JUDGMENT DYNAMICS IN SUPRATHRESHOLD COLOR PSYCHOPHYSICS

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1 JUDGMENT DYNAMICS IN SUPRATHRESHOLD COLOR PSYCHOPHYSICS Galina V. Paramei Leibniz Institute for Occupational Physiology, Ardeystr. 67, D-9 Dortmund Inst. of Psychology, Russian Academy of Sciences, Moscow, Russia Abstract The dynamics of color judgments was investigated, addressing the questions of (i) whether color stimuli/responses, occurring early in a sequence, affect later estimations, and (ii) whether a response scale changes over trials. In different runs, color pairs with either "small" or "large" differences occurred on initial trials. Dissimilarity within pairs was rated by observers using a 0-9 integer. A consistent relationship between (at least five) initial and overall subsequent judgments was found, indicating response assimilation. This primacyassimilation effect for difference judgments is in general agreement with the response consistency principle (Haubensak, 99), though at odds with a primacy-contrast effect demonstrated for absolute judgments. In addition, a scale drift was found: as the series proceeds, the initial assimilation wanes, to then reverse to a response contrast. The present positive (=assimilation) and negative (=contrast) sequential effects are considered to result from two antagonistic criterion-setting mechanisms -- stabilization vs. tracking (Treisman, 98) -- that subserve optimizing stimulus-response mapping under uncertainty. Studies on color perception concentrate primarily on sensory function, reporting averaged measures, and seldom consider the process of judgment. Psychophysical responses undergo though variability that, along with sensory, may be associated with cognitive-judgmental processes (Baird, 997). The present study focuses on judgments of color differences, inquiring whether these are subjected to potent presentation-order effects. Judgments are known to be contingent upon presentation context, resulting in various response-bias effects: compared to a 'veridical' judgment, a response under question may deviate in the direction of contrast from or assimilation to judgments in its vicinity (Marks & Algom, 998). For absolute judgments, one of the well-studied examples is the contrastive frequency-range effect.(parducci & Wedell, 986). The amount of contrast is considered to be related to maximization of stimulus-response differences, since it depends negatively on the number of available categories and positively on the number of judged stimuli. Another example is the primacy effect (e.g., Haubensak, 99; Sokolov, Pavlova & Ehrenstein, 000): the range of initial stimuli appears to influence subsequent responses in a contrastive way. The effect is accounted for by internal consistency of subjects' judgments 9

2 retained over a session. The consistency model is supported by the evidence that a subject's scale reaches its essential form fairly quickly - after 8- initial trials (cf. Haubensak, 99). The above named effects may be considered as special cases of the sequential effect. This describes the sequential dependency between the response (R N ) at trial N and the stimulus (S N- ) and response (R N- ) at trial N-. For absolute judgments, the main finding is that the response is contrasted toward the previous stimulus (or other previous stimuli up to lag of 8). On further trials, the contrast effect may be overshadowed by the assimilation effect between successive responses. Assimilation is considered to rely solely on response-system processes: in the face of uncertainty, a subject is assumed to choose a response close to the previous one (Ward & Lockhead, 97). Compared to quite numerous studies of contextual effects for absolute judgments, those for differential judgments have been rather neglected. One reason for this neglect, as Sailor and Pineda (99) point out, may be the tacit assumption that absolute and relational judgments are affected by a context in a similar way. One should, however, note that relational judgments and the development of a response scale to them depend on spacing of stimulus differences rather than on single stimuli (Mellers & Birnbaum, 98). Few thorough studies of comparative judgments, ensuing Thurstone's theory of categorical judgment, showed that subjects are capable of substantial response bias (see Link, 990). To our knowledge, no studies on contextual effects for differential judgments have been undertaken in the color-vision domain. We reanalyzed data from our 'sensory-directed' work (Paramei & Cavonius, 999), while posing the questions of whether color-difference judgments are: (i) affected by the range of stimuli/responses occurring early in a series; if so, (ii) in which way, contrastive or assimilative, are they biased; (iii) how many initial stimuli are sufficient to induce the effect; (iv) whether the effect is reflected by frequency distribution of response categories; and (v) whether the effect is stable within a long trial series. Method Observers were three normal trichromats, two of whom were experienced in psychophysical tasks. Stimuli were lights representing part of the CIE 976 u'v'-chromaticity diagram. They were displayed in pairs on a computer-controlled monitor (for details see Paramei & Cavonius, 999). Color pairs were defined by u'v'-distances that reflect almost uniformly perceptual differences for a standard (normal trichromatic) observer. Color differences, in terms of u'v'-distances, varied between 0 and 0.6 and were spaced unevenly along a sensory dimension, as is attested by the varying amount (M i ) of the color pairs in the following u'v'-subranges: [0-0.]: M =; [>0.-0.]: M =8; [>0.-0.]: M =; [>0.-0.]: M =; [>0.-0.]: M =6. Procedure. All possible pairs were presented quasi-randomly, twice in a run (N=0), not excluding back-to-back presentations of identical pairs. Each observer participated in five runs; s/he was required to category-rate dissimilarity in a color pair using a 0-9 scale (0 indicating no and 9 maximal subjective difference); no feed-back was given. In different runs, color pairs with either "small" or "large" differences (= u'v'-distances) occurred predominantly on initial trials. 9

3 Results Correlations between color-pair u'v'-distances and corresponding ratings were examined by calculating Spearman s coefficients (r S ). High correlations were obtained: for individual subjects, r S varied between 0.76 and 0.8 (average r S = 0.77). In addition, mean rating for pooled data was calculated:.76 ±.. For the following analysis, the value of.8 was taken as a 'yardstick': at the mean rating on initial trials.8, the run was regarded as small, whereas at the mean initial rating >.8 as large. The primacy effect was investigated by comparing means of ratings on initial and later trials in small vs. large runs using a one-way analysis of variance (ANOVA). The number of initial trials was first taken to equal ten. The results show (Table, data pooled across Ss) that means of the remaining ratings in a run were significantly different for small vs. large runs (p<.0), with the mean difference between the two of approximately 0., thus, manifesting the primacy effect. Moreover, the data indicate that subsequent judgments tended to be smaller with initial small and larger with initial large ratings demonstrating assimilation. The finding was confirmed using a non-parametric Wilcoxon signed rank test for matched pairs. The amount of initial responses sufficient to induce the assimilative primacy effect was further explored (since in previous studies, investigating response bias, eight initial stimuli have been found to affect a subjective scale; e.g., Parducci & Wedell, 986). For this, numbers of initial responses <0 were tested. The results indicate that the effect was still detected, when as few as five initial responses were considered (Table ). This amount of initial trials was taken as the lowest limit and considered in further analysis. Table. Mean ratings in small vs. large runs contingent on the number of initial responses No. of Response Small runs: Large runs: p-value initial responses interval Rating Mean ± SD Rating Mean ± SD -. ± ± ± ± ± ± ± 0..9 ± ± ± ± ± Frequency distribution of response categories (0,..., 9) in small vs. large runs was explored by comparing their relative numbers by one-way ANOVA. The results presented in Figure demonstrate skewing of response distribution: in small runs, the proportion of small (0-) ratings is higher and of large (7-9) ratings is lower, compared to that in the large runs (p<.0). The proportion of medium rating categories did, however, not differ significantly for the counterpart series. 96

4 Figure. Frequency distribution of response categories in runs with small vs. large color differences predominating on initial trials. Observer CRC. The dynamics of subjective scale within single presentation series was examined. For this purpose, response sequences were partitioned into n-trial lags of either 0, 0, 60, or 80 trials. For individual Ss, means of ratings were calculated for each lag, and response development was compared between small vs. large runs (one-way ANOVA). Figure shows a drift of mean responses in the 'small' vs. 'large' run for observer CRC (significant differences are depicted by diamonds). The graphs indicate that at a fine sequence partition (with the tenresponse lag), the performance appears idiosyncratic. However, at a coarser partition (over 0 etc.) certain trends can be revealed. Specifically, the assimilation augments up to about 0 th trial, then -- between approximately 0 th -80 th trials -- assimilation abates, to finally -- at ca. 00 th trial -- turn into contrast. A similar response dynamics was found for the other two Ss, though individual differences in the lag mean values and time course were observed. Averaged over 0 Averaged over 60 CRC Mean rating Averaged over Sequence No. Mean rating Averaged over Sequence No. Figure. Mean ratings of color differences in small ( ) vs. large (Δ) runs as a function of the trial interval. Data for variable partition lags are shown. Observer CRC. 97

5 Discussion The results demonstrate that category judgments of color differences, elicited in a long stimulus series, are prone to a response bias. It manifests itself predominantly as the assimilative primacy effect. Five initial judgments suffice to affect the subsequent responses. The dependence of responses on those occurring early in a run is in general agreement with the consistency principle (Haubensak, 99). However, the found assimilation is at odds with the contrastive primacy effect demonstrated earlier for absolute judgments. The assimilative primacy effect is considered a particular case of sequential dependency, which is assumed to reflect a strategy of stimulus-response mapping employed by subjects under uncertainty (Ward & Lockhead, 97). In our design, the uncertainty was due to (i) tight and unequal spacing of the stimulus pairs on the sensory dimension; (ii) a much larger number of stimuli compared to that of response categories, and (iii) the absence of feedback about the correct response. These factors should result in high variability of the response-category boundaries and, hence, in decreased information transmission (Luce et al., 98; Mori & Ward, 99). As a consequence of high uncertainty, a response assimilation is assumed to occur. Besides, its propagation ensues due to retaining consistency by a subject. A more detailed analysis of the assimilation effect with respect to certain response categories revealed further that the 'edge' categories undergo a stronger bias. This finding is in agreement with a notion of a different effect on judgments of stimuli at the ends than on those in the center of a stimulus range (Parducci, 97). An explanation may be sought, again, in terms of response uncertainty, which has been found to be least at the two ends of the stimulus continuum. Conversely, in the mid-range uncertainty is greater: it shows itself in increased response variability, viz. distributions of mid-categories are expanded along a sensory dimension in both directions, compared to narrower, unidirectional distributions of the edge categories (Garner, 9). The expanded distributions were, conceivably, the reason of an insignificant difference between proportion of medium ratings in the small and large runs. An intensive analysis of the development of the judgment scale indicates that the degree of response assimilation undergoes changes: as the number of trials increases, its strength wanes (Jeastedt, Luce & Green, 977), until assimilation appears to be swamped by contrast. The finding is in accord with higher-order sequential effects emerging in longer stimulus series (Parducci, 97). The drift of the judgment scale indicates dynamic adjustment of response criteria. According to the criterion-setting model of Treisman (98), two antagonistic mechanisms -- stabilization and tracking -- determine the adjustment process: stabilization shifts all response criteria on a trial-by-trial basis toward the sensation of the previous stimulus, while tracking shifts the criteria away from the previous response. Keeping in mind that the reference system sets the initial positions of response criteria in the task, stabilization decays over trials, whereas tracking is less strong, but persistent: this would explain the initially salient assimilation observed in our data, with later aggravation of response contrast. In the framework of the Treisman's model, positive (=assimilation) and negative (=contrast) sequential effects could be considered a manifestation of the adjusting criterion setting. The contextual effects demonstrated here give an insight into the role of cognitive constraints: in the complex judgmental situation the response, driven primarily by the subject's sensation of color difference, becomes biased. The response bias is due to the heuristic principles dynamically employed by the subject -- anchoring to the initial sample, internal consistency and scale adjustment during a longer presentation sequence (cf. Tversky & Kahneman, 97). 98

6 References Baird, J. C. (997). Sensation and Judgment: Complementarity Theory of Psychophysics. Mahwah, NJ: Erlbaum. Garner, W. R. (9). An information analysis of absolute judgments of loudness. Journal of Experimental Psychology, 6, Haubensak, G. (99). The consistency model: A process model for absolute judgments. Journal of Experimental Psychology: Human Perception and Performance, 8, Haubensak, G. (99). Wie entsteht der Häufigkeitseffekt in absoluten Urteilen? Zeitschrift für experimentelle und angewandte Psychologie,, Jesteadt, W., Luce, R. D. & Green, D. M. (977). Sequential effects in judgments of loudness. Journal of Experimental Psychology: Human Perception and Performance,, 9-0. Link, S. (990). The psychological design of psychological experiments. In H.-G. Geissler, M. H. Müller and W. Prinz (Eds.), Psychological Exploration of Mental Structures, -. Toronto a.o.: Hogrefe & Huber. Luce, R. D., Nosofsky, R. M., Green, D. M. & Smith, A. F. (98). The bow and sequential effects in absolute identification. Perception & Psychophysics,, Marks, L. E. & Algom, D. (998). Psychophysical scaling. In M. H. Birnbaum (Ed.), Measurement, Judgment, and Decision Making, San Diego: Academic Press. Mellers, B. A. & Birnbaum, M. H. (98). Loci of contextual effects in judgment. Journal of Experimental Psychology: Human Perception and Performance, 8, Mori, S. & Ward, L. M. (99). Pure feedback effects in absolute identification. Perception & Psychophysics, 7, Paramei, G. V. & Cavonius, C. R. (999). Color spaces of color-normal and abnormal observers reconstructed from response times and dissimilarity ratings. Perception & Psychophysics, 6, Parducci, A. (97). A range-frequency approach to sequential effects in category ratings. In S. Kornblum (Ed.), Attention and Performance, 0-. New York: Academic Press. Parducci, A. & Wedell, H. W. (986). The category effect with rating scales: Number of categories, number of stimuli, and method of presentation. Journal of Experimental Psychology: Human Perception and Performance,, Sailor, K. M. & Pineda, K. M. (99). Evidence for implicit scaling in comparative judgment. Memory & Cognition,, -9. Sokolov, A., Pavlova, M. & Ehrenstein, W. H. (000). Primacy and frequency effects in absolute judgments of visual velocity. Perception & Psychophysics, 6, Treisman, M. (98). A theory of criterion setting: an alternative to the attention band and response ratio hypotheses in magnitude estimation and cross-modality matching. Journal of Experimental Psychology: General,, -6. Tversky, A. & Kahneman, D. (97). Judgment under uncertainty: Heuristics and biases. Science, 8, -. Ward, L. M. & Lockhead, G. R. (97). Response system processes in absolute judgment. Perception & Psychophysics, 9, Acknowledgments: This research was supported in part by the DFG grant 6 RUS 7/0/9S. I am grateful to C. R. Cavonius, W. H. Ehrenstein and G. Haubensak for valuable comments and helpful advice, to A. Siefer and E. Zschiesche for assistance in statistical analysis and to P. Dillmann and U. Lobisch for technical assistance. 99