Sensitive periods in the development of the brain and behavior

Experience exerts a profound influence on the brain and, therefore, on behavior. When the effect of experience on the brain is particularly strong during a limited period in development, this period is referred to as a sensitive period. Such periods allow experience to instruct neural circuits to process or represent information in a way that is adaptive for the individual. When experience provides information that is essential for normal development and alters performance permanently, such sensitive periods are referred to as critical periods. Although sensitive periods are reflected in behavior, they are actually a property of neural circuits. Mechanisms of plasticity at the circuit level are discussed that have been shown to operate during sensitive periods. A hypothesis is proposed that experience during a sensitive period modifies the architecture of a circuit in fundamental ways, causing certain patterns of connectivity to become highly stable and, therefore, energetically preferred. Plasticity that occurs beyond the end of a sensitive period, which is substantial in many circuits, alters connectivity patterns within the architectural constraints established during the sensitive period. Preferences in a circuit that result from experience during sensitive periods are illustrated graphically as changes in a ''stability landscape,'' a metaphor that represents the relative contributions of genetic and experiential influences in shaping the information processing capabilities of a neural circuit. By understanding sensitive periods at the circuit level, as well as understanding the relationship between circuit properties and behavior, we gain a deeper insight into the critical role that experience plays in shaping the development of the brain and behavior.

How the brain processes social information: searching for the social brain

Because information about gender, kin, and social status are essential for reproduction and survival, it seems likely that specialized neural mechanisms have evolved to process social information. This review describes recent studies of four aspects of social information processing: (a) perception of social signals via the vomeronasal system, (b) formation of social memory via long-term filial imprinting and short-term recognition, (c) motivation for parental behavior and pair bonding, and (d) the neural consequences of social experience. Results from these studies and some recent functional imaging studies in human subjects begin to define the circuitry of a "social brain." Such neurodevelopmental disorders as autism and schizophrenia are characterized by abnormal social cognition and corresponding deficits in social behavior; thus social neuroscience offers an important opportunity for translational research with an impact on public health.

Metabolic imprinting of choline by its availability during gestation: implications for memory and attentional processing across the lifespan

A growing body of research supports the view that choline is an essential nutrient during early development that has long-lasting effects on memory and attentional processes throughout the lifespan. This review describes the known effects of alterations in dietary choline availability both in adulthood and during early development. Although modest effects of choline on cognitive processes have been reported when choline is administered to adult animals, we have found that the perinatal period is a critical time for cholinergic organization of brain function. Choline supplementation during this period increases memory capacity and precision of the young adult and appears to prevent age-related memory and attentional decline. Deprivation of choline during early development leads to compromised cognitive function and increased decline with age. We propose that this organizational effect of choline availability may be due to relatively permanent alterations in the functioning of the cholinergic synapse, which we have called 'metabolic imprinting'.

Mechanisms of avian imprinting: a review

Filial imprinting is the process through which early social preferences become restricted to a particular object or class of objects. Evidence is presented showing that filial preferences are formed not only as a result of learning through exposure to an object, but also under the influence of visual and auditory predispositions. The development of these predispositions is dependent upon certain non-specific experience. There is little evidence for an endogenously affected sensitive period for imprinting. It is more likely that the end of sensitivity is a result of the imprinting process itself. Similarly, it is now firmly established that filial and sexual preferences are reversible. Evidence suggests, however, that the first stimulus to which the young animal is exposed may exert a greater influence on filial preferences than subsequent stimuli. The learning process of imprinting is often regarded as being different from conventional associative learning. However, the imprinting object itself can function as a reinforcer. Recent studies have attempted to test predictions from an interpretation of filial imprinting as a form of associative learning. The first results suggest that 'blocking' may occur in imprinting, whilst there is no evidence for 'overshadowing'. Social interactions with siblings and parent(-surrogates) have been shown to affect the formation of filial and sexual preferences. The influence of these interactions is particularly prominent in sexual imprinting, making earlier claims about naïve species-specific biases unlikely. Although auditory stimuli play an important role in the formation of social attachments, there is little evidence for auditory imprinting per se. Auditory preferences formed as a result of mere (pre- or postnatal) exposure are relatively weak and short-lasting. Exposure to visual stimuli during auditory training significantly improves auditory learning, possibly through a process of reinforcement. It is becoming increasingly clear that filial and sexual imprinting are two different (although perhaps analogous) processes. Different mechanisms are likely to underlie the two processes, although there is evidence to suggest that the same brain region is involved in recognition of familiar stimuli in both filial and sexual imprinting. There is little evidence for a direct role of hormones in the learning process of imprinting. Androgen metabolism may be a factor constraining the development of a predisposition in the chick. Research into the neural mechanism of filial imprinting in the chick has revealed that a restricted part of the forebrain (IMHV) is likely to be a site of memory storage.(ABSTRACT TRUNCATED AT 400 WORDS)

Imprinting, learning and development: from behaviour to brain and back

Neural and behavioural analyses have shown that the formation of filial preferences in young, precocial birds involves at least two separate processes. One process is an emerging predisposition to approach stimuli with the characteristics of the natural mother. The other (learning) process of filial imprinting results in chicks preferentially-approaching a stimulus to which they have been exposed and involves forming links between the components of the exposed stimulus. The neural substrate for the predisposition is different from that underlying imprinting, and different regions of the chick brain are involved in distinct aspects of learning about imprinting stimuli.

Effects of restricted lesions of the chick forebrain on the acquisition of filial preferences during imprinting

The effects of placing bilateral lesions in that part of the chick brain (IMHV) which was previously been implicated in imprinting, was studied in young domestic chicks. Twenty-four dark-reared chicks were matched in pairs on the basis of their approach activity during a 30 min period of exposure to one of two visual imprinting stimuli. Both members of the chick pair were then anaesthetized and bilateral lesions were made by radio-frequency coagulation in the IMHV of one chick; the other chick served as a sham-operated control. On the following day each chick was exposed for 2.5 h to the imprinting stimulus to which it had previously been exposed. After training, the preferences of all chicks were measured by comparing their approach to the training stimulus with that to the second stimulus. Sham-operated chicks showed a strong preference for the training stimulus; lesioned chicks showed none. Subsequently the latency of each chick to approach and accurately peck a shiny rod was measured. The two groups of chicks did not differ significantly in this test of visuomotor coordination. The area of tissue damaged by the lesion was reconstructed: IMHV was severely damaged with relatively little damage to other areas of the brain.

Visual imprinting and the neural mechanisms of recognition memory

To understand the neural bases of memory it is necessary to localize the regions storing information. Part of the hyperstriatum ventrale (IMHV) serves such a function for the learning process of imprinting in domestic chicks. Chicks exposed to an object learn its characteristics, and in doing so, the responsiveness of IMHV neurones to that object is selectively enhanced. Imprinting is associated with both pre- and postsynaptic changes in the region. Postsynaptic changes involve increases in the length of the postsynaptic density on dendritic spines and in the numbers of NMDA receptors; presynaptically, converging evidence points to an early and persistent enhancement of neurotransmitter release. Increases in the amounts of certain neural cell adhesion molecules a day after training might serve to stabilize the synaptic changes associated with a particular memory by strengthening pre- to postsynaptic adhesion, and by more strongly interconnecting the cytoskeletal frameworks of the dendritic spine and the synaptic terminal. Learning-related increases in the number of neurones staining positive for the transcription factor Fos in the IMHV give promise of identifying the neurones engaged in memory functions and of analysing their connections.

Social visual engagement in infants and toddlers with autism: early developmental transitions and a model of pathogenesis

Efforts to determine and understand the causes of autism are currently hampered by a large disconnect between recent molecular genetics findings that are associated with the condition and the core behavioral symptoms that define the condition. In this perspective piece, we propose a systems biology framework to bridge that gap between genes and symptoms. The framework focuses on basic mechanisms of socialization that are highly-conserved in evolution and are early-emerging in development. By conceiving of these basic mechanisms of socialization as quantitative endophenotypes, we hope to connect genes and behavior in autism through integrative studies of neurodevelopmental, behavioral, and epigenetic changes. These changes both lead to and are led by the accomplishment of specific social adaptive tasks in a typical infant's life. However, based on recent research that indicates that infants later diagnosed with autism fail to accomplish at least some of these tasks, we suggest that a narrow developmental period, spanning critical transitions from reflexive, subcortically-controlled visual behavior to interactional, cortically-controlled and social visual behavior be prioritized for future study. Mapping epigenetic, neural, and behavioral changes that both drive and are driven by these early transitions may shed a bright light on the pathogenesis of autism.

Learning and memory: regional changes in N-methyl-D-aspartate receptors in the chick brain after imprinting

An extensive series of experiments has implicated a restricted region of the chick forebrain in the learning process of imprinting. The region is the intermediate and medial part of the hyperstriatum ventrale (IMHV). Previous studies have shown that training is associated with an increase in the area of the postsynaptic density of axospinous synapses in the left but not the right IMHV. The postsynaptic density is a site of high receptor density, and at least some axospinous synapses are excitatory. We found that imprinting is associated with a 59% increase in N-methyl-D-aspartate-sensitive binding of the excitatory amino acid L-[3H]glutamic acid in the left IMHV. The increase is probably due to an increased number of binding sites. The profile of sensitivity of the sites to a series of amino-, phosphono-substituted carboxylic acids (2-amino-3-phosphonopropionate to 2-amino-8-phosphonooctanoate) is characteristic of N-methyl-D-aspartate-type receptors. There were no significant effects of training on binding in the right IMHV. The effect of training on left IMHV binding could not be attributed to light exposure, arousal, or motor activity per se but was a function of how much the chicks learned. The changes in the left IMHV could increase the effectiveness of synaptic transmission in a region crucial for information storage and so form a neural basis for recognition memory.

The importance of olfactory bulb noradrenalin for maternal recognition in sheep

Herds of grazing mammals characteristically produce precocial offspring in synchrony, and it is therefore important for the mother to form a rapid recognition of her own offspring to distinguish them from others. In sheep, the ewe forms such a selective bond with her lamb within 2-4 hours of parturition, a bond which is primarily dependent on olfactory sensory recognition. Here we report that the neuronal mechanism whereby the olfactory "imprint" is made is dependent on the centrifugal noradrenergic projections to the olfactory bulbs. Lesioning of this neural pathway prevents the ewe from forming a selective bond with her own lamb, enabling her to adopt alien lambs.

Learning-related changes in Fos-like immunoreactivity in the chick forebrain after imprinting

The intermediate and medial part of the hyperstriatum ventrale (IMHV) is a part of the chick forebrain that is critical for the learning process of imprinting and may be a site of information storage. Chicks were either trained on an imprinting stimulus or dark-reared. Trained chicks were classified as good or poor learners by their preference score (a measure of the strength of imprinting). A monoclonal antibody against the immediate early gene product Fos was applied to sections through IMHV and other forebrain regions. In the IMHV, significantly more immunopositive nuclei were counted in good learners than in poor learners or dark-reared chicks. There was a positive correlation between counts of labeled nuclei and preference score that was not attributable to sensory activity per se, locomotor activity during training, or a predisposition to learn well; rather, the results indicated that the change in Fos immunoreactivity in the IMHV was related to learning. In the hyperstriatum accessorium, significantly fewer immunopositive nuclei were counted in good learners than in poor learners or in dark-reared chicks. In the dorsolateral hippocampal region, more immunopositive nuclei were counted in trained than in dark-reared chicks. No significant effects of training were found in the anterior hyperstriatum ventrale, lobus parolfactorius, neostriatum, medial hippocampal region, or ventrolateral hippocampal region, but counts in this last region were positively correlated with training approach. The results for IMHV implicate Fos or Fos-related proteins in memory processes and pave the way for the identification of the cell types that show the learning-related increase in gene expression.

Unique neural circuitry for neonatal olfactory learning

Imprinting ensures that the infant forms the caregiver attachment necessary for altricial species survival. In our mammalian model of imprinting, neonatal rats rapidly learn the odor-based maternal attachment. This rapid learning requires reward-evoked locus ceruleus (LC) release of copious amounts of norepinephrine (NE) into the olfactory bulb. This imprinting ends at postnatal day 10 (P10) and is associated with a dramatic reduction in reward-evoked LC NE release. Here we assess whether the functional emergence of LC alpha2 inhibitory autoreceptors and the downregulation of LC alpha1 excitatory autoreceptors underlie the dramatic reduction in NE release associated with termination of the sensitive period. Postsensitive period pups (P12) were implanted with either LC or olfactory bulb cannulas, classically conditioned with intracranial drug infusions (P14), and tested for an odor preference (P15). During conditioning, a novel odor was paired with either olfactory bulb infusion of abeta-receptor agonist (isoproterenol) to assess the target effects of NE or direct LC cholinergic stimulation combined with alpha2 antagonists and alpha1 agonists in a mixture to reinstate neonatal levels of LC autoreceptor activity to assess the source of NE. Pups learned an odor preference when the odor was paired with either olfactory bulb isoproterenol infusion or reinstatement of neonatal LC receptor activity. These results suggest that LC autoreceptor functional changes rather than olfactory bulb changes underlie sensitive period termination.

An autoradiographic study of the chick brain after imprinting

On the first day after hatching domestic chicks were exposed to an imprinting stimulus, a horizontal yellow slit of light moving upwards in a window and presented at a rate of 4 slits/sec. Chicks were exposed for either 45 min (undertrained) or 180 min (overtrained) on the first day of hatching (60 or 240 min in the case of 1 pair). On the second day all birds were exposed for a further 63 min. Twenty birds were matched in pairs and each chick received 1.1 muCi [14C]uracil/g body weight injected into the heart region before exposure on day 2. At 150 min after the injection the chicks were decapitated and serial coronal sections of their brains cut; alternate pairs of sections were prepared for autoradiography. The optical density for a number of major anatomical regions was measured. The measurements for each region were averaged over congruent to 0.6 mm 'slabs' of brain and expressed as a percentage of the mean of all measurements for that brain. Standardized mean optical density was significantly greater in undertrained chicks than in overtrained chicks in a part of the medial region of hyperstriatum ventrale (MHV) which extended across two adjacent slabs. The slabs were slightly posterior to the mid-point between anterior and posterior poles of the brain. In the next two, adjacent, anterior slabs the variances for MHV were significantly greater in undertrained chicks than in overtrained chicks. There were no other significant differences between brain regions. Taken together with previous studies these results suggest that the intermediate and medial part of hyperstriatum ventrale is intimately linked with the imprinting process.

Auditory imprinting leads to differential 2-deoxyglucose uptake and dendritic spine loss in the chick rostral forebrain

Newly hatched chicks of the domestic fowl (White Leghorn) were imprinted to an acoustic stimulus (Group I: 400 Hz, 3 bursts per s, Group II: 900 Hz, 2 bursts per s) and tested in a straight runway with loudspeakers behind two opposite goal boxes. Those chicks were considered imprinted which headed for the imprinting stimulus in an approach test and subsequently preferred it to a new stimulus (imprinting stimulus of the other group) in a simultaneous discrimination test. On day 7 after hatching (after the sensitive phase) imprinted chicks and naive controls were injected with 2-[14C]deoxyglucose (2DG) and exposed to the imprinting stimulus. Autoradiographic analysis of their brains revealed 3 well demarcated areas of increased 2DG accumulation in the rostral forebrain of imprinted chicks compared to controls: HAD in the rostral Wulst; MNH, an auditory area in the rostromedial neostriatum and hyperstriatum ventrale; LNH in the rostrolateral neostriatum and hyperstriatum ventrale. Analysis of these brain areas in 7-day-old acoustically imprinted and control animals with the Golgi-Cox method revealed a highly significant reduction of spine frequency of a large neostriatal neuron type in MNH of imprinted chicks. An additional Golgi-Cox analysis was carried out with chicks imprinted on a broody hen, i.e. on the whole spectrum of natural stimuli. In that group spine frequency of the same neuron type was between that of acoustically imprinted and control animals. A hypothesis of filial imprinting is presented which considers spine loss as a crucial mechanism.

Olfactory lateralization in the chick

Chicks using their right nostril (and so with direct olfactory input to the right hemisphere), and presented simultaneously with two objects identical in visual appearance with the rearing object, and differing only in odour, chose that which smelled like the rearing object. Chicks using the left nostril chose equally readily but at random. Earlier work, using similar tests, has shown special interest of the right hemisphere in change in visual properties of familiar stimuli, suggesting that analysis of a wide range of properties of a familiar stimulus may be an important function of the right hemisphere in the chick, with consequent detection of novelty.

Developmental emergence of fear learning corresponds with changes in amygdala synaptic plasticity

Mother-infant attachment is facilitated in altricial rodents through unique neural mechanisms that include impaired neonatal fear conditioning until the time that pups first begin to leave the nest (sensitive period). Here, we confirmed the developmental emergence of odor fear conditioning in neonatal rat pups, and examined synaptic plasticity of inputs to the basolateral amygdala in vitro. Coronal slices through the amygdala were obtained from sensitive (<10 days) and post-sensitive (>10, <19 days) period pups. Field potentials were recorded in the basolateral amygdala in response to stimulation of either the external capsule (neocortical inputs) or fibers from the cortical nucleus of the amygdala (olfactory inputs). The effects of tetanic stimulation were examined in each pathway. In both pathways, tetanic stimulation induce significant long-term synaptic plasticity in post-sensitive period pups, but no significant plasticity in sensitive period pups incapable of learning odor aversions. GABA(A) receptor blockade in post-sensitive period slices reverts synaptic plasticity to sensitive period characteristics. The results suggest that sensitive period deficits in fear conditioning may be related to impaired amygdala synaptic plasticity and the immature state of GABAergic inhibition and/or its modulation in the neonatal amygdala.

Detour behaviour, imprinting and visual lateralization in the domestic chick

Detour behaviour was studied in chicks faced with a vertical-bar barrier behind where an imprinting object (a red ball) was located. Right-eyed chicks took less time to detour the barrier than left-eyed chicks, and binocular chicks showed a bias to detour the barrier on the left side, thus maintaining visual contact with the imprinting object using the lateral field of the right eye, while circling around the barrier. In males, the asymmetries were consistent all along the first two weeks of life, whereas in females they disappeared on days 8 and 11. When tested with a slightly novel version of the original imprinting object (i.e., a ball of a different color), binocular chicks showed a bias to detour the barrier on the right side, thus showing preferential use of the left eye. The same bias occurred when unfamiliar conspecifics were used as goal-objects. Results suggest that cerebral lateralization in birds can directly affect visually-guided motor responses through selective use of the lateral field of vision of the eye contralateral to the hemisphere which has to be put in charge of control of overt behaviour.

Modular organisation and spinal somatosensory imprinting

The withdrawal reflex system has been extensively used as a model system for studies of pain related mechanisms, sensorimotor integration, learning and memory. For a long time, this system was assumed to be organised as a flexion reflex system. However, recent studies indicate that this system has a modular organisation, each module performing a detailed and functionally adapted sensorimotor transformation related to the withdrawal efficacy of its output muscle(s). Each module appears to be a self-organising circuitry that uses sensory feedback on single muscle contractions to adjust its synaptic organisation during development. These findings and their implications for the understanding of higher motor functions as well as clinical aspects will be discussed.

Thyroid hormone determines the start of the sensitive period of imprinting and primes later learning

Filial imprinting in precocial birds is the process of forming a social attachment during a sensitive or critical period, restricted to the first few days after hatching. Imprinting is considered to be part of early learning to aid the survival of juveniles by securing maternal care. Here we show that the thyroid hormone 3,5,3'-triiodothyronine (T(3)) determines the start of the sensitive period. Imprinting training in chicks causes rapid inflow of T(3), converted from circulating plasma thyroxine by Dio2, type 2 iodothyronine deiodinase, in brain vascular endothelial cells. The T(3) thus initiates and extends the sensitive period to last more than 1 week via non-genomic mechanisms and primes subsequent learning. Even in non-imprinted chicks whose sensitive period has ended, exogenous T(3) enables imprinting. Our findings indicate that T(3) determines the start of the sensitive period for imprinting and has a critical role in later learning.