Measuring the perception and metacognition of time.

The ability of humans to identify and reproduce short time intervals (in the region of a second) may be affected by many factors ranging from the gender and personality of the individual observer, through the attentional state, to the precise spatiotemporal structure of the stimulus. The relative roles of these very different factors are a challenge to describe and define; several methodological approaches have been used to achieve this to varying degrees of success. Here we describe and model the results of a paradigm affording not only a first-order measurement of the perceived duration of an interval but also a second-order metacognitive judgement of perceived time. This approach, we argue, expands the form of the data generally collected in duration-judgements and allows more detailed comparison of psychophysical behavior to the underlying theory. We also describe a hierarchical Bayesian measurement model that performs a quantitative analysis of the trial-by-trial data calculating the variability of the temporal estimates and the metacognitive judgments allowing direct comparison between an actual and an ideal observer. We fit the model to data collected for judgements of 750 ms (bisecting 1500 ms) and 1500 ms (bisecting 3000 ms) intervals across three stimulus modalities (visual, audio, and audiovisual). This enhanced form of data on a given interval judgement and the ability to track its progression on a trial-by-trial basis offers a way of looking at the different roles that subject-based, task-based and stimulus-based factors have on the perception of time.

Different rules for binocular combination of luminance flicker in cortical and subcortical pathways.

How does the human brain combine information across the eyes? It has been known for many years that cortical normalization mechanisms implement 'ocularity invariance': equalizing neural responses to spatial patterns presented either monocularly or binocularly. Here, we used a novel combination of electrophysiology, psychophysics, pupillometry, and computational modeling to ask whether this invariance also holds for flickering luminance stimuli with no spatial contrast. We find dramatic violations of ocularity invariance for these stimuli, both in the cortex and also in the subcortical pathways that govern pupil diameter. Specifically, we find substantial binocular facilitation in both pathways with the effect being strongest in the cortex. Near-linear binocular additivity (instead of ocularity invariance) was also found using a perceptual luminance matching task. Ocularity invariance is, therefore, not a ubiquitous feature of visual processing, and the brain appears to repurpose a generic normalization algorithm for different visual functions by adjusting the amount of interocular suppression.

A purely visual adaptation to motion can differentiate between perceptual timing and interval timing.

It is unclear whether our brain extracts and processes time information using a single-centralized mechanism or through a network of distributed mechanisms, which are specific for modality and time range. Visual adaptation has previously been used to investigate the mechanisms underlying time perception for millisecond intervals. Here, we investigated whether a well-known duration after-effect induced by motion adaptation in the sub-second range (referred to as 'perceptual timing') also occurs in the supra-second range (called 'interval timing'), which is more accessible to cognitive control. Participants judged the relative duration of two intervals after spatially localized adaptation to drifting motion. Adaptation substantially compressed the apparent duration of a 600 ms stimulus in the adapted location, whereas it had a much weaker effect on a 1200 ms interval. Discrimination thresholds after adaptation improved slightly relative to baseline, implying that the duration effect cannot be ascribed to changes in attention or to noisier estimates. A novel computational model of duration perception can explain both these results and the bidirectional shifts of perceived duration after adaptation reported in other studies. We suggest that we can use adaptation to visual motion as a tool to investigate the mechanisms underlying time perception at different time scales.

A metacognitive approach to the study of motion-induced duration biases reveals inter-individual differences in forming confidence judgments.

Our ability to estimate the duration of subsecond visual events is prone to distortions, which depend on both sensory and decisional factors. To disambiguate between these two influences, we can look at the alignment between discrimination estimates of duration at the point of subjective equality and confidence estimates when the confidence about decisions is minimal, because observers should be maximally uncertain when two stimuli are perceptually the same. Here, we used this approach to investigate the relationship between the speed of a visual stimulus and its perceived duration. Participants were required to compare two intervals, report which had the longer duration, and then rate their confidence in that judgment. One of the intervals contained a stimulus drifting at a constant speed, whereas the stimulus embedded in the other interval could be stationary, linearly accelerating or decelerating, or drifting at the same speed. Discrimination estimates revealed duration compression for the stationary stimuli and, to a lesser degree, for the accelerating and decelerating stimuli. Confidence showed a similar pattern, but, overall, the confidence estimates were shifted more toward higher durations, pointing to a small contribution of decisional processes. A simple observer model, which assumes that both judgments are based on the same sensory information, captured well inter-individual differences in the criterion used to form a confidence judgment.

Individual differences in first- and second-order temporal judgment.

The ability of subjects to identify and reproduce brief temporal intervals is influenced by many factors whether they be stimulus-based, task-based or subject-based. The current study examines the role individual differences play in subsecond and suprasecond timing judgments, using the schizoptypy personality scale as a test-case approach for quantifying a broad range of individual differences. In two experiments, 129 (Experiment 1) and 141 (Experiment 2) subjects completed the O-LIFE personality questionnaire prior to performing a modified temporal-bisection task. In the bisection task, subjects responded to two identical instantiations of a luminance grating presented in a 4deg window, 4deg above fixation for 1.5 s (Experiment 1) or 3 s (Experiment 2). Subjects initiated presentation with a button-press, and released the button when they considered the stimulus to be half-way through (750/1500 ms). Subjects were then asked to indicate their 'most accurate estimate' of the two intervals. In this way we measure both performance on the task (a first-order measure) and the subjects' knowledge of their performance (a second-order measure). In Experiment 1 the effect of grating-drift and feedback on performance was also examined. Experiment 2 focused on the static/no-feedback condition. For the group data, Experiment 1 showed a significant effect of presentation order in the baseline condition (no feedback), which disappeared when feedback was provided. Moving the stimulus had no effect on perceived duration. Experiment 2 showed no effect of stimulus presentation order. This elimination of the subsecond order-effect was at the expense of accuracy, as the mid-point of the suprasecond interval was generally underestimated. Response precision increased as a proportion of total duration, reducing the variance below that predicted by Weber's law. This result is consistent with a breakdown of the scalar properties of time perception in the early suprasecond range. All subjects showed good insight into their own performance, though that insight did not necessarily correlate with the veridical bisection point. In terms of personality, we found evidence of significant differences in performance along the Unusual Experiences subscale, of most theoretical interest here, in the subsecond condition only. There was also significant correlation with Impulsive Nonconformity and Cognitive Disorganisation in the sub- and suprasecond conditions, respectively. Overall, these data support a partial dissociation of timing mechanisms at very short and slightly longer intervals. Further, these results suggest that perception is not the only critical mitigator of confidence in temporal experience, since individuals can effectively compensate for differences in perception at the level of metacognition in early suprasecond time. Though there are individual differences in performance, these are perhaps less than expected from previous reports and indicate an effective timing mechanism dealing with brief durations independent of the influence of significant personality trait differences.

Efeito da Astrocaryum aculeatum (Tucumã) na toxicidade da Doxorrubicina: modelo experimental in vivo / Effect of Astrocaryum aculeatum (tucumã) on doxorubicin toxicity: in vivo experimental model

Resumo Objetivo Obter o óleo do Astrocaryum aculeatum (A.a) e avaliar a genotoxidade/antigenotoxidade pelo teste do micronúcleo em células do sangue periférico. Métodos O óleo da A.a foi obtido por prensagem hidráulica. Os animais foram camundongos Swiss, machos e saudáveis com 6-7 semanas de idade, 6 por grupo. Teste genotóxico e antigenotóxico as concentrações foram de 500, 1.000 e 2.000 mg/kg por 0,5 mL (via oral), seguidas ou não de injeção intraperitoneal de doxorrubicina (0,3mL - 15 mg/kg por peso corporal), além do grupo negativo (água) e dimetilsufóxido (600 µL). As amostras de sangue periférico foram coletadas 24h e 48h após o tratamento. Resultados Houve redução estatisticamente significativa na frequência de micronúcleos em células policromáticas que variou de 34,72% à 38,19% para os tratamentos de 24h, e de 63,70 à 66,12% para os de 48h. Conclusão O óleo fixo do tucumã apresentou potencial antigenotóxico para as concentrações em tratamentos agudos.

Abstract Objective To obtain the oil of Astrocaryum aculeatum (A.a), and evaluate its genotoxicity/antigenotoxicity activities using the micronucleus test in peripheral blood cells. Methods The oil of Astrocaryum aculeatum was obtained by hydraulic pressing. The animals used were healthy Swiss male mice, at 6-7 weeks of age; there were six per group. The genotoxic and antigenotoxic activity of concentrations were 500, 1,000 and 2,000 mg/kg per 0.5 mL (oral), followed or not followed by intraperitoneal injection of doxorubicin (0.3 mL-15 mg/kg by body weight), in addition to a negative group (water) and dimethyl sulfoxide (600 μL). Peripheral blood samples were collected 24h and 48h after treatment. Results A statistically significant reduction was identified in the frequency of micronuclei in polychromatic cells ranging from 34.72% to 38.19% for 24-hour treatments, and from 63.70% to 66.12% for 48 hour. Conclusion The fixed oil of tucumã presented antigenotoxic potential for the concentrations used in acute treatments.

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.

Changes in apparent duration follow shifts in perceptual timing.

It is well established that the apparent duration of moving visual objects is greater at higher as compared to slower speeds. Here we report the effects of acceleration and deceleration on the perceived duration of a drifting grating with average speed kept constant (10°/s).For acceleration, increasing the speed range progressively reduced perceived duration. The magnitude of apparent duration compression was determined by speed rather than temporal frequency and was proportional to speed range (independent of standard duration) rather than acceleration. The perceived duration reduction was also proportional to the standard length. The effects of increases and decreases in speed were highly asymmetric. Reducing speed through the interval induced a moderate increase in perceived duration. These results could not be explained by changes in apparent onset or offset or differences in perceived average speed between intervals containing increasing speed and intervals containing decreasing speed. Paradoxically, for intervals combining increasing speed and decreasing speed, compression only occurred when increasing speed occurred in the second half of the interval. We show that this pattern of results in the duration domain was concomitant with changes in the reported direction of apparent motion of Gaussian blobs, embedded in intervals of increasing or decreasing speed, that could be predicted from adaptive changes in the temporal impulse response function. We detected similar changes after flicker adaptation, suggesting that the two effects might be linked through changes in the temporal tuning of visual filters.

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.

Duration judgments over multiple elements.

We investigated the limits of the number of events observers can simultaneously time. For single targets occurring in one of eight positions sensitivity to duration was improved for spatially pre-cued items as compared to post-cued items indicating that exogenous driven attention can improve duration discrimination. Sensitivity to duration for pre-cued items was also marginally better for single items as compared to eight items indicating that even after the allocation of focal attention, distractor items can interfere with the encoding of duration. For an eight item array discrimination was worse for post-cued locations as compared to pre-cued locations indicating both that attention can improve duration discrimination performance and that it was not possible to access a perfect memory trace of the duration of eight elements. The interference from the distractors in the pre-cued eight item array may reflect some mandatory averaging of target and distractor events. To further explore duration averaging we asked subjects to explicitly compare average durations of multiple item arrays against a single item standard duration. Duration discrimination thresholds were significantly lower for single elements as compared to multiple elements, showing that averaging, either automatically or intentionally, impairs duration discrimination. There was no set size effect. Performance was the same for averages of two and eight items, but performance with even an average of two items was worse than for one item. This was also true for sequential presentation indicating poor performance was not due to limits on the division of attention across items. Rather performance appears to be limited by an inability to remember or aggregate duration information from two or more items. Although it is possible to manipulate perceived duration locally, there appears to be no perceptual mechanisms for aggregating local durations across space.

Effects of Temporal Features and Order on the Apparent duration of a Visual Stimulus.

The apparent duration of a visual stimulus has been shown to be influenced by its speed. For low speeds, apparent duration increases linearly with stimulus speed. This effect has been ascribed to the number of changes that occur within a visual interval. Accordingly, a higher number of changes should produce an increase in apparent duration. In order to test this prediction, we asked subjects to compare the relative duration of a 10-Hz drifting comparison stimulus with a standard stimulus that contained a different number of changes in different conditions. The standard could be static, drifting at 10 Hz, or mixed (a combination of variable duration static and drifting intervals). In this last condition the number of changes was intermediate between the static and the continuously drifting stimulus. For all standard durations, the mixed stimulus looked significantly compressed (∼20% reduction) relative to the drifting stimulus. However, no difference emerged between the static (that contained no changes) and the mixed stimuli (which contained an intermediate number of changes). We also observed that when the standard was displayed first, it appeared compressed relative to when it was displayed second with a magnitude that depended on standard duration. These results are at odds with a model of time perception that simply reflects the number of temporal features within an interval in determining the perceived passing of time.

Duration expansion at low luminance levels.

Duration distortions have been shown to occur at the time of saccades and following high temporal frequency or contrast adaptation. Under all these conditions, changes in the temporal tuning of M neurons also occur, suggesting that there might be a link between the two phenomena. In order to explore this relationship further, we measured the apparent duration of visual stimuli in the dark, where the temporal impulse response has been reported to lengthen. We first measured a progressive shift and reduction of the occurrence of an apparent motion reversal as we decreased the luminance level, indicating a lengthening of the temporal impulse response. We then measured perceived duration at these luminance levels (0.75, 3, and 50 cd/m(2)) after matching for apparent contrast and temporal frequency. While perceived temporal frequency did not substantially differ across luminance levels, duration appeared expanded at the lowest luminance level relative to the highest by approximately 60 ms. Thus, we have shown that reduced luminance is associated with both a lengthening of the temporal impulse response and a duration expansion, linking the two and providing further evidence for a relationship between changes in the neuronal tuning in the early stages of the visual system and time perception.

Effect of the luminance signal on adaptation-based time compression.

Traditionally, time perception has been considered the product of a central, generic, cognitive mechanism. Recent evidence, however, has shown that high temporal frequency adaptation induces local reductions in the apparent duration of brief intervals suggesting a distributive system with modality-specific sensory components. Here, we examine the effect of the luminance signal on these adaptation-based temporal distortions. Our results show that the luminance signal is crucial to generate duration compression as the effect disappears at isoluminance and that low visibility and task difficulty at isoluminance cannot explain the discrepancy. We also demonstrate that the effects of adaptation on perceived duration are dissociable from those on apparent temporal frequency. These results provide further evidence for the involvement of the magnocellular system in the neural encoding and representation of visual time.

Retinotopic adaptation-based visual duration compression.

Eye movements present the visual system with the challenge of providing the experience of a stable world. This appears to require the location of objects to be mapped from retinal to head and body referenced coordinates. Following D. Burr, A. Tozzi, and M. C. Morrone (2007), here we address the issue of whether adaptation-based duration compression (A. Johnston, D. H. Arnold, & S. Nishida, 2006) takes place in a retinocentric or head-centric frame of reference. Duration compression may be associated with shifts in apparent temporal frequency. However, using an adaptation schedule that minimizes any effect of adaptation on apparent temporal frequency, we still find substantial apparent duration compression. Duration compression remains when the adaptor continuously translates in head-centered coordinates but is fixed on the retina, isolating retinal adaptation. Apparent duration was also measured after a change in gaze direction-a strategy which allows eye-centered and head-centered components of adaptation-induced duration compression to be distinguished. In two different paradigms, we found significant compression was elicited by retinotopic adaptation, with no significant change in apparent duration following spatiotopic adaptation. We also observed no interocular transfer of adaptation. These findings point to an early locus for the adaptation-based duration compression effect.

Contrast gain shapes visual time.

Traditionally time perception has been considered the product of a central, generic, cognitive mechanism. However, evidence is emerging for a distributive system with modality-specific sensory components (Morrone et al., 2005; Johnston et al., 2006). Here we show that fast contrast adaptation, which can be observed in the retina, induces a change in apparent duration. The perceived duration of a subsecond interval containing a 50% luminance contrast drifting pattern is compressed when it follows a high (90%) as compared to a low (10%) contrast interval. The duration effect cannot be attributed to changes in latency at onset relative to offset, can be dissociated from the effect of contrast context on apparent speed or apparent contrast per se and it occurs in a retinocentric frame of reference. The temporal compression is limited to high drift temporal frequencies (≥10 Hz) and is not observed for equiluminant chromatic stimuli. This pattern of results indicates a major role for the magnocellular pathway in the neural encoding and representation of visual time.

The spatial tuning of adaptation-based time compression.

Temporal processing is traditionally dissociated from spatial vision. Recent evidence, however, has shown that adaptation to high temporal frequency (D. Burr, A.Tozzi, & M. C. Morrone, 2007; A. Johnston, D. H. Arnold, & S. Nishida, 2006; A. Johnston et al., 2008) induces spatially specific reductions in the apparent duration of subsecond intervals containing medium frequency drift or flicker. Here we examine the spatial tuning of these temporal adaptation effects. Our results show that duration compression is tightly tuned to the spatial location of the adaptor and can be induced by very narrow adaptors. We also demonstrate that the effects of adaptation on perceived duration are dissociable from those on apparent temporal frequency, which suggests early but separate influences of temporal frequency adaptation on time and speed perception.

Visually-based temporal distortion in dyslexia.

In this study, we show that invisible flicker adaptation reduces the perceived duration of a subsequently viewed stimulus in control subjects, but not in dyslexics. Dyslexics, like controls, show apparent duration compression after 20Hz flicker and show normal shifts in apparent temporal frequency after adaptation. However a subgroup of the test group, scoring low on both a test of phonological skill (spoonerisms) and a test of literacy (NART), show an apparent temporal expansion after 5Hz flicker adaptation, a finding not previously seen in controls. Recent studies have linked genes conferring susceptibility to a cluster of language and sensory deficits to anomalous neural migration, providing a tentative biological basis for dyslexia. However it has proved difficult to establish a clear link between sensory deficits and impaired reading. The results presented here point to an abnormal adaptation response within the early precortical stages of the magnocellular pathway, occurring in tandem with a deficit in word-level cognitive processing, providing psychophysical evidence for anomalous cortico-thalamic circuits in dyslexia.

Fusion of visual and auditory stimuli during saccades: a Bayesian explanation for perisaccadic distortions.

Brief stimuli presented near the onset of saccades are grossly mislocalized in space. In this study, we investigated whether the Bayesian hypothesis of optimal sensory fusion could account for the mislocalization. We required subjects to localize visual, auditory, and audiovisual stimuli at the time of saccades (compared with an earlier presented target). During fixation, vision dominates and spatially "captures" the auditory stimulus (the ventriloquist effect). But for perisaccadic presentations, auditory localization becomes more important, so the mislocalized visual stimulus is seen closer to its veridical position. The precision of the bimodal localization (as measured by localization thresholds or just-noticeable difference) was better than either the visual or acoustic stimulus presented in isolation. Both the perceived position of the bimodal stimuli and the improved precision were well predicted by assuming statistically optimal Bayesian-like combination of visual and auditory signals. Furthermore, the time course of localization was well predicted by the Bayesian approach. We present a detailed model that simulates the time-course data, assuming that perceived position is given by the sum of retinal position and a sluggish noisy eye-position signal, obtained by integrating optimally the output of two populations of neural activity: one centered at the current point of gaze, the other centered at the future point of gaze.

Influence of saccadic adaptation on spatial localization: comparison of verbal and pointing reports.

Under conditions of short-term saccadic adaptation, stimuli presented long before saccadic onset are perceptually mislocalized in space. Here we study whether saccadic adaptation can also affect localization of objects by pointing. We measured localization performance during fixation and after normal saccades and adapted saccades, for a bar presented well before a saccadic eye movement, for both pointing and verbal localization, under open-loop conditions generated by a transient dark period about 300 ms after the presentation of the bar. During fixation and normal saccade, localization performance for verbal report was veridical, while for pointing there was an overestimation of the target eccentricity respect to gaze, in agreement with the idea of separate representations of space for action and perception. During saccadic adaptation, there was a significant shift of both pointing and verbal report localization in the direction of adaptation with similar spatial selectivity for both tasks. These results indicate that saccadic adaptation induces a similar re-calibration of the action map as well as of the perceptual map, suggesting a common site of operation in the transformation from eye-centered to gaze-centered coordinates.

Development of saccadic suppression in children.

We measured saccadic suppression in adolescent children and young adults using spatially curtailed low spatial frequency stimuli. For both groups, sensitivity for color-modulated stimuli was unchanged during saccades. Sensitivity for luminance-modulated stimuli was greatly reduced during saccades in both groups but far more for adolescents than for young adults. Adults' suppression was on average a factor of about 3, whereas that for the adolescent group was closer to a factor of 10. The specificity of the suppression to luminance-modulated stimuli excludes generic explanations such as task difficulty and attention. We suggest that the enhanced suppression in adolescents results from the immaturity of the ocular-motor system at that age.