Optimized CUBIC protocol for three-dimensional imaging of chicken embryos at single-cell resolution.

The CUBIC tissue-clearing protocol has been optimized to produce translucent immunostained whole chicken embryos and embryo brains. When combined with multispectral light-sheet microscopy, the validated protocol presented here provides a rapid, inexpensive and reliable method for acquiring accurate histological images that preserve three-dimensional structural relationships with single-cell resolution in whole early-stage chicken embryos and in the whole brains of late-stage embryos.

Auditory perceptual category formation does not require perceptual warping.

Categorical perception occurs when a perceiver's stimulus classifications affect their ability to make fine perceptual discriminations and is the most intensively studied form of category learning. On the basis of categorical perception studies, it has been proposed that category learning proceeds by the deformation of an initially homogeneous perceptual space ("perceptual warping"), so that stimuli within the same category are perceived as more similar to each other (more difficult to tell apart) than stimuli that are the same physical distance apart but that belong to different categories. Here, we present a significant counterexample in which robust category learning occurs without these differential perceptual space deformations. Two artificial categories were defined along the dimension of pitch for a perceptually unfamiliar, multidimensional class of sounds. A group of participants (selected on the basis of their listening abilities) were trained to sort sounds into these two arbitrary categories. Category formation, verified empirically, was accompanied by a heightened sensitivity along the entire pitch range, as indicated by changes in an EEG index of implicit perceptual distance (mismatch negativity), with no significant resemblance to the local perceptual deformations predicted by categorical perception. This demonstrates that robust categories can be initially formed within a continuous perceptual dimension without perceptual warping. We suggest that perceptual category formation is a flexible, multistage process sequentially combining different types of learning mechanisms rather than a single process with a universal set of behavioral and neural correlates.

A proposed mechanism for rapid adaptation to spectrally distorted speech.

The mechanisms underlying perceptual adaptation to severely spectrally-distorted speech were studied by training participants to comprehend spectrally-rotated speech, which is obtained by inverting the speech spectrum. Spectral-rotation produces severe distortion confined to the spectral domain while preserving temporal trajectories. During five 1-hour training sessions, pairs of participants attempted to extract spoken messages from the spectrally-rotated speech of their training partner. Data on training-induced changes in comprehension of spectrally-rotated sentences and identification/discrimination of spectrally-rotated phonemes were used to evaluate the plausibility of three different classes of underlying perceptual mechanisms: (1) phonemic remapping (the formation of new phonemic categories that specifically incorporate spectrally-rotated acoustic information); (2) experience-dependent generation of a perceptual "inverse-transform" that compensates for spectral-rotation; and (3) changes in cue weighting (the identification of sets of acoustic cues least affected by spectral-rotation, followed by a rapid shift in perceptual emphasis to favour those cues, combined with the recruitment of the same type of "perceptual filling-in" mechanisms used to disambiguate speech-in-noise). Results exclusively support the third mechanism, which is the only one predicting that learning would specifically target temporally-dynamic cues that were transmitting phonetic information most stably in spite of spectral-distortion. No support was found for phonemic remapping or for inverse-transform generation.

The human cerebral cortex flattens during adolescence.

The human cerebral cortex appears to shrink during adolescence. To delineate the dynamic morphological changes involved in this process, 52 healthy male and female adolescents (11-17 years old) were neuroimaged twice using magnetic resonance imaging, approximately 2 years apart. Using a novel morphometric analysis procedure combining the FreeSurfer and BrainVisa image software suites, we quantified global and lobar change in cortical thickness, outer surface area, the gyrification index, the average Euclidean distance between opposing sides of the white matter surface (gyral white matter thickness), the convex ("exposed") part of the outer cortical surface (hull surface area), sulcal length, depth, and width. We found that the cortical surface flattens during adolescence. Flattening was strongest in the frontal and occipital cortices, in which significant sulcal widening and decreased sulcal depth co-occurred. Globally, sulcal widening was associated with cortical thinning and, for the frontal cortex, with loss of surface area. For the other cortical lobes, thinning was related to gyral white matter expansion. The overall flattening of the macrostructural three-dimensional architecture of the human cortex during adolescence thus involves changes in gray matter and effects of the maturation of white matter.

Multivariate activation and connectivity patterns discriminate speech intelligibility in Wernicke's, Broca's, and Geschwind's areas.

The brain network underlying speech comprehension is usually described as encompassing fronto-temporal-parietal regions while neuroimaging studies of speech intelligibility have focused on a more spatially restricted network dominated by the superior temporal cortex. Here we use functional magnetic resonance imaging with a novel whole-brain multivariate pattern analysis (MVPA) to more fully characterize neural responses and connectivity to intelligible speech. Consistent with previous univariate findings, intelligible speech elicited greater activity in bilateral superior temporal cortex relative to unintelligible speech. However, MVPA identified a more extensive network that discriminated between intelligible and unintelligible speech, including left-hemisphere middle temporal gyrus, angular gyrus, inferior temporal cortex, and inferior frontal gyrus pars triangularis. These fronto-temporal-parietal areas also showed greater functional connectivity during intelligible, compared with unintelligible, speech. Our results suggest that speech intelligibly is encoded by distinct fine-grained spatial representations and within-task connectivity, rather than differential engagement or disengagement of brain regions, and they provide a more complete view of the brain network serving speech comprehension. Our findings bridge a divide between neural models of speech comprehension and the neuroimaging literature on speech intelligibility, and suggest that speech intelligibility relies on differential multivariate response and connectivity patterns in Wernicke's, Broca's, and Geschwind's areas.

Phonological generalizations in dyslexia: the phonological grammar may not be impaired.

Dyslexia is commonly attributed to a phonological deficit, but whether it effectively compromises the phonological grammar or lower level systems is rarely explored. To address this question, we gauge the sensitivity of dyslexics to grammatical phonological restrictions on spoken onset clusters (e.g., bl in block). Across languages, certain onsets are preferred to others (e.g., blif ≻ bnif ≻ bdif, where ≻ indicates a preference). Here, we show that dyslexic participants (adult native speakers of Hebrew) are fully sensitive to these phonological restrictions, and they extend them irrespective of whether the onsets are attested in their language (e.g., bnif vs. bdif) or unattested (e.g., mlif vs. mdif). Dyslexics, however, showed reduced sensitivity to phonetic contrasts (e.g., blif vs. belif; ba vs. pa). Together, these results suggest that the known difficulties of dyslexics in speech processing could emanate not from the phonological grammar, but rather from lower level impairments to acoustic/phonetic encoding, lexical storage, and retrieval.

ß- And γ-band EEG power predicts illusory auditory continuity perception.

Because acoustic landscapes are complex and rapidly changing, auditory systems have evolved mechanisms that permit rapid detection of novel sounds, sound source segregation, and perceptual restoration of sounds obscured by noise. Perceptual restoration is particularly important in noisy environments because it allows organisms to track sounds over time even when they are masked. The continuity illusion is a striking example of perceptual restoration with sounds perceived as intact even when parts of them have been replaced by gaps and rendered inaudible by being masked by an extraneous sound. The mechanisms of auditory filling-in are complex and are currently not well-understood. The present study used the high temporal resolution of EEG to examine brain activity related to continuity illusion perception. Masking noise loudness was adjusted individually for each subject so that physically identical sounds on some trials elicited a continuity illusion (failure to detect a gap in a sound) and on other trials resulted in correct gap detection. This design ensured that any measurable differences in brain activity would be due to perceptual differences rather than physical differences among stimuli. We found that baseline activity recorded immediately before presentation of the stimulus significantly predicted the occurrence of the continuity illusion in 10 out of 14 participants based on power differences in γ-band EEG (34-80 Hz). Across all participants, power in the ß and γ (12- to 80-Hz range) was informative about the subsequent perceptual decision. These data suggest that a subject's baseline brain state influences the strength of continuity illusions.

Decoding temporal structure in music and speech relies on shared brain resources but elicits different fine-scale spatial patterns.

Music and speech are complex sound streams with hierarchical rules of temporal organization that become elaborated over time. Here, we use functional magnetic resonance imaging to measure brain activity patterns in 20 right-handed nonmusicians as they listened to natural and temporally reordered musical and speech stimuli matched for familiarity, emotion, and valence. Heart rate variability and mean respiration rates were simultaneously measured and were found not to differ between musical and speech stimuli. Although the same manipulation of temporal structure elicited brain activation level differences of similar magnitude for both music and speech stimuli, multivariate classification analysis revealed distinct spatial patterns of brain responses in the 2 domains. Distributed neuronal populations that included the inferior frontal cortex, the posterior and anterior superior and middle temporal gyri, and the auditory brainstem classified temporal structure manipulations in music and speech with significant levels of accuracy. While agreeing with previous findings that music and speech processing share neural substrates, this work shows that temporal structure in the 2 domains is encoded differently, highlighting a fundamental dissimilarity in how the same neural resources are deployed.

Language universals and misidentification: a two-way street.

Certain ill-formed phonological structures are systematically under-represented across languages and misidentified by human listeners. It is currently unclear whether this results from grammatical phonological knowledge that actively recodes ill-formed structures, or from difficulty with their phonetic encoding. To examine this question, we gauge the effect of two types of tasks on the identification of onset clusters that are unattested in an individual's language. One type calls attention to global phonological structure by eliciting a syllable count (e.g., does medifinclude one syllable or two?). A second set of tasks promotes attention to local phonetic detail by requiring the detection of specific segments (e.g., does medifinclude an e?). Results from five experiments show that, when participants attend to global phonological structure, ill-formed onsets are misidentified (e.g., mdif-->medif) relative to better-formed ones (e.g., mlif). In contrast, when people attend to local phonetic detail, they identify ill-formed onsets as well as better-formed ones, and they are highly sensitive to non-distinctive phonetic cues. These findings suggest that misidentifications reflect active recoding based on broad phonological knowledge, rather than passive failures to extract acoustic surface forms. Although the perceptual interface could shape such knowledge, the relationship between language and misidentification is a two-way street.

Novel localizations of TRPC5 channels suggest novel and unexplored roles: A study in the chick embryo brain.

Mammalian TRPC5 channels are predominantly expressed in the brain, where they increase intracellular Ca2+ and induce depolarization. Because they augment presynaptic vesicle release, cause persistent neural activity, and show constitutive activity, TRPC5s could play a functional role in late developmental brain events. We used immunohistochemistry to examine TRPC5 in the chick embryo brain between 8 and 20 days of incubation, and provide the first detailed description of their distribution in birds and in the whole brain of any animal species. Stained areas substantially increased between E8 and E16, and staining intensity in many areas peaked at E16, a time when chick brains first show organized patterns of whole-brain metabolic activation like what is seen consistently after hatching. Areas showing cell soma staining match areas showing Trpc5 mRNA or protein in adult rodents (cerebral cortex, hippocampus, amygdala, cerebellar Purkinje cells). Chick embryos show protein staining in the optic tectum, cerebellar nuclei, and several brainstem nuclei; equivalent areas in the Allen Institute mouse maps express Trpc5 mRNA. The strongest cell soma staining was found in a dorsal hypothalamic area (matching a group of parvicellular arginine vasotocin neurons and a pallial amygdalohypothalamic cell corridor) and the vagal motor complex. Purkinje cells showed strong dendritic staining at E20. Unexpectedly, we also describe neurite staining in the septum, several hypothalamic nuclei, and a paramedian raphe area; the strongest neurite staining was in the median eminence. These novel localizations suggest new unexplored TRPC5 functions, and possible roles in late embryonic brain development.

Hearing history influences voice gender perceptual performance in cochlear implant users.

OBJECTIVES: The study was carried out to assess the role that five hearing history variables (chronological age, age at onset of deafness, age of first cochlear implant [CI] activation, duration of CI use, and duration of known deafness) play in the ability of CI users to identify speaker gender. DESIGN: Forty-one juvenile CI users participated in two voice gender identification tasks. In a fixed, single-interval task, subjects listened to a single speech item from one of 20 adult male or 20 adult female speakers and had to identify speaker gender. In an adaptive speech-based voice gender discrimination task with the fundamental frequency difference between the voices as the adaptive parameter, subjects listened to a pair of speech items presented in sequential order, one of which was always spoken by an adult female and the other by an adult male. Subjects had to identify the speech item spoken by the female voice. Correlation and regression analyses between perceptual scores in the two tasks and the hearing history variables were performed. RESULTS: Subjects fell into three performance groups: (1) those who could distinguish voice gender in both tasks, (2) those who could distinguish voice gender in the adaptive but not the fixed task, and (3) those who could not distinguish voice gender in either task. Gender identification performance for single voices in the fixed task was significantly and negatively related to the duration of deafness before cochlear implantation (shorter deafness yielded better performance), whereas performance in the adaptive task was weakly but significantly related to age at first activation of the CI device, with earlier activations yielding better scores. CONCLUSIONS: The existence of a group of subjects able to perform adaptive discrimination but unable to identify the gender of singly presented voices demonstrates the potential dissociability of the skills required for these two tasks, suggesting that duration of deafness and age of cochlear implantation could have dissociable effects on the development of different skills required by CI users to identify speaker gender.

Auditory stream biasing in children with reading impairments.

Reading impairments have previously been associated with auditory processing differences. We examined auditory stream biasing, a global aspect of auditory temporal processing. Children with reading impairments, control children and adults heard a 10 s long stream-bias-inducing sound sequence (a repeating 1000 Hz tone) and a test sequence (eight repetitions of two pure tones of 1000 and 1420 Hz in an XYX-XYX... pattern) with a variable delay interval (from 0.09 to 8 s) between the two sequences. Reading-impaired children had a significantly lower proportion of streamed responses than control children and adults. Streamed responses in reading-impaired participants differed according to their musical experience, but musically experienced reading-impaired participants were still significantly different from musically experienced controls. Reading impairments are associated with global differences in auditory integration, and musical experience needs to be considered when investigating auditory processing capabilities.

A proposed neural mechanism underlying auditory continuity illusions.

A numerical thought experiment was conducted to assess whether stimulus-specific, short-term changes in auditory neural responsiveness could explain the formation of auditory objects underlying the auditory continuity illusion. A tonotopic, two-layer feedforward network model with one time constant for synaptic weight augmentation based on firing rate, and an independent time constant for synaptic weight decay was presented with classical continuity illusion stimuli. The results suggest that the continuity illusion could, in principle, be explained by basic, duration-dependent auditory circuit behavior, which could emerge at either early or later stages of processing.

Voice gender perception by cochlear implantees.

Gender identification of human voices was studied in a juvenile population of cochlear implant (CI) users exposed to naturalistic speech stimuli from 20 male and 20 female speakers using two different voice gender perception tasks. Stimulus output patterns were recorded from each individual CI for each stimulus, and features related to voice fundamental frequency and spectral envelope were extracted from these electrical output signals to evaluate the relationship between implant output and behavioral performance. In spite of the fact that temporal and place cues of similar quality were produced by all CI devices, only about half of the subjects were able to label male and female voices correctly. Participants showed evidence of using available temporal cues, but showed no evidence of using place cues. The implants produced a consistent and novel cue to voice gender that participants did not appear to utilize. A subgroup of participants could discriminate male and female voices when two contrasting voices were presented in succession, but were unable to identify gender when voices were singly presented. It is suggested that the nature of long-term auditory categorical memories needs to be studied in more detail in these individuals.

Melanin-concentrating hormone (MCH) neurons in the developing chick brain.

The present study was undertaken because no previous developmental studies exist on MCH neurons in any avian species. After validating a commercially-available antibody for use in chickens, immunohistochemical examinations first detected MCH neurons around embryonic day (E) 8 in the posterior hypothalamus. This population increased thereafter, reaching a numerical maximum by E20. MCH-positive cell bodies were found only in the posterior hypothalamus at all ages examined, restricted to a region showing very little overlap with the locations of hypocretin/orexin (H/O) neurons. Chickens had fewer MCH than H/O neurons, and MCH neurons also first appeared later in development than H/O neurons (the opposite of what has been found in rodents). MCH neurons appeared to originate from territories within the hypothalamic periventricular organ that partially overlap with the source of diencephalic serotonergic neurons. Chicken MCH fibers developed exuberantly during the second half of embryonic development, and they became abundant in the same brain areas as in rodents, including the hypothalamus (by E12), locus coeruleus (by E12), dorsal raphe nucleus (by E20) and septum (by E20). These observations suggest that MCH cells may play different roles during development in chickens and rodents; but once they have developed, MCH neurons exhibit similar phenotypes in birds and rodents.

Getting into shape: optimal ligand gradients for axonal guidance.

During neural development, neurons from downstream, presynaptic regions of the nervous system (such as the retina) send spatially patterned axonal projections to upstream, target regions (the tectum or superior colliculus). A servomechanism model has been proposed to explain the pattern and time-course of axonal growth between these two regions [Honda, H., 1998. Topographic mapping in the retinotectal projection by means of complementary ligand and receptor gradients: a computer simulation study. J. Theor. Biol., 192, 235-246]. Here, we show that a modification of this model incorporating a different criterion for axonal decision-making, called the local optimum rule, is guaranteed to converge to a topographic map under a wide range of conditions encountered during neural development. A theoretical investigation of these conditions leads to new hypotheses regarding map formation.

Dual-route imitation in preschool children.

Imitation can be realized via two different routes: a direct route that translates visual input into motor output when gestures are meaningless or unknown, and a semantic route for known/meaningful gestures. Young infants show imitative behaviours compatible with the direct route, but little is known about the development of the semantic route, studied here for the first time. The present study examined preschool children (3-5years of age) imitating gestures that could be transitive or intransitive, and meaningful or meaningless. Both routes for imitation were already present by three years of age, and children were more accurate at imitating meaningful-intransitive gestures than meaningless-intransitive ones; the reverse pattern was found for transitive gestures. Children preferred to use their dominant hand even if they had to anatomically imitate the model to do this, showing that a preference for specular imitation is not exclusive at these ages.

Cognitive developmental biology: history, process and fortune's wheel.

Biological contributions to cognitive development continue to be conceived predominantly along deterministic lines, with proponents of different positions arguing about the preponderance of gene-based versus experience-based influences that organize brain circuits irreversibly during prenatal or early postnatal life, and evolutionary influences acting through selection on small numbers of genes. This article discusses evolutionary, mechanistic and probabilistic aspects of developmental processes that cognitive scientists need to better integrate. Developmental processes inseparably fuse experience-dependent and experience-independent components, have important stochastic contributions, and exhibit a greater degree of mechanistic continuity between developing and adult nervous systems than previously thought. Their balanced integration leads to new models for "critical or sensitive" period phenomena and behavioral biases. A general understanding of behavioral development - cognitive developmental biology--will require better coordination between comparative animal and human developmental research programs.

Brain switching: studying evolutionary behavioral changes in the context of individual brain development.

Working together at Nogent, Marie-Aimee Teillet, Nicole Le Douarin and the author successfully developed an extension of the quail-chick transplant technique for relating species brain cell differences to behavioral differences. This article reviews the application of the technique to species differences in motor behavior (crowing) and auditory perceptual preferences. Interspecies brain transplants provide a unique means for elucidating general cellular mechanisms which integrate evolutionary and individual experience during the development of complex brain circuitry.

Chick embryos have the same pattern of hypoxic lower-brain activation as fetal mammals.

cFos expression (indicating a particular kind of neuronal activation) was examined in embryonic day (E) 18 chick embryos after exposure to 4 h of either normoxia (21% O2), modest hypoxia (15% O2), or medium hypoxia (10% O2). Eight regions of the brainstem and hypothalamus were surveyed, including seven previously shown to respond to hypoxia in late-gestation mammalian fetuses (Breen et al., 1997; Nitsos and Walker, 1999b). Hypoxia-related changes in chick embryo brain activation mirrored those found in fetal mammals with the exception of the medullary Raphe, which showed decreased hypoxic activation, compared with no change in mammals. This difference may be explained by the greater anapyrexic responses of chick embryos relative to mammalian fetuses. Activation in the A1/C1 region was examined in more detail to ascertain whether an O2-sensitive subpopulation of these cells containing heme oxygenase 2 (HMOX2) may drive hypoxic brain responses before the maturation of peripheral O2-sensing. HMOX2-positive and -negative catecholaminergic cells and interdigitating noncatecholaminergic HMOX2-positive cells all showed significant changes in cFos expression to hypoxia, with larger population responses seen in the catecholaminergic cells. Hypoxia-induced activation of lower-brain regions studied here was significantly better correlated with activation of the nucleus of the solitary tract (NTS) than with that of HMOX2-containing A1/C1 neurons. Together, these observations suggest that (1) the functional circuitry controlling prenatal brain responses to hypoxia is strongly conserved between birds and mammals, and (2) NTS neurons are a more dominant driving force for prenatal hypoxic cFos brain responses than O2-sensing A1/C1 neurons.