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In order to fully understand mammalian sensory systems, it is important to examine the origins of their evolutionary development. The brain of the teleost fishes provides us with an early derived model for the evolutionary development of sensory systems so successful as to underlie the cognitive processes of about half of all vertebrate species (Braford, 1995). The connections of hair-cell derived sensory systems are well known in teleosts as they enter the brainstem and project up to the midbrain, but as they continue up into the diencephalon and telencephalon, our knowledge of their anatomy becomes less firm. There is a good deal of evidence that auditory, mechanosensory and electrosensory information, after leaving the torus semicircularus, relays in the preglomerular complex and other diencephalic nuclei before ascending to the telencephalon (Braford and McCormick, 1993; Emde and Prechtl, 1998; Striedter, 1991; Zupanc, 1996). Whether the segregation of sensory modalities is maintained as this information moves toward the telencephalon, however, as it does in the 'lemniscal pathways' of mammals, or whether there is multimodal processing in the diencephalon is still in question. By making Neurobiotin injections into physiologically defined auditory and mechanosensory regions of the telencephon of mormyrids, Emde and Prechtl (1998) found evidence for multimodal processing in the ventral preglomerular nucleus. The degree to which collaterals or passing fibers were labeled in the process, however, is unclear. Furthermore, the presence of a novel electrosensory system in Osteoglossomorphs and Euteleosts, developed after the putative loss of the primitive electrosensory system of Polypterophorms and Chondrichtyes raises the question of whether these electrosensory systems recruit the mechanisms of the mechanosensory system, from which they are thought to be derived, or whether they have some other connectivity (Braford and McCormick, 1993).
Hence, we will be making small Neurobiotin injections into the nuclei of the preglomerular complex in Xenomystes, an Osteoglossomorph, to examine the degree to which different sensory modes are isolated and the ways in which they are interconnected in the diencephalon. From this, we are hoping gain some insight into whether the novel electrosensory system parallels the mechanosensory system or not...