Cerebral regions processing first- and higher-order motion in an opposed-direction discrimination task P. Dupont, 1,2 G. Sa ´ ry, 1, H. Peuskens 1 and G. A. Orban 1 1 K.U.Leuven, Laboratorium voor Neuro- en Psychofysiologie; Campus Gasthuisberg, B-3000 Leuven, Belgium 2 Centrum voor Positron Emissie Tomografie, Departement Nucleaire Geneeskunde, UZ Gasthuisberg, Herestraat 49, B-3000 Leuven, Belgium Keywords: attention, functional imaging, human, vision Abstract Using PET, we studied the processing of different types of motion in an opposed-direction discrimination task. We used first-order motion and two types of higher-order motion (presented as moving gratings with stripes defined by flickering texture and kinetic boundaries, respectively). In these experiments, we found that all types of motion activate a common set of cortical regions when comparing a direction discrimination task to a detection of the dimming of the fixation point. This set includes left hV3A, bilateral hMT/ V5þ and regions in the middle occipital gyrus, bilateral activations in the posterior and anterior parts of the intraparietal sulcus, bilateral precentral gyrus, medial frontal cortex and regions in the cerebellum. No significant differences were observed between different types of motion, even at low statistical thresholds. From this we conclude that, under our experimental conditions, the same cerebral regions are involved in the processing of first-order and higher-order motion in an opposed-direction discrimination task. Introduction Motion processing is a fundamental capacity of the visual brain and one which has attracted wide interest among psychologists and neuroscientists. Ever since the early perceptual studies on motion perception, it has been suggested that there are multiple mechanisms for motion computation (Braddick, 1974). One theory which has received considerable attention is the proposal that, in addition to a luminance-based first-order mechanism, there is a higher-order mechanism for processing stimulus elements not represented by Four- ier components (Chubb & Sperling, 1988). It is unclear at present to what extent these two mechanisms are distinct at the cortical level. Single-cell studies in cat and monkey visual cortex suggest that there is no distinction, even at early levels in the visual cortex. Neurons in near- extrastriate cortex respond to both first- and higher-order motion, but on average respond much more vigorously and more selectively to first-order than to higher-order motion (Albright, 1992; Zhou & Baker, 1993; O’Keefe & Movshon, 1998). In contrast, lesion studies in patients have provided some support for separate motion mechanisms (Plant et al., 1993; Vaina et al., 1999). Other reports have stressed the similarity of the lesion effects on first- and higher-order motion processing (Greenlee & Smith, 1997; Nawrot et al., 2000). A first attempt to image the visual cortical regions processing first- and second-order motion in humans was made by Smith et al. (1998). These authors compared the activation by second-order motion to that elicited by either first-order motion or by the second-order pattern when stationary. Only a full factorial design, however, can disentangle the effect of stimulus type and motion. Initially we used a factorial design with these two factors (motion and stimulus type) under passive conditions, as used by Smith et al. (1998). These pilot studies were disappointing: motion activation for higher-order stimuli was reduced compared to that of first-order stimuli and this decrease could be accounted for by the difference in saliency between stimuli. Therefore, in the main experiments we used a paradigm in which subjects had to perform an opposed- direction discrimination task (as in patient studies) using first- or higher-order stimuli. As a control task we used a detection task in which an identical visual stimulus was presented but the task required processing a different attribute. Materials and methods Subjects Twenty-two male volunteers (aged between 19 and 38 years) partici- pated in one of the two main PET experiments (n 1 ¼ 14, n 2 ¼ 8). All subjects were strictly right-handed as judged by the Edinburgh inven- tory, were drug-free, had no neurological or psychiatric history and had normal or corrected-to-normal vision and a normal brain structure as visualized with MRI. All scanning procedures were undertaken with the understanding and written consent of each participating subject, in accordance with the Declaration of Human Rights, Helsinki 1975. The Ethical Committee of the medical school, Katholieke Universiteit Leuven, has approved the study. Stimuli Stimuli were square-wave gratings (diameter 58, stripe width 18) presented in central vision on a Philips Brilliance 2120 monitor (70 Hz) hosted by a 486 TIGA workstation. The monitor was mounted above the scanner bed at an angle of 528 relative to the horizontal. The sub- jects viewed the display binocularly in a dimly lit room (0.35 cd/m 2 ) at a distance of 114 cm. We used three types of gratings: (i) a luminance- European Journal of Neuroscience, Vol. 17, pp. 1509–1517, 2003 ß Federation of European Neuroscience Societies doi:10.1046/j.1460-9568.2003.02571.x Correspondence: Professor G.A. Orban, as above. E-mail: Guy.Orban@med.kuleuven.ac.be  Home affiliation: Department of Physiology, University of Sze ´ged, Sze ´ged, Hungary Received 19 September 2002, revised 21 January 2003, accepted 30 January 2003