The discrimination of cat odours by humans

We explored the ability of humans to identify individual cats by smell. Twenty-five cat owners were required to indicate which of two odours (one belonging to their own cat and one belonging to an unfamiliar cat) smelt the strongest, most pleasant, and which belonged to their own cat. Only thirteen (52%) of the participants were able to recognise the odour of their own cat.     

Olfactory hedonic context affects both self-report and behavioural indices of palatability

Prior exposure to either a pleasant or unpleasant context may affect later hedonic judgments of a common target stimulus. We explored whether this effect translates into behaviour in the chemical senses. In experiment 1 participants experienced either a pleasant or unpleasant set of odours or pictures, followed by an unfamiliar odour. After self-report hedonic evaluations of the odour, participants were allowed to drink it in solution, followed by a further evaluation of its flavour. Participants reported liking the odour less after smelling pleasant odours and drank less of it too, relative to the unpleasant context. There was no differential context effect for emotive pictures. Experiment 2 replicated these effects, but also included a no-context control. This revealed that the consumption effect was localised to the pleasant olfactory context, whilst contextual effects for liking ratings were primarily localised to the unpleasant olfactory context. In conclusion, hedonic context affects both self-report and behaviour, but not in the same way.     

Olfactory recognition of individual competitors by means of faeces in horse (<Emphasis Type="Italic">Equus caballus</Emphasis>)

Living in complex social systems requires perceptual and cognitive capacities for the recognition of group membership and individual competitors. Olfaction is one means by which this can be achieved. Many animals can identify individual proteins in urine, skin secretions, or saliva by scent. Additionally, marking behaviour in several mammals and especially in horses indicates the importance of sniffing conspecifics’ faeces for olfactory recognition. To test this hypothesis, we conducted two separate experiments: Experiment 1 addressed the question of whether horses can recognise the group membership of other horses by sniffing their faeces. The horses were presented with four faecal samples: (1) their own, (2) those of other members of their own group, (3) those of unfamiliar mares, and (4) those of unfamiliar geldings. Experiment two was designed to assess whether horses can identify the group member from whom a faecal sample came. Here, we presented two groups of horses with faecal samples from their group mates in random distribution. As controls, soil heaps and sheep faecal samples were used. In experiment one, horses distinguished their own from their conspecifics’ faeces, but did not differentiate between familiarity and sex. In experiment two, the horses from both groups paid most attention to the faeces of the horses from which they received the highest amount of aggressive behaviours. We therefore suggest that horses of both sexes can distinguish individual competitors among their group mates by the smell of their faeces.     

Odour memory and odour hedonics in children

The characteristics of olfactory memory during development were investigated and the hypothesis that the pleasantness of smells may be affected by previous associations with pleasant or unpleasant objects or events was tested. This type of emotional memory was compared in the immediate and long-term recognition of olfactory stimuli. Children from three different age groups (mean ages: 6 years 6 months; 8 years 9 months; and 10 years 5 months) were subdivided into two groups. One group was presented with six different odours, each with a slide depicting a pleasant picture. The other group was presented with the odours accompanied by unpleasant pictures. Immediately after stimulus presentation the subjects underwent a recognition test. One month later the subjects underwent a second recognition test, at the end of which they were required to give an evaluation of the pleasantness of each odour on a nine-point scale. At no age level did the pictures matched to the odours affect the recognition score. Olfactory memory varied with age, chiefly because memory decay increased with age, perhaps because of greater proactive interference. With increasing age more rapid decay was set against better immediate recognition. The hypothesis that the hedonic characteristics of odours are partially learned and are affected by events experienced in other modalities was supported.     

Odour perception: an object-recognition approach

Object recognition is a crucial component of both visual and auditory perception. It is also critical for olfaction. Most odours are composed of 10s or 100s of volatile components, yet they are perceived as unitary perceptual events against a continually shifting olfactory background (i.e. figure-ground segregation). We argue here that this occurs by rapid central adaptation to background odours combined with a pattern-matching system to recognise discrete sets of spatial and temporal olfactory features-an odour object. We present supporting neuropsychological, learning, and developmental evidence and then describe the neural circuitry which underpins this. The vagaries of an object-recognition approach are then discussed, with emphasis on the putative importance of memory, multimodal representations, and top-down processing.     

Experience dependent changes in odour-viscosity perception

One consequence of experiencing flavour — the combination of taste, smell and somatosensation that occurs during ingestion — is that it can result in perceptual changes for the odour component, when this is later smelled alone. One such change is the acquisition of taste-like properties, but whether odours can also acquire somatosensory-like qualities is largely unknown. Participants here were exposed to one odour sampled in a viscous solution, another sampled in a sweet/viscous solution, and a further odour sampled in water. The odour sampled in the sweet/viscous solution was, when later sniffed alone, judged to smell thicker and sweeter, than the other two odours. Similarly, when the sweet/viscous paired odour was added to a viscous solution, the combination was judged as more viscous, than the other two odours — and sweeter when added to a sweet solution. This experiment suggests that odours can acquire tactile-like somatosensory qualities and this may best occur when a taste is present during learning. Recent work indicates that tastes may be superior to somatosensory stimuli alone in promoting flavour binding, a seeming precondition for this type of learning.     

A limit in the processing of components in odour mixtures

We investigated the hypothesis that physiological limitations restrict the ability of humans to identify components in an odour mixture. Subjects were trained to identify the test odours, and were required to detect a single highly familiar odorant in stimuli consisting of one, four, eight, twelve, and sixteen odorants by using a selective-attention procedure. The stimuli were delivered by a computer-controlled sixteen-channel air-dilution olfactometer which provided samples of each of the sixteen odorants to be of equal perceived intensity for each subject. Identification fell to chance level when sixteen odorants were present. It is proposed that the profound loss of information was primarily due to inhibition of olfactory receptor cells by the odorants through competitive mechanisms, and the subsequent loss of odour identity through changes in the spatial code that may be used to identify odorants.     

Horses (Equus caballus) discriminate body odour cues from conspecifics

Knowledge about social recognition and memory in animals can help us to determine appropriate management and husbandry techniques. In this study, we used a habituation–discrimination procedure to investigate the ability of horses (Equus caballus) to distinguish between the body odour samples of unfamiliar conspecifics. To pick up body odour, we rubbed material on the coat of horses and presented these unknown body odours to 16 different conspecifics of the same sex and similar age. The test consisted of two successive two-min presentations of a sample from one individual (e.g. individual ‘A’) and a simultaneous presentation of samples from individual ‘A’ and a novel individual (e.g. individual ‘B’) during a final third presentation. The results showed that horses, regardless of sex, decreased the time they spent investigating conspecific body odour across the initial two presentations—demonstrating habituation. In the final presentation, the results demonstrated successful discrimination of the previously experienced odour because horses investigated the novel olfactory sample (‘B’) significantly more than the pre-exposed sample (‘A’). Taken together, these findings suggest, for the first time, that horses are able to discriminate two stimuli derived from body odours of unfamiliar conspecifics over short period of time.     

Dogs (<Emphasis Type="Italic">Canis familiaris</Emphasis>) adjust their social behaviour to the differential role of inanimate interactive agents

Dogs are able to flexibly adjust their social behaviour to situation-specific characteristics of their human partner’s behaviour in problem situations. However, dogs do not necessarily detect the specific role played by the human in a particular situation: they may form expectations about their partners’ behaviour based on previous experiences with them. Utilising inanimate objects (UMO—unidentified moving object) as interacting agents offers new possibilities for investigating social behaviour, because in this way we can remove or control the influence of previous experience with the partner. The aim of the present study was to investigate whether dogs are able to recognise the different roles of two UMOs and are able to adjust their communicative behaviour towards them. In the learning phase of the experiment, dogs were presented with a two-way food-retrieval problem in which two UMOs, which differed in their physical appearance and abilities, helped the dog obtain a piece of food in their own particular manner. After a short experience with both UMOs, dogs in the test phase faced one of the problems in the presence of both inanimate agents. Overall, dogs displayed similar levels of gazing behaviour towards the UMOs, but in the first test they looked, approached and touched the relevant partner first. This rapid adjustment of social behaviour towards UMOs suggests that dogs may generalise their experiences with humans to unfamiliar agents and are able to select the appropriate partner when facing a problem situation.     

Embryonic olfactory learning in frogs.

We examined the effect of embryonic olfactory experiences on behaviour after hatching in two species of anuran amphibians, Rana temporaria and Rana sylvatica. Odorants (orange and citral) injected into the egg were "learned" by the embryo and subsequently preferred by tadpoles after hatching. The observed preferences were specific to the odours experienced prior to hatching, and tadpoles discriminated between the odour experienced as embryos and a novel odour. The acquired preference was maintained after metamorphosis for those individuals that exhibited a preference as tadpoles. Preferences of tadpoles were also influenced "naturally" by odours present in the water surrounding the developing embryos. The experimentally induced olfactory preferences did not appear to influence the exhibition of sibling recognition abilities. The study represents the first demonstration of "embryonic" learning in amphibians, and the functions and importance of early olfactory learning for amphibians are discussed.     

Are olfactory images sensory in nature?

We investigated the features of olfactory mental images by comparing odour images with perceptual and semantic representations. Participants who were assigned to three groups made similarity judgments about 17 common odours by smelling odours, imagining odours, or on the basis of the meaning of odour source names. In the smelling group, every pair of odours was compared. In the imagining group, imagined odours were compared twice, both before and after associative learning of the odour/name combinations. In the meaning group, the odour source names were compared in terms of general word meanings. Nonmetric multidimensional scaling analysis was applied to each group of similarity data and three-dimensional sensory, mental, and semantic spaces were composed. 17 elements in the mental and semantic spaces were superimposed onto the sensory space by Procrustes rotation. We found that the averaged distances of the 17 elements between the sensory and the mental spaces (either before or after learning) were smaller than those between the sensory and semantic spaces. We suggest that odour images have sensory features, especially after associative learning between perceived odours and their names.     

Olfactory awareness is positively associated to odour memory

We investigated the relationship between olfactory awareness and olfactory proficiency as determined by a set of standardised olfactory tasks. Olfactory awareness was indexed by scores in questionnaires focusing on odour interest, imagery ability, and prevalence of olfactory dreams. Nineteen subjects with high and 20 subjects with low odour awareness were presented with a set of standardised olfactory tasks: odour threshold, episodic odour recognition, and odour identification. The results showed that individuals with high odour awareness excelled in odour memory and identified more odours as compared with the low awareness group. Interestingly, odour naming ability exerted no influence on odour memory. Furthermore, high odour awareness was not related to a more sensitive olfactory sensory system as determined by olfactory threshold measurements.     

Smells like sib spirit: kin recognition in three-spined sticklebacks (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>) is mediated by olfactory cues

The ability to recognise kin has been demonstrated in several animal species. However, the mechanisms of kin recognition often remain unknown. The most frequently discussed sensory modalities to recognise kin are visual, olfactory and acoustical cues. Three-spined sticklebacks (Gasterosteus aculeatus) are able to differentiate between kin and non-kin when presented visual and olfactory cues combined. To elucidate, which cues they use to recognise kin female sticklebacks were given the choice between two identical computer animations of courting stickleback males. Next to one animation, water conditioned by a brother was added, while near the other, water from an unrelated male was added. In half of the experiments, the brother was familiar while in the other half he was unfamiliar to the female. Both scenarios were carried out with both outbred and inbred fish. The results showed that the females adjusted their choice behaviour according to relatedness. Furthermore, they were able to recognise both familiar as well as unfamiliar brothers. Inbreeding did not affect this ability. Hence, three-spined sticklebacks are able to recognise their relatives using olfactory cues alone. The cognitive mechanisms underlying this ability were independent from familiarity and not impaired by inbreeding.     

Visual discrimination of species in dogs (Canis familiaris)

In most social interactions, an animal has to determine whether the other animal belongs to its own species. This perception may be visual and may involve several cognitive processes such as discrimination and categorization. Perceptual categorization is likely to be involved in species characterized by a great phenotypic diversity. As a consequence of intensive artificial selection, domestic dogs, Canis familiaris, present the largest phenotypic diversity among domestic mammals. The goal of our study was to determine whether dogs can discriminate any type of dog from other species and can group all dogs whatever their phenotypes within the same category. Nine pet dogs were successfully trained through instrumental conditioning using a clicker and food rewards to choose a rewarded image, S+, out of two images displayed on computer screens. The generalization step consisted in the presentation of a large sample of paired images of heads of dogs from different breeds and cross-breeds with those of other mammal species, included humans. A reversal phase followed the generalization step. Each of the nine subjects was able to group all the images of dogs within the same category. Thus, the dogs have the capacity of species discrimination despite their great phenotypic variability, based only on visual images of heads.     

Embryonic olfactory learning in frogs

We examined the effect of embryonic olfactory experiences on behaviour after hatching in two species of anuran amphibians, Rana temporaria and Rana sylvatica. Odorants (orange and citral) injected into the egg were “learned” by the embryo and subsequently preferred by tadpoles after hatching. The observed preferences were specific to the odours experienced prior to hatching, and tadpoles discriminated between the odour experienced as embryos and a novel odour. The acquired preference was maintained after metamorphosis for those individuals that exhibited a preference as tadpoles. Preferences of tadpoles were also influenced “naturally” by odours present in the water surrounding the developing embryos. The experimentally induced olfactory preferences did not appear to influence the exhibition of sibling recognition abilities. The study represents the first demonstration of “embryonic” learning in amphibians, and the functions and importance of early olfactory learning for amphibians are discussed.     

How does dominance rank status affect individual and social learning performance in the dog (<Emphasis Type="Italic">Canis familiaris</Emphasis>)?

Dogs can learn effectively to detour around a V-shaped fence after observing a demonstration from either an unfamiliar human or dog demonstrator. We found earlier that there is substantial individual variation between the dogs’ performance, even when using the same experimental conditions. Here, we investigate if the subjects’ relative dominance rank with other dogs had an effect on their social learning performance. On the basis of the owners’ answers to a questionnaire, subjects from multi-dog homes were sorted into groups of dominant and subordinate dogs. In Experiment 1, dominant and subordinate dogs were tested without demonstration and we did not find any difference between the groups—they had similarly low detour performances on their own. In Experiment 2 and 3, dogs from single dog and multi-dog households were tested in the detour task with demonstration by an unfamiliar dog, or human, respectively. The results showed that social learning performance of the single dogs fell between the dominant and subordinate multi-dogs with both dog and human demonstration. Subordinate dogs displayed significantly better performance after having observed a dog demonstrator in comparison to dominant dogs. In contrast, the performance of dominant and subordinate dogs was almost similar, when they observed a human demonstrator. These results suggest that perceived dominance rank in its own group has a strong effect on social learning in dogs, but this effect seems to depend also on the demonstrator species. This finding reveals an intricate organization of the social structure in multi-dog households, which can contribute to individual differences existing among dogs.     

Scented memories of literature

This study examined the effects on recall of story details of congruity or incongruity between the hedonic valence of literary texts and odours inhaled while reading them. During the reading session, 24 undergraduates (12 males and 12 females) read two passages involving positive subject matter and two with negative subject matter while sniffing pleasant or unpleasant odours in a within-subject fully counterbalanced design. Subjects rated their experience of each text on eleven 7-point scales. During the test session 48 hours later, subjects read a two-word title associated with each of the passages and inhaled the odour that was paired with it in the reading session. They also rated their experience on six of the scales that had been used during the reading session. Results showed that hedonic congruence between the passage and the odour fostered enhanced recall during the test session. The combination of positive subject matter and positive odour was reflected in more accurate recall of character details, while pairing negative subject matter and negative odour resulted in more accurate recall of setting details. Regression analysis showed that overall recall accuracy was increased by identifying with the characters in the stories and for passages that were found pleasing and personally meaningful. Consistent with the literature on implicit learning involving odours, recall accuracy varied inversely with perceived odour intensity. Implicit learning involving odours and literary passages is therefore fostered by unity in the reading experience.     

Recognition of human faces by dogs (<Emphasis Type="Italic">Canis familiaris</Emphasis>) requires visibility of head contour

Researchers have suggested that dogs are able to recognise human faces, but conclusive evidence has yet to be found. Experiment 1 of this study investigated whether dogs can recognise humans using visual information from the face/head region, and whether this also occurs in conditions of suboptimal visibility of the face. Dogs were presented with their owner’s and a stranger’s heads, protruding through openings of an apparatus in opposite parts of the experimental setting. Presentations occurred in conditions of either optimal or suboptimal visibility; the latter featured non-frontal orientation, uneven illumination and invisibility of outer contours of the heads. Instances where dogs approached their owners with a higher frequency than predicted by chance were considered evidence of recognition. This occurred only in the optimal condition. With a similar paradigm, Experiment 2 investigated which of the alterations in visibility that characterised the suboptimal condition accounted for dogs’ inability to recognise owners. Dogs approached their owners more frequently than predicted by chance if outer head contours were visible, but not if heads were either frontally oriented or evenly illuminated. Moreover, male dogs were slightly better at recognition than females. These findings represent the first clear demonstration that dogs can recognise human faces and that outer face elements are crucial for such a task, complementing previous research on human face processing in dogs. Parallels with face recognition abilities observed in other animal species, as well as with human infants, point to the relevance of these results from a comparative standpoint.     

Emotional intelligence and social perception

One of the key facets of emotional intelligence (EI) is the capacity of an individual to recognise emotions in others. However, this has not been tested cross-culturally, despite the body of research indicating that people are better at recognising facial affect of members of their own culture. Given the emotion recognition aspect of EI, it would seem that EI should be related to correctly identifying emotion in others regardless of race. In order to test this, a social perception inspection time task was carried out in which participants (41 Caucasian and 46 Far-East Asian) were required to identify the emotion on Caucasian and Far-East Asian faces that were happy, sad, or angry. Results from this study indicate that EI was not related to correctly identifying facial expressions. The results did confirm that participants are better able to recognise people of their own ethnicity, though this was only applicable to negative emotions.     

Scented memories of literature

This study examined the effects on recall of story details of congruity or incongruity between the hedonic valence of literary texts and odours inhaled while reading them. During the reading session, 24 undergraduates (12 males and 12 females) read two passages involving positive subject matter and two with negative subject matter while sniffing pleasant or unpleasant odours in a within‐subject fully counterbalanced design. Subjects rated their experience of each text on eleven 7‐point scales. During the test session 48 hours later, subjects read a two‐word title associated with each of the passages and inhaled the odour that was paired with it in the reading session. They also rated their experience on six of the scales that had been used during the reading session. Results showed that hedonic congruence between the passage and the odour fostered enhanced recall during the test session. The combination of positive subject matter and positive odour was reflected in more accurate recall of character details, while pairing negative subject matter and negative odour resulted in more accurate recall of setting details. Regression analysis showed that overall recall accuracy was increased by identifying with the characters in the stories and for passages that were found pleasing and personally meaningful. Consistent with the literature on implicit learning involving odours, recall accuracy varied inversely with perceived odour intensity. Implicit learning involving odours and literary passages is therefore fostered by unity in the reading experience.