Coding of incremental changes in skin temperature by single warm fibers in the monkey

1. Experiments were designed to answer the question: how well does a single warm fiber innervating the glabrous skin of the monkey's hand resolve incremental changes in the intensity of near-rectangular warming pulses applied to the fiber's receptive field? 2. In these experiments the measure of the warm fiber's capacity to resolve incremental changes in the intensity of successive warming pulses was termed the discriminable stimulus increment (DSI). The DSI is defined as that incremental difference in the intensity of a pair of warming pulses that could be resolved correctly, with a probability of 0.75, by comparing the fiber's responses to these two stimuli. In the specified conditions of the experiment, DSI = 0.67 sigma delta tau/(dR/dI) where sigma delta tau is the standard deviation of the difference in responses of the fiber to pairs of stimuli, and dr/dI is the fiber's sensitivity to incremental stimulus change. (dr/dI) was experimentally determined as the mean rate of change of the fiber's responses to incremental changes in the intensity of the warming pulse. 3. The DSI, as defined above, assumes that the basis for differentiating the stimuli in each pair was that the larger response in the fiber was in each instance generated by the more intense stimulus. A more general form of the DSI was also developed and used to examine the effects on intensity resolution of different discrimination rules that the brain might use. 4. In the experimental analysis the response measure of each warm fiber was the cumulative impulse count over successively longer segments of the stimulus period. With short integration intervals the DSI was high (i.e., intensity resolution was poor), but typically the DSI fell to a plateau level within 2.0--2.5 s of the onset of the warming stimulus. 5. The DSI was measured on 23 warm fibers in Macaca nemestrina for warming pulses with intensities of 0, 2, 4, 6, and 8 degrees C, at T-base levels of 29, 34 (near normal temperature of palmar skin), and 39 degrees C. For most observations the intensity resolution possible from the responses of single warm fibers, measured over this wide variety of stimulus conditions, was less than is achieved by the human observer trained to differentiate comparable warming pulses applied to the skin of the thenar eminence.