# Creating EpochsArray with explicit timestamps instead of sampling frequency **URL:** \ **Category:** Support & Discussions\ **Tags:** epochs\ **Created:** [April 12, 2021, 9:47am UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963 "2021-04-12T09:47:22Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![roman](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/roman/32/378_2.png) [@roman](https://mne.discourse.group/u/roman)\ **Post date:** [April 12, 2021, 9:47am UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/1 "2021-04-12T09:47:22Z")
Hello, I have EEG epochs stored in MatLab files with the following arrays: - allTargets: array of all target epochs with shape (n\_epochs, n\_samples, n\_channels), - allNTargets: array of all non-target epochs, - tSCALE: array with sample timestamps (from range [-0.2;1] seconds) with shape (1, n\_samples). As the `EpochsArray` constructor requires an `Info` object, I created one with `mne.create_info`. Because I don’t have sampling frequency explicitly, I calculated it as _sfreq = n\_samples / 1.2_. However, when I compare the tSCALE array with the `epochsArray.times` array (with `np.allclose`), they are not even close. Is it possible to specify timestamps explicitly? Can I edit the `EpochsArray.times` attribute directly? I am sorry if there is an answer somewhere already. I tried to search for it, but maybe I searched for the wrong keywords. I really appreciate any help you can provide. ---
**Author:** ![drammock](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/drammock/32/4_2.png) [@drammock](https://mne.discourse.group/u/drammock)\ **Post date:** [April 12, 2021, 2:49pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/2 "2021-04-12T14:49:14Z")
just a quick sanity check: did you do `EpochsArray(..., tmin=-0.2)`? Can you tell us more about how “not even close” the arrays are? (do you get a constant value if you subtract one from the other? what value?) ---
**Author:** ![roman](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/roman/32/378_2.png) [@roman](https://mne.discourse.group/u/roman)\ **Post date:** [April 12, 2021, 5:14pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/3 "2021-04-12T17:14:11Z")
> [@drammock](#): > > did you do `EpochsArray(..., tmin=-0.2)` ? Yes, I did > [@drammock](#): > > do you get a constant value if you subtract one from the other? what value? I typed the sampling frequency formula into a calculator and got a result of 511.66… so I tried to round it to 512. When I create the `EpochsArray` with this sampling frequency, the difference is an almost constant vector with a single step in the middle at index 306. I’m not sure if this makes any sense. Please let me know if you need to make something more clear. ---
**Author:** ![drammock](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/drammock/32/4_2.png) [@drammock](https://mne.discourse.group/u/drammock)\ **Post date:** [April 12, 2021, 6:41pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/4 "2021-04-12T18:41:10Z")
that makes sense. Sampling frequency need not be an integer. I suggest when you create the `info` object you pass `sfreq=n_samples / duration` (i.e., whatever expression yielded the number 511.66…), and see if that gives the correct timestamps. ---
**Author:** ![roman](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/roman/32/378_2.png) [@roman](https://mne.discourse.group/u/roman)\ **Post date:** [April 12, 2021, 6:45pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/5 "2021-04-12T18:45:52Z")
That is what I did earlier but then the difference between the arrays is not constant at all. ---
**Author:** ![drammock](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/drammock/32/4_2.png) [@drammock](https://mne.discourse.group/u/drammock)\ **Post date:** [April 12, 2021, 6:50pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/6 "2021-04-12T18:50:00Z")
can you share a link to the relevant files and post a code sample showing what you’ve done so far (i.e., code that loads the files, then creates the info and the EpochsArray)? ---
**Author:** ![roman](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/roman/32/378_2.png) [@roman](https://mne.discourse.group/u/roman)\ **Post date:** [April 13, 2021, 12:09pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/7 "2021-04-13T12:09:03Z")
I created a minimalistic example at colab: > **[Google Colaboratory](https://colab.research.google.com/drive/1H5RoCmOcRVDMMMVHPos2gSD9EEx9x16R?usp=sharing)** Well, it seems like I was fooled by PyCharm’s debugger, which tells me that the difference between arrays is not constant, but it is. Sorry for the confusion. ---
**Author:** ![drammock](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/drammock/32/4_2.png) [@drammock](https://mne.discourse.group/u/drammock)\ **Post date:** [April 13, 2021, 3:24pm UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/8 "2021-04-13T15:24:36Z")
OK, I realized what the problem is. MNE-Python **requires** when creating epochs that there be a sample at _exactly_ time=0. So even though you specify `tmin=-0.2`, you end up with `tmin` equalling -0.19921875 because the time span `tmin` to zero does not accommodate an integer number of samples given your sampling frequency of 512 Hz. Put another way: in your data `sfreq * tmin` = 102.4 samples, so MNE-Python shifts everything by 0.4 samples to make sure we get a sample aligned at exactly zero. The solution is to set `tmin=0` when creating the epochs, and then shift times afterward: ```python import scipy.io import numpy as np from mne import create_info, EpochsArray path = 'example.dat.mat' subject = scipy.io.loadmat(path) orig_times = subject['tSCALE'][0] n_samp = subject['tSCALE'].shape[-1] tmin, tmax = orig_times[[0, -1]] duration = tmax - tmin sfreq = (n_samp - 1) / duration assert sfreq == 1 / np.diff(orig_times)[0] ch_names = [x[0] for x in subject['electrodes'].flat] ch_types = ['eeg'] * len(ch_names) info = create_info(ch_names=ch_names, ch_types=ch_types, sfreq=sfreq) info.set_montage('standard_1020') # transpose to (n_epochs, n_channels, n_samples) data = subject['allTARGETS'].transpose(0, 2, 1) n_epochs = data.shape[0] n_samples = data.shape[2] # create epochs event_no = 1 events_dict = {'target': event_no} events = np.column_stack(( np.arange(0, n_epochs * n_samples, n_samples), np.zeros(n_epochs, dtype=int), np.full((n_epochs, ), event_no) )) epoch = EpochsArray( data, info, tmin=0, # <- changed baseline=(0, -tmin), # <- note this change too events=events, event_id=events_dict ) epoch = epoch.shift_time(tmin, relative=False) np.testing.assert_allclose(orig_times, epoch.times) ``` ---
**Author:** ![roman](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/roman/32/378_2.png) [@roman](https://mne.discourse.group/u/roman)\ **Post date:** [April 14, 2021, 8:29am UTC](https://mne.discourse.group/t/creating-epochsarray-with-explicit-timestamps-instead-of-sampling-frequency/2963/9 "2021-04-14T08:29:13Z")
Okay, good to know there has to be a sample at time=0. Thank you for your help.