Re: perceptual learning (Peter Meijer )

Subject: Re: perceptual learning
From:    Peter Meijer  <peter.b.l.meijer(at)PHILIPS.COM>
Date:    Mon, 3 Apr 2000 13:09:18 +0200

Ward writes > So, if an object in the visual space were turned to sound for a > blind person--would it then not be most useful to have the sound > emanate (perceptually) from the location of the object? Yes, as long as there is only one object or at most a very few objects, this approach would be intuitive and have the advantage of near-instantaneous sensory feedback. Unfortunately, this "spatial" or "3D" sound scheme that is so popular with games doesn't scale and generalize well to arbitrary and more complex images. It breaks down quite rapidly when there are more than a few objects or shapes in a planar ("photographic") view. Basically, it gets hard to avoid that when you have something like a bright spot on your left and an identical bright spot on your right that the combination sounds as a single spot right in front of you. With a simple sine wave to indicate lateral position through ITD and ILD this becomes readily apparent: the ITD and ILD effects cancel each other in a left-right symmetrical situation, while pitch is needed to indicate elevation and is assumed the same for the left and right spot in this example. Using a more complex spectrum for a spot does not help here either as long as left-right symmetry is to be maintained. A "time division multiplexing" scheme (term coined originally by Paul Bach-Y-Rita for his tactile matrix displays, but in our case it could be coined "inverse spectrographic" or something like that) resolves the problem by breaking the symmetry while also allowing for a higher image resolution by spreading the visual information in time, but the resulting dynamic spectral profile may indeed be no longer quite as intuitive, and perceptual latency is now an inevitable consequence or artefact. However, part of the intuitive lateral localization can be restored by adding stereo panning to the time axis of the general "inverse spectrographic" image to sound mapping. More on that below. By the way, if it is mainly for detecting nearest obstacles rather than general vision substitution, then a distance-to-pitch mapping with directional (spatial, 3D) sound can indeed work quite well, and a number of sonar-based mobility devices for blind people are in fact making use of this. There is a publication on the auditory perception of objects by blind persons using sonar devices forthcoming in the JASA of May 2000. > Peter has height mapped to frequency and left to right mapped > to time (would this be right to left in some countries?) Yes, we'll have a special edition for the Middle-East area as soon as the market justifies extended regional support. ;-) > At any rate, it wouldn't be tremendously difficult with headphones > to have that image shift from left to right acoustically as it does > visually. True, and that is what the The vOICe "Learning Edition" does by default: it supplements and reinforces the perception of left-to-right scanning through stereo panning, using ILD and ITD. HRTF would not be very useful here because the spectral shape is already being used to convey visual content in this image to sound mapping. For those interested, an illustration of this scanning + stereo panning is the sound of an artificial image showing one period of a bright sine wave trace and ten little bright squares in a sound sample of just one second=20 duration (22K file size) at the URL or its two-second "slow-motion" version (44K) at One can make spectrograms of these sound samples to check out their visual content against what one hears (using a logarithmic frequency scale for proper results), and use autorepeat or an equivalent option during playback to hear the short samples a number of times. Listeners will likely notice that their brain applies "mental saccades" to focus attention to the different time-frequency components within the sound as the same sound sample is being played repeatedly. Hearing it "all at once" in a single pass appears to be rather hard to an untrained person. > Persons who are blind may well have a heightened auditory sensations > (since all the brain mass most people used to see could be used by > the other senses) A few years ago, Cohen et al. showed (or at least made plausible) through fMRI measurements that congenitally blind people use part of their visual cortex for tactile processing: L. G. Cohen, P. Celnik et al., ``Functional relevance of cross-modal= plasticity in blind humans,'' Nature, vol. 389, pp. 180-183, Sept. 1= 1, 1997. I haven't seen comparable publications on auditory processing in the visual cortex of blind people yet, but it would seem plausible too when conceptually generalizing from various kinds of other neuroscienti= fic experiments where neighbouring cortical areas tend to "take over" areas= of amputated limbs, and with coordination-affecting RSI (not the more common painful variant) apparently mixing signals in normally distinct cortical areas, etc. Another issue is sensory deprivation: to what extent does part of the cortex perhaps degenerate for lack of input from the senses? For=20 instance, most recently a blind man regained partial sight through surgery after over fourty years of near-total blindness, and (for the moment at least) he has significant problems understanding what he sees with his eyes - although he is highly intelligent, and has seen normally until he was three years old. I had the pleasure of meeting this man last year before his surgery. Now these special cases of (problems with) "learning to see" can be of great interest to any attempt to provide some form of vision substitution through technical means, because the whole chain from the purely technical to low-level auditory perception, higher-level processing, neural plasticity and human psychology (for motivation) needs to be covered, and the weakest link could be just about anywhere along the chain. Best wishes, Peter Meijer Seeing with Sound - The vOICe =

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