1. Bjork, E. L., & Cummings, E. M.The “A, not B” search error in Piaget's theory of object permanence: Fact or artifact? Paper presented at the meeting of the Psychonomic Society, Phoenix, Arizona, November 1979.
2. Butterworth, G.Logical competence in infancy: Object permanence or object concept? Paper presented at the meeting of the Society for Research in Child Development, San Francisco, March 1979.
3. Cummings, E. M., & Bjork, E. L.Piaget's Stage IV object concept error: Evidence of perceptual confusion, state change, or failure to assimilate?. Paper presented at the meeting of the Western Psychological Association, Seattle, April 1977.