Electrophysiological and anatomical evidence for a direct projection from the nucleus of the basal optic root to the nucleus rotundus in pigeons

Distribution of calcium-binding proteins in the pigeon visual thalamic centers and related pretectal and mesencephalic nuclei. Phylogenetic and functional determinants

Multichannel processing of environmental information constitutes a fundamental basis of functioning of sensory systems in the vertebrate brain. Two distinct parallel visual systems - the tectofugal and thalamofugal exist in all amniotes. The vertebrate central nervous system contains high concentrations of intracellular calcium-binding proteins (CaBPrs) and each of them has a restricted expression pattern in different brain regions and specific neuronal subpopulations. This study aimed at describing the patterns of distribution of parvalbumin (PV) and calbindin (CB) in the visual thalamic and mesencephalic centers of the pigeon (Columba livia). We used a combination of immunohistochemistry and double labeling immunofluorescent technique. Structures studied included the thalamic relay centers involved in the tectofugal (nucleus rotundus, Rot) and thalamofugal (nucleus geniculatus lateralis, pars dorsalis, GLd) visual pathways as well as pretectal, mesencephalic, isthmic and thalamic structures inducing the driver and/or modulatory action to the visual processing. We showed that neither of these proteins was unique to the Rot or GLd. The Rot contained i) numerous PV-immunoreactive (ir) neurons and a dense neuropil, and ii) a few CB-ir neurons mostly located in the anterior dorsal part and associated with a light neuropil. These latter neurons partially overlapped with the former and some of them colocalized both proteins. The distinct subnuclei of the GLd were also characterized by different patterns of distribution of CaBPrs. Some (nucleus dorsolateralis anterior, pars magnocellularis, DLAmc; pars lateralis, DLL; pars rostrolateralis, DLAlr; nucleus lateralis anterior thalami, LA) contained both CB- and PV-ir neurons in different proportions with a predominance of the former in the DLAmc and DLL. The nucleus lateralis dorsalis of nuclei optici principalis thalami only contained PV-ir neurons and a neuropil similar to the interstitial pretectal/thalamic nuclei of the tectothalamic tract, nucleus pretectalis and thalamic reticular nucleus. The overlapping distribution of PV and CB immunoreactivity was typical for the pretectal nucleus lentiformis mesencephali and the nucleus ectomamillaris as well as for the visual isthmic nuclei. The findings are discussed in the light of the contributive role of the phylogenetic and functional factors determining the circuits׳ specificity of the different CaBPr types.

Looming responses of telencephalic neurons in the pigeon are modulated by optic flow

The movement of animals through space filled with various objects requires the interaction between neuronal mechanisms specialized for processing local object motion and those specialized for processing optic flow generated by self-motion of the animal. In the avian brain, visual nuclei in the tectofugal pathway are primarily involved in the detection of object motion. By contrast, the nucleus of the basal optic root (nBOR) and the pretectal nucleus lentiformis mesencephali (nLM) are dedicated to the analysis of optic flow. But little is known about how these two systems interact. Using single-unit recording in the entopallium of the tectofugal pathway, we show that some neurons appeared to be integrating visual information of looming objects and whole-field motion simulating optic flow. They specifically responded to looming objects, but their looming responses were modulated by optic flow. Optic flow in the nasotemporal direction, typically produced by the forward movement of the bird, only mildly inhibited the looming responses. Furthermore, these neurons started firing later than when the looming object was presented against a stationary background. However, optic flow in other directions, especially the temporonasal direction, strongly inhibited their looming responses. Previous studies have implicated looming-sensitive neurons in predator avoidance behavior and these results suggest that a bird in motion may need less time to initiate an avoidance response to an approaching object.

Tectorotundal connections in turtles: an electron microscopic tracing and GABA-immunocytochemical study

The nucleus rotundus of the turtles Emys orbicularis and Testudo horsfieldi was analysed by axonal tracing methods and post-embedding GABA immunocytochemistry. After injections of horseradish peroxidase or biotinylated dextran amine into the optic tectum, electron microscopic observations showed that the vast majority of ipsilateral tectorotundal axon terminals were small in size, had smooth contours and contained small, round, densely packed synaptic vesicles. These terminals were GABA-immunonegative, often gathered in clusters, and established asymmetrical synaptic contacts with either small- or medium-sized GABA-negative dendritic profiles and with GABA-immunoreactive (GABA-ir) dendrites, which did not contain synaptic vesicles. Occasional GABA-ir-labelled axon terminals were observed; these may arise from the rare GABAergic neurons in the central tectal layer, or from neurons in the ventral pretectal nucleus, which projects both to the optic tectum and nucleus rotundus. In addition to tracer-labelled axon terminals, we observed both GABA-negative and GABA-ir cell bodies and dendrites also labelled by the tracer. No GABA-ir presynaptic dendritic profiles containing synaptic vesicles were observed. The existence in reptiles of reciprocal connections between the nucleus rotundus and the optic tectum as a phylogenetically ancient feedback system is discussed.

Influence of the behavioural context on the optocollic reflex (OCR) in pigeons (Columba livia)

We investigated the effects of several behavioural conditions on the properties of the horizontal optocollic reflex (OCR) in pigeons. The head reflex was triggered by rotating the visual surroundings at different velocities (stimuli steps of 30-300 deg. s-1) and the characteristics of the slow and fast phases of the OCR were analysed during,(i) the `resting condition', in which animals were hung in a harness, (ii) the`standing condition', in which animals were freely standing, (iii) the`walking condition', in which animals were walking on a treadmill at different velocities, and (iv) the `flying condition', in which animals were hung in a harness and subjected to a frontal air-stream, provoking a flying posture.

In the `resting' condition, irregularities were observed in the amplitude of nystagmic beats, in the beating field and in the slow phase velocity (SPV)of the OCR. These irregularities diminished progressively when the behavioural condition changed from `standing' to `walking', and disappeared in the`flying' condition. Correlatively, the working range of the OCR (evaluated by its gain at the plateau of SPV) was progressively extended toward higher stimulation velocities.

The velocity of the fast phases of the OCR (measured for all the conditions except the `walking condition') also increased in correlation with the SPV. The walking speed did not influence the OCR in the treadmill velocity range of 0.20-0.40 m s-1. The presence of a frontal airstream in the`standing condition' did not change the OCR properties. This fact (and other observations discussed in the paper) suggests that the adaptation of the OCR to the behavioural context is mediated by internal signals generated by each behavioural condition.

Differential modulation of thalamic neurons by optokinetic nuclei in the pigeon

The visual system in the pigeon is composed of the tectofugal, thalamofugal and accessory optic pathways. Though their anatomy and physiology have been extensively studied, the functional interactions between these pathways are largely unknown. The present study shows by using multiple electrophysiological techniques that firing activity in the nucleus opticus principalis thalami (OPT) of the thalamofugal pathway is differentially modulated by the pretectal nucleus lentiformis mesencephali (nLM) and the nucleus of the basal optic root (nBOR) of the accessory optic system, two optokinetic nuclei responsible for generating eye movements to stabilize the image on the retina. Reversible inactivation, electrical stimulation, microiontophoresis and receptive field mapping experiments all consistently indicate that the nBOR-OPT pathway is inhibitory and mediated by GABA as a transmitter and its GABAA receptors, whereas the nLM-OPT pathway is excitatory and mediated by glutamate as a transmitter and its NMDA receptors. They also differentially modulate the size and/or responsiveness of receptive fields in OPT cells as well. Numerous electrode tip sites were histologically confirmed in the neural structures under study. The results suggest that these optokinetic nuclei may dually modulate the transfer of visual information from the retina to the telencephalon at the thalamic level during eye movements.

Comparisons of Visual Properties between Tectal and Thalamic Neurons with Overlapping Receptive Fields in the Pigeon

The present study is the first attempt to make comparisons of the visual response properties between tectal and thalamic neurons with spatially overlapping receptive fields by using extracellular recording and computer mapping techniques. The results show that in neuronal pairs about 70% of thalamic cells have excitatory receptive field alone, whereas 85% of tectal cells possess an excitatory receptive field surrounded by an inhibitory receptive field. In 70% of pairs the tectal cells are selective for direction of motion different from that which the thalamic cells prefer. Most thalamic cells prefer high speeds (80-160 degrees/s), whereas tectal cells prefer intermediate (40 degrees/s) or low (10-20 degrees/s) speeds. Photergic and scotergic cells exist in the thalamus but not in the tectum. These results provide evidence that tectal and thalamic cells extract different visual information from the same region of the visual field. The functional significance of these differences is discussed.

Lateralization of magnetic compass orientation in a migratory bird

Lateralization of brain functions, once believed to be a human characteristic, has now been found to be widespread among vertebrates. In birds, asymmetries of visual functions are well studied, with each hemisphere being specialized for different tasks. Here we report lateralized functions of the birds' visual system associated with magnetoperception, resulting in an extreme asymmetry of sensing the direction of the magnetic field. We found that captive migrants tested in cages with the magnetic field as the only available orientation cue were well oriented in their appropriate migratory direction when using their right eye only, but failed to show a significant directional preference when using their left eye. This implies that magnetoreception for compass orientation, assumed to take place in the eyes alongside the visual processes, is strongly lateralized, with a marked dominance of the right eye/left brain hemisphere.

Discharge patterns evoked by depolarizing current injection in basal optic nucleus neurons of the pigeon

The nucleus of the basal optic root of the accessory optic system in birds is involved in optokinetic nystagmus, which stabilizes images on the retina by compensatory movements of the eyes. The present paper studies the physiological and morphological properties of basal optic neurons in the pigeon by using a brain slice preparation and intracellular recordings. Sixty-one cells examined could be categorized into six types based on their firing patterns in response to depolarizing current injection. Type I cells (54%) fire spontaneously and more spikes as current intensity is increased. Type II cells (15%) discharge regular spikes with similar interspike intervals. Type III cells (5%) show an early burst followed by tonic firing. Type IV cells (5%) fire regular bursts with similar interburst intervals. Type V cells (16%) fire a few spikes in a cluster only at onset of current application. Type VI cells (5%) produce a hump-like depolarization or a single spike depending on current intensities. Seventeen cells stained with Lucifer yellow have multipolar or piriform perikarya (15-28 microm) with two to eight primary dendrites. In some cases, an axon is observed to originate from the cell body, traveling dorsolaterally or dorsally. The physiological significance of these findings is discussed.

Visual responses of neurons in the nucleus of the basal optic root to stationary stimuli in pigeons

The nucleus of the basal optic root of the accessory optic system in pigeons is involved in generating optokinetic nystagmus, which stabilizes object images on the retina by compensatory eye movements. Previous studies have indicated that basal optic neurons are selective for the direction and velocity of motion. The present study shows that these optokinetic cells also respond to stationary stimuli and thereby could be categorized into three groups. The first group of cells (69.1%) responds to stationary gratings orthogonal to the preferred direction but not to gratings parallel to the preferred direction. They do not respond to stationary random-dot patterns without any orientational cues. The second group of cells (7.4%) almost equally discharges a series of bursts in response to stationary gratings with any orientations and to random-dot patterns as well. The third group of cells (23.5%) is responsive to motion but not to stationary gratings and random-dot patterns. The receptive field of basal optic cells is composed of an excitatory field and an inhibitory field, both of which overlap or occupy different regions in the visual field. The aforementioned properties may be attributed to the excitatory receptive field, whereas the inhibitory receptive field is functional when visual stimuli are moving in the direction opposite to the preferred direction of basal optic cells. The functional significance of visual responses of optokinetic neurons to stationary patterns is discussed.