PubMed Automatic Term Mapping (ATM) was originally developed to identify author names, journal titles, and MeSH terms in search queries without search tags. In the old schema, author name and journal title fields were not searched when the search terms also matched a MeSH term. This caused thousands of citation-type searches to fail every day. The new ATM addresses this problem by removing those field restrictions. Under the new schema, query terms can be found in both MeSH terms and every PubMed citation field.
For more information visit http://www.nlm.nih.gov/services/pubmed_atm_change.html