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The chemotaxis signalling network in Escherichia coli that controls the locomotion of bacteria is a classic model system for signal transduction. This pathway modulates the behaviour of flagellar motors to propel bacteria towards sources of chemical attractants. Although this system relaxes to a steady state in response to environmental changes, the signalling events within the chemotaxis network are noisy and cause large temporal variations of the motor behaviour even in the absence of stimulus. That the same signalling network governs both behavioural variability and cellular response raises the question of whether these two traits are independent. Here, we experimentally establish a fluctuation-response relationship in the chemotaxis system of living bacteria. Using this relationship, we demonstrate the possibility of inferring the cellular response from the behavioural variability measured before stimulus. In monitoring the pre- and post-stimulus switching behaviour of individual bacterial motors, we found that variability scales linearly with the response time for different functioning states of the cell. This study highlights that the fundamental relationship between fluctuation and response is not constrained to physical systems at thermodynamic equilibrium but is extensible to living cells. Such a relationship not only implies that behavioural variability and cellular response can be coupled traits, but it also provides a general framework within which we can examine how the selection of a network design shapes this interdependence.

Experimental psychotic children, 9 boys and 3 girls (mean age=66.9 mos.) were compared with 5 non-psychotic subnormal boys (mean age=72 mos.) in body imitation, motor-object imitation and spontaneous object use and also for influence of task class and difficulty using each subject's adaptive age as referant. Psychotic children did poorly in body, better in motor-object imitation and best in object use. Controls achieved higher body and motor-object imitation scores. Task difficulty equally affected the performances of subjects and controls. Body imitation failure, traced to neurophysiological deficiencies, combined with severe central language disorders in the psychotic group, precluded non-verbal and verbal interpersonal communication. It is suggested that during early life, psychotic children, unlike the subnormal who have less severe visual-motor or language processing handicaps, cannot use parents' help to interpret environment.