Screen Flickering/Glitching

HI there, So for the past 2 patches or so I have had this kinda screen glitch while playing usually when before the gates drop and I don't know whats happening, my video settings are on medium high, my fps is around 50 - 60 and I have updated all of my software. Can anyone help? it's really annoying me and is really affecting how I'm playing as sometimes it's so bad that I can't even see my minimap or lane

Specs (I think) MacBook Pro (13-inch, Mid 2012) Processor 2.9 GHz Intel Core i7 Graphics Intel HD Graphics 4000 1024 MB