Picture-object recognition in a comparative approach: performance of humans (Homo sapiens) and pigeons (Columba livia) in a rotational invariance and a complementary information task

Abstract

Pigeons and humans are two highly visual species that have evolved separately for about 310 million years (Kumar and Hedges, 1998) and developed largely convergent visual systems due to similar visual needs. To investigatepigeon vision and cognitive abilities twodimensional pictorial stimuli are often used. However, it is not entirely clear, how pigeons perceive such stimuli and whether or not they can associate photographs with real objects. In the present study nine pigeons and eleven humans were trained to discriminate between photographs of two biologically irrelevant objects (“Greebles”). The pigeons were housed in an aviary containing the real Greebles and were trained in wooden chambers where they had to peck on a Plexiglas disk when positive stimuli were presented, thus obtaining food. Humans were trained with the same stimuli presented on a computer screen and had to click with a computer mouse on positive stimuli. Results showed that humans were much faster at learning to discriminate the two Greebles. In the first test, pigeons and humans had to discriminate new rotational views of the Greebles. Humans performed equally well on interpolated test views (i.e. views that lay between the training views) and extrapolated views (i.e. views outside of training range), while pigeons performed better on interpolated than on extrapolated test views. Therefore, it can be concluded that object recognition was viewpointindependent for humans and viewpoint-dependent for pigeons. In the second test, following a procedure by Aust and Huber (2006), pigeons were presented with parts of the Greebles that were not included in training and the first test to see whether they formed associations between the 2D images and the 3D objects in their aviary. They did not discriminate these parts correctly. The test was repeated with three of the test views already used in the second test but presented in different sizes. Discrimination seemed to depend on the visibility of the appendages and might have been based on visual features of the pictures themselves without 71 recognition of what they portrayed. The results of this study were compared to a previous study in which pigeons were trained to discriminate either real Greebles, holograms, or computer images of them. There, too, the real Greebles were installed in the pigeons’ aviary; however, the pigeons trained and tested on computerimages lived in the adjacent aviary and thus only had limited visual contact to them. We wanted to find out whether the more extensive visual contact to the Greebles had any influence on the pigeons’ performance. However, there was no difference in performance between the two groups. This is evidence that the result of the previous study — better performance with real objects and holograms than with computer images — was not based on the fact that pigeons trained with the latter stimulus type had only limited visual access to thereal 3D objects