A table below summarises the test parameters.
The electrode montage used for the N1-P2 cortical response is a Cz (vertex) /mastoid electrode pair. Some loss of response amplitude occurs if a high forehead site is chosen instead of Cz (Vaughan & Ritter, 1970). Either mastoid can be used as the reference site, regardless of test ear and indeed, a slight (√2) reduction in myogenic activity can be achieved by using a linked mastoid arrangement. By convention, a forehead ground is used.
The filter settings (recording bandwidth) depend, of course, on the spectral peak of the N1-P2 response which lies in the range 2 to 5 Hz. Since we are interested in response detection (rather than analysis), a narrow filter bandwidth helps achieve good signal to noise ratio and is optimally 1 Hz to about 15 Hz (30 Hz can be used if this is the lowest available low-pass setting).
The analysis epoch (time base or window) can be in the range 500 to 1000 ms. It is useful to include a pre-stimulus epoch of about 250 ms to assist in the assessment of background activity. As with other ERA tests, it is important to duplicate or triplicate the response, particularly when the response is small, close to threshold.
Although a click or tone pip may be used, the stimulus of choice is a tone burst of the desired audiometric frequency. The response can be detected at all audiometric frequencies although at frequencies above 2kHz, a smaller response is recorded and so the precision of the threshold estimate is probably poorer. The frequency specificity of this stimulus, and of the response it evokes, is almost ideal and far better than that afforded by tone pips used in ABR tests. This is simply a by-product of the number of cycles in the stimulus. The rise time of the tone burst is an important parameter. If this was very short (if we were to abruptly present the tone burst without a gradual rise time) then we would suffer from a loss of frequency specificity which may be important in steeply sloping or notched audiograms. However, the amplitude of the cortical response diminishes if long rise and fall times are used. A good compromise is to have a linear rise time of 10 to 20 cycles (e.g. 10 ms at 1 kHz). The "plateau" of the tone burst also needs to be defined. Very brief plateaus (<25ms) would compromise frequency specificity and also affect the loudness of the stimulus through the process of temporal integration and hence diminish the response (Davis & Zerlin, 1966; Skinner & Jones, 1968). After the first 30-50ms of the stimulus, the response has been evoked, so there is little merit in extending a plateau for much longer than this. Interestingly, many centres use tone bursts of 100 ms or more. Very long tone bursts should be avoided, since the end of the tone burst will also evoke a cortical "off response" as well as slightly and unnecessarily extending the test time. Those centres using long plateau times will argue that they do so in order to intentionally separate the on and off responses. A plateau of around 100 ms (often advocated) should be avoided since in theory, this can cause the destructive overlapping of the onset P2 and offset N1 responses. In practice, these arguments are rather academic and a plateau of either about 50 ms or 200ms is acceptable. A stimulus of this duration allows us to use the calibration reference values available for pure tone audiometry since the extent of temporal integration is small enough to ignore. Until the recent availability of ISO 389-6 (2007) giving reference values for ABR stimuli, this was a great practical advantage over ABR tests for which there was no agreed calibration values - of particular importance in the medico-legal context.
The choice of stimulus repetition rate is critical and represents a compromise between two opposing considerations. On the one hand, we would like to make the rate fast to shorten the test time, especially if we have several frequencies to test. On the other hand, we do not want to degrade the response and so make its identification difficult. A reasonable question to ask is "what is the maximum rate that does not degrade (reduce the amplitude) of the response?". To record a response unaffected by rate effects, we need to keep the rate down to about one stimulus every ten seconds, i.e. 0.1Hz (Appleby, 1964; Davis et al, 1966). Using a rate this slow would make the test very time consuming. Although rates above 0.1Hz diminish the response, the rate that yields the best signal to noise ratio improvement per unit test time is chosen. For cortical responses in adults it is normal to have a repetition rate between 0.5 and 1.0 stimuli per second (1 - 2 seconds between stimuli) (Rapin, 1964; Davis & Zerlin, 1966). In older children 0.25 to 0.5 Hz (2 – 4s between stimuli) is required. At these rates we record a partially adapted response but we do so in a reasonable time. Of course the very first stimulus in an averaging run is un-adapted because it is preceded by silence and is therefore large. The second is somewhat adapted and the third is more so. The amplitude continues to diminish slightly during the average, though the biggest change is at the start of the averaging run (Walter, 1964; Ozesmi et al, 2000).
The above feature plays a part in our choice of the number of sweeps in an average. A very common mistake is to over-average. Averages containing more than 50 sweeps (used to further improve the signal to noise ratio) are often counter-productive, and merely serve to further adapt the response (Henry & Teas, 1968). The number of stimuli required to produce an acceptable response depends upon the size of the response. Stimuli above about 20 dBSL usually produce a clear response after 20 or so stimuli whereas closer to threshold, 30 to 50 stimuli may be required. Replication is essential and for greatest efficiency, the above numbers of sweeps should be distributed across several sub-averages and then combined to form a grand average (e.g. 30 sweeps in total, 10 sweeps in each of 3 sub-averages).
Another way of enhancing response detection is to use a non-rhythmical stimulus and some systems provide the facility for a pseudo-random stimulus rate. This facility used to be common on systems 20 years ago but few systems offer it now - so much for progress! This is also useful in prolonged testing sessions where the response amplitude diminishes due to habituation - a process which can be in part reduced by making the stimulus less predictable (Rapin, 1964; Rothman et al, 1970). Other tactics may involve randomising other aspects of the stimulus, for example the ear under test (Butler, 1972), test frequency or test intensity. Giving the patient a brief break or making them more alert in some other (devious?) way can rejuvenate a flagging response.