The effects of collinearity and orientation on texture visual evoked potentials Alfredo Romani * , Roberto Callieco, Eleonora Tavazzi, Vittorio Cosi Laboratorio Potenziali Evocati, Fondazione ‘Istituto Neurologico Casimiro Mondino’, Via Palestro 3, 27100 Pavia, Italy Accepted 5 March 2003 Abstract Objective: The aim of the research was to study the effects of stimulus orientation at both the local (textons) and the global (segregated elements) level on texture visual evoked potentials (tVEPs). Methods: Two tVEP paradigms were presented to 10 volunteers. The paradigms were characterized by alternating uniform textures (random mixture of square dots and lines) and textures in which stripes of randomly disposed lines segregated from a square dots’ background. In one paradigm, the stripes were horizontal and in the other, vertical. The lines could be either horizontal or vertical in single stimuli of both paradigms. Thus, two stimuli with local/global collinearity and two stimuli without local/global collinearity were available. tVEPs were derived from Oz referenced to the left earlobe and averaged separately for each condition. Segregation-related components were obtained subtracting the traces without segregation from the traces with segregation. Results: A negative segregation component starting at the latency of P1 and extending until the end of N2 characterized the tVEPs, without significant differences among the 4 stimulus conditions. In the presence of local/global collinearity, we found an early modulation of N1 amplitude. This modulation was orientation-dependent, as vertical collinearity increased N1 negativity and horizontal collinearity reduced N1 negativity. Conclusions: Our experiment confirms previous findings about the segregation negativity, which may depend on contextual modulation of V1 neurons by long-range horizontal and feed-back connections. The early effect of collinearity may depend on more local modulatory connections. q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Science Ireland Ltd. All rights reserved. Keywords: Visual evoked potentials; Texture; Collinearity; Orientation; Segregation 1. Introduction Some regions of the visual world are perceived preattentively, i.e. effortlessly and without scrutiny. They are said to ‘pop-out’ from their environment due to some peculiar feature that differentiates them from their neigh- boring elements. Neurophysiological studies have shown that this differentiation or ‘segregation’ of the visual image into functionally coherent, global objects is achieved not only by local detectors of feature gradients, but also by ‘contextual modulation’ of the activity of large ensembles of V1 neurons, whose receptive fields are adjacent (Knierim and Van Essen, 1992), but also far away from the objects’ edges (Lamme, 1995; Zipser et al., 1996). Lamme (1995) demonstrated that the firing of single neurons, as stimulated by identical elements in their classical receptive field (Hubel and Wiesel, 1959), was different according to whether the elements belonged to a ‘figure’ or to the background. This widespread neuronal involvement in the processing of figure/ground perception has justified the use of surface- recorded visual evoked potentials (VEPs) in studies that explore different aspects of preattentive perception in man unintrusively. The most widely adopted stimulus for these experiments consists of textures, which are not only a common feature of the visual world, but which are also ideal candidates for psychophysical and neurophysiological experiments, as they can be easily and independently manipulated at both a local and global level (Julesz, 1981). The main and common finding of all these experiments (Bach and Meigen, 1992, 1999; Caputo et al., 1999; Lamme et al., 1992, 1993; Romani et al., 1999) is that the presence of segregated elements is associated with a negative com- ponent. This ‘segregation negativity’ is superimposed on the ‘low-level’ texture VEP (tVEP) and occurs in a latency 1388-2457/03/$30.00 q 2003 International Federation of Clinical Neurophysiology. Published by Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S1388-2457(03)00075-0 Clinical Neurophysiology 114 (2003) 1021–1026 www.elsevier.com/locate/clinph CLINPH 2002688 * Corresponding author. Tel.: þ 39-0382-380224; fax: þ 39-0382- 380286. E-mail address: alfredo.romani@mondino.it (A. Romani).