On the automaticity and flexibility of covert attention: a speed-accuracy trade-off analysis

Abstract

Exogenous covert attention improves discriminability and accelerates the rate of visual information processing (M. Carrasco & B. McElree, 2001). Here we investigated and compared the effects of both endogenous (sustained) and exogenous (transient) covert attention. Specifically, we directed attention via spatial cues and evaluated the automaticity and flexibility of exogenous and endogenous attention by manipulating cue validity in conjunction with a response-signal speed-accuracy trade-off (SAT) procedure, which provides conjoint measures of discriminability and information accrual. To investigate whether discriminability and rate of information processing differ as a function of cue validity (chance to 100%), we compared how both types of attention affect performance while keeping experimental conditions constant. With endogenous attention, both the observed benefits (valid-cue) and the costs (invalid-cue) increased with cue validity. However, with exogenous attention, the benefits and costs in both discriminability and processing speed were similar across cue validity conditions. These results provide compelling time-course evidence that whereas endogenous attention can be flexibly allocated according to cue validity, exogenous attention is automatic and unaffected by cue validity.

Figure 1

Hypothetical SAT functions. Illustrative SAT functions, plotted in d′ units versus processing time (time of the response cue plus observer's average latency to respond). Top panel: Expected pattern if an experimental factor increases target discriminability only. The functions differ in asymptotic accuracy but are associated with the same intercept (point when accuracy departs from chance) and proportional rate of information accrual. Bottom panel: One expected pattern if the experimental factor alters the speed of information accrual (intercept and rate) only. The functions display disproportional dynamics; they reach a given proportion of their asymptotes at different times. Circles show hypothetical results from a typical RT task plotted in SAT coordinates (abscissa = mean RT; ordinate = accuracy), illustrating that RT differences can reflect differences in discriminability (top panel) or information accrual (bottom panel). The position of the RT points on the corresponding SAT functions is determined by the decision criteria that an observer uses to balance speed and accuracy.

Figure 2

Sequence of events in a single trial. The stimuli were randomly presented at 8 equidistant locations from a central fixation point on an invisible polar grid at 4° eccentricity. The interval between the cue onset and the stimulus onset was optimum to facilitate endogenous or exogenous attention. Eyes were monitored in the endogenous condition to ensure no movements were made. Observers were required to respond whether the target was tilted to the right or the left within 350 ms of the response tone presented at one of 7 times after stimulus onset. Feedback was provided after each trial. All observers were informed of the validity of the cue prior to running in both endogenous and exogenous conditions.

Figure 3

Results: low and high cue validities. Average discrimination accuracy (in d′ units) as a function of processing time in 33% (low) and 100% (high) cue validities for endogenous (top) and exogenous (bottom) conditions. Smooth functions show the best-fitting exponential model (Equation 1) for the valid (red lines), neutral (blue lines), and invalid (green lines) cueing conditions for set sizes 1 (solid lines) and 8 (dashed lines), based on fits of nested models that systematically varied the 3 parameters of Equation 1. In these SAT curves, the rate (valid condition rate indicated by shaded areas) differences across validity for endogenous are clearly shown. The endogenous cue benefit (valid-neutral) increases greatly, i.e., 396 ms, while the exogenous cue benefit increases slightly, i.e., 46 ms, as cue validity increases from 33% to 100%. With respect to discriminability (represented by the asymptotes), the endogenous cue benefit increases 0.44 d′ units while the exogenous cue benefit only increases 0.12 d′ units as validity increases from 33% to 100%. (endogenous: R² for 33% = 0.92, R² for 100% = 0.94; exogenous: R² for 33% = 0.96, R² for 100% = 0.96).

Figure 4

Discriminability differences. Average discriminability differences (in d′ units) for set sizes 1 (dark gray bars) and 8 (light gray bars) as a function of cue validity for endogenous (left) and exogenous (right) conditions. Benefits (solid line bars) are differences between valid and neutral cue asymptotes while costs (dashed line bars) are differences between invalid and neutral invalid cue asymptotes. Benefits significantly increase with validity for both set sizes in endogenous conditions (left panel) while both benefits and costs remain constant across cue validity for exogenous conditions (right panel).

Figure 5

Processing speed differences. Average processing speed differences (in ms) as a function of cue validity for endogenous (left) and exogenous (right) conditions. Benefits (solid line bars) are differences between valid and neutral cue processing speeds while costs (dashed line bars) are differences between invalid and neutral invalid cue processing speeds. Costs significantly increase with validity in endogenous conditions while both benefits and costs remain constant across cue validity for exogenous conditions.