Nonverbal transitive inference: Effects of task and awareness on human performance

We studied human nonverbal transitive inference in two paradigms: with choice stimuli orderable along a physical dimension and with non-orderable choice stimuli. We taught 96 participants to discriminate four overlapping pairs of choice stimuli: A+ B−, B+ C−, C+ D−, and D+ E−. Half of the participants were provided with post-choice visual feedback stimuli which were orderable by size; the other half were not provided with orderable feedback stimuli. In later testing, we presented novel choice pairs: BD, AC, AD, AE, BE, and CE. We found that transitive responding depended on task awareness for all participants. Additionally, participants given ordered feedback showed clearer task awareness and stronger transitive responding than did participants not given ordered feedback. Associative models ( [Wynne, 1995] and [Siemann and Delius, 1998]) failed to predict the increase in transitive responding with increasing awareness. These results suggest that ordered and non-ordered transitive inference tasks support different patterns of performance.     

Transitive inference in non-human animals: an empirical and theoretical analysis

Transitive inference has long been considered one of the hallmarks of human deductive reasoning. Recent reports of transitive-like behaviors in non-human animals have prompted a flourishing empirical and theoretical search for the mechanism(s) that may mediate this ability in non-humans. In this paper, I begin by describing the transitive inference tasks customarily used with non-human animals and then review the empirical findings. Transitive inference has been demonstrated in a wide variety of species, and the signature effects that usually accompany transitive inference in humans (the serial position effect and the symbolic distance effect) have also been found in non-humans. I then critically analyze the most prominent models of this ability in non-human animals. Some models are cognitive, proposing for instance that animals use the rules of formal logic or form mental representations of the premises to solve the task, others are based on associative mechanisms such as value transfer and reinforcement and non-reinforcement. Overall, I argue that the reinforcement-based models are in a much better empirical and theoretical position. Hence, transitive inference in non-human animals should be considered a property of reinforcement history rather than of inferential processes. I finalize by shedding some light on some promising lines of research.     

Associative symmetry and stimulus-class formation by pigeons: The role of non-reinforced baseline relations

Two experiments tested the assumption of Urcuioli's (2008) theory of pigeons’ equivalence-class formation that consistent non-reinforcement of certain stimulus combinations in successive matching juxtaposed with consistent reinforcement of other combinations generates stimulus classes containing the elements of the reinforced combinations. In Experiment 1, pigeons were concurrently trained on symbolic (AB) and two identity (AA and BB) successive tasks in which half of all identity trials ended in non-reinforcement but all AB trials were reinforced, contingent upon either responding or not responding to the comparisons. Subsequent symmetry (BA) probe trials showed evidence of symmetry in one of four pigeons. In Experiment 2, pigeons learned three pair-comparison tasks in which left versus right spatial choices were reinforced after the various sample-comparison combinations comprising AB, AA, and BB conditional discriminations. Non-differentially reinforced BA probe trials following acquisition showed some indication of symmetrical choice responding. The overall results contradict the theoretical predictions derived from Urcuioli (2008) and those from Experiment 2 challenge other stimulus-class analyses as well.     

The role of awareness and working memory in human transitive inference

The human ability to perform transitive inference (TI) is an area of debate from a neurocognitive standpoint. Some studies emphasize a stimulus driven medial-temporal lobe process [Preston, A.R., Shrager, Y., Dudukovic, N.M., Gabrieli, J.D., 2004. Hippocampal contribution to the novel use of relational information in declarative memory. Hippocampus 14, 148-152; Titone, D., Ditman, T., Holzman, P., Eichenbaum, H., Levy, D., 2004. A transitive inference test of relational memory in schizophrenia. Schizophr. Res. 68, 235-247; Van Elzakker, M., O'Reilley, R., Rudy, J., 2003. Transivity, flexibility, conjenctive representation and the hippocampus: an empirical analysis. Hippocampus 13, 334-340] while others emphasize a higher-level frontal lobe strategy that requires the flexible maintenance of information in working memory [Waltz, J., Knowlton, B., Holyoak, K., Boone, K., Mishkin, F., de Menedezes Santos, M., Thomas, C., Miller, B., 1999. A system for relational reasoning in human prefrontal cortex. Psychol. Sci. 10, 119-125]. In two experiments we investigated when and how adults employ different cognitive strategies during TI by evaluating the interaction between task instructions and individual differences in working memory capacity. Participants engaged in a paired discrimination task involving a 6-unit TI hierarchy and were either prior aware, prior unaware or serendipitously aware of the hierarchical relationship among stimulus items. Both prior aware participants and serendipitously aware participants were more likely to engage in a logic-based strategy compared to unaware participants who relied upon stimulus-driven strategies. Individual differences in working memory were associated with the acquisition of awareness in the serendipitously aware group and with the maintenance of awareness in the prior aware group. These findings suggest that the capacity for TI may be supported by multiple neurocognitive strategies, and that the specific strategy employed is dependent upon both task- and participant-related factors.     

Relational reasoning with derived comparative relations: A novel model of transitive inference

A behavior-analytic model of transitive inference (TI) as relational reasoning with derived comparative relations is outlined. Following nonarbitrary relational training and testing to establish contextual functions of “more than” (>) and “less than” (<) for two abstract stimuli, two groups of participants learned a series of contextually controlled more than or less than relations (All-More: E > D > C > B > A; All-Less: A < B < C < D < E). On meeting the training criterion, inferential tests were presented to both groups involving mutually entailed relations (All-More: A < B, B < C, C < D and D < E; All-Less: B > A, C > B, D > C and E > D) and one-step (A < C, B < D, C < E, C > A, D > B and E > C) and two-step (A < D, B < E, D > A and E > B) combinatorially entailed relations. Performance accuracy on the trained and inferential tasks was uniformly high across both groups, with no significant differences observed. In both groups, however, performance accuracy differed significantly on one-step and two-step combinatorially entailed tasks involving the same or different relation to that trained. The present findings demonstrate complex relational reasoning with derived comparative relations, replicate several key effects from the literature on TI and have potential implications for the development of a contemporary behavior-analytic account of TI.     

Prospective timing in pigeons: Isolating temporal perception in the time-left procedure

In the time-left procedure, a task used to study prospective timing, animals choose between two stimuli that signal different delays to reinforcement. Trials begin with one stimulus signaling C seconds to reinforcement and, at different moments since its onset, another stimulus, signaling S seconds to reinforcement, with C > S, is introduced. Optimal performance consists in choosing the stimulus signaling the shorter time to reinforcement. Animals have been found to perform in this optimal way. However, this procedure is complex and variables other than time may be responsible for the results. In two experiments with pigeons we sought to improve the time-left procedure to better isolate the effect of time in the animals’ behavior. We attempted to control for two confounding variables, the asymmetry in the time markers from training to testing and the cost of switching between the two response alternatives. We conclude that in the time-left task pigeons seem indeed to regulate their behavior based on time because, with our improved procedure, they still chose the stimulus associated with the shorter time to food. However, our version of the procedure created new interpretative difficulties, strengthening the idea that the time-left procedure may be too complex to study timing.     

Scrounger numbers and the inhibition of social learning in pigeons

Social foraging can inhibit the learning and performance of food-finding behaviours. Confusion, overshadowing and frequency-dependent payoffs may all contribute to the inhibition, but standard experimental procedures make the separation of these effects difficult. In this study, we combine characteristics of cage and aviary experiments and present either a single naive pigeon or groups of three naive pigeons with a pre-trained producer opening an apparatus in an aviary. All naive birds scrounged on the 3456 openings they witnessed. In a post-test given in the absence of other birds, all single scroungers opened the apparatus, but only one of the group-scrounging pigeons did. Scrounger numbers appear to play an important role in the inhibition of food-finding behaviour, suggesting that confusion is a major component of learning in a social context.     

Matching-to-sample by pigeons: the dissociation of comparison choice frequency from the probability of reinforcement

It has been proposed that comparison choice in matching-to-sample should depend on two factors, the relative probability of reinforcement associated with each of the comparison stimuli and the conditional probability of each comparison stimulus being correct given presentation of one of the samples. DiGian and Zentall [DiGian, K.A., Zentall, T.R., 2007. Matching-to-sample in pigeons: in the absence of sample memory, sample frequency is a better predictor of comparison choice than the probability of reinforcement for comparison choice. Learn Behav. 35, 242-261] have shown that sample frequency together with the probability of choosing each of the comparison stimuli in training can influence comparison choice when delays are introduced, when the number of reinforcements associated with each of the comparison stimuli is equated. Furthermore, Zentall and Clement [Zentall, T.R., Clement, T.S., 2002. Memory mechanisms in pigeons: Evidence of base-rate neglect. J. Exp. Psych.: Anim. Behav. Proc. 28, 111-115] have found that sample frequency can affect comparison choice when delays are introduced independently of the number of choices of each of the comparison stimuli in training and the number of reinforcements associated with each of the comparison stimuli is equated. In the present experiment we found that the probability of choosing each of the comparison stimuli in training can affect comparison choice when delays are introduced, independently of sample frequency and when the number of reinforcements associated with each of the comparison stimuli is equated. Together, these experiments suggest that when the sample is not available, there is a partial dissociation between comparison choice and the probability of reinforcement associated with each of the comparison stimuli.     

Toward a framework for the evaluation of feature binding in pigeons

Pigeons were trained in a new procedure to test for visual binding errors between the dimensions of color and shape. In Experiment 1, pigeons learned to discriminate a target compound from 15 non-target compounds (constructed from four colors and shapes) by choosing one of two hoppers in a two-hopper choice task. The similarity of the target to non-target stimuli influenced choice responding. In Experiment 2, pigeons learned to detect a target compound when presented with a non-target compound within the same trial under conditions of simultaneity and sequentiality. Non-target trials were arranged to allow for the testing of binding errors (i.e., false identifications of the target on certain non-target trials). Transient evidence for binding errors in two of the birds occurred at the start of two-item training, but decreased with training. The experiments represent an important step toward developing a framework for the evaluation of visual feature binding in nonhumans.     

Attention, the presolution period, and choice accuracy in pigeons

Six pigeons were trained first on a color then on a form discrimination; four other pigeons were trained first on form and then on color. One of two colors or one of two forms (sample stimuli) appeared in the center of a touch sensitive monitor for 5 pigeons and in the center and in 16 other locations for 5 other pigeons. A peck anywhere within the region in which the sample stimuli appeared produced two white disks (comparison stimuli), one on the left and one on the lower right corners of the screen. Correct left-right choices provided food. Although of no consequence, the location of pecks in presence of the sample was predictive of the pigeon's subsequent choice. Accuracy, choice of the correct comparison stimulus, was greater when the sample stimuli appeared in the center as well as 16 other locations than when it appeared only in the center. The presolution period, the period of chance accuracy prior to evidence of discrimination learning, was decreased on each task following training on the other task. This evidence of facilitation following an extra-dimensional shift was attributed to continued relevance of the conditions under which the first task was learned. The duration of the presolution period was inversely related to asymptotic accuracy-data accounted for by Heinemann's (1983) theory of information processing during the presolution period.     

The evolution of associative learning: A factor in the Cambrian explosion

The Cambrian explosion is probably the most spectacular diversification in evolutionary history, and understanding it has been a challenge for biologists since the time of Darwin. We propose that one of the key factors that drove this great diversification was associative learning. Although the evolutionary emergence of associative learning required only small modifications in already existing memory mechanisms and may have occurred in parallel in several groups, once this type of learning appeared on the evolutionary scene, it led to extreme diversifying selection at the ecological level: it enabled animals to exploit new niches, promoted new types of relations and arms races, and led to adaptive responses that became fixed through genetic accommodation processes. This learning-based diversification was accompanied by neurohormonal stress, which led to an ongoing destabilization and re-patterning of the epigenome, which, in turn, enabled further morphological, physiological, and behavioral diversification. Our hypothesis combines several previous ideas about the dynamics of the Cambrian explosion and provides a unifying framework that includes both ecological and genomic factors. We conclude by suggesting research directions that would clarify the timing and manner in which associative learning evolved, and the effects it had on the evolution of nervous systems, genomes, and animal morphology.     

Why a neuromaturational model of memory fails: Exuberant learning in early infancy

The characteristics of memory in infants and adults seem vastly different. The neuromaturational model attributes these differences to an ontogenetic change in the basic memory process, namely, to the hierarchical maturation of two distinct memory systems. The early-maturing (implicit) system is functional during the first third of infancy and supports the gradual learning of perceptual and motor skills; the late-maturing (explicit) system supports representations of contextually specific events, relationships, and associations. An alternative model holds that the basic memory process does not change, but what infants and adults select to encode for learning does. This ontogenetic change in selective attention has been mistaken for an ontogenetic shift in the basic memory process. Over the last 25 years, evidence from transfer studies with developing rats and human infants has revealed that the first third of infancy is actually a period of exuberant learning that ends, not coincidentally, at the same age that the late-maturing memory system presumably emerges. This article reviews data from recent studies of sensory preconditioning, potentiation, associative chains, and transitive inference with human infants that support this conclusion—data for which the neuromaturational model cannot account. Fast mapping is a general learning mechanism that accounts for this evidence.     

Review: Current status and future prospects of associative nitrogen fixation in rice

Biological sources of nutrients are gaining importance over the chemical and organic sources from the standpoint of environmental safety and quality, and sustainable agriculture. The nutrient input for a growing rice crop can largely be met by promoting the activities of physiologically diverse microorganisms in the aerobic, anaerobic and interface zones in the ecologically important flooded soils. Associative bacteria contribute from 10 to 80 kg N per hectare per cropping season depending upon the ecosystem, cultural practices and rice variety grown. In addition to N contribution, these bacterial associations can improve the nutrient transformations and contribute to plant growth-promoting effects. Current improved agronomic and crop production management systems greatly affect the contributions of biological sources to the overall soil nutrient status. Azospirillum and other associative bacterial systems have been intensively researched using various evaluation techniques to understand the diazotrophic rhizocoenosis. Researches clearly indicate that these associations are governed by several soil, water, nutrient, agrochemical, plant genotype and other biological factors. Considerable efforts have been made so far in selecting efficient bacterial strains as inoculants and identifying host genotypes which support maximum nitrogenase activity in addition to other beneficial traits of effective associative relationships. Knowledge gained so far on how the N2-fixing system in rice functions suggests the need for providing optimum management practices to ensure greater contribution from the plant-microbe associations. Holistic approaches integrating technological developments and achievements in biological sciences could lead to crop improvement. Research on extending nitrogen-fixing symbiosis to rice using molecular and genetic approaches is underway, albeit at a slow pace. The need for further fine-tuning and developing management practices, innovative approaches to improve rice-bacterial systems and the strategies to sustain the benefits from associative diazotrophy are discussed.     

Scaling relative incentive value: different adjustments to incentive downshift in pigeons and rats

Previous research suggests that pigeons and rats show differences in their behavioral adjustments in spaced-trial, incentive-downshift situations. Also, Papini and Pellegrini [Papini, M.R., Pellegrini, S., 2006. Scaling relative incentive value in consummatory behavior. Learn. Motiv. 37, 357-378] and Pellegrini and Papini [Pellegrini, S., Papini, M.R., 2007. Scaling relative incentive value in anticipatory behavior. Learn. Motiv. 38, 128-154] showed that changes in the rat's lever-pressing performance, runway running, and consumption of sucrose solutions after downshifts in incentive magnitude were a function of the ratio of postshift/preshift incentive magnitudes. Here, two experiments using a Pavlovian autoshaping procedure studied the adjustment of pigeons and rats to changes in incentive magnitude. In Experiment 1, pigeons received light-food pairings, whereas in Experiment 2, rats received lever-sucrose pairings. As a result, key-pecking and lever-pressing developed in each experiment, respectively. Preshift incentive magnitudes were downshifted so as to obtain postshift/preshift ratios of 0.125 and 0.25. Pigeons responded during the postshift phase according to the preshift incentive value and independently of the ratio value. However, rats showed ratio constancy, responding during the postshift in accordance with the postshift/preshift ratio, rather than with the absolute magnitudes of either the preshift or postshift incentives. These results support the comparative hypothesis that the mechanisms underlying ratio constancy during incentive downshifts are unique to mammals.     

Salience modulation and associative inhibition interaction: Short but not long exposure to similar stimuli protects the salience of the unique elements

In three experiments, rats were given preexposure to two similar flavour compounds, AX and BX. Following preexposure, conditioning trials took place in which AX was paired with an illness-induced unconditioned stimulus. Animals that were given short alternated preexposure to AX and BX, showed higher generalization of conditioned aversion to AX to a new compound, AN, than animals that were given blocked preexposure (short and long) and long alternated preexposure (Experiments 1 and 2); and showed less preference for A when they were given a choice between A and X (Experiment 3). These results have been taken to indicate that the salience of the A element is well preserved after short alternated preexposure, but declines when preexposure goes on for some more trials. The results reported support the notion that perceptual learning is a multi-determined phenomenon that depends on salience modulation processes after relatively short preexposure, and on an associative inhibition mechanism after prolonged preexposure.     

Some characteristics of spatial associative memory in the pigeon, Columba livia

Willson and Wilkie (1993) developed a novel procedure to assess pigeons' memory for the spatial location of food. Only one of four locations provided food each daily session. Each location consisted of an illuminated pecking key and grain feeder. Over different days different locations, randomly selected, provided food during a 16-min session. The pigeons tended to revisit the location at which food was found on the previous day thereby demonstrating memory for food-spatial location associations over 24 h. Three experiments were conducted to further investigate this phenomenon. In Experiment 1 the session duration was varied between 4 and 32 min. Longer sessions had no detectable effect on their ability to remember the rewarded location 24 h later, a result that suggests that only brief encounters with food at a particular location are necessary for recall. In Experiment 2 the necessity of an active search for the day's rewarded location was removed; a 5-min period in which only the rewarded key was lit preceded the regular 16-min session. Pecks to the lit key in this 5-min period produced grain on the standard schedule. This manipulation facilitated the pigeons' discovery of food but did not affect their ability to remember the rewarded location, suggesting that the process of search and discovery is not essential to the associative memory process. In Experiment 3, food was available during the complete session (non-depleting condition) or was available only during the first half of the session (depleting condition). No detectable differences in the birds' memory of yesterday's profitable location were found. This suggests that non-depletion of food is not a necessary condition for day-to-day recall of food location. Taken together these findings enlarge our understanding of the spatial associative memory process.     

Impaired associative learning in schizophrenia: behavioral and computational studies

Associative learning is a central building block of human cognition and in large part depends on mechanisms of synaptic plasticity, memory capacity and fronto–hippocampal interactions. A disorder like schizophrenia is thought to be characterized by altered plasticity, and impaired frontal and hippocampal function. Understanding the expression of this dysfunction through appropriate experimental studies, and understanding the processes that may give rise to impaired behavior through biologically plausible computational models will help clarify the nature of these deficits. We present a preliminary computational model designed to capture learning dynamics in healthy control and schizophrenia subjects. Experimental data was collected on a spatial-object paired-associate learning task. The task evinces classic patterns of negatively accelerated learning in both healthy control subjects and patients, with patients demonstrating lower rates of learning than controls. Our rudimentary computational model of the task was based on biologically plausible assumptions, including the separation of dorsal/spatial and ventral/object visual streams, implementation of rules of learning, the explicit parameterization of learning rates (a plausible surrogate for synaptic plasticity), and learning capacity (a plausible surrogate for memory capacity). Reductions in learning dynamics in schizophrenia were well-modeled by reductions in learning rate and learning capacity. The synergy between experimental research and a detailed computational model of performance provides a framework within which to infer plausible biological bases of impaired learning dynamics in schizophrenia.     

Role of stimulus-stimulus pairing in matching-to-sample procedure: cross-species comparison of humans and pigeons

The present experiment examined whether, in a matching-to-sample (MTS) procedure, a relation between two stimuli, a sample and a comparison, could be established as a result of just stimulus–stimulus pairing, even if back up reinforcers were never provided for the conditional relation between the sample and comparison stimuli, but rather only for the comparison stimulus. A procedure called “pseudo matching-to-sample” was used in which, when S1 was presented as a sample stimulus, two comparison stimuli (C1 and C2) were presented, and only responses to C1 were reinforced. Conversely, when S2 was presented, only responses to C3 (and not C4) were reinforced. In other words, organisms experiencing this procedure could discriminate C1 from C2, and C3 from C4, by simple discrimination without regard to the conditional sample stimuli. In order to examine cross-species differences, responding by humans in this procedure was compared to that by pigeons. Although the humans developed a discriminative function for the sample stimuli, that is, the humans’ responding was affected by both the sample stimuli and the reinforcers, responding by the pigeons was affected solely by the reinforcers. These data suggest that, in this procedure, humans (but not pigeons) are able to learn relations among stimuli simply as a result of stimulus–stimulus pairing.     

Configural and elemental processing in associative learning: commentary on Melchers, Shanks and Lachnit

The paper by Melchers, Shanks, and Lachnit (2007) reviews the available evidence suggesting that there is flexible processing, such that some associative learning tasks can be solved either configurally or elementally. We find the evidence provocative but limited in its demonstrated generality and silent with respect to the theoretical mechanisms that might regulate the alleged flexibility of processing. Further research is invited to determine the scope of the variation involved and how best to account for it. At present, theories, either elemental or configural, that include mechanisms to shift the weight assigned to component stimuli appear to be potential candidates for embracing the data.