Induction of a concept description given noisy instances is difficult and is further exacerbated when the concepts may change over time. This paper presents a solution which has been guided by psychological and mathematical results. The method is based on a distributed concept description which is composed of a set of weighted, symbolic characterizations. Two learning processes incrementally modify this description. One adjusts the characterization weights and another creates new characterizations. The latter process is described in terms of a search through the space of possibilities and is shown to require linear space with respect to the number of attribute-value pairs in the description language. The method utilizes previously acquired concept definitions in subsequent learning by adding an attribute for each learned concept to instance descriptions. A program called STAGGER fully embodies this method, and this paper reports on a number of empirical analyses of its performance. Since understanding the relationships between a~ new learning method and existing ones can be difficult, this paper first reviews a framework for discussing machine learning systems and then describes STAGGER in that framework.

Although much effort has been directed toward understanding the neural basis of speech processing, the neural processes involved in the categorical perception of speech have been relatively less studied, and many questions remain open. In this functional magnetic resonance imaging (fMRI) study, we probed the cortical regions mediating categorical speech perception using an advanced brain-mapping technique, whole-brain multivariate pattern-based analysis (MVPA). Normal healthy human subjects (native English speakers) were scanned while they listened to 10 consonant-vowel syllables along the /ba/-/da/ continuum. Outside of the scanner, individuals' own category boundaries were measured to divide the fMRI data into /ba/ and /da/ conditions per subject. The whole-brain MVPA revealed that Broca's area and the left pre-supplementary motor area evoked distinct neural activity patterns between the two perceptual categories (/ba/ vs /da/). Broca's area was also found when the same analysis was applied to another dataset (Raizada and Poldrack, 2007), which previously yielded the supramarginal gyrus using a univariate adaptation-fMRI paradigm. The consistent MVPA findings from two independent datasets strongly indicate that Broca's area participates in categorical speech perception, with a possible role of translating speech signals into articulatory codes. The difference in results between univariate and multivariate pattern-based analyses of the same data suggest that processes in different cortical areas along the dorsal speech perception stream are distributed on different spatial scales.

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Four lines of evidence suggest a plausible link between prenatal cocaine exposure (CE) and specific effects on the mechanisms subserving arousal and attention regulation in infants and preschool-aged children. These are (1) the association of prenatal CE with alterations in monoaminergic system ontogeny; (2) neurobehavioral effects of prenatal CE in animals consistent with an enduring increased level of activity in response to novelty and inhibited exploration and altered responses to stress, suggesting overarousal in the face of novel/stressful situations and disrupted attention and exploration; (3) altered norepinephrine system function in cocaine-exposed human infants; and (4) neurobehavioral findings in infants and preschool-aged children suggestive of disrupted arousal regulation in the face of novelty, increased distractibility, and consequent impaired attention to novel, structured tasks. This paper summarizes findings on response to novel challenges from a cohort of prenatally cocaine-exposed infants and preschool-aged children followed longitudinally since birth. Arousal regulation in the face of novel challenges is operationalized behaviorally as state and emotional reactivity and neurophysiologically as the startle response and heart rate variability. Across different ages and tasks, behavioral and neurophysiological findings suggest that prenatally cocaine-exposed children are more likely to exhibit disrupted arousal regulation. Because the regulation of arousal serves as a gating mechanism to optimize orientation and attention, arousal regulation has important implications for ongoing information processing, learning, and memory. Furthermore, impaired arousal regulation predisposes children to a lower threshold for activation of "stress circuits" and may increase their vulnerability to the developmentally detrimental effects of stressful conditions particularly when such children are also exposed to the chaotic environmental conditions often characterizing substance-abusing families.

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Regional variations and substrates of high-frequency rhythmic activity induced by cholinergic stimulation were studied in hippocampal slices with 64-electrode recording arrays. (1) Carbachol triggered ␤ waves (17.6 Ϯ 5.7 Hz) in pyramidal regions of 75% of the slices. (2) The waves had phase shifts across the cell body layers and were substantially larger in the apical dendrites than in cell body layers or basal dendrites. (3) Continuous, twodimensional current source density analyses indicated apical sinks associated with basal sources, lasting ϳ10 msec, followed by apical sources and basal sinks, lasting ϳ20 msec, in a repeating pattern with a period in the range of 15-25 Hz. (4) Carbachol-induced ␤ waves in the hippocampus were accompanied by 40 Hz (␥) oscillations in deep layers of the entorhinal cortex. (5) Cholinergically elicited ␤ and ␥ rhythms were eliminated by antagonists of either AMPA or GABA receptors. Benzodiazepines markedly enhanced ␤ activity and sometimes introduced a distinct ␥ frequency peak. (6) Twenty Hertz activity after orthodromic activation of field CA3 was distributed in the same manner as carbachol-induced ␤ waves and was generated by a current source in the apical dendrites of CA3. This source was eliminated by high concentrations of GABA A receptor blockers. It is concluded that cholinergically driven ␤ rhythms arise independently in hippocampal subfields from oscillatory circuits involving (1) bursts of pyramidal cell discharges, (2) activation of a subset of feedback interneurons that project apically, and (3) production of a GABA A -mediated hyperpolarization in the outer portions of the apical dendrites of pyramidal neurons.

Simulations were performed of layers I and II of olfactory paleocortex, as connected to its primary input structure, olfactory bulb. Induction of synaptic long-term potentiation by means of repetitive sampling of inputs caused the simulation to organize encodings of learned cues into a hierarchical memory that uncovered statistical relationships in the cue environment, corresponding to the performance of hierarchical clustering by the biological network. Simplification led to characterization of those parts of the network responsible for the mechanism, resulting in a novel, efficient algorithm for hierarchical clustering. The hypothesis is put forward that these corticobulbar networks and circuitry of similar design in other brain regions contain computational elements sufficient to construct perceptual hierarchies for use in recognizing environmental cues.