Spatiotemporal profile of synaptic activation produced by the electrical and visual stimulation of retinal inputs to the optic tectum: a current source density analysis in the pigeon (Columba livia)

Morphological study of the midbrain tectum in ostrich (Struthio camelus) embryo

In this study the morphological features of the optic tectum in ostrich embryo were studied macroscopically and microscopically. After gross anatomical study, fixed specimens of the optic lobes in 30th, 36th and 40th embryonic days were processed for paraffin sections. Sections were stained by Harris haematoxylin and eosin (H&E), Luxol Fast Blue/Cresyl Echt Violet and Malory PTAH dyes. The optic lobes had large volumes even on the 30th embryonic day and increased proportionally to age. The optic lobe consisted of two parts: gray matter (outer) and white matter (inner). The first external layer of the optic lobe e.g. molecular layer consisted of neural fibres, neuroglia and scarce small neurons. The most common appearance of the optic lobes was characterised by small to medium-sized neurons (rounded to pyramid-shaped with large and pale nucleus consistong of obvious nucleoli arranged in three layers whose thickness increased in the deeper one) supported by neuroglia. Larger size neurons and occasionally multipolar neurons were presented in the interior compared with these layers. The lateral mesencephalic nucleus was detectable in the optic lobe base even on 30th embryonic day and was composed of few multipolar neurons supported by neuroglia. The tectal ventricles were lined with simple cuboidal ciliated ependymal cells in the embryonic period. As embryonic age increased, the ratio of tectal ventricle volume to its thickness decreased. Special stainings showed that Nissl bodies and myelin fibres, also glial fibres were available from the 30th embryonic day and that their density, especially myelin fibres density, increased with age.

Spatiotemporal analysis of electrically evoked activity in the chicken optic tectum: a VSDI study

The midbrain is an important processing area for sensory information in vertebrates. The optic tectum and its mammalian counterpart, the superior colliculus, receive multimodal, topographic information and contain a sensory map that plays a role in spatial attention and orientation movements. Many studies have investigated the tectal circuitry by cytochemistry and by characterization of particular cell types. However, only a few studies have investigated network activation throughout the depth of the tectum. Our study provides the first data on spatiotemporal activity profiles in the depth and width of the avian optic tectum. We used an optical imaging approach with voltage-sensitive dyes to investigate population responses at a high temporal and spatial resolution. With the necessary caution due to cell extension across several layers, we can thus link our findings tentatively with the general layout of the avian optic tectum. Single electrical stimuli in the retinorecipient layers 1-4 evoked a complex optical response pattern with two components: a short, strong transient response and a weaker persistent response that lasted several hundred milliseconds. The response started in layer 5 and spread within this layer before it propagated into deeper layers. This is in line with neuroanatomical and earlier physiological data. Analysis of temporal sequence and pharmacological manipulations revealed that these responses were mainly driven by postsynaptic activation. Thus tectal network responses to patterned input can be studied by voltage-sensitive dye imaging.

Generating oscillatory bursts from a network of regular spiking neurons without inhibition

Avian nucleus isthmi pars parvocellularis (Ipc) neurons are reciprocally connected with the layer 10 (L10) neurons in the optic tectum and respond with oscillatory bursts to visual stimulation. Our in vitro experiments show that both neuron types respond with regular spiking to somatic current injection and that the feedforward and feedback synaptic connections are excitatory, but of different strength and time course. To elucidate mechanisms of oscillatory bursting in this network of regularly spiking neurons, we investigated an experimentally constrained model of coupled leaky integrate-and-fire neurons with spike-rate adaptation. The model reproduces the observed Ipc oscillatory bursting in response to simulated visual stimulation. A scan through the model parameter volume reveals that Ipc oscillatory burst generation can be caused by strong and brief feedforward synaptic conductance changes. The mechanism is sensitive to the parameter values of spike-rate adaptation. In conclusion, we show that a network of regular-spiking neurons with feedforward excitation and spike-rate adaptation can generate oscillatory bursting in response to a constant input.

Timing of ascending and descending visual signals predicts the response mode of single cells in the thalamic nucleus rotundus of the pigeon (Columba livia)

Neurons of the pigeon's diencephalic n. rotundus were demonstrated to show visual responses of short and long latency representing ascending signals of the retino-tecto-rotundal system and descending signals from telencephalo-tecto-rotundal fibers. Pigeons thus provide an ideal model to investigate the convergence of ascending and descending visual processing streams at single cell level. Although it is known that rotundal responses of long latency show distinct response characteristics, dependent on the stimulus being presented monocularly or binocularly, the mechanisms underlying these response differences are still unclear. While it is possible that the simultaneity of eye stimulation produces a change of processing, it is also possible that the relative timing and order between ipsilateral and contralateral signals are the decisive variable. To test between both possibilities, we recorded from cells in the pigeon's n. rotundus while providing monocular or binocular visual stimulation and varying the delay and order of eye presentations. We revealed that the precise temporal interaction and order of ascending and descending inputs to the tectum decide about late responses with burst or tonic characteristics. When descending signals reached the tectum before the ascending signals, rotundal cells showed late responses that were characterized by burst activity patterns. When ascending input reached the tectum first, responses with tonic characteristic were observed. These effects might become mediated by intratectal mechanisms, the nucleus ventrolateralis thalami, or the bed nuclei of the tectothalamic tract and might constitute the neural basis of a bihemispheric gating function.

Oscillatory bursts in the optic tectum of birds represent re-entrant signals from the nucleus isthmi pars parvocellularis

Fast oscillatory bursts (OBs; 500-600 Hz) are the most prominent response to visual stimulation in the optic tectum of birds. To investigate the neural mechanisms generating tectal OBs, we compared local recordings of OBs with simultaneous intracellular and extracellular single-unit recordings in the tectum of anesthetized pigeons. We found a specific population of units that responded with burst discharges that mirrored the burst pattern of OBs. Intracellular filling with biocytin of some of these bursting units demonstrated that they corresponded to the paintbrush axon terminals from the nucleus isthmi pars parvocellularis (Ipc). Direct recordings in the Ipc confirmed the high correlation between Ipc cell firing and tectal OBs. After injecting micro-drops of lidocaine in the Ipc, the OBs of the corresponding tectal locus disappeared completely. These results identify the paintbrush terminals as the neural elements generating tectal OBs. These terminals are presumably cholinergic and ramify across tectal layers in a columnar manner. Because the optic tectum and the Ipc are reciprocally connected such that each Ipc neuron sends a paintbrush axon to the part of the optic tectum from which its visual inputs come, tectal OBs represent re-entrant signals from the Ipc, and the spatial-temporal pattern of OBs across the tectum is the mirror representation of the spatial-temporal pattern of bursting neurons in the Ipc. We propose that an active location in the Ipc may act, via bursting paintbrushes in the tectum, as a focal "beam of attention" across tectal layers, enhancing the saliency of stimuli in the corresponding location in visual space.

Asymmetrical modes of visual bottom-up and top-down integration in the thalamic nucleus rotundus of pigeons

The aim of this study was to separate bottom-up and top-down influences within cerebral asymmetries. This was studied in the lateralized visual system of pigeons by recording from single units of the left and right diencephalic nucleus rotundus of the tectofugal pathway while visually stimulating the ipsilateral and/or contralateral eye. Analyses of response latencies revealed rotundal neurons with short and/or late response components. Cells with short latencies very likely represent bottom-up neurons participating in the ascending retinotectorotundal system. Because lidocaine injections into the visual Wulst produced a significant reduction of late response components only, neurons with long latencies were probably activated via a top-down telencephalotectorotundal system. The distribution and response characteristics of bottom-up and top-down neurons provided insight into several asymmetries of ascending and descending pathways. Asymmetries of the ascending retinotectorotundal system (bottom-up) were characterized by longer periods of tonic activation in the left and shorter response latencies in the right rotundus. Left-right differences in these responses probably facilitate faster access to visual input to the right hemisphere and a prolonged processing of this input in the left. The descending telencephalotectorotundal system (top-down) revealed a completely different lateralized organization. This system was characterized by long latency responses that exclusively derived from the left hemisphere, regardless of whether recordings took place in the left or the right rotundus. We assume that asymmetrical modes of visual processing within both hemispheres of the ascending tectofugal system are ultimately directed to left hemispheric forebrain mechanisms that subsequently generate executive control over sensory and motor structures.

The mapping of the visual field onto the dorso-lateral tectum of the pigeon (Columba livia) and its relations with retinal specializations

Most of the physiological studies of the pigeon retino-tectal visual pathway have investigated the accessible tectum, a small dorso-lateral tectal section that can be easily accessed by a simple craniotomy. However, at present we lack a detailed study of the topographical arrangement between the visual field, the retina and the accessible tectum. In particular, it is not known which section of the visual field is mapped onto the accessible tectum, and which of the specialized retinal areas mediates this projection. Here we determined, using local field potential (LFP) recordings and reverse retinoscopy, the shape, size and position in the visual space of the portion of the visual field mapped onto the accessible tectum (called here the accessible visual field, or AVF). Using this data and the mapping of Nalbach et al. [Vis. Res. 30 (4) (1990) 529], the retinal area corresponding to the AVF was determined. Such retinal area was also directly delimited by means of retrograde transport of DiI. The results indicate that the AVF is a triangular perifoveal zone encompassing only 15% of total visual field. The retinal region corresponding to the AVF has the shape of an elongated triangle that runs parallel to the visual equator and contains the fovea, the tip of the pecten, a perifoveal region of the yellow field and a small crescent of the red field. In agreement with this anatomical heterogeneity, visual evoked potentials measured in different parts of the accessible tectum present steep variations in shape and size. These results are helpful to better design and interpret anatomical and physiological experiments involving the pigeon's visual system.