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recognition in the chimpanzee; firstly, it was more
difficult for Ai to recognise human faces than chim-
panzee’s faces, while the opposite was shown for hu-
man subjects (they had more difficulty recognising
chimpanzee’s faces than human faces) and, secondly, it
was more difficult for her to recognise inverted faces or
horizontal faces than to recognise upright faces (a
similar, but more pronounced effect being obtained
with human participants). It must be noted that experi-
ments conducted with macaque monkeys [8,83] failed to
show this inversion effect, but this could be due to
differences in processing rotated complex visual stimuli.
For example, monkeys and some apes could be more
suited to doing this type of rotation because they live in
an environment in which they often hang upside-down
[103], the above findings do not, therefore, necessarily
imply that subjects were unable to recognise the pic-
tures presented. Nevertheless, Swartz [93] showed, with
a visual fixation habituation-dishabituation paradigm,
that infant pigtail macaques (3 months old) could dis-
criminate between colour photographs of faces of three
macaque species (pigtail, cynomolgus, and stumptailed)
when they were presented upright, but not when the
faces were displayed upside-down.
Other experiments using schematic drawings of mon-
keys’ bodies indicate that longtailed macaques were
able to discriminate one monkey from other monkeys,
basing their recognition on the limited information
provided by the black-and-white shape and texture of
their body characteristics [31]. Such findings, however,
do not present any obvious interpretation regarding
picture recognition. Firstly, as stated by the author of
the above study ‘‘it remains questionable whether the
monkey has a knowledge of the representational nature
of the image’’ (Dittrich [31], p. 150). Secondly, we know
from other studies that monkeys can correctly cate-
gorise images of different classes of stimuli by using
some absolute cues which are not constitutive of the to
be categorised stimuli. For example, D’Amato and van
Sant [13] have shown that their monkeys used an
irrelevant red patch to form the ‘person versus non-per-
son’ category. Thus, caution is in order before conclud-
ing that seemingly appropriate classification skills mean
that the animal realises the relationship between a
picture and the real object.
Watanabe and Ito [106] trained pigeons to discrimi-
nate between colour slides of two pigeons’ faces. While
discrimination was apparently effortless for two stimuli
easily discriminated by human observers, it was difficult
with the stimuli which humans also struggled to differ-
entiate. When the S + stimulus was replaced by its
scrambled parts, subjects did not respond; such a reac-
tion seems to indicate that the birds recognised that the
stimuli depicted on the slides represented conspecifics
(which could be recognised only when the faces were
not scrambled).
Lumsden [64] conducted an experiment with one
pigeon; the bird was trained to discriminate one geo-
metric object from two others, after which transfer was
examined when the object, its cut-out photograph, or
its line drawing was shown at various orientations.
Response curves were the same for photographs and
for three-dimensional objects: generalisation was good
at 0, 45, and 135°, poor at 180°, and was absent for 90°.
Although the line drawings were responded to at the
lowest rate, the pattern of responding was similar. In a
subsequent experiment, the pigeon was trained to dis-
criminate between the object and its photograph dis-
played at 45°: the bird then generalised that
discrimination to photos presented at other orienta-
tions. We should note however that there was only one
subject involved in this experiment.
Wilkie et al. [108] trained four pigeons to discrimi-
nate between pictures taken in the vicinity of the loft to
which they had been raised and other areas they had
not visited, with four other pigeons that were not
trained to home being tested as a control. The results
showed that after training with only eight slides, both
groups were able to transfer and then to discriminate
the two categories of slides, but homing pigeons were
better than nonhoming pigeons. In the same paper, the
authors mention the experiment of Honig and Ouellette
[45], in which eight pigeons were taught to discriminate
colour pictures of various views of two ends of a long
room. Following this task, the pigeons had to discrimi-
nate between the two ends of the real room; a feeder
was placed at each end of the of the test room but only
one feeder contained food: for the congruent group, it
was in the same location that had been positive during
the previous slide discrimination procedure, while for
the incongruent group, it was the opposite end of the
room. The subjects in the congruent condition took
consistently less time to find the correct feeder than the
incongruent subjects. In a similar study, Wilkie et al.
cited an unpublished study by Willson et al. [109] in
which eight pigeons were placed outside the laboratory
for 20 min prior to each training session in a picture
discrimination task; for four of the pigeons the place
they had seen outside was presented as the positive
stimuli (‘relevant place’), whereas for the other four
pigeons, the visited place was not pictured at all (‘irrel-
evant place’). The ‘relevant place’ birds acquired the
discrimination more quickly than did the ‘irrelevant
place’ subjects. Such experiments suggest that pigeons
can perceive the correspondence between pictorial stim-
uli and the place they represent.
The above findings provide further information re-
garding the issue of picture recognition. In some exper-
iments, animals showed differential preferences for
pictures and appeared to be able to discriminate be-
tween them. However, they did not treat them as the
conspecifics they represented: for example macaques