Components of visual prior entry

Probabilistic model of onset detection explains paradoxes in human time perception

A very basic computational model is proposed to explain two puzzling findings in the time perception literature. First, spontaneous motor actions are preceded by up to 1-2 s of preparatory activity (Kornhuber and Deecke, 1965). Yet, subjects are only consciously aware of about a quarter of a second of motor preparation (Libet et al., 1983). Why are they not aware of the early part of preparation? Second, psychophysical findings (Spence et al., 2001) support the principle of attention prior entry (Titchener, 1908), which states that attended stimuli are perceived faster than unattended stimuli. However, electrophysiological studies reported no or little corresponding temporal difference between the neural signals for attended and unattended stimuli (McDonald et al., 2005; Vibell et al., 2007). We suggest that the key to understanding these puzzling findings is to think of onset detection in probabilistic terms. The two apparently paradoxical phenomena are naturally predicted by our signal detection theoretic model.

Evaluating comparative and equality judgments in contrast perception: attention alters appearance

Covert attention not only improves performance in many visual tasks but also modulates the appearance of several visual features. Studies on attention and appearance have assessed subjective appearance using a task contingent upon a comparative judgment (e.g., M. Carrasco, S. Ling, & S. Read, 2004). Recently, K. A. Schneider and M. Komlos (2008) questioned the validity of those results because they did not find a significant effect of attention on contrast appearance using an equality task. They claim that such equality judgments are bias-free whereas comparative judgments are bias-prone and propose an alternative interpretation of the previous findings based on a decision bias. However, to date there is no empirical support for the superiority of the equality procedure. Here, we compare biases and sensitivity to shifts in perceived contrast of both paradigms. We measured contrast appearance using both a comparative and an equality judgment. Observers judged the contrasts of two simultaneously presented stimuli, while either the contrast of one stimulus was physically incremented (Experiments 1 and 2) or exogenous attention was drawn to it (Experiments 3 and 4). We demonstrate several methodological limitations of the equality paradigm. Nevertheless, both paradigms capture shifts in PSE due to physical and perceived changes in contrast and show that attention enhances apparent contrast.

Perception of intersensory synchrony: a tutorial review

For most multisensory events, observers perceive synchrony among the various senses (vision, audition, touch), despite the naturally occurring lags in arrival and processing times of the different information streams. A substantial amount of research has examined how the brain accomplishes this. In the present article, we review several key issues about intersensory timing, and we identify four mechanisms of how intersensory lags might be dealt with: by ignoring lags up to some point (a wide window of temporal integration), by compensating for predictable variability, by adjusting the point of perceived synchrony on the longer term, and by shifting one stream directly toward the other.

Video game players show more precise multisensory temporal processing abilities

Recent research has demonstrated enhanced visual attention and visual perception in individuals with extensive experience playing action video games. These benefits manifest in several realms, but much remains unknown about the ways in which video game experience alters perception and cognition. In the present study, we examined whether video game players' benefits generalize beyond vision to multisensory processing by presenting auditory and visual stimuli within a short temporal window to video game players and non-video game players. Participants performed two discrimination tasks, both of which revealed benefits for video game players: In a simultaneity judgment task, video game players were better able to distinguish whether simple visual and auditory stimuli occurred at the same moment or slightly offset in time, and in a temporal-order judgment task, they revealed an enhanced ability to determine the temporal sequence of multisensory stimuli. These results suggest that people with extensive experience playing video games display benefits that extend beyond the visual modality to also impact multisensory processing.

Voluntary attention increases perceived spatial frequency

Voluntary covert attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenology has received little empirical investigation. Involuntary attention increases perceived spatial frequency (Gobell & Carrasco, 2005), but involuntary attention can differ from voluntary attention in its effects on performance in tasks mediated by spatial resolution (Yeshurun, Montagna, & Carrasco, 2008). Therefore, we ask whether voluntary attention affects the subjective appearance of spatial frequency--a fundamental dimension of visual perception underlying spatial resolution. We used a demanding rapid serial visual presentation task to direct voluntary attention and measured perceived spatial frequency at the attended and unattended locations. Attention increased the perceived spatial frequency of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task. In the control experiment, we ruled out response bias as an alternative account by using a lengthened interstimulus interval, which allows observers to disengage attention from the cued location. In contrast to the main experiment, the observers showed neither increased perceived spatial frequency nor improved orientation discrimination at the attended location. Thus, this study establishes that voluntary attention increases perceived spatial frequency. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.

Effect before cause: supramodal recalibration of sensorimotor timing

BACKGROUND

Our motor actions normally generate sensory events, but how do we know which events were self generated and which have external causes? Here we use temporal adaptation to investigate the processing stage and generality of our sensorimotor timing estimates.

METHODOLOGY/PRINCIPAL FINDINGS

Adaptation to artificially-induced delays between action and event can produce a startling percept--upon removal of the delay it feels as if the sensory event precedes its causative action. This temporal recalibration of action and event occurs in a quantitatively similar manner across the sensory modalities. Critically, it is robust to the replacement of one sense during the adaptation phase with another sense during the test judgment.

CONCLUSIONS/SIGNIFICANCE

Our findings suggest a high-level, supramodal recalibration mechanism. The effects are well described by a simple model which attempts to preserve the expected synchrony between action and event, but only when causality indicates it is reasonable to do so. We further demonstrate that this model successfully characterises related adaptation data from outside the sensorimotor domain.

Audio-tactile superiority over visuo-tactile and audio-visual combinations in the temporal resolution of synchrony perception

To see whether there is a difference in temporal resolution of synchrony perception between audio-visual (AV), visuo-tactile (VT), and audio-tactile (AT) combinations, we compared synchrony-asynchrony discrimination thresholds of human participants. Visual and auditory stimuli were, respectively, a luminance-modulated Gaussian blob and an amplitude-modulated white noise. Tactile stimuli were mechanical vibrations presented to the index finger. All the stimuli were temporally modulated by either single pulses or repetitive-pulse trains. The results show that the temporal resolution of synchrony perception was similar for AV and VT (e.g., approximately 4 Hz for repetitive-pulse stimuli), but significantly higher for AT approximately 10 Hz). Apart from having a higher temporal resolution, however, AT synchrony perception was similar to AV synchrony perception in that participants could select matching features through attention, and a change in the matching-feature attribute had little effect on temporal resolution. The AT superiority in temporal resolution was indicated not only by synchrony-asynchrony discrimination but also by simultaneity judgments. Temporal order judgments were less affected by modality combination than the other two tasks.

Perceived timing of vestibular stimulation relative to touch, light and sound

Different senses have different processing times. Here we measured the perceived timing of galvanic vestibular stimulation (GVS) relative to tactile, visual and auditory stimuli. Simple reaction times for perceived head movement (438 +/- 49 ms) were significantly longer than to touches (245 +/- 14 ms), lights (220 +/- 13 ms), or sounds (197 +/- 13 ms). Temporal order and simultaneity judgments both indicated that GVS had to occur about 160 ms before other stimuli to be perceived as simultaneous with them. This lead was significantly less than the relative timing predicted by reaction time differences compatible with an incomplete tendency to compensate for differences in processing times.

Attentional influences on the dynamics of motion-induced blindness

Motion-induced blindness (MIB) is a visual phenomenon in which a highly salient, peripheral, visual target spontaneously disappears from visual awareness (and subsequently reappears) when superimposed on a globally moving background of distracters. Here, we investigated the influence of attention on these fluctuations in perception in two experiments. In the first experiment, directing spatial attention to the MIB target (and thus away from the distracters) led to an increased probability of disappearance of the target. This counter-intuitive effect of attention enhancing disappearance is nonetheless consistent with earlier reports that increased target salience enhances disappearance. Conversely, in a second experiment withdrawing attention from the entire MIB display (both target and distracters) led to a decrease in perceptual disappearances and reappearances, as well as prolonged periods of invisibility. Taken together these findings suggest that the global availability of attention facilitates competition between target and moving distracters, while the local direction of attention toward or away from the target can influence the outcome of that competition. Thus, in common with other related perceptual phenomena, attention has complex effects on the dynamics of target-distracter interactions associated with motion-induced blindness.

Dedicated and intrinsic models of time perception

Two general frameworks have been articulated to describe how the passage of time is perceived. One emphasizes that the judgment of the duration of a stimulus depends on the operation of dedicated neural mechanisms specialized for representing the temporal relationships between events. Alternatively, the representation of duration could be ubiquitous, arising from the intrinsic dynamics of nondedicated neural mechanisms. In such models, duration might be encoded directly through the amount of activation of sensory processes or as spatial patterns of activity in a network of neurons. Although intrinsic models are neurally plausible, we highlight several issues that must be addressed before we dispense with models of duration perception that are based on dedicated processes.

Realignment of temporal simultaneity between vision and touch

Adaptation to temporal asynchrony between senses (audiovisual and audiotactile) affects the subsequent simultaneity or temporal order judgment. Here, we investigated the effects of adaptation to temporal asynchrony between vision and touch. Participants experienced deformation of virtual objects with a fixed temporal lag between vision and touch. In subsequent trials, the visual and haptic stimuli were deformed with variable temporal lags, and the participants judged whether the stimuli became deformed simultaneously. The point of subjective simultaneity was shifted toward the adapted lag. No intermanual transfer of the adaptation effect was, however, found. These results indicate that the perceptual simultaneity between vision and touch is adaptive, and is determined separately for each hand.

Facial affect and temporal order judgments: emotions turn back the clock

The influence of facial affect on the perception of temporal order was examined in the context of the temporal order judgment (TOJ) paradigm. Two schematic faces were presented either simultaneously, or separated by varying stimulus onset asynchronies (SOAs; -100 ms, -34 ms, -17 ms, 17 ms, 34 ms, 100 ms), and participants had to judge which face appeared first. Each schematic face displayed one of three emotions; happy, neutral, or angry. Facial affect was found to influence judgments of temporal order at short SOAs (-17 ms, 0 ms, and 17 ms) but not at the longest SOAs (-100 ms and 100 ms), consistent with the hypothesis that facial affect influences relative onset judgments when they are difficult to make.

Temporal recalibration to tactile-visual asynchronous stimuli

Here we demonstrate that the perceptual system adapts to tactile-visual temporal asynchronies (i.e., temporal recalibration). Participants were exposed to a train of tactile and visual stimuli with a constant time lag (either -100ms, 0ms, or 100ms; with negative values indicating that the tactile stimulus came first). Following exposure, they were presented tactile-visual test stimulus pairs and judged whether the tactile or the visual stimulus was presented first (Temporal Order Judgement). Results show that subjective simultaneity (the PSS) was shifted in the direction of the exposure lag. The results fit reports on auditory-visual temporal recalibration and indicate that the brain adapts to temporal incongruencies between modalities in general.

Unpredictable visual changes cause temporal memory averaging

Various factors influence the perceived timing of visual events. Yet, little is known about the ways in which transient visual stimuli affect the estimation of the timing of other visual events. In the present study, we examined how a sudden color change of an object would influence the remembered timing of another transient event. In each trial, subjects saw a green or red disk travel in circular motion. A visual flash (white frame) occurred at random times during the motion sequence. The color of the disk changed either at random times (unpredictable condition), at a fixed time relative to the motion sequence (predictable condition), or it did not change (no-change condition). The subjects' temporal memory of the visual flash in the predictable condition was as veridical as that in the no-change condition. In the unpredictable condition, however, the flash was reported to occur closer to the timing of the color change than actual timing. Thus, an unpredictable visual change distorts the temporal memory of another visual event such that the remembered moment of the event is closer to the timing of the unpredictable visual change.

Better late than never: how onsets and offsets influence prior entry and exit

The three experiments presented in the paper examine visual prior entry (determining which of two stimuli appeared first) and prior exit (determining which of two stimuli disappeared first) effects with a temporal order judgment (TOJ) task. In addition to using onset and offset targets, the preceding cues also consisted of either onset or offset stimuli. Typical, and equivalent, prior entry effects were found when either onset or offset cues preceded the onset targets. Unexpectedly large prior exit effects where found with the offset targets, with offset cues producing greater capture effects than onset cues. These findings are consistent with the notion that more attention is allocated to searching the visual field when targets are more difficult to find. In addition, the results indicate that attentional control settings may be more likely to occur with more difficult searches. In addition, these findings demonstrate that TOJ tasks provide extremely precise measures of the allocation of attention and are very sensitive to a range of task manipulations.

No effect of auditory-visual spatial disparity on temporal recalibration

It is known that the brain adaptively recalibrates itself to small (∼100 ms) auditory–visual (AV) temporal asynchronies so as to maintain intersensory temporal coherence. Here we explored whether spatial disparity between a sound and light affects AV temporal recalibration. Participants were exposed to a train of asynchronous AV stimulus pairs (sound-first or light-first) with sounds and lights emanating from either the same or a different location. Following a short exposure phase, participants were tested on an AV temporal order judgement (TOJ) task. Temporal recalibration manifested itself as a shift of subjective simultaneity in the direction of the adapted audiovisual lag. The shift was equally big when exposure and test stimuli were presented from the same or different locations. These results provide strong evidence for the idea that spatial co-localisation is not a necessary constraint for intersensory pairing to occur.

Temporal Order is Coded Temporally in the Brain: Early Event-related Potential Latency Shifts Underlying Prior Entry in a Cross-modal Temporal Order Judgment Task

The speeding-up of neural processing associated with attended events (i.e., the prior-entry effect) has long been proposed as a viable mechanism by which attention can prioritize our perception and action. In the brain, this has been thought to be regulated through a sensory gating mechanism, increasing the amplitudes of early evoked potentials while leaving their latencies unaffected. However, the majority of previous research has emphasized speeded responding and has failed to emphasize fine temporal discrimination, thereby potentially lacking the sensitivity to reveal putative modulations in the timing of neural processing. In the present study, we used a cross-modal temporal order judgment task while shifting attention between the visual and tactile modalities to investigate the mechanisms underlying selective attention electrophysiologically. Our results indicate that attention can indeed speed up neural processes during visual perception, thereby providing the first electrophysiological support for the existence of prior entry.

Does attention alter appearance?

Abrupt onsets in the visual field can change the appearance of subsequent stimuli, according to one interpretation, by engaging an attentional mechanism that increases effective stimulus contrast. However, abrupt onsets can also engage capacity-unlimited and thus attention-independent sensory mechanisms. We conducted a series of experiments to differentiate the sensory and attentional accounts. Observers compared the contrasts of uncued low-contrast peripheral targets with simultaneous targets cued by one of three cue types with different sensory attributes: white or black peripheral abrupt onsets and central gaze direction cues devoid of sensory activity near the target locations. Each cue facilitated the perception of perithreshold targets; however, the white abrupt onsets increased the perceived contrast of suprathreshold targets, whereas the black abrupt onsets tended to reduce the perceived contrast, and the gaze direction cues had no significant effect. The effectiveness of the gaze direction cues in automatically orienting attention was demonstrated in a control experiment in which they consistently speeded response times. The results suggest that sensory interaction, and not attention, is responsible for changes in appearance.

Attention delays perceived stimulus offset

Yeshurun and Levy (2003) [Transient spatial attention degrades temporal resolution. Psychological Science, 14, 225 -231.] have suggested that visual attention enhances the activation of the parvocellular system and thus delays the perceived offset of a stimulus. We tested this assumption in a spatial cueing paradigm in which participants responded to stimulus offset. Consistent with this assumption, offset reaction time (RT) was prolonged for attended compared to unattended stimuli. For onset RT, however, we confirmed the well-known spatial cueing effect that attention speeds up the detection of stimulus onset. The results provide direct evidence for the proposal made by Yeshurun and Levy.

Does attention cause illusory line motion?

Illusory line motion (ILM) has been shown to occur when a line is presented with one end next to a previously stimulated location. The line appears to be drawn away from the site of stimulation. It has been suggested that this is because of the allocation of attention to the stimulated site. Using an endogenous attentional manipulation (a central arrow cue) with no differences in the display between the two ends of the line at the time of line presentation or immediately prior, no ILM was detected, though there was a small effect in the opposite direction. Those who have found endogenously induced ILM have used an endogenous cue based on a property of a location marker that indicated the cued location. Changing the method of cuing to one based on a property of a peripheral marker instead of a central arrow produced a small but significant report of ILM. The small magnitude of the effect, participant self-reports, and the absence of the effect in the purely endogenous condition, suggest that this was merely a bias. ILM is not generated by endogenous attention shifts.

Neural basis of auditory-induced shifts in visual time-order perception

Attended objects are perceived to occur before unattended objects even when the two objects are presented simultaneously. This finding has led to the widespread view that attention modulates the speed of neural transmission in the various perceptual pathways. We recorded event-related potentials during a time-order judgment task to determine whether a reflexive shift of attention to a sudden sound modulates the speed of sensory processing in the human visual system. Attentional cueing influenced the perceived order of lateralized visual events but not the timing of event-related potentials in visual cortex. Attentional cueing did, however, enhance the amplitude of neural activity in visual cortex, which shows that attention-induced shifts in visual time-order perception can arise from modulations of signal strength rather than processing speed in the early visual-cortical pathways.

Audiovisual prior entry

It is almost one hundred years since Titchener [E.B. Titchener, Lectures on the Elementary Psychology of Feeling and Attention, Macmillan, New York, 1908] published his influential claim that attending to a particular sensory modality (or location) can speed up the relative time of arrival of stimuli presented in that modality (or location). However, the evidence supporting the existence of prior entry is, to date, mixed. In the present study, we used an audiovisual simultaneity judgment task in an attempt to circumvent the potential methodological confounds inherent in previous research in this area. Participants made simultaneous versus successive judgment responses regarding pairs of auditory and visual stimuli at varying stimulus onset asynchronies (SOAs) using the method of constant stimuli. In different blocks of trials, the participants were instructed to attend either to the auditory or to the visual modality, or else to divide their attention equally between the two modalities. The probability of trials containing intramodal stimulus pairs (e.g., vision-vision or audition-audition) was increased in the focused attention blocks to encourage participants to follow the instructions. The perception of simultaneity was modulated by this attentional manipulation: Visual stimuli had to lead auditory stimuli by a significantly smaller interval for simultaneity to be perceived when attention was directed to vision than when it was directed to audition. These results provide the first unequivocal evidence for the existence of audiovisual prior entry.

Perception of static eye gaze direction facilitates subsequent early visual processing

OBJECTIVE

Using event-related potentials (ERPs), it has been recently shown that a reflexive shift of attention following the observation of a dynamic eye gaze cue enhances and speeds up early visual processing of a target presented at the gazed-at location. Here we investigate whether similar early sensory modulations are also elicited by static gaze cues, or if previously described attentional effects were caused mainly by visual motion cues and not by eye gaze direction per se. Furthermore, we explore if these possible attentional orienting effects reflect facilitation of the processing of cued stimuli, inhibition of the unattended stimuli, or both.

METHODS

Subjects were presented with a face looking to the right or left visual field (VF), or straight away, before the occurrence of a lateralized target to detect. There were 3 conditions in this nonpredictive cueing task: (1) target presented in the VF indicated by the eye gaze direction (congruent); (2) opposite to the eye gaze direction (incongruent); or (3) preceded by a straight gazing face (neutral).

RESULTS

Subjects were faster at detecting congruently than incongruently and neutrally cued targets. Facilitation effects were observed on early ERP components: the occipital P1 and occipito-temporal N1 components were speeded up as early as approximately 100 ms following stimulus onset (P1), and enhanced (P1 and N1) in response to congruent trials, particularly in the right hemisphere.

CONCLUSIONS

Spatial attention triggered by static eye gaze direction produces response facilitations - predominantly lateralized to the right hemisphere - from the early sensory stages of visual processing.

SIGNIFICANCE

This study provides the first evidence of a speeding up and amplification of early visual processing following attention triggered by static eye gaze perception.