In psycholinguistics, language processing refers to the way humans use words to communicate ideas and feelings, and how such communications are processed and understood. Language processing is considered to be a uniquely human ability that is not produced with the same grammatical understanding or systematicity in even human's closest primate relatives.
Throughout the 20th century the dominant model for language processing in the brain was the Geschwind–Lichteim–Wernicke model, which is based primarily on the analysis of brain-damaged patients. However, due to improvements in intra-cortical electrophysiological recordings of monkey and human brains, as well non-invasive techniques such as fMRI, PET, MEG and EEG, an auditory pathway consisting of two parts has been revealed and a two-streams model has been developed. In accordance with this model, there are two pathways that connect the auditory cortex to the frontal lobe, each pathway accounting for different linguistic roles. The auditory ventral stream pathway is responsible for sound recognition, and is accordingly known as the auditory 'what' pathway. The auditory dorsal stream in both humans and non-human primates is responsible for sound localization, and is accordingly known as the auditory 'where' pathway. In humans, this pathway (especially in the left hemisphere) is also responsible for speech production, speech repetition, lip-reading, and phonological working memory and long-term memory. In accordance with the 'from where to what' model of language evolution, the reason the ADS is characterized with such a broad range of functions is that each indicates a different stage in language evolution.
