How does the human visual system compare the speeds of spatially separated objects?

• Published in: PloS one Vol. 15; no. 4; p. e0231959
• Main Authors: Danilova, M V, Takahashi, C, Mollon, J D
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 30-04-2020; Public Library of Science (PLoS)
• Subjects: Arrays; Biology and Life Sciences; Combinatorial analysis; Computer and Information Sciences; Engineering and Technology; Experiments; Humans; Medicine and Health Sciences; Motion detection; Motion Perception - physiology; Neurons; Observers; Psychophysics; Separation; Social Sciences; Space Perception - physiology; Substrates; Time Factors; Visual field; Visual fields; Visual observation; Visual Perception - physiology; Visual signals; Visual stimuli; Visual system; United Kingdom > UK; England
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0231959

We measured psychophysical thresholds for discriminating the speeds of two arrays of moving dots. The arrays could be juxtaposed or could be spatially separated by up to 10 degrees of visual angle, eccentricity being held constant. We found that the precision of the judgments varied little with separation. Moreover, the function relating threshold to separation was similar whether the arrays moved in the same, in opposite or in orthogonal directions. And there was no significant difference in threshold whether the two stimuli were initially presented to the same cerebral hemisphere or to opposite ones. How are human observers able to compare stimuli that fall at well separated positions in the visual field? We consider two classes of explanation: (i) Observers' judgments might be based directly on the signals of dedicated 'comparator neurons', i.e. neurons drawing inputs of opposite sign from local regions of the visual field. (ii) Signals about local features might be transmitted to the site of comparison by a shared 'cerebral bus', where the same physical substrate carries different information from moment to moment. The minimal effects of proximity and direction (which might be expected to influence local detectors of relative motion), and the combinatorial explosion in the number of comparator neurons that would be required by (i), lead us to favor models of type (ii).