Experience and plasticity in the central nervous system

Visual Imprinting in Birds: Behavior, Models, and Neural Mechanisms

Filial imprinting is a process, readily observed in precocial birds, whereby a social attachment is established between a young animal and an object that is typically (although not necessarily) a parent. During a perinatal sensitive period, the young animal learns characteristics of the object (the imprinting stimulus) simply by being exposed to it and will subsequently recognize and selectively approach this stimulus. Imprinting can thus establish a filial bond with an individual adult: a form of social cohesion that may be crucial for survival. Behavioral predispositions can act together with the learning process of imprinting in the formation, maintenance, and modification of the filial bond. Memory of the imprinting stimulus, as well as being necessary for social recognition, is also used adaptively in perceptual classification of sensory signals. Abstract features of an imprinting stimulus, such as similarity or difference between stimulus components, can also be recognized. Studies of domestic chicks have elucidated the neural basis of much of the above behavior. This article discusses (1) principal behavioral characteristics of filial imprinting and related predispositions, (2) theoretical models that have been developed to account for this behavior, and (3) physiological results elucidating the underlying neural mechanisms. Interactions between these different levels of analysis have resulted in advancement of all of them. Taken together, the different approaches have helped define strategies for investigating mechanisms of learning, memory, and perception.

Why are individuals so different from each other?

An important contributor to the differences between individuals derives from their plasticity. Such plasticity is widespread in organisms from the simple to the most complex. Adaptability plasticity enables the organism to cope with a novel challenge not previously encountered by its ancestors. Conditional plasticity appears to have evolved from repeated challenges from the environment so that the organism responds in a particular manner to the environment in which it finds itself. The resulting phenotypic variation can be triggered during development in a variety of ways, some mediated through the parent's phenotype. Sometimes the organism copes in suboptimal conditions trading off reproductive success against survival. Whatever the adaptedness of the phenotype, each of the many types of plasticity demonstrates how a given genotype will express itself differently in different environmental conditions-a field of biology referred to as the study of epigenetics. The ways in which epigenetic mechanisms may have evolved are discussed, as are the potential impacts on the evolution of their descendants.

Thirty years of collaboration with Gabriel Horn

All the collaborative work described in this review was on the process of behavioural imprinting occurring early in the life of domestic chicks. Finding a link between learning and a change in the brain was only a first step in establishing a representation of the imprinting object. A series of overlapping experiments were necessary to eliminate alternative explanations. Once completed, a structure, the intermediate and medial mesopallium (IMM), was found to be strongly linked to the formation of a neural representation of the object used for imprinting the birds. With the site identified, lesion experiments showed that it was necessary for imprinting but not associative learning. Also the two sides of the brain responded differently with the left IMM acting as a permanent store and the right side acting as a way station to other parts of the brain. The collaborative work led to many studies by Gabriel Horn with others on the molecular and cellular bases of imprinting, and also to neural net modelling and behavioural studies with me on the nature of category formation in intact animals.

Analysis of differential gene expression supports a role for amyloid precursor protein and a protein kinase C substrate (MARCKS) in long-term memory

Previous work has identified the intermediate and medial part of the hyperstriatum ventrale (IMHV) as a region of the chick brain storing information acquired through the learning process of imprinting. We have examined in this brain region changes in expression of candidate genes involved in memory. Chicks were exposed to a rotating red box and the strength of their preference for it, a measure of learning, determined. Brain samples were removed approximately 24 h after training. Candidate genes whose expressions were different in IMHV samples derived from strongly imprinted chicks relative to those from chicks showing little or no learning were identified using subtractive hybridization. The translation products of two candidate genes were investigated further in samples from the left and right IMHV and from two other brain regions not previously implicated in imprinting, the left and right posterior neostriatum. One of the proteins was the amyloid precursor protein (APP), the other was myristoylated alanine rich C kinase substrate (MARCKS). In the left IMHV the levels of the two proteins increased with the strength of learning. The effects in the right IMHV were not significantly different from those in the left. There were no effects of learning in the posterior neostriatum. This is the first study to relate changes in the amounts of MARCKS and APP proteins to the strength of learning in a brain region known to be a memory store and demonstrates that the systematic identification of protein molecules involved in memory formation is possible.

Is imprinting such a special case?

As the result of relatively brief exposure to a particular type of object early in life, many birds and mammals will form strong and exclusive attachments to that object. This is known as ‘filial imprinting’. Early experience can also have long-lasting effects on sexual preferences, but the conditions are different from those in which the first attachments are formed. Some of the characteristics of imprinting are undoubtedly because of the naive animal searching for and responding selectively to particular stimuli. But that is not all. At least two types of plastic change seem to be involved: establishing an internal representation of the familiar object and pre-emptive capturing by that representation of the systems controlling filial behaviour and, much later in development, sexual behaviour. The second plastic change is likely to generate the phenomenon of a sensitive period and gives the formation of social attachments some of its other peculiar properties. The first change is likely to be the process used in most forms of recognition. Distinguishing between the sub-processes that underlie an overall change in behaviour serves to make some overdue links between different areas of knowledge about learning which have hitherto been poorly connected.

Electroconvulsive stimulations, learning, and protein changes in the rat brain

Two groups of rats were subjected to 17 training sessions on an operant task demanding the sequential operation of two manipulanda, while two other groups were left with no training experience. Within both the trained and passive groups one was exposed to a series of 12 electroconvulsive stimulations. The series of training and stimulation sessions were concurrent but arranged in such a way that at least 24 h always separated training and stimulation. Upon completion of the behavioural part of the experiment the concentrations of the marker proteins neural cell adhesion molecule (NCAM), D3, synaptophysin, and S100 were estimated in the prefrontal and occipital parts of the cortex, the hippocampus, and in the total forebrain. The electroconvulsively stimulated animals demonstrated severe impairment of learning. The pattern of marker protein concentrations indicated that acquisition and/or performance of the task and exposure to electroconvulsive stimulation were both accompanied by similar patterns of synaptic changes: an increased concentration of small synaptic vesicles in both the prefrontal cortex and the total forebrain and an increased synaptic remodulation in the prefrontal cortex.

Learning-dependent Changes in the Responses to Visual Stimuli of Neurons in a Recognition Memory System

When young chicks are trained by exposing them to a conspicuous object they learn its characteristics. The learning process is known as imprinting. In the present study neuronal activity in a region crucial for imprinting was shown to be affected by training and by the object on which the chicks had been trained. The region is the intermediate and medial part of the left hyperstriatum ventrale (left IMHV). No such effects were found in a visual projection area, the left hyperstriatum accessorium. Domestic chicks were imprinted on either a rotating red box (n=7 chicks) or a rotating blue box (n=8). When the chicks were approximately 48 h old they were anaesthetized and multiple-unit activity was recorded in simultaneous, single penetrations through each of the two regions. Records were also made from eight dark-reared chicks. Whilst recording, the red or blue box, placed in front of the contralateral eye, was switched on to give a total of 20 rotations, the interval between each rotation being 10 s. The alternative stimulus was then presented 20 times. Unit activity in the 3 s before and after stimulus onset was compared and the data for each of the 20 presentations were combined. In the left IMHV 18 out of a total of 115 recording sites (16%) responded significantly to the stimuli; in the left hyperstriatum accessorium 39 out of 126 recording sites (26%) did so. Measures of unit activity at each recording site were combined for a given penetration to provide a 'mean penetration response'. The response to the red box differed from the response to the blue box in the left IMHV of dark-reared chicks. After training with the blue box the response to both boxes was similar to the response to the blue box in dark-reared birds. After training with the red box the response to both boxes was similar to the response to the red box in dark-reared birds. No significant effects were found in the left hyperstriatum accessorium. The two training boxes were virtually identical apart from the differences in colour and brightness. Training appeared to stabilize the response of the visually naive left IMHV to the training stimulus whilst changing its response to the alternative, but similar stimulus. That is, one consequence of training is that the two stimuli are placed in the same category, and this neural change may provide a basis for stimulus generalization. The underlying neural system is modelled and a mechanism that allows such stimuli to be discriminated is proposed.

Neural bases of recognition memory investigated through an analysis of imprinting

Through a learning process known as imprinting, the young of some animals, including the domestic chick, come to recognize an object by being exposed to it. Visually naive chicks vigorously approach a wide range of objects. After an adequate period of exposure to one object chicks selectively approach it in a recognition test. The nervous system of dark-reared chicks is not a tabula rasa, as chicks have predispositions to approach some stimuli rather than others. Nevertheless, visual imprinting leads to changes in a nervous system that may not have been 'marked' by previous visual experience, and so encourages the hope of discovering the neural bases of the learning process. The intermediate and medial part of the hyperstriatum ventrale, a sheet of cells within the cerebral hemispheres, plays a crucial role in visual imprinting, particularly in the memory process of recognition. The cellular and sub-cellular changes that take place in this part of the hyperstriatum ventrale after imprinting are described. The right and left hyperstriatum ventrale regions play different roles in the imprinting process, and evidence is given for the existence of multiple memory systems in the chick brain.

Altered sensitivities of auditory neurons in the rat midbrain following early postnatal exposure to patterned sounds

Pure tone sensitivity of inferior colliculus (IC) neurons in adult rats was studied electrophysiologically following exposure to a frequency-modulated (FM) tone during the first 5 postnatal weeks. The distribution of best frequencies (BF) and minimum thresholds (MT) of 274 single units, when compared to the control, showed an abnormal clustering centered around the region of the audiogram occupied by the FM tone to which the rats had been exposed. This effect was interpreted as the result of activity-dependent changes of IC during development.

Contraction of neuronal branching volume: an anatomic correlate of Pavlovian conditioning

Associative memory of the mollusc Hermissenda crassicornis, previously correlated with changes of specific K+ currents, protein phosphorylation, and increased synthesis of mRNA and specific proteins, is here shown to be accompanied by macroscopic alteration in the structure of a single identified neuron, the medial type B photoreceptor cell. Four to five days after training, terminal arborizations of B cells iontophoretically injected with Ni2+ ions and then treated with rubeanic acid were measured with charge-coupled device (CCD)-digitized pseudocolor images of optical sections under "blind" conditions. Boundary volumes enclosing medial-type B-cell arborizations from classically conditioned animals were unequivocally reduced compared with volumes for naive animals or those trained with unpaired stimuli. Branch volume magnitude was correlated with input resistance of the medial type B-cell soma. Such associative learning-induced structural changes may share function with "synapse elimination" described in developmental contexts.

Memory systems in the chick: dissociations and neuronal analysis

Young domestic chicks learn to recognize the visual characteristics of an object to which they are exposed. A restricted part of the forebrain, the intermediate and medial part of the hyperstriatum ventrale (IMHV) is implicated in this process. This form of exposure learning can be dissociated from (i) the ability to learn certain procedures or skills, and (ii) a predisposition to attend to particular types of naturalistic objects. The first of these dissociations is reminiscent of that found in certain human organic amnesias, whilst the second may have its counterpart in the processes involved in face recognition by infants. The right and left IMHV play different roles in the memory that underlies imprinting. The cellular and molecular processes involved in this form of memory are discussed.

Ontogenetic changes in [3H]-spiroperidol binding sites in posthatch chick brain

The ontogenetic development of [3H]-spiroperidol binding sites was measured in the optic tectum, cerebellum, forebrain base, and forebrain roof of 1-, 4-, and 16-day-old chicks. In the chick optic tectum and cerebellum both the density and the total number of [3H]-spiroperidol binding sites increased from 4- to 16-days-posthatch, but no significant differences were found in either brain area across the initial four posthatch days. In the forebrain base, [3H]-spiroperidol receptor density and total binding increased significantly between 1- and 4-days-posthatch, but at 16-days-posthatch there was a slight decrease in receptor density. Binding sites in the forebrain roof were minimal at all ages. As expected, saturation experiments yielded curvilinear plots indicating the presence of high- and low-affinity binding sites. The high-affinity sites probably reflect dopamine D-2 receptors; whereas, the low-affinity sites may reflect other receptor types, possibly serotonin S-2. These results suggest that large doses of haloperidol, which are normally used in chick behavioral research, may produce behavioral effects by antagonizing multiple receptors.

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.

Calcium-mediated reduction of ionic currents: a biophysical memory trace

Learning behavior similar to vertebrate classical conditioning was demonstrated for the mollusc Hermissenda crassicornis. Postsynaptic membrane changes within well-defined neural systems that mediate the learning play a casual role in recording the learned association for later recall. Specific ionic currents in neural tissue undergo transformations lasting days after associative training with physiologic stimuli. During acquisition the intracellular calcium increases; this increase is accompanied by specific potassium current reduction that lasts for days after conditioning. The increase of calcium enhances calmodulin-dependent phosphorylation of proteins that either regulate or are part of ion channels. These currents and the conditions that precede their transformation occur in many types of vertebrate neurons, and hence this biophysical basis of Hermissenda learning could have relevance for species other than the gastropod studied.

Interocular transfer of pattern discrimination learning in chicks

A previous study found that chicks pecking a key for heat did not show interocular transfer of a pattern discrimination, indicating that the monocularly acquired discrimination was stored as a unilateral engram which was not available to the untrained eye/hemisphere system. In the present study, chicks were trained monocularly on a pattern discrimination and tested for interocular transfer exactly as in the previous experiment, except that a correct pecking response was reinforced by presentation of food. There was good interocular transfer of the discrimination under these conditions. These results are interpreted as indicating that the biological relevance of a learning situation influences the extent of interocular/interhemispheric communication of information. In addition to the findings with respect to transfer, the present study revealed some unexpected laterality effects. Chicks trained first through the right eye (left hemisphere) learned the pattern discrimination faster and showed more savings during the interocular transfer test than chicks trained first through the left eye (right hemisphere). These findings are discussed in terms of possible hemispheric specialization and asymmetry of interhemispheric communication in the avian brain.

Imprinting and cortical plasticity: a comparative review

Results of research on imprinting and developmental neurobiology of the visual cortex are compared to evaluate the evidence for or against a frequently hypothesized linkage of the two phenomena. The comparison reveals striking similarities. In both paradigms a sensitive period exists. Once this sensitive period is over, the storage of early influences from the environment remains stable throughout life. Storage of "natural" stimuli is facilitated by a certain preorganisation of the receiving brain areas. It is stated that the two phenomena are not directly linked, but are two expressions of a developmental process, which may be common for the organisation of the connectivity of single cells as well as for complex neuronal networks as they are likely to be involved in imprinting. This process is basically self-organizing, but can be influenced by superimposed controls. Differences of the stability of storage of external influences might be explained by the difference in the overall amount of morphological alterations, which is large in the young and small in the adult animal. This holds for both the modifiability in the visual cortex and imprinting.

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.

Imprinting. An electron microscopic study of chick hyperstriatum ventrale

Fourteen chicks were hatched and reared in darkness to approximately equal to 21 h when they were exposed to overhead illumination for 0.5 h and then to an imprinting stimulus (a pulsing red light) for 20 min. The chicks were then matched in pairs on the basis of their activity. One member of each pair was returned to the dark and the other was trained for a further 120 min. All chicks were killed approximately equal to 6.5 h after the onset of training and perfused with fixative. Blocks were removed bilaterally from a restricted part of the hyperstriatum ventrale and prepared for electron-microscopy. Various synaptic features were measured, all counts being performed 'blind'. In undertrained chicks the length of the synaptic apposition zone in this part of the left hemisphere was shorter than that in the right by a mean value of 35 +/- 11.4 (SEM) nm. Further training eliminated this difference. No other synaptic measurements were affected by prolonging the period of training.

Effects of rhythmic hyperstriatal stimulation on chicks' preferences for visual flicker

Chicks' preferences for the frequency of a flashing light can be modified by electrically stimulating the medial part of the hyperstriatum ventrale with trains of shocks at that frequency. Chicks stimulated with 1.5 pulse trains s-1 preferred a light flashing at 1.5 s-1 and chicks stimulated at 4.5 trains s-1 preferred a light flashing at 4.5 s-1. Three behavioural measures taken during the preference test, namely measures of movement, latency to approach and distress calls were not significantly influenced by the frequency of brain stimulation.

Role of RNA and protein synthesis in memory formation

A brief review is given of experiments which are concerned with the hypothesis that brain RNA and protein synthesis are directly involved in the establishment of long-term memory. It is concluded that these experiments neither support or refute this hypothesis. A convincing demonstration is lacking of interanimal memory transfer by injection of macromolecular extracts. The majority of experiments which attempt to correlate increased macromolecular synthesis with learning use radioactive precursor methods and these studies do not exclude possible changes in precursor specific activity as the cause of the increased labeling. Although some studies find directly observable changes in brain macromolecules in response to training, their relationship to memory formation is unclear. It is possible that these changes represent only an enhanced production of constitutive macromolecules in response to an increase in cerebral metabolism during training, rather than molecular changes that are directly involved with modifying synaptic connectivity. Inhibitors of cerebral protein synthesis block memory formation, but these drugs are not pharmacologically specific and this complicates the interpretation of these studies.

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.

Biochemical effects of cycloheximide in developing chick brain

The timecourses of inhibition of protein synthesis in forebrain roof 15 min to 24 hr after either intracerebral or peripheral injection of 40 microgram of cycloheximide (CXM), show maximum inhibition (68--85%) 1 hr after injection in 12 hr, 2-day- and 16-day-old chicks. In 2-day-old chicks, the level of free lysine was elevated by around 250% 1 hr after intracerebral injection of CXM. The total radioactivity in the forebrain roof/mg protein following peripheral injection of labelled lysine increased by around 80% compared to saline controls. Following unilateral injection of 20 microgram in 25 microliter CXM into a forebrain hemisphere, the inhibition of protein synthesis spread to the opposite forebrain hemisphere and both optic lobes within 5 min. Smaller volumes did not seem to give a clear unilateral inhibition capable of explaining the unilateral behavioural differences which have been reported. Central injection of CXM in 2- and 16-day-old chicks, resulted in an inhibition of liver protein synthesis of 55% and 40%, respectively. There were also gross signs of pathological effects in the liver and gall bladder. The implications of these effects of CXM are discussed in terms of the use of the drug in behavioural experiments.

Constant and flickering light stimulations produce similar effects on RNA content in visual cells

Adult male rats were illuminated for 2 h with a constant or flickering light of 40 Lx intensity; frequency of flickering was 2 Hz. By means of two-wave-length visible cytospectrophotometry of gallocyanin-stained sections, it was shown that the light stimulation resulted in a marked RNA accumulation in retina ganglion neurons and in the neurons of all the cell layers of visual cortex (with the only exception of the layer VI). In the cells of perineuronal glia, a decrease of the RNA content per cell was found in the retina while no changes were observed in the visual cortex. Effects of constant and flickering light stimulations were qualitatively and quantitatively similar.

Incorporation of amino acids into protein in different brain areas of rat, subjected to enriched and restricted environment

The incorporation of [14C]valine into acid-insoluble brain material from rats living in enriched and restricted environment, respectively, was studied. Quantitative estimations showed higher incorporation into brain protein of frontal, entorhinal, cerebellar and visual cortex and the hippocampus in enriched environment animals compared to restricted environment animals. This finding was confirmed by autoradiography.