Neural Mechanisms Behind the Polygraph
The human nervous system is organized into two primary divisions: the Central Nervous System (CNS), consisting of the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes all other neural structures such as nerves and muscles. For the purpose of understanding the physiological mechanisms underlying the polygraph, attention is directed solely to the PNS, as it governs the involuntary, physiological responses recorded during testing.
The PNS can be further divided into the Somatic Nervous System, responsible for voluntary motor control, and Autonomic Nervous System (ANS), which regulates involuntary processes such as heart rate, respiration, and glandular activity. Since the polygraph measures involuntary physiological changes, the somatic division may be set aside, and focus shifted to the ANS.
While the entirety of the Autonomic Nervous System is under involuntary control, the ANS itself comprises two functionally distinct branches: the Parasympathetic Nervous System (PSNS) responsible for the “Rest and Digest” response and the Sympathetic Nervous System (SNS) or the “Fight or Flight” response. The delineation between these two systems is key to understanding the mechanism behind the polygraph, as they operate in dynamic balance, modulating physiological states according to environmental and psychological demands.
The PSNS, often summarized as the “rest and digest” state, maintains homeostasis under conditions of relative calm. Parasympathetic pathways employ a two-neuron chain in which both preganglionic and postganglionic neurons release acetylcholine (ACh) to induce physiologic changes in effector organs. Resultant effects include: decreased HR, reduced respiratory rate, diminished skin conductance, and other markers of physiological relaxation. During the polygraph examination, deviations from the relative baseline, enacted by the SNS, are measured and recorded.
In contrast, the Sympathetic Nervous System (SNS) mediates our fight or flight response, mobilizing resources in the presence of a perceived threat or stress. The degree of stress is determined psychologically which in turn generates an adequate physiological response. Like the PSNS, sympathetic pathways utilize preganglionic neurons that release ACh. However, postganglionic neurons predominantly release norepinephrine (NE) which bind and activate adrenergic receptors on effector organs throughout the body. Additionally, as the body floods with NE, the adrenal medulla secretes epinephrine into the bloodstream, amplifying the systemic response. Contrary to the PSNS, the SNS primes the body to run or fight, so the systemic changes are generally opposite: increased heart rate, elevated respiratory rate, and heightened skin conductance- precisely the variables captured by polygraph instruments.
By contrasting parasympathetic homeostasis with sympathetic arousal, the polygraph purports to detect deception through autonomic fluctuations. This premise, however, rests on a flawed assumption: that the brain possess dedicated pathways for distinguishing lies from other forms of arousal. In reality, autonomic responses are evolutionarily conserved and nonspecific, triggered indiscriminately by stress, anxiety, fear, or even the pressure of the test itself. Consequently, the validity of the polygraph as a measure of truthfulness remains highly contested.
Domenica Rehmann