This app provides a common Checkerboard Reversal Task to elicit visual evoked potentials (VEPs) in real-time.

What is the basis of this task?

The Checkerboard Reversal Task (CRT) is a technique used to elicit VEPs. The task involves presenting a checkerboard pattern to a participant, where the pattern alternates between black and white squares. The reversal of the checkerboard pattern, from black to white or white to black, creates a change in the visual stimulation and elicits a VEP response originated at the primary visual cortex (located inside the occipital lobe). 

The VEP response is an electrical signal produced by the brain in response to a visual stimulus. It is usually recorded using electrodes attached to the scalp (i.e., via electroencephalography, EEG), which detect changes in electrical activity produced by the brain in response to the visual stimulation. The VEP response provides a measure of the processing of visual information as it travels through the various stages of the visual pathway, including the retina, optic nerve, and visual cortex. The VEP response has several distinct components that reflect the processing of visual information at different stages of the visual pathway, such as P100, N75 or N135. These components of the VEP response provide important information about the processing of visual information in the brain, and can be used to diagnose various visual disorders and evaluate the effectiveness of treatments for visual problems. Thus, the CRT is commonly used in neurophysiology and psychology research to study visual processing and the underlying neural mechanisms involved in perception. The results of this task can provide important information about the functioning of the visual cortex and can help to diagnose certain visual disorders. Additionally, the checkerboard reversal task can also be used to evaluate the effectiveness of various interventions for improving visual function.

Check the following manuscript to learn more: Odom JV, Bach M, Brigell M, Holder GE, McCulloch DL, Mizota A, Tormene AP; International Society for Clinical Electrophysiology of Vision. ISCEV standard for clinical visual evoked potentials: (2016 update). Doc Ophthalmol. 2016 Aug;133(1):1-9. doi: 10.1007/s10633-016-9553-y. Epub 2016 Jul 21. PMID: 27443562.

Relevant functionalities of the app

The app provides a simple but functional CRT to visualize VEPs in real-time. The main functionalities are:

1. Three different reversal colors are supported: black/white, red/green and blue/yellow.
2. CRT can optimize the number of cells to your screen size by pressing the reload button.
3. VEPs are visualized in real time while the averaging of responses is being performed. The 95% confidence interval is also shown by the shaded area. Three different graphs help you to select the channels you want to monitor.
4. The app can be paused and restarted freely.
5. The recorded biosignal, together with the stimuli onsets, can be saved after pressing the stop button.

About signal processing

The signal processing used in this app is very straightforward and simple, yet recommended. Only an infinite impulse response (IIR) Butterworth bandpass filter is applied, followed by a notch if required. Then, EEG epochs of (0, 1/fr) ms from the stimulus onset (where fr is the update_rate) are extracted and normalized using a z-score baseline of (-100, 0) ms. VEPs are computed by averaging the epochs whenever a new reversal is displayed. For more information, check the update_erp function in main.py.

Settings

The configuration window allows to customize the following settings:

• Update rate (default: 2 Hz): changing rate of the reversal pattern, i.e., 2 Hz means that the pattern changes every 500 ms. 
• Bandpass cutoff (default: 0.5-30 Hz): cutoff for the BPF filter. If the sampling rate of your biosignal is less than 60 Hz, update this parameter to avoid aliasing according to Nyquist theorem.
• Notch cutoff (default: 49-51 Hz): cutoff for the notch filter. Notch filter is only applied if the cutoff falls inside the bandpass range of the previous parameter. Important: set it to 59-61 Hz if you live in a place where the electrical network runs at 60 Hz (e.g., USA)
• Baseline window (default: -100-0 ms): window range for the z-score normalization.