"Shepherd's crook" neurons drive and synchronize the enhancing and suppressive mechanisms of the midbrain stimulus selection network.

The optic tectum (TeO), or superior colliculus, is a multisensory midbrain center that organizes spatially orienting responses to relevant stimuli. To define the stimulus with the highest priority at each moment, a network of reciprocal connections between the TeO and the isthmi promotes competition between concurrent tectal inputs. In the avian midbrain, the neurons mediating enhancement and suppression of tectal inputs are located in separate isthmic nuclei, facilitating the analysis of the neural processes that mediate competition. A specific subset of radial neurons in the intermediate tectal layers relay retinal inputs to the isthmi, but at present it is unclear whether separate neurons innervate individual nuclei or a single neural type sends a common input to several of them. In this study, we used in vitro neural tracing and cell-filling experiments in chickens to show that single neurons innervate, via axon collaterals, the three nuclei that comprise the isthmotectal network. This demonstrates that the input signals representing the strength of the incoming stimuli are simultaneously relayed to the mechanisms promoting both enhancement and suppression of the input signals. By performing in vivo recordings in anesthetized chicks, we also show that this common input generates synchrony between both antagonistic mechanisms, demonstrating that activity enhancement and suppression are closely coordinated. From a computational point of view, these results suggest that these tectal neurons constitute integrative nodes that combine inputs from different sources to drive in parallel several concurrent neural processes, each performing complementary functions within the network through different firing patterns and connectivity.

The centrifugal visual system of a palaeognathous bird, the Chilean Tinamou (Nothoprocta perdicaria).

The avian centrifugal visual system, which projects from the brain to the retina, has been intensively studied in several Neognathous birds that have a distinct isthmo-optic nucleus (ION). However, birds of the order Palaeognathae seem to lack a proper ION in histologically stained brain sections. We had previously reported in the palaeognathous Chilean Tinamou (Nothoprocta perdicaria) that intraocular injections of Cholera Toxin B subunit retrogradely label a considerable number of neurons, which form a diffuse isthmo-optic complex (IOC). In order to better understand how this IOC-based centrifugal visual system is organized, we have studied its major components by means of in vivo and in vitro tracing experiments. Our results show that the IOC, though structurally less organized than an ION, possesses a dense core region consisting of multipolar neurons. It receives afferents from neurons in L10a of the optic tectum, which are distributed with a wider interneuronal spacing than in Neognathae. The tecto-IOC terminals are delicate and divergent, unlike the prominent convergent tecto-ION terminals in Neognathae. The centrifugal IOC terminals in the retina are exclusively divergent, resembling the terminals from "ectopic" centrifugal neurons in Neognathae. We conclude that the Tinamou's IOC participates in a comparable general IOC-retina-TeO-IOC circuitry as the neognathous ION. However, the connections between the components are structurally different and their divergent character suggests a lower spatial resolution. Our findings call for further comparative studies in a broad range of species for advancing our understanding of the evolution, plasticity and functional roles of the avian centrifugal visual system.

Laminar segregation of GABAergic neurons in the avian nucleus isthmi pars magnocellularis: a retrograde tracer and comparative study.

The isthmic complex is part of a visual midbrain circuit thought to be involved in stimulus selection and spatial attention. In birds, this circuit is composed of the nuclei isthmi pars magnocellularis (Imc), pars parvocellularis (Ipc), and pars semilunaris (SLu), all of them reciprocally connected to the ipsilateral optic tectum (TeO). The Imc conveys heterotopic inhibition to the TeO, Ipc, and SLu via widespread γ-aminobutyric acid (GABA)ergic axons that allow global competitive interactions among simultaneous sensory inputs. Anatomical studies in the chick have described a cytoarchitectonically uniform Imc nucleus containing two intermingled cell types: one projecting to the Ipc and SLu and the other to the TeO. Here we report that in passerine species, the Imc is segregated into an internal division displaying larger, sparsely distributed cells, and an external division displaying smaller, more densely packed cells. In vivo and in vitro injections of neural tracers in the TeO and the Ipc of the zebra finch demonstrated that neurons from the external and internal subdivisions project to the Ipc and the TeO, respectively, indicating that each Imc subdivision contains one of the two cell types hodologically defined in the chick. In an extensive survey across avian orders, we found that, in addition to passerines, only species of Piciformes and Rallidae exhibited a segregated Imc, whereas all other groups exhibited a uniform Imc. These results offer a comparative basis to investigate the functional role played by each Imc neural type in the competitive interactions mediated by this nucleus.

Pre-pupation behaviour of the aphid parasitoid Aphidius ervi (Haliday) and its consequences for pre-imaginal learning.

Olfactory learning may occur at different stages of insect ontogeny. In parasitoid wasps, it has been mostly shown at adult emergence, whilst it remains controversial at pre-imaginal stages. We followed larval growth of the parasitoid wasp, Aphidius ervi Haliday, inside the host aphid, Acyrthosiphom pisum Harris, and characterised in detail the behaviour of third instar larvae. We found that just before cocoon spinning begins, the third instar larva bites a hole through the ventral side of the mummified aphid exoskeleton. We then evaluated whether this period of exposure to the external environment represented a sensitive stage for olfactory learning. In our first experiment, the third instar larvae were allowed to spin their cocoon on the host plant (Vicia faba L.) surface or on a plastic plate covering the portion of the host plant exposed to the ventral opening. Recently emerged adults of the first group showed a preference for plant volatiles in a glass Y-olfactometer, whereas no preference was found in adults of the second group. In a second experiment, during the period in which the aphid carcass remains open or is being sealed by cocoon spinning, third instar larvae were exposed for 24 h to either vanilla odours or water vapours as control. In this experiment, half of the parasitoid larvae were later excised from the mummy to avoid further exposure to vanilla. Adult parasitoids exposed to vanilla during the larval ventral opening of the mummy showed a significant preference for vanilla odours in the olfactometer, regardless of excision from the mummy. The larval behaviour described and the results of the manipulations performed are discussed as evidences for the acquisition of olfactory memory during the larval stage and its persistence through metamorphosis.