Neural correlates for roughness choice in monkey second somatosensory cortex
(sii).
Pruett, J. R., Jr. , R. J. Sinclair, and H. Burton.
Department of Anatomy and Neurobiology, Washington University School of
Medicine, St. Louis, MO 63110
APStracts 8:0234J, 2001.
This experiment explored the relationship between neural firing patterns in second
somatosensory cortex (SII) and decisions about roughness of tactile gratings. Neural and
behavioral data were acquired while monkeys made dichotomous roughness
classifications of pairs of gratings that differed in groove width (1.07 versus 1.90 mm and
1.42 versus 2.53 mm). A computer-controlled device delivered the gratings to a single
immobilized finger pad. In one set of experiments three levels of contact force (30, 60,
and 90 gms) were assigned to these gratings at random. In another set of experiments
three levels of scanning speed (40, 80, and 120 mm/sec) were assigned to these gratings
at random. Groove width was the intended variable for roughness. Force variation
disrupted the monkeys= groove width (roughness) classifications more than did speed
variation. A sample of 32 SII cells showed correlated changes in firing (positive or
negative effects of both variables) when groove width and force increased (Pruett et al.
2000). While these cells were recorded, the monkeys made roughness classification
errors, confusing wide groove width gratings at low force with narrow groove width
gratings at high force. Three-dimensional plots show how some combinations of groove
width and force perturbed the monkeys= trial-wise classifications of grating roughness.
Psychometric functions show that errors occurred when firing rates failed to distinguish
gratings. A possible interpretation is that when asked to classify grating roughness, the
monkeys based classifications on the firing rates of a sub-set of roughness-sensitive cells
in SII. Results support human psychophysical data and extend the roughness range of a
model (Lederman 1974) of the effects of groove width and force on roughness. One
monkey=s SII neural sample (21 cells) showed significant correlation between firing rate
response functions for groove width and speed (both correlations either positive or
negative). Only that monkey showed a statistically significant interaction between
groove width and speed on roughness classification performance. This additional finding
adds weight to the argument that SII cell firing rates influenced monkey roughness
classifications.
Received 19 May 2000; accepted in final form 21 May 2001
APS Manuscript Number J2019-0.
Article publication pending Am J Physiol
ISSN 1080-4757 Copyright 2001 The American Physiological Society.
Published in APStracts on 31 July 2001