Hemoencephalography

Hemoencephalography (HEG) is a relatively new neurofeedback technique within the field of neurotherapy. Neurofeedback, a specific form of biofeedback, is based on the idea that human beings can consciously alter their brain function through training sessions in which they attempt to change the signal generated by their brain and measured via some neurological feedback mechanism. By so doing, participants increase cerebral blood flow to a specified region of the brain, consequently increasing brain activity and performance on tasks involving that region of the brain.

Both approaches to hemoencephalography, near infrared and passive infrared, are indirect measures of neural activity based on neurovascular coupling. Neurovascular coupling is the mechanism by which cerebral blood flow is matched to metabolic activity. When a region of the cortex is used in a specific cognitive task, neuronal activity in that region increases, consequently increasing local metabolic rate. To keep up with the nutritional and waste removal demands of a higher metabolic rate, cerebral blood flow to the cortical area in use must increase proportionally. Along with the increase in flow, hemoglobin molecules in the blood, which are responsible for the transport and transference of oxygen to tissue throughout the body, must increase the amount of oxygen they deliver to the activated region of the cortex, resulting in a greater local blood oxygenation level. This is also referred to as the haemodynamic response.

Developed by the godfather of hemoencephalography, Dr. Hershel Toomim, near infrared hemoencephalography measures changes in the local oxygenation level of the blood. Similar to functional magnetic resonance imaging, which uses changes in the magnetic properties of blood resulting from oxygenation to form an image of brain activity, NIR utilizes the changes in blood translucence resulting from oxygenation to generate a signal that can be consciously manipulated in neurofeedback sessions. At the most basic level, NIR hemoencephalography shines alternating red (660 nm) and near infra-red (850 nm) light on a specified area of the brain, usually through the forehead. While the skull is largely translucent to these wavelengths of light, blood is not. The red light is used as a probe, while the infrared light provides a relatively stable baseline for comparison. Photoelectric cells in a spectrophotometer device worn on the forehead measure the amount of each wavelength of light reflected by cerebral blood flow in the activated cortical tissue and send the data to a computer, which then calculates the ratio of red to infrared light and translates it into a visual signal of corresponding to oxygenation level on a graphical interface the patient can see. The key nutrient monitored by NIR is oxygen. In NIR, as the ratio of oxygenated hemoglobin (HbO2) to deoxygenated hemoglobin (Hb) increases, the blood becomes less and less translucent and scatters more of the red light, instead of absorbing it. In contrast, the amount of infrared light scattered by the blood is largely impermeable to changes in the oxygenation level of hemoglobin.

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