Visual Neuroscience

Research Articles

The integration of local chromatic motion signals is sensitive to contrast polarity

SOPHIE M. WUERGERa1 c1, ALEXA RUPPERTSBERGa2, STEPHANIE MALEKa3, MARCO BERTAMINIa1 and JASNA MARTINOVICa4

a1 School of Psychology, University of Liverpool, Liverpool, UK

a2 Department of Optometry, University of Bradford, Bradford, UK

a3 Department of Psychology, University of Halle-Wittenberg, Halle, Germany

a4 School of Psychology, University of Aberdeen, Aberdeen, UK

Abstract

Global motion integration mechanisms can utilize signals defined by purely chromatic information. Is global motion integration sensitive to the polarity of such color signals? To answer this question, we employed isoluminant random dot kinematograms (RDKs) that contain a single chromatic contrast polarity or two different polarities. Single-polarity RDKs consisted of local motion signals with either a positive or a negative S or L–M component, while in the different-polarity RDKs, half the dots had a positive S or L–M component, and the other half had a negative S or L–M component. In all RDKs, the polarity and the motion direction of the local signals were uncorrelated. Observers discriminated between 50% coherent motion and random motion, and contrast thresholds were obtained for 81% correct responses. Contrast thresholds were obtained for three different dot densities (50, 100, and 200 dots). We report two main findings: (1) dependence on dot density is similar for both contrast polarities (+S vs. −S, +LM vs. −LM) but slightly steeper for S in comparison to LM and (2) thresholds for different-polarity RDKs are significantly higher than for single-polarity RDKs, which is inconsistent with a polarity-blind integration mechanism. We conclude that early motion integration mechanisms are sensitive to the polarity of the local motion signals and do not automatically integrate information across different polarities.

(Received October 21 2010)

(Accepted February 02 2011)

(Online publication March 23 2011)

Correspondence:

c1 Address correspondence and reprint requests to: Sophie M. Wuerger, School of Psychology, University of Liverpool, Eleanore Rathbone Building, Bedford Street South, Liverpool L697ZA, UK. E-mail: s.m.wuerger@liverpool.ac.uk