Stop Task After-Effects The Extent of Slowing During the Preparation and Execution of Movement Peter G. Enticott, 1 John L. Bradshaw, 1 Mark A. Bellgrove, 2 Daniel J. Upton, 1 and James R.P. Ogloff 1 1 School of Psychology, Psychiatry and Psychological Medicine, Monash University, Australia 2 School of Psychology and Queensland Brain Institute, The University of Queensland, Australia Abstract. In the stop task, response time to the go signal is increased when the immediately preceding trial involves the presentation of a stop signal. A recent explanation suggests that these ‘‘after-effects’’ are due to mechanisms that occur prior to the completion of response selection processes, but it is possible that they instead may reflect a slowed motor response (i.e., deliberate slowing after response selection). The participants completed a novel stop task that allows a differentiation between the time taken to prepare a movement (which incorporates response selection processes) and the time taken to execute a movement (i.e., speed of motor response). If mechanisms underlying stop task after-effects occur prior to the completion of response selection processes, then slowing should only occur during movement preparation. Movement preparation and execution time during go trials were analysed according to the characteristics of the preceding trial. Slowing after a stop trial was found during movement preparation time (regardless of inhibition success on that stop trial), and it further increased during this period when the primary task stimulus was repeated. There was also evidence for general after-effects during movement execution time, but no effect of repetition. These findings support the current theoretical accounts that suggest that repetition-based stop task after-effects are attributable to a mechanism that occurs prior to the completion of response selection processes, and also indicate a possible switch to a more conservative response set (as in signal detection theory terms) that results in deliberate slowing of movement. Keywords: stop task, after-effects, motor preparation, repetition priming, control adjustments The stop task concerns the inhibition of a response after it has been prepared or initiated (Logan, 1994; Logan & Cowan, 1984). While stop task performance is intended to measure behavioural inhibitory control, there are also after-effects associated with the stop task. Broadly speaking, response time to the go signal (trial n) is increased when the immediately preceding trial (trial n À 1) was a stop trial (i.e., involved presentation of the stop signal). This has been found both when the preceding trial involves successful inhibition (signal inhibit trial) (e.g., Kramer et al., 1992, cited in Emeric et al., 2007; Logan, 1994; Rieger & Gauggel, 1999) and also when it involves unsuccessful inhi- bition (signal respond trial) (e.g., Rieger & Gauggel, 1999; Verbruggen, Logan, Liefooghe, & Vandierendonck, 2008). After-effects are further increased when the consecutive tri- als (i.e., stop then go) share primary task stimulus properties (Rieger & Gauggel, 1999), analogous to the negative prim- ing situation. Verbruggen et al. (2008), however, report a stimulus repetition effect only for signal inhibit trials, and no signal inhibit after-effects where stimuli are not repeated. Multiple mechanisms have been proposed to explain these stop task after-effects. Initially, Rieger and Gauggel (1999) suggested that signal respond after-effects could reflect participant strategy and that signal inhibit after-effects (in which primary task stimulus properties were repeated) could reflect, at least in part, an inhibitory mechanism sim- ilar to that encountered in negative priming (Tipper, 2001). More recently, Verbruggen et al. (2008) conducted a series of experiments and concluded that two mechanisms are involved. Firstly, signal respond after-effects are thought to reflect between-trial control adjustments. According to this model, failed inhibition causes the individuals to increase their response threshold, adopting a more conservative response set, thereby improving the likelihood of successful stopping on the subsequent trial should the stop signal appear. This is somewhat analogous to the accounts of post-error slowing (Hajcak, McDonald, & Simons, 2003; Li, Huang, Constable, & Sinha, 2006; Rabbitt, 1966), and control adjustments are thought to occur prior to the presen- tation of trial n. Secondly, signal inhibit after-effects are thought to involve repetition-priming effects. This is some- what similar to negative priming (Tipper, 2001), and Verbruggen et al. cite a role for episodic retrieval (Neill, Valdes, Terry, & Gorfein, 1992). According to episodic retrieval theory, which has been proposed as an explanatory account of the negative priming effect (Neill et al., 1992), a stimulus that is associated with a forbidden or inhibited response is given a ‘‘do not respond’’ tag in memory. Where the next trial involves a go response to that same stimulus, this ‘‘do not respond’’ tag is retrieved and interferes with the (correct) response to the go signal. This interference sub- sequently results in a slowed response. This has been further supported in a recent study that found long-term stop task after-effects (i.e., in stimuli repeated between 1 and 20 trials Ó 2009 Hogrefe & Huber Publishers Experimental Psychology 2009; Vol. 56(4):247–251 DOI: 10.1027/1618-3169.56.4.247