Molyneux's question : vision, touch, and the philosophy of perception / Michael J. Morgan.

  • Morgan, Michael J.
Date:
1977
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Molyneux's question : vision, touch, and the philosophy of perception / Michael J. Morgan. Source: Wellcome Collection.

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    MOLYNEUX'S QUESTION well-known trick in which a line with outward-pointing arrows on the end is made to look longer than one with inward-pointing arrows. It is possible to make a haptic version of this illusion by constructing it in relief, and asking blindfolded people to make judgements of the relative lengths of the lines. Géza Révész investigated a number of illusions in this way, including the Müller-Lyer, and found that in most cases there was a haptic illusion corresponding to the visual one.'^^ Another illusion with a haptic counterpart is the Poggendorff, in which a straight line interrupted by two parallels no longer looks straight. Both Révész and Calabresi report that this is also experienced as a haptic illusion.Over thirty years later, G. Fisher rediscovered the same fact (this happens quite a lot in the study of illusions). Are the haptic illusions in any way the same as the visual ones, or is their resemblance just an accident? Important evidence for a common mechanism is the finding that there is a transfer of decrement between them. If the Müller-Lyer illusion is inspected for long periods of time, the strength of the illusion is decreased. Rudel and Teuber found that this decrement occur¬ red with the haptic illusion also, and moreover that the mag¬ nitude of the visual illusion was decreased by a previous haptic decrement. A possible mechanism for the transfer of decrement, as well as of the existence of haptic illusions themselves, is that the blind¬ folded subject merely 'visualizes' the illusion while apparently 'feeling' it. In other words, this explanation would suggest that there is really only a visual illusion, and the haptic illusion is secondary. If the explanation is correct, haptic illusions should not be experienced by blind people, for they would lack the ability to visualize the figures. This possibility was investigated by Révész.He presented congenitally-blind subjects with G. Révész, 'System der optischen und haptischen Raumtäuschungen', Zeitschrift für Psychologie, 131 (1934) 296-375. Renate Calabresi, Contributo allo studio della illusione di Poggendorff nella periezione tattilo-cinetica (Turin: Giuseppe Anfossi, 1933]. G. H. Fisher, 'AtactilePoggendorff illusion',Nature,Lond., 212 (1966) 105-6. R. G. Rudel and H.-L. Teuber, 'Decrement of visual and haptic Müller-Lyer illusion on repeated trials: a study of cross-modal transfer'. Quarterly Journal of Experimental Psychology, 15 (1963) 125-31. Op. cit., section IV C: 'Unabhängigkeit der haptischen Täuschungen von visuellen Vorstellungen. Versuche an Blindgeborenen'. 194