, 2012). The efficient operation of the saccade system

depends on the ability to exert voluntary control (an endogenous process) over the automatic response to sensory events (an exogenous process). The antisaccade and memory-guided saccade tasks, which have traditionally been used to investigate saccade initiation in PD, involve competition between contradictory processes: subjects must simultaneously suppress and generate an CHIR-99021 clinical trial eye movement. This makes it difficult to establish the origin of impairments in these tasks. To clarify the effect of PD in the saccade system, we elected to use a saccade task that allows the separate measurement of endogenous and exogenous processes in the saccade system and that does

not require suppression of saccades. We adapted a well-known task (Deubel, 2008), in which saccades can be performed with or without a concurrent perceptual discrimination task. Participants are instructed to make a voluntary saccade to a peripheral target location, which HKI-272 mw is indicated by a central arrow cue. Shortly after the onset of the arrow cue, before the saccade is initiated, symbols can appear briefly at the target location and at distractor locations. After each saccade, observers are asked to report the identity of the symbol that appeared at the target location. It has been shown that the concurrent performance of a discrimination task can facilitate saccade initiation (Montagnini & Chelazzi, 2005; Trottier & Pratt, 2005). The brief, pre-saccadic, peripheral symbol-changes can also modulate saccade latencies in this Urease paradigm (Deubel, 2008; van Stockum et al., 2011a). The effect of the discrimination task can be attributed to endogenous processes, because it is due solely to the task instructions and the observer’s intention. The effect of the peripheral symbol-changes can be attributed to exogenous processes, because it is due solely to a change in visual input. When a group of PD patients

and a control group performed reflexive (visually guided) saccades in a variant of this paradigm, the discrimination task reduced saccade latencies more in the PD group than in the control group (van Stockum et al., 2011b). This observation is consistent with reports of hyper-reflexivity in PD (Chan et al., 2005; van Stockum et al., 2008; van Koningsbruggen et al., 2009; Cameron et al., 2012). Moreover, the discrimination task facilitated saccade initiation in the PD group especially in trials with an overlap, where the ongoing presence of the central fixation point (the overlap) had a smaller inhibitory effect in the PD group than in the control group. We suggested therefore that the discrimination task reveals a source of abnormal endogenous saccadic facilitation in PD, which may affect the saccade system globally (van Stockum et al., 2011b).