Sequence detectors as a basis of grammar in the brain

Grammar processing may build upon serial-order mechanisms known from non-human species. A circuit similar to that underlying direction-sensitive movement detection in arthropods and vertebrates may become selective for sequences of words, thus yielding grammatical sequence detectors in the human brain. Sensitivity to the order of neuronal events arises from unequal connection strengths between two word specific neural units and a third element, the sequence detector. This mechanism, which critically depends on the dynamics of the neural units, can operate at the single neuron level and may be relevant at the level of neuronal ensembles as well. Due to the repeated occurrence of sequences, for example word strings, the sequence-sensitive elements become more firmly established and, by substitution of elements between strings, a process called auto-associative substitution learning (AASL) is triggered. AASL links the neuronal counterparts of the string elements involved in the substitution process to the sequence detector, thereby providing a brain basis of what can be described linguistically as the generalization of rules of grammar. A network of sequence detectors may constitute grammar circuits in the human cortex on which a separate set of mechanisms establishing temporary binding and recursion can operate.