experiencer subjects

Experiencer subjects are always animate, usually human. An experiencer experiences a sensory perception or a psychological state.

sensory verb with agent subject>>John looked at the scar./ Maria smelled the tulips. 

sensory verb with experiencer subject>>John saw some blood. / Maris smelled smoke. 

Sometimes the semantic difference between an experiencer subject and an agent subject is reflected in the verb and sometimes it is not. In the case of look at versus see and listen to versus hear, this semantic difference is lexicalized; in other words, the difference in meaning is signaled by different words. In the case of agnetive smell and experiencer smell, the semantic difference is not lexicalized; the verbs take the same form.