Visibility versus accountability in pooling local motion signals into global motion direction

Abstract

The human observer is surprisingly inaccurate in discriminating proportions between two spatially overlapping sets of randomly distributed elements moving in opposite directions. It was shown that observers took into account an equivalent of 74 % of all moving elements when the task was to estimate their relative number, but only an equivalent of 21 % of the same elements when the task was to discriminate between opposite directions. It was concluded that, in the motion direction discrimination task, a large proportion of the signal from all of the elements was inaccessible to the observers, whereas the majority of the signal was accessible in a numerosity task. This type of perceptual limitation belongs to the attentional blindness category, where a strong sensory signal cannot be noticed when processing is diverted by parallel events. In addition, we found no evidence for the common-fate principle, as the ability to discriminate numerical proportions remained the same, irrespective of whether all estimated elements were moving coherently in one direction or unpredictably in opposite directions.

Discrimination of numerical proportions: A comparison of binomial and Gaussian models

“…Rather surprisingly, human observers are extremely inaccurate in discriminating proportion between two spatially overlapping sets of randomly distributed elements moving in opposite directions (Raidvee et al, 2011). In a wide range of set sizes, decisions about motion direction are made as if only a very limited number of elements (in some cases, less than 0.5 %) are taken into account, even if the motion direction of each element in isolation can be determined with near absolute certainty (Raidvee, Averin, & Allik, 2012). In a similar task, where instead of motion direction the elements differed by color, observers were able to discriminate the relative number of red and green dots as if they had taken into account 69 elements from a total of 100 (Tokita & Ishiguchi, 2009) provided that psychometric response curves are interpreted in terms of dot counts used in the decision (Raidvee, Averin, et al, 2012;Raidvee et al, 2011).…”

“…In a wide range of set sizes, decisions about motion direction are made as if only a very limited number of elements (in some cases, less than 0.5 %) are taken into account, even if the motion direction of each element in isolation can be determined with near absolute certainty (Raidvee, Averin, & Allik, 2012). In a similar task, where instead of motion direction the elements differed by color, observers were able to discriminate the relative number of red and green dots as if they had taken into account 69 elements from a total of 100 (Tokita & Ishiguchi, 2009) provided that psychometric response curves are interpreted in terms of dot counts used in the decision (Raidvee, Averin, et al, 2012;Raidvee et al, 2011). However, the same observers' ability to discriminate between the relative number of parallel and converging lines was much poorer, with a precision equal to discrimination decisions made on the basis of no more than two elements out of the 100 available (Tokita & Ishiguchi, 2009).…”