Covert Attention

(Editor:   Pam Marek, Kennesaw State University)

Introduction

"Everyone knows what attention is..Focalization, concentration, of consciousness are of its essence" (James, 1890, Ch. 11, as reprinted in Green, n.d.). Researchers have systematically examined different aspects of attention beyond the characteristics of selective attention captured by the James quote. For example, Posner (1980) discussed the orienting of attention, defined as "the aligning of attention with a source of sensory input or an internal semantic structure stored in memory" (p. 4). Posner postulated that such orienting may precede detection, a conscious awareness of the stimulus itself. Further, although orienting of attention may be accompanied by overt eye movements, Posner also raised the possibility of covert orienting of attention - a shift in the focus of attention that is not accompanied by eye or head movements.

To assess the extent to which orienting might facilitate subsequent stimulus detection, Posner, Nissen, and Ogden (1977) employed a detection paradigm in which participants encountered orienting cues on a selected proportion of trials. To begin each trial, participants fixated their gaze at the center of a computer screen. After viewing either a valid cue (pointing toward the location where a stimulus would appear), an invalid cue (pointing away from the location where the stimulus would appear), or no cue at all, participants responded as quickly as possible to indicate the location of an X flashed on the right or left side of the screen. The interval between the presentation of cue and the appearance of the target X was very brief, varying from 0 ms to 1000 ms. By comparing reaction times for valid and invalid cue conditions to reaction times for the no cue control, the researchers determined the benefits of a correct orienting cue, the costs of a misleading cue, and the time course of shifts of attention.

Posner et al.'s (1977) results suggested that we can quickly orient visual attention to particular spatial areas (space-based attention) in 50 to 150 ms, with additional experiments providing evidence that such orienting may occur independently of changes in eye fixations. Subsequently, Egly, Driver, and Rafal (1994) indicated that people can also orient their attention to particular objects or groups of objects, rather than, or in addition to, particular spatial areas.

Factors such as cue location--either central (endogeneous, as in Posner et al.) or peripheral (exogeneous, as in Egly et al.)--may influence whether attention is space- or object-based. Cue location may also interact with the extent to which attention is initially divided or focused to influence object-based attention effects (Goldsmith & Yeari, 2003). Further, Nougier, Rossi, Alain and Taddei (1996) reported that cue location has also been linked to whether covert shifts of attention are considered voluntary (for central cues) or automatic (for peripheral cues), emphasizing that practice and voluntary strategies can influence the effects of automatic allocation of attention. To demonstrate the benefits and costs of valid and invalid central cues on the time it takes to covertly shift attention to detect the target stimulus, the present experiment is based on Posner et al's design.

Design

Prior to beginning the experiment, you will follow a simple procedure that enables the computer program to present the target stimuli at a precise viewing angle. After calibrating the viewing angle, a necessary prerequisite to accurate assessment of shifts in visual attention, you will complete 20 practice trials. Because the experiment is designed to measure covert attentional shifts, you will be focusing your eyes on the center of the screen throughout the entire experiment. At the beginning of each trial, you will view either an arrow pointing to the left, an arrow pointing to the right, or an X. Shortly thereafter, the borders of one of four rectangles (two on the left of the screen and two on the right) will be illuminated. Your task is to press the "a" key if a rectangle on the left is illuminated and to press the "s" key if a rectangle on the right is illuminated.

After each trial, you will receive feedback about your accuracy and response time. After the practice trials, you will continue with the actual experimental trials. You will complete 90 trials. For trials including an arrow as a cue, the arrow will point in the direction of the rectangle to be illuminated 80% of the time and in the opposite direction 20% of the time. Thus, in the long run, to minimize the time it takes to detect the illuminated target, it would be to your advantage to focus your visual attention in the direction of the arrow, without moving your eyes.

Data Format and Layout

Data are downloadable in three formats (XML, Excel spreadsheet format, and comma delimited for statistical software packages). Figure 1 shows an excerpt from a sample Excel spreadsheet. The first five columns provide classification data (participant ID number, class id, gender, age, and completion date. The next 15 columns indicate the response times in seconds for each delay interval for each type of cue (valid, invalid, or no cue). For example, the first set of data columns provides average response times for invalid cues at delays of 0, 100, 200, 400, and 800 milliseconds.) The next set of columns includes the same delays for valid cues, and the final set contains the delays for the no cue condition.

Sample data image from the Covert Attention experiment
Figure 1

The experiment uses a 3 (type of cue: valid, invalid, no cue) x 5 (delay: 0, 100, 200, 400, 800) repeated-measures designs. Thus, analysis would be accomplished using a factorial repeated-measures ANOVA. If results parallel those of Posner et al. (1977), you will find a main effect of cue. Post-hoc tests or planned comparisons would indicate that reaction time was faster in the valid cue condition than in invalid or no-cue conditions, illustrating the benefit of orienting visual attention in the appropriate direction.

As illustrated in Figure 2 (adapted from Posner et al.), you might also find a type of cue x delay interaction, illustrating the following: (a) Reaction times for the neutral conditions remain relatively constant across all delays; (b) Reaction time for the valid conditions decrease and then level off; and (c) Reaction times for the invalid conditions increase and then level off. This pattern of findings provides insight into the time course of covert attention shifts. For example, if there is no time to covertly orient attention (at 0 delay), there is no difference between the three cue conditions. However, with a small lapse of time (e.g., a lapse of 150 milliseconds), a valid cue reduces detection time whereas an invalid cue increases it. You might consider creating a line graph from your data to determine if it resembles these results.

Covert Attention Posner graph
Figure 1

Applications/Extensions

From the time Posner et al. (1977) introduced the detection paradigm, a variety of researchers have explored the relationship between covert shifts of attention and eye movements. Engbert and Kliegl (2003) found that shifts of attention influenced the timing of microsaccades. Additionally, functional magnetic resonance imaging (fMRI) has revealed considerable overlap of areas activated by covert shifts of attention and by eye movements, but also showed differences in the level of activation for the two tasks (Nobre, Gitelman, Dias, & Mesulam, 2000).

Beyond studies of its relation to eye movement, covert attention has sparked interest in other contexts. For example, Berger, Jones, Rothbart, and Posner (2000) developed a battery of tests in which they used computerized games to track the developmental course of different aspects of attention, including covert orienting, in children. Additionally, Nobre et al. used the fMRI technique to compare brain activity when people directed attention to internal mental images versus actual external locations, whereas Forster and Eimer (2005) used event-related potentials (ERPs) to investigate the influence of covert orienting of attention on the sense of touch. In a clinical application, Bartolomeo and Chokron (2002) have reported that patients with left unilateral neglect (who ignore the left side of objects) experience greater difficulty with exogenous rather than endogenous orienting of attention.

References


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