I had the same problem with a new laptop after upgrading to Windows 10. I found a less invasive solution. I disabled adaptive brightness in the power settings instead. That way I didn't have to change any driver settings. See Disable Adaptive Brightness in Windows to Fix Dark Screen Problems for the fix.
Hope this helps!
Edit: Oops, I see the OP already mentioned this setting.
unfortunately there are some other limitations when upgrading to Windows 10 on Sony Vaio laptops as they have mentioned here (and Sony has not yet released any Win10 compatible drivers & software until either late October or mid-November of 2015).
and Lenovo has NOT tested the Ideapad P400 touch for Windows 10 compatibility as mentioned here (if your Lenovo computer is not listed there, then Lenovo does not support the Win10 OS upgrade on older non-supported computers).
Here's a search of Intel HD Graphics 4000 drivers from the Microsoft Update Catalog site. Latest version they have is v10.18.10.4276 dated 8/17/2015. one could try that one. And be sure to install the latest Cumulative update for Windows 10 to resolve other non-related display problems.
I encountered this problem on my AMD Mobility Radeon HD 5000 series graphics card. Go to 'Device Manager' 'Display Adaptors' and uninstall the Display Adaptor. After uninstallation, Win10 will automatically re-install the correct driver.
It was a worrying thought to uninstall a key component, but my colleague said that there is always a basic generic driver available that will handle basic screen tasks. Luckily for me, he was right!
I found the solution on this Blog. -> http://blog.teksoporte.es/2015/11/le...reglar-el.html
The blog is in spanish but it's easy to follow up. Basicaly you just have to edit a reg key and restart the pc.
Windows 10 240 and 586 both have a grayscale calibration bug. This could be part of your problem.
For me I had to override the color and greyscale with NVidia's control panel.
No idea with AMD or Intel but you should have a similar setup.
Thank you, IvanDavid1! I've been struggling with this issue since upgrading my Dell Studio 1737 laptop in August. It was so bad that I avoided this laptop and turned to my old Win XP desktop for word processing and used my little Win 10 ASUS Transformer and Windows smartphone for everything else. Your solution worked for me. I can see again. You've given me back my laptop.
Thanks for the solution IanDavid1, however, your solution worked partially for me. I chose the Microsoft Basic adapter, and the system demanded a restart. When I restarted it, pretty much everything on the screen was disturbed, and was scattered here & there. I took a gut feeling and selected the HD driver again in the list from where we had selected the Basic driver (from the path given in your thread). I selected the HD driver again and Voila! ... The display was bright again, all the screen items were properly organized as they were before and above all, it is accepting the brightness controls now, I can select the brightness adjustments as I wish. Hope this will help others too.