Your peripheral vision. Also would account when somebody says "I caught it out of the corner of my eye".
Richard- "Dude, i saw Jacob eat your four piece chicken McNuggets"
Dave- "How? We were just making eye contact during this whole talk."
Richard- "I used my perif."
Dave- "Ohh, makes sense. But no biggie, them Nuggets were sprinkled with pet dander."
Your Peripheral vision
"Yeah i caught dem fly kicks outta the corner of my perif. And i had to rock that shit. ya dig?"