Effects of stimulus size and spatial organization on pigeons' conditional same-different discrimination

Abstract

In two experiments, we explored the effects of varying the size and the spatial organization of the stimuli in multi-item arrays on pigeons' same-different discrimination behavior. The birds had previously learned to discriminate a simultaneously presented array of 16 identical (Same) visual items from an array of 16 nonidentical (Different) visual items, when the correct choice was conditional on the presence of another cue: the color of the background (Castro et al., in press). In Experiment 1, we trained pigeons with 7-item arrays and then tested them with arrays containing the same item, but in a variety of sizes. In Experiment 2, we tested the birds with the items grouped in novel locations: the top, the bottom, the left, or the right portions of the display area, which generated different vertical and horizontal alignments. Accuracy scores revealed virtually perfect stimulus generalization across various item sizes and spatial organizations. Reaction times revealed that the birds perceived different sizes of a single icon as the same stimulus (Experiment 1) and that the birds processed vertical arrangements faster than horizontal arrangements (Experiment 2). These results suggest that the pigeons noticed both physical and spatial changes in the stimuli (as shown by their reaction times), but that these changes did not disrupt the birds' discriminating the sameness or differentness of the multi-item arrays (as shown by their accuracy scores).

Figure 1

Example of the two types of trials in Castro, Kennedy, and Wasserman (in press). The color of the background indicates whether the pigeons must choose the Same or the Different array. For example, in the case of the red background, the correct choice was the Same array, whereas in the case of the blue background, the correct choice was the Different array.

Figure 2

Examples of some of the training arrays used in Experiment 1. One Same (identical icon, always the same size) and one Different (seven nonidentical icons, always the same size) array were simultaneously presented on the screen. In the actual experiment, the arrays could appear either on a red or a blue background.

Figure 3

Examples of some of the testing arrays used in Experiment 1. One Same-Uniform array (identical icon, always the same size) and one Same-Varied array (identical icon in seven different sizes) were simultaneously presented on the screen. In the actual experiment, the arrays could appear either on a red or a blue background.

Figure 4

Mean accuracy during the last 5 training days in Experiment 1 for all the 7 different sizes. Left, mean percentage of correct responses. Right, log-transformed reaction time (RT) to choose the correct response. Error bars indicate the standard error of the means.

Figure 5

Mean accuracy as a function of the size of the items in the Same-Uniform array across the 20 testing sessions in Experiment 1. Left, mean percentage of “same” responses. Right, log-transformed reaction time (RT) to choose either “same” or “different.” Error bars indicate the standard error of the means. The dashed line indicates the chance level of performance.

Figure 6

Examples of the training (Center) and testing (H-Bottom, H-Top, H-Top&Bottom, V-Close, V-Far, and V-SameSide) arrays used in Experiment 2. For H-Top&Bottom arrays, on half of the trials the items in the left array were located on the top and the items in the right array were located on the bottom, and the reverse was true for the other half of the trials. For V-SameSide arrays, on half of the trials the items in both arrays were located on the left, and on half of the trials they were located on the right. In the actual experiment, the arrays could appear either on a red or a blue background.

Figure 7

Mean accuracy as a function of the grouping of the items across the 10 testing sessions in Experiment 2. Left, mean percentage of correct responses. Right, log-transformed reaction time (RT) to choose the correct response. Error bars indicate the standard error of the means.