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Abstract When the interval between a warning signal (WS) and an imperative signal (IS), termed the foreperiod (FP), is variable across trials, reaction time (RT) to the IS typically decreases with increasing FP length. Here we examined the auditory filled-FP effect, which refers to a performance decrement after FPs filled with irrelevant auditory stimulation compared to FPs without additional stimulation. According to one account, irrelevant stimulation distracts individuals from processing time and probability information during the FP (distraction-during-FP hypothesis). This should predominantly affect long-FP trials. Alternatively, the filled-FP effect may arise from a failure to shift attention from FP modality to IS modality (attention-to-modality hypothesis). The first hypothesis focuses on preparatory processing, predicting a selective RT increase on long-FP trials, whereas the second hypothesis focuses on target processing, only predicting a global RT increase irrespective of FP length. Across four experiments, a filled-FP (compared to a blank-FP) condition consistently yielded a selective RT increase on long-FP trials, irrespective of FP–IS modality pairing. This pattern of results contradicts the attention-to-modality hypothesis but corroborates the distraction-during-FP hypothesis. More generally, these data have theoretical implications by supporting a multi-process view of temporal preparation under time uncertainty.

. Introduction Time given to prepare a speeded response to an imperative signal (IS) generally improves performance in reaction-time (RT) tasks (Hackley, 2009 and Rolke and Ulrich, 2010). In experiments on effects of temporal preparation, a warning signal (WS) typically announces the start of a trial, which is followed by a blank interval (i.e., the foreperiod, FP), and the IS (Los & Schut, 2008). Individuals are assumed to establish a state of nonspecific preparation during the FP interval in order to optimally process task-relevant information and respond to the IS at the moment of its occurrence (i.e., at the imperative moment). With constant FPs, individuals can synchronize peak preparation with the imperative moment (i.e., the moment of IS occurrence). When, however, FP varies randomly across trials, deterministic synchronization is impossible. That is, under time uncertainty, probability information needs to be processed in addition to time estimation. Responses in such variable-FP paradigms are usually slow in short-FP trials but fast in long-FP trials, yielding a downward-sloping FP–RT function, which is explained by assuming that the time elapsed after the WS is informative, since the conditional probability of IS occurrence monotonously increases during the FP interval (Niemi & Näätänen, 1981, pp. 137–141). Researchers agree that individuals must somehow be capable to convert the objective conditional-probability increase into a subjective expectation; yet, the precise mechanism is still being debated (Bueti et al., 2010, Los and Van den Heuvel, 2001 and Vallesi, Shallice and Walsh, 2007). 1.1. Theoretical models of temporal preparation A strategic account assumes that individuals use the WS as a symbolic time marker to begin with focusing on the task, from which they actively monitor the time flow during the FP interval and increase preparatory state according to the time-related increase in the conditional probability of IS occurrence (Näätänen, 1970 and Rabbitt and Vyas, 1980). Accordingly, manipulations that change the conditional IS probability (e.g., Los and Agter, 2005 and Requin and Granjon, 1969) or explicit information about the impending imperative moment at the beginning of a particular trial (e.g., Correa, Cappucci, et al., 2010, Coull et al., 2000 and Coull and Nobre, 1998) are predicted to cause a change in the FP–RT slope. Strategic preparation is considered to require cognitive control for monitoring conditional IS probability (Requin and Granjon, 1969 and Stilitz, 1972), for shielding against distraction (Dreisbach and Haider, 2008 and Dreisbach and Haider, 2009), and for intentionally enhancing preparatory state (Näätänen & Merisalo, 1977). The critical variable thus is the availability of attentional resources, ensuring the normal operation of preparatory processing at any time during the FP interval. A strategic model implies that when resources are reduced for some reason (e.g., due to insufficient attention, high cognitive load, etc.), these processes should operate less efficiently, and performance thus is predicted to decline under these conditions. This classic account, however, cannot appropriately explain the typical sequential modulation of the FP–RT slope across subsequent trials. In particular, responses on short-FP trials are slower when preceded by a long-FP trial, compared to when preceded by an equally long or shorter one. The effect is asymmetric in that responses only vary on short-FP trials and are unaffected by previous FP length on long-FP trials (e.g., Alegria, 1975a, Karlin, 1959, Langner, Steinborn, et al., 2010, Los et al., 2001, Los and Van den Heuvel, 2001, Steinborn et al., 2008, Steinborn et al., 2009, Steinborn et al., 2010 and Van der Lubbe et al., 2004). A further argument that imposes difficulties for the classic view is that the asymmetry of the sequential FP effect decreases when sensory WS features changes across trials (Steinborn et al., 2009 and Steinborn et al., 2010), indicating that the WS is more than a symbolic marker and also acts as a memory retrieval cue. Vallesi and his collaborators (Vallesi et al., 2009, Vallesi and Shallice, 2007 and Vallesi, Shallice and Walsh, 2007) developed the classic strategic explanation of the variable-FP effect into a dual-process model, which can account for the sequential FP effect. Maintaining the idea of a strategic preparatory process based on conditional-probability monitoring, the sequential FP effect is assumed to arise from a trial-to-trial variation in motor excitation due to the variable spacing (i.e., the temporal distance) of two subsequent responses (Vallesi, Mussoni et al., 2007). That is, responses on short-FPn trials are assumed to be facilitated when following a short-FPn − 1 trial, due to an increase in the motor-activation level. In contrast, responses on short-FPn trials are slowed when following a long-FPn − 1 trial, due to a decrease in the motor-activation level. On long-FPn trials, however, responses are fast, irrespective of FPn − 1, because the motor-activation decrement following long-FPn − 1 trials is compensated by strategic preparation based on conditional-probability monitoring. According to this view, the asymmetry of the sequential FP arises from the combined impact of two different processes: an originally symmetric sequential effect, resulting from different residual activation levels produced by prior responses, is rendered asymmetric by a selective probability-based preparation process during a long-FPn trial. Recently, strategic accounts were challenged by a trace-conditioning model, developed by Los and colleagues (Los and Heslenfeld, 2005, Los et al., 2001 and Los and Van den Heuvel, 2001). This model accounts fo the variable-FP effect and its sequential modulation (i.e., the sequential FP effect) by arguing that the former results from the asymmetry inherent in the latter. In particular, states of peak preparation at critical moments are assumed to be attained by dynamic learning and re-learning of temporal intervals. Elapsed time during the FP is represented as a sequence of time-tagged events (Los et al., 2001, p. 128), with each event capable of being associated with features from external stimuli, internal representations, and responses. The model resembles other trace-conditioning models in related domains, which similarly assume that discrete events along a time line activate each other until target occurrence (e.g., Desmond and Moore, 1991, Dickinson, 1980, Machado, 1997, Moore et al., 1998 and Sutton and Barto, 1981). The WS event is considered to act as a retrieval cue that automatically initiates an activation cascade along this sequence until the IS occurs (Steinborn et al., 2009 and Steinborn et al., 2010). When the IS occurs, an associative link is established between the IS and activated components on the event sequence, increasing the so-called response strength associated with that specific moment. The main predictions of the model are derived from three conditioning rules (Los & Van den Heuvel, 2001, p. 372): Response strength (i.e., preparedness) at a particular moment (1) increases when the IS occurs at that moment, due to excitatory reinforcement, (2) remains unchanged when the IS occurs earlier, and (3) decreases when the IS occurs later, due to extinction. Based on these rules, the model predicts fast responses on short-FP repetition trials, since response strength was reinforced at the same critical moment on the previous trial. Fast responses are also predicted to occur on short–long FP sequences, since the critical moment was not bypassed on the previous trial (and, thus, its previously acquired response strength was not reduced). Conversely, in long–short FP sequences, slow responses are predicted, since the critical moment was bypassed on the previous trial. In sum, the trace-conditioning model explains both the variable-FP effect and its sequential modulation by a set of rules governing associative trial-to-trial learning, which produce asymmetric sequential dependencies that – as a necessary “side-effect” – result in the well-known variable-FP effect. 1.2. Effect of irrelevant stimulation during foreperiods on temporal preparation As mentioned previously, the dual-process model (Vallesi & Shallice, 2007) points to the importance of attentional capacity for tracking time and probability information during the FP. Hence follows that any manipulation that effectively reduces capacity during FP should impair preparatory processing, which should manifest itself in a specific RT increase on long-FPn trials. Empirical support for this prediction is mainly derived from studies comparing group-related individual differences in cognitive-control functions. In particular, subgroups of individuals considered less capable to adequately implement and/or sustain cognitive control have been shown to exhibit a selective RT increase on long-FPn trials (yielding a flattening of the FPn–RT function), compared to matched normal controls. This has been shown for individuals with a variant of attention-deficit disorder (Zahn, Kruesi, & Rapoport, 1991), trait impulsivity (Correa, Trivino, Perez-Duenas, Acosta, & Lupiáñez, 2010), or patients with damage in the right dorsolateral prefrontal cortex (rDLPFC) (Trivino, Correa, Arnedo, & Lupiáñez, 2010). Vallesi, Shallice, and Walsh (2007) provided experimental evidence that the FPn–RT slope, but not the sequential FP effect, is reduced after inhibiting the rDLPFC with transcranial magnetic stimulation (TMS). According to Vallesi et al., decreasing rDLPFC functioning via TMS is equivalent to a reduction of attentional resources. Further, irrelevant stimulation during preparatory processing has been shown to interfere with RT performance in FP experiments. In a pioneering study, Terrell and Ellis (1964) examined temporal preparation in a simple-RT task as a function of concurrent irrelevant stimulation during the FP interval (FP length was 2, 4, 8, or 12 s). In one condition, a visual WS was presented for 1500 ms and followed by a standard (blank) FP until auditory IS presentation. In the other condition, the visual WS remained present after its onset for the entire FP interval. The authors found a global RT increase in the filled-FP compared to the blank-FP condition but no selective RT increase on long-FPn trials. Since the study mainly focused on sustained-attention differences between normal and individuals with mental retardation, it should be noted that normal individuals were more severely affected by the filled-FP condition than retarded ones. Baumeister and Wilcox (1969) replicated these results using an almost identical design. Again, the filled-FP condition yielded an additive RT increase (the filled-FP effect was also larger for normal than for individuals with mental retardation), while there was no interaction with FPn length. The filled-FP effect has also been examined in other studies, and mostly, an RT increase was also observed in normal individuals (e.g., Borst and Cohen, 1989, Cassel and Dallenbach, 1918, Hawkins and Baumeister, 1965 and Kellas and Baumeister, 1968). Accounts of the filled-FP effect have argued that stimulation during the FP interval produces a performance impairment by distracting individuals from maintaining the attentional focus on task processing over the FP. The larger RT increase in normal compared to retarded individuals was explained by a floor effect, assuming that retarded individuals are already deficient in maintaining attention so that no resources can be further “drawn off” by additional challenges (e.g., Baumeister and Wilcox, 1969 and Terrell and Ellis, 1964). This distraction-during-FP hypothesis is further corroborated by research on what is termed the irrelevant-sound effect on delayed-response performance (cf. Beaman, 2005, Jones and Macken, 1993, Macken et al., 2009 and Poulton, 1977). In particular, it has repeatedly been shown that concurrent auditory stimulation during task processing is highly intrusive (and, thus, obligatorily processed) and competes with cognitive task processing. Attempts to shield against irrelevant stimulation has been linked to an activation of the rDLPFC — a brain area involved in the maintenance of attention (cf. Hadlington et al., 2006, Shallice et al., 2008, Sturm and Willmes, 2001 and Vallesi, Shallice and Walsh, 2007). Given that irrelevant sound challenges right prefrontal maintenance networks (Campbell, 2005), a filled FP should also impair processes related to temporal preparation. From the perspective of the dual-process model (Vallesi and Shallice, 2007 and Vallesi, Shallice and Walsh, 2007), it could be argued that concurrent stimulation reduces attentional resources, which may impair individuals to maintain task focus during the FP interval. The degree to which resources are drawn off may depend on the salience of the stimulation (e.g., modality, intensity, or novelty), and it is likely that concurrent stimulation is most effective in the auditory modality (cf. Jones & Macken, 1993). Thus, from a dual-process perspective, an auditory filled-FP condition should effectively induce a selective RT increase on long-FPn trials, which should resemble the RT pattern obtained by Vallesi et al. (2007) applying TMS over the rDLPFC during temporal preparation. In the trace-conditioning model (Los & Van den Heuvel, 2001), sensory stimulation effects are not explicitly incorporated. One could, however, argue that if preparatory processing is fairly automatic, it should not be affected by irrelevant sound (cf. Langner, Steinborn, et al., 2010 and van Lambalgen and Los, 2008). 1.3. Present study The present study examined the effects of an auditorily filled FP on temporal preparation under time uncertainty (i.e., in variable-FP settings). Since applying concurrent stimulation might also be fecund for theorizing about preparation-related phenomena (Clark & Squire, 1999), studying the as-yet poorly understood nature and specific boundary conditions of filled-FP effects becomes even more worthwhile. Moreover, none of the previous studies employing filled FPs sufficiently analyzed sequential effects. To lessen this backlog, we identified critical variables that may account for previous findings and performance differences. As mentioned above, the standard explanation (i.e., the distraction-during-FP hypothesis) argues that irrelevant auditory stimulation during the FP distracts attention from temporal and conditional-probability monitoring (e.g., Terrell & Ellis, 1964). Alternatively, the filled-FP effect may be related to impaired target processing, that is, it could simply arise from a failure to shift attention from the auditory input during FP to the (visual) IS modality. This explanation has also been offered in previous research, particularly to explain additive RT increases (Kellas & Baumeister, 1968). Indeed, recent research has shown that the need for stimulus-driven shifts of attention between sensory modalities (across subsequent trials) in speeded stimulus detection induces behavioral costs as well as additional brain activity, as compared to no-shift conditions (Langner et al., 2011, Quinlan and Hill, 1999 and Spence et al., 2001). We will refer to this hypothesis as attention-to-modality hypothesis. Thus, the standard explanation focuses on preparatory processing, predicting a selective RT increase on long-FP trials, whereas the alternative explanation focuses on target processing, predicting a global RT increase. To test our predictions, four experiments were conducted in which a blank-FP and a filled-FP condition were compared between blocks of trials in a variable-FP paradigm. Across experiments, we used a two-choice RT task with three equiprobable FPs (300, 600, and 900 ms). As alluded to above, according to the dual-process view, supervisory monitoring during FPs depends on attentional resources, and reductions of applicable resources are predicted to affect the FPn–RT slope. According to a trace-conditioning view, the time-tagged event sequence is processed preattentively, and the dynamic associative learning (and re-learning) of temporal contingencies occurs unintentionally. Thus, a decrease in the FPn–RT slope by irrelevant sound would be consistent with the dual-process view, but would provide a challenge for the trace-conditioning view. However, since we cannot exactly determine whether irrelevant sound during the FP is also capable to impair preattentive processing (cf. Greenwald, 1970 and Poulton, 1977), any modulations of the FPn–RT slope may not be taken as strong evidence against the trace-conditioning model. Before applying the filled-FP effect to “test” competing models of variable-FP phenomena, we, therefore, have to determine the nature and specific boundary conditions of the filled-FP effect.