  Some versions of quantum theory treat wave function collapse as a fundamental
physical phenomenon to be described by explicit laws. One motivation is to find
a consistent unification of quantum theory and gravity, in which collapse
prevents superpositions of space-times from developing. Another is to invoke
collapse to explain our perception of definite measurement outcomes. Combining
these motivations while avoiding two different collapse postulates seems to
require that perceptibly different physical states necessarily create
significantly different mass distributions in our organs of perception or
brains. Bassi et al. investigated this question in the context of mass density
dependent spontaneous collapse models. By analysing the mechanism of visual
perception of a few photons in the human eye, they argued that collapse model
parameters consistent with known experiment imply that a collapse would take
place in the eye within the human perception time of ~100ms, so that a definite
state of observing some or no photons would be created from an initial
superposition. I reanalyse their arguments, and note a key problem: they treat
the relevant processes as though they take place in vacuo, rather than in
cytoplasm. This makes a significant difference, since the models imply that
superpositions collapse at rates that depend on the difference between the
coarse grained mass densities of their components. This increases the required
collapse rate, most likely by at least an order of magnitude and plausibly by
significantly more. This casts some doubt on the claim that there are collapse
model parameters consistent with known experiment that imply collapse times of
<~ 100ms within the human eye. A complete analysis would require a very
detailed understanding of the physical chemistry and biology of rod cells at
microscopic scales.
