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Performance in an artificial grammar learning task does not depend on explicit memory for exemplars used during training. Several aspects of the grammar may be learned implicitly, including the frequencies of different letter bigrams and trigrams (chunk strength) and rules governing legal letter strings. These different forms of implicit learning are likely to have different neural substrates. We sought to examine brain activity during an artificial grammar classification task in which grammatical and nongrammatical items were balanced with respect to chunk strength. Four subjects participated in an fMRI study in which they viewed 46 letter strings formed according to the artificial grammar rules. After being positioned in the scanner, subjects were presented with 64 letter strings, half of which were grammatical and half were nongrammatical. The strings were presented for 1 second each, and the subject classified them …