Human intraparietal sulcus (IPS) and competition between exogenous and endogenous saccade plans Elaine J. Anderson, a,b, ⁎ Masud Husain, a,b and Petroc Sumner b,c a Institute of Cognitive Neuroscience, Alexandra House, 17 Queen Square, London WC1N 3AR, UK b Department of Clinical Neuroscience, Imperial College London, Charing Cross Campus, Fulham Palace Road, London W6 8RP, UK c School of Psychology, Cardiff University, Park Place, Cardiff CF10 3AT, UK Received 18 July 2007; revised 19 October 2007; accepted 30 October 2007 Available online 12 November 2007 How are stimulus-driven reflexes generated, and what controls their competition with voluntary action? The saccadic reflex to look towards an abrupt visual onset (prosaccade) has been associated with the retinotectal and magnocellular pathways, which rapidly convey signals to the superior colliculus and cortical eye fields. Such stimulus-driven reflexes need to be suppressed when making an eye movement in the opposite direction (antisaccade), resulting in a cost in saccade latency. We compared the latencies of pro- and anti-saccades elicited by conventional luminance stimuli with those evoked by stimuli visible only to short-wave-sensitive cones (S cones) embedded in dynamic luminance noise. Critically, the retinotectal and magnocellular path- ways are functionally blind to such stimuli. Compared to luminance stimuli, antisaccade latency costs were significantly reduced for ‘S- cone’ stimuli. This behavioural interaction is consistent with reduced competition between reflexive and endogenous saccade plans when S- cone stimuli are employed, while other processes involved in making an antisaccade, such as changing preparatory set or generating an endogenous saccade, are predicted to be equivalent for each kind of stimulus. Using fMRI, we found that activity in the right intraparietal sulcus (IPS) mirrored the behavioural interaction in saccade latencies. Thus, the right IPS appears to index the degree of competition between exogenous and endogenous saccade plans, showing the activity pattern predicted for an area involved in suppressing the saccade reflex. Furthermore, signals recorded from the superior colliculus showed the reverse pattern of responses, consistent with a direct inhibitory influence of IPS on SC. © 2007 Elsevier Inc. All rights reserved. Keywords: Antisaccades; Parietal eye fields; Inhibition Introduction Suddenly appearing objects tend to capture attention and often induce a fast eye-movement (saccade) towards them (Theeuwes et al., 1998). However, we are also able to suppress such reflexes and choose to make a saccade to a different location. Such flexible interplay between fast stimulus-driven behaviour and voluntary goal-directed behaviour remains at the heart of sensori-motor research. A favourite tool for investigating competition between reflexive and goal-directed action plans is the “antisaccade” task, in which the participant is required to make an eye movement away from a visual onset, instead of a “prosaccade” towards it (Hallet, 1978; Munoz and Everling, 2004). The saccadic latency for anti- saccades is generally longer than that for prosaccades, presumably because antisaccades require resolution of conflict between the reflexive urge to look at the stimulus and the endogenous plan to saccade away. In this study, we focus on the source of the senso- rimotor reflex and on the cortical areas that may be responsible for resolving the competition it causes with the required endogenous saccade. The source of the sensorimotor reflex Reflexive saccades are generally associated with the superior colliculus (SC) and cortical areas such as the lateral intraparietal area (LIP) and frontal eye-fields (FEF) (Findlay and Walker, 1999; Munoz and Everling, 2004; Sparks, 1986; Wurtz and Goldberg, 1972). Indeed, neuronal inhibition in these areas appears to be crucial for suppressing unwanted reflexive saccades (Everling et al., 1998; Everling and Munoz, 2000). But while saccadic motor control has been studied in detail, less is known about the source of the early visual signals that trigger reflexive movements. Visual onsets activate SC sensory cells via the retinotectal pathway and via signals from primary visual cortex (Sparks, 1986). These sensory cells may in turn activate saccade-generating cells in the SC (Isa, 2002), although this cannot fully explain collicular motor activity. Saccade-generating cells are also driven by projections from areas such as LIP and FEF, which themselves receive very short latency www.elsevier.com/locate/ynimg NeuroImage 40 (2008) 838 – 851 ⁎ Corresponding author. Institute of Cognitive Neuroscience, Alexandra House, 17 Queen Square, London WC1N 3AR, UK. E-mail address: e.anderson@fil.ion.ucl.ac.uk (E.J. Anderson). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2007.10.046