Perceptual coupling of multiple point-light figures

Abstract

When confronted with multiple bistable stimuli at the same time, the visual system tends to generate a common interpretation for all stimuli. We exploit this perceptual-coupling phenomenon to investigate the perception of depth in bistable point-light figures. Observers indicate the global depth orientation of simultaneously presented point-light figures while the similarity between the stimuli is manipulated. In a first experiment, a higher occurrence of coupled percepts is found for identical figures, but coupling breaks down when either the movement pattern or both the viewpoint and the phase-relation are changed. A second experiment confirms these results, but also demonstrates that different point-light actions can be subject to perceptual coupling as long as they share the same viewpoint and exhibit equivalent degrees of perceptual ambiguity. The data are consistent with an explanation in terms of differential contributions of stored, view-dependent object and action representations and of an interaction between stages processing local stimulus features. The consequences of these results are discussed in the framework of an explicit model for the perception of depth in biological motion.

Acknowledgements

This research was supported by Concerted Research Effort Convention GOA/05/03/TBA, Impulse Program IMPH/06/GHW, and the Fund for Scientific Research of Flanders, of which JV is a research assistant.

Notes

¹Although there are fundamental differences, a similar method of exploiting the bistability to gauge aspects of stimulus similarity has been proposed by Maier, Wilke, Logothetis, and Leopold (2003). In their study, it was shown that temporally interleaving different ambiguous patterns in a series of sequential stimulus presentations (in which observers indicated the perceived alternative for each presented stimulus), stabilizes the percept of the patterns (cf. Leopold, Wilke, Maier, & Logothetis, 2002), but also that occasional reversals in one pattern were coupled with an interleaved pattern and that this coupling was based on stimulus similarity.

²All actions used in these experiments are “loopable” in the sense that the movement pattern can be repeated so that the transition from the final frame back to the first frame of the action proceeds smoothly.

³If, for example, the probability of perceiving a jumping point-light figure as facing the viewer, which when presented in isolation is about .50–.60, would drastically increase, to .80–.90 for instance, by the mere addition of another stimulus, any interpretation in terms of perceptual coupling processes would be severely compromised.

⁴Although not essential to the present question, the inclusion of the response mapping as an independent variable was motivated by the fact that some observers in previous experiments with PLFs noted that the mappings of an FTV response to a key that was spatially closer (like the “d” and “l” keys) and of an FA response to a key that was spatially further (like the “e” and “o” keys) were “easier” than when this was reversed. Of course, this manipulation of spatial compatibility is an extremely subtle one, but given the instability of the visual stimulus and the fact that performed actions can indeed affect the perceptional processes (e.g., Wohlschläger, 2000), worth testing. Moreover, under the assumption that the reverse response mapping, being more difficult, relies to a higher degree on attentional resources, then the comparison between a condition with a “compatible” and “incompatible” stimulus–response mapping could be informative if coupling would indeed originate in nonvisual stages, such as a decisional or motor response stage.

⁵It is worth repeating that the significant effects observed here did not simply result from some general change in the overall perceptual ambiguity of multielement displays. In other words, it is not the case that simply presenting more than a single point-light figure resulted in a general shift towards a specific interpretation of the PLF hereby increasing the probability of coupled percepts. First, the ANOVA on the overall frequency of FTV responses with the number of presented stimuli did not show a significant difference between an item in isolation and in a multi-item display. Second, even if there were an increased tendency to interpret the figures in a particular orientation, then basing the calculation of the expected frequencies on the data of the multielement trials, as was done in the analysis, will only make it more difficult for a perceptual-grouping effect to reach significance. Consider the following situation for a given PLF: P(FTV) in isolation equals .60, then under the assumption of independent processing the expected P(same response) for a two-element display would be .52. However, if using the stimuli in a multielement display would increase the probability of an FTV percept to, say, .80, then calculating the expected P(same response) on the basis of the results of the two-element display would be .72. We explicitly calculated the expected amount of coupling on the basis of the results observed for the actions presented in isolation: Compared to the predicted frequencies based on the multielement displays, only very small differences were found (the average difference for the six stimulus combinations was .006) and compared to the actual observed frequencies, the same fundamental pattern of results was obtained.

⁶Interestingly, the movement pattern that was most susceptible for this strategy of attending to specific features is precisely the action that yielded the strongest effect of synchrony in the present investigation (jumping).