This will be helpful.

Comparison of perception functions:
http://3.bp.blogspot.com/-CsNapEC53Dw/VXnk0SY6hFI/AAAAAAAAK70/sxXtfp24cI4/s1600/Screen%2BShot%2B2015-06-11%2Bat%2B3.42.14%2BPM.png