Can the crossmodal congruency task be a proxy for intuitiveness of sensory feedback in lower-limb amputees?

The lack of sensory feedback from the foot contributes to balance impairments and falls in individuals with a lower-limb amputation. Ongoing research focuses on developing somatosensory neuroprostheses to restore sensation to the missing limb via electrical stimulation; however, participants often report that the sensations produced by the stimulation are unintuitive. The impact of sensory intuitiveness on prosthetic function has not yet been established, in part due to the lack of a reliable metric of intuitiveness. Previous work has proposed a speeded cross-modal congruency task as a means to quantify intuitiveness for upper-limb somatosensory neuroprostheses. Participants verbally indicate the location of evoked sensations while ignoring a visual distractor at the same or another location (congruent or incongruent trials, respectively). The magnitude of slowing of response times for incongruent trials, called the cross-modal congruence effect (CCE), has been used to measure intuitiveness, under the assumption that more intuitive sensations will be more intrusive. This study modified the task to evaluate the intuitiveness of two types of evoked sensations (electrotactile or pneumotactile) in the knee and foot. Fifteen able-bodied individuals completed the modified task. The CCE was higher for pneumotactile stimulation compared to electrotactile stimulation at the knee, but not at the foot. The location dependence of the CCE in the lower extremity suggests that it is not a good proxy for sensory intuitiveness in the lower-limb and thus should not be used for assessing lower-limb somatosensory neuroprostheses.