Relations between selective attention and perceptual segregation by color were investigated in binary-choice reaction time experiments based on the nonsearch paradigm of Eriksen and Eriksen (1974). In focused attention conditions (Experiment 1), noise letters flanking a central target letter caused less interference when they differed from the target in color, although color carried no information as to whether or not a letter was the target. When blocking of trials favored a strategy of dividing attention between target and noise letters (Experiment 2), no benefit accrued from difference between target color and noise color. The results supported an attentional interpretation of the effect of color demonstrated in Experiment 1, implying that perceptual segregation by color improved the efficiency of focusing attention on the target.Recent studies have demonstrated remarkable deficits in subjects' abilities to control their visual information processing by focusing attention on stimuli in a prespecified spatial location (cf. Egeth, 1977; C. W. Eriksen & Schultz, 1979;Shiffrin & Schneider, 1977). A binary-choice reaction time experiment by B. A. Eriksen and C. W. Eriksen (1974) provides a good example. Subjects were presented with displays in which a central target letter appeared alone or flanked by a number of noise letters. The target was always presented directly above the fixation point, and the required response was uncorrelated with the number and type (response compatible, incompatible, or neutral) of noise letters. For all types of noise, reaction time increased as between-letter spacing decreased, but interference was stronger with response-incompatible than with neutral noise and stronger with neutral than with responsecompatible noise. Eriksen and Eriksen concluded that the subject "cannot prevent processing of noise letters occurring within about 1 deg of the target due to the nature of processing channel capacity and must inhibit his response until he is able to discriminate exactly which letter is in the target position" (p. 143).

The time course of visual letter recognition was investigated in a single-stimulus identi®cation experiment. On each trial, a randomly chosen stimulus letter was presented at 1 of 12 equiprobable positions that were equally spaced around the circumference of an imaginary circle centered on ®xation. Exposure duration was varied from 10 to 200 ms, and the letter was followed by a pattern mask. The subject's task was to report the identity of the stimulus letter but refrain from guessing. For the briefest exposures, correct reports never occurred. For longer exposures, the function relating the probability p of recognizing the letter to the duration t of the stimulus exposure was well approximated by an exponential distribution function: pt 1 À expÀv Á t À t 0 , where v is the rate of processing and t 0 is the minimum eective exposure duration. The generality of this ®nding may be limited to cases in which stimuli are highly discriminable and response criteria are conservative. Extensions to Poisson counter or random walk models are considered for cases in which stimuli are confusable.

Two experiments were performed in which drivers' cognitive load was measured by the time taken to complete calculation tasks presented to them while driving on prespecified routes. Experiment 1 was conducted on two different driving routes. Both routes included driving on a highway and driving through a village area. Mean driving speed was Iower and mean calculation time was higher in the village areas than on the adjacent highway sections of the two driving routes. Across successive 100-m intervals of the two driving routes, drivers' cognitive load was found to be inversely related to their driving speed. Experiment 2 tested the same dual-task method on driving routes'including two rural junctions. At both junctions driving speed decreased with decreasing distance to the give-way line and the drivers' cognitive load was highest while approaching and driving through the junctions. It is concluded that the applied dual-task method is sensitive to the variation in drivers' cognitive load caused by variations in the demands of the driving task.

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