Odour detection animals are required to learn many individual odours during training. Most organisations and agencies use single-odour (sequential) training, where animals learn one odour followed by another. However, this method may not be optimal for learning or for detecting target odours when they are mixed with other substances, which is an inevitable occurrence when animals are actively deployed in the field. Here, we used a Go/No-Go procedure to investigate the impact of three different training methods upon rats' ability to identify target-odours alone and within mixtures. A sequential group were trained with odour A followed by odour B or vice versa; a compound group were trained with odours AB presented as a single stimulus; and an intermixed group were trained separately on both odours A and B within a session. Following training, all groups were tested for their responses to A, B, and AB, as well as these odours combined with a novel odour C: AC, BC, ABC. Responses to the test stimuli significantly differed between groups (p = 0.002). The intermixed group generalised significantly better than the sequential (p = 0.005) and compound (p = 0.014) groups; and the compound group generalised significantly better than the sequential group (p = 0.023). These findings have important implications for the training of animals used for odour detection. They provide strong evidence that an intermixed training method may be more effective than the generally employed method of sequential single-odour training.

Visual illusions are objects that are made up of elements that are arranged in such a way as to result in erroneous perception of the objects’ physical properties. Visual illusions are used to study visual perception in humans and nonhuman animals, since they provide insight into the psychological and cognitive processes underlying the perceptual system. In a set of three experiments, we examined whether dogs were able to learn a relational discrimination and to perceive the Müller-Lyer illusion. In Experiment 1, dogs were trained to discriminate line lengths using a two-alternative forced choice procedure on a touchscreen. Upon learning the discrimination, dogs’ generalization to novel exemplars and the threshold of their abilities were tested. In the second experiment, dogs were presented with the Müller-Lyer illusion as test trials, alongside additional test trials that controlled for overall stimulus size. Dogs appeared to perceive the illusion; however, control trials revealed that they were using global size to solve the task. Experiment 3 presented modified stimuli that have been known to enhance perception of the illusion in other species. However, the dogs’ performance remained the same. These findings reveal evidence of relational learning in dogs. However, their failure to perceive the illusion emphasizes the importance of using a full array of control trials when examining these paradigms, and it suggests that visual acuity may play a crucial role in this perceptual phenomenon.

The ability to identify a novel stimulus as a member of a known category allows an organism to respond appropriately towards it. Categorisation is thus a fundamental component of cognition and an essential tool for processing and responding to unknown stimuli. Therefore, one might expect to observe it throughout the animal kingdom and across sensory domains. There is much evidence of visual categorisation in non-human animals, but we currently know little about this process in other modalities. In this experiment, we investigated categorisation in the olfactory domain. Dogs were trained to discriminate between 40 odours; the presence or absence of accelerants formed the categorical rule. Those in the experimental group were rewarded for responding to substrates with accelerants (either burnt or un-burnt) and inhibit responses to the same substrates (either burnt or un-burnt) without accelerants (S+ counterbalanced). The pseudocategory control group was trained on the same stimuli without the categorical rule. The experimental group learned the discrimination and animals were able to generalise to novel stimuli from the same category. None of the control animals were able to learn the discrimination within the maximum number of trials. This study provides the first evidence that non-human animals can learn to categorise non-biologically relevant odour information.