Computational limits don't fully explain human cognitive limitations.

The project of justifying all the limits and failings of human cognition as inevitable consequences of strategies that are actually "optimal" relative to the limits on computational resources available may have some value, but it is far from a complete explanation. It is inconsistent with both common observation and a large body of experimentation, and it is of limited use in explaining human cognition.

Stepwise acquisition of vocal combinatorial capacity in songbirds and human infants.

Human language, as well as birdsong, relies on the ability to arrange vocal elements in new sequences. However, little is known about the ontogenetic origin of this capacity. Here we track the development of vocal combinatorial capacity in three species of vocal learners, combining an experimental approach in zebra finches (Taeniopygia guttata) with an analysis of natural development of vocal transitions in Bengalese finches (Lonchura striata domestica) and pre-lingual human infants. We find a common, stepwise pattern of acquiring vocal transitions across species. In our first study, juvenile zebra finches were trained to perform one song and then the training target was altered, prompting the birds to swap syllable order, or insert a new syllable into a string. All birds solved these permutation tasks in a series of steps, gradually approximating the target sequence by acquiring new pairwise syllable transitions, sometimes too slowly to accomplish the task fully. Similarly, in the more complex songs of Bengalese finches, branching points and bidirectional transitions in song syntax were acquired in a stepwise fashion, starting from a more restrictive set of vocal transitions. The babbling of pre-lingual human infants showed a similar pattern: instead of a single developmental shift from reduplicated to variegated babbling (that is, from repetitive to diverse sequences), we observed multiple shifts, where each new syllable type slowly acquired a diversity of pairwise transitions, asynchronously over development. Collectively, these results point to a common generative process that is conserved across species, suggesting that the long-noted gap between perceptual versus motor combinatorial capabilities in human infants may arise partly from the challenges in constructing new pairwise vocal transitions.

How robust are probabilistic models of higher-level cognition?

An increasingly popular theory holds that the mind should be viewed as a near-optimal or rational engine of probabilistic inference, in domains as diverse as word learning, pragmatics, naive physics, and predictions of the future. We argue that this view, often identified with Bayesian models of inference, is markedly less promising than widely believed, and is undermined by post hoc practices that merit wholesale reevaluation. We also show that the common equation between probabilistic and rational or optimal is not justified.

What can individual differences tell us about the specialization of function?

Can the study of individual differences inform debates about modularity and the specialization of function? In this article, we consider the implications of a highly replicated, robust finding known as positive manifold: Individual differences in different cognitive domains tend to be positively intercorrelated. Prima facie, this fact, which has generally been interpreted as reflecting the influence of a domain-general cognitive factor, might be seen as posing a serious challenge to a strong view of modularity. Drawing on a mixture of meta-analysis and computer simulation, we show that positive manifold derives instead largely from between-task neural overlap, suggesting a potential way of reconciling individual differences with some form of modularity.

The topographic brain: from neural connectivity to cognition.

A hallmark feature of vertebrate brain organization is ordered topography, wherein sets of neuronal connections preserve the relative organization of cells between two regions. Although topography is often found in projections from peripheral sense organs to the brain, it also seems to participate in the anatomical and functional organization of higher brain centers, for reasons that are poorly understood. We propose that a key function of topography might be to provide computational underpinnings for precise one-to-one correspondences between abstract cognitive representations. This perspective offers a novel conceptualization of how the brain approaches difficult problems, such as reasoning and analogy making, and suggests that a broader understanding of topographic maps could be pivotal in fostering strong links between genetics, neurophysiology and cognition.

Information from multiple modalities helps 5-month-olds learn abstract rules.

By 7 months of age, infants are able to learn rules based on the abstract relationships between stimuli (Marcus et al., 1999), but they are better able to do so when exposed to speech than to some other classes of stimuli. In the current experiments we ask whether multimodal stimulus information will aid younger infants in identifying abstract rules. We habituated 5-month-olds to simple abstract patterns (ABA or ABB) instantiated in coordinated looming visual shapes and speech sounds (Experiment 1), shapes alone (Experiment 2), and speech sounds accompanied by uninformative but coordinated shapes (Experiment 3). Infants showed evidence of rule learning only in the presence of the informative multimodal cues. We hypothesize that the additional evidence present in these multimodal displays was responsible for the success of younger infants in learning rules, congruent with both a Bayesian account and with the Intersensory Redundancy Hypothesis.

The roles of item repetition and position in infants' abstract rule learning.

We asked whether 11- and 14- month-old infants' abstract rule learning, an early form of analogical reasoning, is susceptible to processing constraints imposed by limits in attention and memory for sequence position. We examined 11- and 14- month-old infants' learning and generalization of abstract repetition rules ("repetition anywhere," Experiment 1 or "medial repetition," Experiment 2) and ordering of specific items (edge positions, Experiment 3) in 4-item sequences. Infants were habituated to sequences containing repetition- and/or position-based structure and then tested with "familiar" vs. "novel" (random) sequences composed of new items. Eleven-month-olds (N = 40) failed to learn abstract repetition rules, but 14-month-olds (N = 40) learned rules under both conditions. In Experiment 3, 11-month-olds (N = 20) learned item edge positions in sequences identical to those in Experiment 2. We conclude that infant sequence learning is constrained by item position in similar ways as in adults.

Roots, stems, and the universality of lexical representations: evidence from Hebrew.

Is the structure of lexical representations universal, or do languages vary in the fundamental ways in which they represent lexical information? Here, we consider a touchstone case: whether Semitic languages require a special morpheme, the consonantal root. In so doing, we explore a well-known constraint on the location of identical consonants that has often been used as motivation for root representations in Semitic languages: Identical consonants frequently occur at the end of putative roots (e.g., skk), but rarely occur in their beginning (e.g., ssk). Although this restriction has traditionally been stated over roots, an alternative account could be stated over stems, a representational entity that is found more widely across the world's languages. To test this possibility, we investigate the acceptability of a single set of roots, manifesting identity initially, finally or not at all (e.g., ssk versus skk versus rmk) across two nominal paradigms: CéCeC (a paradigm in which identical consonants are rare) and CiCúC (a paradigm in which identical consonants are frequent). If Semitic lexical representations consist of roots only, then similar restrictions on consonant co-occurrence should be observed in the two paradigms. Conversely, if speakers store stems, then the restriction on consonant co-occurrence might be modulated by the properties of the nominal paradigm (be it by means of statistical properties or their grammatical sources). Findings from rating and lexical decision experiments with both visual and auditory stimuli support the stem hypothesis: compared to controls (e.g., rmk), forms with identical consonants (e.g., ssk, skk) are less acceptable in the CéCeC than in the CiCúC paradigm. Although our results do not falsify root-based accounts, they strongly raise the possibility that stems could account for the observed restriction on consonantal identity. As such, our results raise fresh challenge to the notion that different languages require distinct sets of representational resources.

Songbirds work around computational complexity by learning song vocabulary independently of sequence.

While acquiring motor skills, animals transform their plastic motor sequences to match desired targets. However, because both the structure and temporal position of individual gestures are adjustable, the number of possible motor transformations increases exponentially with sequence length. Identifying the optimal transformation towards a given target is therefore a computationally intractable problem. Here we show an evolutionary workaround for reducing the computational complexity of song learning in zebra finches. We prompt juveniles to modify syllable phonology and sequence in a learned song to match a newly introduced target song. Surprisingly, juveniles match each syllable to the most spectrally similar sound in the target, regardless of its temporal position, resulting in unnecessary sequence errors, that they later try to correct. Thus, zebra finches prioritize efficient learning of syllable vocabulary, at the cost of inefficient syntax learning. This strategy provides a non-optimal but computationally manageable solution to the task of vocal sequence learning.

Cognitive architecture and descent with modification.

Against a background of recent progress in developmental neuroscience, some of which has been taken as challenging to the modularity hypothesis of , this article contrasts two competing conceptions of modularity: sui generis modularity, according to which modules are treated as independent neurocognitive entities that owe nothing to one another, and descent-with-modification modularity, according to which current cognitive modules are understood to be shaped by evolutionary changes from ancestral cognitive modules. I argue that sui generis modularity is incompatible with a range of data, from the co-occurrence of deficits to the patterns of activation in neuroimaging studies, but that same range of data is compatible with descent-with-modification modularity. Furthermore, I argue that the latter conception of modularity may have important implications for the practice and conception of fields such as developmental disorders and linguistics.

From semantics to syntax and back again: argument structure in the third year of life.

An essential part of the human capacity for language is the ability to link conceptual or semantic representations with syntactic representations. On the basis of data from spontaneous production, suggested that young children acquire such links on a verb-by-verb basis, with little in the way of a general understanding of linguistic argument structure. Here, we suggest that a receptive understanding of argument structure--including principles linking syntax and conceptual/semantic structure--appears earlier. In a forced-choice pointing task we have shown that toddlers in the third year of life can map a single scene (involving a novel causative action paired with a novel verb) onto two distinct syntactic frames (transitive and intransitive). This suggests that even before toddlers begin generalizing argument structure in their own speech, they have some representation of conceptual/semantic categories, syntactic categories, and a system that links the two.

The scope of linguistic generalizations: evidence from Hebrew word formation.

Does the productive use of language stem from the manipulation of mental variables (e.g. "noun", "any consonant")? If linguistic constraints appeal to variables, rather than instances (e.g. "dog", "m"), then they should generalize to any representable novel instance, including instances that fall beyond the phonological space of a language. We test this prediction by investigating a constraint on the structure of Hebrew roots. Hebrew frequently exhibits geminates (e.g. ss) in its roots, but it strictly constraints their location: geminates are frequent at the end of the root (e.g. mss), but rare at its beginning (e.g. ssm). Symbolic accounts capture the ban on root-initial geminates as *XXY, where X and Y are variables that stand for any two distinct consonants. If the constraint on root structure appeals to the identity of abstract variables, then speakers should be able to extend it to root geminates with foreign phonemes, including phonemes with foreign feature values. We present findings from three experiments supporting this prediction. These results suggest that a complete account of linguistic processing must incorporate mechanisms for generalization outside the representational space of trained items. Mentally-represented variables would allow speakers to make such generalizations.

Top-down influence in young children's linguistic ambiguity resolution.

Language is rife with ambiguity. Do children and adults meet this challenge in similar ways? Recent work suggests that while adults resolve syntactic ambiguities by integrating a variety of cues, children are less sensitive to top-down evidence. We test whether this top-down insensitivity is specific to syntax or a general feature of children's linguistic ambiguity resolution by evaluating whether children rely largely or completely on lexical associations to resolve lexical ambiguities (e.g., the word swing primes the baseball meaning of bat) or additionally integrate top-down global plausibility. Using a picture choice task, we compared 4-year-olds' ability to resolve polysemes and homophones with a Bayesian algorithm reliant purely on lexical associations and found that the algorithm's power to predict children's choices was limited. A 2nd experiment confirmed that children override associations and integrate top-down plausibility. We discuss this with regard to models of psycholinguistic development.

Musicality: instinct or acquired skill?

Is the human tendency toward musicality better thought of as the product of a specific, evolved instinct or an acquired skill? Developmental and evolutionary arguments are considered, along with issues of domain-specificity. The article also considers the question of why humans might be consistently and intensely drawn to music if musicality is not in fact the product of a specifically evolved instinct.

The role of association in early word-learning.

Word-learning likely involves a multiplicity of components, some domain-general, others domain-specific. Against the background of recent studies that suggest that word-learning is domain-specific, we investigated the associative component of word-learning. Seven- and 14-month-old infants viewed a pair of events in which a monkey or a truck moved back and forth, accompanied by a sung syllable or a tone, matched for pitch. Following habituation, infants were presented with displays in which the visual-auditory pairings were preserved or switched, and looked longer at the "switch" events when exposure time was sufficient to learn the intermodal association. At 7 months, performance on speech and tones conditions was statistically identical; at 14 months, infants had begun to favor speech. Thus, the associative component of word-learning does not appear (in contrast to rule-learning, Marcus et al., 2007) to initially privilege speech.