NR2B downregulation in a forebrain region required for avian vocal learning is not sufficient to close the sensitive period for song learning

The neural changes that limit the sensitive period for avian song development are unknown, but neurons in a forebrain region critical for song learning, the lMAN, exhibit experience-driven changes in NMDAR subunit expression that could regulate sensitive period closure. Specifically, NR2B levels in lMAN decrease during song acquisition, potentially reducing synaptic plasticity by decreasing NMDAR EPSC duration and/or affecting NMDAR-coupled intracellular cascades. While rearing birds in isolation extends the sensitive period and also delays the developmental changes in NR2B expression and NMDAR physiology, recent work indicates that a transition to faster NMDAR currents does not preclude further song learning. However, NR2B mRNA expression in isolates remains elevated beyond the age at which NMDAR currents shorten, leaving open the possibility that NR2B levels regulate closure of the sensitive period through effects other than those mediated by NMDAR current duration. To determine whether the experience-driven decrease in NR2B expression in lMAN closes the sensitive period, we promoted this change in gene expression either by treating isolation-reared zebra finches briefly with testosterone (T-isolates) or by allowing males limited access to conspecific song (pre-exposed isolates). We then assessed if these birds could acquire song from tutors after the normal close of the sensitive period. Despite a normal decline in NR2B expression, T-isolate and pre-exposed isolate birds learned tutor songs heard from d65-90, while normally reared birds did not. These findings suggest that the normal decline in NR2B expression with lMAN is not sufficient for sensitive period closure.     

Differences in the complexity of song tutoring cause differences in the amount learned and in dendritic spine density in a songbird telencephalic song control nucleus

In our search for relations between vocal learning and neuron structure in the song control nuclei of songbird forebrains, we tested whether differential experience that leads to differences in adult song repertoire would affect dendritic spine density in HVc (also called high vocal center) and RA (robustus archistriatalis). We tape-tutored juvenile Eastern marsh wrens (Cistothorus palustris) with either 5 or 45 song types. As adults, the small repertoire group had learned mostly 5 or 6 song types, and the large repertoire group had learned 36 to 47. Wrens that learned the large song repertoires had a greater dendritic spine density for the most spiny neurons present in HVc (mean difference, 36%), but not in RA. Recent physiological evidence describes HVc as a premotor area coding syllables, motifs, and higher-order song patterns, and our data now clearly reveal that differences in the size of the song repertoire that is experienced lead to differences both in song learning and in the density of dendritic spines in HVc. In the forebrain song nuclei of these songbirds, as in some other vertebrate systems, differences in learning and performance are associated with differences in synaptic anatomy specifically in the region that organizes the learned pattern.     

Song perception during the sensitive period of song learning in zebra finches (Taeniopygia guttata)

The sensitive period is a special time for auditory learning in songbirds. However, little is known about perception and discrimination of song during this period of development. The authors used a go/no-go operant task to compare discrimination of conspecific song from reversed song in juvenile and adult zebra finches (Taeniopygia guttata), and to test for possible developmental changes in perception of syllable structure and syllable syntax. In Experiment 1, there were no age or sex differences in the ability to learn the discrimination, and the birds discriminated the forward from reversed song primarily on the basis of local syllable structure. Similar results were found in Experiment 2 with juvenile birds reared in isolation from song. Experiment 3 found that juvenile zebra finches could discriminate songs on the basis of syllable order alone, although this discrimination was more difficult than one based on syllable structure. The results reveal well-developed song discrimination and song perception in juvenile zebra finches, even in birds with little experience with song.     

Selective impairment of song learning following lesions of a forebrain nucleus in the juvenile zebra finch

Area X, a large sexually dimorphic nucleus in the avian ventral forebrain, is part of a highly discrete system of interconnected nuclei that have been implicated in either song learning or adult song production. Previously, this nucleus has been included in the song system because of its substantial connections with other vocal control nuclei, and because its volume is positively correlated with the capacity for song. In order to directly assess the role of Area X in song behavior, this nucleus was bilaterally lesioned in both juvenile and adult zebra finches, using ibotenic acid. We report here that lesioning Area X disrupts normal song development in juvenile birds, but does not affect the production of stereotyped song by adult birds. Although juvenile-lesioned birds were consistently judged as being in earlier stages of vocal development than age-matched controls, they continued to produce normal song-like vocalizations. Thus, unlike the lateral magnocellular nucleus of the anterior neostriatum, another avian forebrain nucleus implicated in song learning, Area X does not seem to be necessary for sustaining production of juvenile song. Rather, the behavioral results suggest Area X is important for either the acquisition of a song model or the improvement of song through vocal practice.     

A method for the analysis of complex avian song repertoires

A new method for the analysis of complex avian song repertoires is introduced. The method consists of sending a tape-recorded bird-song signal through five narrow band-pass filters and then to a five-channel high-speed pen recorder. A coding method that allows a researcher to convert the output from the pen recorder to numbers for later analysis by computer is described.     

Preexposure effects in the spatial domain: Dissociation between latent inhibition and perceptual learning

In 4 experiments, rats were given intermixed or blocked preexposure to an array of landmarks that subsequently defined the location of a hidden goal in a Morris pool task. Previous research has shown that intermixed preexposure to pairs of adjacent landmarks retards learning whereas preexposure to individual landmarks facilitates subsequent learning (J. Prados, V. D. Chamizo, & N. J. Mackintosh, 1999). Accordingly, in Experiment 1, intermixed and blocked preexposure to pairs of adjacent landmarks was found to retard learning. In Experiment 2, however, a scheduling effect was found: Rats given intermixed preexposure to the individual landmarks learned faster than rats given blocked or no preexposure. Experiment 3 showed that intermixed (but not blocked) preexposure to pairs of landmarks resulted in a facilitatory effect when preexposure and test were carried out in different contexts. Experiment 4 replicated within a single experiment the main results observed in Experiments 1 and 3. This pattern of results suggests that intermixed preexposure engages learning processes other than latent inhibition that facilitate subsequent learning of the navigation task.     

Contrasting effects of motor and visual spatial learning tasks on dendritic arborization and spine density in rats

Rats were trained in four different learning tasks including the Morris-water task, a T-maze delayed nonmatch-to-sample task, a skilled unilateral reaching task, and a skilled bilateral string-pulling task. At the end of training the brains were harvested and stained using a Golgi-Cox procedure. Learning the spatial navigation task produced increased dendritic length and branching as well as decreased spine density in layer III pyramidal cells in occipital cortex. Learning the T-maze task increased dendritic branching in layer III medial but not orbital frontal cortex pyramidal cells and increased spine density in both regions. The motor learning tasks produced increased dendritic length and branching in layer V pyramidal cells in the forelimb cortex in the hemisphere contralateral to the trained limb in the unilateral skilled reaching task and in both limbs in the bilateral skilled pulling task. There were no changes in spine density in layer V in the motor tasks, but there was a decrease in spine density in layer III in the unilateral reaching task. Spatial and motor learning thus produce different patterns of change in layer III cortical pyramidal neurons. Furthermore, changes in spine density and dendritic length and branching are not tightly correlated and can increase and/or decrease independently of one another in learning tasks.     

Acoustic and perceptual categories of vocal elements in the warble song of budgerigars (Melopsittacus undulatus)

The warble songs of budgerigars (Melopsittacus undulatus) are composed of a number of complex, variable acoustic elements that are sung by male birds in intimate courtship contexts for periods lasting up to several minutes. If these variable acoustic elements can be assigned to distinct acoustic-perceptual categories, it provides the opportunity to explore whether birds are perceptually sensitive to the proportion or sequential combination of warble elements belonging to different categories. By the inspection of spectrograms and by listening to recordings, humans assigned the acoustic elements in budgerigar warble from several birds to eight broad, overlapping categories. A neural-network program was developed and trained on these warble elements to simulate human categorization. The classification reliability between human raters and between human raters and the neural network classifier was better than 80% both within and across birds. Using operant conditioning and a psychophysical task, budgerigars were tested on large sets of these elements from different acoustic categories and different individuals. The birds consistently showed high discriminability for pairs of warble elements drawn from between acoustic categories and low discriminability for pairs drawn from within acoustic categories. With warble elements reliably assigned to different acoustic categories by humans and birds, it affords the opportunity to ask questions about the ordering of elements in natural warble streams and the perceptual significance of this ordering.     

Dissociation between judgments and outcome-expectancy measures in covariation learning: a signal detection theory approach

A number of studies using trial-by-trial learning tasks have shown that judgments of covariation between a cue c and an outcome o deviate from normative metrics. Parameters based on trial-by-trial predictions were estimated from signal detection theory (SDT) in a standard causal learning task. Results showed that manipulations of P(c) when contingency (deltaP) was held constant did not affect participants' ability to predict the appearance of the outcome (d') but had a significant effect on response criterion (c) and numerical causal judgments. The association between criterion c and judgment was further demonstrated in 2 experiments in which the criterion was directly manipulated by linking payoffs to the predictive responses made by learners. In all cases, the more liberal the criterion c was, the higher judgments were. The results imply that the mechanisms underlying the elaboration of judgments and those involved in the elaboration of predictive responses are partially dissociable.     

Dissociation of sensory facilitation and decision bias in statistical learning

ABSTRACT

The detection of regularities in the sensory environment, known as statistical learning, is an important brain function that has been observed in many experimental contexts. In these experiments, statistical learning of patterned sensory stimulation leads to improvements in the speed and/or accuracy with which subsequent stimuli are recognized. That is, statistical learning facilitates the transformation of sensory stimuli into motor responses, but the mechanism by which this occurs is unclear. Statistical learning could improve the efficiency of sensory processing, or it could bias responses toward particular outcomes. The distinction is important, as these different hypotheses imply different functions and different neural substrates for statistical learning. Here we address this problem by studying statistical learning as a decision-making process, which allows us to leverage the extensive computational literature on this topic. Specifically we describe a method for applying the Diffusion Decision Model (DDM) to isolate different sensory and cognitive processes associated with decision-making. The results indicate that statistical learning improves performance on a visual learning task in two distinct ways: by altering the efficiency of sensory processing and by introducing biases in the decision-making process. By fitting the parameters of the DDM to data from individual subjects, we find that the prominence of these two factors differed substantially across the population, and that these differences were predictive of individual performance on the psychophysical task. Overall, these results indicate that different cognitive processes can be recruited by statistical learning, and that the DDM is a powerful framework for detecting these influences.     

A sensitive period for the acquisition of a nonnative phonological system

Immigrants who had learned English at various ages and who had been in the United States for various amounts of time were judged for degree of accent in English. It was found that age at arrival was a strong predictor of degree of accent, while length of stay had very little effect. Other practice and motivational factors were related to accent only by virtue of their correlation with age at arrival. It was suggested that a sensitive period exists for the acquisition of a nonnative phonological system.This article was adapted from a doctoral dissertation for the Department of Psychology and Social Relations, Harvard University, 1973 (unpublished), which was supported by NSF Grant No. GS-36263 and the Radcliffe Institute.     

Subregion-specific dendritic spine abnormalities in the hippocampus of Fmr1 KO mice

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and is caused by the lack of fragile X mental retardation protein (FMRP). In the brain, spine abnormalities have been reported in both patients with FXS and Fmr1 knockout mice. This altered spine morphology has been linked to disturbed synaptic transmission related to altered signaling in the excitatory metabotropic glutamate receptor 5 (mGluR5) pathway. We investigated hippocampal protrusion morphology in adult Fmr1 knockout mice. Our results show a hippocampal CA1-specific altered protrusion phenotype, which was absent in the CA3 region of the hippocampus. This suggests a subregion-specific function of FMRP in synaptic plasticity in the brain.     

考古发现与先秦易学

本文从考古发现的一些才料,考证了先秦易学的若干问题.认为,古贞与鼎同字,贞亦训“定”,鼎是“定命”的象征.殷墟卜辞“卜王”与《周易》卦辞中的“安贞”同义。卜辞中有“由王事”,即“从王事”。《周易》爻辞中有“从王事”.《周易》卦爻辞中屡言“我”,取象与“从王事”全同.卜辞中有神名“(虫虫)”,指太一神。包山楚简中有神名“二天子”,应是伏牺、神农。郭店楚简《成之闻之》中的“大帝”即为“易”.张家山汉简《盖庐》出现了“太极”的概念,是研究古代思想的珍贵资料。     

Dissociation between priming and recognition in the expression of sequential knowledge

Exposure to a repeating sequence of target stimuli in a speeded localization task can support both priming of sequence-consistent responses and recognition of sequence components. Here, a task is introduced in which measures of priming and recognition are obtained concurrently, and it is demonstrated that priming of sequence-consistent responses occurs even when test stimuli are not recognized. The results show that sequence knowledge can be expressed in the absence of conscious recognition. However, we also show that this result is consistent with a simple model in which priming and recognition depend on exactly the same underlying memory strength variable.     

Evidence for a sensitive period in the effects of early life stress on hippocampal volume

Exposure to stress has been causally linked to changes in hippocampal volume (HV). Given that the hippocampus undergoes rapid changes in the first years of life, stressful experiences during this period may be particularly important in understanding individual differences in the development of the hippocampus. One hundred seventy‐eight early adolescents (ages 9–13 years; 43% male) were interviewed regarding exposure to and age of onset of experiences of stress; the severity of each stressful event was rated by an objective panel. All participants underwent structural magnetic resonance imaging, from which HVs were automatically segmented. Without considering the age of onset for stressful experiences, there was a small but statistically significant negative association of stress severity with bilateral HV. When considering the age of onset, there was a moderate and significant negative association between stress severity during early childhood (through 5 years of age) and HV; there was no association between stress severity during later childhood (age 6 years and older) and HV. We provide evidence of a sensitive period through 5 years of age for the effects of life stress on HV in adolescence. It will be important in future research to elucidate how reduced HV stemming from early life stress may contribute to stress‐related health outcomes.     

概率贴现和时间贴现在人脑中的分离

本研究运用ERP技术来检验概率贴现和时间贴现是否反映了相同的神经机制过程。在本研究中,我们在一个刺激中同时操纵概率贴现（风险vs.安全）和时间贴现（1个月后vs.今天）水平。通过对两个过程上的ERN以及全时间段的ERP分析来确定它们是否反映了相同的认知过程。结果发现,这两者在ERN上存在差异,同时这两个效应在ERP的时程和出现的电极点上都存在差异。这些差异表明这两者可能包含着不同的神经机制过程。     

知觉与行为的分离在视错觉研究中的验证

知觉与行为的分离是指人的视觉对物体的知觉与对行为的控制属于两个不同的系统。最早人们从临床病例中发现存在知觉与行为的分离,以后Aglioti等利用铁钦纳错觉实验证实正常人中也存在这一分离现象。一些研究者将两者的分离看作是两个不同视觉皮层通路（腹侧知觉系统和背侧视觉运动系统）的结果。围绕这一分离现象的实验和假说引发了不少的争论。该文回顾了十年来利用错觉实验所做的众多验证性研究,分析了支持和否定分离现象存在的各类证据,对一些重要结果和观点进行了总结,并由此提出了自己的看法。     

Tuning the brain by learning and by stimulation of the nucleus basalis

Although it is well-established that the cerebral cortex is a substrate for learning, memory and higher cognitive functions, rather less is known about the mechanisms by which experiences are acquired and stored in the cortex. The role of the basal forebrain cholinergic system (BFCS) in learning-induced plasticity is underlined by a recent report by Kilgard and Merzenich[1]. In this article I will discuss the findings of Kilgard and Merzenich in the context of other developments in our understanding of the BFCS and its role in learning-induced plasticity. However, before the discussion I would like to provide some essential background information.