Discriminating time intervals presented in sequences marked by visual signals

This article presents the results of three experiments on the discrimination of time intervals presented in sequences marked by brief visual signals. In Experiment 1A (continuous condition), the participants had to indicate whether, in a series of 2–4 intervals marked by 3–5 visual signals, the last interval was shorter or longer than the previous one(s). In Experiment 1B (discontinuous condition), the participants indicated whether, in a presentation of two series of 1–3 intervals, with each series being marked by 2–4 signals, the intervals of the second sequence were shorter or longer than those of the first. Whenever one, two, or three standard intervals were presented, the difference threshold was as high at 150 msec as it was at 300 msec with the continuous method but increased monotonically from 150 to 900 msec with the discontinuous method. With both methods, the increase was well described by Weber’s law&#x2014the Weber fraction was roughly constant&#x2014between 600 and 900 msec (Experiment 2), whereas between 900 and 1,200 msec (Experiment 3), the Weber fraction increased.     

Duration discrimination of empty and filled intervals marked by auditory and visual signals

Experiments 1 and 2 compared, with a single-stimulus procedure, the discrimination of filled and empty intervals in both auditory and visual modalities. In Experiment 1, in which intervals were about 250 msec, the discrimination was superior with empty intervals in both modalities. In Experiment 2, with intervals lasting about 50 msec, empty intervals showed superior performance with visual signals only. In Experiment 3, for the auditory modality at 250 msec, the discrimination was easier with empty intervals than with filled intervals with both the forced-choice (FC) and the single stimulus (SS) modes of presentation, and the discrimination was easier with the FC than with the SS method. Experiment 4, however, showed that at 50 and 250 msec, with a FC-adaptive procedure, there were no differences between filled and empty intervals in the auditory mode; the differences observed with the visual mode in Experiments 1 and 2 remained significant. Finally, Experiment 5 compared differential thresholds for four marker-type conditions, filled and empty intervals in the auditory and visual modes, for durations ranging from .125 to 4 sec. The results showed (1) that the differential threshold differences among marker types are important for short durations but decrease with longer durations, and (2) that a generalized Weber’s law generally holds for these conditions. The results as a whole are discussed in terms of timing mechanisms.     

Discrimination of time intervals presented in sequences: spatial effects with multiple auditory sources

This article discusses two experiments on the discrimination of time intervals presented in sequences marked by brief auditory signals. Participants had to indicate whether the last interval in a series of three intervals marked by four auditory signals was shorter or longer than the previous intervals. Three base durations were under investigation: 75, 150, and 225 ms. In Experiment 1, sounds were presented through headphones, from a single-speaker in front of the participants or by four equally spaced speakers. In all three presentation modes, the highest different threshold was obtained in the lower base duration condition (75 ms), thus indicating an impairment of temporal processing when sounds are presented too rapidly. The results also indicate the presence, in each presentation mode, of a 'time-shrinking effect' (i.e., with the last interval being perceived as briefer than the preceding ones) at 75 ms, but not at 225 ms. Lastly, using different sound sources to mark time did not significantly impair discrimination. In Experiment 2, three signals were presented from the same source, and the last signal was presented at one of two locations, either close or far. The perceived duration was not influenced by the location of the fourth signal when the participant knew before each trial where the sounds would be delivered. However, when the participant was uncertain as to its location, more space between markers resulted in longer perceived duration, a finding that applies only at 150 and 225 ms. Moreover, the perceived duration was affected by the direction of the sequences (left-right vs. right-left).     

Duration discrimination of empty time intervals marked by intermodal pulses

In 1973, Rousseau and Kristofferson reported that short empty intermodal time intervals marked by a light flash and a brief tone were poorly discriminated by subjects, and that AT,5 was constant over a large range of durations. It led them to suggest that short intramodal empty intervals, marked by stimuli from the same sensory modality, might be handled by a “more efficient mechanism” to which intermodal intervals would not have access. Unfortunately, their study lacked the basic evidence needed to make a strong statement: no direct comparison between inter- and intramodal duration discrimination and no within-subject discrimination function were available. To clarify these two issues, three experiments were performed. The data indicate that intermodal time intervals are discriminated more poorly than intramodal ones, and that intermodal duration discrimination functions follow Weber’s law. Analysis of data from different experiments lead to the conclusion that inter- and intramodal intervals are timed by a common timekeeper and that intermodal intervals induce a large noise component in the timekeeping operation.

     

Perception of acoustically presented time series with varied intervals

Data from three experiments on serial perception of temporal intervals in the supra-second domain are reported. Sequences of short acoustic signals (“pips”) separated by periods of silence were presented to the observers. Two types of time series, geometric or alternating, were used, where the modulus 1 + δ of the inter-pip series and the base duration T_b (range from 1.1 to 6 s) were varied as independent parameters. The observers had to judge whether the series were accelerating, decelerating, or uniform (3 paradigm), or to distinguish regular from irregular sequences (2 paradigm). “Intervals of subjective uniformity” (isus) were obtained by fitting Gaussian psychometric functions to individual subjects' responses. Progression towards longer base durations (T_b = 4.4 or 6 s) shifts the isus towards negative δs, i.e., accelerating series. This finding is compatible with the phenomenon of “subjective shortening” of past temporal intervals, which is naturally accounted for by the lossy integration model of internal time representation. The opposite effect observed for short durations (T_b = 1.1 or 1.5 s) remains unexplained by the lossy integration model, and presents a challenge for further research.     

Discrimination of time intervals marked by brief acoustic pulses of various intensities and spectra

Experienced observers were asked to identify, in a four-level 2AFC situation, the longer of two unfilled time intervals, each of which was marked by a pair of 20-msec acoustic pulses. When all the markers were identical, high-level (186-dB SPL) bursts of coherently gated sinusoids or bursts of band-limited Gaussian noise, a change in the spectrum of the markers generally did not affect performance. On the other hand, for 1-kHz tone-burst markers, intensity decreases below 25 dB SL were accompanied by sizable deterioration of the discrimination performance, especially at short (25-msec) base intervals. Similarly large changes in performance were observed also when the two tonal markers of each interval were made very dissimilar from each other, either in frequency (frequency difference larger than 1 octave) or in intensity (level of the first marker at least 45 dB below the level of the second marker). Time-difference thresholds in these two latter cases were found to be nonmonotonically related to the base interval, the minima occurring between 40- and 80-msec onset separations.     

Perception of signals presented in the periphery of the visual field

Two experiments were performed which tested the type of information that can be extracted from the periphery of the visual field, i.e. content information or position information. In both studies two digits were compared, one presented at the left and one at the right of the subject's meridian, seperated by a binocular visual angle, comprising either an eye-field (45°) or a head-field display (100°). In the first experiment the right signal was presented in separate blocks of either degraded or undegraded digits. In the case of undegraded digits, processing efficiency was better in the eye-field than in the head-field. With degraded digits this eye-field advantage was significantly smaller, but did not altogether disappear, suggesting that information about both the content and the position of digits in the periphery is acquired in the eye-field as opposed to the head-field. In the second experiment degraded and undegraded digits occured randomly within the same block of trials. This was done to investigate whether peripheral information is acquired automatically or through controlled processing. The results indicate a small eye-field advantage, about equal for both degraded and undegraded digits. This supports the controlled processing hypothesis, at least with regard to the acquisition of content information from the periphery.     

Discrimination of two neighboring intra- and intermodal empty time intervals marked by three successive stimuli

We investigated the discrimination of two neighboring intra- or inter-modal empty time intervals marked by three successive stimuli. Each of the three markers was a flash (visual—V) or a sound (auditory—A). The first and last markers were of the same modality, while the second one was either A or V, resulting in four conditions: VVV, VAV, AVA and AAA. Participants judged whether the second interval, whose duration was systematically varied, was shorter or longer than the 500-ms first interval. Compared with VVV and AAA, discrimination was impaired with VAV, but not so much with AVA (in Experiment 1). Whereas VAV and AVA consisted of the same set of single intermodal intervals (VA and AV), discrimination was impaired in the VAV compared to the AVA condition. This difference between VAV and AVA could not be attributed to the participants' strategy to perform the discrimination task, e.g., ignoring the standard interval or replacing the visual stimuli with sounds in their mind (in Experiment 2). These results are discussed in terms of sequential grouping according to sensory similarity.     

Short-term memory for simultaneously presented visual and auditory signals in the pigeon

A series of divided-attention experiments in which matching to the visual or auditory component of a tone-light compound was compared with matching to visual or auditory elements as sample stimuli were carried out. In 0-s delayed and simultaneous matching procedures, pigeons were able to match visual signals equally well when presented alone or with a tone; tones were matched at a substantially lower level of accuracy when presented with light signals than when presented as elements. In further experiments, it was demonstrated that the interfering effect of a signal light on tone matching was not related to the signaling value of the light, and that the prior presentation of light proactively interfered with auditory delayed matching. These findings indicate a divided attention process in which auditory processing is strongly inhibited in the presence of visual signals.     

Memory and learning for visual signals in time and space

Vision is often characterized as a spatial sense, but what does that characterization imply about the relative ease of processing visual information distributed over time rather than over space? Three experiments addressed this question, using stimuli comprising random luminances. For some stimuli, individual items were presented sequentially, at 8 Hz; for other stimuli, individual items were presented simultaneously, as horizontal spatial arrays. For temporal sequences, subjects judged whether each of the last four luminances matched the corresponding luminance in the first four; for spatial arrays, they judged whether each of the right-hand four luminances matched the corresponding left-hand luminance. Overall, performance was far better with spatial presentations, even when the entire spatial array was presented for just tens of milliseconds. Experiment 2 demonstrated that there was no gain in performance from combining spatial and temporal information within a single stimulus. In a final experiment, particular spatial arrays or temporal sequences were made to recur intermittently, interspersed among, non-recurring stimuli. Performance improved steadily as particular stimulus exemplars recurred, with spatial and temporal stimuli being learned at equivalent rates. Logistic regression identified several shortcut strategies that subjects may have exploited while performing our task.     

Judgments of the duration of visually marked empty time intervals: Linking perceived duration and sensitivity

The capability of subjects to categorize (as short or long) visually marked empty time intervals was investigated in three experiments. Two visual signals, located 18° to the left (L) and to the right (R) of a fixation point in the visual field, established four marking conditions, two unilaterally presented (L-L and R-R) and two bilaterally presented (L-R and R-L). In Experiments 1 and 2, the results show that discrimination is better with unilateral sequences than with bilateral sequences and that the perceived duration is longer with an L-R than with an R-L sequence. In addition, Experiment 2 shows that, in comparison with a condition in which Markers 1 and 2 remain identical for a complete session, varying the markers from trial to trial does not decrease discrimination. Also, Experiment 2 shows that discrimination is better when both visual markers are presented at fovea than it is in the unilateral conditions. Experiment 3 shows that bilateral intervals are perceived as being longer and are better discriminated than are intervals marked by an intermodal sequence (auditory-visual or visual-auditory). The general discussion reports the implications of having different perceived duration and sensitivity levels, in various marker-type conditions, for an internal-clock hypothesis. Some implications of these results for a lateralized-timer hypothesis are also discussed.     

Response time to the second of two signals following varied vs constant intersignal intervals

The Ss made choice responses under three experimental conditions: (1) a variable interval between the warning signal and the signal to respond, (2) a constant interval between the warning signal and the signal to respond, and (3) omission of the warning signal. With variable intersignal intervals (lSI), response time (RT) was inversely related to lSI at least when the latter was in the 10 to 225 msec range. When lSI was constant its duration had no perceptible effect on RT. RT tended to be shorter with constant lSI. independent of its duration, than when the warning signal was omitted altogether.     

Pigeons presented with sequences of light flashes use behavior to count but not to time

On randomly ordered trials, pigeons were presented with either a blue or a white key that flashed red for 200 ms at a fast (2 flashes/s), medium (1 flash/s), or slow (0.5 flashes/s) rate. The blue key signaled a fixed-interval (FI) schedule in which the first response after 20 s was reinforced, and the white key signaled a fixed-number (FN) schedule in which the first response after 20 flashes was reinforced. In Experiments 1 and 2, pigeons showed depressed responding to the flash on FI-cued trials and accelerated responding to the flash on FN-cued trials. When the response key was periodically blacked out in Experiments 3 and 4, counting but not timing was eliminated.     

Discrimination of time intervals bounded by tone bursts

Trained listeners had to discriminate, in a four-level 2AFC paradigm, the duration of a time interval bounded by pairs of brief tone bursts. The time intervals ranged from 10 to 100 msec. When the tone-burst markers were similar in their intensity (86 dB SPL) and frequency (1 kHz), the just noticeable time difference was found to be monotonically related to the base interval. On the other hand, when the intensity of the first marker was severely attenuated (by 50 dB) or when a large (2-octave) difference was introduced between the frequencies of the two markers, the time discrimination functions became nonmonotonic. A similar, albeit slight, departure from monotonicity was also achieved by making the second marker much longer than the first (300 msec vs. 10 msec). The nonmonotonicity of these time discrimination functions is compared to the well-documented nonmonotonicity that may be observed for voice onset time (VOT) discrimination functions.     

Intertrial intervals and visual habituation in neonates

Eighteen 3-day-old human neonates were shown a 12 by 12 black-white checkerboard target for 45-sec. trials with either a 10-, 20-, or 30-sec. intertrial interval until their visual fixation time decreased to a set criterion for habituation. On subsequent recovery trials, a 2 by 2 black-white checkerboard target received significantly longer fixations from boys in the 10-sec. and girls in the 20-sec. conditions. 30-sec. intervals, however, produced little habituation and recovery. The implications of these findings are discussed.     

Selective strategies in the assimilation of successively presented signals

It is found that performance in experiments on the psychological refractory period is highly affected by instructions. In the present experiment subjects were instructed either to handle the signals successively or to group them. Both instructions were obeyed. This seems to indicate that the human organism has various strategies available. Which strategy is actually applied is likely to depend on the structure of the experimental situation, and partly also, on momentary preference. This may explain the variety of results in the literature on this subject.

Finally, it is found that, when two signals are presented simultaneously, the total reaction time is considerably shorter if the signals are “grouped” than if they are “handled successively.” This difference disappears at interstimulus intervals of 0.2 sec. and 0.4 sec. The hypothesis is put forward, that the gain in time at the former case is due to simultaneous perceptual processing of the signals.     

Information processing and estimation of short time intervals.

Duration estimates were assessed by the method of reproduction with filled reproduction intervals. Mental arithmetic, reading and mirror-image drawing were used in pairs as initial and/or reproduction tasks. All nine possible pairs of tasks were used in a 9 X 5 X 5 mixed design with five Ss per task pair and five interruption intervals for each initial task. Results indicated that, when arithmetic was used as the initial task, Ss underestimated the duration of the initial interval. When arithmetic was used as the reproduction task, Ss overestimated the duration of the initial interval. A significant correlation was obtained between arithmetic outputs and the lengths of the duration estimates. Results are interpreted as supportive of Burnside's (1971) interpretation of Ornstein's (1969) storage-size hypothesis.