By watching the actions of others and comprehending the intent conveyed through the acts, individuals constantly learn from their social interactions and adapt accordingly. In humans a specific class of neurons, known as mirror neurons, comprise a system believed to be a factor in the understanding of actions and intentions, as well as in language procurement, learning by imitation, and development of empathy. It has been well established that the mirror system shows greater activation during observation of familiar movements, and can even be activated through listening to familiar sounds alone without any accompanying visual cues. While this clearly indicates that visual perception is not a requisite for activation of mirror neurons, it does not preclude the possibility that activation is ultimately due to visually based mental imagery that has been triggered by the auditory stimuli. So is functional sight vital to the activation of mirror neurons and thus to the ability to learn through imitation and interact successfully in the external world? Or can the visually impaired still retain use of their mirror neuron networks, and thereby learn from and effectively "see" the actions of others? Emiliano Ricciardi of the University of Pisa, in conjunction with his colleagues, designed a novel experiment in order to ascertain the correct answer.
The study consisted of eight blind participants and fourteen participants with normal vision. Of the eight blind participants, seven were congenitally blind and one lost vision at the age of two, yet had no memory of any visual experience. Each subject was asked to listen to twenty aural descriptions of hand-motor actions and ten environmental sound samples, such as of a rainstorm. The participants were also asked to perform motor pantomimes of specific actions upon hearing a spoken explanation of the movements and the items involved. Upon completion of the task, participants were asked to identify the sounds they heard, and rate their competency in performing the associated actions. The sighted participants were additionally asked to view action or environmental images, as well as pantomime action words that appeared on the screen. During all phases, every participant was imaged using fMRI technology, in order to observe changing brain activity throughout the tasks. Both groups of participants demonstrated equal capabilities in correct execution of the given motor movements and recognition of the various auditory stimuli.
The imaging results from Ricciardi's study revealed an overlap of brain activation between specific areas when participants listened to action sounds and when the actions were physically performed. This overlap isolated a left-lateralized mirror neuron cortical network that was comprised of premotor, temporal, and parietal areas in both blind and sighted subjects. Auditory presentation of familiar actions and movements caused greater overall activation of the mirror system network in both subject groups, compared to the activation elicited by unfamiliar sounds/actions.
These results were incredibly significant as they provided evidence that the mirror system can normally develop without vision through processing of non-visual information concerning actions. Individuals who were born sightless, and thus were never exposed to any visual stimuli, still possessed functioning motor-based mirror neuron networks that were triggered by auditory cues describing actions. Thus, blind individuals can in essence "see" the actions of others, in the sense that cortical activation of identical areas occurs in them as does in sighted people in response to observation/interpretation of action. This implies that mirror neuron networks are based upon abstract and supramodal sensory portrayals of actions, thereby allowing sightless individuals to understand external actions and learn through imitation of others as successfully as those without impaired vision. Ricciardi has achieved the seemingly impossible and presented the blind with the gift of sight... at least from a cortical perspective.
Original article can be found at: http://www.jneurosci.org/content/29/31/9719.full?sid=948c8b50-3d40-470f-b0dd-ca5c939b2ec0