Abstract

The Necker cube (NC) figure appears to switch between two 3-D perceptual alternatives,
`cube up' (CU) and `cube down' (CD). However, experienced observers not only report CU and
CD but also a myriad of 2-D percepts. Four experienced observers recorded perceptual changes
that occurred during observation periods lasting 5 min. In one period they recorded when the
CU percept began and finished, thereby giving the number and duration of CU percepts; this
was repeated for CD and 2-D percepts. The sum of the durations for the percept of CU was
about the same as that for CD; taken together CU and CD percepts accounted for between a
third and a half of an observation period. The sum of the 2-D percepts accounted for between
a half and two-thirds of an observation period. The distributions of CU and CD percept dura-
tions were similar and were modelled adequately by gamma distributions. These percepts tended
to be brief (about 1 s) compared to the far more stable, and normally distributed, 2-D percepts.
The difference between the characteristics of CU/CD and 2-D percepts is consistent with a
system that operates at at least two different tiers of perceptual change. At one level, changes
can occur between 2-D and 3-D interpretations. Further changes can occur within either the
3-D or 2-D interpretations, such that if a 3-D interpretation has been accepted then there can be
further changes between CU and CD at this subordinate level. Most models of perceptual change
only consider this subordinate level.
^ Effects of top ^ bottom polarity and