[Eeglablist] importing events from mff files

[Delorme, Arnaud]

Dear Roy,


> 1) As a prelude, event latencies in eeglab for continuous data are said to
> be in sample points (
> https://sccn.ucsd.edu/wiki/Chapter_03:_Event_Processing#Event_processing_from_the_Matlab_command_line).
> However, when importing events using *pop_importevent*, the *latency* field
> is required to be in seconds. So it appears that the term "latency" refers
> to (slightly but importantly) different things in different contexts. Is
> this correct?

The latency unit is specified in seconds (so could be milliseconds if you set it to 0.001). For MFF file, this is microSeconds. If you have samples in the file you are importing, the documentation indicate to enter NaN and then no conversion will be applied.

> 2) Turning to mff: mff event files (Events****.xml) appear to show event
> timing in clock time. What is the origin of this clock time? More
> importantly, can it (still) be trusted in the context of data
> discontinuities? (As a side note, if anyone has any documentation with
> details of the mff format, especially event timing, that would be very
> welcome.)

The clock time down to microseconds of events is correct. When discontinuities are present, this does not affect the clock time of the events which is absolute. 

However, then the file epochs.xml will contain the segment and the function mff_import.m (not mff_importevent.m) will adjust the latencies of the events imported by mff_importevent.m based on that information to add the discontinuities for EEGLAB.

> 3) I've tried using the *mff_importevents *function from the MFFMatlabIO
> (v3.4) plugin. While it runs, I can't be sure it's working as intended.
> It's unclear a) what the input argument *begTime* is supposed to hold

“begTime” is the beginning time of the experiment as defined in the field “recordTime” of the file “info.xml”

> b)
> how returned latencies are represented (seconds, sample points?),

Samples.

> c)
> whether data discontinuities are accounted for (I suspect not since there
> is no reference to the epochs.xml file).

No, discountinuities are only accounted for after the file is processed by mff_import.m because discontinuities are stored in another xml file “epochs.xml.”

Best wishes,

Arno