Sowon Hahn

Ageing, fitness and neurocognitive function

In the ageing process, neural areas, and cognitive processes, do not degrade uniformly. Executive control processes and the prefrontal and frontal brain regions that support them show large and disproportionate changes with age. Studies of adult animals indicate that metabolic and neurochemical functions improve with aerobic fitness. We therefore investigated whether greater aerobic fitness in adults would result in selective improvements in executive control processes, such as planning, scheduling, inhibition and working memory. Over a period of six months, we studied 124 previously sedentary adults, 60 to 75 years old, who were randomly assigned to either aerobic (walking) or anaerobic (stretching and toning) exercise. We found that those who received aerobic training showed substantial improvements in performance on tasks requiring executive control compared with anaerobically trained subjects.

Our Eyes do Not Always Go Where we Want Them to Go: Capture of the Eyes by New Objects

Observers make rapid eye movements to examine the world around them. Before an eye movement is made, attention is covertly shifted to the location of the object of interest. The eyes typically will land at the position at which attention is directed. Here we report that a goal-directed eye movement toward a uniquely colored object is disrupted by the appearance of a new but task-irrelevant object, unless subjects have a sufficient amount of time to focus their attention on the location of the target prior to the appearance of the new object. In many instances, the eyes started moving toward the new object before gaze started to shift to the color-singleton target. The eyes often landed for a very short period of time (25–150 ms) near the new object. The results suggest parallel programming of two saccades: one voluntary, goal-directed eye movement toward the color-singleton target and one stimulus-driven eye movement reflexively elicited by the appearance of the new object. Neuroanatomical structures responsible for parallel programming of saccades are discussed.

Task coordination and aging : explorations of executive control processes in the task switching paradigm

A number of models of cognitive aging suggest that older adults exhibit disproportionate performance decrements on tasks which require executive control processes. In a series of three studies we examined age-related differences in executive control processes and more specifically in the executive control processes which underlie performance in the task switching paradigm. Young and old adults were presented with rows of digits and were required to indicate whether the number of digits (element number task) or the value of the digits (digit value task) were greater than or less than five. Switch costs were assessed by subtracting the reaction times obtained on non-switch trials from trials following a task switch. Several theoretically interesting results were obtained. First, large age-related differences in switch costs were found early in practice. Second, and most surprising, after relatively modest amounts of practice old and young adults switch costs were equivalent. Older adults showed large practice effects on switch trials. Third, age-equivalent switch costs were maintained across a two month retention period. Finally, the main constraint on whether age equivalence was observed in task switching performance was memory load. Older adults were unable to capitalize on practice under high memory loads. These data are discussed in terms of their implications for both general and process specific cognitive aging models.

Influence of Attentional Capture on Oculomotor Control

Previous research has shown that when searching for a color singleton, top-down control cannot prevent attentional capture by an abrupt visual onset. The present research addressed whether a task-irrelevant abrupt onset would affect eye movement behavior when searching for a color singleton. Results show that in many instances the eye moved in the direction of the task-irrelevant abrupt onset. There was evidence that top-down control could neither entirely prevent attentional capture by visual onsets nor prevent the eye from starting to move in the direction of the onset. Results suggest parallel programming of 2 saccades: 1 voluntary goal-directed eye movement toward the color singleton target and 1 stimulus-driven eye movement reflexively elicited by the abrupt onset. A neurophysiologically plausible model that can account for the current findings is discussed.

Attentional and oculomotor capture by onset, luminance and color singletons

In three experiments we investigated whether attentional and oculomotor capture occur only when object-defining abrupt onsets are used as distractors in a visual search task, or whether other salient stimuli also capture attention and the eyes even when they do not constitute new objects. The results showed that abrupt onsets (new objects) are especially effective in capturing attention and the eyes, but that luminance increments that do not accompany the appearance of new objects capture attention as well. Color singletons do not capture attention unless subjects have experienced the color singleton as a search target in a previous experimental session. Both abrupt onsets and luminance increments elicit reflexive, involuntary saccades whereas transient color changes do not. Implications for theories of attentional capture are discussed.

Attentional capture and aging: Implications for visual search performance and oculomotor control.

Two studies examined potential age-related differences in attentional capture. Subjects were instructed to move their eyes as quickly as possible to a color singleton target and to identify a small letter located inside it. On half the trials, a new stimulus (i.e., a sudden onset) appeared simultaneously with the presentation of the color singleton target. The onset was always a task-irrelevant distractor. Response times were lengthened, for both young and old adults, whenever an onset distractor appeared, despite the fact that subjects reported being unaware of the appearance of the abrupt onset. Eye scan strategies were also disrupted by the appearance of the onset distractors. On about 40% of the trials on which an onset appeared, subjects made an eye movement to the task-irrelevant onset before moving their eyes to the target. Fixations close to the onset were brief, suggesting parallel programming of a reflexive eye movement to the onset and goal-directed eye movement to the target. Results are discussed in terms of age-related sparing of the attentional and oculomotor processes that underlie attentional capture.

Further Evidence for the Division of Attention Among Non-contiguous Locations

Models of visual attention have, with few exceptions, suggested that attention is deployed to unitary regions of visual space. Kramer and Hahn (1995) recently reported that attention is considerably more flexible than previously believed, such that under some conditions attention may be focused on multiple non-contiguous areas of the visual field. In the five studies reported here, we examined the boundary conditions on the ability to divide attention among different locations in visual space. In each of the studies, subjects performed a same-different matching task with target letters that were presented on opposite sides of a set of distractor letters. Experiments 1, 2 and 3 provide further support for our proposal that subjects can concurrently attend to non-contiguous locations as long as new objects do not appear between the attended areas. Experiment 4 examined whether the disruption of multiple attentional foci was the result of the capture of attention by new objects per se, or by task-irrelevant ...

Top-down guidance in visual search for facial expressions.

Using a visual search paradigm, we investigated how a top-down goal modified attentional bias for threatening facial expressions. In two experiments, participants searched for a facial expression either based on stimulus characteristics or a top-down goal. In Experiment 1, participants searched for a discrepant facial expression in a homogenous crowd of faces. Consistent with previous research, we obtained a shallower response time (RT) slope when the target face was angry than when it was happy. In Experiment 2, participants searched for a specific type of facial expression (allowing a top-down goal). When the display included a target, we found a shallower RT slope for the angry than for the happy face search. However, when an angry or happy face was present in the display in opposition to the task goal, we obtained equivalent RT slopes, suggesting that the mere presence of an angry face in opposition to the task goal did not support the well-known angry face superiority effect. Furthermore, RT distribution analyses supported the special status of an angry face only when it was combined with the top-down goal. On the basis of these results, we suggest that a threatening facial expression may guide attention as a high-priority stimulus in the absence of a specific goal; however, in the presence of a specific goal, the efficiency of facial expression search is dependent on the combined influence of a top-down goal and the stimulus characteristics.

Age-related effects of attentional and oculomotor capture by onsets and color singletons as a function of experience

The present experiment examined the degree to which experience with different stimulus characteristics affects attentional capture, particularly as related to aging. Participants were presented with onset target/color singleton distractor or color singleton target/onset distractor pairs across three experimental sessions. The target/distractor pairs were reversed in the second session such that the target in the first session became the distractor in the second and third sessions. For both young and old adults previous experience with color as a target defining feature influenced oculomotor capture with task-irrelevant color distractors. Experience with sudden onsets had the same effect for younger and older adults, although capture effects were substantially larger for onset than for color distractors. Experience-based capture effects diminished relatively rapidly after target and distractor-defining properties were reversed. The results are discussed in terms of top-down and stimulus-driven effects on age-related differences in attentional control.

Attentional flexibility and aging : you don't need to be 20 years of age to split the beam

Whether young and old adults were able to selectively attend to noncontiguous locations in the visual field and ignore physically interspersed distractor stimuli was examined. Participants decided whether 2 letters matched or mismatched. Target letter locations were precued by square boxes on an imaginary circle centered on fixation. Distractors were located between the 2 targets. Young and old were unable to ignore the distractors when the targets and distractors were presented as onset stimuli; however both young and old were able to ignore the distractors when the target and distractors were presented as non-onset stimuli. The time course of attentional allocation was equivalent for young and old. Results are discussed in terms of models of visual selective attention and the flexibility of attentional control.