This project explores the nature of the relationships which exist between the conceptual characteristics of visual and auditory perception and the semantic and syntactic behaviour of the Spanish and French perception verbs ver/voir, oír/entendre, mirar/regarder and escuchar/écouter. Based on the theoretical framework of Cognitive Grammar it is argued in the first part of the study that visual and auditory perception are different as to the degree of agentivity of the perceiver, the level of dynamicity of the stimulus and the perfectivity of the perception event itself. By focusing on these extralinguistic traits, it is shown that as opposed to what has been suggested in the literature, the perception verbs are not necessarily bad examples of semantically transitive verbs. Moreover, it is explained why the perception verbs have caused a large polemic relating to their lexical aspect. The second part of the study presents syntactic evidence of the dependency between the perception modalities and the linguistic traits of the perception verbs. Quantitative research is used to demonstrate that the internal structure of the infinitive complements after the visual perception verbs (e.g. Veo bailar a María, I see Mary dance) is not identical to the structure of the complements after the auditory perception verbs (e.g. Oigo a los niños correr, I hear the children run). The analysis of three syntactic phenomena in Spanish and French, namely the syntactic position of the subordinated participant, his case markings and the verbal agreement in the pronominal infinitive construction, shows that visual perception verbs prototypically select objects and non-clausal infinitive complements whereas the auditory perception verbs select events and clausal infinitive complements.