|
Article Excerpt
Ageing has been associated with deficits in inhibitory processing that cause people to become distracted by elements that are unrelated to the current task or goal (Hasher & Zacks, 1988). Measures of eye movement behaviour have been used to investigate age-related increases in distraction. One such task that measures distraction and inhibitory processing is the antisaccade task. During the antisaccade task, a target is flashed to one side of a central fixation. When the target disappears, instead of looking at the location of the target, as would be the pre-potent response, the viewer is instructed to make an anti-saccade--initiate a saccade of equal amplitude (i.e. to the same location) in the opposite direction of where the cue was presented. If the viewer fails to inhibit responding to the cue, an eye movement towards the cue, a prosaccade, will be generated before the anti-saccade (Roberts, Hager, & Heron, 1994). This task requires the ability to inhibit the reflexive response of initiating a saccade towards a target and is tied to frontal lobe function; older adults and patients with frontal lobe damage make more errors than young adults (Olincy, Ross, Young, & Freedman, 1997). The effects of ageing on saccade latency in the antisaccade task is less clear, with some studies reporting age-related increases in saccade latency (Nieuwenhuis, Ridderinkhof, de Jong, Kok, & van der Molen, 2000; Olincy, et al., 1997; Munoz, Broughton, Goldring, & Armstrong, 1998) and others reporting saccadic latencies similar to that of younger adults (Butler, Zacks, & Henderson, 1999). Whether effects of ageing are seen on measures of saccadic accuracy or latency in the antisaccade task may be related to the amount of task demands that are placed on the subject; that is, if cognitive load is relatively low, age-related differences in accuracy or latency may not be observed, but if working memory demands are taxed, such as in a dual-task situation (e.g. make a pro/antisaccade and identify the target at that location; Butler et al., 1999), then age-related changes in performance may be observed (Eenshuistra, Ridderinkhof, & van der Molen; 2004; Cassavaugh, Kramer, & Peterson, 2004).
Oculomotor capture is another paradigm that has also recently been used to measure age-related increases in distraction (Colcombe et al., 2003; Kramer, Hahn, Irwin, & Theeuwes, 1999). In this paradigm, subjects are required to make a saccade to a colour singleton target presented in an array and identify the letter located inside it. On half of the trials, a sudden onset occurred. If the abrupt onset is distracting, eye movements should be made to the onset, rather than to the target. Both younger and older subjects were distracted by the abrupt onset, making saccades to the distractor before correcting and making a saccade to the target (Kramer et al, 1999). In an initial study (Kramer et al., 1999), older and younger adults performed similarly; however, in a follow-up study, older adults made more misdirected saccades than younger adults when the onset was particularly salient (Kramer, Hahn, Irwin, & Theeuwes, 2000), suggesting that age-related differences in oculomotor capture may depend on the salience of stimuli, and/or the subjects' awareness of the distractors. Alternatively, age-related differences in the oculomotor capture task may be observed depending on whether subjects are explicitly instructed to refrain from making an eye movement to the distractor, thereby establishing a goal representation of inhibitory control that may decay faster in older than in younger adults (Colcombe et al, 2003).
Work using oculomotor capture suggests that it is a powerful paradigm with which to investigate distraction and the integrity of inhibitory processing in young and older adults. Moreover, this work suggests that there is parallel programming of two saccades; a goal-directed saccade to the target singleton and a reflexive saccade to the distractor. That is, the latencies of saccades that went directly to a target were longer in the presence of a distractor than when a distractor was not present (Godijn & Theeuwes, 2002a, 2002b; Irwin, Colcombe, Kramer, & Hahn, 2000). This could occur if multiple eye movements were programmed in parallel, one to the target and one to the distractor. Even though the resulting activation is higher for the target-directed saccade, thereby causing a saccade to be directed to the target, the saccade is initiated with a higher latency due to the lateral inhibition from the saccade programme to the distractor (Godijn & Theeuwes, 2002b).
Further work has demonstrated that the use of a precue can reduce, or even eliminate, the amount of capture that occurs by distractors (non-targets). In Theeuwes, Kramer, Hahn, and Irwin et al. (1998), a precue was provided prior to the onset of the display that informed viewers where the target would appear. The logic here was that if subjects were provided with enough time to programme their goal-directed eye movement via the precue, the distraction from the abrupt onset would be minimal and subjects would make fewer reflexive saccades towards the distractor. Indeed, subjects appeared to use the precue to advance attention to the target location prior to the onset of the cue and were thus minimally distracted by the abrupt onset. Age-related interference effects are observed in visual search conditions in which the target can occupy varied and random locations, but these age-related distractor interference effects are reduced in non-search...
|
