  Theories including a collapse mechanism have been presented various years
ago. They are based on a modification of standard quantum mechanics in which
nonlinear and stochastic terms are added to the evolution equation. Their
principal merits derive from the fact that they are mathematically precise
schemes accounting, on the basis of a unique universal dynamical principle,
both for the quantum behavior of microscopic systems as well as for the
reduction associated to measurement processes and for the classical behavior of
macroscopic objects. Since such theories qualify themselves not as new
interpretations but as modifications of the standard theory they can be, in
principle, tested against quantum mechanics. Recently, various investigations
identifying possible crucial test have been discussed. In spite of the extreme
difficulty to perform such tests it seems that recent technological
developments allow at least to put precise limits on the parameters
characterizing the modifications of the evolution equation. Here we will simply
mention some of the recent investigations in this direction, while we will
mainly concentrate our attention to the way in which collapse theories account
for definite perceptual process. The differences between the case of reductions
induced by perceptions and those related to measurement procedures by means of
standard macroscopic devices will be discussed. On this basis, we suggest a
precise experimental test of collapse theories involving conscious observers.
We make plausible, by discussing in detail a toy model, that the modified
dynamics can give rise to quite small but systematic errors in the visual
perceptual process.
