Telencephalic projections to the nucleus of the basal optic root and pretectal nucleus lentiformis mesencephali in pigeons

Development of the horizontal optocollic reflex in juvenile barn owls (Tyto furcata pratincola)

Adult barn owls and primates possess an almost symmetric monocular rotational horizontal optocollic reflex. In primates, the reflex is initially asymmetric and becomes symmetric with time after birth. The condition in barn owls has not been studied so far. Here, we present data on the development of this reflex in this bird. We tested juvenile barn owls from the time before they open their eyes after hatching to the time they reach adult feather length. Wide-field visual patterns served as stimuli. They were presented at different rotational speeds in binocular and monocular settings. The binocular horizontal optocollic responses of juvenile barn owls were symmetric and adult-like on the first day that the birds responded to the stimulus. The monocular responses showed different rates of development in respect to stimulus velocity and stimulus direction. For velocities up to 20 deg/s, the monocular reflex was also adult-like on the first day that the birds responded to the stimulus. An initially higher asymmetry for 30 deg/s compared to adults disappeared within about two weeks. The development at even higher velocities remained unclear.

Response properties of optic flow neurons in the accessory optic system of hummingbirds versus zebra finches and pigeons

Optokinetic responses function to maintain retinal image stabilization by minimizing optic flow that occurs during self-motion. The hovering ability of hummingbirds is an extreme example of this behavior. Optokinetic responses are mediated by direction-selective neurons with large receptive fields in the accessory optic system (AOS) and pretectum. Recent studies in hummingbirds showed that, compared with other bird species, 1) the pretectal nucleus lentiformis mesencephali (LM) is hypertrophied, 2) LM has a unique distribution of direction preferences, and 3) LM neurons are more tightly tuned to stimulus velocity. In this study, we sought to determine if there are concomitant changes in the nucleus of the basal optic root (nBOR) of the AOS. We recorded the visual response properties of nBOR neurons to large-field-drifting random dot patterns and sine-wave gratings in Anna's hummingbirds and zebra finches and compared these with archival data from pigeons. We found no differences with respect to the distribution of direction preferences: Neurons responsive to upward, downward, and nasal-to-temporal motion were equally represented in all three species, and neurons responsive to temporal-to-nasal motion were rare or absent (<5%). Compared with zebra finches and pigeons, however, hummingbird nBOR neurons were more tightly tuned to stimulus velocity of random dot stimuli. Moreover, in response to drifting gratings, hummingbird nBOR neurons are more tightly tuned in the spatiotemporal domain. These results, in combination with specialization in LM, support a hypothesis that hummingbirds have evolved to be "optic flow specialists" to cope with the optomotor demands of sustained hovering flight.NEW & NOTEWORTHY Hummingbirds have specialized response properties to optic flow in the pretectal nucleus lentiformis mesencephali (LM). The LM works with the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) to process global visual motion, but whether the neural response specializations observed in the LM extend to the nBOR is unknown. Hummingbird nBOR neurons are more tightly tuned to visual stimulus velocity, and in the spatiotemporal domain, compared with two nonhovering species.

Pretecto- and ponto-cerebellar pathways to the pigeon oculomotor cerebellum follow a zonal organization

Both birds and mammals have relatively large forebrains and cerebella. In mammals, there are extensive sensory-motor projections to the cerebellum through the pontine nuclei originating from several parts of the cerebral cortex. Similar forebrain-to-cerebellum pathways exist in birds, but the organization of this circuitry has not been studied extensively. Birds have two nuclei at the base of the brainstem that are thought to be homologous to the pontine nuclei of mammals, the medial and lateral pontine nuclei (PM, PL). Additionally, birds are unique in that they have a pretectal nucleus called the medial spiriform nucleus (SpM) that, like the pontine nuclei, also receives projections from the forebrain and projects to the oculomotor cerebellum (OCb; folia VI to VIII). The OCb also receives input from the pretectal nucleus lentiformis mesencephali (LM), which analyzes visual optic flow information resulting from self-movement. In this study, we used single or double injections of fluorescent tracers to study the organization of these inputs from PM, PL, SpM and LM to the OCb in pigeons. We found that these inputs follow a zonal organization. The most medial zone in the OCb, zone A1, receives bilateral inputs from the lateral SpM, PL and LM. Zones A2 and C receive a bilateral projection from the medial SpM, and a mostly contralateral projection from PM and LM. We discuss how the pathway to zone A1 processes mainly visuo-motor information to spinal premotor areas, whereas the pathways to zone A2/C processes somato-motor and visuo-motor information and may have a feedback/modulatory role.

Pretectal projections to the oculomotor cerebellum in hummingbirds (Calypte anna), zebra finches (Taeniopygia guttata), and pigeons (Columba livia)

In birds, optic flow is processed by a retinal-recipient nucleus in the pretectum, the nucleus lentiformis mesencephali (LM), which then projects to the cerebellum, a key site for sensorimotor integration. Previous studies have shown that the LM is hypertrophied in hummingbirds, and that LM cell response properties differ between hummingbirds and other birds. Given these differences in anatomy and physiology, we ask here if there are also species differences in the connectivity of the LM. The LM is separated into lateral and medial subdivisions, which project to the oculomotor cerebellum and the vestibulocerebellum. In pigeons, the projection to the vestibulocerebellum largely arises from the lateral LM; the projection to the oculomotor cerebellum largely arises from the medial LM. Here, using retrograde tracing, we demonstrate differences in the distribution of projections in these pathways between Anna's hummingbirds (Calypte anna), zebra finches (Taeniopygia guttata), and pigeons (Columba livia). In all three species, the projections to the vestibulocerebellum were largely from lateral LM. In contrast, projections to the oculomotor cerebellum in hummingbirds and zebra finches do not originate in the medial LM (as in pigeons) but instead largely arise from pretectal structures just medial, the nucleus laminaris precommissuralis and nucleus principalis precommissuralis. These species differences in projection patterns provide further evidence that optic flow circuits differ among bird species with distinct modes of flight.

The retinal projection to the nucleus lentiformis mesencephali in zebra finch (Taeniopygia guttata) and Anna's hummingbird (Calypte anna)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are retinal recipient nuclei involved in the analysis of optic flow and the generation of the optokinetic response. In both pigeons and chickens, retinal inputs to the nBOR arise from displaced ganglion cells (DGCs), which are found at the margin of the inner nuclear and inner plexiform layers. The LM receives afferents from retinal ganglion cells, but whether DGCs also project to LM is a matter of debate. Previous work in chickens had concluded that DGCs do not project to LM, but a recent study in pigeons found that both retinal ganglion cells and DGCs project to LM. These findings leave open the question of whether there are species differences with respect to the DGC projection to LM. In the present study, we made small injections of retrograde tracer into the LM in a zebra finch and an Anna's hummingbird. In both cases, retrogradely labeled retinal ganglion cells and DGCs were observed. These results suggest that a retinal input to the LM arising from DGCs is characteristic of most, if not all, birds.

Differential projections of the densocellular and intermediate parts of the hyperpallium in the pigeon (Columba livia)

The visual Wulst in birds shows a four-layered structure: apical part of the hyperpallium (HA), interstitial part of HA (IHA), intercalated part of hyperpallium (HI), and densocellular part of hyperpallium (HD). HD also connects with the hippocampus and olfactory system. Because HD is subjacent to HI, the two have been treated as one structure in many studies, and the fiber connections of HD have been examined by afferents and efferents originating outside HD. However, to clarify the difference between these two layers, they need to be treated separately. In the present study, the fiber connections of HD and HI were analyzed with tract-tracing techniques using a combination of injections of cholera toxin subunit B (CTB) for retrograde tracing and biotinylated dextran amine (BDA) for anterograde tracing. When the two tracers were bilaterally injected in HD, a major reciprocal connection was seen with the dorsolateral subdivision (DL) of the hippocampal formation. When CTB and BDA were bilaterally injected in HI, strong reciprocal connections were found between HI and HA. Next, projection neurons in HD and HI were examined by double staining for CTB combined with vesicular glutamate transporter 2 (vGluT2) mRNA in situ hybridization. After CTB was injected in DL or HA, many neurons revealed CTB+/vGluT2+ in HD or HI, respectively. Furthermore, in situ hybridization showed that DL and HA contained neurons expressing various subunits of ionotropic glutamate receptors: AMPA, kainate, and NMDA types. These results suggest that glutamatergic neurons in HD and HI project primarily to DL and HA, respectively.

Expression of calcium-binding proteins in cerebellar- and inferior olivary-projecting neurons in the nucleus lentiformis mesencephali of pigeons

In the avian brain, the optokinetic response is controlled by two retinal-recipient nuclei: the nucleus of the basal optic root (nBOR) of the accessory optic system and the pretectal nucleus lentiformis mesencephali (LM). Although considered sister nuclei because of their similar response properties and function, there are both similarities and differences with respect to efferent projections and neurochemistry. Both nBOR and LM project to the cerebellum (Cb) directly as mossy fibers but also indirectly via the inferior olive (IO). In a previous report, we showed that the cerebellar- and inferior olivary-projecting neurons in nBOR of pigeons differentially express the calcium-binding proteins calretinin (CR) and parvalbumin (PV). Both CR and PV are expressed in the somata of LM neurons, although the latter is not as prevalent, and whether expression of CR and PV reflects cerebellar and IO projections is not known. In this report, by combining retrograde neuronal tracing from the Cb and IO with fluorescent immunohistochemistry, we examined the expression of these calcium-binding proteins in the pigeon LM. Half (52%) of the cerebellar-projecting neurons were CR+ve, but only 15% were PV+ve. Almost all (>95%) these PV+ve cells also expressed CR. In contrast, few of the IO-projecting neurons expressed CR or PV (≤5%). This is strikingly similar to what we observed in nBOR and reveals that calcium-binding protein expression is concordant with projection patterns in two nuclei that share similar functions.

Programmed acute electrical stimulation of ventral tegmental area alleviates depressive-like behavior

Depressive disorders affect approximately 5% of the population in any given year. Antidepressants may require several weeks to produce their clinical effects. Despite progress being made in this area there is still room and a need to explore additional therapeutic modes to increase treatment effectiveness and responsiveness. Herein, we examined a new method for intervention in depressive states based on deep brain stimulation of the ventral tegmental area (VTA) as a source of incentive motivation and hedonia, in comparison to chemical antidepressants. The pattern of stimulation was fashioned to mimic the firing pattern of VTA neurons in the normal rat. Behavioral manifestations of depression were then monitored weekly using a battery of behavioral tests. The results suggest that treatment with programmed acute electrical stimulation of the VTA substantially alleviates depressive behavior, as compared to chemical antidepressants or electroconvulsive therapy, both in onset time and longitudinal effect. These results were also highly correlated with increases in brain-derived neurotrophic factor mRNA levels in the prefrontal cortex.

Directional responses of visual wulst neurones to grating and plaid patterns in the awake owl

The avian retinothalamofugal pathway reaches the telencephalon in an area known as visual wulst. A close functional analogy between this area and the early visual cortex of mammals has been established in owls. The goal of the present study was to assess quantitatively the directional selectivity and motion integration capability of visual wulst neurones, aspects that have not been previously investigated. We recorded extracellularly from a total of 101 cells in awake burrowing owls. From this sample, 88% of the units exhibited modulated directional responses to sinusoidal gratings, with a mean direction index of 0.74 +/- 0.03 and tuning bandwidth of 28 +/- 1.16 degrees . A direction index higher than 0.5 was observed in 66% of the cells, thereby qualifying them as direction selective. Motion integration was tested with moving plaids, made by adding two sinusoidal gratings of different orientations. We found that 80% of direction-selective cells responded optimally to the motion direction of the component gratings, whereas none responded to the global motion of plaids, whose direction was intermediate to that of the gratings. The remaining 20% were unclassifiable. The strength of component motion selectivity rapidly increased over a 200 ms period following stimulus onset, maintaining a relatively sustained profile thereafter. Overall, our data suggest that, as in the mammalian primary visual cortex, the visual wulst neurones of owls signal the local orientated features of a moving object. How and where these potentially ambiguous signals are integrated in the owl brain might be important for understanding the mechanisms underlying global motion perception.

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.

A comparison of ventral tegmental neurons projecting to optic flow regions of the inferior olive vs. the hippocampal formation

The ventral tegmental area (catecholaminergic group A10) is a midbrain region characterized by concentrated dopaminergic immunoreactivity. Previous studies in pigeons show that the ventral tegmental area provides a robust projection to the hippocampal formation and to the medial column of the inferior olive. However, the distribution, morphology, and neurochemical content of the neurons that constitute these projections have not been resolved. In this study, we used a combination of retrograde tracing techniques and immunofluorohistochemistry to address these issues. Retrograde tracers were used to demonstrate that the distribution of ventral tegmental area neurons projecting to the hippocampus and the inferior olive overlap in the caudo-ventral ventral tegmental area. The hippocampus- and inferior olive-projecting ventral tegmental area neurons could not be distinguished based on morphology: most neurons had small- to medium-sized multipolar or fusiform soma. Double-labeling with fluorescent retrograde tracers revealed that the hippocampus- and medial column of the inferior olive-projecting neurons were found intermingled in the ventral tegmental area, but no cells were double labeled; i.e. individual ventral tegmental area neurons do not project to both the hippocampal formation and medial column of the inferior olive. Finally, we found that a minority (8.2%) of ventral tegmental area neurons providing input to the hippocampus were tyrosine hydroxylase-immunoreactive, whereas none of the inferior olive-projecting neurons were tyrosine hydroxylase positive. Combined, our findings show that the projections to the hippocampus and olivocerebellar pathway arise from intermixed subpopulations of ventral tegmental area neurons with indistinguishable morphology but only the hippocampal projection involves dopaminergic neurons. We suggest that equivalent projections from the ventral tegmental area to the hippocampal formation and inferior olive exist in mammals and discuss their potential role in the processing of optic flow and the analysis of self-motion.