Picture recognition in animals and humans

Abstract

The question of object–picture recognition has received relatively little attention in both human and comparative psychology; a paradoxical situation given the important use of image technology (e.g. slides, digitised pictures) made by neuroscientists in their experimental investigation of visual cognition. The present review examines the relevant literature pertaining to the question of the correspondence between and/or equivalence of real objects and their pictorial representations in animals and humans. Two classes of reactions towards pictures will be considered in turn: acquired responses in picture recognition experiments and spontaneous responses to pictures of biologically relevant objects (e.g. prey or conspecifics). Our survey will lead to the conclusion that humans show evidence of picture recognition from an early age; this recognition is, however, facilitated by prior exposure to pictures. This same exposure or training effect appears also to be necessary in nonhuman primates as well as in other mammals and in birds. Other factors are also identified as playing a role in the acquired responses to pictures: familiarity with and nature of the stimulus objects, presence of motion in the image, etc. Spontaneous and adapted reactions to pictures are a wide phenomenon present in different phyla including invertebrates but in most instances, this phenomenon is more likely to express confusion between objects and pictures than discrimination and active correspondence between the two. Finally, given the nature of a picture (e.g. bi-dimensionality, reduction of cues related to depth), it is suggested that object–picture recognition be envisioned in various levels, with true equivalence being a limited case, rarely observed in the behaviour of animals and even humans.

Introduction

Researchers in animal and in human cognition frequently use photographs or slides in place of real objects in their studies of categorisation, face recognition, etc., but paradoxically, there are few experiments either with animals or humans that have explicitly addressed the question of the equivalence between an object and its picture. In other words, it is not obvious that animal and human subjects do really interpret the 2-D stimuli as the 3-D objects they represent. For example, the success obtained in training pigeons [40], [63] or monkeys [13], [80], [112] to categorise photographic slides does not prove that the animals understand what the pictures they categorise actually represent. In fact, as we will see in this paper, some studies have demonstrated that this is not the case, while others have shown that the establishment of some equivalence between the real object and its pictorial representation is dependent upon both the stimulus’ dimensions and experimental and/or motivational conditions. The present review tries to take stock of this question by examining the available literature for humans (mostly infants) and nonhuman subjects.

This review will first examine experiments concerning humans and will subsequently consider studies with nonhuman subjects, with the latter being classified into three categories. The first category comprises of cases of convincing demonstrations in which animals are able to treat pictures like the stimuli they represent; we can assume that a picture is recognised when animals react to a picture as they would react, spontaneously or after some training, to the real object. Of course, such reactions may vary according to the type of presented pictures: social behaviour with pictures of conspecifics, fear with threatening stimuli, predator behaviours with pictures of prey, etc.; spontaneous responses and transfer of various acquired responses (naming, categorisation, discrimination, cross-modal matching, etc.) in other cases. The second category encompasses the experiments which could indicate the existence of picture recognition but are not really demonstrations because the experimental design is questionable (for example when only one subject is involved) or the results (subjects’ preferences or time viewing, discrimination of individuals or species, or various spontaneous behaviours) are not necessarily specifically elicited by the presented stimuli. The third category includes those experiments which show that animals may have difficulties with picture recognition.

In addition, two subclasses may be distinguished among the studies that have used pictures of living or inanimate objects with animal subjects; the first class referring to the studies examining learned reactions to stimuli (as it is often the case when the subjects are primates or birds), while the second class of studies measures spontaneous or natural reactions to the stimuli (this type of study is frequently seen in experiments involving lower vertebrates or invertebrates). In this latter case, a very salient feature of the releasing stimulus can suffice to induce the reaction. For example, a male redbreast reacts to a lure (e.g. a red tuft of red feathers) as if it were a real conspecific, even if the lure does not look like a bird [61]; a colour photograph of a male conspecific may induce a similar reaction but it is not certain whether, in such cases, the whole stimulus has to be processed and recognised. However, when a subject is trained to respond to real stimuli and then transfers its response to pictures of those stimuli, or is able to use video images to acquire some information about the nature of a real object, this suggests that the most significant features of the pictured stimuli are considered and recognised. Therefore, as picture processing can differ as a function of the kind of response (spontaneous or learned), these two classes will be considered separately.

A third classificatory key concerns the issue of the stimuli presented, that is, whether the image is static (photography, slide, digitised picture) or a motion picture, which of course implies some movement and often sound and may thus greatly facilitate the subjects’ reaction to the stimuli. For example, movement is well known as releasing predatory behaviour [11], or may play an important role in the courtship of many species (see for example Refs. [35], [94]).