The many directions of time

Face adaptation induces duration distortion of subsequent face stimuli in a face category-specific manner

Studies have shown that duration perception depends on several visual processes. However, the stages of visual processes that contribute to duration perception remain unclear. This study examined the effects of categorical differences in face adaptation on perceived duration. In all the experiments, we compared the perceived durations of human, monkey, and cat faces (comparison stimuli) after adapting to a human face. Results revealed that the human comparison stimuli were perceived shorter than the monkey and cat comparison stimuli (categorical face adaptation on duration perception [CFAD]). The difference between the face categories disappeared when the adapting stimulus was rendered unrecognizable by phase scrambling, indicating that adaptation to low-level visual properties cannot fully account for the CFAD effect. Furthermore, CFAD was preserved but attenuated when the adapting stimulus was inverted or a 1,000-ms interval was inserted before the comparison stimuli, which implied that CFAD occurred as long as the adapting stimulus was perceived as a face and not simply based on conceptual category processes. These findings indicate that face adaptation affects perceived duration in a category-specific manner (the CFAD effect) and highlights the involvement of visual categorical processes in duration perception.

The effect of space on subjective time is mediated by apparent velocity

In contrast to the intuitive and traditional assumption of a centralized and universal neural clock, many recent studies have provided evidence against this idea. Here, we investigated whether subjective duration is estimated by a mechanism that tracks the trajectory of a moving object. Such a mechanism would integrate over the velocity and the spatial distance the object traveled to derive its duration. We exposed observers to a moving object that covered either a small or large spatial distance. We found that subjective duration decreased after this exposure when intervals were tested that were defined by stimuli covering a large, but not by stimuli covering a small, spatial distance. We compared the effects of our velocity exposure to previously used adaptation to a drifting grating and found a dependence of spatial distance only for velocity exposure. The finding that temporal estimations decrease after exposure to fast-moving stimuli selectively at large distances suggests that subjective duration is derived from measurements of velocity and space.

Short-term global motion adaptation induces a compression in the subjective duration of dynamic visual events

Apparent duration can be manipulated in a local region of visual field by long-term adaptation to motion or flicker (Johnston, Arnold, & Nishida, 2006). These effects show narrow spatial tuning (Ayhan, Bruno, Nishida, & Johnston, 2009), as well as retinotopic position dependency (Bruno, Ayhan, & Johnston, 2010), supporting early locus in the visual pathway. Here, we introduce a new effect using RDK as a short-term visual adaptor and demonstrate that a brief, subsecond range adaptation induces a significant subjective duration compression (∼10%) on a subsequently presented test stimulus (RDK pattern) only for global motion patterns drifting at 50% motion coherence but not for those drifting at 0% coherence, suggesting a higher level area as a source of origin. In another set of experiments using a plaid stimulus as the adaptor and gratings as the tests, we report again a significant duration compression following a brief motion adaptation, although the effect does not seem to be consistently selective for a particular direction of the standard test relative to that of the plaid adaptor (two-dimensional motion) or its components (one-dimensional motion). Finally, we conduct an experiment using shutter glasses and find that the effects of a short-term adaptor presented monocularly to one eye transfer to the nonadapted eye, providing evidence for the interocular transfer. In a series of control experiments, we also show that the duration effects cannot be explained by adaptation-induced changes in perceived speed, perceived onset-and-offset, and attentional resource allocation. Overall, the duration compression effect requiring motion coherence in RDK, persisting in plaid stimulus, and showing interocular transfer imply explicit genuine mechanisms mediating duration effects in the higher level motion areas.

Motion-induced compression of perceived numerosity

It has been recently proposed that space, time, and number might share a common representation in the brain. Evidence supporting this idea comes from adaptation studies demonstrating that prolonged exposure to a given stimulus feature distorts the perception of different characteristics. For example, visual motion adaptation affects both perceived position and duration of subsequent stimuli presented in the adapted location. Here, we tested whether motion adaptation also affects perceived numerosity, by testing the effect of adaptation to translating or rotating stimuli moving either at high (20 Hz) or low (5 Hz) speed. Adaptation to fast translational motion yielded a robust reduction in the apparent numerosity of the adapted stimulus (~25%) while adaptation to slow translational or circular motion (either 20 Hz or 5 Hz) yielded a weaker but still significant compression. Control experiments suggested that none of these results could be accounted for in terms of stimulus masking. Taken together, our results are consistent with the extant literature supporting the idea of a generalized magnitude system underlying the representation of numerosity, space and time via common metrics. However, as changes in perceived numerosity co-varied with both adapting motion profile and speed, our evidence also suggests complex and asymmetric interactions between different magnitude representations.

Automatic detection of the duration of visual static and dynamic stimuli

The perception of the passing of time is fundamental to conscious experience. The duration of a sensory stimulus is one of its defining attributes, but it is not clear how this is encoded in the brain. This work explores whether the duration of a visual stimulus is an attribute that the brain can automatically adapt to and use to predict future stimulus durations. Visual mismatch negativity (vMMN) is an ERP component elicited, even when the stimuli are unattended, when an 'unexpected' visual stimulus appears amongst a series of expected stimuli in an 'oddball' paradigm. As such vMMN has been suggested to show that the violation of a pattern in a sequence has been automatically detected. To date, vMMN has only been measured to differences in the visual durations of static on/off stimuli, placed near to the centre of the visual field. Our study measures vMMN to test whether duration is encoded automatically for static stimuli against a blank background and moving stimuli against a static background, whilst attention is directed to a different spatial location using a continuous, attention demanding task. VMMN elicited in response to the shorter duration for both stimuli shows that the brain detects the differences of duration even in the absence of focussed spatial attention. For the motion stimulus a larger difference in duration was needed. We conclude that duration is encoded automatically in the visual cortex and is an attribute that can be adapted to, and form the basis of predictions.

Duration compression induced by visual and phonological repetition of Chinese characters

Our prior experience heavily influences our subjective time. One of such phenomena is repetition compression, that is, repeated stimuli are perceived shorter than novel stimuli. However, most of the studies on repetition compression used identical stimuli, leaving the question whether similar repetition effects could take place in phonological and semantic level repetition. We used Chinese characters to manipulate different levels of repetition in a duration discrimination task. We replicated earlier findings that repetition of visual identical characters shortened the apparent duration and found the repetition compression was spatially independent. Phonological repetition also caused the duration compression though the effect was weaker than the visual repetition. However, we observed no duration compression during the semantic repetition. The results suggest that repetition compression is mediated by visual and phonological representation of a stimulus in an early stage in processing hierarchy. We explained our findings according to the framework of predictive coding.

Perceived duration of brief visual events is mediated by timing mechanisms at the global stages of visual processing

There is a growing body of evidence pointing to the existence of modality-specific timing mechanisms for encoding sub-second durations. For example, the duration compression effect describes how prior adaptation to a dynamic visual stimulus results in participants underestimating the duration of a sub-second test stimulus when it is presented at the adapted location. There is substantial evidence for the existence of both cortical and pre-cortical visual timing mechanisms; however, little is known about where in the processing hierarchy the cortical mechanisms are likely to be located. We carried out a series of experiments to determine whether or not timing mechanisms are to be found at the global processing level. We had participants adapt to random dot patterns that varied in their motion coherence, thus allowing us to probe the visual system at the level of motion integration. Our first experiment revealed a positive linear relationship between the motion coherence level of the adaptor stimulus and duration compression magnitude. However, increasing the motion coherence level in a stimulus also results in an increase in global speed. To test whether duration compression effects were driven by global speed or global motion, we repeated the experiment, but kept global speed fixed while varying motion coherence levels. The duration compression persisted, but the linear relationship with motion coherence was absent, suggesting that the effect was driven by adapting global speed mechanisms. Our results support previous claims that visual timing mechanisms persist at the level of global processing.

Perceived Duration Increases with Contrast, but Only a Little

Recent adaptation studies provide evidence for early visual areas playing a role in duration perception. One explanation for the pronounced duration compression commonly found with adaptation is that it reflects adaptation-driven stimulus-specific reduction in neural activity in early visual areas. If this level of stimulus-associated neural activity does drive duration, then we would expect a strong effect of contrast on perceived duration as electrophysiological studies shows neural activity in early visual areas to be strongly related to contrast. We employed a spatially isotropic noise stimulus where the luminance of each noise element was independently sinusoidally modulated at 4 Hz. Participants matched the perceived duration of a high (0.9) or low (0.1) contrast stimulus to a previously presented standard stimulus (600 ms, contrast = 0.3). To achieve perceptually equivalent durations, the low contrast stimulus had to be presented for longer than the high contrast stimulus. This occurred when we controlled for stimulus size and when we adjusted for individual differences in perceived temporal frequency. Further, we show that the effect cannot be explained by shifts in perceived onset and offset and is not explained by a simple contrast-driven response bias. The direction of our results is clearly consistent with the idea that level of neural activity drives duration. However, the magnitude of the effect (~10% duration difference over a 0.9-0.1 contrast reduction) is in marked contrast to the larger duration distortions that can be found with repetition suppression and the oddball effect; particularly when these may be associated with smaller differences in neural activity than that expected from our contrast difference. Taken together, these results indicate that level of stimulus-related neural activity in early visual areas is unlikely to provide a general mechanism for explaining differences in perceived duration.

The duration compression effect is mediated by adaptation of both retinotopic and spatiotopic mechanisms

The duration compression effect is a phenomenon in which prior adaptation to a spatially circumscribed dynamic stimulus results in the duration of subsequent subsecond stimuli presented in the adapted region being underestimated. There is disagreement over the frame of reference within which the duration compression phenomenon occurs. One view holds that the effect is driven by retinotopic-tuned mechanisms located at early stages of visual processing, and an alternate position is that the mechanisms are spatiotopic and occur at later stages of visual processing (MT+). We addressed the retinotopic-spatiotopic question by using adapting stimuli - drifting plaids - that are known to activate global-motion mechanisms in area MT. If spatiotopic mechanisms contribute to the duration compression effect, drifting plaid adaptors should be well suited to revealing them. Following adaptation participants were tasked with estimating the duration of a 600ms random dot stimulus, whose direction was identical to the pattern direction of the adapting plaid, presented at either the same retinotopic or the same spatiotopic location as the adaptor. Our results reveal significant duration compression in both conditions, pointing to the involvement of both retinotopic-tuned and spatiotopic-tuned mechanisms in the duration compression effect.

Multiple channels of visual time perception

The proposal that the processing of visual time might rely on a network of distributed mechanisms that are vision-specific and timescale-specific stands in contrast to the classical view of time perception as the product of a single supramodal clock. Evidence showing that some of these mechanisms have a sensory component that can be locally adapted is at odds with another traditional assumption, namely that time is completely divorced from space. Recent evidence suggests that multiple timing mechanisms exist across and within sensory modalities and that they operate in various neural regions. The current review summarizes this evidence and frames it into the broader scope of models for time perception in the visual domain.

Adaptation-Induced Compression of Event Time Occurs Only for Translational Motion

Adaptation to fast motion reduces the perceived duration of stimuli displayed at the same location as the adapting stimuli. Here we show that the adaptation-induced compression of time is specific for translational motion. Adaptation to complex motion, either circular or radial, did not affect perceived duration of subsequently viewed stimuli. Adaptation with multiple patches of translating motion caused compression of duration only when the motion of all patches was in the same direction. These results show that adaptation-induced compression of event-time occurs only for uni-directional translational motion, ruling out the possibility that the neural mechanisms of the adaptation occur at early levels of visual processing.

Numerical Context and Time Perception: Contrast Effects and the Perceived Duration of Numbers

In the current study, we examined how the contextual repetition of magnitude information presented in either symbolic (Arabic digits) or nonsymbolic (numerosities) formats impacted on the perceived duration of a later occurring target number. The results of the current study demonstrated a time-magnitude bias in which, on average, large magnitude target numbers were judged to last for longer durations relative to small magnitude target numbers, regardless of notation (symbolic number and numerosity). Furthermore, context effects were found, in which a greater discrepancy in the target's magnitude from the initial context led to longer perceived duration ratings. However, this was found to be asymmetrical, occurring only for large magnitude targets. Additionally, the type of context effect was shown to be determined by whether the context was presented in the same notation as the target or a different notation.

High temporal frequency adaptation compresses time in the Flash-Lag illusion

Previous research finds that 20 Hz temporal frequency (TF) adaptation causes a compression of perceived visual event duration. We investigate if this temporal compression affects further time-dependent percepts, implying a further functional role for duration perception mechanisms. We measure the effect of 20 Hz flicker adaptation on Flash-Lag, an illusion whereby an observer perceives a moving object displaced further along its trajectory compared to a spatially localized briefly flashed object. The illusion scales with object speed; therefore, it has a fixed temporal component. By comparing adaptation at 5 Hz and 20 Hz we show that 20 Hz TF adaptation reduces perceived Flash-Lag magnitude significantly, with no effect at 5 Hz, whereas the opposite pattern of adaptation was seen on perceived speed. There is a significant effect of 20 Hz adaptation on the perceived duration of a moving bar. This suggests that 20 Hz TF adaptation has compressed the fixed temporal component of the Flash-Lag illusion, implying the mechanism underlying duration perception also has effects on judging spatial relationships in dynamic stimuli.

Direction-contingent duration compression is primarily retinotopic

Previous research has shown that prior adaptation to a spatially circumscribed, oscillating grating results in the duration of a subsequent stimulus briefly presented within the adapted region being underestimated. There is an on-going debate about where in the motion processing pathway the adaptation underlying this distortion of sub-second duration perception occurs. One position is that the LGN and, perhaps, early cortical processing areas are likely sites for the adaptation; an alternative suggestion is that visual area MT+ contains the neural mechanisms for sub-second timing; and a third position proposes that the effect is driven by adaptation at multiple levels of the motion processing pathway. A related issue is in what frame of reference - retinotopic or spatiotopic - does adaptation induced duration distortion occur. We addressed these questions by having participants adapt to a unidirectional random dot kinematogram (RDK), and then measuring perceived duration of a 600 ms test RDK positioned in either the same retinotopic or the same spatiotopic location as the adaptor. We found that, when it did occur, duration distortion of the test stimulus was direction contingent; that is it occurred when the adaptor and test stimuli drifted in the same direction, but not when they drifted in opposite directions. Furthermore the duration compression was evident primarily under retinotopic viewing conditions, with little evidence of duration distortion under spatiotopic viewing conditions. Our results support previous research implicating cortical mechanisms in the duration encoding of sub-second visual events, and reveal that these mechanisms encode duration within a retinotopic frame of reference.

Motion-direction specificity for adaptation-induced duration compression depends on temporal frequency

Adapting to a 20 Hz oscillating grating reduces the apparent duration of a 10 Hz drifting grating displayed subsequently in the same location as the adaptor. The effect is orientation-independent as it remains once the adaptor is rotated 90° relative to the tests (Johnston, Arnold, & Nishida, 2006). However, it was shown that, for random dots moving at 3°/s, duration compression follows adaptation only when the adaptor and test drift in the same direction, and it disappears when they drift in opposite directions (Curran & Benton, 2012). Here, we explored the relationship between the relative motion direction of adaptor and test and the strength of duration compression for a wider range of speeds and for narrow-band stimuli (temporal frequencies between 3 and 18 Hz). We first measured perceived temporal frequency for the same stimuli after adaptation, and we used these estimates to match the apparent rate of the adapted and unadapted tests in the duration task. We found that, whereas at 3 Hz the effect of adaptation in the opposite direction on duration is marginal, at higher frequencies there is substantial duration compression in the opposite direction. These results indicate that there may be two contributions to apparent duration compression: a cortical contribution sensitive to orientation and motion direction at a wide range of temporal frequencies and a direction-independent subcortical contribution, which is revealed at higher frequencies. However, while direction specificity implies cortical involvement, subcortical orientation dependency and the influence of feedback to subcortical areas should not be ignored.

Time dilation caused by static images with implied motion

The present study examined whether implicit motion information from static images influences perceived duration of image presentation. In Experiments 1 and 2, we presented observers with images of a human and an animal character in running and standing postures. The results revealed that the perceived presentation duration of running images was longer than that of standing images. In Experiments 3 and 4, we used abstract block-like images that imitated the human figures used in Experiment 1, presented with different instructions to change the observers' interpretations of the stimuli. We found that the perceived duration of the block image presented as a man running was longer than that of the image presented as a man standing still. However, this effect diminished when the participants were told the images were green onions (objects with no implied motion), suggesting that the effect of implied motion cannot be attributed to low-level visual differences. These results suggest that implied motion increases the perceived duration of image presentation. The potential involvement of higher-order motion processing and the mirror neuron system is discussed.