Pulse-discharging, weakly electric fish actively electrolocate by emitting electric organ discharges and sensing changes provided by objects on transepidermal self-generated electric fields. In this way they create a series of discrete electric images on a cutaneous electroreceptive mosaic (Lissmann, 1958; cf. Bullock, 1986 cf. Bullock, , 1999Bastian, 1986). In this study we examine how fish discriminate between electrosensory images of different contrast. This kind of analysis requires unambiguous definition and measurement of the stimulus (input) and of the related performance of a sensory system (output; Marr, 1982). Our recent knowledge of electric image generation mechanisms allowed us to control and measure the electrosensory image (Caputi and Budelli, 1995;Rasnow, 1996;Caputi et al., 1998;Stoddard et al., 1999;Nelson and MacIver, 1999;Budelli and Caputi, 2000;Sicardi et al., 2000;Caputi et al., 2003). Whereas the input is a clearly defined physical entity, the output of a sensory system can be considered as a broad spectrum of 'intangible facts'. Although sensation and perception may exist independently of any behavioural response, only behaviour can be measured objectively (Spector, 2000). So, we restricted our research to the analysis of an orienting behavior ('a specific behavioural act directed towards the extraction of information from the environment'; Sokolov, 1990) elicited by changes in stimulus contrast, aiming to infer electrosensory processing mechanisms.Pulse gymnotids show a typical orienting behavior, the novelty response (Lissmann, 1958;Szabo and Fessard, 1965;Larimer and McDonald, 1968;Bullock, 1969; cf. Hopkins, 1983;Kramer, 1990;Moller, 1995). This behavior consists of a transient shortening of the inter-electric organ discharge (EOD) interval triggered by changes in nearby impedance. It has been frequently used to test a fish's electrolocation ability and to assess the effects of reafferent and exafferent input on pacemaker frequency (Bullock, 1969;Heiligenberg, 1980;Grau and Bastian, 1986;Hall et al., 1995;Zellick and von der Emde, 1995;Post and von der Emde, 1999).After studying novelty responses evoked by a short-circuit in the presence of different amounts of noise, Heiligenberg (1980)

The fast electrosensory pathway (FEP) of gymnotiform fish is mediated by tuberous electroreceptor organs innervated by ganglion cells that synapse with spherical cells of the electrosensory lateral line lobe (ELL). Spherical cells project to the magnocellular mesencephalic nucleus. The electrosensory environment was represented somatotopically within ELL. The mandibular (MN) and the supraorbital (SON) nerves projected to rostral ELL (occupying 19-28% and 4-10%, respectively), and the posterior branch of the anterior lateral line nerve (pALLN) projected to caudal ELL (occupying 56-64%). Labeling with horseradish peroxidase or biotinylated dextran-amine demonstrated three kinds of synaptic endings coupling primary afferents to spherical cells: multiple synaptic knobs, medium-sized calyxes, and very large calyxes. Multiple synaptic knobs arose from MN and SON primary afferents and were found in a narrow rostral area covering the centrolateral (CLS) and lateral (LS) segments of ELL. Medium and large calyxes, proceeding from the same nerves, predominated in the remaining parts of the three segments of ELL containing spherical cells. Calyx-type endings were also found in the LS-occupying regions in which the pALLNs projected. Calyx-type endings formed gap junctions but also contained vesicles and showed submembrane specializations typical of chemical synapses. The postsynaptic spherical cells were linked by dendrosomatic gap junctions and were also contacted by unlabeled en passant synaptic boutons, whose fine structure suggested chemical transmission. Electrophysiological studies indicated that spherical cell responsiveness diminished after electrosensory stimulation. This apparently inhibitory phenomenon may be subserved by the unlabeled synaptic boutons, which possibly originate from interneurons that have yet to be identified.

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