By synthesizing sounds in the middle of the VOT values of [b] and [p] and having listeners determine whether they were [b] or [p], psycholinguists have been able to study how we perceive sounds. They found that listeners hear two distinct sounds, despite the variation; there was only a small boundary region between the sounds when participants were unsure of what they were listening to.
Later research has shown that categorical perception may also occur in animals, such as chinchillas.
To try this out yourself with the sounds 'ba', 'da' and 'ga', visit http://www.ling.gu.se/~anders/KatPer/Applet/index.eng.html.
Crystal, D. (1987). The Cambridge Encyclopedia of Language. Cambridge CB2 1RP: Cambridge University Press.