There now exist a wide range of sensors for acquiring signals from the human body in non-invasive ways. Some of those in use date back a few decades and many new technologies have enabled different sensors designs in recent years. Sensors systems that are portable, safe and low-cost are becoming possible. This is an overview that focuses on the technology behind sensors currently used by psychophysiologists with the objective to illuminate the choices available for acquiring signals that inform us about cognitive and other mental processes. In previously published encyclopaedic-type reviews of psychophysiology the focus has been on what is measured, not how it is measured. By focussing on how the sensors and sensor system work, this review aimed to provide users of these technologies with information which will help decide on the appropriate sensor to use, as well as facilitate innovation and development of new sensors.
The human body provides a number of signal sources that can be non-invasively detected at or near the skin surface. The definition of a non-invasive sensor is similar to that of Khalil (1999), “a device that comes in contact with, or remotely senses, a human body part, without protrusion through membranes or sampling a body fluid for analysis external to the body part.” The invasiveness of CAT, PET and MRI scans excludes them from this domain of sensing technologies. However, we do consider as non-invasive a skin patch that could potentially assay components trapped in the outer layers of the dermis. Also sensors, which could analyse saliva or urine samples we also consider as non-invasive if done external to the body. Many of these body surface signals have been associated with processes of interest to psychophysiologists. The electrocardiogram (ECG) is a good example. Electrodes are placed against the skin for a recording. The electrical discharges associated with the beating heart propagating to these sites are transduced into voltage fluctuations and conveyed to a viewing and/or storage device. Cognitive, behavioural and emotional differences within subjects or between groups (eg treatments) can be associated with differences in parameters extracted from the ECG signal, such as heart rate, heart rate variability, spectral analysis and component analysis.
There is a range of psychological phenomena associated with changes in non-invasive signals acquired near or on the body surface and excellent reference books exist in this area (Cacioppo et al., 2000, Stern et al., 2001). However, these texts emphasize the human physiological functions being measured, rather than the sensor technologies directly. In other words, they emphasize what is measured by psychophysiologists (for example Cacioppo et al., 2000) with secondary information on how the sensors themselves operate.
Hence, there is a lack of a review that makes the technology of how the sensors work accessible to the psychophysiologist who is not generally trained as an engineer or physicist. The goals of this paper were threefold: (a) to provide an overall scheme for sensor classification (b) to provide a general over-view of the processes involved in signal acquisition, including how the sensors within a class work as well as precautions in their use and (c) to describe current trends in sensor development and their integration into portable systems. The next decade will see commercially available wearable (and wireless) sensors systems that can monitor many interesting psychophysiological variables.
Tim Cutmore, Daniel James 2005