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gate subjects’ spontaneous reactions to biologically sig-
nificant stimuli. Some empirical evidence for distin-
guishing between the two interpretations can be found
in the studies with sheep cited above, in particular,
when the sheep stopped expressing interest in the pic-
ture of a conspecific when it realised that it was not a
‘true’ sheep [100]. In the experiments involving some
sort of training (e.g. our category of acquired re-
sponses), the same difficulty arises in deciding whether
responses express merely the confusion between the
pictures and their referents or true recognition. For
example, the pigeons tested by Watanabe [105] were
able to differentiate food versus non-food, or real ob-
jects versus pictures, but these same subjects were un-
able to simultaneously perform both kinds of
discrimination. Similar interpretation problems occur
concerning successful cross-modal matching perfor-
mance by nonhuman primates; it is by no means obvi-
ous that these subjects know that the picture they see is
not the real object (e.g. [16]).
Even for human infants and children, the distinction
between objects and pictures can be unclear. Deloache
et al. [25] showed that 9-month-old infants (from two
very different cultures regarding their familiarity with
pictures) were quite similar in their manual investiga-
tion of colour photographs: they touched them and
tried to pick them up off the page as if they were real
objects. However, a subsequent experiment clearly
demonstrated that the infants were able to discriminate
between pictures and real objects, in the sense that they
preferentially grasped real objects. A possible explana-
tion is that the infants investigated the pictures to try to
understand their two-dimensional nature and their
properties; pictures appear like real objects in some
ways, but they are not. As concluded by the authors,
‘‘physically grasping at pictures helps infants begin to
mentally grasp the true nature of pictures’’ ([25], p.
210). This manual investigation is gradually replaced by
pointing at the pictures, which constitutes the predomi-
nant behaviour at 19 months of age. The question of
equivalence between an object and its picture can also
be raised regarding young children who had to retrieve
a hidden toy from a location shown on a picture.
Deloache and Burns [23] (and see above) showed that
24-month-olds were unable to perform such a task,
although children of the same age and even infants can
recognise pictures of familiar individuals (e.g. their
mother). Moreover, an experiment with 30-month-old
children has shown that they could retrieve an object
whose location was shown to them on a picture but,
perhaps surprisingly, these same children could not
solve this task when the information was provided
using a scale model; a task which might appear easier
given the high degree of iconicity that exists between
the symbol (the model) and the reality [22]. It seems
that this surprising difference comes from the percep-
tion of the model; for the children of this age group, the
model is an interesting object per se, not the image of
another thing. In the same way, when 2-year-old chil-
dren could see a video of a demonstrator hiding an
object, they retrieved the object if they thought that
they were looking through a window but not when they
knew that they were watching the same scene on video.
For the authors, the children did not treat the televised
information as real information, although they could
perceive and understand that information [24].
An important point concerning picture recognition is
illustrated by the above examples and it is emphasised
by the expression used by Deloache et al. [24]: the
development of pictorial competence involves three
‘Rs’, namely representation, referent, and relation. In
effect, it is not enough to see a picture only as a piece
of paper or as the object represented because real
pictorial competence requires understanding the rela-
tion between pictures and their referents. Such an un-
derstanding is observed when 19-month-old children,
instead of trying to grasp pictures, will point and try to
name them [25].
It is also possible for some animals to establish
object-picture correspondence and at the same time to
show evidence that the two are not perceived as being
the same. In fact, a clear case of a possible recognition
of the difference between an object and its picture can
be found with a linguistically trained chimpanzee (e.g.
the example of Vicki mentioned above) who showed an
ability to refer to objects represented on pictures (by
using a token) without confusing them with real objects
(see also Ref. [89]). A related instance is provided in
our study with olive baboons [5]; upon seeing cut-out
pictures of food objects shown to them in rotation, the
baboons never attempted to grab them (as they did
with real food), although they were still able to ade-
quately categorise them with respect to the food versus
non-food categories.
In any case, the decision concerning the nature of the
relationship perceived by animals between real world
stimuli and their pictorial representation (being a pho-
tograph, a digitised picture or a video) will remain
problematic. In his recent review of the use of video
images in animal behaviour experiments, d’Eath [20]
observed that even if an animal responds adequately
upon viewing biologically relevant video stimuli, a fur-
ther demonstration that such a response is produced as
a specific reaction to the particular stimuli used by the
experimenter has to be provided. In this respect, some
studies (e.g. [86]) have shown that an unrelated class of
visual stimuli, and even the screen itself, can elicit a
seemingly adapted response! In other words, systematic
controls are required before concluding that the presen-
tation of pictorial stimuli has the same behavioural
effects as would the objects they represent.
A final remark is in order; namely, that it is surpris-
ing to observe that the questions we have addressed in
this review have received relatively little attention in the