Discrete confidence levels revealed by sequential decisions.

Humans can meaningfully express their confidence about uncertain events. Normatively, these beliefs should correspond to Bayesian probabilities. However, it is unclear whether the normative theory provides an accurate description of the human sense of confidence, partly because the self-report measures used in most studies hinder quantitative comparison with normative predictions. To measure confidence objectively, we developed a dual-decision task in which the correctness of a first decision determines the correct answer of a second decision, thus mimicking real-life situations in which confidence guides future choices. While participants were able to use confidence to improve performance, they fell short of the ideal Bayesian strategy. Instead, behaviour was better explained by a model with a few discrete confidence levels. These findings question the descriptive validity of normative accounts, and suggest that confidence judgments might be based on point estimates of the relevant variables, rather than on their full probability distributions.

Unifying Visual Space Across the Left and Right Hemifields.

Visual space is perceived as continuous and stable even though visual inputs from the left and right visual fields are initially processed separately within the two cortical hemispheres. In the research reported here, we examined whether the visual system utilizes a dynamic recalibration mechanism to integrate these representations and to maintain alignment across the visual fields. Subjects adapted to randomly oriented moving lines that straddled the vertical meridian; these lines were vertically offset between the left and right hemifields. Subsequent vernier alignment judgments revealed a negative aftereffect: An offset in the same direction as the adaptation was required to correct the perceived misalignment. This aftereffect was specific to adaptation to vertical, but not horizontal, misalignments and also occurred following adaptation to movie clips and patterns without coherent motion. Our results demonstrate that the visual system unifies the left and right halves of visual space by continuously recalibrating the alignment of elements across the visual fields.

Time constancy in human perception.

Estimated time contracts or dilates depending on many visual-stimulation attributes (size, speed, etc.). Here we show that when such attributes are jointly modulated so as to respect the rules of perspective, their effect on the perceived duration of moving objects depends on the presence of contextual information about viewing distance. We show that perceived duration contracts and dilates with changes in the retinal input associated with increasing distance from the observer only when the moving objects are presented in the absence of information about the viewing distance. When this information (in the form of linear perspective cues) is present, the time-contraction/dilation effect is eliminated and time constancy is preserved. This is the first demonstration of a perceptual time constancy, analogous to size constancy but in the time domain. It points to a normalization of time computation operated by the visual brain when stimulated within a quasi-ecological environment.

Agency alters perceptual decisions about action-outcomes.

Humans experience themselves as agents, capable of controlling their actions and the outcomes they generate (i.e., the sense of agency). Inferences of agency are not infallible. Research shows that we often attribute outcomes to our agency even though they are caused by another agent. Moreover, agents report the sensory events they generate to be less intense compared to the events that are generated externally. These effects have been assessed using highly suprathreshold stimuli and subjective measurements. Consequently, it remains unclear whether experiencing oneself as an agent lead to a decision criterion change and/or a sensitivity change. Here, we investigate this issue. Participants were told that their key presses generated an upward dot motion but that on 30 % of the trials the computer would take over and display a downward motion. The upward/downward dot motion was presented at participant's discrimination threshold. Participants were asked to indicate whether they (upward motion) or the computer (downward motion) generated the motion. This group of participants was compared with a 'no-agency' group who performed the same task except that subjects did not execute any actions to generate the dot motion. We observed that the agency group reported seeing more frequently the motion they expected to generate (i.e., upward motion) than the no-agency group. This suggests that agency distorts our experience of (allegedly) caused events by altering perceptual decision processes, so that, in ambiguous contexts, externally generated events are experienced as the outcomes of one's actions.

Temporal frequency of events rather than speed dilates perceived duration of moving objects.

In everyday life moving objects often follow irregular or repetitive trajectories for which distinctive events are potentially noticeable. It is known that the perceived duration of moving objects is distorted, but whether the distortion is due to the temporal frequency of the events or to the speed of the objects remains unclear. Disentangling the contribution of these factors to perceived duration distortions is ecologically relevant: if perceived duration were dependent on speed, it should contract with the distance from the observer to the moving objects. Here, we asked observers to estimate the perceived duration of an object rotating at different speeds and radii and found that perceived duration dilated with temporal frequency of rotations, rather than speed (or perceived speed, which we also measured). We also found that the dilation was larger for two than for one object, but the increase was not large enough to make perceived duration independent of the number of objects when expressed as a function of the local frequency (the number of times an object crossed a given location per time unit). These results suggest that perceived duration of natural stimuli containing distinctive events doesn't depend on the distance of the events to the observer.

Time dilates more with apparent than with physical speed.

The perceived duration of a moving stimulus correlates positively with its speed. It is not known whether such duration dilation depends on the physical or apparent speed. Here we show the latter to be true. The perceived duration of a shortly presented (500, 900, 1300 ms) Gabor patch whose carrier moved at 1°/s in a direction opposite to a background of random black dots rigidly moving at 3°/s appeared to last 20% longer and to drift 240% faster than the same Gabor carrier moving in the same direction as the random-dot background. Assessment of the perceived speed of each of the two configurations relative to a moving Gabor patch in the absence of the moving background allowed the comparison of the observed duration dilation with that obtained as a function of the corresponding physical speeds, which should have yielded a dilation of only 7%, i.e., three times less. In line with the proposal that perceived duration correlates with the strength of the neural response evoked by the stimuli to be timed, the present data can be accounted for by the increased responsiveness of antagonistic center-surround motion-receptive fields when stimulated with center-surround antagonist motions.

A Refresher of the Original Bloch's Law Paper (Bloch, July 1885).

In 1885, Adolphe-Moïse Bloch asked the following simple question "Is there a law describing the relationship between the duration of a light and its perceived intensity?" Based on a series of experiments using a Foucault regulator and a candle, Bloch concluded that "when the lighting duration varies from 0.00173 to 0.0518 seconds (…) the [visible] light is markedly in inverse proportion to its duration"-his famous law. As this law pertains to the more general and hotly debated question of accumulation of sensory information over time, it is timely to offer the public a full translation of Bloch's original paper (from French) and to present it within the context of contemporary research.

Summary statistics for size over space and time.

A number of studies have investigated how the visual system extracts the average feature-value of an ensemble of simultaneously or sequentially delivered stimuli. In this study we model these two processes within the unitary framework of linear systems theory. The specific feature value used in this investigation is size, which we define as the logarithm of a circle's diameter. Within each ensemble, sizes were drawn from a normal distribution. Average size discrimination was measured using ensembles of one and eight circles. These circles were presented simultaneously (display times: 13-427 ms), one at a time, or eight at a time (temporal-frequencies: 1.2-38 Hz). Thresholds for eight-item ensembles were lower than thresholds for one-item ensembles. Thresholds decreased by a factor of 1.3 for a 3,200% increase in display time, and decreased by the same factor for a 3,200% decrease in temporal frequency. Modeling and simulations show that the data are consistent with one readout of three to four items every 210 ms.

A unified comparison of stimulus-driven, endogenous mandatory and 'free choice' saccades.

It has been claimed that saccades arising from the three saccade triggering modes-stimulus-driven, endogenous mandatory and 'free choice'-are driven by distinct mechanisms. We tested this claim by instructing observers to saccade from a white or black fixation disc to a same polarity (white or black) disc flashed for 100 or 200 ms presented either alone (Exo), or together with an opposite (Endo) or same (EndoFC) polarity disc (blocked and mixed sessions). Target(s) and distractor were presented at three inter-stimulus intervals (ISIs) relative to the fixation offset (ISI: -200, 0, +200 ms) and were displayed at random locations within a 4°-to-6° eccentricity range. The statistical analysis showed a global saccade triggering mode effect on saccade reaction times (SRTs) with Endo and EndoFC SRTs longer by about 27 ms than Exo-triggered ones but no effect for the Endo-EndoFC comparison. SRTs depended on both ISI (the "gap-effect"), and target duration. Bimodal best fits of the SRT-distributions were found in 65% of cases with their count not different across the three triggering modes. Percentages of saccades in the 'fast' and 'slow' ranges of bimodal fits did not depend on the triggering modes either. Bimodality tests failed to assert a significant difference between these modes. An analysis of the timing of a putative inhibition by the distractor (Endo) or by the duplicated target (EndoFC) yielded no significant difference between Endo and EndoFC saccades but showed a significant shortening with ISI similar to the SRT shortening suggesting that the distractor-target mutual inhibition is itself inhibited by 'fixation' neurons. While other experimental paradigms may well sustain claims of distinct mechanisms subtending the three saccade triggering modes, as here defined reflexive and voluntary saccades appear to differ primarily in the effectiveness with which inhibitory processes slow down the initial fast rise of the saccade triggering signal.

Time in perspective.

Perceptions of time and space are subject to strong contextual effects. Like their physical counterparts, they appear to be bound together. The perceived spatial extent of a constant retinal extent increases with its perceived distance from the observer. The perceived duration of a moving object increases with its covered angular trajectory. It follows that the perceived duration of moving objects covering identical angular trajectories should also increase with distance. Using three-dimensionally rendered balls rolling for 600 ms, 900 ms, and 1,200 ms and covering 5.5°, 11°, and 22° trajectories in fronto-parallel planes of a linear-perspective scene, we showed that perceived duration dilates by up to 50% as the fronto-parallel plane of the rolling ball recedes from the observer. Such time dilation is mostly contributed to by the smaller size of the distant ball. As in a three-dimensional world, objects' sizes and their covered trajectories per time unit decrease with distance, and as the two factors lead to opposite perceived-duration effects, the results suggest a form of time constancy in a three-dimensional world.

Recognition criteria vary with fluctuating uncertainty.

In distinct experiments we examined memories for orientation and size. After viewing a randomly oriented Gabor patch (or a plain white disk of random size), observers were given unlimited time to reproduce as faithfully as possible the orientation (or size) of that standard stimulus with an adjustable Gabor patch (or disk). Then, with this match stimulus still in view, a recognition probe was presented. On half the trials, this probe was identical to the standard. We expected observers to classify the probe (a same/different task) on the basis of its difference from the match, which should have served as an explicit memory of the standard. Observers did better than that. Larger differences were classified as "same" when probe and standard were indeed identical. In some cases, recognition performance exceeded that of a simulated observer subject to the same matching errors, but forced to adopt the single most advantageous criterion difference between the probe and match. Recognition must have used information that was not or could not be exploited in the reproduction phase. One possible source for that information is observers' confidence in their reproduction (e.g., in their memory of the standard). Simulations confirm the enhancement of recognition performance when decision criteria are adjusted trial-by-trial, on the basis of the observer's estimated reproduction error.

Decision criteria in dual discrimination tasks estimated using external-noise methods.

According to classical signal detection theory (SDT), in simple detection or discrimination tasks, observers use a decision parameter based on their noisy internal response to set a boundary between "yes" and "no" responses. Experimental paradigms where performance is limited by internal noise cannot be used to provide an unambiguous measure of the decision criterion and its variability. Here, unidimensional external noise is used to estimate a criterion and its variability in stimulus space. Within this paradigm, the criterion is defined as the stimulus value separating the two response alternatives. This paradigm allows the assessment of interactions between criteria assigned to different targets in dual tasks. Previous studies suggested that observers' criteria interacted or even collapsed to one (hence, nonoptimal) criterion. An alternative interpretation of those results is that observers equated their false alarm (FA) rates. The external-noise method enables the confrontation of the two hypotheses. It is shown that the variability of observers' criterion in stimulus space is about 1.6 times their measured sensory threshold, suggesting that the presence of external noise increases decision uncertainty. Observers' stimulus criterion settings are close to SDT predictions in single tasks, but not in dual tasks where the two criteria tend to "attract" each other. Observers maintain distinct FA rates even when SDT predicts equal rates. Observers trained in psychophysics or provided with basic notions of SDT exemplified with the present experimental design manage to better separate their criteria in some conditions.

Ticks per thought or thoughts per tick? A selective review of time perception with hints on future research.

The last decade underwent a revival of interest in the perception of time and duration. The present short essay does not compete with the many other recent reviews and books on this topic. Instead, it is meant to emphasize the notion that humans (and most likely other animals) have at their disposal more than one time measuring device and to propose that they use these devices jointly to appraise the passage of time. One possible consequence of this conjecture is that the same physical duration can be judged differently depending on the reference 'clock' used in any such judgment. As this view has not yet been tested empirically, several experimental manipulations susceptible to directly test it are suggested. Before, are summarized a number of its latent precursors, namely the relativity of perceived duration, current trends in modeling time perception and its neural and pharmacological substrate, the experimental literature supporting the existence of multiple 'clocks' and a selected number of experimental manipulations known to induce time perception illusions which together with many others are putatively accountable in terms of alternative clock readings.

Location and identity memory of saccade targets.

While the memory of objects' identity and of their spatiotopic location may sustain transsaccadic spatial constancy, the memory of their retinotopic location may hamper it. Is it then true that saccades perturb retinotopic but not spatiotopic memory? We address this issue by assessing localization performances of the last and of the penultimate saccade target in a series of 2-6 saccades. Upon fixation, nine letter-pairs, eight black and one white, were displayed at 3° eccentricity around fixation within a 20° × 20° grey frame, and subjects were instructed to saccade to the white letter-pair; the cycle was then repeated. Identical conditions were run with the eyes maintaining fixation throughout the trial but with the grey frame moving so as to mimic its retinal displacement when the eyes moved. At the end of a trial, subjects reported the identity and/or the location of the target in either retinotopic (relative to the current fixation dot) or frame-based(1) (relative to the grey frame) coordinates. Saccades degraded target's retinotopic location memory but not its frame-based location or its identity memory. Results are compatible with the notion that spatiotopic representation takes over retinotopic representation during eye movements thereby contributing to the stability of the visual world as its retinal projection jumps on our retina from saccade to saccade.

On the perceptual/motor dissociation: a review of concepts, theory, experimental paradigms and data interpretations.

With its roots in Ungerleider and Mishkin's (1982) uncovering of two distinct - ventral and dorsal - anatomical pathways for the processing of visual information, and boosted by Goodale and Milner's (1992; Milner and Goodale, 1995) behavioral study of patients with lesions of either of these pathways, the perception-action dissociation became a standard reference in the sensorimotor literature. Here we present briefly the anatomical, neuropsychological and, more extensively, the psychophysical evidence favoring such dissociation and pit it against counteracting evidence as well as against potential methodological and conceptual pitfalls. We also discuss classes of models accounting for a number of 'dissociation' results and conclude that the most general and parsimonious one posits the existence of one single processing stream that accumulates information up to a decision criterion modulated by stimulation conditions, response mode (motor vs. verbal/perceptual), task constraints (speeded vs. free time responses) and the nature of the task (detection, discrimination, temporal order judgment, etc.). The reviewed evidence is not meant to refute or validate the hypothesis of a perceptual-motor dissociation. Rather, its main objective is to show that, beyond its self-evidence, such dissociation is difficult if not impossible to test.

Introspective duration estimation of reactive and proactive motor responses.

The metajudgment of motor responses refers to our ability to evaluate the accuracy of our own actions. Can humans metajudge the duration of their Reaction Times (RTs) to a light-flash and the accuracy of their reproduction of a reference time interval bounded by two light flashes (Anticipatory Response Time, ART)? A series of four distinct experiments shows that RT_Meta and ART_Metajudgments are possible but with accuracies about x2.4 and x3 poorer than the corresponding RT and ART ones. In order to reveal the origin of this drop in performance, we ask whether a visual feedback synchronous with subjects' key-presses could improve performance. We show that overall the presence of a visual feedback does not significantly improve metajudgment accuracy although such a trend is noticeable in ART_Meta. We then compare these performances with the passive perceptual estimation of the played back (Pb) RT and ART time intervals when bounded by two (RT_Pb) and three (ART_Pb) light flashes. We show that RT_Meta and RT_Pb accuracies are close to equal, but that ART_Meta is about x2 less accurate than ART_Pb which in turn is x1.5 less accurate than ART. The latter observation fails however to reach statistical significance hence not sustaining proposals that active time estimation is more reliable than passive one. The whole dataset is accounted for by a clock-type model where duration estimation performance is limited by four noise sources (visual, clock-count, motor and proprioceptive+efference copy) plus one proper to ART_Meta task. It is proposed that the latter reflects the impossibility for the time-counting system to use the same time origin more than once.

Adaptation and prolonged inhibition as a main cause of motion-induced blindness.

Motion-induced blindness (MIB) is one of the most enigmatic perceptual disappearance phenomena. Here we suggest that MIB may be caused by the combined effects of two distinct adaptation processes: one shared with two other non-MIB configurations and entailing a response-gain reduction, and a second, MIB-specific transient-to-sustained incremental inhibition causing a contrast-gain reduction. Response-gain reduction is evidenced by brightness-tracking experiment where the 1-minute brightness time course of an MIB target is compared to the time courses of the same target superimposed on a static mask (SM) and on no mask at all (absent mask; AM). MIB and SM yield about the same brightness time courses with a faster initial drop and reaching a lower plateau than AM. While the frequency of phenomenal suppressions and their duration are very much reduced under SM and AM conditions, they increase as for MIB within the first 5-15 s of inspection and level off thereafter. Contrast-gain reduction over time is evidenced in a detection experiment showing that MIB target thresholds are higher and increase more steeply with inspection time than AM or SM thresholds. The interplay between these noisy adaptation and prolonged inhibition processes may well account for MIB's specificity.

Comparison of perceptual and motor decisions via confidence judgments and saccade curvature.

This study investigated the effects on perceptual and motor decisions of low-contrast distractors, presented 5 degrees on the left and/or the right of the fixation point. Perceptual decisions were assessed with a yes/no (distractor) detection task. Motor decisions were assessed via these distractors' effects on the trajectory of an impending saccade to a distinct imperative stimulus, presented 10 degrees above fixation 50 ms after the distractor(s). Saccade curvature models postulate that distractors activate loci on a motor map that evoke reflexive saccades and that the distractor evoked activity is inhibited to prevent reflexive orienting to the cost of causing a saccade curvature away from the distractor. Depending on whether or not each of these processes depends on perceptual detection, one can predict the relationships between saccades' curvature and perceptual responses (classified as correct rejections, misses, false alarms, and hits). The results show that saccades curve away from distractors only when observers report them to be present. Furthermore, saccade deviation is correlated (on a trial-by-trial basis) with the inferred internal response associated with the perceptual report: the stronger the distractor-evoked perceptual response, the more saccades deviate away from the distractor. Also in contrast with a supersensitive motor system, perceptual sensitivity is systematically higher than the motor sensitivity derived from the distributions of the saccades' curvatures. Finally, when both distractors are present (and straight saccades are expected), the sign of saccades' curvature is correlated with observers' perceptual bias/criterion. Overall the results point to a strong perceptual-motor association.

Comparison of perceptual and motor latencies via anticipatory and reactive response times.

To compare the timing of perceptual and motor decisions, distinct tasks have been designed, all of which have yielded systematic differences between these two moments. These observations have been taken as evidence of a sensorimotor dissociation. Inasmuch as the distinction between perceptual and motor decision moments is conceptually warranted, this conclusion remains debatable, since the observed differences may reflect the dissimilarity between the stimulations/tasks used to assess them. Here, we minimize such dissimilarities by comparing response time (RT) and anticipatory RT (ART), an alternative technique with which to infer the relative perceptual decision moments. Observers pressed a key either in synchrony with the third of a sequence of three stimuli appearing at a constant pace (ART) or in response to the onset of this third stimulus presented at a random interval after the second (RT). Hence, the two stimulation sequences were virtually identical. Both the mean and the variance of RT were affected by stimulus intensity about 1.5 times more than were the mean and the variance of ART. Within the framework of two simple integration-to-bound models, these findings are compatible with the hypothesis that perceptual and motor decisions operate on the same internal signal but are based on distinct criteria, with the perceptual criterion lower than the motor one.