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Probability and amplitude of novelty responses as a function of the change in contrast of the reafferent image in G. carapo
Department of Neurofisiología Comparada, Instituto de Investigaciones Biológicas Clemente Estable, Associated Unit of Facultad de Ciencias, Av. Italia 3318, Montevideo, Uruguay, CP 11600
* Author for correspondence (e-mail: angel{at}iibce.edu.uy)
Accepted 10 December 2002
Pulse electric fish evaluate successive electrosensory images generated by self-emitted electric discharges, creating a neural representation of the physical world. Intervals between discharges (system resolution) are controlled by a pacemaker nucleus under the influence of reafferent signals. Novel sensory stimuli cause transient accelerations of the pacemaker rate (novelty responses). This study describes quantitatively the effect of changes in contrast of reafferent electrosensory signals on the amplitude and probability of novelty responses. We found that: (i) alterations of a single image in an otherwise homogeneous series cause a novelty response; (ii) the amplitude of the elicited novelty response is a linear function of the logarithm of the change in image contrast; (iii) the parameters of this function, threshold and proportionality constant, allowed us to evaluate the transference function between change in stimulus amplitude and the amplitude of the novelty response; (iv) both parameters are independent of the baseline contrast; (v) the proportionality constant increases with the moving average of the contrast of hundreds of previous images. These findings suggest that the electrosensory system (i) calculates the difference between each reafferent electrosensory image and a neural representation of the past electrosensory input (`template'); (ii) creates the comparison template in which the relative contribution of every image decreases with the incorporation of successive images. We conclude that contrast discrimination in the electrosensory system of G. carapo obeys the general principle of appreciating any instantaneous input by the input's departure from a moving average of past images.
Key words: contrast discrimination, contrast adaptation, electric fish, Gymnotus carapo, fovea, short term memory, sensory representation
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