Although the acceptable ratio of signal-to-noise is 2:1 for most recordings, the ratios for single and multiple neuron firing recordings with MEAs typically range between 3:1 and 10:1, in both rodent and nonhuman primates (Opris et al. 2011, 2012a, b). For in vivo electrochemical recording, MEA recordings achieve improved signal-to-noise by self-referencing noise subtraction (Burmeister and Gerhardt 2001), but can detect changes in glutamate as low as 0.1 μM/s.

The good signal-to-noise ratios of single cell recording achieved by the Pt MEAs allows for very clean isolation of cell waveforms. Extracellular neuron action potentials (spikes) were identified and isolated on-line by separation of extracellular action potential waveform duration and amplitude and confirmed via off-line analysis using principal components analysis and autocorrelation to confirm that (a) single neurons were isolated, (b) all spikes of a given neuron were identified within 95 % confidence limits, and (c) neurons detected simultaneously on adjacent sites were counted only once. In daily recording sessions we were able to isolate up to 4 units per pad (Opris et al. 2011), but 2 units per pad was most common.

To discriminate neurons recorded simultaneously we usually employ a cluster separation method (using Neuroexplorer from www.​Plexon.​com), such as Principal Component Analysis (PCA) in real time or by offline sorting. We also perform peak-to-peak amplitude separations for cells at each pad where at least two well separated clusters of cells were recorded. All 8 pads discriminated the temporal pattern of neuron spiking from clusters of simultaneously recorded cells (Opris et al. 2011).

The durability of MEAs has continuously increased from 3 weeks (Moxon et al. 2004) to up to several years (Deadwyler et al. 2013). A good MEA used in chronic recording can provide reliable units for 6 months to 3 years. In acute recordings the MEAs can be reused many times with very high yield. For example a MEA with 8 pads can provide up to 32 reliable cells per session while a 4 shank MEA with 32 pads can yield up to 128 units in both acute (on a session base) and chronic (on long term base) recordings.

A good recording resolution comes from the distance between adjacent pads needed for isolation of separate units which is 30 μm. According to anatomic measures by Buxhoeveden and Casanova (2002) and quantitative analysis based on columnar neuron recordings by Mahan and Georgopoulos (2013), this is the ideal separation between pads for biomorphic minicolumnar recordings. Nevertheless, occasionally a cell may fall in between two adjacent pads, and thus one ends up recording the same unit with the adjacent pads. Neurons detected simultaneously on adjacent sites are isolated on only one pad and used for further analysis. On the chemical recording aspect, we have demonstrated that MEAs that are chemically modified for glutamate, acetylcholine, choline, lactate and glucose measures have fast temporal resolution (<1 s), excellent spatial resolution (microns), low detection limits (≤100 nM for all analytes) and cause minimal damage (50–100 μm) to surrounding brain tissue.

Neurons (Opris et al. 2011, 2012a, b, 2013) were recorded in Layer 2/3 and Layer 5 and from the dorsal bank of the arcuate sulcus in the dorsolateral prefrontal/premotor areas of prefrontal cortex (PFC). Figure 17.3c shows examples of biomorphic MEA recordings with the W3 MEAs in the dorsal premotor region during go trials of the go/no-go task in nonhuman primates. The waveforms recorded at the vertically separated (1,350 μm) recording sites on the MEA indicate individually isolated neuron firing exhibiting task-related discharges within different cortical cell layers, presumably supragranular Layer 2/3 cells shown at locations 2a–d and infragranular Layer 5 cells shown at MEA site locations 5a–d, 6a–f. Corresponding Post-Event Histograms (PEHs) in Fig. 17.4c illustrate the firing patterns of the same neurons, identified in terms of MEA recording pad location (i.e., 2a, 2b, 5a, 5b, 5a, 6b, etc.) synchronized to target image presentations within the task. The MEA routinely recorded RS (define?) cells from both Layer 2/3 and Layer 5/6 simultaneously in the same session, and as indicated in Fig. 17.4c, most cells showed a significant either increase or decrease (noted by asterisks) in activity related to go/no-go task events.

Implementation of the newly applied MEA technology described here (Burmeister et al. 2000; Hampson et al. 2004; Moxon et al. 2004) allowed simultaneous recording from neurons at two different anatomically and physiologically distinct recording sites in the sensorimotor areas of PFC. The biomorphic design of the MEA allowed identification and designation of activity within a dual laminar arrangement of orthogonal MEA recording sites separated by 1,350 μm, the precise distance between Layer 2/3 and Layer 5 in the PFC Arcuate sulcus (Fig. 17.4a). This recording arrangement allowed unique comparisons of the simultaneous activity of cells from (i) each separate cortical layer and (ii) within the same layer but at adjacent physically separate locations on the MEA (Fig. 17.4b). The differential firing to task-related events of identified PFC cells in both Layer 2/3 and Layer 5 is described in detail in Figs. 17.3–17.5 in terms of individual and mean discharge patterns of simultaneously recorded neurons in each layer during either go or no-go trials.

Our recent results (Opris et al. 2011, 2012a, b, 2013) demonstrate many of the functional “operations” described above in which PFC cortical neurons in both the supragranular and the infragranular layers differentially encoded sensory stimuli relevant to performance of a simple go/no-go task. In addition, firing to task-related events was shown to be encoded between layers by synchronized firing of cell pairs comprising apparent functional “minicolumns” connecting the supragranular and the infragranular layers. This assessment was made possible by use of biomorphic MEAs designed to sample neuronal activity in a manner that would reveal the demonstrated functional columnar organization of the PFC. The results provide new insight into the manner in which underlying PFC microcircuitry integrates convergent sensory, cognitive and motor signals from other brain regions to select and control task-related behavioral response tendencies in primate brain (Opris et al. 2011, 2013).

Amperometric measurements of L-glutamate using biomorphic MEAs provide spatially and temporally resolved measures of neuromolecules in the central nervous system of rats, mice and non-human primates (Talauliker et al. 2011). Although the functional capabilities of MEAs have been previously documented for both anesthetized and freely-moving paradigms, the performance enabling intrinsic physical properties of the MEA device have not heretofore been presented. In these studies, spectral analysis confirmed that the MEA recording sites were primarily composed of elemental platinum (Pt°). In keeping with the precision of the photolithographic process, scanning electron microscopy revealed that the Pt recording sites have unique microwell geometries postfabrication. Atomic force microscopy demonstrated that the recording surfaces have nanoscale irregularities in the form of elevations and depressions, which contribute to increased current per unit area that exceeds previously reported microelectrode designs. The ceramic substrate on the back face of the MEA was characterized by low nanoscale texture and the ceramic sides consisted of an extended network of ridges and cavities. Thus, individual recording sites have a unique Pt° composition and surface profile that has not been previously observed for Pt-based MEAs. These features likely impact the physical chemistry of the device, which may influence adhesion of biological molecules and tissue as well as electrochemical recording. This is likely contributing as well to the unique electrophysiological recording capabilities of the MEAs for single and multi-unit neuronal activity (see above).

A major advantage of microdialysis is the ability to measure multiple neurotransmitters simultaneously (assuming good separation of peaks during offline detection by high performance liquid chromatographic-based methods). Our laboratory has successfully demonstrated the ability to measure acetylcholine and choline simultaneously (Burmeister et al. 2008), which is an essential element of the enzymes involved. Ideally, the long-term goal is to design an MEA to measure multiple neurotransmitters from the same brain structure. As previously mentioned, photolithographic methods allow us to design MEAs with well-defined, highly reproducible geometric configurations that can be patterned onto the front and back of a ceramic wafer. By using both sides of the ceramic wafer we can greatly enhance the recording site density of an MEA. We have designed several 16 site MEAs, one of which is shown in Fig. 17.2b. By increasing the recording site density we can selectively coat recording sites for self-referencing measurements of different analytes in the same brain region.

The ability to simultaneously measure local field potentials (LFPs) and neurochemical dynamics would greatly enhance our understanding of neuromodulation. This would have profound implications for studying behavior or disorder related changes in the CNS. Until recently, few studies were capable of achieving simultaneous electrophysiological and electrochemical measures from the same recording preparation. And, those that did either switched between neuronal spike recording and the in vivo electrochemical recordings (Williams and Millar 1990; Stamford et al. 1993; Cheer et al. 2005) or required the use of two separate recording systems (Zhang et al. 2009, 2010). This made the experimental setups cumbersome and expensive while making data analysis and interpretation difficult. However, Zhang and colleagues (2009, 2010) are the first to demonstrate simultaneous LFPs and local neurochemical measurements using a single MEA coupled with constant potential amperometry. They successfully demonstrated that the high frequency component of the amperometric signal resembled LFPs. Using a single set-up and biosensor for the simultaneous acquisition of electrophysiological and neurochemical information can greatly enhance the field of in vivo electrochemistry (Zhang et al. 2009, 2010).

Due to great diversity of cell types in neural circuits, it is critical to be able to analyze how these different kinds of cells work together. An emerging approach with broad implications for basic and clinical neuroscience is based on optogenetic stimulation (Gradinaru et al. 2007; Tye and Deisseroth 2012). Recent developments in optogenetics based on optical manipulation of activity in neural circuits with light-sensitive rhodopsins, such as the Chlamydomonas channelrhodopsin-2 (ChR2) are now capable to illuminate the inter-laminar microcircuits at the millisecond-scale, with cell type-specific optical perturbations in nonhuman primates (Diester et al. 2011; Han 2012), opening up new possibilities for repair and augmentation. In recent years, MEAs with integrated optical fibers use light-assisted perturbation and recording of local neural circuits during animal behavior (Royer et al. 2010; Marshel and Deisseroth 2013; Bernsterin and Boyden 2011).

This aspect will be described in another chapter, here we just briefly touch on the relevance of MEAs in neural prosthetics.