Vision in Toads - Modulatory Loops and Evolutionary Perspectives

Modulatory Loops and Evolutionary Perspectives

Having analyzed neuronal processing streams in brain structures (pretectal, tectal, medullary) that mediate between visual stimuli and adequate behavioral responses in toads, Ewert and coworkers examined various neural loops that — in connection with certain forebrain structures (striatal, pallial, thalamic) — can initiate, modulate or modify stimulus-response mediation (Ewert and Schwippert 2006). For example, in the course of associative learning the toad’s visual prey schema can be modified to include non-prey objects. After lesions to a telencephalic structure involved in learning — the posterior ventromedial pallium — this learning effect fails and prey recognition shows again its species-specific selectivity. The posterior part of the ventromedial pallium is homologous to the hippocampus of mammals which is also involved in learning processes. Both in anuran amphibians and mammals striatal efferents are, for example, involved in directed attention, i.e. gating an orienting response towards a sensory stimulus. The anuran striatum is homologous to a portion of the amniote basal ganglia.

From an evolutionary point of view it is important to note that the tetrapod vertebrates share a common pattern of homologous brainstem and forebrain structures (e.g., see Reiner et al. 1998; González et al. 1999; Kenigfest et al. 2002). Neuroethological, neuroanatomical, and neurochemical investigations suggest that the neural networks underlying essential functions — such as attention, orienting, approaching, avoidance, associative or non-associative learning, and basic motor skills — have, so to speak, a phylogenetic origin in homologous structures of the amphibian brain.

From a neural network approach, it is reasonable to ask whether the toad’s ability to classify moving objects by special configuration cues — object’s dimension parallel vs. perpendicular to the direction of motion — is unique in the animal kingdom. Developmental studies suggest that this detection principle is an adaptation in terrestrial amphibians to their biotope and thus addressed to objects that are moving on land. Actually, it was found that this specific detection ability fails to occur in aquatic frogs (Rana esculenta). In common toads (Bufo bufo) it was shown that this detection property matures during the metamorphosis of the aquatic tadpoles in the course of their transition to terrestrial life (independent of food experience). Interestingly, comparable detection principles are discovered in amphibious fish (Periophthalmus koelreuteri) and in insect (Sphodromantis lineola). In mammals erect body postures, for example, may address a threat signal to a rival. This suggests that the configuration-algorithm responsible for the distinction between profitable (e.g., prey-like) vs. dangerous (e.g., threat-like) may be implemented by quite different neural networks. Studies in artificial neuronal nets support this presumption (for an example see Ewert 2004).