Please cite this article in press as: Chapman, C. S., et al. Mental blocks: fMRI reveals top-down modulation of early visual cortex when obstacles interfere with grasp planning. Neuropsychologia (2011), doi:10.1016/j.neuropsychologia.2011.02.048 ARTICLE IN PRESS G Model NSY-4046; No. of Pages 15 Neuropsychologia xxx (2011) xxx–xxx Contents lists available at ScienceDirect Neuropsychologia journal homepage: www.elsevier.com/locate/neuropsychologia Mental blocks: fMRI reveals top-down modulation of early visual cortex when obstacles interfere with grasp planning Craig S. Chapman a , Jason P. Gallivan b , Jody C. Culham a,b , Melvyn A. Goodale a,b,∗ a Department of Psychology, University of Western Ontario, London, Ontario N6A 5C2, Canada b Neuroscience Program, University of Western Ontario, London, Ontario N6A 5C2, Canada article info Article history: Received 26 August 2010 Received in revised form 27 January 2011 Accepted 23 February 2011 Available online xxx Keywords: fMRI Grasping Obstacle interference Top-down modulation Intraparietal sulcus Visual cortex abstract When grasping an object, the fingers, hand and arm rarely collide with other non-target objects in the workspace. Kinematic studies of neurological patients (Schindler et al., 2004) and healthy participants (Chapman and Goodale, 2010a) suggest that the location of potential obstacles and the degree of inter- ference they pose are encoded by the dorsal visual stream during action planning. Here, we used a slow event-related paradigm in functional magnetic resonance imaging (fMRI) to examine the neural encoding of obstacles in normal participants. Fifteen right-handed participants grasped a square target object with a thumb-front or thumb-side wrist-posture with (1) no obstacle present, (2) an obstacle behind the target object (interfering with the thumb-front grasp), or (3) an obstacle beside the tar- get object (interfering with the thumb-side grasp). Within a specified network of areas involved in planning, a group voxelwise analysis revealed that one area in the left posterior intraparietal sulcus (pIPS) and one in early visual cortex were modulated by the degree of obstacle interference, and that this modulation occurred prior to movement execution. Given previous reports of a functional link between IPS and early visual cortex, we suggest that the increasing activity in the IPS with obsta- cle interference provides the top-down signal to suppress the corresponding obstacle coding in early visual areas, where we observed that activity decreased with interference. This is the first concrete evidence that the planning of a grasping movement can modulate early visual cortex and provides a unifying framework for understanding the dual role played by the IPS in motor planning and attentional orienting. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Even after the visuomotor system has solved the difficult prob- lem of selecting a specific target from the many that occupy our cluttered environment, non-target objects can function as obstacles that significantly alter the trajectory of the movement (Chapman & Goodale, 2008, 2010a, 2010b; Mon-Williams, Tresilian, Coppard, & Carson, 2001; Tresilian, 1998, 1999). This suggests the brain must flexibly encode objects to both attract and repel move- ments depending on whether the object is a potential target or a potential obstacle. Studies of patients with damage to the dorsal visual stream (Goodale & Milner, 1992) have implicated the posterior parietal cortex as being critical for obstacle encod- ing (McIntosh, McClements, Dijkerman, Birchall, & Milner, 2004; McIntosh, McClements, Schindler, et al., 2004; Rice et al., 2008; ∗ Corresponding author at: Department of Psychology, University of Western Ontario, London, Ontario N6A 5C2, Canada. Tel.: +1 519 661 2111x82070; fax: +1 519 661 3961. E-mail address: mgoodale@uwo.ca (M.A. Goodale). Rice, McIntosh, et al., 2006; Schindler et al., 2004). Specifically, optic ataxic patients with damage to dorsal stream structures show significantly less deviation away from obstacles than normal partic- ipants (Schindler et al., 2004). Using functional magnetic resonance imaging (fMRI), we sought to identify whether the dorsal stream is also involved in encoding obstacles in normal individuals. Studying real actions in the MRI environment is difficult due to the spatial constraints and the artifacts introduced by hand motion (Culham, 2006). To overcome these difficulties we adapted an obstacle task that interferes with grasping movements (which are easier to perform with less space, Tresilian, 1998) for use in a slow event-related paradigm. fMRI paradigms using delay periods have isolated planning responses in eye-movement tasks (Curtis, Cole, Rao, & D’Esposito, 2005; Curtis & Connolly, 2008; Curtis & D’Esposito, 2006; Curtis, Rao, & D’Esposito, 2004; Ikkai & Curtis, 2008) and have demonstrated preparatory activity in attentional cueing paradigms (Bressler, Tang, Sylvester, Shulman, & Corbetta, 2008; Kastner, Pinsk, De Weerd, Desimone, & Ungerleider, 1999; Serences, Yantis, Culberson, & Awh, 2004; Sylvester, Jack, Corbetta, & Shulman, 2008). Importantly, separating instructions with delay periods has also been used to isolate movement planning responses 0028-3932/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2011.02.048