Effects of background structure on the discrimination of configurational moving prey dummies by toadsBufo bufo (L.)

Summary

The prey-catching activity of the toadBufo b. bufo was measured in response to various configurational moving two-dimensional black or white stimuli contrasting with various homogeneous or structured backgrounds (Figs. 1 to 5).Area discrimination was tested with squares of different sizes,configurational discrimination with stripes of equal size and different configuration (‘worm’ or ‘antiworm’), andlength discrimination with worm-like stripes of different length.

Signal Extraction. A 2×5 mm² wormlike stripe of a luminanceL _s≈36 cd×m⁻², for example, was practically not resolved from a homogeneous background of almost the same luminanceL _b = 36. But it could be well detected if the background was structured ($\bar L_B = 36$); at a given luminance ($\bar L_B \approx 36$) the releasing value of the grey stimulus (L _s= 36) increased to some extent with structure width of the background (Fig. 1 c-e).

Configurational Discrimination. The basic stimulus response relationships were maintained, if Configurational stimuli were moved against black, white, grey or structured backgrounds. However, in the range of short stripes (xl= 5 mm) the worm/antiworm discrimination was significantly better withblack stimuli moving against white background than forwhite against black (curvesa andc in Figs. 2A and 3A). The Configurational selectivity in response to white wormlike and antiwormlike stripes increased if stimuli were moved against a grey or a structured background (curvesa andc in Figs. 2 and 3B-E forxl≧5 mm).

Length Discrimination. Relatively short stripes (close to the square configuration) and relatively long ones were discriminated less well than those in a medium size range. The optimal discrimination range was different for black stimuli (range: 3≦xl ₁≦6.5 mm) and white stimuli (range: 5≦xl ₁≦ 10 mm) (Fig. 4A, B). Discrimination in this range could be enhanced to some extent if the background was structured. These results were consistent for artificially (Fig. 4A) and naturally structured backgrounds (Fig. 5A-C).

The results show that stimulus discrimination in toads depends largely on the values of Configurational parameters. The background structure may have distinct effects on stimulus perception, i.e., either masking or facilitation.