The Impact of Stochastic and Deterministic Sounds on Visual, Tactile and Proprioceptive Modalities

Stimulus localization and particularly directional hearing can be considered as methods for investigating neural activity and they have proven to be useful tools for research in physiology and psychology. Human directional hearing techniques have been reflected upon way back by Von Bekesy in Austrian forests [1]. For example, he observed that some of the roads took a perfectly straight course through deep, dark woods. He could not imagine how such straight roads had been cut through the forest when the usual optical methods used by road surveyors would seem to be useless in this case. Further some of these roads were very old and probably built before the introduction of the theodolite. Many of these roads were laid out by an acoustic method. How did they do it? A man stationed at the starting point noted the direction of the sound produced by someone at the other end blowing a horn. The first man then walked toward the sound source, marking the threes on the way. It turned out that this method produced a straight line from start to finish [1]. From this observation Bekesy was motivated to perform a series of studies on stimuli localization not limited to hearing but also to vibration sensations on the skin, electrical pulses on the tongue and odors through the nose as well. Strikingly, his results showed an underlying ubiquitous mechanism present in the different stimuli localization modalities. For instance, the effect on localization of the time delay between two stimuli on the skin, the tongue, the two nostrils in the nose and the two ears, presented the same dynamics [2-4]. These results were quite exciting because it showed that, in humans, the senses work similarly for stimuli localization although the basic underlying neural pathways are not the same. It was this kind of general principle on stimuli localization that motivated us in the search for more general principles related to how senses interact to generate multisensory perceptions but with a special emphasis on auditory stimulation. This is known as multisensory integration and its study is very important because it is the foundation of how humans bind all the information coming from the senses to generate a coherent percept. We began by studying something that we called cross-modal stochastic resonance. This consists