Rhesus monkey (Macaca mulatta) cool sensitivity measured by a signal detection method

Two rhesus monkeys were trained to detect cool stimuli (decrements in skin temperature from the adapted temperature) presented to the palm of the left hand after the skin had been preadapted to a 33°C temperature. The procedure was analogous to the signal detection “yes-no” method used with human observers. Receiver operating characteristic curves were generated by varying the a priori probability of stimulus oecurrence. The proportion of hits and of false alarms increased with increases in the a priori probability of stimulation. The points of isosensitivity of both subjects for cool intensities of 0.3°C, but not at greater intensities, yielded straight lines with slopes of approximately one when plotted on normal-normal coordinates. The values of the d’_s detectability index decreased from 3.52 to 0.75 for one subject and from 2.81 1to 1.28 for the other as the stimulus intensity was decreased from 0.8° to 0.15° cooling from the adapting temperature. Cool thresholds for each subject of 0.19° and 0.12°C were computed from classical psychometric functions when threshold was defined as 50% hits at a 50% probability of stimulus occurrence. These thresholds are comparable to published reports of the cool threshold for the rhesus.