TOUCH PSYCHOPHYSIOLOGY L. Négyessy PPKE, 2010

2 Haptic exploration of local shape Static stimuli 1-2 mm 2,8 mm min. 0,5 mm  : 3% 0,17 mm

3 Thresholds I Braille dots’ height: 500 µm

4 6 x 18 = 108 blunt pins 230 Hz Vibrating stimuli OPTACON

5 Thresholds II Discriminabitlity (d’) of complex waveforms for low frequencies d’

6 The 4 channel model - psychophysics Threshold

7 Sensory Afferents Conduct.: m/s (Hand scan: mm/s) The 4 channel model - neurophysiology

8 Receptive field organization (finger pad) SAIRAPSAII ? RF size 1 mm mm 2 diffuse Aff. denz. 100 cm cm / finger? (low) Diverg. (RF area) 4-16 (5 mm 2 ) 1:1 Converg. 1:12-71:1 Adequ. stim. Strain energy density (point, edge, curve) Slip, load force High freq. vibration Skin stretch function Form, textureGrip control, fine discr. Distant events, tool use Hand shape

9 Form perception: dots, edges, curves Braille reading Aperiodic grating SAI channel

10 Receptive field characteristics relevant to form perception RAI & SAI channels

11 Surround supression skin mechanics

12 Perception of texture: roughness

13 Fine texture perception: SAI spatial variation* *Mean absolute difference in firing rates between SAI afferents with RFs separated by ~2mm

14 SAI spatial variation code for fine textures (0.2-1mm)

15 Summary of form & texture perception coding of spatial features Evidences of SA1 specialization for the representation of spatial information: SA1 responses to stimulus elements on a surface are independent of the force of application. SA1-receptive fields grow minimally (relative to RA receptive fields) with increasing indentation depth. SA1 afferents possess a response property, surround suppression, which confers response properties similar to those produced by surround inhibition in the central nervous system. This response property is a consequence of sensitivity to strain energy density, not a synaptic mechanism. SA1 spatial resolution is affected minimally by changes in scanning velocity at velocities up to at least 80 mm s–1. SA1 afferents are at least ten times more sensitive to dynamic than to static stimuli. SA1 responses to repeated skin indentation are practically invariant: the variability is about one impulse per trial regardless of the number of action potentials evoked. The RA system has greater sensitivity but poorer spatial resolution and limited dynamic range.

16 Vibrotactile perception: flutter, vibration

17 RA channel Periodic Stimulus Firing rate Periodic St Periodicity (IS interval) Aperiodic St Firing rate Periodic+aperiodic St Only firing rate firing rate periodicity monkey ideal obs. Thresholds ratios: psychometric/neurometric thresholds

18 P channel 10 nm skin motion at 200 Hz ideal obs. of St motion

19 Response to vibrating stimulus

20 P channel intense filtering (at nearly 60 dB per decade) of low-frequency stimuli respond to stimuli less than 100–150 Hz with a phase-locked, Poisson discharge, therefore a whole population firing randomly but at a rate proportional to the instantaneous stimulus amplitude can represent the stimulus waveform accurately RA channel RA neurons of S1, like their afferent fibers, fire periodically, in phase with mechanical oscillations RA neurons modulate their firing rates as a function of the stimulus frequency Flutter is encoded by firing rate of RA neurons Summary of vibrotactile perception - coding of temporal features

21 Adaptation Peripheral mechanisms

22 Time course of adaptation and recovery

23 RA interference in spatial processing

24 Tool use

25 Coding object size

26 Grasping and manipulation

27 Grasping and manipulation

28 SUMMARY The 4 channel model of vibrotactile discrimination RF correlates of the 4 channel model Elements of form perception Texture (roughness) perception Vibrotactile perception Object manipulation