Do domestic dogs (<Emphasis Type="Italic">Canis lupus familiaris</Emphasis>) perceive the Delboeuf illusion?

In the last decade, visual illusions have been repeatedly used as a tool to compare visual perception among species. Several studies have investigated whether non-human primates perceive visual illusions in a human-like fashion, but little attention has been paid to other mammals, and sensitivity to visual illusions has been never investigated in the dog. Here, we studied whether domestic dogs perceive the Delboeuf illusion. In human and non-human primates, this illusion creates a misperception of item size as a function of its surrounding context. To examine this effect in dogs, we adapted the spontaneous preference paradigm recently used with chimpanzees. Subjects were presented with two plates containing food. In control trials, two different amounts of food were presented in two identical plates. In this circumstance, dogs were expected to select the larger amount. In test trials, equal food portion sizes were presented in two plates differing in size: if dogs perceived the illusion as primates do, they were expected to select the amount of food presented in the smaller plate. Dogs significantly discriminated the two alternatives in control trials, whereas their performance did not differ from chance in test trials with the illusory pattern. The fact that dogs do not seem to be susceptible to the Delboeuf illusion suggests a potential discontinuity in the perceptual biases affecting size judgments between primates and dogs.     

Context specificity of inhibitory control in dogs

Across three experiments, we explored whether a dog’s capacity for inhibitory control is stable or variable across decision-making contexts. In the social task, dogs were first exposed to the reputations of a stingy experimenter that never shared food and a generous experimenter who always shared food. In subsequent test trials, dogs were required to avoid approaching the stingy experimenter when this individual offered (but withheld) a higher-value reward than the generous experimenter did. In the A-not-B task, dogs were required to inhibit searching for food in a previously rewarded location after witnessing the food being moved from this location to a novel hiding place. In the cylinder task, dogs were required to resist approaching visible food directly (because it was behind a transparent barrier), in favor of a detour reaching response. Overall, dogs exhibited inhibitory control in all three tasks. However, individual scores were not correlated between tasks, suggesting that context has a large effect on dogs’ behavior. This result mirrors studies of humans, which have highlighted intra-individual variation in inhibitory control as a function of the decision-making context. Lastly, we observed a correlation between a subject’s age and performance on the cylinder task, corroborating previous observations of age-related decline in dogs’ executive function.     

Domestic dogs conceal auditory but not visual information from others

A number of studies have shown that dogs are sensitive to a human’s perspective, but it remains unclear whether they use an egocentric strategy to assess what humans perceive. We investigated whether dogs know what a human can see and hear, even when the dogs themselves are unable to see the human. Dogs faced a task in which forbidden food was placed in a tunnel that they could retrieve by using their paw. Whereas the dogs could not see the experimenter during their food retrieval attempts, the experimenter could potentially see the dog’s paw. In the first experiment, dogs could choose between an opaque and a transparent side of the tunnel, and in the second experiment, they could choose between a silent and a noisy approach to the tunnel. The results showed that dogs preferred a silent approach to forbidden food but they did not hide their approach when they could not see a human present. We conclude that dogs probably rely on what they themselves can perceive when they assess what the human can see and hear.     

Domestic dogs’ (Canis familiaris) choices in reference to agreement among human informants on location of food

When interacting with others, informants may offer conflicting information or information of varying accuracy. Recent research suggests that young children do not trust all informants equally and are selective in both whom they solicit for information and whose claims they support. We explored whether domestic dogs (Canis familiaris) are similarly sensitive to agreement among informants. To this end, we utilized a common human gesture, pointing, to which recent research suggests dogs are sensitive. We conducted two experiments in which an experimenter secretly hid food in one of two clear containers while the dog was distracted. Next, a small group moved to indicate the food's location using stationary points positioned above the containers. In Experiment 1, two experimenters moved to stand behind the non-baited container, while a third experimenter moved to stand behind the baited container. Then, all directed one static point at the container in front of them. Experiment 2 exactly resembled Experiment 1 with the exception that the single experimenter standing behind the baited container directed two static points at the container (one with each hand). Dogs chose the container indicated by the majority in Experiment 1 significantly more often than chance, but chose the container indicated by the minority in Experiment 2 significantly more often than chance. This suggests that the number of points, not the number of people, more strongly influenced dogs' choices.     

Sensing sociality in dogs: what may make an interactive robot social?

This study investigated whether dogs would engage in social interactions with an unfamiliar robot, utilize the communicative signals it provides and to examine whether the level of sociality shown by the robot affects the dogs’ performance. We hypothesized that dogs would react to the communicative signals of a robot more successfully if the robot showed interactive social behaviour in general (towards both humans and dogs) than if it behaved in a machinelike, asocial way. The experiment consisted of an interactive phase followed by a pointing session, both with a human and a robotic experimenter. In the interaction phase, dogs witnessed a 6-min interaction episode between the owner and a human experimenter and another 6-min interaction episode between the owner and the robot. Each interaction episode was followed by the pointing phase in which the human/robot experimenter indicated the location of hidden food by using pointing gestures (two-way choice test). The results showed that in the interaction phase, the dogs’ behaviour towards the robot was affected by the differential exposure. Dogs spent more time staying near the robot experimenter as compared to the human experimenter, with this difference being even more pronounced when the robot behaved socially. Similarly, dogs spent more time gazing at the head of the robot experimenter when the situation was social. Dogs achieved a significantly lower level of performance (finding the hidden food) with the pointing robot than with the pointing human; however, separate analysis of the robot sessions suggested that gestures of the socially behaving robot were easier for the dogs to comprehend than gestures of the asocially behaving robot. Thus, the level of sociality shown by the robot was not enough to elicit the same set of social behaviours from the dogs as was possible with humans, although sociality had a positive effect on dog–robot interactions.     

Use of experimenter-given cues in dogs

Since the observations of O. Pfungst the use of human-provided cues by animals has been well-known in the behavioural sciences (“Clever Hans effect”). It has recently been shown that rhesus monkeys (Macaca mulatta) are unable to use the direction of gazing by the experimenter as a cue for finding food, although after some training they learned to respond to pointing by hand. Direction of gaze is used by chimpanzees, however. Dogs (Canis familiaris) are believed to be sensitive to human gestural communication but their ability has never been formally tested. In three experiments we examined whether dogs can respond to cues given by humans. We found that dogs are able to utilize pointing, bowing, nodding, head-turning and glancing gestures of humans as cues for finding hidden food. Dogs were also able to generalize from one person (owner) to another familiar person (experimenter) in using the same gestures as cues. Baseline trials were run to test the possibility that odour cues alone could be responsible for the dogs’ performance. During training individual performance showed limited variability, probably because some dogs already “knew” some of the cues from their earlier experiences with humans. We suggest that the phenomenon of dogs responding to cues given by humans is better analysed as a case of interspecific communication than in terms of discrimination learning. Received: 30 May 1998 / Accepted after revision: 6 September 1998     

Do dogs (Canis familiaris) understand invisible displacement?

Domestic dogs (Canis familiaris) perform above chance on invisible displacement tasks despite showing few other signs of possessing the necessary representational abilities. Four experiments investigated how dogs find an object that has been hidden in 1 of 3 opaque boxes. Dogs passed the task under a variety of control conditions, but only if the device used to displace the object ended up adjacent to the target box after the displacement. These results suggest that the search behavior of dogs was guided by simple associative rules rather than mental representation of the object's past trajectory. In contrast, Experiment 5 found that on the same task, 18- and 24-month-old children showed no disparity between trials in which the displacement device was adjacent or nonadjacent to the target box.     

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.     

The use of experimenter-given cues by South African fur seals (<Emphasis Type="Italic">Arctocephalus pusillus</Emphasis>)

Dogs can use a variety of experimenter-given cues such as pointing, head direction, and eye direction to locate food hidden under one of several containers. Some authors have proposed that this is a result of the domestication process. In this study we tested four captive fur seals in a two alternative object choice task in which subjects had to use one of the following experimenter-given cues to locate the food: (1) the experimenter pointed and gazed at one of the objects, (2) the experimenter pointed at only one of the objects, (3) the experimenter gazed at only one of the objects, (4) the experimenter glanced at only one of the objects, (5) the experimenter pointed and gazed at one of the objects but was sitting closer to one object than to the other, (6) the experimenter pointed only with the index finger at one of the objects, (7) the experimenter presented a replica of one of the objects. The fur seals were able to use cues which involved a fully exposed arm or a head direction, but failed to use glance only, the index finger pointing and the object replica cues. The results showed that a domestication process was not necessary to develop receptive skills to cues given by an experimenter. Instead, we hypothesize that close interactions with humans prior to testing enabled fur seals to use some gestural cues without formal training. We also analyzed the behavior of the seals depending on the level of difficulty of the task. Behavioral signs of hesitation increased with task difficulty. This suggests that the fur seals were sensitive to task difficulty.     

Search behavior in various breeds of adult dogs (Canis familiaris): object permanence and olfactory cues.

Human analog tests of object permanence were administered to various breeds of adult dogs (Canis familiaris). Experiment 1 showed that the performance of terriers, sporting, and working dogs did not differ. Dogs succeeded in solving invisible displacement problems, but performance was lower than in visible displacement tests. Familiarity with the task had some influence because invisible displacement tests were more successful if they were preceded by visible displacement tests. In Experiment 2, odor cues from the target object and the hiding screens were available or were masked. Results confirmed that success was lower in invisible than in visible displacement tests and that these problems were solved on the basis of representation of visual information rather than on the basis of olfactory cues or of local rule learning. Dogs are compared with other species that display Stage 6 object permanence.     

Dogs are able to solve a means-end task

Dogs, although very skilled in social-communicative tasks, have shown limited abilities in the domain of physical cognition. Consequently, several researchers hypothesized that domestication enhanced dogs’ cognitive abilities in the social realm, but relaxed selection on the physical one. For instance, dogs failed to demonstrate means-end understanding, an important form of relying on physical causal connection, when tested in a string-pulling task. Here, we tested dogs in an “on/off” task using a novel approach. Thirty-two dogs were confronted with four different conditions in which they could choose between two boards one with a reward “on” and another one with a reward “off” (reward was placed next to the board). The dogs chose the correct board when (1) both rewards were placed at the same distance from the dog, when (2) the reward placed “on” the board was closer to the dog, and (3) even when the reward placed “off” the board was much closer to the dog and was food. Interestingly, in the latter case, dogs did not perform above chance, if instead of a direct reward, the dogs had to retrieve an object placed on the board to get a food reward. In contrast to previous string-pulling studies, our results show that dogs are able to solve a means-end task even if proximity of the unsupported reward is a confounding factor.     

Comprehension of human pointing gestures in young human-reared wolves (<Emphasis Type="Italic">Canis lupus</Emphasis>) and dogs (<Emphasis Type="Italic">Canis familiaris</Emphasis>)

Dogs have a remarkable skill to use human-given cues in object-choice tasks, but little is known to what extent their closest wild-living relative, the wolf can achieve this performance. In Study 1, we compared wolf and dog pups hand-reared individually and pet dogs of the same age in their readiness to form eye-contact with a human experimenter in an object-choice task and to follow her pointing gesture. The results showed that dogs already at 4 months of age use momentary distal pointing to find hidden food even without intensive early socialization. Wolf pups, on the contrary, do not attend to this subtle pointing. Accordingly in Studies 2 and 3, these wolves were tested longitudinally with this and four other (easier) human-given cues. This revealed that wolves socialized at a comparable level to dogs are able to use simple human-given cues spontaneously if the human’s hand is close to the baited container (e.g. touching, proximal pointing). Study 4 showed that wolves can follow also momentary distal pointing similarly to dogs if they have received extensive formal training. Comparing the wolves to naïve pet dogs of the same age revealed that during several months of formal training wolves can reach the level of dogs in their success of following momentary distal pointing in parallel with improving their readiness to form eye-contact with a human experimenter. We assume that the high variability in the wolves’ communicative behaviour might have provided a basis for selection during the course of domestication of the dog.     

Visual perspective taking by dogs (Canis familiaris) in a Guesser–Knower task: evidence for a canine theory of mind?

We tested domestic dogs (N = 16) in a Guesser–Knower task in which they chose between possible locations for hidden food indicated by human informants. In four experiments, the perceptual access of the Guesser and Knower to the hidden food baiting was manipulated. When informants had differing perceptual access to the baiting, dogs preferred the location indicated by the Knower from the start of testing (Experiment 1), even when baiting was done by a third experimenter (Experiments 2–3). However, when there was no difference in perceptual access and both informants either knew or did not know the food location, dogs had no preference between the informants (Experiment 4). Controls ruled out alternative explanations in terms of associative learning, unintentional and olfactory cues. Analysis of individual data showed no significant heterogeneity across dogs, and results were not correlated with age or sex. Dogs’ performances were superior to those of nonhuman primates in previous studies. Although a mentalistic explanation is not required, results add to evidence that dogs have a remarkable sensitivity to cues related to humans’ attentional state, which enables them to respond as if they had a functional theory of mind in the Guesser–Knower task with human informants.     

Can expectancies produce placebo effects for implicit learning?

The placebo effect is an important phenomenon whereby real changes occur in response to an otherwise inert intervention. Despite increasing research attention, it remains unclear exactly which processes are amenable to placebo effects. The current study tested whether an instructional manipulation could produce placebo effects on a nonconscious cognitive task, namely implicit learning. Four hundred and sixty-four university students completed a visual search task while smelling an odor or no odor, in alternating blocks. Unknown to them, the task contained a contingency whereby on half the trials the target's location was cued by the pattern of distractors, which was achieved by repeating some configurations of targets and distractors. Prior to the task, participants received positive, negative, or no information about the odor's possible effects on performance. Those given positive information demonstrated faster reaction times on cued trials than other participants. Those given negative information showed slower reaction times on cued trials compared with participants given no information. Further, the cuing effect appeared to be nonconscious, with participants' ability to recognize the repeated configurations equivalent to chance and no evidence that performance on a recognition test was related to the magnitude of the cuing effect. This suggests that instructional manipulations can produce placebo effects on some nonconscious processes.     

Differential preparatory cardiovascular responses to aversive and appetitive behavioral conditioning

Dogs were required to press a response panel either to avoid shock or obtain food. During a one-hour interval immediatelypreceding performance on the avoidance task, blood pressure increased while heart rate decreased. In contrast, both blood pressure and heart rate increased during the one-hour interval immediatelypreceding performance on the food task. During both the shock-avoidance and food performancesper se, however, elevations in heart rate and blood pressure were observed, though the avoidance task produced less consistent heart rate changes. These effects were observed (1) with different individual dogs on each of the two training procedures, and (2) with the same dogs exposed successively to both aversive and appetitive behavioral conditioning.     

Waiting for more: the performance of domestic dogs (Canis familiaris) on exchange tasks

Five domestic dogs (Canis familiaris) were tested in a cooperative exchange task with an experimenter, as previously tested in non-human primates. In the first task, the dogs exchanged to maximise payoffs when presented with food items of differing quality. All consistently exchanged lower-value for higher-value rewards, as determined by their individual food preference, and exchanges corresponded significantly with the spontaneous preferences of three dogs. Next, all subjects demonstrated an ability to perform two and three exchanges in succession, to gain both qualitative and quantitatively increased rewards (group mean = 72 and 92% successful triple exchanges, respectively). Finally, the ability to delay gratification over increasing intervals was tested; the dogs kept one food item to exchange later for a larger item. As previously reported in non-human primates, there was considerable individual variation in the tolerance of delays, between 10 s and 10 min for the largest rewards. For those who reached longer time lags (>40 s), the dogs gave up the chance to exchange earlier than expected by each subject’s general waiting capacity; the dogs anticipated delay duration and made decisions according to the relative reward values offered. Compared to primates, dogs tolerated relatively long delays for smaller value rewards, suggesting that the socio-ecological history of domestic dogs facilitates their performance on decision-making and delay of gratification tasks.     

Invisible displacement understanding in domestic dogs (<Emphasis Type="Italic">Canis familiaris</Emphasis>): the role of visual cues in search behavior

Recently, (Collier-Baker E, Davis JM, Suddendorf T (2004) J Comp Psychol 118:421–433) suggested that domestic dogs do not understand invisible displacements. In the present study, we further investigated the hypothesis that the search behavior of domestic dogs in invisible displacements is guided by various visual cues inherent to the task rather than by mental representation of an object’s past trajectory. Specifically, we examined the role of the experimenter as a function of the final position of the displacement device in the search behavior of domestic dogs. Visible and invisible displacement problems were administered to dogs (N = 11) under two conditions. In the Visible-experimenter condition, the experimenter was visible whereas in the Concealed-experimenter condition, the experimenter was visibly occluded behind a large rigid barrier. Our data supported the conclusion that dogs do not understand invisible displacements but primarily search as a function of the final position of the displacement device and, to a lesser extent, the position of the experimenter.     

My owner,right or wrong: the effect of familiarity on the domestic dog’s behavior in a food-choice task

Dogs are strongly influenced by human behavior, and they readily form bonds with specific humans. Yet these lines of inquiry are not often combined. The goal of this study was to investigate whether such bonds would play a role in how dogs behave in response to human signals. Using various types of signals, we compared dogs’ use of information from a familiar human (their owner) versus an unfamiliar human when choosing between two food containers. In some conditions, the owner indicated a container that gave food and a stranger indicated a container that did not; in other conditions, this was reversed. Dogs more often chose the container indicated by or nearest to their owner, even when this container never yielded a food reward. In two conditions, dogs chose at chance: a control condition in which both pointers were strangers and a condition in which the owner and stranger sat reading books and provided no social signal. This is the first study to directly compare owners to strangers in a single food-choice situation. Our results suggest that dogs make decisions by attending preferentially to social signals from humans with whom they have become familiar.     

Effects of repeated acquisitions and extinctions on response rate and pattern

The procedure developed by R. A. Rescorla (2002) was used to study the effects of repeated acquisitions and extinctions of head entry responses into a food cup by rats. In each of 4 20-session phases, food was delivered at the end of particular 30-s auditory and visual stimuli, but not at the end of different 30-s auditory and visual stimuli. Based on response rates to individual stimuli and compound stimuli, the increase in response rate in acquisition occurred more rapidly than the decrease in extinction. Acquisition, but not extinction, occurred faster after successive transitions between acquisition and extinction. Temporal gradients of responding developed during acquisition and remained during extinction. Conclusions based on mean response rate, temporal gradients, and transfer tests were consistent.     

Comprehension of human pointing gestures in horses (Equus caballus)

Twenty domestic horses (Equus caballus) were tested for their ability to rely on different human gesticular cues in a two-way object choice task. An experimenter hid food under one of two bowls and after baiting, indicated the location of the food to the subjects by using one of four different cues. Horses could locate the hidden reward on the basis of the distal dynamic-sustained, proximal momentary and proximal dynamic-sustained pointing gestures but failed to perform above chance level when the experimenter performed a distal momentary pointing gesture. The results revealed that horses could rely spontaneously on those cues that could have a stimulus or local enhancement effect, but the possible comprehension of the distal momentary pointing remained unclear. The results are discussed with reference to the involvement of various factors such as predisposition to read human visual cues, the effect of domestication and extensive social experience and the nature of the gesture used by the experimenter in comparative investigations.