Читать книгу The Behavior of Animals - Группа авторов - Страница 48

The visual field of toad’s retina is mapped—via retinal ganglion cell axons along the optic nerve—inter alia mainly in the contralateral optic tectum and pretectal thalamus (Figure 2.9A ). If, in the absence of retinal input, a locus in the tectum of a free-moving toad was excited by trains of electrical impulses delivered by an implanted electrode, the toad responded with orienting or snapping. Probably, the electrical stimulus excited neurons mediating information on prey recognition (Ewert 1974, 1984). In the pretectal thalamus, focal electrical stimulation elicited avoidance, such as ducking or jumping or freezing associated with secretion of skin poison glands—all-up behaviors known to be released by airborne or ground predators.

A neuroimaging technique allows one to check the regional neural activities in response to prey or predator stimuli (Finkenstädt et al. 1985). If ¹⁴C-labeled 2-deoxy-D-glucose, ¹⁴C-2DG, was administered systemically to the toad, active neurons were confusing the 2-deoxy-D-glucose with glucose, hence taking it up, but failing in decomposing it like glucose. The more active neurons were, the greater the storage of ¹⁴C-2DG and thus the radioactivity measured in brain sections later on (Figure 2.9B; see also Suggested Reading, Movie A1).

Figure 2.9 Bd shows a color-coded autoradiographic image of a transverse section through the midbrain of a toad snapping toward a prey-like stripe moving in the binocular field. Strong radioactivity was focused bilaterally on the ventrolateral tectum: “snapping-evoking areas” (Figure 2.9 Bd, VT). In a toad escaping from a moving large square, the overall radioactivity was high, strongest in tectal and pretectal/thalamic structures (Figure 2.9 Bb, DT and TH). This substantiates Tinbergen’s prediction that “neural orchestration” of the whole brain may participate in a stimulus-response. In a toad becoming stiffened toward a threat-like stripe moving in the right visual field, moderate radioactivity in the corresponding left pretectal thalamus was stronger than in the optic tectum (Figure 2.9 Bc).