Pigeons and humans are more sensitive to nonaccidental than to metric changes in visual objects

Abstract

Humans and macaques are more sensitive to differences in nonaccidental image properties, such as straight vs. curved contours, than to differences in metric properties, such as degree of curvature [Biederman, I., Bar, M., 1999. One-shot viewpoint invariance in matching novel objects. Vis. Res. 39, 2885-2899; Kayaert, G., Biederman, I., Vogels, R., 2003. Shape tuning in macaque inferior temporal cortex. J. Neurosci. 23, 3016-3027; Kayaert, G., Biederman, I., Vogels, R., 2005. Representation of regular and irregular shapes in macaque inferotemporal cortex. Cereb. Cortex 15, 1308-1321]. This differential sensitivity allows facile recognition when the object is viewed at an orientation in depth not previously experienced. In Experiment 1, we trained pigeons to discriminate grayscale, shaded images of four shapes. Pigeons made more confusion errors to shapes that shared more nonaccidental properties. Although the images in that experiment were not well controlled for incidental changes in metric properties, the same results were apparent with better controlled stimuli in Experiment 2: pigeons trained to discriminate a target shape from a metrically changed shape and a nonaccidentally changed shape committed more confusion errors to the metrically changed shape, suggesting that they perceived it to be more similar to the target shape. Humans trained with similar stimuli and procedure exhibited the same tendency to make more errors to the metrically changed shape. These results document the greater saliency of nonaccidental differences for shape recognition and discrimination in a non-primate species and suggest that nonaccidental sensitivity may be characteristic of all shape-discriminating species.

Fig. 1

The four shapes used in Experiment 1. The left panel lists the nonaccidental properties for each shape. The dashed lines indicate potential locations of the brick wall, an irrelevant feature of the pictorial stimuli in the present discrimination.

Fig. 2

Percentage of confusion errors plotted by the number of shared nonaccidental properties in Experiment 1. The dashed line indicates chance level (33%) and asterisks indicate values significantly different from chance.

Fig. 3

The three trios of stimuli used in Experiment 2. Each trio included a target shape (the choice of which was reinforced) plus metric (MEM) and nonaccidental (NAM) modifications (the choice of which was not reinforced).

Fig. 4

Percentage of choices of the target shape, the MEM shape, and the NAM shape measured over 36 discrimination training sessions. The top panel shows the data for the Pigeon Partial Group and the bottom panel shows the data for the Pigeon Consistent Group. The dashed line indicates chance discrimination performance (33%).

Fig. 5

Percentage of metric errors measured over 36 discrimination training sessions for the Pigeon Consistent Group and the Pigeon Partial Group. The dashed line indicates chance discrimination performance (50%).