i\'m using android studio 2.1.1 in ubuntu 14.04.Now my question is,i want to run the program through my phone without emulator. so i chose the target as usb dev
Note: I had problem on Windows 7 but it might help you as well..
I had problem with android studio detecting my phone(Acer Liquid Zest 4G), tried restarting android studio, switching back and forth between PTP and MTP, OS was able to detect device normally.
So what I did was, in Developer Options i enabled USB debugging, USB connection is in PTP mode, then from phone manufacturer's site (you can find site for your phone here: https://developer.android.com/studio/run/oem-usb.html), I downloaded USB driver for my phone model, installed driver and android studio was able to detect my phone(there was no need for restart).
I will repeat again, you must have USB Debugging enabled in Developer Options, otherwise it won't work. Hope it helps.