Comparative Psychophysics: Some Contextual Effects in Birds and Humans

Three different types of psychophysical context effects have been studied in comparative experiments with animals and humans. The main context variables investigated were: (1) range of the test series; (2) asymmetry of training to test stimuli (anchor effects); and (3) frequency distribution of the test stimuli. A two-stimulus, two response training procedure, followed by various generalization tests, was used. All subjects (19 chickens and 128 humans) were trained and tested with cubes of different sizes. The psychometric functions support the general assumption that perception in birds undergoes psychophysical context effects similar to that observed in humans. However, while all three variables affected the judgments of human subjects, the choices of chickens and human infants were not strongly affected by the frequency distribution of the test stimuli. These data suggest that two factors are responsible for the three contextual effects investigated: a basic perceptual factor invariant across species and age groups and a cognitive component