Multisensory integration
The binding problem stemmed from unanswered questions about how mammals (particularly higher primates) generate a unified, coherent perception of their surroundings from the cacophony of electromagnetic waves, chemical interactions, and pressure fluctuations that forms the physical basis of the world around us.
Notwithstanding the existence of Gestalt psychology schools that advocate a holistic approach to the operation of the brain,[5][6] the physiological processes underlying the formation of percepts and conscious experience have been vastly understudied.
This effect was also found later in individuals with undamaged brains by Krakov[15] and Hartmann,[16] as well as the fact that the visual acuity could be improved by other type of stimuli.
A feature of this syndrome is the great permeability to crossmodal effects between visual, tactile, auditive stimuli as well as muscular effort to improve the perception, also decreasing the reaction times.
[29] This view has been backed up by computational modeling of such a Bayesian inference from signals to coherent representation, which shows similar characteristics to integration in the brain.
[42] From this perspective, Shared intentionality contributes to the formation of percepts and conscious experiences, solving the binding problem, or at least complements one of the mechanisms noted above.
[43] The contributions of Barry Stein, Alex Meredith, and their colleagues (e.g."The merging of the senses" 1993,[44]) are widely considered to be the groundbreaking work in the modern field of multisensory integration.
Gestalt theory, dominant in the late 19th and early 20th centuries espoused two general principles: the 'principle of totality' in which conscious experience must be considered globally, and the 'principle of psychophysical isomorphism' which states that perceptual phenomena are correlated with cerebral activity.
Just these ideas were already applied by Justo Gonzalo in his work of brain dynamics, where a sensory-cerebral correspondence is considered in the formulation of the "development of the sensory field due to a psychophysical isomorphism" (pag.
[53] In patients studied by Gonzalo,[18] with lesions in the parieto-occipital cortex, the decrease in the reaction time to a given stimulus by means of intersensory facilitation was shown to be very remarkable.
fMRI studies have shown that there is crossmodal activation in early, low level visual areas, which was qualitatively similar to the perception of a real flash.
[69] The structure receives afferents directly from the retina, as well as from various regions of the cortex (primarily the occipital lobe), the spinal cord and the inferior colliculus.
The structure contains a high proportion of multisensory neurons and plays a role in the motor control of orientation behaviours of the eyes, ears and head.
[53] Receptive fields from somatosensory, visual and auditory modalities converge in the deeper layers to form a two-dimensional multisensory map of the external world.
[71] Single neurons in the macaque putamen have been shown to have visual and somatosensory responses closely related to those in the polysensory zone of the premotor cortex and area 7b in the parietal lobe.
[76] Hubel and Wiesel showed that receptive fields and thus the function of cortical structures, as one proceeds out from V1 along the visual pathways, become increasingly complex and specialized.
[81] In contrast, the dorsal auditory pathway, projecting from the temporal lobe is largely concerned with processing spatial information, and contains receptive fields that are topographically organized.
[94] Stein, London, Wilkinson and Price (1996) analysed the perceived luminance of an LED in the context of spatially disparate auditory distracters of various types.
Concurrently in species of monkey, newborns are endowed with a significant complement of multisensory cells; however, along with cats there is no integration effect apparent until much later.
[53] This delay is thought to be the result of the relatively slower development of cortical structures including the AES; which as stated above, is essential for the existence of the integration effect.
[93] Furthermore, it was found by Wallace (2004) that cats raised in a light deprived environment had severely underdeveloped visual receptive fields in deep layers of the SC.
These cats displayed a marked improvement in their ability to localize stimuli through audition; and consequently also showed increased neural connectivity between V1 and the auditory cortex.
[98] Some multisensory abilities are present from early infancy, but it is not until children are eight years or older before they use multiple modalities to reduce sensory uncertainty.
Infants who were 8–10 months old showed significantly decreased response times when the source was presented through both visual and auditory information compared to a single modality.
[99] Follow-up work by Gori et al. (2012) showed that, at all ages, vision-size perceptions are near perfect when viewing objects within the haptic workspace (i.e. at arm's reach).
[103] Indeed, studies have shown that visuo-haptic integration fails in adults when there is a perceived spatial separation, suggesting sensory information is coming from different targets.
[111][112] Ernst (2008) suggests that adults can obtain this knowledge from previous experiences to quickly determine which sensory sources depict the same target, but young children could be deficient in this area.
Nardini et al. (2010) provides evidence that children's (aged 6 years) response latencies are significantly lower when stimuli are presented in multi-cue over single-cue conditions.
Considering the haste of real-world events, this strategy may prove necessary to counteract the general slower processing of children and maintain effective vision-action coupling.
Although the current commercially available prosthetic devices mainly focusing in implementing the motor side by simply uses EMG sensors to switch between different activation states of the prosthesis.
