time and space

[Neil Todd <todd@xxxxxxxxxxxxxxxxxx>]

Dear Pierre, Peter, Al, Charles, etc. and List

If I may reiterate some points I made earlier related to my own
modelling work. The issue of temporal resolution and the
relative temporal acuity of the auditory and visual systems
becomes more clear if we move out of the time-domain and into
the frequency-domain. In doing so we realise that it makes a
lot of sense for the brain to represent temporal information
"spatially". When we do this the relationship between vision
and audition also becomes more clear.

By representing temporal information topographically at an
early stage of the auditory system (CN, IC), to a certain
extent this may still be available to the cortex even though in
terms of phase locking of individual units they may only go up
to 100 Hz at most in primary cortex and most only go as high as
about 20 Hz. The existence of an approximately orthogonal
cochleotopic/periodotopic arrangement in the cortex has
recently been shown by Gerald Langner using MEG in humans and
single unit recordings in Gerbils. Remarkably though, the
cortical periodotopic range goes above that of the IC.

Once we have this 3-D "space-time" representation of the input
to the cortex, analogous to (x,y,t) of the visual cortex, then
by multi-scale spatio-temporal decomposition, e.g. by 3-D
space-time Gabor filters such as used in models of visual
motion, the flow of auditory information can be represented by
a suitably tuned and oriented propulation of filters in a 3-D
scale-space. The problem with the time dimension of a 3-D Gabor
is that it has a Gaussian envelope which is acausal and
therefore physically impossible.  However,  existence of
dynamic spatio-temporal RFs has been shown in the cortex by
both Christoph Schriener and Shihab Shamma. So this idea is not
a complete fantasy. Certainly in terms of modelling, whilst
computationally highly intensitve, produces some extremely
interesting results.

If we accept the above then by considering the relative ecology
of auditory and visual motion any difference that we see would
be perfectly natural.  The brain during maturation "tunes in"
to the temporal properties of the environment. Although there
is some overlap between the range of temporal-frequencies, as
evidenced in lip-reading speech, there are also important
differences. I would suggest that the answer to many of our
questions might be found by looking at the comparative temporal
properties of natural auditory and visual signals.

Neil