In times of physical or emotional stress, the body undergoes unconscious physiological changes mediated by the autonomic nervous system. Image Source: Flickr user Andrew Imanaka.
While law enforcement agencies around the country are adopting Computerized Voice Stress Analysis (CVSA) technology, there remains some skepticism within the community about its reliability. But if you understand the science behind the system, you will realize that the CVSA is a technology that can be trusted. In particular, understanding the physiology behind the vocal changes CVSA measures will relieve any concerns about the validity of CVSA technology as a valuable tool for law enforcement.
During questioning, the CVSA measures changes in a subject’s voice in order to determine whether they are under stress. Stress affects the autonomic nervous system, which is responsible for a wide range of physiological processes, including the physiological tremor—undulations in the muscles that are present in the vocal cords. Changes in physiological tremors are then detected by the CVSA, and based on the resulting graph, law enforcement officials can assess the veracity of the subject’s answers to particular questions.
Introduction to the Autonomic Nervous System
The nervous system is divided into two parts: the central nervous system and the autonomic nervous system. The central nervous system controls most functions of the body and the mind. In contrast, the autonomic nervous system functions below the level of conscious awareness, and it regulates unconscious physiological processes like breathing and the beating of the heart.
From there, the autonomic nervous system is further divided between the sympathetic nervous system and the parasympathetic nervous system. The parasympathetic nervous system is responsible for the regulation of body processes when a person is at rest, so it dominates during periods of relaxation. However, when a person is under stress, the sympathetic nervous system kicks in automatically, initiating the unconscious physiological processes that are responsible for the “fight-or-flight” response.
It can help to start by thinking about the situation in terms of physical stress. When you are sitting down, your heart beats at a steady rate, and the parasympathetic nervous system is in control. But if you jump up and run across a football field, the sympathetic nervous system takes over, and your heart pumps faster in order to sustain the effort. Once you stop running, the parasympathetic nervous system restores the balance by bringing your heart rate back down to its normal rate.
The same thing happens when you are under mental or emotional stress. If you are feeling calm and relaxed, the parasympathetic nervous system dominates. But when you start to get angry, scared, or excited, sympathetic nervous system processes begin to have effects that you are unable to consciously control. One of these effects—the physiological tremor—is measured by CVSA technology.
Physiological Tremor Under Periods of Relaxation and Stress
The physiological processes of speech are highly complex, involving multiple organs and tissues acting in coordination with each other. The fundamental voice frequencies come from the vocal cords, and like other muscles in the body, the muscles controlling the membranes of the vocal cords are subject to the slight oscillations of physiological tremor. Changes in the movement of these muscles alter how the vocal cords vibrate, creating sub-audible shifts in voice frequency.
At the same time, the muscles that control the vocal cords are also subject to autonomic nervous system processes, modulated by either the sympathetic or parasympathetic nervous system, depending on whether the person is undergoing stress or not. During periods of relaxation, when the parasympathetic nervous system is in control, physiological tremors are occurring constantly. However, when a subject starts to experience stress, the muscles controlling the vocal cords tighten up, so there are fewer physiological tremors.
Documenting Physiological Tremor on CVSA Charts
Humans can’t hear physiological tremor under conditions of relaxation. Similarly, when the tremors start to diminish under periods of stress, the changes to the subject’s voice are typically not audible. That’s where the CVSA comes in. This technology can detect changes in the frequency modulation (FM) frequency, which makes it possible to determine whether physiological tremors are occurring when a subject is speaking and therefore whether the parasympathetic nervous system or the sympathetic nervous system is in control. If it is the latter, a law enforcement professional can conclude the subject was under stress when answering the question, which may be a sign of deception.
By making it possible to determine whether the parasympathetic nervous system or sympathetic nervous system is controlling unconscious physiological processes, the technology can provide law enforcement professionals with a reliable indicator of subject stress and possible deception during questioning. Therefore, the science behind the CVSA demonstrates that it can be an invaluable tool, whether your agency conducts criminal investigations, general screening examinations, or covert analysis.
Please reach out to us at NITV Federal Services to learn more about our CVSA systems and training programs.