Specifications

Extracellular Recording from MEAs

3.3 Signal Types

3.3.1 Single Unit Activities

Usually several cells are plated onto a MEA. The waveform of a single unit spike depends on the

signal source, the geometry of the extracellular space, and the distance of the signal source to the

electrode. The property of a waveform derived from a single neuron is reproducible over time and

therefore specific for that neuron. That is, the differences of waveforms from separate signal

sources can be used to distinguish the activities and to sort spikes into single unit spikes. Thus, you

can acquire single unit data from multiple cells in parallel by recording from a single electrode.

You have to discriminate between independent activities and network responses. Responses of

cells on a MEA triggered by a chemical, electrical, or light stimulus can be either statistically

independent or show a specific pattern. The latter is quite interesting for studying the role of cells

and different tissues in a pathway. MEA recording allows such studies under controlled

experimental settings and is much easier and less labor intensive than an in vivo experiment.

3.3.2 Local Field Potentials

If the dendrite soma axes of the active cells are aligned, the waveforms from multiple units on a

MEA overlay and form a compound potential, or local field potential (LFP). The higher the activity,

that is, the spike rate, the higher is the amplitude of the LFP. A modulation of the stimulus results

in a higher frequency of action potentials that will result in a graded multi-unit response. LFPs

often show a high signal-to-noise ratio, which is very beneficial for the analysis.

If dendrites are arranged in a nonparallel or radial fashion forming a closed field, the waveforms

may cancel each other out, when the neurons fire in synchrony.

3.4 Recording and Stimulation

The MEA sensor is placed directly into the small-sized MEA amplifier. When the amplifier is closed,

the contact pins in the lid of the amplifier are pressed onto the MEA contact pads. The very close

location of the amplifier to the MEA sensor is very favorable concerning a high signal-to-noise

ratio. The amplifier is connected with a single cable to the data acquisition computer.

You can then simply ground bad or unwanted electrodes by toggling small switches on the

amplifier. You can also connect a stimulus generator to any electrode(s) for stimulation.

Stimulating electrodes are grounded to prevent a saturation of the amplifier.

If you use the new MEA preamplifier with blanking circuit, software controls allow you to select

any electrode on a MEA for stimulation and recording. A user defined, typically 500 μs long,

blanking signal switches off stimulating electrodes during stimulation and thus removes stimulus

artifacts.

With the MEA_Select software, it is easy to change the electrode selection during the experiment,

for example, to use stimulating electrodes for recording and vice versa. It is also possible to use

the same electrode for recording shortly after stimulation provided that you use a dedicated

biphasic pulse protocol that compensates for the slight DC offset that a stimulating electrode

always shows after stimulation.