Anterior decerebration blocks visual habituation in the larval salamander (Ambystoma punctatum)

Organization of the sensory input to the telencephalon in the fire-bellied toad, Bombina orientalis

The functional organization of sensory activity in the amphibian telencephalon is poorly understood. We used an in vitro brain preparation to compare the anatomy of afferent pathways with the localization of electrically evoked sensory potentials and single neuron intracellular responses in the telencephalon of the toad Bombina orientalis. Anatomical tracing showed that the anterior thalamic nucleus innervates the anterior parts of the medial, dorsal, and lateral pallia and the rostralmost part of the pallium in addition to the subpallial amygdala/ventral pallidum region. Additional afferents to the medial telencephalon originate from the thalamic eminence. Electrical stimulation of diverse sensory nerves and brain regions generated evoked potentials with distinct characteristics in the pallium, subpallial amygdala/ventral pallidum, and dorsal striatopallidum. In the pallium, this sensory activity is generated in the anterior medial region. In the case of olfaction, evoked potentials were recorded at all sites, but displayed different characteristics across telencephalic regions. Stimulation of the anterior dorsal thalamus generated a pattern of activity comparable to olfactory evoked potentials, but it became similar to stimulation of the optic nerve or brainstem after bilateral lesion of the lateral olfactory tract, which interrupted the antidromic activation of the olfactohabenular tract. Intracellular bimodal sensory responses were obtained in the anterior pallium, medial amygdala, ventral pallidum, and dorsal striatopallidum. Our results demonstrate that the amphibian anterior pallium, medial amygdala/ventral pallidum, and dorsal striatopallidum are multimodal sensory centers. The organization of the amphibian telencephalon displays striking similarities with the brain pathways recently implicated in mammalian goal-directed behavior.

Measures of phototaxis and movement detection in the larval salamander

In this investigation, we describe two precise tests of visual function that integrate quasinatural situations with time-lapse video recording and infrared computerized monitoring of activity to assess movement detection and phototaxic tendencies, respectively. Four groups of larvae from A. punctatum, A. tigrinum, A. mexicanum, and a mutant albino axolotl were tested in an alley containing light and dark halves and lined with infrared sensors to monitor their phototaxic response. A. punctatum showed no phototaxic tendency, while the other three groups displayed a strong negative phototaxic response. Enucleation of the eyes in mutant albinos eliminated the negative phototaxis. Visual detection of motion was tested by videorecording the behavior of A. punctatum, A. mexicanum, and the mutant albino axolotl larvae while they explored a large bowl with 6 small vials on the perimeter, one of which contained white worms. A. punctatum rapidly approached the worm vial and engaged in intense predatory behavior. A. mexicanum responded to the presence of worms very slowly and rapidly lost interest. Albino axolotls displayed no visual recognition of the worms. The results indicate that visual function can be precisely determined in larval salamanders utilizing behavioral measures consistent with the animal's natural tendencies.