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fer occurred with objects (for all four pigeons), with
photographs (for three of the four pigeons) and with
silhouettes (for all four pigeons), but not with line
drawings (none of the four pigeons). Four birds tested
only with objects were then re-tested without reversal
using black-and-white photographs and showed a posi-
tive transfer and, in addition, subsequent training to
discriminate objects from their photographs showed
that this was fairly easily obtained. These results sug-
gest that the birds did not confound objects and their
pictures and, altogether, these experiments also attest
that prior experience is not necessary for picture
recognition.
Delius [26] trained eight pigeons to discriminate
spherical objects from nonspherical objects and follow-
ing this training, seven birds out of eight were able to
transfer to black-and-white photographs, colour photo-
graphs, or drawings of these objects, as well as to
photographs of novel objects; the overall transfer to
black-and-white photographs was best, that to draw-
ings intermediate, and that to colour photographs
worst. The author suggested that the pigeons did worst
with colour photographs because their colour vision is
at least pentachromatic and the colour photography
was matched to the trichromatic colour vision of hu-
mans. Watanabe [105] trained 12 experimentally naive
pigeons to discriminate, for one half, between real
objects (edible and nonedible objects) and their photo-
graphs, and for the remaining half between food and
non-food objects in the same set of stimuli (real objects
and their photographs). The pigeons performed high on
either task and showed generalisation to novel stimuli
for the two tasks, hence demonstrating that pigeons
were able to treat photographs like real objects, and to
discriminate between them, according to their training
and categorical judgement.
Jitsumori and Ohkubo [51] found that four experi-
mentally naive pigeons trained to discriminate right-
side-up and upside-down orientations of colour slides
of natural scenes depicting humans, transferred this
discrimination to new slides of the same kind. Both the
orientations of the human figures and of the back-
ground scenes controlled this discrimination, but when
these slides were oriented in the opposite direction, the
background orientation cue was the dominant feature.
The birds were also able to categorise by orientation
natural objects (humans, apes, monkeys and birds) on a
white background, indicating that the subjects recog-
nised the objects presented.
3.2.2. Reactions to motion pictures
Infant chimpanzees [68] and even baboons [102] were
able, after limited experience, to match what they ob-
served on a television screen to events occurring else-
where in order to determine the location of a hidden
goal object in a familiar outdoor field. Along similar
lines of research, Menzel et al. [69] showed that two
chimpanzees could use mirrors or live video images to
move their hands in the appropriate direction and make
contact with target (food) objects.
It has been demonstrated that fear reactions can also
be learned by use of videotaped demonstrators. For
example, naive rhesus monkeys acquired a fear of
snakes through watching videotapes of conspecifics re-
acting fearfully to snakes; note however, that the mon-
keys did not acquire a fear of flowers through watching
videotapes of monkeys reacting fearfully to flowers [12].
To summarise, it appears that the experiments pre-
sented in this section reveal that picture recognition is
possible by animals, even without previous experience.
Thus, spontaneous adapted responses were displayed to
significant stimuli photographs (prey, predators, or
conspecifics) by monkeys. Other mammals (sheep)
showed adapted responses to slides of conspecifics and
similar responses to pictures of conspecifics were also
evident in other species (birds, lizards, fishes and even
in some invertebrates) when the pictures were presented
in motion; such spontaneous responses to these stimuli
did not require any previous training with pictures.
Apes, monkeys and pigeons were also able to transfer
acquired responses from objects to pictures in various
tasks, however, it is difficult to know in such cases
whether familiarisation with the pictures is necessary
for their recognition. The methods, species and main
findings of the papers that have reported some evidence
of picture recognition in animals are outlined in Table
2.
4. Studies with animals: experiments that could indicate
picture recognition
This section reviews those experiments which provide
some cues indicating the presence of picture recognition
but that may not constitute real proof of such an
ability.
4.1. Spontaneous responses to pictures
As in the previous section, studies that used still and
motion pictures will be considered in turn.
4.1.1. Reactions to still pictures
Humphrey [47] used visual stimuli, either plain fields
of light colour, or photographs or films, and a simple
choice procedure with two adolescent rhesus monkeys;
subjects could push two buttons to choose one of the
two stimuli presented on a screen. Humphrey inter-
preted rhesus monkeys’ preferences in terms of ‘interest’
(determined by the information content in the stimuli)
and ‘pleasure’ (determined by features such as colour
and brightness), with these two factors determining the