[Eeglablist] How do I account for 100ms delay (from sound card) when looking at averaged files?

Stephen Politzer-Ahles spa268 at nyu.edu
Tue Oct 21 19:53:30 PDT 2014

Hello Michelle,

There are various ways to do this, and it depends somewhat on what you plan
on doing with the data. The first question is whether or not you need to
re-process the data. If you have a 100ms delay and you baseline-corrected
from -200ms to 0ms (for example), then with the delay your baseline is
really -300ms to -100ms, so you may or may not want to correct that.
Likewise, if you epoched from -200ms to 500ms (for example) and you really
need your epoch to go out to 500ms, then you would need to redo this
because you really epoched from -300ms to 400ms (after accounting for the
delay). If these things don't matter to you, then you may not need to redo
any of the processing. If you do need to redo processing, you could
probably accomplish this by either 1) using a loop to iterate over each
event and change the event's latency (see
http://sccn.ucsd.edu/wiki/Chapter_03:_Event_Processing#Event_latencies for
information on how the event latency is represented); or 2) not actually
change the events, but just choose epoch sizes that account for that (e.g.,
epoching from "-100ms" to "600ms), which is equivalent to -200 to 500ms
after accounting for the delay).

If you don't need to reprocess, or you reprocess without actually changing
the event latencies, then one thing you can do is just adjust your analyses
later on (for example, changing the x-axis of plots to account for the
shifted latency, and adjusting y your window of analysis--e.g. doing
statistics on the "200ms - 300ms" time window if what you really want is
the 100ms - 200ms window). Others on the list might have other suggestions
for ways to actually change the times within EEGLAB without having to go
back and re-process. It may depend on your format (e.g. whether or not you
plan on using STUDY) and what EEGLAB functions you plan on using later.


Stephen Politzer-Ahles
New York University, Abu Dhabi
Neuroscience of Language Lab

On Tue, Oct 21, 2014 at 6:05 PM, Downes, Michelle <michelle.downes at ucl.ac.uk
> wrote:

>     How do I account for 100ms delay (from sound card) when looking at
> averaged files? I've been told I can write a loop but I am not an
> experienced programmer and can't see anything online!
>  Thank you!
>     *Michelle Downes*
>  Michelle Downes, Msc
>  PhD Student
>  Cognitive Neuroscience and Neuropsychiatry Section
>  Developmental Neurosciences Programme
>  UCL Institute of Child Health
> London
> WC1N 1EH
>  Tel: +44(0)20 7905 2644
> Email: michelle.downes at ucl.ac.uk
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