Tracking memory's trace

Abstract

There is strong converging evidence that the intermediate and medial part of the hyperstriatum ventrale of the chick brain is a memory store for information acquired through the learning process of imprinting. Neurons in this memory system come, through imprinting, to respond selectively to the imprinting stimulus (IS) neurons and so possess the properties of a memory trace. Therefore, the responses of the intermediate and medial part of the hyperstriatum ventrale neurons to a visual imprinting stimulus were determined before, during, and after training. Of the total recorded population, the proportions of IS neurons shortly after each of two 1-h training sessions were significantly higher (approximately 2 times) than the pretraining proportion. However, Ϸ4.5 h later this proportion had fallen significantly and did not differ significantly from the pretraining proportion. Nevertheless, Ϸ21.5 h after the end of training, the proportion of IS neurons was at its highest (approximately 3 times the pretraining level). No significant fluctuations occurred in the proportions of neurons responding to the alternative stimulus. In addition, nonmonotonic changes were found commonly in the activity of 230 of the neurons tracked individually from before training to shortly after the end of training. Thus the pattern of change in responsiveness both at the population level and at the level of individual neurons was highly nonmonotonic. Such a pattern of change is not consistent with simple models of memory based on synaptic strengthening to asymptote. A model is proposed that accounts for the changes in the population responses to the imprinting stimulus in terms of changes in the responses of individual neurons.

A Bird’s Eye View of Sleep-Dependent Memory Consolidation

“…In one study, the maximal proportion of ISselective neurons was observed at 25 h after IS exposure. The largest increase in ISselective neurons occurred between 8 and 25 h after exposure, which indicated a possible role of sleep in the consolidation of the IS memory (Horn et al 2001). Later work confirmed the necessity of sleep.…”

The international society for developmental psychobiology annual meeting symposium: Impact of early life experiences on brain and behavioral development

“…Experimental analysis in an animal model for filial imprinting (Lorenz, 1935;Hess, 1959), that is, the very first neonatal emotional learning event in vertebrates including human and nonhuman primates, which results in the formation of an emotional bond to the mother or caregiver, has revealed dramatic changes of synaptic connectivity in prefrontal forebrain regions in relation to this learning process. Domestic chicks, which were imprinted on an artificial acoustic or visual stimulus representing the mother, showed increased synaptic densities in an associative forebrain region (Horn, Bradley, & McCabe, 1985;Horn, Nicols, & Brown, 2001) but decreased densities of excitatory spine synapses in two other higher associative forebrain regions (Bock & Braun, 1998, 1999aWallhausser & Scheich, 1987). The reduction of spine synapses which was observed after acoustic filial imprinting critically depends on the activation of glutamatergic NMDA receptors (Bock, Schnabel, & Braun, 1997;Bock & Braun, 1999a), and appears to reflect a synaptic network which responds with a potentiated response upon recall of the learned, emotionally relevant stimulus.…”

Birdsong memory and the brain: In search of the template

“…In an earlier electrophysiological study, Horn and collaborators (Horn et al, 2001;Horn, 2004) enquired whether there was evidence for strengthening Hebbian 'neural assemblies' (Hebb, 1949) in the IMM during imprinting. Such neural assemblies would involve functional coupling between neurons that respond to the imprinting stimulus.…”