Techniques in Data Analysis: General Principles

MANJIT SANGHERA AND ROBERT GROSSMAN

With the resurgence of pallidotomy and, more recently, with the advent of deep brain stimulation of the thalamus and the subthalamic nucleus in patients with refractory Parkinson’s disease and other movement disorders, performing microelectrode recording to localize the globus pallidus, the thalamus, and the subthalamic nucleus has become an integral part of the neurosurgical procedure. The electrophysiological properties of pallidal and subthalamic nucleus neurons can be used to localize these areas, allowing the accurate placement of lesions or stimulating electrodes within the target areas.

Electrophysiological data collected during the neurosurgical procedure can be analyzed off-line and used to support models of corticobasal ganglia circuitry in PD and other movement disorders. These electrophysiological data, in combination with anatomical, histochemical, pharmacological, positron, and functional MRI studies have led to a better understanding of neural circuits involved in movement disorders. The purpose of this chapter is to give an overview of electrophysiological data analysis: its acquisition, its reduction, and the information that can be extracted from these analyses.

One of the most common artifacts that can mask neuronal signals is the presence of 60 Hz and its second and third harmonics (240 Hz and 480 Hz, respectively) arising in the AC power supply to the lights and to the equipment in the operating room. The presence of this interference or “hum” will impede the visual and auditory recognition of spikes, making it difficult to collect data and to conduct subsequent off-line data analysis. Extracellularly recorded action potentials are in the μV or low mV range, whereas stray electrical signals are often in the tens of mV range. Much of the 60 Hz artifact can be eliminated by turning off nonessential electrical equipment. Proper grounding of the patient, the operating room equipment, and the recording equipment is essential. It is important to avoid “ground loops” in which there are multiple connections of different resistance between the patient, ground, and various pieces of equipment. In some cases, grounding the electrically powered operating room table will reduce 60 Hz. In other cases, grounding the table will increase 60 Hz. It is worthwhile to try a variety of grounding configurations of the equipment and to try removing equipment connected to the patient, such as EKG leads, which may be producing an “antenna effect.” An ohmmeter can be used to determine which equipment is grounded and to examine the electrical resistance pathway from the patient to ground, before and after the ground lead is attached to the patient. Fluorescent lights generate a considerable amount of electromagnetic noise and should be turned off if possible. In some circumstances, it is feasible to construct a Faraday cage around the operative field, which can reduce 60 Hz and other electrical noise. In general, removing 60 Hz in the operating room requires as much art as it does science.

If grounding and shielding cannot eliminate 60 Hz artifacts, a 60 Hz notch filter may be incorporated into the recording system to eliminate unwanted frequencies. Such filters may be passive or active. The active filters may be thought of as “learning” the pattern of interference, then introducing a signal to counteract the interference. One drawback of active filters is that they may reintroduce unwanted interference for a brief period of time if the recording conditions are changed, until the new situation is “learned.” In contrast, passive filters may not be as efficient, but they may be depended on to greatly attenuate 60 Hz interference.

Most oscilloscopes are designed to display a negative potential recorded at an electrode and led to the active input, with respect to ground, as a downward deflection on the screen. It is useful to calibrate the recording system with signals of known polarity and amplitude to be certain of the amplification of the system and the polarity display conventions that are being used.

In extracellular recordings, low-pass (high-frequency cutoff) and high-pass (low-frequency cutoff) filters are employed to remove high-frequency and low-frequency interference, respectively. Filters for microelectrode recording are usually set to attenuate all signals below ~300 Hz and all signals above ~2 kHz. Although these filter settings will remove unwanted potentials and noise, there may also be some distortion of the morphology of the spikes. It is important to recognize these effects with respect to the electrical signal of interest.