Processing of first-order motion in marmoset visual cortex is influenced by second-order motion NICK BARRACLOUGH, 1 CHRIS TINSLEY, 2 BEN WEBB, 3 CHRIS VINCENT, 4 and ANDREW DERRINGTON 4 1 Department of Psychology, University of Hull, East Yorkshire, United Kingdom 2 Department of Experimental Psychology, University of Bristol, Bristol, United Kingdom 3 School of Psychology, University of Nottingham, Nottingham, United Kingdom 4 The School of Psychology, University of Newcastle, Newcastle, United Kingdom (Received October 7, 2005; Accepted June 1, 2006! Abstract We measured the responses of single neurons in marmoset visual cortex ~ V1, V2, and the third visual complex! to moving first-order stimuli and to combined first- and second-order stimuli in order to determine whether first-order motion processing was influenced by second-order motion. Beat stimuli were made by summing two gratings of similar spatial frequency, one of which was static and the other was moving. The beat is the product of a moving sinusoidal carrier ~first-order motion! and a moving low-frequency contrast envelope ~second-order motion!. We compared responses to moving first-order gratings alone with responses to beat patterns with first-order and second-order motion in the same direction as each other, or in opposite directions to each other in order to distinguish first-order and second-order direction-selective responses. In the majority ~72%, 67093! of cells ~ V1 73%, 45062; V2 70%, 16023; third visual complex 75%, 608!, responses to first-order motion were significantly influenced by the addition of a second-order signal. The second-order envelope was more influential when moving in the opposite direction to the first-order stimulus, reducing first-order direction sensitivity in V1, V2, and the third visual complex. We interpret these results as showing that first-order motion processing through early visual cortex is not separate from second-order motion processing; suggesting that both motion signals are processed by the same system. Keywords: Primate, Physiology, V1, Luminance, Contrast Introduction We can perceive motion as a result of either the movement of luminance-defined spatial structures in a visual stimulus, or the movement of higher-order spatial structures, for example contrast envelopes or texture borders, ~ Badcock & Derrington, 1985; Der- rington & Badcock, 1985; Chubb & Sperling, 1988!. The two types of motion are called, respectively, first-order motion and second-order motion, ~Cavanagh & Mather, 1989!. First-order motion information abounds in our moving natural environment, often accompanied by second-order motion information. Second- order motion information can be used to break camouflage and evaluate movement when first-order motion information is unreli- able or even absent. There is substantial disagreement in the scientific literature whether first-order motion and second-order motion are processed separately. Motion is processed at multiple and distinct stages within the visual system ~ Zeki, 1990; Movshon & Newsome, 1996! and this have additionally confused the issue. In this study we address the question of whether first-order motion processing in V1, V2, and the third visual complex is influenced by an additional second-order motion signal. Derrington and Badcock ~1985! found that the discrimination of the direction of motion of first-order and second-order stimuli were different. Sensitivity to a moving second-order stimulus was lower and the necessary temporal resolution of the second-order stimulus was also lower than that for a moving first-order stimulus. The authors also found that second-order motion stimuli do not elicit a motion after-effect and proposed that the human visual system uses different mechanisms to process first- and second- order motion ~ Badcock & Derrington, 1985; Derrington & Bad- cock, 1985!. Many other psychophysical studies have been used to argue that processing systems for first-order motion and second- order motion are largely independent ~ Harris & Smith, 1992; Mather & West, 1993; Ledgeway & Smith, 1994; Lu & Sperling, 1995; Nishida et al., 1997; Scott-Samuel & Georgeson, 1999!. Separate processing of first-order motion and second-order motion has also received some support from physiological ~ Zhou & Baker, 1993, 1994; Mareschal & Baker, 1998a! and neurological studies ~ Vaina & Cowey, 1996; Greenlee & Smith, 1997!. Address correspondence and reprint requests to: Nick Barraclough, University of Hull, Department of Psychology, East Yorkshire HU6 7RX, United Kingdom. E-mail: n.barraclough@hull.ac.uk Visual Neuroscience ~2006!, 23, 815–824. Printed in the USA. Copyright © 2006 Cambridge University Press 0952-5238006 $16.00 DOI: 10.10170S0952523806230141 815