The cutaneous code: breaking down neural patterns and optimal noise

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Reduced skin input from the feet causes deficits in the sense of body position (proprioception) and declines in the muscular reflexes that control balance. Efforts to augment skin input have taken on several forms, primarily vibration or electrical stimulation to enhance skin input (stochastic resonance; SR). To date, SR vibration technologies have used a large range of frequencies to target all four of the cutaneous mechanoreceptors that reside in the skin (Slow adapting; SA and Fast Adapting FA, type I and II). However, work in our lab has shown there are stronger reflex connections from specific subclasses of afferents (SA type I and FA type I) to motor neurons of the arms and legs, suggesting these mechanoreceptors may have a larger role in balance control. As a result, it would be more effective to focus SR frequencies to enable targeted receptor activation of these relevant subclasses. The first important step is to identify which receptors are activated during natural movements. In the first Aim of the proposal we will identify the fundamental firing characteristics of skin in response to natural load and shear. This project extends our recent work where we examined individual receptor firing to isolated vibration frequencies, which are components of real input. This previous work aimed to better understand how to best activate individual receptor subclasses. By understanding which receptors respond to natural load we can target these receptors to enhance sensory feedback. The second Aim of the proposal is to generate isolated activation of select mechanoreceptors using streamlined SR. In our previous work we have shown that SA and FA receptors in the foot skin have specific frequency bands to which they are most easily activated. Aim 2 will establish whether refined frequency bands (with limited frequencies) will enable targeted activation of specific receptors. Specialized SR may have the ability to improve balance outcomes by targeting individual skin receptors that are relevant for movement and balance control. Finally, we acknowledge that skin receptors, particularly on the feet, are not activated in isolation during natural movement. We aim to enhance specific receptors in our work, and as such it is necessary to understand how the receptors work together to provide the code. The third Aim will examine the excitatory and inhibitory connections between receptor subclasses to help optimize sensory activation. Together this work will identify specific receptors in the skin that we can target for enhanced sensory feedback. The ability to selectively enhance skin receptors in isolation will enable augmented feedback from receptors that are most relevant for balance control. This work can also contribute to the advancement of current insole technologies, improve the sensory interface for prosthetic users, and contribute to the development of navigation devices (gaming, military).