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Biomedical Signal and Image Processing
AEP can be due to damages either in the ear or in the CNS. Further tests will verify
the exact source of the abnormality observed in the AEP.
Certain pathological neurological sensory conditions as well as mechanical pathological auditory conditions can be treated by surgical methods. The importance of
the AEPs can be further realized knowing that the AEPs serve as the main diagnostic tool to detect these conditions.
10.4.2 SOMATOSENSORY-EVOKED POTENTIALS
SEPs are accomplished by the application of electric impulses administered to the
skin of the arms or legs. The response of the CNS to these stimuli is then measured
and analyzed to identify the sensitivity and functionality of the neuromuscular system under study. It is often the case that the stimuli are applied to the arm or leg that
is closest to the muscular system under study.
Under normal conditions, these electric impulses delivered through electrodes
pass through the skin and result in motor neuron actions. These neuron actions are
typically followed by muscle contractions. The muscle contractions are stimulated
by delivery of currents in the range of 25–50 mA. The electric impulses are usually
delivered in pulse mode to avoid saturation and muscle fatigue. The pulse durations
are varied between 100 μs and 1 ms. It is standard procedure to apply the stimulus at
various distances from the location where the EPs are recorded.
The measured time delay between the stimulus and EP is often treated as the
main indication of pathological characteristics. A long observed delay indicates that
the neural pathways involved in the transmission of the stimuli to the brain are somehow damaged. The SEPs are typically used as the main diagnostic tools to identify
neuromuscular disorders. They are also utilized to measure the depth of anesthesia
in some specialized surgeries.
10.4.3 VISUAL-EVOKED POTENTIALS
VEPs are EEG signals measured when the patient is subjected to the visual stimuli.
The applied visual stimuli often contain high-contrast patterns stretching the entire
field of view. Such stimuli typically generate sensory potentials that are transmitted
to the brain and are reflected in the measured EEG. The main part of the brain that
registers VEPs is the primary visual cortex, located on the rear of the head in the
occipital lobes of the brain. However, other areas of the brain have been found to
show EPs as well.
During the course of measuring VEPs, there cannot be any distractions, especially other visual disturbances, because the brain stimulus needs to be responding
only to the applied visual stimulus. A standard visual stimulus is the use of a checkerboard that changes the location of the black and white squares. Other stimuli are
flashes of light that turn on and off with a certain frequency and other light-intensity
patterns. One aspect of designing useful visual stimuli is calibration and standardization of the stimuli. The aspects of the stimuli that need to follow certain standards
include the light color(s), the light intensity, the pupil diameter, and the degree of eye
fixation on the stimulus.
Biomedical Signal and Image Processing
AEP can be due to damages either in the ear or in the CNS. Further tests will verify
the exact source of the abnormality observed in the AEP.
Certain pathological neurological sensory conditions as well as mechanical pathological auditory conditions can be treated by surgical methods. The importance of
the AEPs can be further realized knowing that the AEPs serve as the main diagnostic tool to detect these conditions.
10.4.2 SOMATOSENSORY-EVOKED POTENTIALS
SEPs are accomplished by the application of electric impulses administered to the
skin of the arms or legs. The response of the CNS to these stimuli is then measured
and analyzed to identify the sensitivity and functionality of the neuromuscular system under study. It is often the case that the stimuli are applied to the arm or leg that
is closest to the muscular system under study.
Under normal conditions, these electric impulses delivered through electrodes
pass through the skin and result in motor neuron actions. These neuron actions are
typically followed by muscle contractions. The muscle contractions are stimulated
by delivery of currents in the range of 25–50 mA. The electric impulses are usually
delivered in pulse mode to avoid saturation and muscle fatigue. The pulse durations
are varied between 100 μs and 1 ms. It is standard procedure to apply the stimulus at
various distances from the location where the EPs are recorded.
The measured time delay between the stimulus and EP is often treated as the
main indication of pathological characteristics. A long observed delay indicates that
the neural pathways involved in the transmission of the stimuli to the brain are somehow damaged. The SEPs are typically used as the main diagnostic tools to identify
neuromuscular disorders. They are also utilized to measure the depth of anesthesia
in some specialized surgeries.
10.4.3 VISUAL-EVOKED POTENTIALS
VEPs are EEG signals measured when the patient is subjected to the visual stimuli.
The applied visual stimuli often contain high-contrast patterns stretching the entire
field of view. Such stimuli typically generate sensory potentials that are transmitted
to the brain and are reflected in the measured EEG. The main part of the brain that
registers VEPs is the primary visual cortex, located on the rear of the head in the
occipital lobes of the brain. However, other areas of the brain have been found to
show EPs as well.
During the course of measuring VEPs, there cannot be any distractions, especially other visual disturbances, because the brain stimulus needs to be responding
only to the applied visual stimulus. A standard visual stimulus is the use of a checkerboard that changes the location of the black and white squares. Other stimuli are
flashes of light that turn on and off with a certain frequency and other light-intensity
patterns. One aspect of designing useful visual stimuli is calibration and standardization of the stimuli. The aspects of the stimuli that need to follow certain standards
include the light color(s), the light intensity, the pupil diameter, and the degree of eye
fixation on the stimulus.