Spike Synchronization in Hippocampal Cultures Using Hebbian Learning

Abstract

Event Abstract Back to Event Spike Synchronization in Hippocampal Cultures Using Hebbian Learning Víctor Lorente Sánchez1*, Jose Manuel Ferrández Vicente1, Felix De La Paz2 and Eduardo Fernández3 1 Universidad Politécnica de Cartagena, Spain 2 UNED, Departamento de Inteligencia Artificial, Spain 3 Universidad Miguel Hernández, Instituto de Bioingeniería, Spain The use of dissociated cortical neurons cultured onto microelectrode arrays represents a useful experimental model to characterize both the spontaneous behaviour of neuronal populations and their activity in response to electrical and pharmacological changes. Learning is a natural process that needs the creation and modulation of sets of associations between stimuli and responses. Many different stimulation protocols have been used to induced changes in the electrophysiological activity of neural cultures looking for achieve learning [1-11] and low-frequency stimulation has brought good results to researchers enhancing bursting activity in cortical cultures [8,9]. Hebbian learning describes a basic mechanism for synaptic plasticity wherein an increase in synaptic efficacy arises from the presynaptic cell's repeated and persistent stimulation of the postsynaptic cell. The theory is commonly evoked to explain some types of associative learning in which simultaneous activation of cells leads to pronounced increases in synaptic strength. Basically the efficiency of a synaptic connection is increased when presynaptic activity is synchronous with post-synaptic activity. In this work, we use different stimulation protocols following Hebb’s Law to create adjacent physical or logical connections between adjacent electrodes in the connectivity graphs. The connected electrodes show synchronized activity before and after a new stimulation is applied. Dissociated cultures of hippocampal CA1-CA3 neurons were prepared from E17.5 sibling embryos. Microelectrode arrays (Multichannel systems, MCS) consisted of 60 TiN/SiN planar round electrodes (200 μm electrode spacing, 30 μm electrode diameter) arrange in a 8x8 grid were used. A total of 24 cultures were used in five experiments of 2-3 weeks duration. In every experiment 4-5 cultures were stimulated with a specific electrical stimulation protocol. Low frequency current stimulation and tetanization voltage stimulation were the main protocols used in this study. Experiments were started when neural cultures had 14 Days in Vitro (DIV) and were carried out during 2-3 weeks. In every experiment, the electrophysiological activity of the cultures was previously analysed and connectivity diagrams based on cross-correlation were obtained for each culture. Two pairs of electrodes with an acceptable spiking activity and no logical connections between them were selected for stimulation. Low-frequency current stimulation and tetanic stimulation had both an impact on the electrophysiological responses of the cultures. Raster plots showed that all of the stimulations provided induce changes in the firing frequency of the cultures. Instantaneous firing frequency graphs shows that stimulated electrodes start firing in more separated period of times after stimulation but each firing period last longer. In addition, interspike intervals show the same results observed in the spiking periods but also it can be seen analytically the ISI decrease both in value and dispersion. Both effects are related to the neural stimulation, which modulates the firing capabilities of the cultures. Connectivity diagrams based on cross-correlation between electrodes showed some kind of connections reorganization after stimulations, concentrating them in a few electrodes. Furthermore, adjacent physical or logical connections in the connectivity graph following Hebb’s law appeared in some pairs of stimulated electrodes. Electrodes with created connections between them can distinctly be detected with the instantaneous firing frequencies graphs (Figure 1). The firing periods of the electrodes from the connected pairs follow exactly each other, whereas the firing periods of the not connected pairs of electrodes do not match. Furthermore, the electrodes of connected pairs change both the firing periods after stimulation. This features indicates that there exits a strong connection between them. Low-frequency stimulation induces permanent changes in most experiments using different values of current amplitude and stimulation patterns. Persistent and synchronous stimulation of relevant adjacent electrodes may be used for strengthen the efficiency of their connectivity graph. These processes may be used for imposing a desired behaviour over the network dynamics. Figure 1 Acknowledgements This work is being funded by grant 12361/FPI/09 from Séneca Foundation, Science and Technology Agency from the region of Murcia and by the project 2010V/PUNED/0011 from Universidad Nacional de Educación a Distancia.