Facial sensitivity to rates of temperature change: neurophysiological and psychophysical evidence from cats and humans.

The dynamic responses in a thermal afferent pathway to rates of temperature change have been studied in anaesthetized cats. Recordings were made in the caudal trigeminal nucleus from neurones with a synaptic input from facial cold receptors. Five rates of cooling and warming ranging from 0.05 degrees C/sec to 1 degree C/sec were applied to the receptive fields of the neurones. Several measures of the dynamic response were computed but the most representative was the maximum rate during cooling or the minimum rate during warming. During cooling the maximum rate increased with increasing cooling rates between 0.05 degrees C/sec and 0.25 degrees C/sec, but did not increase at faster rates. Minimum activity during warming reached near zero at rates of 0.25 degrees C/sec and faster. The total number of impulses generated during cooling or absent during warming was unrelated to rate of temperature change. The same thermal stimuli were applied to the cheeks of human subjects. They were able to sense cooling or warming changes at 0.05 degrees C/sec. They could also distinguish the faster of two cooling changes when these were slow, but not when they were fast. Warming rates could not be distinguished, except from an adapting temperature of 35 degrees C, when warm receptors would have been activated. There was good agreement between the responses of the cat neurones and the human sensations. Slow rates of cooling could be detected or distinguished. Fast rates appeared to saturate the neuronal and sensory mechanisms.