Perceptual Learning during Filial Imprinting: Evidence from Transfer of Training Studies

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.

Representation in development: from a model system to some general processes

The view that filial imprinting might serve as a useful model system for studying the neurobiological basis of memory was inspired, at least in part, by a simple idea: acquired filial preferences reflect the formation of a memory or representation of the imprinting object itself, as opposed to the change in the efficacy of stimulus-response pathways, for example. We provide a synthesis of the evidence that supports this idea; and show that the processes of memory formation observed in filial imprinting find surprisingly close counterparts in other species, including our own.

Gene expression profile in cerebrum in the filial imprinting of domestic chicks (Gallus gallus domesticus)

In newly hatched chicks, gene expression in the brain has previously been shown to be up-regulated following filial imprinting. By applying cDNA microarrays containing 13,007 expressed sequence tags, we examined the comprehensive gene expression profiling of the intermediate medial mesopallium in the chick cerebrum, which has been shown to play a key role in filial imprinting. We found 52 up-regulated genes and 6 down-regulated genes of at least 2.0-fold changes 3h after the training of filial imprinting, compared to the gene expression of the dark-reared chick brain. The up-regulated genes are known to be involved in a variety of pathways, including signal transduction, cytoskeletal organization, nuclear function, cell metabolism, RNA binding, endoplasmic reticulum or Golgi function, synaptic function, ion channel, and transporter. In contrast, fewer genes were down-regulated in the imprinting, coinciding with the previous data that the total RNA synthesis increased associated with filial imprinting. Our data suggests that the filial imprinting involves the modulation of multiple signaling pathways.

Generalization gradients and representation modes after absolute and relative discrimination learning in young chickens

Perceptual relativity has become a central issue in animal psychophysics. In order to assess how the interplay of training experience and stimulus dimension might affect perceptual relativity, we investigated the role of 'absolute' and 'relative' training on the learning and representation of stimuli from two dimensions that might favor absolute or relative encoding to a different degree. Young chicks learned to discriminate 3D-objects by either color or size. During 'absolute' training always the choice of the same stimulus of a simultaneously presented pair was reinforced, while choice of the larger (smaller) or greener (bluer) stimulus was reinforced during 'relative' training. Overall, discrimination learning was faster with relative training, but size learning profited more from 'relative' training than color learning. Post-training generalization tests revealed a combined effect of training experience and stimulus dimension: a higher amount of absolute encoding occurred with absolute training and color learning, while relative choices prevailed with relative training and size learning.

In-vivo gene transfer into newly hatched chick brain by electroporation

Newly hatched domestic chicks serve as ideal models for studies of the neural basis of behavioral plasticity, particularly for understanding the mechanisms of learning such as filial imprinting. To elucidate the molecular basis and gene functions involved in learning, we developed an in-vivo gene-transfer system in the brain of a living chick using electroporation. When green fluorescent protein-encoding plasmids were transfected to a chick brain, green fluorescence was clearly observed, and expression at the protein level was confirmed by immunoblotting. Most of the transfected brain cells were neuronal cells with dendrites. This neuron-selective electroporation system will facilitate the analysis of gene functions in the living chick brain and provide further clues as to the molecular mechanisms of avian learning.

The evolution of intelligence: adaptive specializations versus general process

Darwin argued that between-species differences in intelligence were differences of degree, not of kind. The contemporary ecological approach to animal cognition argues that animals have evolved species-specific and problem-specific processes to solve problems associated with their particular ecological niches: thus different species use different processes, and within a species, different processes are used to tackle problems involving different inputs. This approach contrasts both with Darwin's view and with the general process view, according to which the same central processes of learning and memory are used across an extensive range of problems involving very different inputs. We review evidence relevant to the claim that the learning and memory performance of non-human animals varies according to the nature of the stimuli involved. We first discuss the resource distribution hypothesis, olfactory learning-set formation, and the 'biological constraints' literature, but find no convincing support from these topics for the ecological account of cognition. We then discuss the claim that the performance of birds in spatial tasks of learning and memory is superior in species that depend heavily upon stored food compared to species that either show less dependence upon stored food or do not store food. If it could be shown that storing species enjoy a superiority specifically in spatial (and not non-spatial) tasks, this would argue that spatial tasks are indeed solved using different processes from those used in non-spatial tasks. Our review of this literature does not find a consistent superiority of storing over non-storing birds in spatial tasks, and, in particular, no evidence of enhanced superiority of storing species when the task demands are increased, by, for example, increasing the number of items to be recalled or the duration of the retention period. We discuss also the observation that the hippocampus of storing birds is larger than that of non-storing birds, and find evidence contrary to the view that hippocampal enlargement is associated with enhanced spatial memory; we are, however, unable to suggest a convincing alternative explanation for hippocampal enlargement. The failure to find solid support for the ecological view supports the view that there are no qualitative differences in cognition between animal species in the processes of learning and memory. We also argue that our review supports our contention that speculation about the phylogenetic development and function of behavioural processes does not provide a solid basis for gaining insight into the nature of those processes. We end by confessing to a belief in one major qualitative difference in cognition in animals: we believe that humans alone are capable of acquiring language, and that it is this capacity that divides our intelligence so sharply from non-human intelligence.

Early learning and the development of filial preferences in the chick

Newly hatched domestic chicks (Gallus gallus domesticus) rapidly form a social preference for a conspicuous stimulus to which they are exposed. The learning process involved is known as filial imprinting. When chicks are exposed to an audio-visual compound stimulus, both auditory and visual learning are enhanced. The enhancement of visual imprinting is virtually abolished when chicks are exposed separately to the auditory element, either before or after training with the audio-visual compound. Simultaneous exposure to the two elements of the compound is superior to sequential exposure in achieving the enhancement of visual learning. These results are unlike Pavlovian conditioning, but are consistent with an interpretation of imprinting as a form of within-event learning, where links are formed between the representations of the elements of the compound, that can be weakened by separate exposure to an element. Apart from imprinting, chicks may show a developing predisposition to approach stimuli resembling conspecifics. The predisposition emerges in dark-reared chicks given some non-specific experience during a sensitive period, and is expressed as a relatively general preference for stimuli with a head and neck region. In the natural situation, the animal's response may be biased by the predisposition, and through imprinting it then learns the characteristics of individuals.

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.

Neurophysiological investigations of a recognition memory system for imprinting in the domestic chick

The responsiveness of neurons in a region of the chick brain involved in the learning process of imprinting, the right intermediate and medial hyperstriatum ventrale (right IMHV), has been investigated in unanaesthetized, trained and untrained chicks. The results demonstrate that neuronal responsiveness in this region reflects a variety of behavioural consequences of imprinting and is markedly altered as a result of the learning process. Groups of chicks (nine in each group) were either dark-reared or trained (imprinted) by exposure to a rotating red box or a rotating blue cylinder. Recordings of single or small groups of neurons were subsequently made from 156 sites in the right IMHV while the 2-day-old chicks were free to move in a running wheel. There was a highly significant increase in the proportion of sites responsive to the stimulus used to train the birds compared to the proportion responsive to that stimulus in dark-reared birds (30 and 9% respectively). These changes were found when either the red box or the blue cylinder was used to train the bird, the changes being similar for both stimuli. There was also a significant increase in the mean magnitude of the change in neuronal activity on stimulus presentation for the training stimulus compared to the same stimulus when not used in the bird's training. No significant effects of the training experience of the chicks were found upon either the magnitude of evoked activity or the proportion of sites responsive to a rotating stuffed jungle fowl or the sound of the maternal call. The presence of the training stimulus was selectively signalled by the response at certain sites. At other sites there was response generalization across stimulus shape or colour. A comparison with results for the left IMHV demonstrates both similarities and differences in neuronal responsiveness between the two regions. In both regions imprinting selectively enhances neuronal responsiveness to the training stimulus. However, for trained birds the mean proportion of sites responding to whichever of the red box or the blue cylinder was not used in the bird's training was significantly lower in the right than the left IMHV. These results are discussed in relationship to previously reported asymmetries in the response of the right and left IMHV regions to imprinting. A model is introduced to explain the physiological findings. The effects of training on right IMHV neuronal function are consistent with a long-term role for this region in the recognition memory of imprinting.