Near-Infrared Spectrometry for the Measurement of Central Nervous System Activation: A Brief Demonstration of an Emerging Behavioral Assessment Tool

Article excerpt

Behavior analysts are familiar with the use of electrodermal activity as a dependent measure of central nervous system activation. In addition, behavior analysts have increasingly turned to direct measures of brain activation, such as electroencephalography and event-related potentials. Recent developments in the field of bioengineering, however, have produced a new and exciting brain-activation recording device known as near infrared spectrometry, or NIRS. The current paper reports a demonstration of its use in a traditional respondent conditioning paradigm. Specifically, a male volunteer was exposed to a conditioning paradigm designed to produce both an eliciting stimulus for fear and a relief stimulus. Conditioning effects were assessed using electrodermal activation as well as blood volume changes in the frontal lobe, recorded by NIRS. The results of the demonstration show that both electrodermal and NIRS measures can successfully identify conditioning effects without necessarily tracking each other on a trial-by-trial basis. It is suggested that NIRS is an inexpensive, non-invasive technique for the assessment of learning and behavior at the neural level.

Key words: central nervous system, NIRS, electrodermal activation, learning and behavior assessment

**********

Electrodermal activity (EDA; otherwise known as the Galvanic Skin Response) represents perhaps the most reliable index of general autonomic arousal (Dawson, Schell, & Fillon, 1990). The system was discovered by Fere (1888), who found that by passing a small current across two electrodes, through the body of a human subject, he could measure fluctuations in skin resistance in response to a variety of external stimuli. Thus, the EDA method was essentially a measure of potential electrical difference across any two points in the human body, measured in millivolts. The EDA method proved so reliable as a measure of sympathetic nervous system activity that such figures as C. G. Jung (1906) used it in assessing the emotional content of verbal stimuli for his patients. It has now been well established experimentally that EDA is an excellent index of physiological responses to discrete and tonic stimuli and a good correlate of most other psycho-physiological measures (see Cacioppo & Tassinory, 2000).

It is often thought that EDA is caused by the increased sweating that occurs during periods of autonomic activity. The truth is a little more complicated. Specifically, it has been known for almost a century that the EDA response occurs about 1 second before the appearance of sweat at the electrode placement site on the epidermis (Darrow, 1927). Early research also established that increased blood flow and blood pressure are not direct causes of EDA insofar as electrodermal responses can be shown to diverge from both of these measures under certain conditions (Darrow, 1927). Further complication is added by the fact that different types of sweat glands function in different ways to affect the EDA response. Specifically, eccrine glands respond largely to thermoregulatory stimuli, with the exception of those eccrine glands on the palm of the hands (Fowles, 1986). Appocrine glands, which are dense in the genital areas and armpits and less dense on the palms of the hands, are less well understood but are thought to respond largely to emotional stimuli. The stimuli that trigger apocrine sweating are still debated but are usually of emotional/psychological significance and do not usually occasion thermoregulatory activity (Shields, MacDowell, Fairchild, & Campbell, 1987).

While palmar sweating certainly contributes to the EDA response, it is now widely accepted that the sympathetic nervous system primarily controls EDA (rather than sweating itself). This view is supported by the strong correlation between sympathetic action potentials and skin conductance responses (SCRs) at normal room temperatures (Wallin, 1981). Thus, electrodermal activity is a complex function of the activity of the central and peripheral nervous systems and sweat gland activity. …