If you suspect it refers to a specific author with the last name starting with "Ey" (like Eysenck), search for their 1994 publications on Google Scholar.
In many student psychology documents and course notes, the citation [EYS94] or [CITATION Eys94] is used to reference the widely used cognitive psychology textbook by Michael W. Eysenck (specifically editions published around 1994). eys94.rar
Look at the reference list of the source document you pulled this from to see if "EYS94" is listed as a short-hand citation. If you suspect it refers to a specific