What is it?
Somatosensory Evoked Potentials (SSEP) are a neurophysiological assessment technique invented to measure the speed and quality of electrical information carried from a peripheral nerve to the parietal, or somatosensory cortex, to help a clinician to identify neurological disorders. However, over the decades, researchers realized that these electrical potentials were capable of facilitating neuroplasticity through the manipulation and adaptation of somatosensory maps (1-3) via a non-invasive, electrical, neuromuscular stimulator delivering an asymmetrical biphasic pulse (4,5) over a specific point on the skin. This therapy has been termed Non-Invasive Neuromodulation (NINM), or more specifically, Repetitive Peripheral Somatosensory Stimulation (RPSS). Regardless of what it is called, the stimulation does not cause pain, but instead is reported to feel like a light tapping, tingling, or thumping over the contact point.
Why is it important?
The targeted area of the brain, called the somatosensory system, is comprised of modalities of touch, vibration, temperature, pain and kinesthesia (sense of movement). Because the perception of all external and internal stimuli is integrated in the brain, the evoked potentials generated in the periphery are relayed through pathways involving receptors in the skin, nuclei in the spinal column, brainstem, thalamus, and cerebral cortex (primary somatosensory cortex, primary motor cortex) (5-7). Located in the primary somatosensory cortex (parietal lobe), as well as the primary motor cortex (frontal lobe), is a topographical map, known as the homunculus, reflecting the amount of cortex dedicated to a to a specific body part and its particular function (8,9).
It is important that these body maps be accurate because of the many subsequent areas associated, coordinated and integrated with the sensations mentioned above. These areas include, but are not limited to, the vestibular system (balance/sensing motion), frontal lobe (executive function/motor planning) (1-3,10), posterior parietal cortex (spatial orientation), and cerebellum (motor coordination/error processing) (11). Dysfunction in any of these areas may cause these maps to be altered, skewed, or misrepresented, resulting in symptomology of spatial disorientation, postural abnormalities, dizziness, oculomotor dysfunction, movement disorders, motor planning, etc. (1.3,10,11).
How does it work?
In addition to modulating and correcting somatotopic maps, NINM stimulation has also been researched and proven to have a positive effect on cognitive states (24-26), blood pressure/autonomic function, balance and stability (21,22), and even visual acuity, when applied to specific areas, such as, locations on the face tongue, and wrist. This phenomenon occurs through a series of reflex-arcs that integrate in the brainstem, the primary region for consciousness and autonomic regulation.
The median nerve, located on the palmar, thumb-side of the wrist (12,13), when stimulated, provides a peripheral entryway to the Ascending Reticular Activating System (consciousness center) located in the brainstem (autonomic regulation). It makes sense then, that stimulation to this pathway has been shown to be sufficient to cause clinical improvement in many conditions involving altered states of consciousness, such as, Alzheimer’s Disease and even comatose patients (12-15).
The Trigeminal Nerve (Cranial Nerve 5), originating in the brainstem, innervates the face through three separate branches. Stimulation of any of these branches (Ophthalmic, Maxillary, Mandibular) stimulates a reflex known as the Trigeminal-Cardial Reflex (TCR). Within seconds after initiation or stimulation, powerful and differentiated activations of sympathetic pathways are engaged, attenuating physiological homeostatic responses, such as baroreceptors and chemoreceptor reflexes for blood pressure regulation (16-20).
Stimulation to the tongue has been shown to provide stimulation to an area of the brainstem specifically within the region of the pons (33). This region is important because of the nuclei that originate here, including vestibular, trigeminal, and solitary nuclei. The trigeminal nuclei receive somatosensory information from the tongue while the solitary nuclei receive information about taste, but because of such complex and coordinated interactions between these structures, including but not limited to, co-modulation of visual, vestibular (balance/stability), visceral sensory (sensations from organs) and pain signals, SSEP to the tongue could be involved in the resolution of various symptoms such as hypersensitivity to visual stimuli in balance, anxiety disorders, balance dysfunction in migraine disorders, and interoception (physiological sense of well-being) (21,22, 27-32).
How does it help?
Likewise, the presence of any of the aforementioned symptomatology could be a subclinical que or biomarker indicating the need for somatosensory evoked potential over a specific area of the body.