Pigeons were required to discriminate between identical and different pairs of lights in a same/different signal-detection task. Ifthe two lights projected onto the stimulus field, which was mounted behind the center response key in a three-key chamber, were identical in intensity, a single peck on the left key was reinforced with food. If the two lights differed in intensity, rightkey pecks were reinforced. Each pigeon experienced all possible pairs (45) of 10 levels of light intensity. The percentage of correct responses was taken as an ordinal measure of the brightness difference between the lights constituting a pair, and was used to determine interval scales of brightness in the pigeon. The brightness scale for pigeons was similar to that obtained from human subjects in judging brightness differences.
371Although the relation between brightness and luminance has been studied extensively in humans (Marks, 1966(Marks, , 1968(Marks, , 1971(Marks, , 1972 there have been only a few attempts to determine the function relating brightness to luminance in animals. Herrnstein and Van Sommers (1962), in attempting to determine if a power function (Stevens's law) related brightness to luminance, trained pigeons to respond at different rates to lights of different intensities. The response rates at these training intensities were constrained such that response rate was a power function of intensity. When testing at intermediate intensities, Herrnstein and Van Sommers found that the intermediate stimuli elicited response rates that were consistent with a power law formulation. However, as Blough (1965) pointed out, the power law was, in a very direct sense, "built into" the response measure. Furthermore, when Blough analyzed Herrnstein and Van Sommer's data, he found that response rate was closer to a logarithmic, rather than a power, function of intensity. In general, to use response rate to measure sensation in a technique such as this, one must assume that response rate is proportional to sensory magnitude. Considering the number of factors that are known to influence response rate, the probability of such a correspondence is impossible to assess.Boakes (1969) employed a bisection technique to scale brightness. In this procedure, pigeons were trained to peck a right key for a bright stimulus and a left key for a dim one. Intermediate stimuli were then presented to determine the stimulus that produced equal pecking on both keys. Presumably, this stimulus bisected the sensory interval. But, as Blough and Blough (1977) probability of a side-key response and influence the location of the bisection point. Since there is no easy way to control or compensate for such potential response biases in the bisection procedure, they pose a problem for the use of bisection techniques in animal psychophysics.In the experiments reported here, brightness scales were obtained from pigeons using nonmetric scaling techniques. Pigeons were required to discriminate between identical and different pairs of lights in a same/different sig...
