Sensitivity to prediction error in reach adaptation

MK Marko, AM Haith, MD Harran, and R Shadmehr (2012) Journal of Neurophysiology           

Abstract  It is thought that the brain predicts sensory consequences of a movement and compares it to the actual sensory feedback.  When the two differ, an error signal is formed, driving adaptation.  If one is sensitive to error, that error produces large amounts of adaptation.  In contrast, if sensitivity to error is low, then that error produces only a small amount of adaptation.  Here, we show that sensitivity to error is not constant, but declines as a function of error magnitude.  We performed an experiment in which humans made reaching movements and randomly experienced an error in both their visual and proprioceptive feedback.  Proprioceptive errors were created using force fields, and visual errors were formed by perturbing the cursor trajectory to create a visual error that was smaller, the same size, or larger than the proprioceptive error.  We measured single trial adaptation and calculated sensitivity to error, i.e., the ratio of the change in motor commands to error size.  We found that for both sensory modalities, sensitivity decreased with increasing error size.  A reanalysis of a number of previously published psychophysical results also exhibited this feature.  Finally, we asked how the brain might encode sensitivity to error.  We reanalyzed previously published probabilities of cerebellar complex spikes (CS) and found that this probability declined with increasing error size.  From this we posit that a CS may be representative of the sensitivity to error, and not error itself, a hypothesis which may explain conflicting reports about CSs and their relationship to error.