| Dear All, I have received a few ‘private’ replies to my post last month asking about pros and cons of auditory stimulus equipment for EEG recordings (http://www.auditory.org/mhonarc/2014/msg00184.html); I have also had a couple queries whether I would mind sharing these. So here is a synopsis of the replies that I have gotten, which may be informative for some. Please jump in and correct if any of this is inaccurate or incomplete. Previously, I had heard from someone who had done a comparison of insert earphones (3MTM E-A-RTONE), headphones (Beyerdynamic TD-48), and a loudspeaker, using a melon; all three ways of stimulus delivery produced some sort of artifact (and in a way the insert earphones produced larger artifacts than headphones, though that might have been due to the placement of the transducers). But a melon is apparently not such a good model for EEG; a conductive medium, e.g. a bucket of salt water, would perhaps be better, lest the leads end up acting like antennae. Again, if anyone has done or is aware of a comprehensive comparison, I for one would be very interested (and I know others would be too). For looking at brainstem or oscillatory activity in particular it seems like insert earphones, e.g. the Etymotics (e.g. https://www.etymotic.com/pro/er3a.aspx), are the way to go, since these arguably introduce the smallest amount of acoustic signal correlated corruption by virtue of the last part being a plastic tube (i.e. there is no transducer near the head or ear). For brainstem recordings in particular, higher frequencies are of interest that might overlap with those of the acoustic stimuli - this is not so much an issue when looking at low-frequency ERPs - and it is therefore vital to exclude these from interfering with the recording. However, since in those insert earphones the last part is a tube, the signal fidelity drops for frequencies > 4 kHz. Another potential disadvantage of insert earphones is that one needs to take great care fitting them into the ear canal to a) achieve a good seal and b) avoid one of them coming out over the course of the experiment, which introduces a lateralized sound percept. Similarly, apparently some EEG caps have relatively small cuts to fit the ears through, and fitting insert earphones might be difficult in this case. Headphones have comparatively better overall sound quality and are widely used. One thing to look out for is to use supra-aural instead of circumaural ear cushions, and secondly that the bracket/bow must not touch any leads. They might also start to feel uncomfortable for long experiments. It seems no-one has used the Sensimetrics S14 earphones in an EEG set up. Hope this was helpful to some, Tobias -- Tobias Overath, PhD Duke Institute for Brain Sciences P.O. Box 91003 Duke University Durham, NC 27708 U.S.A. |