Twenty-four men were given the task of judging whether to drive through gaps which might be larger or smaller than the car. They were also given a telephoning task of checking the accuracy of short sentences. Interference between the concurrently performed tasks was investigated. Telephoning mainly impaired judgments of 'impossible' gaps (p < .01). The control skills employed in steering through 'possible' gaps were not reliably degraded, although speed of driving was reduced (p < .01). Driving increased errors (p<.01) and prolonged response times (p < .005) on the sentence-checking task. It is concluded that telephoning has a minimal effect on the more automatized driving skills, but that perception and decision-making may be critically impaired by switching between visual and auditory inputs J The authors would like to thank the United Kingdom Ministry of Transport for providing some of the apparatus, most of Ss, and two research assistants. The General Post Office kindly provided the radiophone equipment and a technical assistant. The experimental car and financial assistance were supplied by the Medical Research Council. Thanks are also due to the Ministry of Defence (Air) for permitting part of an airfield to be used as a test track. The research was carried out uhder^the general direction of R. Conrad.

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850 unsophisticated Os each made multiple estimates of darkness or fractional estimates of lightness on the same gray variables against a white standard called 1 or 100, respectively. On a normal plot the median estimates were approximately linear against reflectance. The multiple estimates were linear against the fractional estimates, whereas a reciprocal relationship is predicted by transducer theory. The slope depended upon the order of the variables; the ratio of the greatest to the smallest slope ranged between 2.1 and 1.2 to 1 (p < .01 or better). Conclusion: unsophisticated Os can estimate sensory magnitudes systematically, but the nature and size of the units they use are determined by present and prior experimental conditions.

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1200 unsophisticated O's each made 2 numerical estimates (NEs) of the magnitude of the reflectance of a variable gray with respect to a standard typically called 10. With standards of high reflectance, variables with reflectances still higher gave the larger exponents. The reverse was the case with standards of low reflectance. The effects increased as the range of numbers available to O was reduced. Averaging exponents from multiple and fractional estimates produced larger means when the reflectance of the standard was high. Fractional NEs were influenced by prior multiple NEs and vice versa. These results raise doubts about the generality of direct measurements of sensory magnitude. (16 ref.)

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