[Eeglablist] Correct for EGI latency error functions

Mon Oct 12 06:09:22 PDT 2015

Regarding the latency issue, there were actually two announcements about the latency issue from EGI (they are attached below).
The original notice says that there was a significant timing delay in most of Net Amp models but later they changed it.
I don’t know Isaiah’s codes reflect the most recent announcement from EGI.
In their second announcement, EGI said that they found latency delay only in NetAmp 300 model with some specific versions of Netstation.

See below.

*** Original announcement ****

Electrical Geodesics, Inc. Advisory Notice:
Anti-Alias Filter Effects on EEG Timing

This advisory notice is to alert users of EGI’s GES 300 and 400 series amplifiers to the delay of EEG data inherent in the anti-aliasing filters of these amplifiers. These delay values have not been effectively communicated to EGI customers and need to be considered when aligning events (such as events generated from E-Prime or from digital inputs) to the EEG.

The digital Finite Impulse Response filters used prior to A/D conversion delay the EEG by a fixed interval. This interval is known for each sampling rate and amplifier model:

Sampling Rate

NA 300

NA 400

NA 405

NA 410

1000 s/s

8 ms

36 ms

36 ms

13 ms

500 s/s

18 ms

66 ms

66 ms

34 ms

250 s/s

36 ms

112 ms

112 ms

76 ms

125 s/s

72 ms

Without adjustment, the EEG is delayed with respect to real time and to event markers that are aligned with real time. This delay does not affect timing of events manually entered by the user with respect to the EEG during acquisition.

Particularly for the Net Amps 400, 405, and 410 models at lower sampling rates, the delays may be significant in considering EEG and ERP timing. To adjust for this delay, users must add a positive value in the “Offset Segment” field in the segmentation tool, in addition to E-Prime Event or digital input event (DIN) offsets.

This adjustment is only required for real time events that are recorded during acquisition. Events entered during review or from the operations of tools after data acquisition are already aligned correctly with the EEG.

To make the delays clear to each user, both the EGI technical manuals and EGI Support training will be modified to underscore the importance of including these delays in the event alignment and segmentation process. In addition, the Net Station software (version 5.1.1) will provide an option for automatic adjustment.

<http://cts.vresp.com/c/?ElectricalGeodesicsI/25e92832cf/6774d3b846/1bb8b10916/utm_content=gfrishkoff%40gsu.edu&utm_source=VerticalResponse&utm_medium=Email&utm_term=&utm_campaign=Advisory%20Notice%20-%20Anti-Alias%20Filter%20Effects%20on%20EEG%20Timing>Please contact EGI support<mailto:supportteam at egi.com> for any questions.

*** Second announcement ****

Electrical Geodesics, Inc.
Update to Anti-Alias Filter Effects on EEG Timing Advisory Notice

This is an update to an advisory notice issued to users of EGI’s GES 300 and 400 series amplifiers on the delay of EEG data inherent in the anti-alias filters of these amplifiers.

In the original advisory notice, it was noted that the digital Finite Impulse Response filters used after analog-to-digital conversion delay the EEG by a fixed interval by the following values:

Sampling Rate

NA 300

NA 400

NA 405

NA 410

1000 s/s

 8 ms (0.52)

 36 ms (0.63)

 36 ms (0.52)

 13 ms (0.52)

500 s/s

 18 ms (0.63)

 66 ms (1.03)

 66 ms (1.03)

 34 ms (1.10)

250 s/s

 36 ms (2.07)

 112 ms (0)

 112 ms (0)

 76 ms (2.07)

125 s/s

 72 ms (2.53)

Without adjustment, the EEG is delayed with respect to real time and to event markers that are aligned with real time. This delay does not affect timing of events manually entered by the user with respect to the EEG during acquisition.

The delay values in the original Advisory Notice were verified with the most recent versions of Net Station 4.5 and 5.0, representing the software versions that are most widely used by our customer base. The original Advisory Notice was issued immediately after this verification effort.

Since releasing the original Advisory Notice on 28 August 2014, we have continued our verification testing to confirm these delay values in all other versions of Net Station associated with the NA 300 amplifier, in order to provide our customers not using Net Station 4.5 or 5.0 with more specific guidance.

With the NA 300 amplifier, we found that the filter delay is only present in Net Station version 4.4 and later; events are correctly aligned with the EEG data when acquired with earlier versions of Net Station (4.3.1 and earlier). Furthermore, we found that for data recorded with Net Station versions 4.4 through 4.5.7, the correction values for the default 250 s/s sampling rate (which is likely used by the majority of customers) is 8 ms.

For data acquired with the NA 400, NA 405 and NA 410 amplifiers, there is no modification to the delay values presented in the original Advisory Notice.

Starting with Net Station 4.4, all data files contain an amplifier serial number. This information can be used to determine whether adjustments are required for a given data file acquired with an NA 300 amplifier. Contact EGI support for information on how to locate the amplifier serial number (if present) in the data file.

EGI will release a tool to adjust the latency delay in affected files in the coming weeks. In addition, the Net Station software (version 5.1.2) will provide an option for automatic alignment during acquisition.

<http://cts.vresp.com/c/?ElectricalGeodesicsI/becac2c8a8/2eeb100ff4/ec13d0112a/utm_content=ascherme%40uvm.edu&utm_source=VerticalResponse&utm_medium=Email&utm_term=&utm_campaign=Update%20to%20Advisory%20Notice%20-%20Anti-Alias%20Filter%20Effects%20on%20EEG%20Timing>Please contact EGI support<mailto:supportteam at egi.com> for any questions.

From: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Makoto Miyakoshi
Sent: Saturday, October 10, 2015 5:53 PM
To: Isaiah Innis <isainnis at indiana.edu>
Cc: EEGLAB List <eeglablist at sccn.ucsd.edu>
Subject: Re: [Eeglablist] Correct for EGI latency error functions

Dear Isaiah,

Thank you for sharing this info. I heard of it from Elise, one of visitors here. EGI users, please pay attention!

Makoto

On Fri, Oct 9, 2015 at 9:48 AM, Isaiah Innis <isainnis at umail.iu.edu<mailto:isainnis at umail.iu.edu>> wrote:
Hello eeglab list:

Those of us who use EGI hydro-cel systems received "Advisory Notice: Anti-Alias Filter Effects on EEG Timing" warning that there is a fixed latency delay present for any event codes sent to the EEG during recording. The EEG is delayed with respect to the events, and needs to be corrected by either shifting the EEG forward or the events back. While it is possible to correct this within NetStation, since our lab analyzes data exclusively using eeglab I wrote a series of functions to correct the delay within eeglab. It is my hope that others may find this beneficial. However, I am a matlab novice and would like to see if anyone on the list has any comments or suggestions.

You can download a .zip archive with the functions here: <https://www.dropbox.com/sh/2udz5qrdb79b2b8/AAAS6Bws_O65gJnpnATS1azza?dl=0>.

If you have any questions or criticisms upon reviewing the functions, please contact me at: isainnis at indiana.edu<mailto:isainnis at indiana.edu>
If there is interest, I will make this a plugin for ease of use.

Thank you,
--
Isaiah Innis
Indiana University '13
EEG Technician, IUB IRF

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--
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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