That's what I said. Imperfective is often used in negations to deny the mere fact of such action even being performed, let alone finished to any result. Of course, given the nature of such actions as hearing and seeing (involuntary), there isn't much of a difference. The perfective one just adds a tiny-tiny shade of meaning: maybe you are talking about one particular occasion, or maybe you tried to see/hear, but not to any result.
That is, in the Past. Remember than a conjugated form of a perfective verb means future tense, i.e. "Я тебя не услышу" is "I won't hear you".
Imperfective is also used in permissions or prohibition (i.e. doing an action is allowed or not allowed: it doesn't make sense to only forbid to finish the action, rather than the whole process in general)