Cockroach Art

A review in the Nov. 28 issue of Nature of an exhibition: "The Art of Arthropods". An excerpt:

A Californian entomologist uses insects as living paintbrushes to create abstract art. After loading water-based, non-toxic paints on to the tarsi and abdomens of insects, Steven Kutcher directs his bugs to create their 'masterpieces'.

Kutcher controls the direction and movement of his arthropods — such as hissing cockroaches (pictured), darkling beetles and grasshoppers — by their response to external lighting. The result is controlled and random movements, created in a co-authorship between the artist — with predetermined ideas about colour, form, shape and creative flexibility — and his living brushes.

Kutcher's art is more than just a novelty, because it reveals the hidden world of insect footprints. "When an insect walks on your hand, you may feel the legs move but nothing visible remains, only a sensation," he says. "These works of art render the insect tracks and routes visible, producing a visually pleasing piece."

For more see http://www.BugArtbySteven.com

Roach Robots

Here is a moment of visual relief:

Monkeys judge inequity like humans

DeWaal and collaborators expand on previous observations to show that capuchin monkeys get really pissed off if they worked harder for a reward than another monkey, and then see that monkey get a greater reward! This suggests that our human resentment of inequity in rewards my have a very ancient origin in primate behavior. Here is their abstract:

Without joint benefits, joint actions could never have evolved. Cooperative animals need to monitor closely how large a share they receive relative to their investment toward collective goals. This work documents the sensitivity to reward division in brown, or tufted, capuchin monkeys (Cebus apella). In addition to confirming previous results with a larger subject pool, this work rules out several alternative explanations and adds data on effort sensitivity. Thirteen adult monkeys exchanged tokens for rewards, showing negative reactions to receiving a less-favored reward than their partner. Because their negative reaction could not be attributed to the mere visibility of better rewards (greed hypothesis) nor to having received such rewards in the immediate past (frustration hypothesis), it must have been caused by seeing their partner obtain the better reward. Effort had a major effect in that by far the lowest level of performance in the entire study occurred in subjects required to expend a large effort while at the same time seeing their partner receive a better reward. It is unclear whether this effort–effect was based on comparisons with the partner, but it added significantly to the intensity of the inequity response. These effects are as expected if the inequity response evolved in the context of cooperative survival strategies.

Kewl Bio-inspired Robotics...

The Nov. 16 issue of Science has a special section on Robotics. I thought this graphic from the article by Pfeifer et al. - "Self-Organization, Embodiment, and Biologically Inspired Robotics" - was fascinating. It describes several biologically inspired robots.


Figure: Self-organization, dynamics, and materials in bio-inspired robotics. (A) Smooth transition between swimming and walking. This amphibious salamanderlike robot (~80 cm long) embeds a spinal cord model that explains the ability of salamanders to switch between swimming and walking. The locomotion model is built by extending a primitive neural circuit for swimming by phylogenetically more recent limb oscillatory centers. (B) Rich sensory stimulation through proper sensor morphology. This robot (~7 cm in diameter) owes its sophisticated sensory capacities to the specific arrangement, shape, and material characteristics of its whiskers. Natural whiskers from rodents (such as the ones used on this robot) are far superior to whiskers built from other materials in terms of richness of the signals relayed to the neural system. (C) Self-stabilizing rapid hexapod locomotion. This robot (~15 cm long) moves with a bouncing gait, achieving rapid (over 4 body lengths per second) locomotion. Its legs are built with compliant pneumatic actuators, which yield self-stabilization through mechanical feedback. (D) Passive dynamics–based walking. Designed to work on a slope as a dynamic walker, this robot (~45 cm tall) exploits dynamics and morphology (in particular, the shape and length of the body and feet) to achieve stable walking. The robot's natural dynamics serves as the target dynamics for a reinforcement learning mechanism, enabling the robot to quickly learn to walk on flat ground. (E) Self-stabilizing vertical takeoff through materials and morphology. Inspired by flies, this ultralight (60 mg, 3-cm wingspan) ornithopter (a device that flies by flapping its wings) generates sufficient lift to take off vertically (power is supplied externally). A large part of the control is delegated to the morphological and material properties of the robot. Compliant structures are driven into resonance to produce a large wing stroke, and flexible material is used in the wing hinges to allow for passive rotations of the wings. (F) Agile wall-climbing through materials. The bio-inspiration for this palm-sized robot is provided by the gecko and its uncanny climbing talents. The robot's tri-foot (three-footed wheel) is equipped with a polymer dry adhesive material, which to some extent has contact properties comparable to those of its biological analog. The robot can flexibly navigate on smooth vertical and even inverted surfaces. (G) Morphing through localized self-reconfiguration. This self-reconfigurable robot is composed of active (actuated, black) and passive (nonactuated, white) cubic modules (~400 g, ~60 to 65 mm side length). The modules connect to each other through hooks, which enables the robot to change its morphology in a large number of ways. The picture shows the metamorphosis from a four-legged (quadruped) structure to a linear (snakelike) structure. (H) Global movement through local interaction dynamics. The individual wheel-like modules (~10 cm in diameter) constituting this robot are equipped with spokelike parts driven by linear actuators. The wheels lie horizontally on the ground and attach to neighboring modules by Velcro. Although no module can move on its own, by using neural oscillators as drivers for the actuators and through the physical coupling between the units, a coordinated global wave of activation can be induced in clusters of more than 30 modules, which leads to forward movement, even though there is no global control.

The instinct to swarm

Groups of social animals whose individual members follow simple sets of rules do surprising things. This NY Times article by Carl Zimmer in the Nov. 13 science section quotes Ian Couzin, a mathematical biologist at Princeton: “No matter how much you look at an individual army ant...you will never get a sense that when you put 1.5 million of them together, they form these bridges and columns. You just cannot know that.” The article notes the simple models that predict swarming behavior by setting the population density that which individuals switch from going their own way to following others. It also describes experiments using human subjects to test Couzin's models.

Many take our brains to be a more massive and complex version of the "hive minds" displayed by groups of bees, ants, birds and fish. Brain modelers assign relatively simple properties to their model neurons and then watch amazing patterns emerge when their whole society of neurons is fired up to interact.

Rationalizing our choices - an early evolutionary origin

When we humans make a choice, we protect our self esteem by rationalizing that it was the correct one, even in the face of evidence to the contrary. It turns out Capuchin monkeys do the same thing. In a kind of "why didn't someone think of trying this before?" experiment, a group of Yale psychologists offered the monkeys several different colors of M&M candies. Once a monkey was observed to show an equal preference for three colors of M&M’s — say, red, blue and green — he was given a choice between two of them. If he chose red over blue, his preference changed and he downgraded blue. When he was subsequently given a choice between blue and green, it was no longer an even contest — he was now much more likely to reject the blue. Thus the monkeys are dealing with cognitive dissonance ('should I choose the blue or the green?') by downgrading or eliminating one of the options. They performed a similar experiment with little children, obtaining similar results. The fact that children and primates show the same behavior as adults suggests that this rationalization behavior is largely unconscious, and may have appeared in evolution earlier than previously thought.

Single cells in monkey brain trained to associate numbers with their symbols

An interesting study from Diester and Nieder showing single nerve cell activity that might be the primitive cognitive precursor that ultimately has given rise to symbolic thinking in linguistic humans. Their abstract:

The utilization of symbols such as words and numbers as mental tools endows humans with unrivalled cognitive flexibility. In the number domain, a fundamental first step for the acquisition of numerical symbols is the semantic association of signs with cardinalities. We explored the primitives of such a semantic mapping process by recording single-cell activity in the monkey prefrontal and parietal cortices, brain structures critically involved in numerical cognition. Monkeys were trained to associate visual shapes with varying numbers of items in a matching task. After this long-term learning process, we found that the responses of many prefrontal neurons to the visual shapes reflected the associated numerical value in a behaviorally relevant way. In contrast, such association neurons were rarely found in the parietal lobe. These findings suggest a cardinal role of the prefrontal cortex in establishing semantic associations between signs and abstract categories, a cognitive precursor that may ultimately give rise to symbolic thinking in linguistic humans.

Another window into the minds of chimps and humans

Rilling et al. compare resting-state brain activity in humans and chimpanzees:

In humans, the wakeful resting condition is characterized by a default mode of brain function involving high levels of activity within a functionally connected network of brain regions. This network has recently been implicated in mental self-projection into the past, the future, or another individual's perspective. Here we use [18F]-fluorodeoxyglucose positron emission tomography imaging to assess resting-state brain activity in our closest living relative, the chimpanzee, as a potential window onto their mental world and compare these results with those of a human sample. We find that, like humans, chimpanzees show high levels of activity within default mode areas, including medial prefrontal and medial parietal cortex. Chimpanzees differ from our human sample in showing higher levels of activity in ventromedial prefrontal cortex and lower levels of activity in left-sided cortical areas involved in language and conceptual processing in humans. Our results raise the possibility that the resting state of chimpanzees involves emotionally laden episodic memory retrieval and some level of mental self-projection, albeit in the absence of language and conceptual processing.

In an Ultimatum Game, Chimps, but not humans, are rational maximizers

Another interesting bit of work from Tomasello's group (PDF here), in which they devised an ingenious apparatus for a mini-ultimatum game, a reduced form of the ultimatum game in which proposers are given a choice between making one of two pre-set offers which the responder can then accept or reject. The proposer had as one option an amount that would typically be rejected by a human responder as unfair, namely 80% for the proposer and 20% for the responder. The most important finding was that responders tended to accept any offer. These results support the hypothesis that other-regarding preferences and aversion to inequitable outcomes, which play key roles in human social organization, distinguish us from our closest living relatives. Here is their abstract, slightly edited:

Traditional models of economic decision-making assume that people are self-interested rational maximizers. Empirical research has demonstrated, however, that people will take into account the interests of others and are sensitive to norms of cooperation and fairness. In one of the most robust tests of this finding, the ultimatum game, individuals will reject a proposed division of a monetary windfall, at a cost to themselves, if they perceive it as unfair. Here we show that in an ultimatum game, humans' closest living relatives, chimpanzees (Pan troglodytes), are rational maximizers and are not sensitive to fairness.

Figure: Illustration of the testing environment. The proposer, who makes the first choice, sits to the responder's left. The apparatus, which has two sliding trays connected by a single rope, is outside of the cages. (A) By first sliding a Plexiglas panel (not shown) to access one rope end and by then pulling it, the proposer draws one of the baited trays halfway toward the two subjects. (B) The responder can then pull the attached rod, now within reach, to bring the proposed food tray to the cage mesh so that (C) both subjects can eat from their respective food dishes (clearly separated by a translucent divider).

Baboon Metaphysics

Nicholas Wade comments (PDF here) on the work of Cheney and Seyfarth, whose most recent book has the title of this post. They have carried out an ingenious series of experiments in the wild to probe how the baboon's mind keeps track of transient relationships:

Royal is a cantankerous old male baboon whose troop of some 80 members lives in the Moremi Game Reserve in Botswana. A perplexing event is about to disturb his day....From the bushes to his right, he hears a staccato whoop, the distinctive call that female baboons always make after mating. He recognizes the voice as that of Jackalberry, the current consort of Cassius, a male who outranks Royal in the strict hierarchy of male baboons. No hope of sex today....But then, surprisingly, he hears Cassius’s signature greeting grunt to his left. His puzzlement is plain on the video made of his reaction. You can almost see the wheels turn slowly in his head:...“Jackalberry here, but Cassius over there. Hmm, Jackalberry must be hooking up with some one else. But that means Cassius has left her unguarded. Say what — this is my big chance!”...The video shows him loping off in the direction of Jackalberry’s whoop. But all that he will find is the loudspeaker from which researchers have played Jackalberry’s recorded call.

Although Baboons excel at the skills required for maintaining social networks regulated by matrilineal lines and dominance hierarchies, there is no evidence that they attribute beliefs or ideas to other animals, or that 'they know that they know.' They provide an example of what sort of social and cognitive complexity is possible in the absence of language and a theory of mind.

Ape language slips reveal category knowledge storage.

Michael Erard summarizes efforts (PDF here) to analyze 'language' errors of apes for insight into the covert mental processes of animals - analogous to such work done on human language errors He focuses on the work of Lyn, who was the first to apply the study of errors to bonobos . Kanzi and a female bonobo, Panbanisha, who now live at the Great Ape Trust in Des Moines, Iowa, can comprehend instructions and descriptions in spoken English, and they can respond by using 384 lexigrams, which they touch on a keyboard.

Lyn found that Kanzi and Panbanisha have arranged hundreds of lexigrams in their minds in a complex, hierarchical manner based mainly on their meaning. She coded the relations between all 1497 sample-error pairs along seven dimensions, including whether the lexigrams looked alike, had English words that sounded alike, or referred to objects in the same category. She found that the errors were not random but patterned. If the lexigram stood for "blackberry," the error was more likely than chance to sound like blackberry, be edible, be a fruit, or be physically similar. Errors were also more likely to be associated with more than one category. For example, "cherries" are both edibles and fruits, and the word sounds like the correct one, "blackberries." All this indicated to Lyn that mental representations of the lexigrams must be stored not as simple one-to-one associations but in more complex arrangements. This suggests that, given the chance, bonobos and other apes can acquire systems of meaning that are closer than anyone has thought to what humans do, and that some aspects of language acquisition are not unique to humans.

Evolving size of the social brain.

Dunbar and Shultz ask why primates have such large brains, compared to their body mass, compared with other animals. Here is their abstract, followed by a central clip from their article:

The evolution of unusually large brains in some groups of animals, notably primates, has long been a puzzle. Although early explanations tended to emphasize the brain's role in sensory or technical competence (foraging skills, innovations, and way-finding), the balance of evidence now clearly favors the suggestion that it was the computational demands of living in large, complex societies that selected for large brains. However, recent analyses suggest that it may have been the particular demands of the more intense forms of pairbonding that was the critical factor that triggered this evolutionary development. This may explain why primate sociality seems to be so different from that found in most other birds and mammals: Primate sociality is based on bonded relationships of a kind that are found only in pairbonds in other taxa.

Figure - In anthropoid primates, mean social group size increases with relative neocortex volume (indexed as the ratio of neocortex volume to the volume of the rest of the brain). Solid circles, monkeys; open circles, apes. Regression lines are reduced major axis fits.


The important issue in the present context is the marked contrast between anthropoid primates and all other mammalian and avian taxa (including, incidentally, prosimian primates): Only anthropoid primates exhibit a correlation between social group size and relative brain (or neocortex) size. This quantitative relationship is extremely robust; no matter how we analyze the data (with or without phylogenetic correction, using raw volumes, or residuals or ratios against any number of alternative body or brain baselines) or which brain data set we use (histological or magnetic resonance imaging derived, for whole brain, neocortex, or just the frontal lobes), the same quantitative relationship always emerges. This suggests that, at some early point in their evolutionary history, anthropoid primates used the kinds of cognitive skills used for pairbonded relationships by vertebrates to create relationships between individuals who are not reproductive partners. In other words, in primates, individuals of the same sex as well as members of the opposite sex could form just as intense and focused a relationship as do reproductive mates in nonprimates. Given that the number of possible relationships is limited only by the number of animals in the group, primates naturally exhibit a positive correlation between group size and brain size. This would explain why, as primatologists have argued for decades, the nature of primate sociality seems to be qualitatively different from that found in most other mammals and birds. The reason is that the everyday relationships of anthropoid primates involve a form of "bondedness" that is only found elsewhere in reproductive pairbonds.

Nonhuman primates perceive human goals

Hauser and collaborators do a clever experiment to demonstrate that several primates can make inferences about a human experimenters goal that cannot be explained by simple associative learning. This means that our capacity to infer rational, goal-directed action derives from capabilities present in monkeys ~40 million years ago. Here is their abstract and a figure showing the basic idea of the experiment.

Humans are capable of making inferences about other individuals' intentions and goals by evaluating their actions in relation to the constraints imposed by the environment. This capacity enables humans to go beyond the surface appearance of behavior to draw inferences about an individual's mental states. Presently unclear is whether this capacity is uniquely human or is shared with other animals. We show that cotton-top tamarins, rhesus macaques, and chimpanzees all make spontaneous inferences about a human experimenter's goal by attending to the environmental constraints that guide rational action. These findings rule out simple associative accounts of action perception and show that our capacity to infer rational, goal-directed action likely arose at least as far back as the New World monkeys, some 40 million years ago.


Figure: During each trial, an experimenter presented subjects with two potential food containers, performed an action on one, and then allowed the subject to select one of the containers. In the intentional condition, the experimenter reached directly for and grasped the container. In the accidental condition, the experimenter flopped his hand onto the container with palm facing upwards in a manner that appeared, from a human perspective, accidental and non–goal-directed (13). If non-human primates fail to distinguish between intentional and accidental actions when making inferences about others' goals, attending to the mere association of the hand and container, then they should show the same pattern of searching in both conditions—that is, approach the experimenter-contacted container. However, if they distinguish between intentional and accidental actions, then they should selectively inspect the container targeted by the experimenter'sintentional action but not that targeted by accidental action.

A "language gene" in echolocating bats

Slightly altered abstract from Li et al.:

FOXP2 is a transcription factor implicated in the development and neural control of orofacial coordination, particularly with respect to vocalisation. [Thus, it is not really a "language gene" as indicated in many popular press reports.] Observations that orthologues show almost no variation across vertebrates yet differ by two amino acids between humans and chimpanzees have led to speculation that recent evolutionary changes might relate to the emergence of language. Echolocating bats face especially challenging sensorimotor demands, using vocal signals for orientation and often for prey capture. To determine whether mutations in the FoxP2 gene could be associated with echolocation, we sequenced FoxP2 from echolocating and non-echolocating bats as well as a range of other mammal species. We found that contrary to previous reports, FoxP2 is not highly conserved across all nonhuman mammals but is extremely diverse in echolocating bats. We detected divergent selection (a change in selective pressure) at FoxP2 between bats with contrasting sonar systems, suggesting the intriguing possibility of a role for FoxP2 in the evolution and development of echolocation. We speculate that observed accelerated evolution of FoxP2 in bats supports a previously proposed function in sensorimotor coordination.

Social cognitive skills unique to humans...

From Tomasello's group in Leipzig comes an article (PDF here), arguing for a distinctively human social cognitive intelligence rather a more "general intelligence" as distinguishing humans from the great apes. Here is their abstract:

Humans have many cognitive skills not possessed by their nearest primate relatives. The cultural intelligence hypothesis argues that this is mainly due to a species-specific set of social-cognitive skills, emerging early in ontogeny, for participating and exchanging knowledge in cultural groups. We tested this hypothesis by giving a comprehensive battery of cognitive tests to large numbers of two of humans' closest primate relatives, chimpanzees and orangutans, as well as to 2.5-year-old human children before literacy and schooling. Supporting the cultural intelligence hypothesis and contradicting the hypothesis that humans simply have more "general intelligence," we found that the children and chimpanzees had very similar cognitive skills for dealing with the physical world but that the children had more sophisticated cognitive skills than either of the ape species for dealing with the social world.

Did Alex really "want" a cracker?

New York Times science writer George Johnson, who is one very intelligent guy, has done a nice piece on the capabilities and history of Alex the parrot (PDF here). I was unaware of several of the behaviors that had been noted in Alex:

“Want a nut!” Alex demanded. The interview was over. “Want a nut!” he repeated. “Nnn ... uh ... tuh.”...Dr. Pepperberg was flabbergasted. “Not only could you imagine him thinking, ‘Hey, stupid, do I have to spell it for you?’ ” she said. “This was in a sense his way of saying to us, ‘I know where you’re headed! Let’s get on with it.’ ”....She is quick to concede the impossibility of proving that the bird was actually verbalizing its internal deliberations. Only Alex knew for sure.

Next to infinity, one of the hardest concepts to grasp is zero. Toward the end of his life Alex may have been coming close. In a carnival shell game, an experimenter would put a nut under one of three cups and then shuffle them around. Alex would pick up the cup where the prize was supposed to be. If it wasn’t there he’d go a little berserk — a small step, maybe, toward understanding nothingness.

A bigger leap came in an experiment about numbers, in which the parrot was shown groups of two, three and six objects. The objects within each set were colored identically, and Alex was asked, “What color three?”.... “Five,” he replied perversely (he was having a bad attitude day), repeating the answer until the experimenter finally asked, “O.K., Alex, tell me, ‘What color five?’ ”....“None,” the parrot said....Bingo. There was no group of five on the tray. It was another of those high huneker moments. Alex had learned the word “none” years before in a different context. Now he seemed to be using it more abstractly....Dr. Pepperberg reported the result with appropriate understatement: “That zero was represented in some way by a parrot, with a walnut-sized brain whose ancestral evolutionary history with humans likely dates from the dinosaurs, is striking.”

Want to avoid snakes?..Heat your tail.

Prey species have evolved a number of tricks to avoid or deceive predators, involving movement, visual, sound, or smell cues. Now infrared cues get added to the list. Rundus et al. have found that California ground squirrels have evolved a clever trick to deceive snakes, who use infrared (heat) detectors in sizing up their potential prey. The squirrel heats its tail as it shakes it, thus giving off the amount of heat expected from a larger animal and making the snake think that it is larger than it really is. Because larger squirrels are more likely to directly attack snakes, the snake thus is more cautious and less likely to strike.

In memoriam - Alex the Parrot

Sad news, reported in the Sept. 11 NY Times. Alex the talking parrot passed away of natural causes last week at the age of 31. I have heard Irene Pepperberg (former wife of a vision colleague of mine) give talks about Alex over a 25 year period. Irene taught Alex to learn scores of words, which he could put into categories, and to count small numbers of items, as well as recognize colors and shapes. His cognitive and language skills appeared to be about as competent as those in trained primates, and like them, he showed no evidence of having the recursive logic capabilities required for grammar and working with digital numbers.

Here is a brief video of Alex performing in his prime:

The smell of an alpha male....

Pheromones influence sexual behavior and reproduction in rodents. Mak et al report that:

...the pheromones of dominant (but not subordinate) males stimulate neuronal production in both the olfactory bulb and hippocampus of female mice, which are independently mediated by prolactin and luteinizing hormone, respectively. Neurogenesis induced by dominant-male pheromones correlates with a female preference for dominant males over subordinate males, whereas blocking neurogenesis with the mitotic inhibitor cytosine arabinoside eliminated this preference. These results suggest that male pheromones are involved in regulating neurogenesis in both the olfactory bulb and hippocampus, which may be important for female reproductive success.

I keep wondering if we won't be finding evidence for a version of this effect (perhaps more subtle) in humans... would the cheerleader, like the female rat in the box below, be more likely to hang out with the star quarterback if she had smelled his sweaty jersey a day earlier??

An illustration from the summary review by DiRocco and Xia:

Figure legend: Dominant male pheromones stimulate neurogenesis in females.
(a) Female mice exposed to dominant male pheromones spent more time sniffing the dominant male, whereas females exposed to subordinate male pheromones did not show any preference. (b) Exposing female mice to pheromones from dominant males led to increased neurogenesis in the subventricular zone (SVZ) and dentate gyrus (DG). Pheromones signal the main olfactory epithelium (MOE)–main olfactory bulb (MOB) axis, which relays the signal to the hypothalamus (HYP)–pituitary (PIT) axis, leading to the release of luteinizing hormone (LH) and prolactin (PRL). LH appeared to stimulate neurogenesis in the dentate gyrus of the hippocampus, whereas prolactin induced neurogenesis in the SVZ and MOB. It is hypothesized that pheromone-induced neurogenesis may underlie female mating preference for the dominant male. NC, nasal cavity; RMS, rostral migratory stream; green circles, newborn neurons.

Discontinuities between Human and Animal Cognition

Premack offers a stimulating brief essay (PDF here) pointing out that recent cognitive studies finding abilities in animals once thought unique to humans should not lead us to confuse similarity with equivalence, for the human brain has nerve cell types and connections not found in any other animals. He examines eight cognitive areas to argue that dissimilarities are large. Here is his abstract:

Microscopic study of the human brain has revealed neural structures, enhanced wiring, and forms of connectivity among nerve cells not found in any animal, challenging the view that the human brain is simply an enlarged chimpanzee brain. On the other hand, cognitive studies have found animals to have abilities once thought unique to the human. This suggests a disparity between brain and mind. The suggestion is misleading. Cognitive research has not kept pace with neural research. Neural findings are based on microscopic study of the brain and are primarily cellular. Because cognition cannot be studied microscopically, we need to refine the study of cognition by using a different approach. In examining claims of similarity between animals and humans, one must ask: What are the dissimilarities? This approach prevents confusing similarity with equivalence. We follow this approach in examining eight cognitive cases—teaching, short-term memory, causal reasoning, planning, deception, transitive inference, theory of mind, and language—and find, in all cases, that similarities between animal and human abilities are small, dissimilarities large. There is no disparity between brain and mind.

Another major article on this topic is in draft form for Brain and Behavioral Sciences: "Darwin’s mistake: explaining the discontinuity between human and nonhuman minds," by Derek C. Penn, Keith J. Holyoak and Daniel J. Povinelli.
Their abstract:

Over the last quarter-century, the dominant tendency in comparative cognitive psychology has been to emphasize the similarities between human and nonhuman minds and to downplay the differences as “one of degree and not of kind” (Darwin 1871). In the present paper, we argue that Darwin was mistaken: the profound biological continuity between human and nonhuman animals masks an equally profound discontinuity between human and nonhuman minds. To wit, there is a significant discontinuity in the degree to which human and nonhuman animals are able to approximate the higher-order, systematic, relational capabilities of a physical symbol system (Newell 1980). We show that this symbolic-relational discontinuity pervades nearly every domain of cognition and runs much deeper than even the spectacular scaffolding provided by language or culture alone can explain. We propose a representational-level specification of where human and nonhuman animals’ abilities to approximate a PSS are similar and where they differ. We conclude by suggesting that recent symbolic-connectionist models of cognition shed new light on the mechanisms that underlie the gap between human and nonhuman minds.