IS THERE INHIBITION OF RETURN FOR ISOLUMINANT COLORS? Luiz Henrique M. do Canto-Pereira 1 , Galina V. Paramei 2 , Ronald D. Ranvaud 1 and Hermann J. Müller 3 1 Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of São Paulo, São Paulo, Brazil 2 Institute of Psychology, Darmstadt University of Technology, Darmstadt, Germany 3 Department of Psychology, Ludwig-Maximilians-University, Munich, Germany Abstract In the present study we replicate experiments of Lupiañez et al. (1997) who addressed the issue of inhibition of return with respect to the color of stimuli. In contrast to that study, using non-isoluminant stimuli, we employed two pairs of isoluminant colors varying along either the red-green or blue-yellow cardinal axes of a color space – deutan or tritan confusion lines in the CIE 1976. Participants (normal trichromats; n=8) were exposed successively to the color pairs; their task was to respond to the second (target) stimulus by key pressing, whereby simple reaction times were measured. In different trial stimulus onset asynchrony (SOA) was varied: 100, 400, 700, 1000 and 1300 ms. At no SOA inhibition of return was found; instead, facilitation of return was demonstrated, this especially pronounced at the short SOA=100 ms. We found that participants were overall faster in the invalid condition when compared to the valid condition (F=25.986, p=0.001). We hypothesized that when isoluminants colors are employed as both cue and target, Inhibition of Return can’t be establish. Attention has a crucial role in the process of identifying and selecting relevant objects in a visual environmentof an organism. This process should be rapid in order to move attention from one location to another and, thus, improve discrimination and overall efficiency at the attended location. Immediately following an event at a peripheral location, there is facilitation in time in the processing of other stimuli near that location, this is said to reflect a reflexive shift of attention towards the source of stimulation. However, after attention has been removed from such peripheral location, a delayed responding to stimuli displayed there subsequently is registered. This inhibitory effect was coined by Posner and Cohen (1984) as inhibition of return (IOR). It is suggested that IOR encourages orienting towards novel locations and, hence, might facilitate foraging and other search behaviors (Klein, 2000; Riggio et al., 2004). Despite its relatively recent “discovery”, IOR was and still is the focus of considerable amount of research in the last years. The time course of this phenomenon is closely related to the attentional resources that the involved task requires. For instance, in a simple luminance-detection task, IOR starts around 225 ms after the onset of the target (Posner & Cohen, 1984); but when the task is more attentional demanding, IOR occur much later (Lupiáñez & Milliken, 1999) and could last for several seconds. With respect to the color dimension, some authors expressed doubts on whether the IOR phenomenon could occur (Klein & Taylor, 1994; Terry et al., 1994). However, later a color-based IOR was indeed demonstrated (Law et al., 1995; Lupiáñez et al., 1997). The Opponent Color Theory of the 19th century physiologist Ewald Hering (1964) derived by the analysis of subjective human color vision. Certain colors are not perceived together, i.e. they do not mix; for instance we never see bluish-yellows or reddish- greens.