Semantic memory is a general knowledge of concepts and it is part of our long term memory system. Collins & quillian developed a hierarchical model of semantic memory. In this model concepts are represented by nodes, and these nodes are connected to each other through links. So for example bird would be linking to animal and features such as can fly, has feathers, chirps, etc. would all be linked to bird. One way to test this model is through semantic priming. Priming tasks have revealed that when the prime is semantically linked to the target word, our lexical access is faster. The term stimulus onset asynchrony (soa) refers to the time lag between the onset of the end of the prime and the onset of the stimulus. So priming semantically related words increases facilitation and improves reaction times in lexical decision tasks. When the prime of a word is semantically unrelated there is an inhibition effect.