Ensemble coding of muscle stretches in afferent populations containing different types of muscle afferents

Ensemble coding of simple mechanical stimuli (small sinusoidal stretches) was studied in simultaneously recorded mixed ensembles of primary- and secondary muscle spindle afferents (MSAs), and Golgi tendon organ (GTO) afferents recorded from L7-S1 dorsal root filaments. The experiments were made on 48 recorded afferents (29 primary MSAs, 6 secondary MSAs and 13 GTO afferents) in chloralose anaesthetised cats. For the analyses, we used a combination of principal component analysis and algorithms for quantification of stimulus discrimination. Mixed ensembles of primary- and secondary MSAs, and GTO afferents, discriminated significantly better between different muscle stretches than ensembles of only one or two types of these afferents. All kinds of ensembles showed a successive increase in discriminative ability with increased ensemble size and this ability seemed to level at larger populations. However, the increase in discriminative ability was significantly greater for the mixed ensembles. It is hypothesised that the main reason for the greater discriminative ability achieved by mixed ensembles, might be that the variation in response profiles (sensitivity tuning) among the individual afferents of the mixed ensemble will be larger than that for ensembles of only one type of afferent. Finally, the results in the present study give experimental support to some of the teleological arguments in favour of the ensemble coding theory.