Individual Wi-Fi signal strength indicators missing

I can see the WiFi icon in my menu bar, but when I click on the icon to choose a router signal strength icons aren't displayed.

Picture of my WiFi menu: http://i.imgur.com/5rsGv.png. As you can see, none of the have signal strength indicators next to them, but their security status is stil displayed.

I've tried hiding/unhiding WiFi from the menu bar, restarting my computer, turning WiFi off and on, using "Assist Me" diagnostics and assistant, and searched for similar problems on google but I've had no luck fixing the problem.

15" Macbook Pro - Early 2011. Running Lion 10.7.4

This problem has persisted for almost a month.