Correlation of spatial event plots with simulated population responses of mechanoreceptive fibers

The experimental setup for generating spatial event plots (SEPs) from single mechanoreceptive fibers of the skin was computationally simulated. The generic fibers used in the simulations were similar to the rapidly adapting fibers (RAs), and had variable refractoriness and receptive-field size. The speed, lateral shift, and the contact width of the drum scanned across the receptive field of the fiber are adjustable parameters. The stimulus patterns used on the drum mimicked stimuli used by several other investigators. These were dot patterns, grating patterns, and the letter "E". First, the effects of simulation parameters on the SEPs were studied. The simulation output confirms the results of physiological experiments that SEPs contain information on the spatiotemporal resolution of the fiber. The next series of simulations involved generating SEPs of fibers obtained from the same or varying spatial distributions of receptive fields. Three hypothetical distributions were used: homogeneous rectangular, uniformly random, and Gaussian. The momentary population response at each case was found using the technique by Johansson and Vallbo (Brain Res 184: 353-366, 1980). The population responses were not isomorphic images of the stimulus patterns due to the variations in field sizes and locations. However, every fiber, no matter which distribution it came from, generated almost identical SEPs given similar response properties. Furthermore, the SEPs looked like the outline of the stimulus. These observations show that SEPs do not contain information about the population response. Therefore, reconstructing the population response using SEPs can result in misleading conclusions on central-nervous-system processing and should be viewed cautiously when formulating psychophysical/physiological linking hypotheses.