'Vision in Toads' provides detailed information and factual summaries regarding the neural correlates of visually guided prey catching and threat avoidance behavior (neuroethology) in a phylogenetically basal terrestrial vertebrate species.Toad Video: Image Processing in the Visual System of the Common Toad — Behaviour, Brain Function, Artificial Neuronal Net. https://doi.org/10.3203/IWF/C-1805eng It sheds light on central themes such as visual feature recognition, the 'go functions' of coded triggering systems,Ewert J.-P. (1997) Neural correlates of key stimulus and releasing mechanism: a case study and two concepts. In: Trends in Neurosciences 20(8), 332-339 the generation of behavioral patterns,Ewert J.-P., Framing E.M., Schürg-Pfeiffer E., Weerasuriya A. (1990) Responses of medullary neurons to moving visual stimuli in the common toad. I.Characterization of medial reticular neurons by extracellular recording. In: Journal of Comparative Physiology A 167, 495-508 as well as their modification through individual experience.Ewert J.-P., Dinges A.W., Finkenstädt T. (1994) Species-universal stimulus responses, modified through conditioning, reappear after telencephalic lesions in toads. In: Naturwissenschaften 81, 317-320 At the same time, it provides historical context, key figures, and concepts of evolutionary perspectives. For example, evolution has endowed the phylogenetically basal anuran brain for configurational pattern recognition with the ability to detect the orientation of moving visual contrast boundaries by implicit computationStevens K.H. (1987) Implicit versus explicit computation. In: Behavioral and Brain Sciences 10: 387-388 using an intelligent, efficient algorithm — thereby bypassing an explicit method that takes advantage of the immense neuroarchitecture characteristic of the striate cortex in mammals (see also: feature detection (nervous system)).
Since the 1960s, Jörg-Peter Ewert and his research group TH Darmstadt, Univ Kassel. have been conducting studies with the common toad Bufo b. bufo (L.) to investigate in depth the neural basis of visually guided prey recognition and the distinction between prey and enemy. The neurobehavioral analysis of the toad's visual system has become one of the 'models' in vertebrate neuroethology.Ewert J.-P., Capranica R.R., Ingle D.J., (Eds.)(1983)Advances in Vertebrate Neuroethology. Proceedings of the Advanced Study Institute „Advances in Vertebrate Neuroethology" at Univ. Kassel, Germany, August 1981. Science Series A, Vol.56, Plenum Press 1983, Publisher Springer New York, NY, pp. 1238 This example has received constructive feedback from distinguished authorities in this field, who have offered valuable insights from various perspectives in an Open Peer Commentary in the journal Behavioral and Brain SciencesEbbesson, S.O.E. (1987)Prey-catching in toads: An exceptional neuroethological model. In: Behavioral and Brain Sciences 10(3), 337-405Baerends G.P. (1987)Ethology and physiology: A happy marriage. In: Behavioral and Brain Sciences 10(3),369-370Fentress R.D. (1987)More than meets the eye. In: Behavioral and Brain Sciences 10(3), 378-379Kondrashev S.L. (1987)Neuroethology and color vision in amphibians. In: Behavioral and Brain Sciences 10(3), 385Goodale M.A. (1987)The complete visual system: From input to output. In: Behavioral and Brain Sciences 10(3), 379-380Comer C.M. (1987)Sensori-motor functions: What is a command, that a code may yield it?. In: Behavioral and Brain Sciences 10(3), 372DiDomenico R. and EatonR.C. (1987)Toward a reformulation of the command concept. In: Behavioral and Brain Sciences 10(3), 374-375Stevens K.H. (1987)Implicit versus explicit computation. In: Behavioral and Brain Sciences 10(3), 387-388Arbib M.A. (1987)Advantages of experimentation in neuroscience. In: Behavioral and Brain Sciences 10(3), 368-369Ehret G. (1987)Networks with evolutionary potential. In: Behavioral and Brain Sciences 10(3), 376-377Camhi J.M. (1987) How is the toad not like a bug? In: Behavioral and Brain Sciences 10(3), 371-372Doty R.W. (1987)Has the greedy toad lost its soul; and if so, what was it?. In: Behavioral and Brain Sciences 10(3), 375Dennett D. (1987)Eliminate the middle toad! In: Behavioral and Brain Sciences 10(3), 372-374, see also Viewpoint in Trends in Neurosciences.Ewert J.-P. (1997)Neural correlates of key stimulus and releasing mechanism: a case study and two concepts. In: Trends in Neurosciences 20(8), 332-339Marin O., Gonzalez A., Smeets W.J.A.J. (1998)Amphibian basal ganglia control of tectal function: a complex matter. Comment to Viewpoint EwertTINS20(8). In: Trends in Neurosciences 21(8), 336Ewert J.-P. (1998)Reply abstract to Comment MarinTINS21(8). In: Trends in Neurosciences 21(8), 336-337
Ewert's workJ.-P. Ewert (1967)Untersuchungen über die Anteile zentralnervöser Aktionen an der taxisspezifischen Ermüdung beim Beutefang der Erdkröte (Bufo bufo L.). (Investigations on the central nervous influences to the taxis-specific habituation of the common toad (Bufo bufo L.). In: Z. vergl. Physiol. 57, 263-298.J.-P. Ewert (1968)Der Einfluss von Zwischenhirndefekten auf die Visuomotorik im Beute- und Fluchtverhalten der Erdkröte (Bufo bufo L.). (The influence of diencephalic lesions to visuomotor processes of the prey-catching and escape behavior in common toads Bufo bufo L.). In: Z. verg. Physiol. 61, 41-70.Ewert J.-P. (1974) The neural basis of visually guided behavior. In: Scientific American 230(3), 34-42Ewert J.-P. (1980) Neuroethology: an introduction to the neurophysiological fundamentals of behavior. In: Springer-Verlag, Berlin/Heidelberg/New YorkEwert J.-P. (2004) Motion perception shapes the visual world of amphibians. In: Prete F.R. (ed.) Complex Worlds from Simpler Nervous Systems. Cambridge, MA, MIT Press, pp. 117–160Burghagen H., Ewert J.-P. (2022)Stimulus perception. In: Bolhuis J.J., Giraldeau L.-A., Hogan J.A. (eds.) The Behavior of Animals: Mechanisms, Function, and Evolution. 2nd Edn. John Wileys & Sons, Inc, pp.12-47 yielded several important discoveries. In general, his research revealed the specific neural circuits for recognition of complex visual stimuli. Specifically, he identified two interacting —serial and parallel information processing— regions of the brain,Ewert, J.-P., Seelen, W.v. (1973) Neurobiologie und System-Theorie eines visuellen Muster-Erkennungsmechanismus bei Kröten (Neurobiology and systems theory of a visual pattern recognition mechanism in toads). In: Kybernetik 14, 167–183 the mesencephalic optic tectum and the diencephalic pretectal thalamic region, that were responsible for discriminating prey from non-prey and revealed the neural pathways that connect them. Furthermore, he found that the neural mechanisms are plastic and adaptable to varying environments and conditions (e.g., discussed by CarewCarew T.J. (2000) Feature analysis in toads. In: Behavioral Neurobiology, Sunderland, MA, Sinauer, 95-119 and ZupancZupanc G. (2004) Behavioral neurobiology: An Integrative approach. In: Oxford University Press. pp. 121-132).
This article is dedicated to the Zeitschrift für vergleichende Physiologie (later renamed Journal of Comparative Physiology A JCPA) on the occasion of its centennial anniversaryZupanc G.K. (2024)Centennial issue. In: J Comp Physiol A 210, 103–107. Of all the publications by Ewert and his coworkers on the neuroethology of visually guided behaviors in toads and frogs,Ewert et al.: Research papers 1967-2022. a total of 47 research papers have appeared in the Z. vgl. Physiol. and JCPA since 1967.
Natural toad behavior
Vision in toads is a complex matter. Toads respond to objects depending on: a) what they see (recognizing the object; e.g., prey?), b) where the object is located in the visual field (localizing the object; e.g., choice of capture strategy?), and c) the toad's motivation, i.e. its interest, which can be influenced by endogeneous factors such as hunger, the time of day, and season, as well as event-related individual experiences.
Common toads Bufo bufo bufo (L.) are mainly active at dusk and dawn. In toads, rod photoreceptors —which are responsible for twilight vision— are partly involved in color vision: At the absolute visual threshold, their peculiar retinal dual rod system enables a blue-sensitive and green-sensitive limited color perceptionYovanovich C.A.M., Koskela S.M., Nevala N., Sergei L., Kondrashev S.L., Kelber A., Donner K. (2017)The dual rod system of amphibians supports colour discrimination at the absolute visual threshold. In: Philos Trans R Soc Lond B Biol Sci 372 (1717): 20160066. which may help toads to navigate their environment, especially in the mating season. Chromatic mechanisms determine social interactions of toads during mate choice,Kondrashev S. L. (2023)Trichromatic vision in toads: evidence from preference for colour objects during mate choice. In: Behaviour, 160(8-9) 753-784 as neuroethologist S.L. Kondrashev and his research group have convincingly demonstrated by studying the mating behavior of male toads, which showed a preference for bluish in response to color pairs displayed on an LCD monitor. During prey-catching and threat-avoidance behaviors, however, toads rely on the perception of movement, contrast, size, and configuration (gestalt) of objects. Their perfect discrimination of configural features of contrast boundaries remains unchangedEwert J.-P., Arend B., Becker V., Borchers H.W. (1979)Invariants in configurational prey selection by Bufo bufo (L.). In: Brain, Behavior and Evolution 16 (1), 38–51Gestalt perception in the common toad Bufo bufo. 1) Innate prey recognition. https://doi.org/10.3203/IWF/C-1430eng with respect to changes in color.
Toads' annual life cycle is divided into different seasonal periods: winter for hibernation, spring for mating, summer for prey hunting as well as threat avoiding, and fall for migration to wintering grounds. In spring, when the nighttime temperature no longer drops below 6 °C, the time has come for multisensory perception that allows common toads to interact socially in search of mates. They leave the soft forest floor, where they had burrowed up to 50 cm deep to hibernate, and then embark on a migration of almost 600 m per day over up to 5 km to their spawning grounds, where they grew up. During migration, the smaller males grab the heavier females as they pass by and cling to them, riding on their backs to reach the pond where mating takes place after several males have fought over each female.
In the context of a main season, behavioral objectives and their underlying motivations are commensurate. However, a high level of motivation can substantially influence the attribution of a visual object to a particular meaning category and vice versa. For example, during the mating season, male common toads are attracted to the large swollen bodies of the females, which are suitable for grasping and carrying a male in piggyback mode. In the event of a male, driven by testosterone eager to mate cannot find a female in the pond quickly, it will even utilize an oversize piece of floating tree bark as a substitute by intensely clasping it. Their sensory organs now focus on tactile sensations of their body skin and on communication through mating calls, while vision does not yet play a dominant role.