A great deal is known about the stimulus/response characteristics of individual nerve cells in a variety of systems. However, detailed understanding of the correlation between neural activity patterns and the Neural Code from a group of neurons has not been attainable due to several technology limitations. Some of the main issues are: electrodes, recording instruments, and analysis software.

Electrode technology has progressed steadily from wire electrodes, bed of nails, and tetrodes to silicon micro-machined electrodes. The development of electrodes gave researchers the ability to record large numbers of neurons to understand their behavior. Recording instruments such as DATAMAX II (developed with support from NIH and neuroscientists) make it possible to capture multi-unit data for signal processing. Commercial programs, such as Nex*, provide the ability to analyze data gathered by DATAMAX II. Researchers are now able to use these multiple tools as a complete solution to research the Neural Code.

Figure 1. 31 Channel Neocortical Activity Above the Hippocampus.
Courtesy  of Dr  Buzsaki, CMBN, Rutgers University 
Recording instruments have been a bottleneck to advancing the understanding of the Neural Code. The traditional methods, converting action potentials to event time or collecting a few data points from a threshold, work well when you can isolate cells with individual electrodes. However, when recording multi-site electrodes, invariably the signal reflects the interaction of multiple neurons (Figure 1). It is difficult to separate individual units from a complex action potential caused by overlapping multiple units. Field potentials, although at a lower frequency, have also been proven to influence the characteristics of action potentials. The extraction of the Neural Code, therefore, is a complex subject and requires gathering all the available information to decipher its secrets. A recorder provides the advantage of storing the data from experiments and having it available for decoding using multiple and yet-to-be developed analysis tools. Recorders thus become powerful tools working side by side with other real-time instruments in the quest for discovery. 

Recorder Technology 

Traditional tape recorders are easy to use but often have channel limitations and take time to play back data. Data acquisition systems can be difficult to use and typically have low resolution (12 bit, 70 dB dynamic range). This limits their ability to capture the full fidelity of the signal for processing. In addition, accurate signal reproduction is often hindered in digital sampling systems due to variable time delays between channels, lack of proper anti-aliasing filters to eliminate high frequency noise, and the absence of differential inputs to effectively eliminate common mode 60 Hz noise. 

DATAMAX II was developed specifically for large channel-count recordings and to collect valid data under any condition, while remaining easy to use. It was designed to operate as simply as a tape recorder, with the flexibility of a data acquisition system. This 16 bit system provides a wide 90 dB dynamic range to capture both spikes and field potentials, with a selectable sampling rate from 100 Hz to 200 kHz per channel.                      Figure 2: Nex Unit Detection Using Principal Component Analysis and Cluster Sorting     
 DATAMAX II utilizes one A/D per channel, thereby eliminating multiplexing and guaranteeing simultaneous sampling of all A/D converters. This built-in multiple A/D concept provides an added layer of dependability. In the unlikely event that any one A/D fails, neither the operation nor data integrity of the remaining channels will be affected. DATAMAX II includes differential inputs, programmable anti-aliasing filters, and real-time calibration to insure you get accurate and valid data for analysis. Real-time display in Scope, Spectrum, DVM, and Bar Chart modes provides visual feedback regarding the health of your signals during recording. In addition, DATAMAX II is a Windows NT based system, making it network capable for high speed data download to other workstations, or control from remote workstations.

DATAMAX II has a graphics playback capability to display the raw data, letting you select the desirable portion for export to third party analysis software.  Some popular data conversion formats include ASCII, Binary,  DADiSP and MATLAB file formats.  Electrophysiology specific programs such as Nex are available to separate spikes based on thresholds, principal components, and cluster cutting to identify spikes as event times (Figure 2). Nex includes a wide range of analysis tools such as: Histograms (rate, spike interval, peri-event, auto/cross correlation, instant frequency, etc.), Raster, Poincare Maps, PSD and others to process spike data. 

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