The influence of interactions between the antagonistic centre and surround areas of receptive fields on the electrical activities of medulla neurones was examined in the larval swallowtail butterfly Papilio xuthus. Weak signals from the surround had a significant depressive effect on the maintained discharge, which increased for on-centre cells or decreased for off-centre cells in response to illumination of the centre. Moderation of the maintained discharge suppressed saturation of the response and extended the graded response range of the neurones. Surround illumination also reduced irregular fluctuations in the membrane potentials and variability in the discharge rate of impulses. The results indicate that the spatial inhibitory mechanism is important for the larval visual system in order to increase the efficiency of signal processing.

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