Читать книгу The Behavior of Animals - Группа авторов - Страница 52

Snapping a hive-bee, the painful/distasteful incident—conditioned with bee’s appearance—prevents a toad from catching such bees again (Cott 1936).

Can a toad be trained to catch a threatening stimulus? Usually, toads are frightened by a moving hand. Feeding a toad daily with a mealworm presented in the experimenter’s hand, the toad associated the hand with food and became tame (Brzoska & Schneider 1978). Approximately after a fortnight of hand-feeding—once a day—the moving hand alone released snapping and, generalizing, a moving large square or threat-like stripe was included to the toad’s prey category. The species-specific prey recognition was extended by individual experience—in terms of classic ethology, a “modified IRM” developed (Ewert 1997; see also Further Reading, Movie A3).

How can this modification be explained? Recalling the “window hypothesis,” we suggest in the telencephalon a neural structure that—during hand-feeding training—becomes sensitized to the contiguous presentation of prey and threat signals. In the learning phase, the sensitized neurons inhibit threat detection, which leads to a sort of “disinhibition” in prey detection.

How was this checked out? The study passed three steps: Step 1: a toad—left eye covered—was trained by hand-feeding. Step 2: during prey-catching of the trained toad—eyes uncovered—toward a moving large square, the ¹⁴C-2DG-uptake rose in the posterior ventromedial pallium of the left telencephalon (Figure 2.9 Ba, MP). This structure obtains visual information from the right eye (uncovered during training) and projects to thalamic/pretectal regions. This telencephalic pallial structure is homologous to the mammalian hippocampus known to be involved in learning. Step 3: after lesions to that pallial structure, the training effects disappeared and the property of species-specific prey recognition—classic ethologically speaking, the “IRM”—reappeared (Ewert et al. 1994).

Summarizing, prey-catching releasing systems in toads contain—inter alia—midbrain neurons with species-specific prey-selective characteristics. Ontogenetically speaking, configurational prey selection is present after toad’s metamorphosis in the context of aquatic-to-terrestrial transition. It can be modified by individual experience, e.g., via a forebrain-loop involving ventromedial pallium, the “primordium hippocampi.”

There are promising studies concerning modulatory functions of diencephalic pretectal/thalamic and hypothalamic nuclei on the stimulus-response pathways that mediate prey-catching and threat-avoiding (e.g., see Ewert & Schwippert 2006; Islam et al. 2019; Prater et al. 2020).