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This article reports some contextual effects of fish hovering in small environments on active imaging. Foveal electrosensory images of objects and their corresponding evoked responses in the electrosensory lobe are altered in amplitude and waveform when the fish are inside tubes. The article describes: (i) the physical basis of the changes imposed by small environments on electric images, (ii) the field potential responses at the electrosensory lobe of chronically implanted animals when entering and leaving tubes, and (iii) the effect of context on object discrimination. Biophysical analysis indicates that tubes cause a change in the efficiency of a previously described pre-receptor/post-effector mechanism responsible for the electric 'illumination' of nearby objects (as mirrors change the illumination of visual scenes). Field potential responses at the electrosensory lobe showed two components corresponding to the fast- and slow-electrosensory pathways respectively: (a) an early spike following the input without adaptation; (b) a series of waves lasting the rest of the cycle and exhibiting different degrees of adaptation. Discrimination experiments showed that fish react to changes in image rather than to changes in object resistance. The amplitude of the novelty responses evoked by similar changes in the total energy of electric images was constant despite the large change in basal stimulus amplitude and waveform caused by hovering in the tubes. These facts may be explained by the presence of adaptive responses observed at the slow pathway in the electrosensory lobe.

Contextual effects of small environments on the electric images of objects and their brain evoked responses in weakly electric fish