The role of category density in pigeons' tracking of relevant information

Abstract

Prior categorization studies have shown that pigeons reliably track features that are relevant to category discrimination. In these studies, category exemplars contained two relevant and two irrelevant features; therefore, category density (specifically, the relevant to irrelevant information ratio) was relatively high. Here, we manipulated category density both between and within subjects by keeping constant the amount of relevant information (one feature) and varying the amount of irrelevant information (one or three features). One group of pigeons started with low-density training, then proceeded to high-density training, and finally returned to low-density training (Low-High-Low); a second group of pigeons started with high-density training and then proceeded to low-density training (High-Low). The statistical density of the category exemplars had a large effect on pigeons' performance. Training with high-density exemplars greatly benefitted category learning. Accuracy rose faster and to a higher level with high-density training than with low-density training; the percentage of relevant pecks showed a very similar pattern. In addition, high-density training (in the Low-High-Low group) led to an increase in performance on the more difficult low-density task, an observation reminiscent of the easy-to-hard effect. These results illuminate factors affecting pigeons' accuracy and tracking of relevant information in visual categorization.

Keywords: Categorization; Category structure; Easy-to-hard effect; Pigeons; Selective attention; Statistical density.

Figure 1.

Examples of Category A and Category B low-density exemplars. All of the low-density exemplars contained only one relevant feature (the rainbow for Category A and the green spiral for Category B) and three irrelevant features, so the ratio of relevant to irrelevant features was low (1:3) and, therefore, their statistical density was relatively low. On half of the exemplars, the features were presented in the corners of the square display, whereas on the other half of the exemplars, the features were presented in the center of the lines forming the square.

Figure 2.

Examples of Category A and Category B high-density exemplars. All of the high-density exemplars contained one relevant feature (the rainbow for Category A and the green spiral for Category B) and one irrelevant feature, so the ratio of relevant to irrelevant features was high (1:1) and, therefore, their statistical density was relatively high. On half of the exemplars, the features were presented in the corners of the square display, whereas on the other half of the exemplars, the features were presented in the center of the lines forming the square.

Figure 3.

Mean percentage of correct responses for both groups Low-High-Low and High-Low throughout all the training phases. Note that group High-Low started training in the high-density phase. The dashed line, at 50%, represents the chance level for correct responses. Error bars indicate the standard error of the means (± 1 SEM).

Figure 4.

Mean percentage of correct responses for both groups on their respective first 50 training sessions: low-density training for group Low-High-Low and high-density training for group High-Low. The dashed line, at 50%, represents the chance level for correct responses. Error bars indicate the standard error of the means (± 1 SEM).

Figure 5.

Mean percentage of relevant pecks for both groups Low-High-Low and High-Low throughout all the training phases. Note that group High-Low started training in the high-density phase. The dashed line at 25% represents the chance level for training with low-density exemplars, whereas the dashed line at 50% represents the chance level for training with high-density exemplars. Error bars indicate the standard error of the means (± 1 SEM).

Figure 6.

Mean percentage of relevant pecks for both groups on their respective first 50 training sessions: low-density training for group Low-High-Low and high-density training for group High-Low. The dashed line at 25% represents the chance level for group Low-High-Low, whereas the dashed line at 50% represents the chance level for group High-Low. Error bars indicate the standard error of the means (± 1 SEM).