1. Comparison of Spatial and Temporal Discrimination Performance across Various Difficulty Levels J.E. THROPP, J.L. SZALMA, & P.A. HANCOCK Department of Psychology and Institute for Simulation and Training University of Central Florida, Orlando, FL Acknowledgments This research was supported by a Multidisciplinary University Research Initiative (MURI) program grant from the Army Research Office, Dr. Elmar Schmeisser, Technical Monitor (Grant# DAAD19-01-1-0621).The views expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the Department of the Army, Department of Defense, or the US Government. Discussion The results of this study indicate that the functions describing the relation between performance and discrimination difficulty are different for the spatial and temporal dimensions. Although this will be dependent upon range of stimuli used (i.e., milliseconds/millimeters vs. minutes/meters), prior research has demonstrated that discrimination performance is similar across levels of the base stimulus selected within the millisecond range (e.g., see Allan, Kristofferson, & Wiens, 1971). However, there were large individual differences in performance and in the rate of change in performance as a function of discrimination difficulty, consistent with prior observation (e.g., see Thropp et al., 2004). For instance, some individuals tend to perform better on one task than the other, while others are more consistently high or low in performance levels on both tasks. Future work is needed to establish what variables account for such differences within and between the two task dimensions. The current study represents an initial investigation of the psychophysical relations between spatial and temporal dimensions. Further research should evaluate whether changes in performance along one dimension influence performance on the other dimension. However, the current results suggest that the performance effects of increasing task load may be linear for both the information structure and information rate dimensions of the Hancock and Warm (1989) model. The spatial discrimination task required participants to identify which of two vertical lines was taller. A trial consisted of presenting each stimulus sequentially, followed by a screen prompting a response by the observer regarding whether the taller of the two lines was presented first or second. The shorter line was held constant at 32mm across the levels of difficulty. The difficulty of the discrimination was manipulated by increasing the 32mm line by increments of.5, 1.0, 1.5, 2.0, 2.5, 3.0, or 3.5mm. These represent increments relative to the shorter line of 1.56, 3.12, 4.69, 6.25, 7.81, 9.38, and 10.94%, respectively. In the temporal discrimination task, participants were required to determine which of two lines, presented sequentially, was longer in duration. The shorter duration was held constant across conditions at 300ms, and the difficulty of the discrimination was manipulated by increasing the presentation time of the line by increments of 10ms, 40ms, 70ms, 100ms, 130ms, 160ms, and 190ms. These represent increments relative to the shorter duration of 3.33, 13.33, 23.33, 33.33, 43.33, 53.33, and 63.33%, respectively. All stimuli were presented on a PC monitor and the presentation times were controlled by the clock mechanism in SuperLab for Windows. Spatial and temporal stimulus values were selected from pilot work previous experiments regarding spatial and temporal perception. For both tasks, participants responded by pressing the ‘1’ key on the keyboard if the first line was the tall line (in the spatial discrimination task) or the line was presented for the longer duration (in the temporal discrimination task), or by pressing the ‘2’ key if the second line was the tall line or presented for the longer duration. For each of seven difficulty conditions observers experienced 30 trials. The taller or longer stimulus was presented first in 15 trials and second in the other 15 trials in each 30-trial block, and the order in which trials appeared was randomized. Participants completed the spatial and temporal tasks in blocks. Half of the participants experienced the spatial block first and the other half experienced the temporal block first. The orders in which the difficulty conditions were presented varied across participants and were determined by using a Latin Square. Results The mean proportion correct scores for the spatial and temporal discrimination tasks are displayed as a function of difficulty level (% delta between stimulus values in each condition) in Figure 2. It can be seen in the figure that the performance varies from near chance (.5) in the most difficult (i.e., smallest change to be discriminated) conditions to approximately.9 in the easiest conditions. In addition, participants achieved better performance in the spatial than in the temporal task. These impressions were confirmed via regression analysis of performance on discrimination difficulty for each participant in each task. The average regression equations are shown in Figure 2. Tests of the differences between the regression parameters for the two tasks indicated that both the slope and intercept of the functions were significantly different from one another. For the regression weights, t(35)=18.032, p<.001. For the intercepts, t(35)=3.447, p<.01. Note that although the relation between discrimination performance and discrimination difficulty varies as a function of task, there are also large individual differences in performance among participants. These are shown for the temporal and spatial tasks in Figures 3 and 4, respectively. References Allan, L.G., Kristofferson, A.B., & Wiens, E.W. (1971). Duration discrimination of brief light flashes. Perception & Psychophysics, 9, 327-334. Hancock, P. A. & Warm, J. S. (1989). A dynamic model of stress and sustained attention. Human Factors, 31, 519-537. Hancock, P.A., Szalma, J. L. & Weaver, J. L. (2004). The distortion of perceptual space-time under stress. Proceedings of the 23rd Annual Army Science Conference, Orlando, FL (IP-23). Ross, J. M., Szalma, J. L., Thropp, J. E., & Hancock, P. A. (2003). Performance, workload, and stress correlates of temporal and spatial task demands. Proceedings from the Annual Human Factors and Ergonomics Society Meeting, 47, 1712-1716. Thropp, J. E., Szalma, J. L. & Hancock, P. A. Performance Operating Characteristics for Spatial and Temporal Discriminations: Separate or Common Capacities? Presentation given September 24, 2004 at 48th Annual Meeting of the Human Factors and Ergonomics Society, New Orleans, LA. Figure 4. Performance on the spatial discrimination task as a function of discrimination difficulty Abstract Spatial and temporal information may be perceptually related, such that performance on spatial and temporal discrimination tasks may draw upon common resource capacities. However, the degree of this relationship is not clear. Multiple levels of difficulty in spatial and temporal discrimination tasks were manipulated in the current study in order to examine how performance changes along the two dimensions as a function of discrimination difficulty. It was found that performance across difficulty levels differed for the spatial and temporal tasks, such that although both tasks showed a linear relationship between performance and discrimination difficulty, performance was superior in the spatial condition and improved at a faster rate. Individual differences in performance level across the two dimensions were also observed. Introduction The Hancock and Warm (1989) model of human adaptation under various levels of stress and task load established the importance of understanding the role played by task characteristics. Within the context of that model, these characteristics are represented on two axes: spatial elements (information structure) and temporal elements (information rate; see Figure 1). However, what is not clear is the degree to which these dimensions are related perceptually, and how changes along one dimension affect an individual’s position along the other dimension. It has been suggested (Hancock, Szalma, & Weaver, 2002) that information processing demands along these dimensions may draw on common resource capacities. However, recent studies presented at this conference have indicated that the relation between these dimensions may not be so simple (Ross, Szalma, Thropp, & Hancock, 2003; Thropp, Szalma, & Hancock, 2004). For instance, in a perceptual discrimination study deriving performance operating characteristics to examine resource sharing between spatial and temporal tasks, Thropp et al. (2004) reported that the functions obtained varied substantially across participants. In the current study we sought to examine the relations between these dimensions psychophysically by comparing multiple levels of discrimination difficulty across the two dimensions. While such data do not directly test hypotheses derived from the Hancock and Warm model, it provides an additional window into the relation between these two dimensions. That is, the experiment determined the relative discriminability of spatial and temporal stimuli across a range of values. It is hoped that this will provide the basis for future investigation into the perceptual mechanisms underlying spatial and temporal processing. Figure 1. Adaptation under Stress. After Hancock & Warm (1989). Method Participants Participants were 36 undergraduate students (10 males, 26 females, mean age 22.2 years, SD = 4.8 years) enrolled at the University of Central Florida, all with normal or corrected to normal vision. Participants were compensated by payment of $15 for an experimental session lasting one hour and forty five minutes. Experimental Design Observers engaged in both a spatial and a temporal discrimination task, and experienced seven levels of discrimination difficulty within each task type. This yielded a 2 (task type) by 7 (discrimination difficulty) within-subjects design. A two alternative forced choice procedure was employed for both tasks. Figure 2. Proportion correct as a function of discrimination difficulty (% difference between stimuli) for spatial and temporal tasks. Note. Error bars are standard errors. Figure 3. Performance on the temporal discrimination task as a function of discrimination difficulty