Synchronization failures

hi! we use macBook pro mid 2015 CATALINA
we have been facing a bunch errors in our paradigm since we updated the psychtoolBox to the latest release and matlab from 2015 to 2020B. The experiments were running ok until wednesday this week. Thanks to Mario I was able to pass an error on the message and now PTB is claiming for an error in Synchronization. Before i proceed and make things worst for my experiment i want to consult if you think i should downgrade the version of PsychtoolBox to the release on 2015. ( i do not know if that version still being served so please i apologise if i am asking a basic question) otherwise. or if you think my late 2015 macbook pro running on Catalina Intel Iris Pro 1536 MB graphics may be corrected for the synchronization by adding , deleting or modifying some lines of code in my program. ( if you think so please direct me to step by step instructions as to correct the vbl retrace? I have been searching for a long time with no success. it is of course totally my fault because i am totally blind and my digital search skills are very poor. I do apologise for that. i will improve! the error is pasted below this line.

PTB-INFO: Using modified PortAudio V19.6.0-devel, revision unknown
Initialize Eyelink Connection and Data file
Eyelink: Opening Eyelink in DUMMY mode
Initialize Display Screen
PTB-INFO: Retina display. Enabling panel fitter for scaled Retina compatibility mode.

PTB-INFO: This is Psychtoolbox-3 for Apple OS X, under Matlab 64-Bit (Version 3.0.17 - Build date: Dec 17 2020).
PTB-INFO: OS support status: OSX 10.15 minimally supported and tested.
PTB-INFO: Type ‘PsychtoolboxVersion’ for more detailed version information.
PTB-INFO: Most parts of the Psychtoolbox distribution are licensed to you under terms of the MIT License, with
PTB-INFO: some restrictions. See file ‘License.txt’ in the Psychtoolbox root folder for the exact licensing conditions.

PTB-INFO: For information about paid priority support, community membership and commercial services, please type
PTB-INFO: ‘PsychPaidSupportAndServices’.

PTB-INFO: OpenGL-Renderer is Intel Inc. :: Intel Iris Pro OpenGL Engine :: 2.1 INTEL-14.7.8
PTB-INFO: Renderer has 1536 MB of VRAM and a maximum 1536 MB of texture memory.
PTB-INFO: VBL startline = 1800 , VBL Endline = -1
PTB-INFO: Beamposition queries unsupported or defective on this system. Using basic timestamping as fallback.
PTB-INFO: Timestamps returned by Screen(‘Flip’) will be therefore less robust and accurate.
PTB-INFO: Measured monitor refresh interval from VBLsync = 16.671358 ms [59.983115 Hz]. (299 valid samples taken, stddev=0.712793 ms.)
PTB-INFO: Small deviations between reported values are normal and no reason to worry.

WARNING: Couldn’t compute a reliable estimate of monitor refresh interval! Trouble with VBL syncing?!?


One or more internal checks (see Warnings above) indicate that synchronization
of Psychtoolbox to the vertical retrace (VBL) is not working on your setup.

This will seriously impair proper stimulus presentation and stimulus presentation timing!
Please read ‘help SyncTrouble’ for information about how to solve or work-around the problem.
You can force Psychtoolbox to continue, despite the severe problems, by adding the command
Screen(‘Preference’, ‘SkipSyncTests’, 1); at the top of your script, if you really know what you are doing.

Error using BlackHole (line 167)
See error message printed above.

Last Error: Error using BlackHole (line 167)
See error message printed above. ()

i forgot to say that we also tried everything listed on help synchronization thanks you so much!

Psychtoolbox 3.0.17 should not be the problem, but more part of the solution, as PTB versions older than 3.0.16 lack the advanced workarounds to deal with macOS Catalina’s brokeness. And at least on my MBP 2017 with 10.15.7 Cataline and Matlab R2020b, things work fine.

The general problem is that you try to use an Apple product under macOS for serious work - something i strongly advise against. There’s no worse choice than Apple hw or software for tasks that need visual timing precision, accuracy or performance.

If you don’t need precise visual timing, you could just disable all timing and checks, as explained in the output of PTB about the synchronization failure.

Other than that, it is pretty possible that the problem is not fixable on your machine with macOS. Upgrading to Linux would be the optimal solution for great timing, but that may not be that straightforward, as modern Apple hardware is rather hostile to alternative operating systems, another reason to stay away from Apple. Most trouble starts around MBP 2016 and later though, i think.

It looks like the desktop compositor might be kicking in for some reason, despite all our hacks to prevent that, which would kill timing. Proper use of the PsychtoolboxKernelDriver can help to diagnose that – only diagnose, not fix it though.

However, further assistance on this topic from myself, without any guarantee for a resolution of your issues, would require your lab to buy priority support, as explained by typing the command PsychPaidSupportAndServices.


Thank you so much Mario!
As you kindly said Mac betrayed us after all the upgrades.

We will buy the support! We went to the webpage and filled a small form to request information on your services which we filled and now we will await for the prices to proceed. Please share with us if we did the right step or if there is another direct form for payment on the webpage to get the license key that my screenreader did not find.
Looking forward to give it a try ( im aware that there is no Guarantee but I am looking forward to try)!
All the best!

Hi Wanda,

Thanks for your support. You are probably looking for this option:

Where you can buy one or multiple hours at once (just increase quantity). Your support is very important for PTB, so thanks a lot.

All the best,

Thank you so much!
I bought it! It downloaded a zip file that contains three images. Is that right?
All the best

Those images in the zip files are probably the fan-stickers. For support you need to use the PsychPaidSupportAndServices function to generate an authentication token. That function will ask you for the product order id and license key and then generate the token for you to post here. Both product order id and key are part of the materials you received, at least of the invoice you got.

That said, i do not have any information about any new licenses sold after 9th March, and the processing time for new sold license keys can be on the order of 5-10 business days, which is why we advise to not buy them in the last minute, but ahead of time like a fire insurance. It could take a while.


Hi Thank you so much! We did pay Mario. Me and my student will get in touch with you shortly!
All the best