Within the last few years a number of investigators have studied a variety of psychological functions in the monkey-indeed, the monkey is becoming a preferred animal in audition and other fields. Yet although "hearing" is a factor in many psychological tests with monkeys, the most basic auditory function, namely, the intensity threshold, of the monkey is not very precisely known, while other important functions, such as pitch and intensity discrimination, are enitrely uninvestigated.This study is in inquiry, using a motor conditioning method, into the auditory acuity of the late-infant and pre-adolescent Old World monkey. To find the auditory acuity we determine the lower absolute intensity threshold for each of a number of representative frequencies over the audible range. One simply reduces step-wise the intensity of a tone of a particular frequency until the subject fails to respond to it. After several repetitions of this procedure, appropriate psychophysical methods are employed to fix the threshold precisely. Auditory acuity is then represented by a plot, the so-called audiogram, relating the physical energy of the tone at threshold intensity, to tone-frequency.Wendt (7) has investigated the auditory acuity of monkeys in a careful study. Most of his general conclusions are confirmed by the present study, and it is likely that Wendt's paper will remain a model for students of primate audition. Several considerations, however, make it desirable to repeat some of the earlier work. In the first place, Wendt was unable to present his results in terms of actual physical units. He was able to approximate this by comparing results from his monkeys with results from five human subjects in the same apparatus. However, one has to assume with Wendt that his human subjects possessed exactly the same thresholds as did Sivian and White's (6) human subjects whose thresholds were known in terms of dynes per cm 2 . Further, one has to assume that the apparatuses of Wendt and of Sivian and White were equivalent as to purity of tone, magnitude of intensity steps, standing waves, and the like. It is clear that it would be valuable to have at hand measurements of the physical characteristics of the stimulus with which to speak more precisely of the monkey's auditory acuity. This paper presents audiograms in terms of the physical stimulus.In the second place, the thresholds which Wendt reports might have been somewhat different if measured by more usual psychophysical methods. As he decreased the tonal intensity, Wendt termed as a threshold that intensity setting at which the animal last made a correct response. It is evident that his mean
