[Mne_analysis] shifting time-scale in RAW data

[Eric Larson]

I use Steve's suggestion with Martin's modification (shifting event
samples) for a while and it works well.

One other related to consider is that it's also a good idea to take into
account any delay or jitter between the trigger and the stimulus or
condition it is designed to represent. For example, both auditory and
visual stimulus delivery can have an associated delay (or usually worse,
jitter) due to software and/or hardware, so it's not a bad idea to get a
handle on that, too.

Cheers,
Eric



On Tue, Apr 29, 2014 at 6:25 AM, Lau Møller Andersen
<lau.andersen at cnru.dk>wrote:

> Thanks, Martin
>
> Luckily, I did have a sampling frequency of 1 kHz, but I will use your
> suggestion because of its generality.
>
> Thanks
>
> Lau
> Den 29/04/2014 kl. 15.21 skrev Martin Luessi <mluessi at nmr.mgh.harvard.edu
> >:
>
> On 04/29/14 05:54, Stephen Politzer-Ahles wrote:
>
> Hi Lau,
>
> I thought there was a function for this on raw data but I can no longer
> find it (so I too would be interested to hear what others have to say).
> But it should be possible to do this in sort of a hack-ish way by just
> shifting the times of the events in the event table. For example:
>
> # Get the event list events = mne.find_events( raw, min_duration = .002 )
>
> # Adjust the trigger latencies
> events[:,0] = [ x + 25 for x in events[:,0] ]
>
>
> Note that the first column in "events" contains the time in samples, not
> milliseconds. So this will be incorrect unless your sampling frequency
> is 1kHz. To shift it by 25ms I would use
>
> import numpy as np
>
> events[:,0] += np.round(25e-3 * raw.info['sfreq'])
>
> Best,
>
> Martin
>
>
> Alternatively, I suppose you could just pull out bigger epochs (e.g., 25
> ms longer than you intend) in the beginning, then use evoked.shift_time
> later, and make sure to leave off the first 25 ms of the epoch in your
> plotting and analysis. But yes, it would be great if there is a cleaner
> way to do this.
>
> Best,
> Steve
>
>
>
> Stephen Politzer-Ahles
> New York University, Abu Dhabi
> Neuroscience of Language Lab
> http://www.nyu.edu/projects/politzer-ahles/
>
>
> On Tue, Apr 29, 2014 at 1:34 PM, Lau Møller Andersen
> <lau.andersen at cnru.dk <mailto:lau.andersen at cnru.dk <lau.andersen at cnru.dk>>>
> wrote:
>
>    Dear list
>
>    I am aware of how to shift the time-scale in evoked files:
>
> http://martinos.org/mne/stable/auto_examples/plot_shift_evoked.html#example-plot-shift-evoked-py
>
>    But is it possible to do it already on the raw file? (I have a
>    trigger delay of 25 msec.)
>
>    The reason for this being preferable in my view is that if you do it
>    after having epoched, you effectively include 25 msec of data before
>    the epoch that does not belong to the epoch. Furthermore, you
>    exclude 25 msec of data at the end of the epoch that does belong in
>    the epoch.
>
>    Best Wishes
>
>    Lau
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