General discussion

The first study presented in this chapter investigated the use of bilateral tactile cues to induce endogenous orienting. In both a detection and discrimination task participants responded significantly faster on valid compared to invalid trials (Figure 2.1.2). Suggesting that participants were able to endogenously attend to the location predicted by the bilateral cue. A key element of Study 1 was to establish a viable tactile cue to direct attention to a tactile location, without also eliciting exogenous cueing effects and/or influences from other modalities. Previous studies of tactile attention have been unable to disentangle endogenous and exogenous effects due to the use of unilateral cues which may evoke both attention mechanisms (Cohen et al., 2005; Lloyd et al., 1999, although see: Chica et al., 2007; Forster & Gillmeister, 2011); and Chapter V for alternative cueing methods), or using vision to direct attention to tactile targets (Forster & Eimer, 2005; Posner, 1978; Spence, Pavani, et al., 2000). Expanding the findings from Study 1 using these bilateral cues, the second study aimed to investigate the relationship between endogenous and exogenous attention. In Study 2 the endogenous effect from Study 1 was replicated in that participants were able to use the bilateral cues to facilitate RTs at the endogenously attended location. The endogenous facilitation effect was present at all SOAs between exogenous cue and target (250, 550, 850, & 1350 ms; see Figure 2.2.2.) demonstrating an endogenous cueing effect over 2 seconds long. The results from Study 2 showed that the exogenous cue only influenced RTs when the SOA was short (250 ms). Moreover, there was no interaction between endogenous and exogenous attention effects suggesting that endogenous and exogenous attention are separate mechanisms. In other words, the effect of

exogenous attention seen at the 250 ms SOA condition was the same regardless if the target appeared at the endogenously attended or unattended location.

The lack of interaction between endogenous and exogenous attention is partly in line with a similar double cueing study in the visual modality. Similar to the present results, Berger et al. (2005) found that endogenous orienting facilitated RTs at the attended location. Moreover, when there was a short SOA (100 ms) between the exogenous cue and target there was facilitation of exogenously valid targets. At longer SOA the participants’ demonstrated IOR, with longer RTs for valid compared to invalid trials. This biphasic pattern of results was the same across all their experiments involving target detection. Their results showed no interaction between the attention mechanisms, even when at long SOA, endogenous and exogenous attention demonstrated opposite effects. However, when they increased task demand and participants’ discriminated targets in contrast to simple target detection, an interaction at particular SOAs between the two attention mechanisms appeared. When targets were preceded by either the shortest (100 ms) or longest (1000 ms) SOA, Berger et al. found facilitation and IOR respectively for their exogenous cue. These effects did not interact with the endogenous attention effect. However, at intermediate SOAs (200 ms & 300 ms) there was no effect of exogenous orienting, but importantly, this interacted with endogenous attention. Berger and colleagues concluded that the more difficult discrimination task increased attentional resources required which led to the interaction of endogenous and exogenous attention. They further suggested that there was no interaction at the shortest SOA (100 ms) as endogenous and exogenous attention did not compete. Thus, at short SOA both mechanisms lead to facilitation of validly cued targets. The lack of interaction between endogenous attention and IOR at the long SOA was explained as IOR being an effect too robust to integrate with the endogenous process. A direct comparison between findings from vision research and the present Study 2 - investigating touch - should be made with caution as there are clear differences between the two modalities. For example, in an exogenous cue-target detection task there is an early facilitation period for validly cued targets in vision (SOA less than approximately 250 ms) before IOR becomes apparent. However, this facilitation period is not present in similar exogenous tactile detection tasks (e.g., Lloyd et al., 1999). If the target needs to be discriminated, then the results are more similar across the two modalities. In both vision (e.g., Lupiáñez et al., 1997) and touch (e.g., Miles et al., 2008) there is a biphasic pattern with early facilitation of exogenously valid targets, followed by IOR. The results of Study 2

demonstrated early facilitation period of exogenous valid over invalid trials (see Figure 2.2.2.). However, no inhibition period followed the early facilitation, hence, not demonstrating the biphasic behavioural pattern. Moreover, there was no interaction between endogenous and exogenous attention at the more intermediate SOAs which showed no effect of exogenous attention.

Berger et al. (2005) proposed the interaction between endogenous and exogenous attention appeared due to increased task demand in their discrimination task, and thus increased attentional load. The present second study also employed a discrimination task. Evidence that the present discrimination task was more difficult compared to a detection task also in touch can be taken from Study 1. This was demonstrated by increased RTs and errors for discrimination task compared to the simple target detection. However, it is possible that the discrimination task was not difficult enough in Study 2 to require endogenous and exogenous attention to interact. This simplicity may be indicated by the fact that the error rates were rather low, amounting to on average 5% of all trials. This is in line with the suggestion made by Berger et al. that when the task is simple (detection task in their study) the attentional resources are not exhausted and the two modes of orienting occur in separation, and they interfere only when task demands are higher. In other words, it is possible that the present tactile discrimination task was too simple to elicit any interaction between endogenous and exogenous attention.

Conversely, there was no indication that the longer SOAs in Study 2 elicited IOR. In contrast to the hypothesis that the lack of interaction between endogenous and exogenous attention effects was a result of the task being too easy, it has been suggested that easier discrimination tasks allows for more IOR (Cheal & Chastain, 1999). Moreover, the absence of IOR influences at the longer SOA contrasts recent tactile discrimination studies of exogenous attention. Brown and colleagues (Brown et al., 2010; Miles et al., 2008) demonstrated facilitation at early SOAs (150 ms and 350 ms), no difference at 540 ms, and IOR at 1000 ms. The 1350 ms SOA between exogenous cue and target in Study 2 is well within the time range previously demonstrated to elicit IOR in an exogenous discrimination task. Several possible hypotheses could account for the lack of IOR effect. It could be that endogenously orienting towards a tactile location eliminates and masks any IOR. This would contrast Berger et al.'s (2005) conclusion that IOR is inexorable and not affected by endogenous attention. It may also be possible that endogenously attending delays the

development of IOR beyond the longest SOA measured in the present task. In other words, in previous tactile exogenous discrimination tasks the IOR develops at around 1000 ms post cue onset (Brown et al., 2010; Miles et al., 2008). By including endogenous orienting in the task the additional attention resources required may delay the onset of IOR even further. A range of longer SOAs, above 1350 ms between exogenous cue and target would be required to investigate this hypothesis. A third possibility may be that endogenous attention is completely re-oriented during the time window between exogenous cue and target. Thus, the attention is initially drawn towards the exogenous cue. When the SOA is short (250 ms) there is not sufficient time for the endogenous attention to fully re-orient back to the attended location. This in turn leads to an effect of exogenous attention. At longer SOAs, the irrelevant cue may initially attract attention away from the endogenously attended location. However, there is sufficient time to fully re-orient covert endogenous attention back to the endogenous valid location, and eliminating any effects of exogenous attention.

Berger and colleagues (2005) proposed five different models which can account for the relationship between endogenous and exogenous attention (see introduction of this chapter). They concluded the model which fits their findings best is that endogenous and exogenous attention are separate mechanisms, at least until task demands and attentional load is increased at which point they interact. The results from the present study are in line with the conclusion of separate mechanisms for the two types of orienting also in touch. Whether endogenous and exogenous attention failed to interact due to the simplicity of the task remains unclear. However, increasing task difficulty in the discrimination task may provide an answer.