The crossmodal congruency task as a measure of intuitiveness of sensory feedback in the lower limb

Problem Statement
Individuals with lower-limb amputations suffer from functional deficits, including balance and gait impairments, due to the loss of somatosensory feedback from their prostheses. While neuroprosthetic advances using electrical stimulation of residual limb nerves or the spinal cord can restore sensations, these evoked sensations are often non-intuitive (e.g., paresthetic buzzing rather than natural touch). To maximize the efficacy of these devices, restored sensations must be intuitive and seamlessly integrated into the sensorimotor network. However, quantifying the "intuitiveness" of these evoked sensations remains a significant challenge. Current research relies heavily on subjective "naturalness" ratings, which are highly variable and do not necessarily reflect how well a sensation integrates with perceptual pathways governing motor performance. While the Crossmodal Congruency Effect (CCE) task has been proposed as an objective metric for multisensory integration in upper-limb amputees, its feasibility and validity for assessing sensory feedback in the lower limb have not been established.

Methodology
This study aimed to validate the CCE task as a measure of sensory feedback intuitiveness in the lower limb using fifteen able-bodied participants. The experimental design compared two sensory modalities:

1. Pneumatic Tactile Stimulation: Delivered via a Galileo system, recruiting slowly adapting type 1, rapidly adapting, and PC fibers to mimic natural skin indentation and pressure changes.
2. Electrical Stimulation: Delivered via a DS8R stimulator, recruiting large populations of afferent fibers simultaneously.

Stimuli were applied at two anatomical locations: the knee (mid-thigh) and the foot (dorsal foot). To ensure valid comparisons, the study implemented several control measures:

• Intensity Matching: Electrical stimulus amplitudes were adjusted for each participant and location until the perceived intensity matched the fixed pneumatic stimulus.
• Visual Reaction Time Control: A preliminary test confirmed that visual reaction times were not significantly affected by the varying distances to the knee and foot LEDs.
• Synchronization: Hardware delays were measured and compensated to ensure target (somatosensory) and distractor (visual) stimuli were synchronized within <1ms.

The CCE Task: Participants performed a speeded-response task where they verbally identified the location of a somatosensory target (knee or foot) while ignoring a simultaneous visual distractor (LED).

• Congruent Trials: Target and distractor appeared at the same location.
• Incongruent Trials: Target and distractor appeared at different locations.
• Metrics: The primary outcome was the Crossmodal Congruency Reaction Time ($CCERT$), defined as the difference in reaction time between incongruent and congruent trials. A higher $CCERT$ indicates greater multisensory integration (and thus, theoretically, greater intuitiveness). Secondary metrics included error rates ($CCE_{Error}$) and EEG band power analysis (Delta and Theta bands) in the parietal region.

Key Results

• Differentiation of Modalities: The CCE task successfully differentiated between pneumatic and electrical stimuli at the knee. Specifically, 14 of 15 participants exhibited a higher $CCERT$ for pneumatic stimuli compared to electrical stimuli, supporting the hypothesis that pneumatic stimuli are more intuitively integrated.
• Location Dependency: This differentiation was not observed at the foot. Results at the foot were inconsistent, with only 3 participants showing higher $CCERT$ for pneumatic stimuli and 5 showing the opposite trend.
• Individual Variability: Significant CCE scores were present in only 67% of participants overall. The effect was not consistent across both stimulus modalities for the same individual, suggesting high inter-subject variability.
• Naturalness Correlation: There was no significant correlation between $CCERT$ scores and subjective naturalness ratings. This indicates that the CCE score is not a direct proxy for perceived naturalness.
• EEG Findings: EEG analysis revealed variability in cortical biomarkers. While incongruent trials showed higher parietal power in Delta and Theta bands at the knee (consistent with prior literature), the foot showed the opposite pattern, though these differences were not statistically significant across the group.
• Fatigue and Reaction Time: No significant correlation was found between fatigue levels or baseline reaction time variability and the $CCERT$ scores.

Significance and Claims
The paper concludes that while the CCE task shows promise as a measure of multisensory integration for the lower limb, its application is currently limited by location-specific inconsistencies and a lack of correlation with subjective naturalness.

• Modest Validation: The study validates the CCE paradigm for the knee but highlights that it does not generalize to the foot under the current experimental conditions.
• Metric Limitations: The authors assert that the CCE score is not simply a proxy for "naturalness" ratings, as the two measures did not correlate.
• Individual vs. Group Analysis: The study emphasizes that for neuroprosthetic applications, individual-level analysis is critical, as group-level averages may mask the fact that the CCE effect is absent in a significant portion of individuals.
• Future Requirements: The authors state that before the CCE task can be implemented to measure intuitiveness in lower-limb amputees, external factors affecting the CCE (such as spatial distance between target and distractor, spatial acuity differences between body regions, and response conflict mechanisms) must be better understood and controlled. The paper does not claim the CCE is the definitive solution but rather a tool requiring further development and validation for lower-extremity use.