Endogenous and exogenous interaction

1.12.1 Double cueing-paradigm

Investigating how endogenous and exogenous attention interact has been explored using different paradigms. One way is to combine the two versions of the Posner paradigm into a double-cueing paradigm (see also Chapter II). This paradigm includes an endogenous and exogenous cue in the same trial. An endogenous cue initially indicates where to attend, for example using a central arrow directing attention to a peripheral location. When attention is engaged, an exogenous cue, which is irrelevant to the task, appears. Following the exogenous cue a target appears, typically to the same or opposite location to the exogenous cue. Early studies showed that when endogenous attention is fully engaged, the exogenous cue does not affect the behaviour (Theeuwes, 1991; Yantis & Jonides, 1990). In other words, engaging endogenous attention can lead to filtering out irrelevant stimuli from reaching our awareness. More recently, using a double-cueing paradigm van der Lubbe and Postma (2005) presented participants with a central arrow directing attention to one side or the other (endogenous cue), followed by a irrelevant peripheral flash (exogenous cue), and then a peripheral target. They demonstrated faster response times when the exogenous cue was at the same side as the target, thus suggesting the irrelevant stimuli captured attention, even when endogenous attention was engaged. Moreover, they found the same effect with an auditory exogenous cue indicating that irrelevant cues attract attention even in a highly focused state in a multisensory setting. However, that an exogenous cue can attract attention when the endogenous system is engaged does not necessarily indicate how the two mechanisms interact.

In a series of experiments, Berger, Henik, and Rafal (2005) investigated whether the two mechanisms interact or operate independently. They found endogenous cues to facilitate response times at attended compared to unattended location. Moreover, they found that the exogenous cue could elicit IOR for validly cued targets. Although these two effects were

opposite, meaning that the target was facilitated by endogenous attention and at the same time inhibited by IOR, they did not interact. In other words, the effect of exogenous attention was the same regardless of whether the target appeared at an endogenously attended or unattended location. However, this was only true when the target was to be detected. When they increased the task difficulty, Berger and colleagues demonstrated that endogenous and exogenous mechanisms interacted. They concluded that the attention mechanisms operate independently under low task demands. Increasing task demands leads to an interaction as the two types of attention compete for shared resources (see introduction to Chapter II for a more detailed discussion).

1.12.2 Assessing the automaticity of exogenous attention

The effects of task demand upon attention have also been researched using a dual task paradigm. Typically a participant will engage in one task and the ability of concurrently presented irrelevant stimuli will be measured to see whether they capture attention. Such a paradigm intuitively relates to situations commonly encountered in our everyday lives. It is easy to imagine how focusing on one task, for example reading this introduction, influences how well irrelevant stimulus such as a buzzing fly attracts our attention. It has been especially demonstrated that varying the attentional, perceptual and cognitive load in one task affects the ability for exogenous stimuli to enter our awareness (see Santangelo & Spence, 2008, for a review).

The load theory of selective attention suggests perception is a limited capacity process. The perception of exogenous stimuli will only proceed as long as sufficient attentional resources remain available (Lavie, 1995; Lavie, Hirst, de Fockert, & Viding, 2004). Under a high perceptual load condition where our attentional capacity is fully engaged in processing task relevant information, then there is no spare capacity to process irrelevant stimuli. On the contrary, when we engage in a task with low perceptual load, any capacity which has not been utilized in the relevant task is left over to automatically process task irrelevant stimuli. It should be highlighted that the load theory of attention distinguishes between perceptual and cognitive load. Increased perceptual load, as mentioned above, leads to decreased processing of irrelevant stimuli. However, cognitive load (sometimes also referred to as working memory load) has the opposite effect. That is, increased cognitive load in a central task (for example memorizing a larger set of numbers during a trial) leads to the increased distracting effect of irrelevant stimuli compared to during low

cognitive load (Lavie, 2005). This thesis will however focus on perceptual and attentional load rather than cognitive or working memory load.

The load theory of selective attention also provides a model for the longstanding debate of when selective attention occurs. Whether attentional selection affects the perceptual process at an early (Broadbent, 1958) or a late stage (Deutsch & Deutsch, 1963). In other words, early selection suggests that unattended stimuli are filtered out at an early stage of processing allowing only selected stimuli to be perceived and recognized. Late selection proposes that all incoming sensory events receive equal perceptual processing and attention operates at a late stage of processing to regulate information into our awareness. The load model incorporates both early and late views as it suggests that attentional selection is an adaptive filtering mechanism which is not fixed at early or late stages of processing. The bottleneck of selection varies according to the amount of concurrently presented information and attentional load. In other words, the theory proposes high load to filter out irrelevant stimuli early in the selection process. When load is low it result in a late selection process (Lavie, 1995; Lavie et al., 2004).

Santangelo and Spence (2007) investigated the effects of varying visual perceptual load upon the influence of irrelevant tactile stimuli (see Chapter VI for a similar paradigm). Tactile stimuli were delivered to the hands - an exogenous tactile cue to the left or right was followed by a target to the same or opposite hand (a typical tactile exogenous version of the Posner paradigm). In one task the participant simply had to respond to the tactile target whilst viewing a centrally located fixation cross. In a second task the fixation cross was replaced with a string of letters presented on a screen (a rapid serial visual presentation; RSVP). Embedded in the letters was a number which served as a visual target. The participant in this task had to detect a visual and tactile target. This dual task involved a higher load placing more demand on our endogenous attention. Santangelo and Spence found the exogenous tactile cue only influenced response times in the low load task (with no RSVP stream). When participants’ load was increased by including searching for a number embedded in a string of letters, then the exogenous cue had no effect upon response times. It was concluded that the exogenous cue was filtered out in the condition where perceptual processing demand was high. Employing an RSVP stream to manipulate perceptual load has also demonstrated similar effects where high load reduces the influence of how well irrelevant visual (Santangelo, Botta, Lupiáñez, & Spence, 2011) and auditor

stimuli (Santangelo, Ho, & Spence, 2008) capture attention. In other words, increased load may lead to filtering out exogenous stimuli.

1.12.3 To what extent are irrelevant stimuli filtered out during varying load?

In the behavioural studies, the reduced attention effect in the high compared to low load tasks is taken as evidence that the exogenous cue fails to capture attention. This is due to the high attentional demands required in the central task. In behavioural tasks a target is needed in addition to the exogenous cue, as otherwise there is no way of measuring the effect of the irrelevant stimuli (the cue). Investigating the ERPs allows us to directly investigate how irrelevant stimuli are processed when perceptual load is high or low. For example, O’Connell, Schneider, Hester, Mattingley, and Bellgrove (2011) presented participants with a central RSVP stream as well as flashing irrelevant visual stimuli in the periphery. O’Connell and colleagues found the amplitude for the P2 and P3 components elicited by the irrelevant stimuli decreased as a function of increasing load. Thus, indicating that the peripheral stimuli processing is diminished with increasing central load. Similar findings have also been demonstrated in neuroimaging studies (e.g., Rees, Frith, & Lavie, 1997; Schwartz et al., 2005). Using fMRI, Schwartz et al. (2005) presented participants with a central RSVP stream of letters. The activation in the primary visual cortex (V1) for task-irrelevant checkerboard stimuli in the periphery was decreased by higher perceptual load in the central task. The effect of load also increased for successive extra striate areas (V2, V3, and V4). This has been taken as evidence that higher perceptual load, which in turn increases the attentional demands, filters out irrelevant stimuli early in the perceptual process (Lavie, 1995).

Although perceptual load studies typically do not use the terminology exogenous and endogenous attention it seems logical that perceptual load research concerns similar processes. That is, the focus on a central task through high versus low perceptual load varies the endogenous attention, and the task irrelevant peripheral stimuli are exogenous. The load theory proposes that higher perceptual load increases the attentional demands which in turn decreases the ability of irrelevant stimuli to capture our attention (Lavie, 1995). A central task (e.g., an RSVP stream) will engage our endogenous attention. Increased endogenous attention leads to reduced influence of exogenous attention (van der Lubbe & Postma, 2005). In other words, the load theory proposes how endogenous and exogenous attention may interact.