hindbrain at the level of the anterior medullary velum. Either the right MLF or caudal oculomotor nucleus was cut (n = 6 and 4, respectively), and biocytin crystals were applied for 20 min. Car- tilage was replaced, the skin was tightly sutured, and the animals were resuscitated. They were again anesthetized after 48 –72 h and perfused with 4% formaldehyde and 0.5% glutaraldehyde. The midbrain and hindbrain were dissected out, gelatin embedded, and sectioned at 75 m. The sections were processed with the use of the avidin-biotin peroxidase complex including NiCo intensifica- tion (18). In all cases, many heavily labeled vestibular neurons were found in the contralateral descending/posterior octaval subnucleus (Fig. 2A) and ipsilateral anterior octaval subnucleus (not illustrated). However, in all experiments, neurons were distributed throughout the contralateral hindbrain from the dorsal to ventral surface (Fig. 2B, D). In particular, a distinct subgroup of neurons largely ven- trolateral to the abducens nucleus (Fig. 2E) was reminiscent of the contralateral abducens internuclear neurons described in all other vertebrates (Fig. 1). Neurons were also distributed dorsally up to, and surrounding, the MLF; these neurons could be envisioned as analogous to the prepositus nucleus described in mammals. Clearly, neurons ipsilateral to either the oculomotor or MLF bio- cytin label were either sparse or absent (Fig. 2C, F). Significant terminal arborization appeared throughout the ipsilateral Abd nu- cleus (Fig. 2F) that conceivably derived from axon collaterals of ascending vestibular neurons. If so, then individual vestibular neurons might contact a pair of synergistic horizontal extraocular motoneurons (see Fig. 1). Neurons were also found in the pretec- tum, midbrain, and cerebellar nuclei that are, in all probability, correlated primarily with vertical eye movements and gaze (not illustrated). Overall, the distribution pattern of neurons throughout the brainstem of the elasmobranchs was surprisingly more exten- sive than expected and only different in anatomical detail from that observed in other vertebrates (6, 19 –21). In conclusion, the evolutionary significance of a proposed di- vergence in elasmobranch oculomotor organization remains uncer- tain. It is parsimonious to conclude that elasmobranchs have used the same genes located in conserved hindbrain segments to gen- erate novel neurons and connections contributing to conjugate horizontal eye movements (1, 3, 4, 18) (see Fig. 1). Single-cell experiments that correlate structure with function will precisely delineate the uniqueness of of these brainstem neurons. 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Bull. 203: 238 –239. (October 2002) Error-Driven Motor Learning in Fish John Montgomery (Leigh Marine Laboratory, University of Auckland, Auckland, New Zealand), Guy Carton, and David Bodznick 1 The cerebellum is considered a motor control structure, yet it shares strong anatomical similarities with the cerebellar-like nuclei of the fish hindbrain, which are clearly sensory and process elec- trosensory and lateral line information. Can these contrasting roles be brought together in a single framework, providing insight into cerebellar function? The striking anatomical similarity between the cerebellum and the cerebellar-like sensory nuclei is that they share a molecular layer organization in which the apical dendrites of the projection neurons receive input from many thousands of parallel fibers (1, 2). Our hypothesis is that, in both systems, plasticity of the synapses between the parallel fibers and the projection neurons allows the formation of a parallel fiber composite that can subtract unwanted activity in the projection neurons and their downstream targets. In cerebellar-like structures, this subtraction can be di- 1 Department of Biology, Wesleyan University, Middletown, CT. 238 REPORTS FROM THE MBL GENERAL SCIENTIFIC MEETINGS