Interval timing with gaps and distracters: evaluation of the ambiguity, switch, and time-sharing hypotheses

Gaps and distracters were presented during the timed signal to examine whether the stop/reset mechanism is activated by (a) changes in the timed signal (switch hypothesis), (b) ITI-like events (ambiguity hypothesis), or (c) processes concurrent with the timing process (time-sharing hypothesis). While the switch and ambiguity hypotheses predict that rats should time through (ignore) distracters, the time-sharing hypothesis predicts that extraneous events (e.g., gaps and distracters) delay timing by causing working memory to decay in proportion to the events' salience. The authors found that response functions were displaced by both gaps and distracters, in accord with the time-sharing hypothesis. Computer simulations show that the time-sharing and memory-decay hypotheses can mechanistically address present data, and reflect different levels of the same model.     

Interval timing with gaps: gap ambiguity as an alternative to temporal decay

C. V. Buhusi, D. Perera, and W. H. Meck (2005) proposed a hypothesis of timing in rats to account for the results of experiments that have used the peak procedure with gaps. According to this hypothesis, the introduction of a gap causes the animal's memory for the pregap interval to passively decay (subjectively shorten) in direct proportion to the duration and salience of the gap. Thus, animals should pause with short, nonsalient gaps but should reset their clock with longer, salient gaps. The present authors suggest that the ambiguity of the gap (i.e., the similarity between the gap and the intertrial interval in both appearance and relative duration) causes the animal to actively reset the clock and prevents adequate assessments of the fate of timed intervals prior to the gap. Furthermore, when the intertrial interval is discriminable from the gap, the evidence suggests that timed intervals prior to the gap are not lost but are retained in memory.     

Time-sharing in rats: effect of distracter intensity and discriminability

Interruptions (gaps) and unfamiliar events (distracters) during a timed signal delay the timed response of humans and other animals. To explore this phenomenon, we manipulate the intensity of auditory distracters (Experiment 1), and we dissociate the role of distracter intensity, distracter similarity with the intertrial interval, and dissimilarity from the timed auditory signal (Experiment 2). When the intertrial interval and the timed signal were silent, the delay in response after an auditory distracter increased with its intensity: Rats ignored (ran through) a 40-dB distracter, stopped timing during a 75-dB distracter, and reset after a 100-dB distracter. However, when timing was signaled by a 70-dB noise, rats reset both for 40- and 100-dB distracters, stopped for both 55- and 85-dB distracters, and run for the 70-dB distracter. Data are accounted for by a time-sharing model assuming 2 concurrent processes-time accumulation and memory decay controlled by the discriminability of the interrupting event-whose interplay results in a continuum of responses, from run to reset.     

Temporal integration as a function of signal and gap intensity in rats (Rattus norvegicus) and pigeons (Columba livia)

Previous data suggest that rats (Rattus norvegicus) and pigeons (Columba livia) use different interval-timing strategies when a gap interrupts a to-be-timed signal: Rats stop timing during the gap, and pigeons reset their timing mechanism after the gap. To examine whether the response rule is controlled by an attentional mechanism dependent on the characteristics of the stimuli, the authors manipulated the intensity of the signal and gap when rats and pigeons timed in the gap procedure. Results suggest that both rats and pigeons stop timing during a nonsalient gap and reset timing after a salient gap. These results also suggest that both species use similar interval-timing mechanisms, influenced by nontemporal characteristics of the signal and gap.     

Timing for the absence of a stimulus: the gap paradigm reversed

Contrary to data showing sensitivity to nontemporal properties of timed signals, current theories of interval timing assume that animals can use the presence or absence of a signal as equally valid cues as long as duration is the most predictive feature. Consequently, the authors examined rats' behavior when timing the absence of a visual or auditory stimulus in trace conditioning and in a "reversed" gap procedure. Memory for timing was tested by presenting the stimulus as a reversed gap into its timed absence. Results suggest that in trace conditioning (Experiment 1), rats time for the absence of a stimulus by using its offset as a time marker. As in the standard gap procedure, the insertion of a reversed gap was expected to "stop" rats' internal clock. In contrast, a reversed gap of 1-, 5-, or 15-s duration "reset" the timing process in both trace conditioning (Experiment 2) and the reversed gap procedure (Experiment 3). A direct comparison of the standard and reversed gap procedures (Experiment 4) supported these findings. Results suggest that attentional mechanisms involving the salience or content of the gap might contribute to the response rule adopted in a gap procedure.     

Timing in pigeons: effects of the similarity between intertrial interval and gap in a timing signal

Previous research suggests that when a fixed interval is interrupted (known as the gap procedure), pigeons tend to reset memory and start timing from 0 after the gap. However, because the ambient conditions of the gap typically have been the same as during the intertrial interval (ITI), ambiguity may have resulted. In the present experiment, the authors found that when ambient conditions during the gap were similar to the ITI, pigeons tended to reset memory, but when ambient conditions during the gap were different from the ITI, pigeons tended to stop timing, retain the duration of the stimulus in memory, and add to that time when the stimulus reappeared. Thus, when the gap was unambiguous, pigeons timed accurately.     

Expectancy in humans in multisecond peak-interval timing with gaps

In two experiments, the peak-interval procedure was used with humans to test effects related to gaps in multisecond timing. In Experiment 1, peak times of response distributions were shorter when the gap occurred later during the encoding of the criterion time to be reproduced, suggesting that gap expectancy shortened perceived durations. Peak times were also positively related to objective target durations. Spreads of response distributions were generally related to estimated durations. In Experiment 2, peak times were shortest when gaps were expected but did not occur, confirming that the shortening effect of gap expectancy is independent of the gap occurrence. High positive start-stop correlations and moderate positive peak-time-spread correlations showed strong memory variability, whereas low and negative start-spread correlations suggest small response-threshold variability. Correlations seemed not to be influenced by expectancy. Overall, the peak-interval procedure with gaps provided relevant information on similarities and differences in timing in humans and other animals.     

Rear-view perception in driving: Distance information is privileged in the selection of safe gaps

Selecting a safe gap before merging into the traffic is a crucial driving skill that relies on images provided by rear-view mirrors or, recently, camera-monitor systems. When using these visual aids, some drivers select dangerously small gaps to cut in front of faster vehicles. They may do so because they base their decision either on information about distance or object size, or on miscalculated information about time-to-passage (TTP). Previous experiments have been unable to compare the role of TTP, speed, and distance information for drivers’ gap selection, as they did not investigate them in the same experimental regime. The present experiments seek to determine the perceptual variables that guide drivers’ rearward gap selection. Using short videos of an approaching vehicle filmed from three different camera heights (low, conventional, high), a total of 61 subjects either made gap safety decisions (Experiment I), or estimated the TTP, speed, and distance of an approaching vehicle (Experiment II). An effect of camera height was found for gap selection, TTP, and distance estimation, but not for speed estimation. For the high camera position, smaller gaps were selected as safe, TTP estimates were longer, and the distance to the approaching vehicle was perceived as farther. An opposite pattern was found for the low camera. Regression analyses suggested that distance is an important player. The subjects strongly relied on distance information when estimating TTP, and perceived distance dominated subjects’ gap selection. Thus, drivers seem to employ distance-based strategies when selecting safe gaps in rear-view mirrors or monitors.     

Visual marking and the perception of salience in visual search

In the present study, the gap paradigm originally developed by Watson and Humphreys (1997) was used to investigate whether the process of visual marking can influence the perceptual salience of a target in visual search. Consistent with previous studies (Watson & Humphreys, 1997), the results showed that search was not affected by the presence of the preceding distractors when the target was relatively low in salience. This finding suggests that visual marking can increase the efficiency of visual search by decreasing the size of the search set. However, more important, the results also showed that search was affected by the presence of the preceding distractors when the target was relatively high in salience. This finding suggests that visual marking may be limited in its ability to increase the perceptual salience of the target. Together, the results of the present study suggest that the effectiveness of visual marking may vary as a function of search context.     

Object salience is transiently represented whereas object presence is not: evidence from temporal order judgment

We investigated the temporal dynamics in the processing of object salience and object presence by using a temporal-order-judgment task. In two experiments, observers were presented with displays containing one or two orientation singletons embedded in a background of homogeneously oriented lines. Observers had the task to indicate the temporal order of two colour changes which occurred either at two singleton locations differing in salience (experiment 1) or at two locations differing in singleton presence (experiment 2). Singleton displays were presented for a short or a long duration prior to the colour changes. The results demonstrated that salience affects the perceived temporal order of two colour changes more strongly after a short than after a long presentation duration (experiment 1). Singleton presence affects the perceived temporal order of two colour changes equally after a short and a long presentation duration (experiment 2). The results suggest that object salience and object presence are differently represented in the visual system.     

Auditory-visual conflicts in the perceived duration of lights, tones and gaps

The perceived duration of a short tone (1,000 or 1,500 msec) was longer than that of a separately presented light of equal length. Thus, when light and tone were presented simultaneously, there was a conflict in perceived duration. In that case, the perceived duration of an interval filled with both light and tone was close to that of an interval filled with tone alone. A silent gap in otherwise continuous tone was perceived as longer than a gap in otherwise continuous light, and the perceived duration of a gap occurring simultaneously in both light and tone was close to that of a gap in tone alone. Thus, auditory dominance occurred under the preceding conditions-that is, auditory-visual conflicts in perceived duration, whether occurring between filled intervals or gaps, were resolved in favor of the auditory modality. Visual dominance occurred only under one condition, in which the intensity of tone was reduced, and in which the perceived duration of a 500-msec light was longer than that of a 500-msec tone. The finding of auditory dominance in the perception of time runs counter to the results of studies of sensory conflicts in spatial perception, where vision typically dominates audition and touch.     

Structural salience and the nonaccidentality of a gestalt

We perceive structure through a process of perceptual organization. Here we report a new perceptual organization phenomenon-the facilitation of visual grouping by global curvature. Observers viewed patterns that they perceived as organized into collections of curves. The patterns were perceptually ambiguous such that the perceived orientation of the patterns varied from trial to trial. When patterns were sufficiently dense and proximity was equated for the predominant perceptual alternatives, observers tended to perceive the organization with the greatest curvature. This effect is tantamount to visual grouping by maximal curvature and thus demonstrates an unprecedented effect of global structure on perceptual organization. We account for this result with a model that predicts the perceived organization of a pattern as function of its nonaccidentality, which we define as the probability that it could have occurred by chance. Our findings demonstrate a novel relationship between the geometry of a pattern and the visual salience of global structure.     

Dynamic visual acuity and coincidence-anticipation timing by experienced and inexperienced women players of fast pitch softball

This study examined the relationship between dynamic visual acuity and coincidence-anticipation timing in 16 inexperienced and 16 experienced women's fast pitch softball players. Pearson-product correlations indicated a low relationship between dynamic visual acuity and coincidence-anticipation timing. The correlations for dynamic visual acuity and coincidence anticipation between experienced and inexperienced dynamic visual acuity were not significant. A significant difference was found between the mean dynamic visual acuity of the two groups, i.e., experienced players had better dynamic visual acuity than inexperienced players. Analysis of variance of constant errors, variable errors, and absolute errors of coincidence anticipation indicated no significant differences between groups or across the three accuracy scores. The interaction between experience and accuracy was not significant.     

Influence of Visual and Haptic Cues on Podokinetic After-Rotation

ABSTRACT

Active countercircling on a rotating platform for 15 min causes individuals to involuntarily circle in the same direction when they step in place on firm ground. This is referred to as podokinetic after-rotation (PKAR). It is unclear how interjecting brief periods of visual or haptic inputs for a stable orientation reference affects PKAR. The authors studied this issue in 16 healthy individuals who participated in three sessions each. Following active countercircling, participants attempted to step in place for 30 min on firm ground. In two of three sessions, participants received full visual input or made fingertip contact with a stationary object for 30 s during 30 min of ongoing PKAR. All participants slowed or stopped rotating during the presence of visual or haptic inputs and resumed PKAR after removal of these inputs. Exponential functions fitted to angular trunk velocity versus time plots revealed no significant differences across conditions (p > .05). The preservation of PKAR after brief exposure to a visual or haptic reference is consistent with a slowly decaying velocity storage that is not reset or dumped after exposure to conflicting visual or haptic cues.     

Timing of the CS-US Interval by Pigeons in Trace and Delay Autoshaping

Evidence for timing during Pavlovian trace and delay conditioning trials was sought by exposing pigeons to differentially cued trial durations of 18, 24, and 60 sec. For a delay group, one of three visual patterns (CSs) was presented on a key for the entire trial; for a trace group, each CS was 12 sec in duration, creating trace gaps of 6, 12, or 48 sec. Intertrial interval duration was 48 sec. The most informative data were provided by measures of time spent proximal to the CS (proximity). Stimulus control was evident in that proximity was inversely related to trial duration for both groups. Evidence for timing was obtained from the relation of relative proximity during each CS to relative elapsed trial time for individual birds. The obtained superposition of the three functions for each bird is consistent with a scalar timing process and implies that subjects learned the temporal relation between each CS and the unconditioned stimulus. Associative status of the CSs differed between groups and among CS-US intervals, as reflected in proximity data during a higher-order test in which the CSs served as reinforcers. The results are consistent with a two-dimensional model of Pavlovian conditioning according to which the associative strength accruing to a CS is orthogonal to the temporal information encoded for that stimulus.     

Privileged directions for subjective contours: horizontal and vertical versus tilted

Subjective contours and brightness enhancement in Ehrenstein-like situations are affected by pattern orientation. If a classic Ehrenstein pattern (with four inducing elements for every gap at intersection points) is observed, a number of anomalous illusory patches usually appear in these gaps, but if the same pattern is observed tilted by 45 degrees the patches disappear and it is possible to see an illusory grid of horizontal and vertical 'streets'. These two perceptual results are mutually exclusive. In a Koffka-cross variant of this pattern, the illusory patches, which are usually square, appear more rounded in the tilted pattern. All these results were confirmed in two experiments by means of a magnitude estimation procedure. It is suggested that the formation of a subjective contour is easier along horizontal and vertical directions and more difficult in an oblique direction, and that this phenomenon, as well as other visual acuity oblique effects, depends in part on the basic functioning of the visual system at the level of sensation.     

Retinal and extraretinal information in movement perception: how to invert the Filehne illusion

During a pursuit eye movement made in darkness across a small stationary stimulus, the stimulus is perceived as moving in the opposite direction to the eyes. This so-called Filehne illusion is usually explained by assuming that during pursuit eye movements the extraretinal signal (which informs the visual system about eye velocity so that retinal image motion can be interpreted) falls short. A study is reported in which the concept of an extraretinal signal is replaced by the concept of a reference signal, which serves to inform the visual system about the velocity of the retinae in space. Reference signals are evoked in response to eye movements, but also in response to any stimulation that may yield a sensation of self-motion, because during self-motion the retinae also move in space. Optokinetic stimulation should therefore affect reference signal size. To test this prediction the Filehne illusion was investigated with stimuli of different optokinetic potentials. As predicted, with briefly presented stimuli (no optokinetic potential) the usual illusion always occurred. With longer stimulus presentation times the magnitude of the illusion was reduced when the spatial frequency of the stimulus was reduced (increased optokinetic potential). At very low spatial frequencies (strongest optokinetic potential) the illusion was inverted. The significance of the conclusion, that reference signal size increases with increasing optokinetic stimulus potential, is discussed. It appears to explain many visual illusions, such as the movement aftereffect and center-surround induced motion, and it may bridge the gap between direct Gibsonian and indirect inferential theories of motion perception.     

Effects of salience are short-lived

ABSTRACT— A salient event in the visual field tends to attract attention and the eyes. To account for the effects of salience on visual selection, models generally assume that the human visual system continuously holds information concerning the relative salience of objects in the visual field. Here we show that salience in fact drives vision only during the short time interval immediately following the onset of a visual scene. In a saccadic target-selection task, human performance in making an eye movement to the most salient element in a display was accurate when response latencies were short, but was at chance when response latencies were long. In a manual discrimination task, performance in making a judgment of salience was more accurate with brief than with long display durations. These results suggest that salience is represented in the visual system only briefly after a visual image enters the brain.     

Converging sources of evidence on spoken and perceived rhythms of speech: cyclic production of vowels in monosyllabic stress feet

The article reviews the literature from psychology, phonetics, and phonology bearing on production and perception of syllable timing in speech. A review of the psychological and phonetics literature suggests that production of vowels and consonants are interleaved in syllable sequences in such a way that vowel production is continuous or nearly so. Based on that literature, a hypothesis is developed concerning the perception of syllable timing assuming that vowel production is continuous. The hypothesis is that perceived syllable timing corresponds to the times sequencing of the vowels as produced and not to the timing either of vowel onsets as conventionally measured or of syllable-initial consonants. Three experiments support the hypothesis. One shows that information present during the portion of an acoustic signal in which a syllable-initial consonant predominates is used by listeners to identify the vowel. Compatibly, this information for the vowel contributes to the vowel's perceived duration. Finally, a measure of the perceived timing of a syllable correlates significantly with the time required to identify syllable-medial vowels but not with time to identify the syllable-initial consonants. Further support for the proposed mode of vowel-consonant production and perception is derived from the literature on phonology. Language-specific phonological conventions can be identified that may reflect exaggerations and conventionalizations of the articulatory tendency for vowels to be produced continuously in speech.     

Temporal ventriloquism in a purely temporal context

This study examines how audiovisual signals are combined in time for a temporal analogue of the ventriloquist effect in a purely temporal context, that is, no spatial grounding of signals or other spatial facilitation. Observers were presented with two successive intervals, each defined by a 1250-ms tone, and indicated in which interval a brief audiovisual stimulus (visual flash + noise burst) occurred later. In "test" intervals, the audiovisual stimulus was presented with a small asynchrony, while in "probe" intervals it was synchronous and presented at various times guided by an adaptive staircase to find the perceived temporal location of the asynchronous stimulus. As in spatial ventriloquism, and consistent with maximum likelihood estimation (MLE), the asynchronous audiovisual signal was shifted toward the more reliably localized component (audition, for all observers). Moreover, these temporal shifts could be forward or backward in time, depending on the asynchrony order, suggesting perceived timing is not entirely determined by physical timing. However, the critical signature of MLE combination--better bimodal than unimodal precision--was not found. Regardless of the underlying model, these results demonstrate temporal ventriloquism in a paradigm that is defined in a purely temporal context.