Language production and language comprehension require a mental lexicon, a mental store of information that includes semantic information, syntactic information and details of word form. The mental lexicon is organised by morphemes (1), frequency of use (2), lexical neighbours (3) and semantic relationships between words (4). Lexical selection refers to integrating words into the full sentence.

A morpheme is the smallest meaningful unit in language. A phoneme is the smallest unit of sound that can make a difference in meaning. Each language has different phonemes. An infant can distinguish between all phonemes. People lose this ability later in life.

Speech production occurs in several steps:

1. Conceptual preparation
   The preparation of the speech concept.
2. Lexical selection
   Linking the concept to a word
3. Morphological encoding
   Encoding the word in morphemes
4. Phonological encoding
   Encoding the morphemes in phonemes
5. Phonetic encoding
   The preparation of pronunciation
6. Articulation
   The pronunciation of the word

Language perception can be either auditory (spoken language) or visually (written language). It occurs in several steps:

1. Analysis of the word
2. Input code
3. Activation of word form
4. Use of mental lexicon
5. Conceptual activation
6. Understanding of the concept

Orthographic input code can be changed into phonological input code, as most people use their inner voice to read.

Language is lateralized to the left-hemisphere surrounding areas of the Sylvian fissure in most people. Wernicke’s area is involved in assigning meaning to words, assigning words to concepts and lexical activation. Broca’s area is involved in production of speech, syntactic integration and lexical selection. The arcuate fasciculus connects Broca’s area and Wernicke’s area and is involved in repetition. The right-hemisphere is involved in the prosody of language and metaphorical meanings.

Broca’s aphasia occurs with damage to Broca’s area and surrounding regions and leads to deficits in speech-production, syntax and grammar. Wernicke’s aphasia occurs with damage to Wernicke’s area and surrounding regions and leads to deficits in assigning meaning to words and language comprehension. Apraxia refers to deficits in motor planning of articulation. Semantic paraphasia refers to problems in speech-production with picking out the right word and occurs in Wernicke’s aphasia.

Lesions in the anterior parts of the inferior and medial temporal cortex lead to deficits involving naming living things. The dissociation between naming non-living and living things may be due to the complexity of the features that help distinguish a living thing from another living thing and a non-living thing from another non-living thing.

The superior temporal gyrus is involved in general auditory processing of phonemes. There is no distinction between phonemes and non-phoneme sounds. The superior temporal sulcus is involved in linguistic processing of phonemes. Areas that are more sensitive to speech are located more ventrolateral in or to the superior temporal sulcus. Occipitotemporal regions in the brain are involved in letter perception. The inferior frontal gyrus is involved in lexical selection and is influenced by cohort size and cohort competition. The bigger the cohort (number of potential words), the more competition and the more difficult lexical selection will be.

The segmentation problem refers to the difficulty of hearing separate words in a continuous stream of speech. The prosody of language helps with the segmentation of words.

Words that are semantically incongruent in the sentence evoke an N400 wave (a negative ERP), ±400ms after the incongruent word. Words that are syntactically incongruent in the sentence evoke a P600 wave (a positive ERP), ±600ms after the incongruent word.