Sensory integration during reaching: the effects of manipulating visual target availability.

When using visual and proprioceptive information to plan a reach, it has been proposed that the brain combines these cues to estimate the object and/or limb's location. Specifically, according to the maximum-likelihood estimation (MLE) model, sensory inputs are combined such that more reliable inputs are assigned a greater weight (Ernst and Banks in Nature 415:429-433, 2002). In this paper, we examined if the brain is able to adjust which sensory cue it weights the most. Specifically, we asked if the brain changes how it weights sensory information when the availability of a visual cue is manipulated. Twelve healthy subjects reached to visual (V), proprioceptive (P), or visual + proprioceptive (VP) targets under different visual delay conditions (e.g., on V and VP trials, the visual target was available for the entire reach; it was removed with the go signal, or it was removed 1 s before the go signal). To establish which sensory cue subjects weighted the most, we compared endpoint positions achieved on V and P reaches to VP reaches. Results indicated that subjects combined visual and proprioceptive cues in accordance with the MLE model when reaching to VP targets. Moreover, subjects' reaching errors to visual targets increased with longer visual delays (particularly in the vertical direction). However, there was no change in reach variability with longer delays, and subjects did not reweight visual information as the availability of visual information was manipulated. Thus, a change in visual environment is not sufficient to cause the brain to reweight how it processes sensory information.