recollect the episodes in which they learned them, also learned and retained the ability to read mirror-reversed words efficiently, yet were severely impaired in recognizing those words, which give evidences about the neurological basis differences in procedural and declarative knowledge. Researchers also found that some normal subjects, like amnesic patients, showed substantial procedural learning in the absence of explicit declarative knowledge. Even though declarative knowledge may influence performance on a procedural task, procedural and declarative knowledge may be acquired separately, one does not need to have knowledge of one type in order to build the other type of knowledge. The influence caused by declarative knowledge might be due to the facilitation to a process of pathway activation that is outside of conscious awareness. If the prime is highly predictive of the target, the amount of facilitation is increased because of an active, conscious, attentional effect that is superimposed on the pathway activation. Therefore, if and when subjects develop explicit declarative knowledge of procedure, they can use this knowledge to form attentional expectancies regarding the next item in this procedure. Activation Lashley (1951) proposed that behavioral sequences are typically controlled with central plans, and the structure of the plans is hierarchical. Some evidences also support this hypothesis. Same behaviors can have different functional interpretations depending on the context in which they occur. The same sound pattern can be interpreted differently depending on where it occurs in a sentence, for example, “there” and “their”. Such contextual dependence is only possible with functionally overarching states of the sort implied by hierarchical plans. The initiation time of a movement sequence and the inter-response times of the sequence elements can increase with its length. Further, inter-response times can depend on the size of the phrase that is about to be generated. The larger the phrase, the longer the inter-response time. Such data have been interpreted in terms of ‘decoding’ or ‘unpacking’ hierarchical plans into their constituents. Moreover, learning difficulties changes with the easiness of behavioral sequences. Finally, long-term learning of skills is naturally characterized by the process of forming ever larger hierarchical units or ‘chunks'. People learn control structures for successively larger units of behavior, with newly learned routines calling up or relying on more elementary routines, like learning to play simple notes before being able to play piano concerto. As for process of behavior plan forming, Rosenhaum et al. (2007) proposed that plans are