Neuroscience Letters 541 (2013) 83–86 Contents lists available at SciVerse ScienceDirect Neuroscience Letters jou rn al h om epage: www.elsevier.com/locate/neulet Motor inhibition of return can affect prepared reaching movements C.D. Cowper-Smith a , G.A. Eskes b , D.A. Westwood c,∗ a Department of Psychology and Neuroscience, Dalhousie University, 1355 Oxford Street, Halifax, Nova Scotia, Canada, B3H 4J1 b Department of Psychiatry, Dalhousie University, 5909 Veterans Memorial Lane, Halifax, Nova Scotia, Canada, B3H 2E2 c School of Health and Human Performance, Dalhousie University, 6230 South Street, Halifax, Nova Scotia, Canada, B3H 3J5 h i g h l i g h t s ◮ Previous work suggests IOR results from sensory/attentional or motor programming processes. ◮ We show that motor IOR affecting reaching can arise from response execution processes. ◮ Our result confirms that motor IOR can be observed outside of the oculomotor system. a r t i c l e i n f o Article history: Received 14 October 2012 Received in revised form 1 February 2013 Accepted 14 February 2013 Keywords: Inhibition of return Motor control Reaching Execution Attention a b s t r a c t Inhibition of return (IOR) is a widely studied phenomenon that is thought to affect attention, eye move- ments, or reaching movements, in order to promote orienting responses toward novel stimuli. Previous research in our laboratory demonstrated that the motor form of saccadic IOR can arise from late-stage response execution processes. In the present study, we were interested in whether the same is true of reaching responses. If IOR can emerge from processes operating at or around the time of response execution, then IOR should be observed even when participants have fully prepared their responses in advance of the movement initiation signal. Similar to the saccadic system, our results reveal that IOR can be implemented as a late-stage execution bias in the reaching control system. © 2013 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Inhibition of return (IOR) refers to a delay in responding to visual targets appearing in a location previously occupied by a cue or another target, and is typically observed when the time between the onset of the two stimulus events is greater than approximately 300 ms. Early accounts of the phenomenon ascribed IOR to the pres- ence of an inhibitory mechanism that discourages the return of eye movements, spatial attention, or both to recently attended loca- tions, perhaps to increase the efficiency of visual search behavior [8,10,21]. Further research indicated that a motor form of IOR can be observed [4,7,14,19,21], for example, when consecutive responses are signaled by central stimuli, an observation that cannot easily be explained by sensory or attentional mechanisms [6,18,19,21]. Using central stimuli, the motor form of IOR has been reported for saccadic eye movements and more recently, reaching movements [e.g.,3, 14, 19, 20]. While evidence for IOR is usually based on ∗ Corresponding author. Tel.: +1 902 494 1164. E-mail addresses: David.Westwood@DAL.CA, ccowpers@gmail.com (D.A. Westwood). reaction time (RT), this measure alone cannot reveal the stage at which IOR arises in the stimulus-response sequence. In a recent experiment with saccadic eye movements [4], we demonstrated that the motor form of IOR can arise from processes operating at or around the time of response execution; even when participants could prepare a saccadic movement in advance, the execution of that response was delayed when it was preceded by a saccade in the same direction compared to a saccade in the opposite direction. Although the motor form of IOR can arise from late-stage execution processes within the saccadic control system, it remains unclear whether IOR can be similarly implemented at this late stage within the reaching control system. This question is important for gaining a clearer understanding of the mechanism(s) underlying IOR. For example, if IOR operates as a late-stage execu- tion bias only within the oculomotor system, it would suggest the presence of a relatively specialized or unique mechanism designed to influence eye movements; alternatively, if IOR can operate as a late-stage execution bias in multiple effector systems, it would suggest the presence of a more generalized mechanism. In the present investigation, we therefore examined whether motor IOR can be observed in late-stage response execution processes when reaching, rather than saccadic responses are required. If IOR was present, we expected to observe the defining pattern of IOR, where 0304-3940/$ – see front matter © 2013 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neulet.2013.02.033