Cuckoos versus reed warblers: Adaptations and counteradaptations

“Sign stimuli” elicit specific patterns of behavior when an organism's motivation is appropriate. In the toad, visually released prey-catching involves orienting toward the prey, approaching, fixating, and snapping. For these action patterns to be selected and released, the prey must be recognized and localized in space. Toads discriminate prey from nonprey by certain spatiotemporal stimulus features. The stimulus-response relations are mediated by innate releasing mechanisms (RMs) with recognition properties partly modifiable by experience. Striato-pretecto-tectal connectivity determines the RM's recognition and localization properties, whereas medialpallio-thalamo-tectal circuitry makes the system sensitive to changes in internal state and to prior history of exposure to stimuli. RMs encode the diverse stimulus conditions referring to the same prey object through different combinations of “specialized” tectal neurons, involving cells selectively tuned to prey features. The prey-selective neurons express the outcome of information processing in functional units consisting of interconnected cells. Excitatory and inhibitory interactions among feature-sensitive tectal and pretectal neurons specify the perceptual operations involved in distinguishing the prey from its background, selecting its features, and discriminating it from predators. Other connections indicate stimulus location. The results of these analyses are transmitted by specialized neurons projecting from the tectum to bulbar/spinal motor systems, providing a sensorimotor interface. Specific combinations of such projective neurons – mediating feature- and space-related messages – form “command releasing systems” that activate corresponding motor pattern generators for appropriate prey-catching action patterns.

Neuroethology of releasing mechanisms: Prey-catching in toads

Within the framework of interpersonal and cognitive theories of depression, we investigated whether the perception of facial emotions was associated with subsequent relapse into depression. The 23 inpatients with major depression who remitted (65 admitted patients) were studied at admission (T0), at discharge (T1), and 6 months thereafter to assess relapse. They judged schematic faces with respect to the expression of positive and negative emotions. Six patients (26.1%) relapsed. High levels of perception of negative emotions in faces, either assessed at T0 or at T1, were associated with relapse. Moreover, subjects saw more negative emotions in depressed than in remitted state. Significant results were confined to ambiguous faces, i.e., faces expressing equal amounts of positive and negative emotions. Our data support the hypothesis that a bias toward the perception of others' facial emotions as negative is an enduring vulnerability factor to depression relapse and depressed mood amplifies this negative bias in perception.

G.P. Baerends

Papers

The External Morphology of the Egg and Its Variability