Discriminating time intervals presented in sequences marked by visual signals

A perceptual glitch in serial perception generates temporal distortions

Precisely estimating event timing is essential for survival, yet temporal distortions are ubiquitous in our daily sensory experience. Here, we tested whether the relative position, duration, and distance in time of two sequentially-organized events-standard S, with constant duration, and comparison C, with duration varying trial-by-trial-are causal factors in generating temporal distortions. We found that temporal distortions emerge when the first event is shorter than the second event. Importantly, a significant interaction suggests that a longer inter-stimulus interval (ISI) helps to counteract such serial distortion effect only when the constant S is in the first position, but not if the unpredictable C is in the first position. These results imply the existence of a perceptual bias in perceiving ordered event durations, mechanistically contributing to distortion in time perception. We simulated our behavioral results with a Bayesian model and replicated the finding that participants disproportionately expand first-position dynamic (unpredictable) short events. Our results clarify the mechanisms generating time distortions by identifying a hitherto unknown duration-dependent encoding inefficiency in human serial temporal perception, something akin to a strong prior that can be overridden for highly predictable sensory events but unfolds for unpredictable ones.

Two attentive strategies reducing subjective distortions in serial duration perception

Humans tend to perceptually distort (dilate/shrink) the duration of brief stimuli presented in a sequence when discriminating the duration of a second stimulus (Comparison) from the duration of a first stimulus (Standard). This type of distortion, termed “Time order error” (TOE), is an important window into the determinants of subjective perception. We hypothesized that stimulus durations would be optimally processed, suppressing subjective distortions in serial perception, if the events to be compared fell within the boundaries of rhythmic attentive sampling (4–8 Hz, theta band). We used a two-interval forced choice (2IFC) experimental design, and in three separate experiments tested different Standard durations: 120-ms, corresponding to an 8.33 Hz rhythmic attentive window; 160 ms, corresponding to a 6.25 Hz window; and 200 ms, for a 5 Hz window. We found that TOE, as measured by the Constant Error metric, is sizeable for a 120-ms Standard, is reduced for a 160-ms Standard, and statistically disappears for 200-ms Standard events, confirming our hypothesis. For 120- and 160-ms Standard events, to reduce TOEs it was necessary to increase the interval between the Standard and the Comparison event from sub-second (400, 800 ms) to supra-second (1600, 2000 ms) lags, suggesting that the orienting of attention in time waiting for the Comparison event to onset may work as a back-up strategy to optimize its encoding. Our results highlight the flexible use of two different attentive strategies to optimize subjective time perception.

Perceptual timing precision with vibrotactile, auditory, and multisensory stimuli

The growing use of vibrotactile signaling devices makes it important to understand the perceptual limits on vibrotactile information processing. To promote that understanding, we carried out a pair of experiments on vibrotactile, auditory, and bimodal (synchronous vibrotactile and auditory) temporal acuity. On each trial, subjects experienced a set of isochronous, standard intervals (400 ms each), followed by one interval of variable duration (400 ± 1-80 ms). Intervals were demarcated by short vibrotactile, auditory, or bimodal pulses. Subjects categorized the timing of the last interval by describing the final pulse as either "early" or "late" relative to its predecessors. In Experiment 1, each trial contained three isochronous standard intervals, followed by an interval of variable length. In Experiment 2, the number of isochronous standard intervals per trial varied, from one to four. Psychometric modeling revealed that vibrotactile stimulation produced poorer temporal discrimination than either auditory or bimodal stimulation. Moreover, auditory signals dominated bimodal sensitivity, and inter-individual differences in temporal discriminability were reduced with bimodal stimulation. Additionally, varying the number of isochronous intervals in a trial failed to improve temporal sensitivity in either modality, suggesting that memory played a key role in judgments of interval duration.

Sandwiched visual stimuli are perceived as shorter than the stimulus alone

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.

Multiple Looks of Auditory Empty Durations Both Improve and Impair Temporal Sensitivity

Discrimination of two neighboring empty durations that are marked by three successive sounds is improved when the presentation of the first (standard, S) duration is repeated before that of the second (comparison, C), as SSSSC. This improvement in sensitivity, called the multiple-look effect, has been explained by a statistical model regarding variability. This model assumes that the perceived duration of the standard is averaged across observations (within a trial within an individual). The increasing of the number of observations thus reduces the standard error of the mean perceived duration. Alternatively, the multiple-look effect is attributed to the listener’s prediction based on regular rhythm. Listeners perceive regular rhythm during the repetition of the standard, predict the timing of subsequent sounds, and detect a sound that is displaced from the predicted timing. These models were tested in the present experiment in which the main factor was a temporal separation between the standard and the comparison; i.e., these durations were adjacent to each other as SSSSC or separated by a temporal blank as SSSS_C. The results differed between stimulus structures. First, the multiple-look effect was replicated in the SSSSC condition (yielding a higher performance than SC), but disappeared in SSSS_C (having no difference with S_C). Second, no multiple-look effect occurred in CSSSS (no difference with CS), and moreover, an impairment effect was observed in C_SSSS (a lower performance than C_S). Finally, discrimination was improved in SSSS_CCCC compared with SSSSCCCC, the effect being kept even when sounds were aligned at irregular intervals. These findings are not consistent with those expected from the statistical model because the temporal separation should have produced no effects if the number of standards had been a sole parameter determining the multiple-look effect. The prediction-based model can explain the first finding; inserting a blank between the standard and the comparison violates the listener’s prediction based on regular rhythm, thus reducing the multiple-look effect. However, it did not expect the other findings and required revisions. Notably, the second finding indicates that the formation of regular rhythm can impair temporal discrimination. In other words, an inversed multiple-look effect occurs.

The Beat to Read: A Cross-Lingual Link between Rhythmic Regularity Perception and Reading Skill

This work assesses one specific aspect of the relationship between auditory rhythm cognition and language skill: regularity perception. In a group of 26 adult participants, native speakers of 11 different native languages, we demonstrate a strong and significant correlation between the ability to detect a "roughly" regular beat and rapid automatized naming (RAN) as a measure of language skill (Spearman's rho, -0.47, p < 0.01). There was no such robust relationship for the "mirror image" task of irregularity detection, i.e., the ability to detect ongoing small deviations from a regular beat. The correlation between RAN and regularity detection remained significant after partialling out performance on the irregularity detection task (rho, -0.41, p, 0.022), non-verbal IQ (rho, -0.37, p < 0.05), or musical expertise (rho, -0.31, p < 0.05). Whilst being consistent with the "shared resources model" in terms of rhythm as a common basis of language and music, evolutionarily as well as in individual development, the results also document how two related rhythm processing abilities relate differently to language skill. Specifically, the results support a universal relationship between rhythmic regularity detection and reading skill that is robust to accounting for differences in fluid intelligence and musical expertise, and transcends language-specific differences in speech rhythm.

The Structure of Sensory Events and the Accuracy of Time Judgments

We investigated how does the structure of empty time intervals influence temporal processing. In experiment 1, the intervals to be discriminated were the silent durations marked by two sensory signals, both lasting 10 or 500 ms; these signals were two identical flashes (intramodal: VV), or one visual flash (V) followed by an auditory tone (A) (intermodal: VA). For the range of duration under investigation (standards = 0.2, 0.6, 1, or 1.4 s), the results indicated that both the marker length and sensory mode influenced discrimination, but no interaction between these variables or between one of these variables and standard duration was significant. In experiment 2, we compared, for each of four marker-type conditions (VV, AA, VA, AV; and standard = 1 s), intervals marked by two 10 ms signals with intervals marked by unequal signal length (markers 1 and 2 lasting 10 and 500 ms, or 500 and 10 ms). As in experiment 1, the results revealed significant marker-mode and marker-length effects, but no significant interaction between these variables. Experiment 3 showed that, for the same conditions as in experiment 2, perceived duration is not influenced by marker length and that the variability of interval reproductions does not depend on the perceived duration of intervals. The results are discussed in the light of a single-clock hypothesis: marker-length and marker-mode effects are presented as being non-temporal sources of variability associated mainly with sensory and memory processes.

Sounds Modulate the Perceived Duration of Visual Stimuli via Crossmodal Integration

Previous studies have shown that the perceived duration of visual stimuli can be strongly distorted by auditory stimuli presented simultaneously. In this study, we examine whether sounds presented separately from target visual stimuli alter the perceived duration of the target’s presentation. The participants’ task was to classify the duration of the target visual stimuli as perceived by them into four categories. Our results demonstrate that a sound presented before and after a visual target increases or decreases the perceived visual duration depending on the inter-stimulus interval between the sounds and the visual stimulus. In addition, three tones presented before and after a visual target did not increase or decrease the perceived visual duration. This indicates that auditory perceptual grouping prevents intermodal perceptual grouping, and eliminates crossmodal effects. These findings suggest that the auditory–visual integration, rather than a high arousal state caused by the presentation of the preceding sound, can induce distortions of perceived visual duration, and that inter- and intramodal perceptual grouping plays an important role in crossmodal time perception. These findings are discussed with reference to the Scalar Expectancy Theory.

Perceived duration decreases with increasing eccentricity

Previous studies examining the influence of stimulus location on temporal perception yield inhomogeneous and contradicting results. Therefore, the aim of the present study is to soundly examine the effect of stimulus eccentricity. In a series of five experiments, subjects compared the duration of foveal disks to disks presented at different retinal eccentricities on the horizontal meridian. The results show that the perceived duration of a visual stimulus declines with increasing eccentricity. The effect was replicated with various stimulus orders (Experiments 1-3), as well as with cortically magnified stimuli (Experiments 4-5), ruling out that the effect was merely caused by different cortical representation sizes. The apparent decreasing duration of stimuli with increasing eccentricity is discussed with respect to current models of time perception, the possible influence of visual attention and respective underlying physiological characteristics of the visual system.

About the (non)scalar property for time perception

Approaching sensation scientifically is relatively straightforward. There are physical attributes for stimulating the central nervous system, and there are specific receptors for each sense for translating the physical signals into codes that brain will recognize. When studying time though, it is far from obvious that there are any specific receptors or specific stimuli. Consequently, it becomes important to determine whether internal time obeys some laws or principles usually reported when other senses are studied. In addition to reviewing some classical methods for studying time perception, the present chapter focusses on one of these laws, Weber law, also referred to as the scalar property in the field of time perception. Therefore, the question addressed here is the following: does variability increase linearly as a function of the magnitude of the duration under investigation? The main empirical facts relative to this question are reviewed, along with a report of the theoretical impact of these facts on the hypotheses about the nature of the internal mechanisms responsible for estimating time.

Introduction to the neurobiology of interval timing

Time is a fundamental variable that organisms must quantify in order to survive. In humans, for example, the gradual development of the sense of duration and rhythm is an essential skill in many facets of social behavior such as speaking, dancing to-, listening to- or playing music, performing a wide variety of sports, and driving a car (Merchant H, Harrington DL, Meck WH. Annu Rev Neurosci. 36:313-36, 2013). During the last 10 years there has been a rapid growth of research on the neural underpinnings of timing in the subsecond and suprasecond scales, using a variety of methodological approaches in the human being, as well as in varied animal and theoretical models. In this introductory chapter we attempt to give a conceptual framework that defines time processing as a family of different phenomena. The brain circuits and neural underpinnings of temporal quantification seem to largely depend on its time scale and the sensorimotor nature of specific behaviors. Therefore, we describe the main time scales and their associated behaviors and show how the perception and execution of timing events in the subsecond and second scales may depend on similar or different neural mechanisms.

What is the best and easiest method of preventing counting in different temporal tasks?

The aim of the present study was to determine the best and easiest method of suppressing spontaneous counting in a temporal judgment task. Three classic methods used to avoid counting--instructions not to count, articulatory suppression, and administration of an interference task--were tested in temporal generalization, bisection, and reproduction tasks with two duration ranges (1-4 and 2-8 s). All the three no-counting conditions prevented participants from counting, counting leading to estimates that were more accurate and less variable and to violations of the fundamental scalar property of timing. With regard to the differences between the no-counting conditions, the interference task distorted time perception more strongly and increased variability in temporal estimates to a greater extent than did articulatory suppression, as well as the no-counting instructions condition. In addition, articulatory suppression produced more noise in behavioral outcome than did the no-counting instruction condition. In sum, although all methods have disadvantages, the instructions not to count actually constitute the simplest and more efficient method of preventing counting in timing tasks. However, further studies must now concentrate on the role of explicit instructions in our experience of perception.

Temporal predictions based on a gradual change in tempo

Previous studies investigating sensitivity to step changes in tempo and prediction of tone onset time have generally utilized isochronous sequences. This study investigates subjects' ability to detect deviations from a gradual change in the tempo of a tone sequence (experiment 1) and their judgment of the perceptually optimal timing of this tone (experiment 2). In experiment 1, inter-onset-intervals within pairs of eight-tone sequences followed a geometric progression to create a gradual tempo change. In one sequence, the final tone was presented either earlier or later than specified by the progression. Subjects performed well at detecting deviations that exaggerated the tempo progression but poorly when it was counteracted. Experiment 2 used similar pairs except that the final tone was always presented earlier in one sequence than the other. Final interval length was adaptively adjusted to subjects' judgments; it was adjudged in best agreement with the progression when its length was roughly half way between the mathematically correct value and the length of the penultimate interval. The data support "multiple-look" and entrainment models of tempo sensitivity and suggest that temporal prediction is based less on the tempo contour of a whole sequence than on the duration of the preceding interval.

Multiple-look effects on temporal discrimination within sound sequences

The multiple-look notion holds that the difference limen (DL) decreases with multiple observations. We investigated this notion for temporal discrimination in isochronous sound sequences. In Experiment 1, we established a multiple-look effect when sequences comprised nine standard time intervals (S) followed by an increasing number of comparison time intervals (C), but no multiple-look effect when one trailing C interval was preceded by an increasing number of S intervals. In Experiment 2, we extended the design. There were four sequential conditions: (a) 9 leading S intervals followed by 1, 2, …, or 9 C-intervals; (b) 9 leading C intervals followed by 1, 2, …, or 9 S intervals; (c) 9 trailing C-intervals preceded by 1, 2, …, or 9 S-intervals; and (d) 9 trailing S-intervals preceded by 1, 2, …, or 9 C-intervals. Both the interval accretions before and after the tempo change caused multiple-look effects, irrespective of the time order of S and C. Complete deconfounding of the number of intervals before and after the tempo change was accomplished in Experiment 3. The multiple-look effect of interval accretion before the tempo change was twice as big as that after the tempo change. The diminishing returns relation between the DL and interval accretion could be described well by a reciprocal function.

Behavioral and in vivo electrophysiological evidence for presymptomatic alteration of prefrontostriatal processing in the transgenic rat model for huntington disease

Cognitive decline precedes motor symptoms in Huntington disease (HD). A transgenic rat model for HD carrying only 51 CAG repeats recapitulates the late-onset HD phenotype. Here, we assessed prefrontostriatal function in this model through both behavioral and electrophysiological assays. Behavioral examination consisted in a temporal bisection task within a supra-second range (2 vs.8 s), which is thought to involve prefrontostriatal networks. In two independent experiments, the behavioral analysis revealed poorer temporal sensitivity as early as 4 months of age, well before detection of overt motor deficits. At a later symptomatic age, animals were impaired in their temporal discriminative behavior. In vivo recording of field potentials in the dorsomedial striatum evoked by stimulation of the prelimbic cortex were studied in 4- to 5-month-old rats. Input/output curves, paired-pulse function, and plasticity induced by theta-burst stimulation (TBS) were assessed. Results showed an altered plasticity, with higher paired-pulse facilitation, enhanced short-term depression, as well as stronger long-term potentiation after TBS in homozygous transgenic rats. Results from the heterozygous animals mostly fell between wild-type and homozygous transgenic rats. Our results suggest that normal plasticity in prefrontostriatal circuits may be necessary for reliable and precise timing behavior. Furthermore, the present study provides the first behavioral and electrophysiological evidence of a presymptomatic alteration of prefrontostriatal processing in an animal model for Huntington disease and suggests that supra-second timing may be the earliest cognitive dysfunction in HD.

Role of olivocerebellar system in timing without awareness

The timing of events can be implicit or without awareness yet critical for task performance. However, the neural correlates of implicit timing are unknown. One system that has long been implicated in event timing is the olivocerebellar system, which originates exclusively from the inferior olive. By using event-related functional MRI in human subjects and a specially designed behavioral task, we examined the effect of the subjects' awareness of changes in stimulus timing on the olivocerebellar system response. Subjects were scanned while observing changes in stimulus timing that were presented near each subject's detection threshold such that subjects were aware of such changes in only approximately half the trials. The inferior olive and multiple areas within the cerebellar cortex showed a robust response to time changes regardless of whether the subjects were aware of these changes. Our findings provide support to the proposed role of the olivocerebellar system in encoding temporal information and further suggest that this system can operate independently of awareness and mediate implicit timing in a multitude of perceptual and motor operations, including classical conditioning and implicit learning.

Implicit, predictive timing draws upon the same scalar representation of time as explicit timing

It is not yet known whether the scalar properties of explicit timing are also displayed by more implicit, predictive forms of timing. We investigated whether performance in both explicit and predictive timing tasks conformed to the two psychophysical properties of scalar timing: the Psychophysical law and Weber's law. Our explicit temporal generalization task required overt estimation of the duration of an empty interval bounded by visual markers, whereas our temporal expectancy task presented visual stimuli at temporally predictable intervals, which facilitated motor preparation thus speeding target detection. The Psychophysical Law and Weber's Law were modeled, respectively, by (1) the functional dependence between mean subjective time and real time (2) the linearity of the relationship between timing variability and duration. Results showed that performance for predictive, as well as explicit, timing conformed to both psychophysical properties of interval timing. Both tasks showed the same linear relationship between subjective and real time, demonstrating that the same representational mechanism is engaged whether it is transferred into an overt estimate of duration or used to optimise sensorimotor behavior. Moreover, variability increased with increasing duration during both tasks, consistent with a scalar representation of time in both predictive and explicit timing. However, timing variability was greater during predictive timing, at least for durations greater than 200 msec, and ascribable to temporal, rather than non-temporal, mechanisms engaged by the task. These results suggest that although the same internal representation of time was used in both tasks, its external manifestation varied as a function of temporal task goals.

FMRI investigation of cross-modal interactions in beat perception: audition primes vision, but not vice versa

How we measure time and integrate temporal cues from different sensory modalities are fundamental questions in neuroscience. Sensitivity to a "beat" (such as that routinely perceived in music) differs substantially between auditory and visual modalities. Here we examined beat sensitivity in each modality, and examined cross-modal influences, using functional magnetic resonance imaging (fMRI) to characterize brain activity during perception of auditory and visual rhythms. In separate fMRI sessions, participants listened to auditory sequences or watched visual sequences. The order of auditory and visual sequence presentation was counterbalanced so that cross-modal order effects could be investigated. Participants judged whether sequences were speeding up or slowing down, and the pattern of tempo judgments was used to derive a measure of sensitivity to an implied beat. As expected, participants were less sensitive to an implied beat in visual sequences than in auditory sequences. However, visual sequences produced a stronger sense of beat when preceded by auditory sequences with identical temporal structure. Moreover, increases in brain activity were observed in the bilateral putamen for visual sequences preceded by auditory sequences when compared to visual sequences without prior auditory exposure. No such order-dependent differences (behavioral or neural) were found for the auditory sequences. The results provide further evidence for the role of the basal ganglia in internal generation of the beat and suggest that an internal auditory rhythm representation may be activated during visual rhythm perception.

Duration discrimination in crossmodal sequences

Four duration-discrimination experiments were carried out to compare crossmodal and unimodal timing conditions. For all experiments, participants were presented with two sequences, each consisting of 1 or 4 time intervals (marked by 2 or 5 signals), and asked to indicate whether the interval(s) of the second sequence was (were) shorter or longer than the interval(s) of the first. Markers in the first and second sequences were, respectively, tones and flashes (experiment 1), flashes and tones (experiment 2), both flashes (experiment 3), and both tones (experiment 4). In all modality conditions, except when using only tones (experiment 4), increasing the number of repetitions of the variable interval reduced duration-discrimination thresholds, independently of whether the fixed interval was presented first or second within the sequence pair. Moreover, judgments about sequence timing were best for tones-tones sequence pairs, worst for flashes-flashes sequence pairs, and intermediate for crossmodal (flashes-tones or tones-flashes) sequences. Finally, presenting a fixed interval in the first sequence resulted in better discrimination than presenting a variable interval in the first sequence. Implications for theories of timing are discussed.

Spatial effects on temporal categorisation

We examined the influence of spatial factors in temporal processing. Participants categorised as short or long empty intervals marked by two brief flashes delivered from locations differing in height and depth (experiment 1), or from two of three locations on a vertical plane (experiment 2). The perceived duration of intervals, as determined by the point of subjective equality, was affected by the height and depth of the signals (experiment 1). Experiment 2 showed that the point of fixation plays a critical role in perceived duration. The duration of an interval located in the upper visual field is perceived as longer when participants fixate the higher visual source and shorter when the fixation point is set in the middle; this latter result also generally applies when the fixation point is in the lower source. Finally, for the sensitivity level, there was a significant segment (upper versus lower) x direction (descending versus ascending) interaction in experiment 1; a similar interaction effect varied according to the fixation point in experiment 2. In experiment 2, the Weber fractions were around 0.22. Most results can be explained in terms of the need to shift attention from one visual source--for marking time intervals--to another.

The context of temporal processing is represented in the multidimensional relationships between timing tasks

In the present study we determined the performance interrelations of ten different tasks that involved the processing of temporal intervals in the subsecond range, using multidimensional analyses. Twenty human subjects executed the following explicit timing tasks: interval categorization and discrimination (perceptual tasks), and single and multiple interval tapping (production tasks). In addition, the subjects performed a continuous circle-drawing task that has been considered an implicit timing paradigm, since time is an emergent property of the produced spatial trajectory. All tasks could be also classified as single or multiple interval paradigms. Auditory or visual markers were used to define the intervals. Performance variability, a measure that reflects the temporal and non-temporal processes for each task, was used to construct a dissimilarity matrix that quantifies the distances between pairs of tasks. Hierarchical clustering and multidimensional scaling were carried out on the dissimilarity matrix, and the results showed a prominent segregation of explicit and implicit timing tasks, and a clear grouping between single and multiple interval paradigms. In contrast, other variables such as the marker modality were not as crucial to explain the performance between tasks. Thus, using this methodology we revealed a probable functional arrangement of neural systems engaged during different timing behaviors.

Stimulus complexity and prospective timing: clues for a parallel process model of time perception

Whereas many studies have considered the role of attention in prospective timing, fewer have established relations between movement complexity and prospective timing. The present study aims at assessing to what extent motion complexity interferes with prospective timing and at delineating a neuropsychophysical plausible model. We have thus designed a visual paradigm presenting stimuli in sequential pairs (reference comparison interval). Stimuli are motionless or moving according to different complexities, and stimulus complexities are intermixed within each pair. To prevent a possible attention-sharing effect, no concurrent task was required. Our study suggests that movement complexity is a key component of duration perception, and that the relative judgement of durations depends on spatio-temporal features of stimuli. In particular, it shows that movement complexity can bias subjects' perception and performance, and that subjects detect that comparison intervals are longer than reference before their end. In the discussion, we advocate that the classical internal clock model cannot easily account for our results. Consequently, we propose a model for time perception, based on a parallel processing between comparison interval perception and the reconstruction of the reference duration.

Effects of visual flicker on subjective time in a temporal bisection task

This experiment investigated the effects of visual flicker on subjective time in humans using a temporal bisection task. A 200-800 ms duration range and 400-1600 ms duration range were presented. Each duration range was presented separately in three different conditions: (1) filled stimuli were presented in both the training and the testing phases, (2) flickering stimuli were presented in the training phase and filled stimuli were presented in the testing phase, and (3) filled stimuli were presented in the training phase and flickering stimuli were presented in the testing phase. Psychophysical functions displacements and bisection point values suggested that flicker increased the speed of the clock; however the direction of the displacement and bisection point changes depended on the phase of the task in which the flicker was presented. This result agrees with the specific storage in either working or reference memory components of Scalar Expectancy Theory of the increased number of pulses from the clock. Weber fractions and difference limens suggested that flicker did not affect subjects' temporal sensitivity.

Neural mechanisms of the time-order error: an MEG study

The time-order error (TOE) refers to the influence of presentation order on performance accuracy in a discrimination task. Despite it being a well-documented perceptual bias, the underlying mechanisms have not been studied. In this study, observers were trained on a two-interval forced-choice procedure. The stimuli presented for discrimination were a standard, consisting of four tones presented at a 5-Hz rate, and targets, consisting of various rates higher than 5 Hz. Psychometric functions were measured for discrimination of the trained standard and targets, a novel standard of 13 Hz with higher target rates; and the trained 5 Hz standard with novel targets with rates below 5 Hz. Discrimination did not improve with training; in fact, accuracy declined when standard was presented in the first interval during the session, resulting in a TOE. The TOE was specific to the 5-Hz standard generalizing to the novel targets slower than 5 Hz, but not to the 13-Hz STANDARD. Analysis of the event-related magnetic field responses (ERFs) revealed a waveform to the whole stimulus, rather than to each tone in the train. Although ERFs in the second interval were attenuated independent of stimulus type, the M300 component in the second interval was attenuated only when the standard was first, but remained of equivalent magnitude when the standard was second. This was observed only in the two 5-Hz conditions. Combined, these results suggest that the TOE reflects the emergence of an internal representation of the standard, and that the M300 is potentially a neural correlate of plasticity.

Tempo sensitivity in isochronous tone sequences: The multiple-look model revisited

Factors affecting tempo sensitivity in isochronous tone sequences were investigated in two experiments. Participants listened to tones in sequence conditions in which the number of time intervals in isochronous standard and comparison sequences was varied, and they were asked to judge the tempo of the comparison relative to the standard. When the duration of the standard interval was held constant, tempo sensitivity was affected by the number of comparison intervals, but not by the number of standard intervals. In contrast, when the duration of the standard interval was varied randomly from trial to trial, tempo sensitivity was affected by the number of intervals in both sequences. The present findings are discussed in the context of a generalized multiple-look model that posits independent contributions of both sequences to tempo sensitivity. Quantitative model fits suggest that the relative contribution of the number of the standard intervals to tempo thresholds depends on (1) the availability of a stable long-term referent for the standard tempo and (2) a priori knowledge about the number of standard intervals.

About hemispheric differences in the processing of temporal intervals

The purpose of the present study was to identify differences between cerebral hemispheres for processing temporal intervals ranging from .9 to 1.4 s. The intervals to be judged were marked by series of brief visual signals located in the left or the right visual field. Series of three (two standards and one comparison) or five intervals (four standards and one comparison), marked by sequences of 4 or 6 signals, were compared. While discrimination, as estimated by d', was significantly better in the 4-standard than in the 2-standard condition when stimuli were presented in the left visual field (LVF), this number-of-standard effect on discrimination varied with the difficulty levels when the signals were presented in the LVF. Moreover, the discrimination levels were constant for the different base durations with stimuli presented in the LVF, but not with stimuli presented in the right visual field. This article discusses the implication of these findings for the study of hemispheric dominance for temporal processing and for a single-clock hypothesis.

Perceived duration of expected and unexpected stimuli

Three experiments assessed whether perceived stimulus duration depends on whether participants process an expected or an unexpected visual stimulus. Participants compared the duration of a constant standard stimulus with a variable comparison stimulus. Changes in expectancy were induced by presenting one type of comparison more frequently than another type. Experiment 1 used standard durations of 100 and 400 ms, and Experiments 2 and 3 durations of 400 and 800 ms. Stimulus frequency did not affect perceived duration in Experiment 1. In Experiments 2 and 3, however, frequent comparisons were perceived as shorter than infrequent ones, and discrimination performance was better for infrequent comparisons. Overall, this study supports the notion that infrequent stimuli increase the speed of an internal pacemaker.

A computer program for Spearman-Kärber and probit analysis of psychometric function data

PMETRIC is a computer program for the analysis of observed psychometric functions. It can estimate the parameters of these functions, using either probit analysis (a parametric technique) or the Spearman-Kärber method (a nonparametric one). For probit analysis, either a maximum likelihood or a minimum chi 2 criterion may be used for parameter estimation. In addition, standard errors of parameter estimates can be estimated via bootstrapping. The program can be used to analyze data obtained from either yes-no or m-alternative forced-choice tasks. To facilitate the use of PMETRIC in simulation work, an associated program, PMETGEN, is provided for the generation of simulated psychometric function data. Use of PMETRIC is illustrated with data from a duration discrimination task.

Depression, attention, and time estimation

Depression is known to affect several cognitive functions, but little is known about the effect of this neuropsychological disorder on timing tasks. In the present experiment, 15 depressed and 20 non-depressed participants, classified on the basis of the Beck Depression Inventory, were tested on attentional and on temporal processing tasks. On the Continuous Performance Test, depressed participants made more omissions, but not more erroneous responses, than non-depressed participants. As well, discrimination of relatively long intervals (1120 vs 1280 ms) was poorer for the depressed group, which was not the case for discrimination of brief durations (80 vs 120 ms, and 450 vs 550 ms). Finally, there was a significant difference between groups regarding the variability of 1- or 10-s interval productions made with continuous series of finger taps. The attentional requirements of long-interval processing seems to be a critical factor in depression-induced deficits of temporal processing.

Modeling Effects of Rhythmic Context on Perceived Duration: A Comparison of Interval and Entrainment Approaches to Short-Interval Timing

Relative merits of interval and entrainment conceptions of the internal clock were assessed within a common theoretical framework by 4 time-judgment experiments. The timing of tone onsets marking the beginning and ending of standard and comparison time intervals relative to a context rhythm were manipulated: onsets were on time, early, or late relative to the implied rhythm, and 2 distinct accuracy patterns emerged. A quadratic ending profile indicated best performance when the standard ended on time and worst performance when it was early or late, whereas a flat beginning profile (Experiments 1-3) indicated uniform performance for the 3 expectancy conditions. Only in Experiment 4, in which deviations from expected onset times were large, did significant effects of beginning times appear in time-discrimination thresholds and points of subjective equality. Findings are discussed in the context of theoretical assumptions about clock resetting, the representation of time, and independence of successive time intervals.