time and space (Neil Todd )

Subject: time and space
From:    Neil Todd  <todd(at)HERA.PSY.MAN.AC.UK>
Date:    Wed, 4 Jun 1997 11:36:19 +0100

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

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