Elements of simulation
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Sensory perception might be defined, in part, as that activity whereby changes in the environment are detected. From this point of view, fluctuations in a stimulus are more important than the mean stimulus intensity. In this paper, we are concerned primarily with the chemical senses in which the stimulus consists of molecules undergoing random motion in a fluid. We extend Smoluchowski's model of Brownian motion to include an analysis of the variance of observed density fluctuations (variance because we are concerned with changes rather than mean values). A computer simulation of Brownian motion in two dimensions was developed and run on the Cray supercomputer. It is shown that density fluctuations in the computer-simulated data fall within the constraints calculated from the analysis of variance, but that published experimental data deviate somewhat from the theoretical constraints. We then apply the computer simulations to show that successive density measurements by olfactory cells are probably highly correlated, which can produce an olfactory ''illusion'' during the early moments of the process of smelling.