Symmetry and transitivity of conditional relations in monkeys (Cebus apella) and pigeons (Columba livia)

In Experiment 1 six monkeys were tested with discriminative relations that were backward relative to their training in a 0-second conditional (“symbolic”) matching procedure. Although there was some indication of backward associations, the evidence was generally weak, and statistical evaluations did not reach conventional significance levels. Unlike children, who show backward associations to the point of symmetry, monkeys and pigeons display at best only weak and transient backward associations. In Experiment 2 associative transitivity was assessed across two sets of conditional matching tasks. All four monkeys tested demonstrated strong transitivity. In contrast, in Experiment 3 there was no evidence of transitivity in three pigeons tested under conditions closely comparable to those of Experiment 2. These results may identify some key features of interspecies differences and contribute to analyses of serial learning in animals.     

Conditional relations by monkeys: Reflexivity, symmetry, and transitivity

Two cynomolgous macaques categorized six colors into two groups of three after conditional discrimination training (zero-delay symbolic match-to-sample). The procedures resulted in the establishment of relations among the elements of each set-relations that were not specifically trained and that can be characterized by the properties of reflexivity, symmetry, and transitivity. Each set of colors was related to a characteristic pattern of responding: One response pattern involved temporal duration (press and hold the response keys); the second response pattern entailed repeated pressing and releasing of the response keys (fixed ratio 8). Six combinations of two colors were trained, three combinations from each set. After discriminative performance stabilized for each monkey, they were tested with 10 additional color combinations, all of which differed from the training combinations. The conditional relations established between test combinations can be characterized as stimulus equivalence. The training procedures were analogous to the procedure of using category names, and have implications for understanding the function of language in the formation of equivalence classes.     

Tests of symmetry in three-way contingency tables

Three-dimensional contingency tables are analyzed, with one variable (e.g., sex) as a factor, and with a natural relation between the other variables (e.g. left and right eye vision). Models of special interest, like symmetry and proportional symmetry between the related variables, and homogeneity across the factor levels, are investigated. Maximum likelihood estimators of parameters and partitions of chi-square goodness-of-fit statistics are explicitly presented; the independence of certain models is noted, and an example is discussed.     

An update on the search for symmetry in nonhumans

Sidman et al.'s (1982) failure to find evidence for symmetry (bidirectional associations between stimuli) in monkeys and baboons set the stage for decades of work on emergent relations in nonhumans. They attributed the failure to the use of procedures that did not (1) promote stimulus control based on the relation between the sample and correct comparison and (2) reduce control by irrelevant stimulus features. Previous reviews of symmetry in nonhumans indicated that multiple exemplar training and successive matching might encourage appropriate stimulus control. This review examined 16 studies that investigated symmetry in 94 subjects, including pigeons, rats, capuchin monkeys, and baboons. Several studies used alternative training procedures to minimize sources of irrelevant stimulus control, and many combined multiple exemplar training with other procedural modifications. Symmetry was observed in approximately 30% of subjects. Studies that reported the strongest evidence for symmetry used successive matching-to-sample procedures that included training on both symbolic and identity relations, and studies finding mixed evidence employed alternative methods. These studies highlight the challenge in creating training procedures that promote symmetry and the need to assess the underlying sources of control on positive demonstrations.     

Sea lions and equivalence: expanding classes by exclusion

Experiments have shown that human and nonhuman subjects are capable of performing new arbitrary stimulus-stimulus relations without error. When subjects that are experienced with matching-to-sample procedures are presented with a novel sample, a novel comparison, and a familiar comparison, most respond by correctly selecting the novel comparison in the presence of the new sample. This exclusion paradigm was expanded with two California sea lions that had previously formed two 10-member equivalence classes in a matching-to-sample procedure. Rather than being presented with a novel sample on a given trial, the sea lions were presented with a randomly selected familiar member of one class as the sample. One of the comparisons was a randomly selected familiar member of the alternative class, and the other was a novel stimulus. When required to choose which comparison matched the sample, the subjects reliably rejected the familiar comparison, and instead selected the unfamiliar one. Next, the sea lions were presented with transfer problems that could not be solved by exclusion; they immediately grouped the new stimuli into the appropriate classes. These findings show that exclusion procedures can rapidly generate new stimulus relations that can be used to expand stimulus classes.     

Stimulus control topographies and tests of symmetry in pigeons

Pigeons were tested for symmetry after A-B training under conditions designed to avoid problems that may prevent its emergence, namely the change of stimulus location in testing relative to training and the lack of requisite discrimination training. In Experiment 1, samples appeared in two locations during baseline training to minimize the impact of stimulus location. Experiments 2 and 3 included multiple-location training along with additional identity and symbolic matching training, respectively, to explicitly train all of the simultaneous and successive stimulus discriminations required for testing. Experiment 4 provided reinforcement for symmetrical matching relations with some stimulus sets (with multiple-location training) prior to symmetry testing with different sets. In all experiments, pigeons showed no evidence of symmetry despite the fact that baseline (A-B) matching transferred to novel locations. Additional tests for reflexivity (Experiment 2) yielded similar outcomes. These results indicate that the change in stimulus location is not the sole reason that pigeons do not show symmetry and increase the plausibility of arguments that symmetry and other indexes of stimulus equivalence may be beyond the capabilities of the pigeon.     

Testing for symmetry in the conditional discriminations of language-trained chimpanzees

If subjects are taught to match Stimulus A to B and then, without further training, match B to A, they have passed a test of symmetry. It has been suggested that non-humans' lack of success on symmetry tests might be overcome by giving them a history of symmetry exemplar training, that is, by directly teaching a large number of conditional relations (e.g., AB, CD, EF,...) and also directly training the "reverse" of these relations (e.g., BA, DC, FE,...). The chimpanzee subjects of the present study, Sherman, Austin, and Lana, had already received extensive symmetry exemplar training as a result of attempts to teach a selection-based language system of lexigrams. The present study systematically subjected 2 of these chimps (Sherman and Lana), for the first time, to standard symmetry tests in controlled conditions. Both chimps failed the tests, even when their correct responses on test trials were reinforced. The findings do not support the exemplar training hypothesis, and cast doubt upon whether the chimps can pass tests of stimulus equivalence.     

The role of the human extrastriate visual cortex in mirror symmetry discrimination: a TMS-adaptation study

The human visual system is able to efficiently extract symmetry information from the visual environment. Prior neuroimaging evidence has revealed symmetry-preferring neuronal representations in the dorsolateral extrastriate visual cortex; the objective of the present study was to investigate the necessity of these representations in symmetry discrimination. This was accomplished by the use of state-dependent transcranial magnetic stimulation, which combines the fine resolution of adaptation paradigms with the assessment of causality. Subjects were presented with adapters and targets consisting of dot configurations that could be symmetric along either the vertical or horizontal axis (or they could be non-symmetric), and they were asked to perform a symmetry discrimination task on the targets while fixating the center of the screen. TMS was applied during the delay between the adapter and the test stimulus over one of four different sites: Left or Right V1/V2, or left or right dorsolateral extrastriate cortex (DLO). TMS over both Left and Right DLO reduced the adaptation effect in detecting vertical and horizontal symmetry, although the Left DLO effect on horizontal symmetry and the Right DLO effect on both vertical and horizontal symmetry were present only when considering subjects who showed a behavioral adaptation effect in the baseline No-TMS condition. Application of TMS over the Left or Right V1/V2 did not modulate the adaptation effect. Overall, these data suggest that both the Left and Right DLO contain neuronal representations tuned to mirror symmetry which play a causal role in symmetry discrimination.     

The impact of face skin tone on perceived facial attractiveness: A study realized with an innovative methodology

This study aimed to assess the impact of target faces’ skin tone and perceivers’ skin tone on the participants’ attractiveness judgment regarding a symmetrical representative range of target faces as stimuli. Presented with a set of facial features, 240 Mozambican adults rated their attractiveness along a continuous scale. ANOVA and Chi-square were used to analyze the data. The results revealed that the skin tone of the target faces had an impact on the participants’ attractiveness judgment. Overall, participants preferred light-skinned faces over dark-skinned ones. This finding is not only consistent with previous results on skin tone preferences, but it is even more powerful because it demonstrates that the light skin tone preference occurs regardless of the symmetry and baseline attractiveness of the stimuli.