Some puzzling findings in multiple object tracking (MOT): II. Inhibition of moving nontargets

Abstract

We present three studies examining whether multiple object tracking (MOT) benefits from the active inhibition of nontargets, as proposed in Pylyshyn (2004, Visual Cognition). Using a probe-dot technique, the first study showed poorer probe detection on nontargets than on either the targets being tracked or in the empty space between objects. The second study used a matching nontracking task to control for possible masking of probes, independent of target tracking. The third study examined how localized the inhibition is to individual nontargets. The result of these three studies led to the conclusion that nontargets are subject to a highly localized object-based inhibition. Implications of this finding for the FINST visual index theory are discussed. We suggest that we need to distinguish between the differentiation (or individuation) of enduring token objects and the process of making the objects accessible through indexes, with only the latter being limited to 4 or 5 objects.

Acknowledgments

The research reported here was supported by NIH Grant 1R01 MH60924. The author wishes to acknowledge the assistance of Amir Amirrezvani, Ashley Black, John Dennis, Charles King, and Carly Leonard, for help with the experiments.

Notes

¹There is no distinction between targets and nontargets in the control (nontracking) condition. However, to meet the analysis of variance requirement that scores in different conditions be independent, we divided these probe detection scores at random for purposes of the analysis (in fact since the algorithm for generating the displays for the control condition is the same as that for the tracking condition, except that the ‘target’’ subset did not flash, and the algorithm itself designated half of the circles as “targets” and the other half as “nontargets”). This division of circles into a notional set of “targets” and “nontargets” was not applied to the graphs so that adventitious differences are not distracting. The graphs simply showed the means for all circles under both “target” and “non-target” bars for the control condition.

²Of course if observers made systematic eye movements in tracking targets these eccentricity results would not apply. Although they were asked to keep looking at the fixation cross, many volunteers indicated in the debriefing questionnaire that they had moved their eyes during tracking. If fixations followed targets, or groups of targets, then it remains possible that the superior probe detection performance on targets might be attributed to a residual eccentricity effect due to superior detection in the region of fixation. However this would not account for the pattern of probe detection performance observed in these studies, particularly for the similarity of inhibition of nontargets relative to empty space and for the steep increase in probe detection performance between nontarget and near nontarget locations found in Experiment 3.

³The FeatureGate model (Cave, 1999) bears a certain similarity to the FINST model, especially with respect to speculations about possible neural implementations (Pylyshyn, 2003, pp. 270–279). However there is a basic difference between the two approaches in that the FINST mechanism assumes a limited number of direct (nonlocation-mediated) pointers, which helps to account for the data of MOT and other evidence discussed in (Pylyshyn, 2001, 2003).

⁴There is a terminological issue here concerning how to refer to the clusters that are perceptually distinguished and tracked. In the preceding I have referred to these as “tokens” on the grounds that it is a neutral term, but the term “individual” (and the process of “individuating”) is somewhat more appropriate since it implies that each token is not only distinct from other tokens, but has an enduring existence. Because distinct tokens are merged through a correspondence operation they reflect enduring entities in the world. But this terminological policy is in conflict with the usage of these terms in philosophy (Strawson, 1963) where individuating requires appeal to conceptual properties in order to distinguish one from another. In the present view, by contrast, individuation precedes the encoding of properties. Perhaps the most common way to refer to such individuals in vision science is to refer to them as “visual objects” or even “proto-objects” without implying that properties of these individuals are encoded (the term “individuate” as well as “object” is also used in this way in cognitive development; see Leslie, Xu, Tremolet, & Scholl, 1998).