Neuroscience Poster Session
Discussion and Conclusion
About the used parameters, and as regards specifically to the stimulus presentation, it should be said that, although some authors (Gorga, 1991) explain that the stimulus polarity can affect low frequencies (being the alternating one not very appropriate because of the difficulty to get good recordings and advising therefore fixed polarity). We obtained in the previous recordings, good identification of the waves with the use of alternating polarity and at the same time the presence of external interference was avoided (Malinoff and Spivak, 1990).
As regards the filters, we maintained those that used for the click, since we had obtained very satisfactory identification results with them, and this allowed us to carry out in a practical way both tests with no need to modify parameters.
A controversial fact that we had previously discussed was the intensity that we would be using for the study that would provide us objective data regarding the recording values that we would obtain. Because of this, we carried out a previous valuation of subjective threshold for each one of the frequencies with the four used logon stimuli; the results that we obtained from this audiometry study led us to use 40 dB p.e. SPL as the minimum value, which was placed in all the frequencies in an average of 10 dB above the subjective auditory threshold, and this would take us to obtain objective values of the measurable recordings.
About the results we obtained, on the one hand we have assessed excellent correlation of the waves identification, especially regarding wave V, which allows us to continuously study its evolution in latency. On the other hand, the evident and reliable obtained recordings with an easy identification of waves I and III leads us to assess the interlatencies and therefor the permeability of the auditory pathway. Nevertheless, when dealing with the identification of waves, we may find greater difficulties than the ones that can be found in the recordings with click, and especially in low frequencies, a problem already stated by other authors (Laukli and Mair, 1986; Ito et al, 1988; Purdy et al, 1989; Telian and Kileny, 1989).
We must insist on the fact (as highlighted in previous works) that the latencies increase as the frequencies decrease, and this is justified by the smallest spectral implication of the frequency. By the reflection to travelling wave. By the partial correspondence between the stimulus spectrum and the effective spatial distribution in the cochlea (Hyde, 1985). And finally by the increase in the time of sensorial analysis in low frequencies, even with Mesencephalon fibres of longer itinerary for these frequencies (McAnally and Calford, 1992; Woods et al, 1993).
One of the main reasons of criticism expressed over the year regarding studies with frequency stimuli has been the fact that they are difficult to identify with low intensities in low frequencies. Apart from that, the answers could be influenced by high frequencies of cochlea (Stapells and Picton, 1981; Beattie et al, 1994). We have never intended to debate about something already stated and expressed by others authors. But only to present some objective and factual data evaluation in recordings that we obtained with logon stimuli, requiring degree likely to happen in low frequencies, as regards its possible spectral implication of high frequencies in the obtained results.
Consequently, from our point of view, the non-availability of ipsilateral masking for the performing of frequency stimuli technique like the one used is not an indispensable requirement. The obtained recordings can be easily identified and measured at higher levels of the threshold, and it can be used in the objective valuation of the audition, intended for non co-operative patients, and providing in that way a more direct approach than the click to the real state of the patient's audition (Frattali et al, 1995).
The second clinical contribution would be the use of this technique for retrocochlear pathologies diagnosis, already manifested by other authors, in relation to the use of other frequency stimuli, especially the 1.000 Hz stimulus, in cases where the click valuation is not clear (Telian and Kileny,1989; Robier et al, 1992; Fowler and Mikami, 1992; Conjin et al, 1993).
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|Mata, J; Ruiz Carmona, E; (1998). Brainstem Auditory Evoked Potentials With Logon Stimuli. Frequency-Specific Study. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Available at URL http://www.mcmaster.ca/inabis98/neuroscience/mata0857/index.html|
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