300
W.E Dolphin
Electrophysiologic potentials are transduced from the animal's scalp for
conveyance to the subsequent electronic instrumentation via electrodes.
The transfer of electrical current from biologic tissue to a conductive electrode requires a low impedance electrical contact between the two elements. Increased impedance due to poor contact results in poor signal
quality, hence a degraded response. Of particular concern is complete electrical isolation of each recording electrode, which requires special attention
in the highly conductive saltwater medium.
In many experiments subdermal needle electrodes, as widely used with
human recordings, have been used (e.g., Popov and Supin 1990). Needle
electrodes have the advantage of insuring low impedance contact and do
not appear to be painful to the animals; a major disadvantage of such
recording techniques is that the electrode insertion site must remain above
the water in order to preserve electrical isolation and, therefore the animal
must maintain a position with the melon and dorsal region above the water
surface. Additionally, precautions must be taken to minimize the possibility of infection at the insertion site. An alternative to needle electrodes is
the use of silver or gold disc electrodes imbedded within latex suction cups
(e.g., Dolphin et al. 1995). While impedances are somewhat higher than
obtained with needle electrodes and require cleaning of the skin surface to
remove dead or flaking epidermis, the advantages include easy and free
placement of suction cup electrodes anywhere on the body surface and the
animals may be completely submerged to position themselves within
the water column. Suction cup electrodes are totally noninvasive, thereby
reducing risk of infection, and completely isolate each electrode from the
surrounding saltwater medium and each other.
As discussed below, the amplitudes of AEPs recorded from the scalp are
quite small, generally in the microvolt range, and thus must be amplified
prior to analog-to-digital conversion. Additionally, the desired response
signal is "contaminated" with ongoing EEG and myogenic potentials, often
orders of magnitude larger in amplitude than the AEP of interest. Therefore, although it is necessary to amplify the EEG signal to optimize analogto-digital conversion, it is also important to exclude, reject, or at least
minimize contributions of sources other than the desired response signal.
A differential recording technique, using two active and one reference
electrode, is often of assistance in this process. Differential amplification
changes the sign (i.e., inverts) the input of one active electrode relative to
that of the other active electrode, thereby effectively subtracting signals that
are common to both electrodes. The premise underlying differential recording is that electrical potentials emanating from sources close to the recording site (i.e., auditory structures) which are different (and out of phase)
between the two active electrodes are amplified and thereby enhanced,
whereas potentials that are common and in phase at each recording site,
presumably generated by sources far from the recording site (e.g., myogenic
activity and background noise), will be canceled.
W.E Dolphin
Electrophysiologic potentials are transduced from the animal's scalp for
conveyance to the subsequent electronic instrumentation via electrodes.
The transfer of electrical current from biologic tissue to a conductive electrode requires a low impedance electrical contact between the two elements. Increased impedance due to poor contact results in poor signal
quality, hence a degraded response. Of particular concern is complete electrical isolation of each recording electrode, which requires special attention
in the highly conductive saltwater medium.
In many experiments subdermal needle electrodes, as widely used with
human recordings, have been used (e.g., Popov and Supin 1990). Needle
electrodes have the advantage of insuring low impedance contact and do
not appear to be painful to the animals; a major disadvantage of such
recording techniques is that the electrode insertion site must remain above
the water in order to preserve electrical isolation and, therefore the animal
must maintain a position with the melon and dorsal region above the water
surface. Additionally, precautions must be taken to minimize the possibility of infection at the insertion site. An alternative to needle electrodes is
the use of silver or gold disc electrodes imbedded within latex suction cups
(e.g., Dolphin et al. 1995). While impedances are somewhat higher than
obtained with needle electrodes and require cleaning of the skin surface to
remove dead or flaking epidermis, the advantages include easy and free
placement of suction cup electrodes anywhere on the body surface and the
animals may be completely submerged to position themselves within
the water column. Suction cup electrodes are totally noninvasive, thereby
reducing risk of infection, and completely isolate each electrode from the
surrounding saltwater medium and each other.
As discussed below, the amplitudes of AEPs recorded from the scalp are
quite small, generally in the microvolt range, and thus must be amplified
prior to analog-to-digital conversion. Additionally, the desired response
signal is "contaminated" with ongoing EEG and myogenic potentials, often
orders of magnitude larger in amplitude than the AEP of interest. Therefore, although it is necessary to amplify the EEG signal to optimize analogto-digital conversion, it is also important to exclude, reject, or at least
minimize contributions of sources other than the desired response signal.
A differential recording technique, using two active and one reference
electrode, is often of assistance in this process. Differential amplification
changes the sign (i.e., inverts) the input of one active electrode relative to
that of the other active electrode, thereby effectively subtracting signals that
are common to both electrodes. The premise underlying differential recording is that electrical potentials emanating from sources close to the recording site (i.e., auditory structures) which are different (and out of phase)
between the two active electrodes are amplified and thereby enhanced,
whereas potentials that are common and in phase at each recording site,
presumably generated by sources far from the recording site (e.g., myogenic
activity and background noise), will be canceled.