Multisensory integration (MI) is a non-linear process that binds information from multiple sensory stimuli. MI is a complex process in which our brain binds the different sensory stimuli that contribute to create a unified perception of the real world beyond our perceptual limitations. This perception is the only reality we have. Researchers have tried to define the human sensory stimulus span from threshold to ceiling.

Researchers have tested humans exposed to stimuli in the different senses, and measured auditory, tactile, and visual thresholds. These kinds of studies have demonstrated that if two weak (close to threshold level) stimuli are applied together, perception is facilitated. In other words, the weak stimuli seem stronger, louder, or brighter – as the case may be. This happens when the multisensory stimuli are presented within a dynamic temporal window. This is known as the law of inverse-effectiveness law (Stein and Meredith, 1993), which also demonstrated that this kind of benefit is not possible if one of the stimuli is clearly supra-threshold. This means that perceptual enhancement takes place through the MI mechanism when we apply weak, subthreshold, deterministic and coincident signals to the subject. However, there is a MI phenomenon that cannot be described by the law of inverse-effectiveness: crossmodal stochastic resonance.

Stochastic resonance (SR) is a non-linear phenomenon whereby the addition of noise can improve the detection of weak stimuli (Moss et al., 2004). An optimal amount of noise will result in the maximum enhancement, whereas further increases in noise intensity will result in reduced detection. The phenomenon does not occur in linear systems, which are better characterised by the fact that the addition of noise to either the system or the stimulus will result in decreases in signal quality. The SR signature is that the signal-to-noise ratio, which is proportional to the system’s sensitivity, is an inverted U-like function of different noise levels. The SR phenomenon was thought to exist only in stochastic, non-linear, dynamical systems but it also exists in another form referred to as ‘threshold SR’ or ‘non-dynamical SR’. This form of stochastic resonance results from the concurrence of a near threshold stimulus and noise. These conditions are omnipresent in nature, as well as in a variety of man-made systems, which accounts for the observation of SR in many fields and conditions.

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