D. Alan Stubbs

Attention and interference in prospective and retrospective timing

Subjects listened to a series of musical selections and then judged the duration of each selection. Some subjects were informed beforehand that timing was involved (prospective timing) whereas others were informed afterwards (retrospective timing). Half the groups performed a concurrent proofreading task during stimulus presentation. The results showed a trade-off between temporal and nontemporal task performance: prospective-timing groups were more accurate in judging time and were worse at proofreading, whereas retrospective-timing groups were relatively poor at judging time but better at proofreading. This pattern is consistent with Michons notion of an essential equivalence between temporal and nontemporal processing, and supports the predictions of an attentional allocation model of timing. The proofreading task interfered both with prospective and with retrospective timing, and both types of time judgments were influenced in the same way by effects of stimulus context. These results imply that similar timing processes operate under prospective and retrospective conditions.

Discrimination of stimulus duration and d-amphetamine in pigeons: a psychophysical analysis.

Pigeons discriminated visual stimulus duration in a psychophysical choice procedure. Following short durations, one of two responses was reinforced; following long durations, the other response was reinforced. Discrimination accuracy decreased as a function of increasing dose level of d-amphetamine. Decrements in accuracy were greater for two of three pigeons following long-than following short-stimulus durations. Position response biases increased as dose level incraesed. Similar effects of the drug on behavior occurred over two temporal ranges of stimulus durations studied.

The Psychophysics of Retrospective and Prospective Timing

In two experiments, different groups of subjects heard four musical selections and then estimated the duration of each selection. Some groups made retrospective time estimates while others made prospective estimates. In both experiments, analyses of the psychophysical relation between perceived and actual duration showed that the slopes of straight-line fits were flatter and accounted for a smaller proportion of the variance under retrospective as compared with prospective conditions. In addition, in experiment 1, retrospective subjects were less accurate in rank ordering the selections from longest to shortest. There was also a serial-order effect, with selections estimated longer when they occurred early in the sequence. In experiment 2 the slopes decreased as the selections in a series became longer. Both retrospective and prospective estimates also exhibited a context effect, in that estimates of a given selection were influenced by the relative durations of the other three selections in the series. The results on inaccurate retrospective judgments raise questions about prior research on stimulus factors and retrospective timing. However, similarities under retrospective and prospective conditions suggest that timing under these conditions, although different in some respects, reflects a similar process.

A new set of illusions--the dynamic luminance-gradient illusion and the breathing light illusion.

A novel set of illusions that break brightness constancy and size constancy at the same time is reported. The illusions occur when observers move towards or away from these patterns. Many variations of these phenomena and a possible explanation are discussed.

Perceptual compromise between apparent and veridical motion indices: the unchained-dots illusion

A new motion illusion based on dot-trajectory misperception is presented. The illusory effect can not be explained by the aperture problem unlike some previous illusions characterised by misperception of motion direction. We propose an explanation in terms of ‘perceptual compromise’ between an original type of apparent motion and the veridical motion. Several demonstrations are presented in support of that hypothesis.

On discriminating temporal relations: Is it relational?

Pigeons were presented on each trial with a pair of keylight stimuli that varied in duration. One of two subsequent choices was reinforced, depending on which of the two stimuli was longer. For some pairs, the duration of one stimulus was predictive of relative duration, but for other pairs, absolute duration was unpredictive. Choice responses depended on relative differences between the stimuli, but were also controlled to some degree by absolute duration of the second member of the pair. Individual differences in control by absolute and relative duration were evident. Those pigeons whose behavior was most influenced by absolute duration showed poorer transfer to a different set of duration pairs.

Scaling of events spaced in time

Pigeons were trained to peck on a key, which could be lit by red or green light, and produce feeder-light stimuli intermittently. On some trials, food followed the fourth feeder flash providing the key color was red, while on other trials food followed the sixteenth flash providing the color was green. The change in color from red to green was produced by a peck to a second, changeover key. Pigeons typically responded in the presence of red until four or more flashes occured and then, if food had not been delivered, changed the main-key color and responded on the green key. Following training, the variable-interval schedule arranging-feeder light events was changed to longer and shorter values to alter the amount of time (and number of responses) between events. Data from these test days indicate that the change from red to green was influenced by the number of events, but also by the time elapsed and/or responses emitted since the onset of a trial. The results suggest multiple sources of related information and stimulus control when events and behavior occur over time.

Perception of linear and nonlinear trends: using slope and curvature information to make trend discriminations.

This study investigated several factors influencing the perception of nonlinear relationships in time series graphs. To model real-world data, the graphed data represented different underlying trends and included different sample sizes and amounts of variability. Six trends (increasing and decreasing linear, exponential, asymptotic) were presented on four graph types (histogram, line graph, scatterplot, suspended bar graph). The experiment assessed how these factors affect trend discrimination, with the overall goal of judging what types of graphs lead to better discrimination. Six participants (two psychology professors, four psychology graduate students) viewed graphs on a computer screen and identified the underlying trend. All participants were familiar with the types of trends presented and were aware of the purpose of the experiment. Analysis indicated higher accuracy when variability was lower and sample size was higher. Choice accuracy was higher for nonlinear trends and was highest when line graphs were used.

Second-order schedules: Manipulation of brief-stimulus duration at component completion

In a second-order schedule, fixed-interval components were reinforced according to a variable-interval schedule. A brief stimulus accompanied the completion of each fixed interval. Brief-stimulus duration was varied across conditions from 0.5 to 8 sec. Patterning was greater the longer the duration of the stimulus. Additionally, exposure to relatively long brief-stimulus durations enhanced patterning upon reexposure to shorter brief-stimulus durations.

Detection of Sample Differences from Dot Plot Displays

Cleveland and McGill [10] concluded that dot plots are effective when one judges position along a common scale. We assessed the ability of graph readers to detect sample mean differences in multipanel dot plots. In Experiment 1, plots containing vertically arranged panels with different sample sizes and levels of variability were presented. Sensitivity was greater with large samples and low variability. In Experiment 2, sensitivity depended on the location of the comparison sample, with vertical and superimposed arrays yielding greater sensitivity than horizontal or diagonal arrays. Horizontal arrays also produced a bias to judge data in right-most panels as having higher means. Experiment 3 showed that ordering of data had little effect on sensitivity or bias. The results suggest that good graph design requires attention to how the specific features of a graphical format influence perceptual judgments of data