Biomechanical Texture Coding and Transmission of Texture Information in Rat Whiskers

Abstract

Classically, texture discrimination has been thought to be based on ‘global’ codes, i.e. frequency

(signal analysis based on Fourier analysis) or intensity (signal analysis based on averaging),

which both rely on integration of the vibrotactile signal across time and/or space. Recently, a

novel ‘local’ coding scheme based on the waveform of frictional movements, discrete short-

lasting kinematic events (i.e. stick-slip movements called slips) has been formulated. In the first

part of my study I performed biomechanical measurements of relative movements of a rat

vibrissa across sandpapers of different roughness. My major finding is that the classic global

codes convey some information about texture identity but are consistently outperformed by the

slip-based local code. Moreover, the slip code also surpasses the global ones in coding for active

scanning parameters. This is remarkable as it suggests that the slip code would explicitly allow

the whisking rat to optimize perception by selecting goal-specific scanning strategies. I therefore

provide evidence that short stick-slip events may contribute to the perceptual mechanism by

which rodent vibrissa code surface roughness.

In the second part, I studied the biomechanics of how such events are transmitted from tip to

follicle where mechano-transduction occurs. For this purpose, ultra-fast videography recording

of the entire beam of a plucked rat whisker rubbing across sandpaper was employed. I found

that slip events are conveyed almost instantly from tip to follicle while amplifying moments by a

factor of about 1000. From these results, I argue that the mechanics of the whisker serve as a

passive amplification device that faithfully represents stick-slip events to the neuronal

receptors. Using measures of correlation, I moreover found that amongst the kinematic

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variables, acceleration portrays dynamic variables (forces) best. The time series of acceleration

at the base of the whisker provided a fair proxy to the time series of forces (dynamical variables)

acting on the whisker base. Acceleration measurements (easily done via videography) may

therefore provide an access to at least the relative amplitude of forces. This may be important

for future work in behaving animals, where dynamical variables are notoriously difficult to

measure.