Easy Setup: Include Chronos triggers in your EEG recording

  • ERP
  • auditory attention
  • synchronization

In this blog we’ll show you how to use Chronos with PRO line mobile devices in your study.

PROX EEG amplifier and Chronos device setup look

Chronos is a USB-based device offering solutions for many stages of your research. Such as – collecting responses, presenting sound, sending triggers and more.

Chronos features an onboard sound card, which eliminates delay issues in audio playback. Consumer sound cards often lack consistent audio timing. This can lead to differences between devices and cause delays in sound presentation.

Chronos also provides many digital inputs/outputs for external communication.

Connecting 2 devices

Connection between PRO devices and Chronos is established using a TTL wire. To be more specific, we use 1-bit TTL triggers to synchronize actions from 2 devices.

Let’s first cover the connection, and then we’ll give some practical examples.

Chronos AUX I/O Breakout Cable

With your Chronos device, you will receive an AUX I/O breakout cable. You get 2 digital outputs, but for 1-bit TTL triggers we only need one.

Input-Output cable used for connecting PROX and Chronos device
Chronos AUX I/O breakout cable. We used OUT15 and IN15 for 1-bit TTL communication.

PRO/PROX input

All PRO devices have 1-bit TTL input on the EEG cap. This cap input is in the form of a 2.5mm audio jack.

Tip of the audio jack is the Receiver line (Rx) and the ring is the Transmitter (Tx). We’ll connect OUT15 to Rx, and IN15 to Tx. In our setup, Chronos will be sending TTLs to the EEG cap.

How to connect Chronos digital inputs to audio jack that goes into EEG cap
To include TTL triggers from Chronos in EEG data, we use a 2.5mm jack. Jack tip is the Rx line, and jack ring is Tx. So, we connect digital output from Chronos to Rx, and digital input to Tx.

Once you connect the 2 devices, you’ll need a PC to control them both. For Chronos you’ll use E-Prime software for communication. With PRO/PROX device you’ll need mbtStreamer.

final setup - Chronos, PROX amplifier and EEG cap, all connected to one PC
Final setup – EEG cap and Chronos connected via I/O cable, and a PC with all necessary software. We’ll use PROX device as an example in text below.

Example no.1: send triggers when there’s a button response

In this example, Chronos will present a TTL trigger when:

  • we present a stimulus in E-Prime (sound or image)
  • a person responds to the stimulus using Chronos buttons

PROX device will then detect the trigger and include it in the EEG recording.

E-Prime trial procedure for example number 1
Procedure for one trial in E-Prime, for our first example. We added a sound as the Stimulus, and a fixation period after each trial.

Connect Chronos to PC via USB and run E-Prime. Chronos add-on is available by default. Go to Experiment Object Properties window and add a Chronos device.

Add Chronos device in Experiment Object Properties, for your E-Prime experiment

In Chronos properties, make sure to set Digital Output 15 (DO 15) to 1 at experiment start.

Set the digital output we'll use to 1 at the beginning of experiment

During the experiment, DO 15 can either be 1 (high) or 0 (low). So how does 1-bit TTL detection work with PROX device?

Our devices will only detect a drop in voltage from high to low. This means we need to ensure DO15 drops to 0 each time we want a TTL trigger.

Let’s say we’re presenting a sound to the participant using Chronos and a set of headphones. We give them the instruction to press the button after a certain sound. We’d like to send a TTL trigger for both stimulus and response.

In Stimulus properties window, we need to add Events which will trigger changes in DO15. First, we’ll add Stimulus.OnsetTime which represents the moment a stimulus is presented.

Then, add an Action that will happen with this Event. Select DigitalOutResetBit and type 15 in Custom: field.

Reset function will set DO15 bit to 0. Because of the “voltage drop” from 1 to 0, PROX device will detect it as a TTL trigger.

Reset digital output to 0 once a stimulus is presented

Now, we need to set DO15 to 1 again because we want to send a TTL trigger for the next action. We’ll use  Stimulus.ActionTime as an Event to set DO15 to 1 – just select DigitalOutSetBit action.

ActionTime represents a timestamp at which the hardware’s driver returns control back to E-Prime. Period between OnsetTime and ActionTime is around 1 millisecond.

Set digital output to 1 after the stimulus is already presented.

When the participant presses a Chronos button, we’ll add the DigitalOutResetBit action. A TTL trigger is then detected by PROX.

Reset digital output to 0 once participant presses a button as a response to stimuli

We added a short period of fixation, just to denote the beginning of a new trial. When we present the fixation cross, DO15 will be set to 1.

Check out our diagram to help understand how one trial works:

Diagram - example 1

Record TTL with EEG

TTL triggers are recorded directly on PRO/PROX device memory. You can also visualize them alongside EEG in mbtStreamer software.

Example no.2: send triggers when audio is detected

In this example, we’ll present a word or question to the participant. Then, we expect an audible reaction from them (to say the word, phrase, sentence or give an answer).

Chronos will send a TTL trigger when:

  • we present the phrase
  • it detects a sound using a microphone. Also, we’ll record the detected sound.

Check out this representation of one experiment trial:

Diagram - example 1

In Experiment Object Properties click on Sound in the list of devices. Change API field to Chronos – this means Chronos will handle audio in your experiment.

Set Chronos to control audio in your experiment.

We want to start recording audio when our participant starts talking. So, in Chronos Device Properties, in Audio In tab, we’ll set StartMode to onset. You can set Onset Threshold for detecting sound changes to any percentage you’d like. This depends on how sensitive you want the mic to be.

StopMode indicates the end of audio recording, and this should be set to normal.

EPrime setting up microphone thresholds

Since we want to record the participant’s response for 5 seconds, we’ll add a wait function after stimulus. We’ll set the waiting duration to 5s.

E-Prime trial procedure for example number 2
Procedure for one trial in E-Prime, for our second example. We’ll add a stimulus in the form of a picture of a word/phrase. After the participant starts talking, we’ll wait 5s. We add a fixation period after each trial.

In Stimulus Properties, you’ll need to set Response Options i.e. buttons. Set value “6” that signifies the Chronos’ pseudo button. You won’t use any buttons, but for audio to be used as a trigger, this needs to be set.

Set a pseudo button that is associated with audio detection

While in this window, navigate to Task Events tab. We’ll add all the actions related to audio recording and sending TTL triggers. We also use DO15 in this example.

example 2 - events and action setup
Top-left: On Stimulus.OnsetTime we’ll add AudioInRecord action. This means that the audio recording can only start if the stimuli is presented. Once the participant speaks, the recording will start. Top-right: We want to send a TTL trigger when the stimulus word/phrase is presented. To detect a TTL trigger, we need to reset DO15 bit. So, for Stimulus.OnsetTime event we also need to add the DigitalOutResetBit action. Bottom-left: Once the participant starts talking, our stimulus ends. That’s why we want to Reset DO15 bit at that point. We want to denote the moment our participant responded. Once we reset DO15 bit, a TTL trigger is detected. So, add DigitalOutResetBit action to Stimulus.FinishTime event. Bottom-right: For the next trial, we need to set the DO15 bit i.e. set it to 1. We do this the same way as for previous example. To Stimulus.ActionTime we add DigitalOutSetBit action.

If you need any help setting up, you can reach out to us by writing an email to support@mbraintrain.zendesk.com, or submit a request on our support page.

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