than orangutans versus other primates/humans, or animals versus non-animals/foods. In fact, animals versus non-animals was the more difficult discrimi-nation for the orangutans, although, just as in Vonk and MacDonald (2002), food versus animal discrimination seemed to be easily acquired.
On all problems large individual differences were found. This might reflect not only individual differences in learning, but in interpreting the stimulus photographs as well. It is feasible that some pictures made sense so some subjects but not to oth-ers. Interestingly, the adult subjects did not perform unequivocally better than the 2 year old orangutan, which in turn performed better than the 4 year old gorilla.
General exposure to pictorial material during one’s lifetime, which is unavoidable in captive settings, did not seem to have influenced pictorial ability. Experience with the species and objects in the pictures did not seem to have affected performance overall, but is likely to account for some of the variation between individuals.
Using the discrimination task in a touchscreen setup as above, Vonk (2002) did also test concepts for social relationships. Two of the orangutans and the gorilla Zuri were tested on their ability to choose mother-infant pairs as opposed to individuals in other types of configurations, as well as single individuals. Positive and negative choice photographs depicted the same species. In this task orangutans and gorillas were used as stimuli in separate sessions, while unfamiliar species were placed in mixed stimuli sets. In training sessions all subjects performed on par with training in the previous studies, reaching criterion of 80% correct in 20 to 130 trials.
Only one of the animals, an orangutan, showed good transfer to novel photo-graphs. The two remaining subjects were above chance but did not reach criterion in the first transfer sessions. In subsequent sessions all three subjects performed well on novel photographs of unfamiliar species. The orangutans, but not the gorilla, were thrown by a photograph of an adult male gorilla with an infant. It was categorised as a mother-infant pair. Zuri, although reared by humans, had experienced conspecifics of both sexes. This episode highlights the necessity of experience with the depicted stimuli, as well as being able to transfer that experience to its recognised version in a photograph. It was also found that Zuri sometimes preferred to choose gorillas of her own age, regardless of context. This effect did not transfer to other species. This is another example of how the real world can affect the pictorial world. Perhaps es-pecially when viewed in a reality mode. When pictures are truly pictures it makes little sense to prefer to indicate one before another, especially in a non-communicative context.
is little use in relying on a purely visual strategy. Different features become useful depending on the specific discrimination that has to be made and, in addition, this has to be accomplished with novel photographs. This has implications also for the pictorial recognition that is necessary. In order to appreciate a social relationship in a picture one must recognise the individuals that form this relationship. Patches of e.g.
red and oblong shapes could in theory form relationships in the discrimination ver-sion of the task described above. However, it is a farfetched assumption, and it is even less probable in a conditional matching task.69
The four matching principles in the present study were mother-infant pairs, so-cial groups, mated pairs, and siblings. In each trial the sample photograph displayed one of these configurations and after a brief blackout period a matching photograph of the same configuration and a non-matching photograph of another configuration came onscreen. Species were mixed across the stimuli set, i.e. a group of gorillas could form a match with a group of birds to the exclusion of a sibling pair of chim-panzees. The subjects were, as usual, rewarded for the right choice with food.
All subjects performed above chance overall within 30 trials, which is exceedingly fast. However, they differed in the matching rules they were able to acquire. All three performed well on matching pictures that contained a mother with her off-spring or two siblings. But only one of the orangutans could match on the concept
“social group” and the second one on “mated pairs.” That the gorilla was able to match only pictures that contained young individuals could perhaps be ascribed to her preference for such photographs rather than matching in accordance with social concepts (Vonk, 2002). Whether the sample and the match contained the same spe-cies or not did not seem to influence any of the subjects, which strongly suggests that matching did take place on more than on a purely visual surface level.
Although Vonk (2002), beside ages and sexual dimorphism, mentions the per-ceived activity in the photograph as a clue for interpreting the social relationship between the depicted animals, this might not be possible for all pictures and activi-ties. For sure, a play activity hints to a sibling relationship and a mating activity hints to a mated pair, but this entails that an otherwise dynamic activity can be read into static pictures. That this can be done by a given subject is not necessarily the case, although it is certainly a possibility for many activities. In pictures playing is for example often accompanied by play faces, and, as shall be made clear below, reading emotion into photographs seems to be possible even for pictorial novices.
Furthermore, if one is able to identify individuals in photographs one is auto-matically inclined to make out what activity they are involved in. There is a drive for a complete perception, but what one comes up with might differ depending on which mode of picture perception one is working in. “Frozen in an awkward posi-tion” is perhaps a common conclusion within a reality mode. However, it is prema-ture to view degree of perceived dynamic activity in picprema-tures as a defining feaprema-ture for any of the modes, at least for photographic stimuli, before studies targeting this spe-cific question have been made.
One of the orangutans and the gorilla were indeed tested on further dynamic content in a third experiment in Vonk (2002). Asked to match in DMTS according
69 One need not assume that the specific concept that is intended by the researcher, e.g. “mother,”
necessarily is the relationship perceived and acted on by the subject either.
to the concepts “sleeping,” “grooming,” “eating” and “playing,” the gorilla matched readily by the first session and for all categories. The orangutan took three exposures to the stimuli (there was no transfer test in this setup) and performed above chance only on “sleeping” and “eating.” “Sleeping” was also the most successful category for the gorilla. That “sleeping” was marginally easier than “eating,” “grooming” or
“playing” might be due to the fact that one does not have to read very much into a picture in order to see the similarity between two scenes that depict sleeping indi-viduals.
The photographs were in no way simplified by reduction of irrelevant back-grounds, or chosen for their prototypical looks. They were rather intentionally made more difficult to discriminate without thorough interpretation. They were for ex-ample balanced so that no single feature was unique for a particular category, and species were again mixed. One could thus expect to find playing individuals with closed eyes or grooming individuals with play faces. No data is given for perform-ance on individual pictures so we do not know if certain manipulations rendered them more difficult than others.
It is noteworthy that overall performance with activity photographs seems to have been slightly better than performance in the experiment that measured concepts for social relationships. Perhaps this reflects that social concepts are more farfetched for a subject to attribute to pictures in an experiment than are individual activities.
Whatever the case it is noteworthy that subjects without much experience with pho-tographs, and definitely no social communicative experience with pictures, are quite able to attribute some dynamic content to them. This illustrates the power of pic-tures, i.e. to evoke one’s knowledge of the real world from exemplars that mirrors only some of that knowledge. But since these studies do not target the question of differentiation and reference, this power can be attributed to reality mode process-ing.
It can easily be argued that when matching photographs of different objects, or pho-tographs of objects to different views of the same object, reality mode processing cannot be excluded. The task requires neither differentiation nor reference. The same is of course true for matching identical pictures where there is even no need to recognise the content of the pictures at all and pure surface mode processing would in theory suffice for matching to occur.
Categorical matching on the other hand, as shown above, can often exclude sur-face mode processing. A further example is Tanaka (2001). Five chimpanzees at the Primate Research Institute in Kyoto, experienced in both MTS and picture tasks, could categorise colour photographs of flowers, trees, weeds and ground (dirt etc.) on a categorical level. However, when the comparison stimuli were all (there were four of them) from the same category as the sample the subjects performed signifi-cantly worse than when only the match was from the same experimenter-defined category as the sample. This means that the subjects had problems, either conceptu-ally or visuconceptu-ally, to pinpoint among similar exemplars the one that was the closest match to the sample. However, these mistakes on a computer screen do not neces-sarily mean that the same mistakes would be made in real life. Distinguishing one type of plant from another is a crucial ability for a foraging animal. It might rather
mean that the grounds for matching photographs did not work on a detailed, e.g.
plant species, level.
Another experimental paradigm that address categorisation, often with the help of pictures, is to teach subjects the concepts of “same” and “different,” and then let them apply these on groups of stimuli, be it objects or pictures of objects, creatures etc. There are two versions of this paradigm. Same can mean “perceptually identi-cal,” in which case there is no need at all to see what a picture depicts, only to dis-criminate patterns from each other. An example is Wasserman et al. (2001) where baboons judged arrays of clip-art to contain either the same or different images. An-other sense in which “sameness” is used is to mean “categorically identical.” In this version you must avoid using identical stimuli. We have seen several examples of this type of similarity in experiments already. (It will be further discussed in section 11.3.)
The problem with the concepts “same” and “different,” is that one cannot be sure of what exactly it is that is judged to be the same or different in the view of the sub-ject. There is also little reason to assume that the criteria for sameness or difference are stable across entire experiments. In theory the subject can use novel criteria for each set of comparison stimuli.
Brown and Boysen (2000) argue that categorisation experiments that involve nondifferential reinforcement more accurately reflect subjects’ natural categories, while reinforced paradigms on the other hand can induce a specific categorisation during the course of testing. This is certainly true, but even without differential rein-forcement the nature of the task is bound to structure the categorisation that is ap-plied. Just being exposed to stimuli pitted against each other forces one to discrimi-nate between them, and thereby categorise, in some way or the other. This becomes further removed from “natural” categorisation when pictorial stimuli are used, where properties that would perhaps otherwise be used for categorisation is not captured in the picture.
That a pictorial cat is judged to be similar or different from a pictorial tiger does therefore not tell us what the subject thinks about the relationship between real cats and tigers, only that they found some basis to judge similarity or difference in the discrimination made. That a specific pair consisting of a gorilla and a chimpanzee is unequivocally judged by five chimpanzee subjects to be “the same” in Brown and Boysen (2000) does not mean that this particular gorilla and chimpanzee would not be seen as very different entities in real life. In fact, the reason they were judged to be “the same” was, according to Brown and Boysen, probably that they were sitting in very similar poses. Other chimpanzee – gorilla pairs were not categorised as “the same.” If the same two animals were to sit alike in a field outside of the subjects’
enclosures they might not be judged to be “the same,” but “different,” or “the same”
but for completely different reasons. Maybe their colour was the same at that occa-sion. If this discrimination had been preceded by several trials on species discrimina-tion there is a chance that this criterion would transfer also to the next pair, but with nondifferential reinforcement it is just as likely that it would not. The gorilla and the chimpanzee that were sitting alike in Brown and Boysen (2000), had in fact
been preceded by several trials on chimpanzee – gorilla discriminations where the subjects had, seemingly, responded categorically according to species.
The five chimpanzees (there was a sixth naïve control subject) in Brown and Boy-sen (2000) had learned the concepts of “same” and “different” previously to being tested with photographs of animals. They were for example able to judge Arabic numerals and arrays of dots as being either the same or different. It is not said if this is the only same/different training that they had had, neither what their previous experience with pictures were. In the present experiment they were required to judge if colour photographs of house cats, chimpanzees, gorillas, tigers and fish were the same or different, within, and across species categories. Seven images for each cate-gory were used, but two images were only pitted against each other once during the whole of testing. Two symbols represented the choices “same” and “different” re-spectively.
The subjects did not seem to respond on the basis of surface features, such as size, but since they performed on average in accordance with the experimenter defined species categories on “only” 69% of the trials, they are unlikely to have responded on the basis of species membership on all occasions. The implications mentioned above probably accounts for this.70 But still, they must have made some assessment if we are to believe that they fully understood the concepts “same” and “different.” If this assessment was always based on the animal content of the pictures, and never on surface features, is impossible to say. Even when animal categories were appreciated, we cannot be sure of which aspects of the animals that were used for the same/different judgements. After all, e.g. chimpanzees were judged to be “the same”
to fish about 30% of the time. Even if content was fully recognised in the pictures we can unfortunately not conclude that anything but reality mode was applied to them. Colour photographs on a computer screen were used, and there was neither a requirement for differentiation, nor reference, inherent in the task.