Somatosensory evoked potential correlates of psychophysical magnitude estimations for air-puff stimulation of the foot in man

Short air-puff stimuli were applied to the sole of the right foot to obtain both psychophysical and neurophysiological responses. The detection threshold (So) was first determined, and six levels of stimulus intensity above threshold were adopted for magnitude estimation. Somatosensory evoked potentials (SEPs) were also recorded over the foot projection area (2 cm posterior to Cz) for the six stimulus intensities. Six components (N40, P45, N55, P70, N80 and P90) were recorded within 100 ms following stimulation. A power function with an exponent of 0.94 provided an adequate description of the magnitude estimation values as a function of stimulus intensity, as was verified by the high correlation coefficient (r = 0.87; P < 0.001). Similarly, stimulus-amplitude functions of P45-N55, N55-P70 and P70-N80 SEP components were well represented by power functions with exponents of 0.62, 0.63 and 0.78, respectively. The SEP latencies as a function of stimulus intensity had negative power functions. The latency functions of the P45 and N55 components had the largest negative power exponents (-0.17 and -0.15) and showed the highest negative correlations (r= -0.70 and-0.71, respectively) with the stimulus intensity. These results suggest that both the amplitude and the latency information encoded in the SEPs may contribute to the magnitude estimation of the stimulus.