Vision in toads
He then rotated a small stripe (bar) of contrasting cardboard (acting as a visual 'dummy') around the vessel to mimic either prey-like or threat-like stimuli; see Video.
By changing characteristics of the visual stimulus in a methodical manner, Ewert was able to comprehensively study the key features that determine behavior.
By means of a different experimental setup it was shown that the worm vs. anti-worm discrimination is independent (invariant) of the direction the object moves in the toad's visual field.
However, this tendency is plastic and reverses seasonally, where black objects against a white background are much more effective at eliciting prey-catching behavior in the fall and winter (Ewert 1980).
To understand the neural mechanisms underlying the toad's behavioral responses, Ewert performed a series of recording and stimulation experiments.
Likewise, when a spot on the tectum was electrically stimulated, the toad would turn toward a corresponding part of its visual field, providing further evidence of the direct spatial connections.
The discharge patterns of these neurons – recorded in freely moving toads – "predict" prey-catching reactions, e.g. the tongue flip of snapping.
In combination with additional projection neurons, prey-selective cells contribute to the ability of the tectum to initiate orienting behavior and snapping, respectively.
More specifically, electrical triggering the thalamic-pretectal region initiates a variety of protective movements such as eyelid closing, ducking and turning away (Ewert 1974, 2004).
These and other experiments suggest that pathways, involving axons of type TH3 cells, extend from the pretectal thalamus to the tectum, suitable to modulate tectal responses to visual stimuli and to determine prey-selective properties due to inhibitory influences.
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).
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.
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.
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.
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.
Previously thought to be characteristic of solely primates and mammals with front-facing eyes, the ability to process depth information from multiple visual points in space has been determined to be possessed by most amphibians, namely, frogs and toads (Nityananda and Read 2017).
This theory would mean that stereopsis has evolved independently at least four times to account for stereo vision being present in mammals, birds, amphibians, and some invertebrates.
