Pretecto-tectal interactions: effects of lesioning and stimulating the pretectum on field potentials in the optic tectum of salamanders in vitro

Lesions of the dorsal striatum impair orienting behaviour of salamanders without affecting visual processing in the tectum

In amphibians, visual information in the midbrain tectum is relayed via the thalamus to telencephalic centres. Lesions of the dorsal thalamus of the salamander Plethodon shermani result in impairment of orienting behaviour and in modulation of spike pattern of tectal neurons. These effects may be induced by an interruption of a tectum-thalamus-telencephalon-tectum feedback loop enabling spatial attention and selection of visual objects. The striatum is a potential candidate for involvement in this pathway; accordingly, we investigated the effects of lesioning the dorsal striatum. Compared to controls and sham lesioned salamanders, striatum-lesioned animals exhibited a significantly lower number of orienting responses toward one of two competing prey stimuli. Orienting towards stimuli was impaired, while the spike pattern of tectal cells was unaffected, because both in controls and striatum-lesioned salamanders the spike number significantly decreased at presentation of one prey stimulus inside the excitatory receptive field and another one in the surround compared to that at single presentation inside the excitatory receptive field. We conclude that the dorsal striatum contributes to orienting behaviour, but not to an inhibitory feedback signal onto tectal neurons. The brain area engaged in the feedback loop during visual object discrimination and selection has yet to be identified. Information processing in the amphibian striatum includes multisensory integration; the striatum generates behavioural patterns that influence (pre)motor processing in the brainstem. This situation resembles the situation found in rats, in which the dorsolateral striatum is involved in stimulus-response learning regardless of the sensory modality, as well as in habit formation.

The role of the dorsal thalamus in visual processing and object selection: a case of an attentional system in amphibians

In amphibians, the midbrain tectum is regarded as the visual centre for object recognition but the functional role of forebrain centres in visual information processing is less clear. In order to address this question, the dorsal thalamus was lesioned in the salamander Plethodon shermani, and the effects on orienting behaviour or on visual processing in the tectum were investigated. In a two-alternative-choice task, the average number of orienting responses toward one of two competing prey or simple configural stimuli was significantly decreased in lesioned animals compared to that of controls and sham-lesioned animals. When stimuli were presented during recording from tectal neurons, the number of spikes on presentation of a stimulus in the excitatory receptive field and a second salient stimulus in the surround was significantly reduced in controls and sham-lesioned salamanders compared to single presentation of the stimulus in the excitatory receptive field, while this inhibitory effect on the number of spikes of tectal neurons was absent in thalamus-lesioned animals. In amphibians, the dorsal thalamus is part of the second visual pathway which extends from the tectum via the thalamus to the telencephalon. A feedback loop to the tectum is assumed to modulate visual processing in the tectum and to ensure orienting behaviour toward visual objects. It is concluded that the tectum-thalamus-telencephalon pathway contributes to the recognition and evaluation of objects and enables spatial attention in object selection. This attentional system in amphibians resembles that found in mammals and illustrates the essential role of attention for goal-directed visuomotor action.

Iterative cooperation between parallel pathways for object and background motion

In a typical visual scene, one or more objects move relative to a larger background, which can itself be in motion as a result of the observer's eyes moving with respect to the outside world. Here we show that accurate estimation of the background motion from an image velocity field can be accomplished through an iterative cooperation between two modules: one that specializes in calculating a weighted average velocity and another one calculating a velocity contrast map. We build on our analysis to provide a model for the tectum-pretectum loop in the nonmammalian midbrain. Our model accounts for some of the known properties of the tectal neurons (sensitivity to relative motion) and pretectal neurons (sensitivity to whole-field motion). It also agrees with our knowledge of the pretectotectal projection (divergent and inhibitory), and with the results of lesion studies in which the pretectal input to the tectum was removed, leading to hyperactivity of the tectal neurons and the animal. Our model also makes a testable prediction regarding the tectopretectal projection, i.e., that the presence of a larger object and a bigger discrepancy between the directions of motion for the object and the background lead to a larger error by the pretectum in estimating the background motion when the tectal input is abolished.

Ultrastructure of neuropeptide-Y immunoreactive elements in the superior colliculus of cat

Whereas the presence of neuropeptide-Y (NPY) in the superior colliculus (SC) has been established, its participation in the ultrastructural organisation of the neuronal networks in the SC has not been studied. Accordingly, in the present paper light and electron microscopic NPY immunohistochemical studies were performed on the SC of cat. NPY fibres were found to be present predominantly in the superficial grey layer (SGL) of the SC, though a few small NPY cells were found in both the deeper and the upper layers. Ultrastructural observations revealed that the NPY nerve endings establish almost exclusively axo-dendritic synaptic contacts in the SGL of the SC. Thus, the presumably inhibitory impact of the NPY terminals is exerted through the dendrites of the SGL neurons, and not directly to the retinal axons, as thought previously.

Neuropeptides and amphibian prey-catching behavior

In mammals, a number of hypothalamic neuropeptides have been implicated in stress-induced feeding disorders. Recent studies in anurans suggest that stress-related neuropeptides may act on elemental aspects of visuomotor control to regulate feeding. Corticotropin-releasing hormone (CRH) and alpha-melanocyte-stimulating hormone, potent an orexic peptides in mammals, inhibit visually-guided prey-catching in toads. Neuropeptide Y (NPY), an orexic peptide in mammals, may be an important neuromodulator in inhibitory pre-tectal-tectal pathways involved in distinguishing predator and prey. Melanocortin, NPY and CRH neurons project onto key visuomotor structures within the amphibian brain, suggesting physiological roles in the modulation of prey-catching. Thus, neuropeptides involved in feeding behavior in mammals influence the efficacy of a visual stimulus in releasing prey-catching behavior. These neuropeptides may play an important role in how frogs and toads gather and process visual information, particularly during stress.

Neural modulation of visuomotor functions underlying prey-catching behaviour in anurans: perception, attention, motor performance, learning

The present review points out that visuomotor functions in anurans are modifiable and provides neurophysiological data which suggest modulatory forebrain functions. The retino-tecto/tegmento-bulbar/spinal serial processing streams are sufficient for stimulus-response mediation in prey-catching behaviour. Without its modulatory connections to forebrain structures, however, these processing streams cannot manage perceptual tasks, directed attention, learning performances, and motor skills. (1) Visual prey/non-prey discrimination is based on the interaction of this processing stream with the pretectal thalamus involving the neurotransmitter neuropeptide-Y. (2) Experiments applying the dopamine agonist apomorphine in combination with 2DG mapping and single neurone recording suggest that prey-catching strategies in terms of hunting prey and waiting for prey depend on dose dependent dopaminergic adjustments in the neural macronetwork in which retinal, pretecto-tectal, basal ganglionic, limbic, and mesolimbic structures participate. (3) Visual response properties of striatal efferent neurones support the concept that ventral striatum is involved in directed attention. (4) Various modulatory loops involving the ventral medial pallium modify prey-recognition in the course of visual or visual-olfactory learning (associative learning) or are responsible for stimulus-specific habituation (non-associative learning). (5) The circuits suggested to underlie modulatory forebrain functions are accentuated in standard schemes of the neural macronetwork. These provide concepts suitable for future decisive experiments.