The Use of Brain Electrophysiology Techniques to Study Language: A Basic Guide for the Beginning Consumer of Electrophysiology Information

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Abstract. This article provides a basic background for the professional who is interested in utilizing event-related potential (ERP) approaches to study language processes but has little background in or knowledge about the technique. First, a brief history of the emergence of this technology is presented, followed by definitions, a theoretical overview, and a practical guide to conducting ERP studies. The basis for choice of electrode positions, equipment characteristics (e.g., filter settings), and analyses are also discussed. Finally, examples of language studies that utilize this information in a research study are provided.

There has been a long-standing interest by researchers and theorists in brain-behavior relations. This interest has led to the development and adaptation of instruments and methodologies that measure the brain's responses for use in studying developmental issues. Techniques such as electroencephalography (EEG), event-related potentials (ERP), and brainstem-evoked response (BSER) all share a common approach to cortical electrophysiology -- scalp electrodes are used to detect electrical activity generated by the brain. These techniques can provide insights into brain-behavior developmental issues that complement and supplement information obtained through more traditional behavioral measures.

This article reviews the history of cortical electro-physiological approaches to investigate brain-behavior relations. An overview of the procedure is provided along with rationales for various components of it. We briefly also comment on how these approaches compare to other techniques, such as Positron Emission Tomography (PET), Magnetic Resonance Imaging (MRI), and functional Magnetic Resonance Imaging (fMRI). A final section will describe the current use of ERP techniques in longitudinal research to investigate the relation between brain functioning in infancy and subsequent cognitive and language development in school-aged children, with a focus on reading and reading disability.

What Are Event-Related Potentials?

In order to understand ERPs (event-related potentials) or Eps (evoked potentials) some general background on the electroencephalography or EEG is useful. In the general EEG (electroencephalogram) technique, electrodes attached to the scalp allow physicians to measure the brain's electrical activity or EEG. The idea behind EEG is straightforward. The human body utilizes electricity in its operation, somewhat like cable cars, computers, and CD players use electricity. One major source and processing station of that electricity is the brain, or more specifically, the neurons that make up much of what we call the brain. Every time a group of neurons perform some function, they generate a small amount of electrical energy, which spreads throughout the brain to other neurons. However, a portion of this electrical signal passes through the brain and travels through the skull where it can be measured on the scalp. By positioning electrodes on the scalp's surface, EEG amplifiers magnify the minute electrical discharges (perhaps 5-10 [micro]V, or millionths of a volt) that occur. Such electrical currents on the scalp thus reflect and indicate the activity level of groups of neurons within the brain. The end result is a readout of the ongoing electrical discharges (in the form of continuous brain waves) that were produced during activity (e.g., sleeping, listening to music, reading) over some period of time.

An EEG field at the scalp can only be recorded if large numbers of neurons are active at the same time, and they must all be aligned in the same orientation. If they are not aligned, that is, they are laminar, then the positive and negative fields of differently aligned neurons tend to cancel each other out. Subcortical brain structures, such as the basal ganglia, may be highly active electrically, but do not appear to contribute to the scalp EEG because they are not aligned with cortical brain structures. Therefore, it is generally assumed that the great majority of the electrical fields contributing to the scalp EEG represent cortical electrical sources. The general belief through two centuries is that measurements of such currents can provide insights not only about the basic neurophysiology of the brain but also behavioral information concerning how and in what manner the brain is involved in sensing, thinking about, and interacting physically and cognitively with the environment throughout development.

One limitation of the EEG is that because it is continuous and ongoing, it is difficult to determine the specific stimuli or events that produce variation or change in the EEG pattern. ERPs, on the other hand, overcome this limitation by simply focusing on a portion (usually about one second) of the ongoing EEG electrical activity that is repeatedly time-locked to the beginning of when a stimulus (e.g., sound, picture) is presented to someone. Time-locking refers to the fact that researchers only record the part of an EEG wave that follows the word, sound, or picture stimulus in time. Repetition refers to the fact that researchers repeatedly present the same stimulus in order to average out the random and nonstimulus-related background electrical activity that is inherent in the ongoing EEG and does not reflect the brain's response to that stimulus.

Consider the following analogy: Suppose that you were a seismologist who wanted to test an idea about earthquakes by dropping pebbles into a pond. In order to test the impact of a pebble in that pond, you would have to measure the ripples produced beginning when the pebble first contacted the water in the pond. You would then continue to record the ripples produced by that impact until they died away. This is the notion of time-locking. However, we know that ponds are already naturally full of ripples from wind and other disturbances (e.g., fish and turtles moving, frogs jumping in, etc.), thus it is difficult to measure the effects of a single pebble drop. In this analogy, the pond's ripples are like ongoing EEG, and the pebble's ripples are like an ERP. In order to "wash" out (i.e., average out) these background ripples caused by other factors, one would have to sequentially drop many individual pebbles and take the average of each pebble's ripples produced over that time. This kind of repetition leads to "averaging out" information unrelated to the pebble's impact.

This analogy is useful for thinking about how to interpret ERPs. The basic idea behind the ERP methodology is that different stimuli of interest (e.g., words, pictures, thoughts, sounds, etc.) cause different brain waves, just like different-sized and -shaped pebbles cause different waves in a pond. These differences can be used just like any other dependent measure in research on language processing. For example, they can be used like behavioral measures of text comprehension rates, reading time, phonological discrimination, and so on.

Among the many advantages of the technique are its noninvasiveness and the fact that it can be used across the life span with virtually identical procedures. Thus ERP technology has the potential to provide a powerful tool to study changes in brain-behavior relations and functions across the life span.

Research over the past century has demonstrated that the ERP can be used to effectively study both general and specific aspects of the organism's response to eliciting events in the external as well as the internal environment (Molfese, 1978a, 1978b). The ERP can also be used to study an individual's perceptions and decisions during tasks or following a learning situation (Molfese, 1983; Nelson & Salapatek, 1986; Ruchkin, Sutton, Munson, Silver, & Macar, 1981). Given that the EP technique does not require a planned and overt response from which it is recorded, it is particularly well suited for the neuropsychological study of early infant and child language development (Molfese, Freeman, & Palermo, 1975). In addition, as noted above, the ERP can provide very fine temporal information (one ms or less) regarding the brain's response to an eliciting input, such as a speech sound. Finally, the ERP has some gross-level spatial resolution capabilities that permit a basis for speculations concerning the distribution of brain mechanisms that subserve functions such as language.

History

Attempts to record the brain's electrical activity and relate it to behavior date from at least the time of Richard Caton (1875), who recorded evoked potential responses (ERPs) from an electrode placed directly on the surface of a rabbit's exposed brain. Other early brain-recording techniques involved immersion, in which patients were required to place each of their limbs in separate buckets of saline solution. The buckets were used as "electrodes" to detect electrical signals conveyed via cables to an amplifying system that magnified the millionth of a volt signals recorded from the scalp, which could then be penned onto a chart recorder.

By the mid-1920s, plate electrodes were developed that could be applied directly to the skin. Unfortunately, any movement of the plate over the skin, no matter how small, produced large artifacts that distorted or obscured the brain responses. Later, scientists constructed a floater type of electrode that required a conducting paste (electrolyte) to be placed between the skin and the …