There is no doubt that optogenetic tools caused a paradigm shift in many fields of neuroscience. These tools enable rapid and reversible intervention with a specific neuronal circuit and then the impact on the remaining circuit and/or behavior can be studied. However, so far the ability of these optogenetic tools to interfere with neuronal signal transmission in the time scale of milliseconds has been used much less frequently although they may help to answer a fundamental question of neuroscience: how important temporal codes are to information processing in the brain. This perspective paper examines why optogenetic tools were used so little to perturb or imitate temporal codes. Although some technical limitations do exist, there is a clear need for a systems approach. More research about action potential pattern formation by interactions between several brain areas is necessary in order to exploit the full potential of optogenetic methods in probing temporal codes.
The introduction of optogenetic methods has already made a dramatic impact on many fields of neuroscience. To many neuroscientists, it all started with a demonstration of light evoked electrical responses in neurons that expressed bacteria-derived, light-sensitive channelrhodopsins (Boyden et al., 2005), although the true beginnings can be traced back to at least few years earlier (Nagel et al., 2002; Zemelman et al., 2002; Miesenbock, 2011). Some first experiments employing these new light sensitive tools did not provide breathtaking insights into the brain mechanisms (Miesenbock, 2011) but the years that followed the introduction of optogenetic methods saw a change in our understanding about many well-known phenomena. For example, for years scientists tried to explain the success of deep brain stimulation in the treatment of Parkinson’s disease by the neuronal activity changes in stimulation areas (Dostrovsky and Lozano, 2002; Lozano et al., 2010). However, optogenetic methods showed that the activation of en passant axons during deep brain stimulation could ameliorate the disease symptoms in mouse models (Gradinaru et al., 2009; Kravitz et al., 2010; Deisseroth, 2014). Similarly, many hypotheses on the role of interneurons in the cortex could be tested only with the advent of optogenetic tools (Cardin et al., 2009; Sohal et al., 2009; Letzkus et al., 2011; Lee et al., 2013; Pfeffer et al., 2013; Bortone et al., 2014). However, in one area of neuroscience the impact of these methods was somehow limited. A major issue in the field of neural coding is how important temporal codes are to information processing in the brain. In this perspective paper I will argue that optogenetic methods have a potential to go beyond simple correlation between a stimulus and a response that so far was almost the only method to demonstrate the importance of temporal codes.
It’s quite likely that in neurons the main information unit is an action potential. To a certain extent, this conclusion follows from the first experiments indicating that the shape of action potentials is rather stereotypical and does not depend much on the stimulus (Adrian and Zotterman, 1926b). Thus, other factors such as rate and timing of action potential occurrence encode the features of sensory stimuli. Rate codes convert stimulus parameters into the number of action potentials fired during a specified time interval (Figure 1). Temporal codes convey sensory input information through specific spike times, either of a single neuron or a neuronal population. In the simplest form of temporal coding the degree of coincidence or synchrony but not the overall number of action potentials carries information about sensory inputs (Figure 1). However, other temporal characteristics of spike sequences such as burst number, variance of spike frequency or any reproducible sequence of time intervals between action potentials may encode stimulus features (Izhikevich, 2006; Kostal et al., 2007).
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Front. Syst. Neurosci., 21 December 2015 | http://dx.doi.org/10.3389/fnsys.2015.00174