International Journal of Comparative Psychology

The perception of the distance of objects with respect to an observer (egocentric distance) and the perception of the relative distance (depth) between external points was found to be optimised in the pigeon visual system according to the optical and retinal constraints of the eye. Each of these perceptual capacities is mediated by different binocular mechanisms in the frontal field, both of which appear to be designed for a stationary world. This is particularly evident in the egocentric distance estimation that occurs during the reaching movement when pecking. Here both the saccadic nature of the head movement and the convergence eye movements appear to allow constant retinal stimulation. This system of vergence signal alone is inadequate for depth perception which is instead mediated by retinal disparity. Stereopsis in the pigeon appears to be more effective for pattern decoding than for absolute spatial perception.

Adult and young gerbils and spiny mice were tested for their ability to discriminate between a short and a long arm in a T-maze. Animals were given 20 training trials/day for 5 days. The gerbils' performance improved from about 45% correct responses on Day 1 to about 80% on Day 5; the performances of the spiny mice stayed at about 40-50% over the training period. There were no age differences. These findings complement those of an earlier study in which spiny mice did demonstrate depth perception on a visual cliff while the gerbils did not. Both sets of data are interpreted from an evolutionary perspective which relates depth perception ability to the organism's natural ecological niche: gerbils are burrowing animals and spiny mice are surface and rock dwellers.

In the scaling experiment proper, 8 rats had to reproduce 10 randomly presented time intervals ranging from 1.3 to 20 s. The beginning of the reproduction was separated from the end of the standard by a 300 ms interruption of the sound indicating the durations. The rat determined the length of the reproduction by pressing a lever, thereby terminating the sound. The scaling model and the final result of the experiment are briefly described. However, before this final phase of the experiment, the rats had to learn to attend to the interruption. This was achieved in Phase 2 of the experiment by defining lever presses during presentation of the standard as incorrect behavior, which was penalized by withholding the reward and lengthening the actual standard duration by 4 s. Scrutiny of the Phase 2 data revealed two unexpected— serendipitous—findings. 1. The rats learned that they had committed an error (a faulty lever press during the standard) before they learned to suspend their lever presses; shorter response latencies for to-be-rewarded than for not-to-be-rewarded trials clearly indicated: "Knowing before doing." 2. A study of the distribution of first lever presses during the standard showed (a) that these lever presses were not evenly distributed (the hypothesis of a negative exponential distribution was rejected), and, more interestingly, (b) that 7 of the 8 rats hardly ever pressed the lever during a certain interval (for 5 of the rats the interval 3.3-4.5 s): "Temporal holes in the latency distributions." It pays to look not only at data when learning has been accomplished, but also during acquisition!

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.

Data are presented showing that humans and non-humans are severely limited in their ability to identify stimuli that vary along a single dimension. Increasing the dimensionality of spatially undifferentiated stimuli improves performance. However, this improvement is trivial compared to that observed when spatially complex stimuli, such as pictures, are presented for identification. The theoretical account of memory and decision processes presented here suggests that, while the number of items that can be held in working memory varies among species, the number of items that can be identified depends more upon the characteristics of the stimuli than upon the organism making the identification.