Introduction

Electrodes are the first technical interface in a system for recording bioelectrical potentials. The electrochemical and biological processes at the material–tissue interface determine the signal transfer properties and are of utmost importance for the long-term behavior of a chronic implant. Here, “electrode” is used for the whole device that consists of one or multiple active recording sites, a substrate that carries these active sites, as well as interconnections, wires, insulation layers and the connectors to the next stage of a complete recording system, whether it is wire bound or wireless. The application of the electrodes in fundamental neuroscience, diagnosis, therapy, or rehabilitation determines their target specifications. The most important factors are the application site, extracorporal device or implant, acute or chronic contact, size of the electrode (device) and the recording sites, number of active sites on a device, geometrical arrangement of electrodes and type of signal to be recorded. They influence the selection process of electrodes suitable for an envisioned application and help engineers as well as neuroscientists to choose the very best materials for the active sites, substrate and insulation and the most appropriate manufacturing technique. The properties of the recorded signals are also strongly related to this selection process since the tailoring of the transfer characteristics helps to pick up the “right” signal components and to ignore, neglect and reject the “wrong” electrical potentials that might be due to the body itself or the surrounding environment or interference caused by noise from the electrode sites and the amplifier of the recording system.

Introduction

Modern neuroimaging and computational neuroscience are two recent neuroscience disciplines that are very important for understanding brain mechanisms. Optical imaging gives the opportunity of observing the brain in activity at the level of large populations of neurons with high resolution. Many types of optical imaging techniques exist, but only two are usually used in vivo (see Grinvald et al., 1999, for a detailed review): the first is based on intrinsic optical signals and records brain activity indirectly, the second is based on voltage-sensitive dyes (VSDs) and reports postsynaptic neuronal activation in real time. In this review, we focus on the second technique, aiming at a better understanding of the origin of the optical signal. Extensive reviews of VSDI have been published elsewhere (Roland, 2002; Grinvald and Hildesheim, 2004).

This amazing technique is based on complex interaction with the system which is not yet fully understood. Indeed, the recorded signal (VSD signal) originates from a large amount of intermingled neuronal and glial membrane components and it seems difficult to isolate the contributions from the different components. Combined intracellular recording with VSDI has demonstrated a linear correspondence between the VSD signal and membrane potential of an individual neuron, but so far no studies have focused on what exactly the VSD signal actually measures when applied to a cortical population in vivo.

Experimental approaches are not really feasible because the available methodologies do not offer the possibility to inspect simultaneously all the components that may contribute to the signal.

Introduction

Extracellular recordings have been, and still are, the main workhorse when measuring neural activity in vivo. In single-unit recordings sharp electrodes are positioned close to a neuronal soma, and the firing rate of this particular neuron is measured by counting spikes, that is, the standardized extracellular signatures of action potentials (Gold et al., 2006). For such recordings the interpretation of the measurements is straightforward, but complications arise when more than one neuron contributes to the recorded extracellular potential. For example, if two firing neurons of the same type are at about the same distance from their somas to the tip of the recording electrode, it may be very difficult to sort the spikes according to from which neuron they originate.

The use of two (stereotrode (McNaughton et al., 1983)), four (tetrode (Recce and O'Keefe, 1989;Wilson andMcNaughton, 1993; Gray et al., 1995; Jog et al., 2002)) or more (Buzsáki, 2004) close-neighbored recording sites allows for improved spike sorting, since the different distances from the electrode tips or contacts allow for triangulation. With present recording techniques and clustering methods one can sort out spike trains from tens of neurons from single tetrodes and from hundreds of neurons with multi-shank electrodes (Buzsáki, 2004).

Information about spiking is typically extracted from the high-frequency band (≳500 Hz) of extracellular potentials. Since these high-frequency signals generally stem from an unknown number of spiking neurons in the immediate vicinity of the electrode contact, this is called multi-unit activity (MUA).

Introduction

Functional magnetic resonance imaging (fMRI) allows the non-invasive measurement of neural activity nearly everywhere in the brain. The structural predecessor, MRI, was invented in the early 1970s (Lauterbur, 1973) and has been used clinically since the mid-1980s to provide high-resolution structural images of body parts, including rapid successions of images for example of the beating heart. However, it was the advent of blood oxygenation level dependent (BOLD) functional imaging developed first by Ogawa et al. (1990) that made the method crucial especially for the human neurosciences, leading to a vast expansion of both the method of fMRI as well as the field of human neurosciences. fMRI is now a mainstay of neuroscience research and by far the most widespread method for investigations of neural function in the human brain as it is entirely harmless, relatively easy to use, and the data are relatively straightforward to analyze. It is therefore no surprise that fMRI has provided a wealth of information about the functional organization of the human brain. While many publications initially confirmed knowledge derived from invasive animal experiments or from clinical studies, it is now frequently fMRI that opens up a new field of investigation that is then later followed up by invasive methods.

It is important to note that fMRI does not measure electrical or neurochemical activity directly. Physically, it relies on decay time-constants of water protons, which are affected by brain tissue and the concentration of deoxyhemoglobin.

Over the past 30 years calcium-sensitive fluorescent dyes have emerged as powerful tools for optical imaging of cell function. Calcium ions subserve a variety of essential functions in all cell types. For example, changes in intracellular free calcium concentration ([Ca2+]i) underlie fundamental cellular processes such as muscle contraction, cell division, exocytosis, and synaptic plasticity. Most of these processes rely on the steep gradient of calcium ion concentration that is actively maintained across the plasma membrane. Moreover, cells store calcium ions in intracellular organelles, enabling them to release a surge of Ca2+ into the cytosol where and when needed. Calcium ions act through molecular binding to various Ca2+-binding proteins, inducing conformational changes and thereby activating or modulating protein function. The development of optical reporters of calcium concentration has opened great opportunities to read out [Ca2+]i directly as a crucial intracellular messenger signal. A major application of calcium indicators is the quantitative study of a specific calcium-dependent process X, for example, neurotransmitter release, with the goal to reveal the function X = X([Ca2+]i). However, this is not the only type of application. Because neuronal excitation in the form of receptor activation or generation of action potentials typically is linked to calcium influx, calcium indicators are also used to reveal neural activation patterns, either within the dendritic tree of individual cells or within cell populations.

Introduction

Intracellular recording is the measurement of voltage or current across the membrane of a cell. It typically involves an electrode inserted in the cell and a reference electrode outside the cell. The electrodes are connected to an amplifier to measure the membrane potential, possibly in response to a current injected through the intracellular electrode (current clamp), or the current flowing through the intracellular electrode when the membrane potential is held at a fixed value (voltage clamp). Ionic and synaptic conductances can be measured indirectly with these two basic recording modes. While spike trains can be recorded with extracellular electrodes (see Chapter 4), subthreshold events in single neurons can only be recorded with intracellular electrodes. Intracellular recordings have been used for many applications: measuring membrane potential distribution in vivo (DeWeese et al., 2003), membrane potential correlations between neurons (Lampl et al., 1999), changes in effective membrane time constant with network activity (Pare et al., 1998; Leger et al., 2005), excitatory and inhibitory synaptic conductances in response to visual stimulation (Borg-Graham et al., 1998; Anderson et al., 2000; Monier et al., 2003), current–voltage relationships during spiking activity (Badel et al., 2008), the reproducibility of neuron responses (Mainen and Sejnowski, 1995) dendritic computation mechanisms (Stuart et al., 1999), gating mechanisms in thalamocortical circuits (Bal and McCormick, 1996), oscillations of membrane potential (Engel et al., 2001; Volgushev et al., 2002), stimulus-dependent modulation of the spike threshold (Azouz and Gray, 1999; Henze and Buzsaki, 2001; Wilent and Contreras, 2005), and many others.

In the nineteenth century, Julius Bernstein invented an ingenious device called the “differential rheotome,” a rotating wheel which could record the time course of action potentials (see Chapter 3). Since then, many sophisticated techniques have been introduced to measure correlates of neural activity: measurements of electricity produced by single neurons (Chapters 3 and 4) or multiple neurons (Chapters 5–7 and 9), measurements based on brain metabolism (Chapters 8 and 11) or on calcium dynamics (Chapter 10). These techniques are always more or less indirect measurements of neural activity, and they have diverse spatial and temporal resolutions, and spatial scales. Each chapter in this book has described the quantitative relationship between neural activity (e.g. membrane potential or synaptic activity) and the measured quantity, as it is currently understood. This effort serves two purposes: to give a better understanding and interpretation of the measurements, and to help enhance existing techniques or develop new ones. To conclude this book, the authors of all the chapters describe ongoing developments in their field, open questions to be addressed, and new emerging techniques.

Extracellular recording

Substrate-integrated microelectrode arrays (MEAs) are planar arrays of microelectrodes used to record electrical activity in neuronal cell cultures or acute brain slices (Taketani and Baudray, 2006; Egert et al., 2010; Gross, 2010). While their history goes back to the 1970s, the rapid development of photolithographic techniques (stimulated by the needs of the computer industry) has now made prefabricated high-density MEA chips a popular research tool.