while 32% of participants could for PLD. Note, however, that the current experiment focused on using a variety of stimulus types to investigate general differences between SJs and TOJs. In doing so we have also shown that differences between the two tasks are not based on the potential confound that when stimulus duration increases with increasing COA SJs but not TOJs can be achieved by focusing on these duration differences. Our results highlight clear differences between SJs and TOJs for all stimulus types tested including BFD and PLD to which the confound does not apply since their stimulus duration was a constant 3 seconds at all COA levels. Therefore, the current data further indicates that the tasks are supported by different perceptual mechanisms and circumvents a potential confound to similar conclusions previously made 
. Future studies are required to fully understand the intricacies of how these tasks differ, for example, in their use of specific stimulus properties and their potentially different supporting neural mechanisms.
We propose that, in the context of a within subjects design, differences in exclusion rates between SJs and TOJs about the same stimulus type can be used as an outcome measure. Although we are not aware of this having been done before in synchrony perception research, it is common for data to be flagged as noisy and excluded (e.g., 
). Moreover, such exclusion rates vary widely, even under very similar experimental settings and using the same stimulus type 
. Our proposal relies strongly on interpreting data exclusion as reflecting a participant's inability to achieve the task for the COA levels presented. However, it could be argued that it simply reflects the result of fitting an inappropriate psychometric function to the data. This alternative interpretation cannot conclusively be ruled out, however, observation of the excluded TOJ data showed that the exclusion resulted from the participants not achieving the task properly, either because their responses were random or because they were biased toward one response (, S1
). Such responses indicate a lack of sensitivity to COA rather than an inappropriate fitting. Attempting to fit them with the appropriate psychometric function for the task produces a relatively flat function with a large standard deviation and ultimately an unacceptable goodness-of-fit. Petrini and colleagues ( of 
) presented the only other illustrations of excluded data that we are aware of, and also found similar patterns of random and biased TOJ responses to a PLD stimulus from some participants. If we assume therefore, that not being able to fit a participant's data represents a lack of sensitivity to the independent variable and hence an inability to achieve the task, then data removal can be regarded as an outcome measure rather than simply noisy data that should be removed from further analysis.
Overall, the evidence presented does not allow us to definitively confirm our hypothesis that participants are less sensitive to COA during a TOJ task than during an SJ task. In fact, significantly smaller TIWs for TOJ/BF than SJ/BF indicate the opposite effect. However, we have highlighted that there are subjective task demand differences between TOJs and SJs for all stimulus types presented, and, for some stimulus types at least, participants are less sensitive to COA when trying to discriminate cue order as opposed to when attempting to discriminate synchrony from asynchrony. We propose that mean TIWs calculated after excluding noisy data, while informative, are not sufficient to successfully capture differences in task demands between TOJs and SJs and that other measures are required to do so.
Despite insufficient evidence to support hypothesis 1, significant support for both hypotheses 2 and 3 provides strong, converging evidence 
that TOJs and SJs involve different perceptual mechanisms. Confirming our second hypothesis, that TOJ PSSs would be audio-leading while SJ PSSs would be video-leading, the PSS of every TOJ/stimulus combination was indeed an audio-leading COA, while the PSS of every SJ/stimulus combination was a video-leading COA. This result reflects the overall summary of previous synchrony perception research provided by Van Eijk et al. 
, that average audio-leading PSSs are “almost exclusively” reported for TOJs, whereas SJ average PSSs are generally video-leading.
In support of our third hypothesis, that neither PSS nor TIW values would correlate across the two tasks, no significant association was found across the tasks for either of these measures. A lack of correlation between TOJs and SJs had previously been reported for other audiovisual stimuli 
and for a variety of other crossmodal stimuli 
. If SJs and TOJs resulted from the same perceptual mechanism we would expect an association between the performance measures derived from them. The evidence presented here clearly shows that there is no such association. Moreover, training on one of the tasks would be expected to transfer to the other if they shared the same perceptual mechanism; however, this transfer of training does not occur 
. Therefore, this converging evidence across different hypotheses and experiments supports the conclusion that the perceptual mechanisms of SJs and TOJs are different.
One surprising result of the current experiment concerned the SJ/FV combination, in which participants were not better at detecting audio-leading asynchrony than video-leading. It is a classical result in psychophysics literature that in SJ tasks, participants are much better at detecting asynchrony when the audio cue leads the visual compared to the other way round 
. We again provide evidence for this classical effect in all stimulus types except FV (). Not finding this effect for the FV stimuli is surprising and inconsistent with previous work using speech stimuli 
. Furthermore, in a separate unpublished study with the same speech stimuli as used here, we did find that participants were better at audio-leading detection during an SJ task. The only differences between that study and the current experiment are different participants, and that here participants completed SJ and TOJ blocks in the same experimental run. Note that none of the other related work, just discussed, had this task switching in the same experimental run either. Therefore, the task switching in the current experiment may have influenced participants' SJs for FV stimuli, the result of which may have been no difference in sensitivity to asynchrony between audio- and video-leading conditions. If this is true, however, the same task switching did not influence SJ or TOJ performance for PLD, as the results of this stimulus type were consistent with previous data using the same stimuli without a task switching design 
. Looking for evidence of this commonly found effect (better audio-leading detection than video-leading) across the different tasks provided more evidence that SJs and TOJs should not be regarded as representing the same underlying process of synchrony perception. Participants were not better at audio-leading detection during the TOJ task for any stimulus type; in fact, when they differed at all (mainly at lower COA levels), audio-leading performance was actually worse than video-leading ().
Neuroscientists have used manipulations of temporal synchrony to explore synchrony perception and multisensory integration in general 
. For example, several functional magnetic resonance imaging (fMRI) studies have explored how the brain responds to synchronous and asynchronous versions of audiovisual stimuli (e.g., 
). There are implications from the results of the current study for the interpretation of such experiments and for the design of future neuroimaging work. It is not uncommon for participants in these fMRI experiments to be asked to perform either an orthogonal task, i.e., one not related to synchrony, or even no task at all 
. This could be problematic, as under such experimental settings there is no evidence as to whether participants focused on the temporal order or the simultaneity/successiveness of the cues; it is even possible that different participants may have focused on different factors or that they changed their focus throughout. Since our results clearly indicate that SJs and TOJs are supported by different perceptual mechanisms, it is important to control the task performed by participants to ensure their attention is focused on the same factor for the entire experiment. The best way to do this is to give participants a specific task related to synchrony perception, potentially a TOJ, an SJ or alternative tasks previously outlined (e.g., 
). What is most important is that whichever task is chosen, in either neuroimaging or psychophysics studies, the results should be interpreted in relation to the task and stimulus; furthermore, synchrony perception has many factors and a single task is most likely not sufficient to explore them 
. Finally, an interesting question raised by the current psychophysics study is whether neuroimaging techniques can be used to define the location and functional properties of the different mechanisms supporting these two tasks.