Point Of Subjective Equality Research Articles

Young Women do it Better: Sexual Dimorphism in Temporal Discrimination.

The temporal discrimination threshold (TDT) is the shortest time interval at which two sensory stimuli presented sequentially are detected as asynchronous by the observer. TDTs are known to increase with age. Having previously observed shorter thresholds in young women than in men, in this work we sought to systematically examine the effect of sex and age on temporal discrimination. The aims of this study were to examine, in a large group of men and women aged 20–65 years, the distribution of TDTs with an analysis of the individual participant’s responses, assessing the “point of subjective equality” and the “just noticeable difference” (JND). These respectively assess sensitivity and accuracy of an individual’s response. In 175 participants (88 women) aged 20–65 years, temporal discrimination was faster in women than in men under the age of 40 years by a mean of approximately 13 ms. However, age-related decline in temporal discrimination was three times faster in women so that, in the age group of 40–65 years, the female superiority was reversed. The point of subjective equality showed a similar advantage in younger women and more marked age-related decline in women than men, as the TDT. JND values declined equally in both sexes, showing no sexual dimorphism. This observed sexual dimorphism in temporal discrimination is important for both (a) future clinical research assessing disordered mid-brain covert attention in basal-ganglia disorders, and (b) understanding the biology of this sexual dimorphism which may be genetic or hormonal.

Relationship between Cornea and Lid Margin Position on the Appearance of Strabismus.

Previous strabismus appearance studies using images from real face photographs could not preclude the effect of other facial landmarks, such as position and contour of brow, nose, and facial outline, from pure effect of ocular deviation itself. With conceptualized drawings of eyes, we simulated strabismus deviation and collected the responses of observers. The experiments consisted of three sessions: (1) one-eye, (2) two-eye cornea, and (3) two-eyelid deviation simulations. On a monitor, eight different ocular deviations in increments of 6 prism diopters of horizontal or vertical deviations were simulated. Nine ophthalmology residents were asked to choose between esotropia and exotropia for horizontal simulations and hypotropia or hypertropia for vertical simulations. Psychometric function fitting was performed. Detection sensitivity, point of subjective equality, and 95% thresholds were calculated. Between cornea and lid deviation simulations, there was no difference in 95% thresholds, points of subjective equality, and sensitivity (all p values > 0.10), reflecting that the relative location of cornea and lid was the determinant factor in recognizing ocular deviation. The sensitivities of vertical deviation simulations were higher than that of horizontal in two-eye cornea and lid comparisons (p values < 0.04) but not in one-eye (p value = 0.37). Our results suggest that the appearance of strabismus depends not on the absolute ocular alignment but rather on the reciprocal relationship of cornea and lid margin. Vertical misalignment was more apparent than horizontal misalignment.

Non-parametric bootstrapping method for measuring the temporal discrimination threshold for movement disorders

Objective. Recent studies have proposed that the temporal discrimination threshold (TDT), the shortest detectable time period between two stimuli, is a possible endophenotype for adult onset idiopathic isolated focal dystonia (AOIFD). Patients with AOIFD, the third most common movement disorder, and their first-degree relatives have been shown to have abnormal visual and tactile TDTs. For this reason it is important to fully characterize each participant’s data. To date the TDT has only been reported as a single value. Approach. Here, we fit individual participant data with a cumulative Gaussian to extract the mean and standard deviation of the distribution. The mean represents the point of subjective equality (PSE), the inter-stimulus interval at which participants are equally likely to respond that two stimuli are one stimulus (synchronous) or two different stimuli (asynchronous). The standard deviation represents the just noticeable difference (JND) which is how sensitive participants are to changes in temporal asynchrony around the PSE. We extended this method by submitting the data to a non-parametric bootstrapped analysis to get 95% confidence intervals on individual participant data. Main results. Both the JND and PSE correlate with the TDT value but are independent of each other. Hence this suggests that they represent different facets of the TDT. Furthermore, we divided groups by age and compared the TDT, PSE, and JND values. The analysis revealed a statistical difference for the PSE which was only trending for the TDT. Significance. The analysis method will enable deeper analysis of the TDT to leverage subtle differences within and between control and patient groups, not apparent in the standard TDT measure.

Subjective evaluation of loudness and preference for noise containing audible tones

Noise in living environments often contains unwanted tonal components that contribute to the unpleasant and intrusive character of a sound and usually lead to an increased annoyance. In the measurement and assessment of noise immissions after the German DIN 45681:2005-3 standard, the higher annoyance due to audible tonal components is covered by so called tone adjustments. Depending on the SNR of a tonal component within each critical band, the level of a sound can be charged with a penalty of up to 6 dB. In this study, the psychoacoustic penalty levels for sounds containing tonal components are directly determined in listening tests. The points of subjective equality (PSEs) for loudness and preference are determined using a matching procedure. The level of the test sound with tonal components is varied until it is equally loud / preferred as a tone-free reference sound that is constant in level. The level difference between test and reference sound at the PSEs is a direct measure of the penalty level. In listening tests with pure multi-tone sounds the penalty levels were found to be well above 6 dB, indicating that for these stimuli the German DIN standard is underestimating the penalty level.

Investigation of interocular blur suppression using luminance-modulated and contrast-modulated noise stimuli.

Presenting two sufficiently dissimilar images, one to each eye, may result in interocular suppression. The present study measured interocular suppression depth and extent in binocularly normal participants when blurring one eye only with varying dioptric lens powers (+0.5, +1, +2, and +4 D). Visual stimuli consisted of eight concentric rings of alternate polarity, divisible into eight sectors, within the central circular 24° visual field. Binocular "ring" stimuli therefore consisted of 64 individually testable dichoptic sectors. Using a two-alternative forced choice paradigm with a staircase procedure, signal strength of each dichoptic sector in the blurred eye was adjusted to perceptually match that of the surrounding ring from the nonblurred eye, determining the point of subjective equality. Rings were defined by differences in luminance (L), luminance-modulated noise (LM), or contrast-modulated noise (CM). Suppression depth was similar irrespective of sector location within the visual field and increased with increasing difference in interocular blur. Adding dynamic noise (LM vs. L stimuli) reduced the effect of blur on measured suppression depth. Significantly deeper suppression was measured for CM than for LM stimuli, both created using dynamic noise, the difference increasing at higher levels of interocular blur. As binocularity is disrupted with interocular blur, this result suggests that CM envelope combination may be processed by later mechanisms receiving binocular input than those required for the processing of LM stimuli. Differences in suppression depth between LM and CM stimuli could not be attributed to differences in spatial summation properties, stimulus visibility, noise modulation, or differential effects on blur discriminability.

Action can amplify motion-induced illusory displacement.

Local motion is known to produce strong illusory displacement in the perceived position of globally static objects. For example, if a dot-cloud or grating drifts to the left within a stationary aperture, the perceived position of the whole aperture will also be shifted to the left. Previously, we used a simple tracking task to demonstrate that active control over the global position of an object did not eliminate this form of illusion. Here, we used a new iPad task to directly compare the magnitude of illusory displacement under active and passive conditions. In the active condition, participants guided a drifting Gabor patch along a virtual slalom course by using the tilt control of an iPad. The task was to position the patch so that it entered each gate at the direct center, and we used the left/right deviations from that point as our dependent measure. In the passive condition, participants watched playback of standardized trajectories along the same course. We systematically varied deviation from midpoint at gate entry, and participants made 2AFC left/right judgments. We fitted cumulative normal functions to individual distributions and extracted the point of subjective equality (PSE) as our dependent measure. To our surprise, the magnitude of displacement was consistently larger under active than under passive conditions. Importantly, control conditions ruled out the possibility that such amplification results from lack of motor control or differences in global trajectories as performance estimates were equivalent in the two conditions in the absence of local motion. Our results suggest that the illusion penetrates multiple levels of the perception-action cycle, indicating that one important direction for the future of perceptual illusions may be to more fully explore their influence during active vision.

Real-time modulation of visual feedback on human full-body movements in a virtual mirror: development and proof-of-concept.

BackgroundVirtual reality (VR) provides interactive multimodal sensory stimuli and biofeedback, and can be a powerful tool for physical and cognitive rehabilitation. However, existing systems have generally not implemented realistic full-body avatars and/or a scaling of visual movement feedback. We developed a “virtual mirror” that displays a realistic full-body avatar that responds to full-body movements in all movement planes in real-time, and that allows for the scaling of visual feedback on movements in real-time. The primary objective of this proof-of-concept study was to assess the ability of healthy subjects to detect scaled feedback on trunk flexion movements.MethodsThe “virtual mirror” was developed by integrating motion capture, virtual reality and projection systems. A protocol was developed to provide both augmented and reduced feedback on trunk flexion movements while sitting and standing. The task required reliance on both visual and proprioceptive feedback. The ability to detect scaled feedback was assessed in healthy subjects (n = 10) using a two-alternative forced choice paradigm. Additionally, immersion in the VR environment and task adherence (flexion angles, velocity, and fluency) were assessed.ResultsThe ability to detect scaled feedback could be modelled using a sigmoid curve with a high goodness of fit (R2 range 89-98%). The point of subjective equivalence was not significantly different from 0 (i.e. not shifted), indicating an unbiased perception. The just noticeable difference was 0.035 ± 0.007, indicating that subjects were able to discriminate different scaling levels consistently. VR immersion was reported to be good, despite some perceived delays between movements and VR projections. Movement kinematic analysis confirmed task adherence.ConclusionsThe new “virtual mirror” extends existing VR systems for motor and pain rehabilitation by enabling the use of realistic full-body avatars and scaled feedback. Proof-of-concept was demonstrated for the assessment of body perception during active movement in healthy controls. The next step will be to apply this system to assessment of body perception disturbances in patients with chronic pain.

The effect of personal grooming on self-perceived body image.

Grooming behaviours, including the application of fragranced products, are thought to reflect a means of managing social impressions and self-image. Although application of deodorants has previously been shown to make individuals appear more confident to others, few studies have specifically examined the psychological effects of such rituals on the wearer. Here, we investigated how grooming behaviours affect self-perceived body image, a central component of an individual's self-image. In two separate experiments, using a psychophysical forced choice task, male and female participants with a normal Body Mass Index (BMI) indicated whether projected life-size images of their own body were bigger or smaller than their actual size. In the experimental condition, participants applied a fragranced deodorant before performing the task, whereas in the control condition, no product was applied. Our dependent measures were the point of subjective equality (PSE), the size at which participants report their body is subjectively equal to their actual body size, and the difference limen (DL), the amount of change in body size distortion necessary for it to be reliably detected. These measurements provide an index of attitudinal and perceptual components of body image, respectively. Both male and female participants who, at baseline, overestimated their body size, made significantly more accurate judgments about their body size, as measured by the PSE, following application of a fragranced deodorant or antiperspirant than they did in the control condition. This effect was seen in the absence of differences in perceptual sensitivity to changes in body size (DL) across groups and conditions. People who underestimated their body size did not show this effect. Of note, both male and female overestimators had a significantly larger BMI than underestimators. These results demonstrate that the attitudinal component of body image is malleable and can be influenced by everyday grooming routines, suggesting such behaviours have psychological benefits for both genders, beyond their basic hygiene function. However, there are individual differences in people's susceptibility to these effects, perhaps reflecting variability in self-esteem.

Modeling depth from motion parallax with the motion/pursuit ratio.

The perception of unambiguous scaled depth from motion parallax relies on both retinal image motion and an extra-retinal pursuit eye movement signal. The motion/pursuit ratio represents a dynamic geometric model linking these two proximal cues to the ratio of depth to viewing distance. An important step in understanding the visual mechanisms serving the perception of depth from motion parallax is to determine the relationship between these stimulus parameters and empirically determined perceived depth magnitude. Observers compared perceived depth magnitude of dynamic motion parallax stimuli to static binocular disparity comparison stimuli at three different viewing distances, in both head-moving and head-stationary conditions. A stereo-viewing system provided ocular separation for stereo stimuli and monocular viewing of parallax stimuli. For each motion parallax stimulus, a point of subjective equality (PSE) was estimated for the amount of binocular disparity that generates the equivalent magnitude of perceived depth from motion parallax. Similar to previous results, perceived depth from motion parallax had significant foreshortening. Head-moving conditions produced even greater foreshortening due to the differences in the compensatory eye movement signal. An empirical version of the motion/pursuit law, termed the empirical motion/pursuit ratio, which models perceived depth magnitude from these stimulus parameters, is proposed.

Duration perception of time-varying sounds: The role of the amplitude decay and rise-time modulator

It is well known that ramped (rising energy) sounds are perceived as longer in duration than damped (falling energy) sounds that are time-reversed, but otherwise identical versions of one another (Schlauch, Ries & DiGiovanni, 2001; Grassi & Darwin, 2006). This asymmetry has generally been attributed to the under-estimation of damped sound duration, rather than the over-estimation of ramped sound duration. As previous literature most commonly employs exponential amplitude modulators, in the present experiment, we investigate whether altering the nature of this amplitude decay- or rise-time modulator (linear or exponential) would influence this typically observed perceptual asymmetry. Participants performed an adaptive, 2AFC task that assessed the point of subjective equality (PSE) between a standard tone with a constant ramped/damped envelope, and a comparator tone with a “flat,” steady state envelope whose duration varied according to a 1-up, 1-down rule. Preliminary results replicated previous findings that ramped sounds are perceived as longer than their time-reversed, damped counterparts. However, for sounds with a linear amplitude modulator, this perceptual asymmetry is partially accounted for by ramped tone over-estimation, contrasting previous findings in the literature conducted with exponential amplitude modulators.

Effects of virtual speaker density and room reverberation on spatiotemporal thresholds of audio-visual motion coherence.

The present study examined the effects of spatial sound-source density and reverberation on the spatiotemporal window for audio-visual motion coherence. Three different acoustic stimuli were generated in Virtual Auditory Space: two acoustically “dry” stimuli via the measurement of anechoic head-related impulse responses recorded at either 1° or 5° spatial intervals (Experiment 1), and a reverberant stimulus rendered from binaural room impulse responses recorded at 5° intervals in situ in order to capture reverberant acoustics in addition to head-related cues (Experiment 2). A moving visual stimulus with invariant localization cues was generated by sequentially activating LED's along the same radial path as the virtual auditory motion. Stimuli were presented at 25°/s, 50°/s and 100°/s with a random spatial offset between audition and vision. In a 2AFC task, subjects made a judgment of the leading modality (auditory or visual). No significant differences were observed in the spatial threshold based on the point of subjective equivalence (PSE) or the slope of psychometric functions (β) across all three acoustic conditions. Additionally, both the PSE and β did not significantly differ across velocity, suggesting a fixed spatial window of audio-visual separation. Findings suggest that there was no loss in spatial information accompanying the reduction in spatial cues and reverberation levels tested, and establish a perceptual measure for assessing the veracity of motion generated from discrete locations and in echoic environments.

Deficient local biological motion perception in migraineurs: Results from a duration discrimination paradigm

Migraine ranks as the third most common disease in the world and has caused significant losses of daily life abilities. Previously, people gave more attention to the pain of migraines and usually ignored the impairments of cognitive function in migraineurs. In the present study, a duration discrimination paradigm was used to assess the global and local biological motion perception in migraineurs and healthy controls. In the experiment, biological motion sequences and inanimate motion sequences (the inverted biological motion sequences) were sequentially presented on a screen. Observers were instructed to make a two-alternative forced choice to accurately indicate which interval (the first or the second) appeared longer. The stimuli involved global biological motion sequences and local biological motion sequences. The statistical analyses were conducted on the points of subjective equality that were obtained by fitting a psychometric function to each individual observer’s data. In migraineurs, global biological motion signals lengthened the perceived temporal duration (as occurs in normal people), whereas local biological motion signals did not have this temporal dilation effect. The results indicated that patients with migraine showed a deficit in local biological motion perception, whereas their global biological motion perception was comparable to that of healthy subjects.

Testing the absolute-tempo hypothesis: context effects for familiar and unfamiliar songs.

In two experiments, we investigated context effects on tempo judgments for familiar and unfamiliar songs performed by popular artists. In Experiment 1, participants made comparative tempo judgments to a remembered standard for song clips drawn from either a slow or a fast context, created by manipulating the tempos of the same songs. Although both familiar and unfamiliar songs showed significant shifts in their points of subjective equality toward the tempo context values, more-familiar songs showed significantly reduced contextual bias. In Experiment 2, tempo pleasantness ratings showed significant context effects in which the ordering of tempos on the pleasantness scale differed across contexts, with the most pleasant tempo shifting toward the contextual values, an assimilation of ideal points. Once again, these effects were significant but reduced for the more-familiar songs. The moderating effects of song familiarity support a weak version of the absolute-tempo hypothesis, in which long-term memory for tempo reduces but does not eliminate contextual effects. Thus, although both relative and absolute tempo information appear to be encoded in memory, the absolute representation may be subject to rapid revision by recently experienced tempo-altered versions of the same song.

Biological motion perception in patients with Parkinson’s disease

Objective To explore the biological motion perception in Parkinson’s disease (PD). Methods 45 individuals with idiopathic PD were compared with 45 matched healthy controls (HCs) using a duration discrimination task. Results The point of subjective equality(PSE) was negative value (-0.27±0.17) for health controls (HCs), and there was significant difference compared with PSE=0 by one sample t test (t=10.96, P<0.01). Compared with HCs, the PSE for PD patients (-0.14±0.30) significantly decreased (t=2.63, P=0.01). When further dividing PD into early stages (stage 1-2) and late stages (stage 3-4), significant difference was found between late-stage PD patients(0.02±0.39)and HCs (t=4.07, P=0.008), but not between early-stage PD patients(-0.24±0.14)and HCs(t=0.84, P=0.405). Conclusion There is biological motion perception disorder in PD patients and it is related to the severity of PD. Key words: Parkinson’s disease; Biological motion; Cognition; Mirror neuron system

Using reaction times and binary responses to estimate psychophysical performance: an information theoretic analysis

As the strength of a stimulus increases, the proportions of correct binary responses increases, which define the psychometric function. Simultaneously, mean reaction times (RT) decrease, which collectively define the chronometric function. However, RTs are traditionally ignored when estimating psychophysical parameters, even though they may provide additional Shannon information. Here, we extend Palmer et al's (2005) proportional-rate diffusion model (PRD) by: (a) fitting individual RTs to an inverse Gaussian distribution, (b) including lapse rate, (c) point-of-subjective-equality (PSE) parameters, and, (d) using a two-alternative forced choice (2AFC) design based on the proportion of times a variable comparison stimulus is chosen. Maximum likelihood estimates of mean RT values (from fitted inverse Gaussians) and binary responses were fitted both separately and in combination to this extended PRD (EPRD) model, to obtain psychophysical parameter values. Values estimated from binary responses alone (i.e., the psychometric function) were found to be similar to those estimated from RTs alone (i.e., the chronometric function), which provides support for the underlying diffusion model. The EPRD model was then used to estimate the mutual information between binary responses and stimulus strength, and between RTs and stimulus strength. These provide conservative bounds for the average amount of Shannon information the observer gains about stimulus strength on each trial. For the human experiment reported here, the observer gains between 2.68 and 3.55 bits/trial. These bounds are monotonically related to a new measure, the Shannon increment, which is the expected value of the smallest change in stimulus strength detectable by an observer.

Subjective Distortions of Time Influence Rapid Information Accrual

One common observation about time perception is that it is subjective, dependent on factors such as attention rather than only the objective passing of time measured by a clock. For example, unexpected and emotional events are often experienced as passing in slow motion, as if time subjectively expands. Such time dilation effects have been measured in the laboratory using an oddball paradigm, in which an infrequent stimulus is perceived to last longer than the standard stimuli in the rest of the sequence. In contrast, time compression for the oddball occurs when the duration of the standard items in the sequence is relatively brief (Tse et al., 2004). One possible explanation for temporal expansion is that increasing the state of arousal increases the rate of internal clocks, which then leads to the accumulation of more “ticks” and, thus, longer perceived durations. Another possibility, however, is that mental processes in general (rather than just mental clocks) are more rapid or efficient during such events, leading to more cognitive processing and hence the impression of more time passing. Here, we investigated whether the amount of information processed for a briefly presented display changes when time is perceived as distorted. Specifically, we used an enumeration task in order to measure the amount of information accrual over time.In this experiment, there were two separate blocks of trials, with standard stimuli presented at a fixed duration of either 70 (compression condition) or 1050ms (expansion condition). Each standard stimulus was each made up of a variable number of green dots. On each trial, a red oddball stimulus of varying numerosity (1-14 items) was presented. The duration of the oddball varied on each trial in both the compression (30 to 150ms) and expansion (650 to 1250ms) conditions. Observers were instructed to count the number of dots within the oddball and to judge its relative duration with respect to the standards on that trial.Consistent with previous results, oddballs were reliably perceived as temporally distorted, either expanded for the long standard stimulus duration blocks (point of subjective equality (PSE) ∼840ms) or compressed for the short standard stimulus duration blocks (PSE∼100ms). On a single trial level, enumeration was more accurate when temporal expansion occurred (oddball erroneously judged longer than the standards) and worse when time was subjectively shorter (illusory compression of the oddball). The effect of temporal distortion was maximal for intermediate set sizes for which duration influences information accrual, beyond the subitizing range but still within the range of accurate counting.These findings provide strong evidence for an integration model of time perception, in which the amount of information accrual over an interval determines the perceived duration of the event. Overall, the results show that subjective time distortions are not just epiphenomenal, but can reflect real changes in sensory processing of the incoming stimulus. In other words, when time subjectively slows down during emotional events, it might allow for processing more information in the same objective temporal duration. Such short-term plasticity in information processing rate could be evolutionarily advantageous in optimizing perception and action during critical moments.

What Makes an Oddball Odd? Evidence from a Spatially Predictable Temporal Oddball Paradigm

The temporal oddball effect (Birngruber, Schröter, and Ulrich, in press; Pariyadath and Eagleman, 2007; Schindel, Rowlands, and Arnold, 2011) describes the finding that rare, deviant stimuli (oddballs) are temporally overestimated as compared to standards of equal physical duration. In a typical oddball paradigm, oddballs are presented at a random position within a stream of repeated standard stimuli. While the standards’ duration is constant, oddball duration varies from trial to trial. After each trial, participants are asked to judge whether the oddball was shorter or longer in duration than the standards. In order to explain the oddball effect, it has been assumed that oddballs as compared to standards either attract more attention (Tse, Intriligator, Rivest, & Cavanagh, 2004) or increase the level of arousal (Ulrich, Nitschke, & Rammsayer, 2006). Nevertheless, it is not entirely clear which aspects make an oddball “odd”.In the present study, we modified the classical paradigm in order to disentangle different factors that could drive the oddball effect. In each trial, a stream of five stimuli was presented, whereby the first four stimuli were always standards (e.g., blue disks) of a constant standard duration (800ms) and the last stimulus was a comparison stimulus of varying duration (480-1120ms; in nine steps). The comparison stimulus could either be another standard (e.g., another blue disk) or an oddball (e.g., a red disk). Participants’ task was to judge whether the last stimulus in the stream was shorter or longer than the four preceding stimuli. In order to make counting of the stimuli unnecessary, we presented the stimuli in a spatially predictable format on the screen, that is, on an imagined clock face. Stimuli appeared in clockwise order at the 12, 3, 6, and 9 o’clock positions (standards), and again at the 12 o’clock position (comparison). In half of all trials, a standard stimulus was presented as comparison whereas in the other half of trials an oddball stimulus was presented as comparison. This allowed for a direct comparison of the subjective duration of oddballs and standards. In two separate blocks of trials, we manipulated how many different oddball colors could appear. In one block of trials, only a single oddball color was used (e.g., red) whereas in the other block of trials, four different oddball colors were used (cyan, green, magenta, and yellow) and presented equally often. Points of subjective equality (PSE) were derived from the individual psychometric functions as measures of perceived duration. Preliminary data show that even though all stimuli evoked PSEs smaller than the objective standard duration, no oddball effect was present in either block. Possible explanations for this result pattern will be discussed.

Perception of Duration with Irregularly Filled Intervals

The perception of interval duration can be distorted by a multitude of factors related to stimulus characteristics. One of them is the filled-duration illusion, where filled intervals are perceived to be longer than their empty counterparts. The filled-duration illusion has been shown to occur with intervals consisting of one continuous signal (e.g., Wearden et al., 2007). Most notably, however, the illusion also occurs with intervals containing several short signals (“fillers”) in between a beginning and an end marker (e.g., Thomas & Brown, 1974). Only few investigations have looked at the effect of the filler signal characteristics on the perception of duration. The present work focused on the role of regular versus irregular spacing of filler signals in interval duration judgments. Stimuli were auditory filled intervals between 0.5 and 1.5 s duration, marked by five 10ms 1000Hz filler tones. Five different levels of irregularity in the timing of the fillers were defined, ranging from regular (that is, spacing between every two neighboring fillers is equal) to 50% of the regular spacing. In a two-interval forced choice task, participants judged which of two intervals appeared longer. One interval was always regular, while the other one was irregular. The serial position of the physically longer interval as well as the irregular interval were counterbalanced and randomized. Psychometric functions were fitted to the proportion of responses plotted as a function of the difference in duration. The point of subjective equality and the just noticeable difference were estimated. While the variation in irregularity did not affect duration discrimination sensitivity, results indicate that irregular intervals are judged to last shorter than regular ones of the same physical duration and this effect increases with the amount of irregularity. Furthermore in trials where the regular interval precedes the irregular one, irregular intervals were specifically judged shorter than their regular counterparts, when the first interval between fillers was significantly longer than the second one. These findings suggest that duration perception of filled intervals can be affected by the properties and regularity of the individual filler stimuli, opening the road to a new interpretation of the reasons behind the filled-duration illusion. Implications for a computational model of interval duration perception are discussed considering the perception of filler stimulus timing to be affected by the expectation built up from previously presented stimuli.

Time Contracts when the Mind Wanders

Mind-wandering refers to the act of engaging in task-unrelated thoughts. Mind-wandering is known to impact a wide array of cognitive functions but whether and how it affects time perception is unknown. We expected that allocation of attention to task- unrelated thoughts during interval timing would have the consequence of reduced attention to, and a concomitant perceived contraction of, timed intervals. Thirty-four participants completed a sub-second visual oddball task in which infrequent stimuli of varying duration were embedded within a train of homogeneous standard stimuli of constant duration. Participants judged whether the oddball was shorter or longer in duration than the standards and whether their attention was “on-task” (focused on the task) or “off-task” (directed toward task-unrelated thoughts or the non-task environment) during the stimulus train. Individual participants’ probabilities of a long response p(long) at each oddball interval were fitted with logistic functions separately for on- task and off-task trials. The oddball duration corresponding to the 50% threshold on the psychometric function (t(p(long) = 0.5; the point of subjective equality [PSE]) was used as a measure of perceived duration whereas the Weber fraction (WF), which is the difference limen [(t(p(long) = 0.75) – t(p(long) = 0.25))/2] standardized by the PSE, was used as a measure of temporal precision. Participants reported being off-task in 18% of trials. As predicted, they exhibited larger PSEs, reflecting greater underestimation of oddball intervals, when off-task than when on-task. This effect was robust with 81% of participants displaying temporal contraction during off-task trials. The magnitude of change in PSEs during off-task states was unrelated to the frequency of off-task state reports, finger-response patterns, or participants’ awareness of an association between being off-task and pressing short (as assessed in a post-experiment debriefing session). Participants also exhibited a reduction in temporal precision when mind-wandering, as reflected in larger WFs for off-task than on-task trials. These results demonstrate that mind- wandering is associated with a perceived contraction of temporal intervals.

The effect of filled and empty intervals on clock and memory processes in pigeons

According to the mixed memory model (Penney, Gibbon, & Meck, Journal of Experimental Psychology: Human Perception and Performance, 26, 1770-1787, 2000), different clock rates for stimuli with different nontemporal properties must be stored within a single reference memory distribution in order to detect a difference between the clock rates of the different signals. In Experiment 1, pigeons were trained in a between-subjects design to discriminate empty intervals (bound by two 1-s visual markers) and filled intervals (a continuous visual signal). The intervals were signaled by different visual stimuli, and they required responses to different sets of comparison stimuli. Empty intervals were judged as being longer than filled intervals. The difference between the point of subjective equality (PSE) for the empty intervals and the PSE for the filled intervals increased proportionally as the magnitudes of the anchor duration pairs were increased from 2 and 8 s to 4 and 16 s. In Experiment 2, the pigeons were trained to discriminate intervals signaled by the absence of houselight illumination (Group Empty) or the presence of houselight illumination (Group Filled). The psychophysical timing functions for these intervals were identical to each other. The results of Experiment 1 indicate that memory mixing is not necessary for detecting a timing difference between empty and filled intervals in pigeons. The results of Experiment 2 suggest that the nature of the stimuli that signal the empty and filled intervals impacts how pigeons judge the durations of empty and filled intervals.