Size-constancy phenomenon in the prey-catching behaviour of the common toad (Bufo bufo L.)

Neuroethological Analysis of the Innate Releasing Mechanism for Prey-Catching Behavior in Toads

“…Experimental Group: Two square prey dummies, A and B, of equal size (2.5mm x 2.5mm) are moved at constant angular velocity in the same horizontal direction -one above the other -with constant interstimulus distance, it. In this case the releasing value of AB (compared to that of either A or B) sharply decreases with increasing interstimulus distance ( Fig.7F) (Ewert et al, 1970). Thus, interstimulus distance dependent inhibitory processes perpendicular to the movement direction inhibit the toad's prey-catching activity toward a double stimulus of this kind (cf.…”

“…For relatively small distance the double stimulus AB is more effective than the singly moved one A (or B) (Fig.7A). By increasing i, the releasing value of AB will decrease (Ewert et al, 1970). We can conclude that the prey-catching orienting activity of toads is dependent on excitatory and inhibitory processes resulting from different interstimulus distances in the movement direction of the stimulus (cf.…”

“…This phenomenon can be investigated quantitatively in experiments where configurationally neutral objects, i.e., squares of different sizes, are moved around the toad (Fig.1B) and the stimulus distance d between the toad and the stimulus is varied in different experimental groups (Ewert and Gebauer, 1973). As shown in Fig.6A, the relationship between prey-catching orienting activity (R) and edge length (l ) of the square stimulus can be approximated by the function R = -Bllg(l/l') I +k (orienting responses/min) in which B, k and l' are constants, L' being the edge length of the optimal stimulus.…”

Ontogenetic Aspects on Visual 'Size-Constancy' Phenomena in the Midwife Toad <i>Alytes obstetricans </i>(Laur.)

“…periments in which the distances between a frog and prey dummies of dif ferent sizes were varied, Ingle concluded that it is the absolute rather than the visual angular size of the prey object that guides prey catching. This phenomenon has also been observed and studied in more detail in the common toad Bufo bufo [Ewert and Gebauer, 1973]. Kondrashev [1976] investigated the influence of the visual stimulus size on the breeding be havior of anuran males and observed that toads (B. bufo) are able to de termine the actual size of stationary stimuli (balls and flat discs) irrespec tive of their angular size.…”

“…But also the last possibility (3) should be con sidered to explain the results in one-eyed toads: during brief head move ments, toads may store information for stimulus distance measurements and may evaluate them on a trigonometrical basis. Ewert and Gebauer [1973] found in B. bujo that 'size constancy' in snapping behavior is seen only in animals with binocular vision: one-eyed toads snap primarily at objects of a constant angular size (A.a « 16°) independent of their dis tance from the prey object, so far as have been investigated. Hubei and Wiesel [1971] recorded from neurons in area 18 of mon key's visual cortex, which respond only to visual stimuli presented at par ticular depths.…”