1. Barker, R. Ecology and motivation. In M. Jones (Ed.) Nebraska Symposium on Motivation, Vol. 8, pp. 1-48. Lincoln, Nebraska: University of Nebraska, 1960.
2. Helson, H. Some problems in motivation from the point of view of the theory of adaption level. In D. Levine (Ed.), Nebraska Symposium on Motivation, Vol. 14, pp. 137-182. Lincoln, Nebraska: University of Nebraska, 1966.