Interacoustics Eclipse Additional Information page 229

Eclipse Additional Information Page 219
Epoch: The recording obtained after each stimulus. It is these Epochs that are averaged together to provide
the resulting waveform. This term is also referred to as timebase or window.
Filter: The electrical potentials picked up by the electrodes contain a range of frequencies. Depending on
the type of measurement we are interested in, only a limited range of frequencies has our interest. To re-
move the possible contamination caused by the presence of the unwanted frequencies these are removed
by the application of filters.
This unit has two different types of filters – a basic analogue filter at the Preamplifier level, and a digital filter
applicable to the averaged recorded waveform when displayed.
The digital filters come as low pass (which allows the lower frequencies to pass through, but removes the
high frequencies which give detail to the waveform) and high pass (letting the high frequencies pass, but re-
moving the lower frequencies). As the waves themselves are often partly made up of low frequencies, the
low pass filter may sometimes be set to "none", to obtain maximum deviation of the waves.
The digital filters of this unit can be applied to any waveform you choose to display, as all waveforms are
stored in the database in their raw format without any digital filtering being applied.
Gain: The EEG picked up by the electrodes is an extremely small signal – often ranging around 20µV (20
millionths of a Volt). A defined amount of gain (amplification) must be applied to this weak signal by the Pre-
amplifier so it can reach the level required by the digital circuits in the ABR unit. As these digital circuits vary
between different types of ABR units, no direct gain comparison between different manufactures makes
much sense. This unit therefore also gives the corresponding µV level for which a given amplification is in-
tended. This sensitivity normally ranges from 20µV to 40µV for most patients, and with this unit it corre-
sponds to a gain setting ranging from 98 to 92 dB respectively. See also Rejection.
High Pass Filter: See Filter.
Interlatency Time: The time which elapses between two peaks on a waveform. If this time is prolonged, this
could be an indication of the presence of a tumour or other malfunctions of the auditory nerve system. In par-
ticular, the interlatency time between ears is an easy parameter to judge – just remember that unilateral or
asymmetric hearing loss can cause the stimulus reaching the poorer ear to be heard lower effective level
this itself causes a natural prolongation of wave III and wave V on the poorer ear's response latencies when
compared to the better ear. Latency corrections for such hearing asymmetries have been published in the
research literature (Selters & Brankmann, 1977).
Latency: The time after stimulus onset at which the wave peaks occur. This is a very important diagnostic
measurement. The timing of ABR wave III and especially wave V is used for neurologic screening tests.
They may then be compared to a normative data material for Pass/Refer evaluation.
Latency Time naturally increases when stimulus intensity is lowered, stimulus frequency is lowered, and age
is lower than 18 months. Neonates tested with stimuli close to their threshold thus have quite long latency
times for wave V. Interpretation of latencies therefore must take these systematic effects into account. One
popular otoneurological measurement that is relatively immune to the effects of stimulus intensity, hearing
loss, age and gender is the difference in latencies of ABR waves I & V (the I-V inter-peak interval).
LBK15: The LBK15 it is possible to route the stimulus signal back to the electrodes, this way obtaining a
complete functional check of the stimulus generation as well as of the data acquisition system. The imped-
ance checking circuitry can be tested.
LL: Late Latencies. See ALLR
LLR: Late Latency Response. See ALLR
Low Pass Filter: See Filter
ML: Middle Latency Response. See AMLR.
MLR: Middle Latency Response. See AMLR