Conclusions and future direction

The studies presented in this thesis contribute to the field of tactile attention with novel behavioural and ERP findings. This thesis presents studies which for the first time

investigated the neural correlates of exogenous tactile attention and IOR. The first study of this thesis (Chapter II) demonstrated that exogenous irrelevant stimuli affects response times during an endogenous task. However, the two mechanisms did not interact suggesting the two orienting processes operate independently, at least during simple task demands. All ERP studies demonstrated an ADAN waveform in the cue-target interval in response to the unilateral cues. That is, enhanced negativity for the hemisphere contralateral to the stimulated hand. None of the exogenous tasks demonstrated an LDAP. This suggests that exogenous attention and IOR, at least in the present studies, is somatotopically coded. Adding endogenous orienting to unilateral cue (i.e. lateralized cues indicated the most likely target location instead of just acting as distractors; endogenous predictive task – Chapter V) increased the ADAN effect. Furthermore, the ADAN component was eliminated when the exogenous and endogenous orienting were in conflict (endogenous counter-predictive task – Chapter V). These results suggest the ADAN waveform can be made up of both endogenous and stimulus driven effects. An LDAP was only present during endogenous attention and when orienting was shifted from one side to the other.

A main conclusion to be drawn from the ERPs in the post-target time window is that not one component was demonstrated to be directly linked to IOR. The P100 component, which has in vision been proposed to reflect IOR processing, was not consistently modulated by attention when IOR was present in the studies across this thesis. The N80 was consistently modulated by attention with enhanced negativity for invalid compared to valid trials. Although not directly linked to behaviour, this modulation likely reflects early effects of exogenous rather than endogenous attention. The later components – N140 and Nd – showed to be directly linked to endogenous behavioural effects. This was demonstrated by significant correlations between response time and ERP effects in both the endogenous predictive and counter-predictive tasks (Chapter V). The perceptual load study (Chapter VI) demonstrated that increasing load leads to irrelevant stimuli being filtered out, as demonstrated by behavioural effects. The somatosensory evoked ERPs demonstrated the locus of selective attention to occur relatively early. The P100 of the irrelevant stimuli (the cue) was significantly smaller in the high versus the low perceptual load task. It can be concluded that varying visual perceptual load clearly affects tactile processing, demonstrated through both behavioural and ERP effects.

Both endogenous and exogenous versions of the Posner paradigm (Posner, 1978, 1980) were use in the studies presented in this thesis. The primary reason for choosing this paradigm was to provide results comparable with previous research. Previous ERP studies investigating visual IOR have employed a similar paradigm with peripheral cues and targets. Within tactile attention research the Posner paradigm is the most widely used method studying endogenous and exogenous attention. However, the cue-target paradigm has limitations. The exogenous nature of the tactile stimuli can be debated. The exogenous cue is arguably not the same as an unexpected tap on the shoulder which exogenous stimuli often is compared to. The cue in the cue-target paradigm is certainly to some degree expected. In the experiments in this thesis it appears at either one of only two locations. It may therefore be well founded to question whether results using a Posner paradigm in a laboratory setting reflects unexpected exogenous stimuli as it appears in the real world (see Kingstone, Smilek, Ristic, Kelland Friesen, & Eastwood, 2003, for further criticism of the Posner cueing paradigm). Future research may want to explore other paradigms investigating the neural correlates of exogenous tactile attention. For example, using paradigms where the stimuli are less expected which may be achieved by increasing the possible locations where stimuli could appear. The more locations should decrease the expectancy of exogenous stimuli and therefore better reflect exogenous attention as it may appear in a more natural setting.

A next stage would be to investigate how exogenous attention is processed across modalities. Behavioural studies have demonstrated crossmodal links for IOR (Ferris et al., 2008; Spence, Lloyd, et al., 2000). ERP studies have explored cross-modal links using endogenous attention paradigms (e.g., Eimer & Driver, 2000). However, no study has investigated the ERPs following exogenous cross-modal studies involving touch. It may be of particular interest to explore the cue-target interval during cross-modal exogenous orienting. Based upon endogenous attention research, the ADAN has been suggested to reflect a supramodal attentional control mechanisms (Seiss et al., 2007). Particularly interesting may be for future research to explore the cue-target waveforms in an exogenous cross-modal setting.

Continuing research into the neural correlates of tactile processing and attention may be of great practical and medical benefit. Medically unexplained symptoms (MUS) – symptoms lacking in any physical explanation - are common in medical settings (Nimnuan, Hotopf, &

Wessely, 2001). It has been proposed that disturbances in attention play a crucial role in MUS and chronic pain states (Brown, 2004; Crombez, van Damme, & Eccleston, 2005; Deary, Chalder, & Sharpe, 2007). Recently, Brown et al. (2010) demonstrated that people high in somatoform dissociation did not demonstrate the same cueing pattern as people with low somatoform dissociation. Specifically, people scoring high in somatoform dissociation did not demonstrate IOR in a tactile discrimination task (similar paradigm to that presented in Chapter III). Thus, emerging behavioural evidence suggest particular clinical populations demonstrate impaired processing using such cue-target paradigms as presented in this thesis. To map out the neural correlates of tactile attention may therefore benefit diagnosis of MUS. Before the leap of using ERPs as a diagnostic application can be made, it is imperative for future research to continue to map out reliable ERP patterns relating to exogenous and endogenous attention.

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