Investigations into visually-induced somatic amplification

Abstract

Studies into tactile detection have found that a simultaneous light stimulus affects judgments of the presence or absence of a near threshold tactile stimulus, leading to higher numbers of false alarms, whereby participants report feeling a touch in the absence of a stimulus, as well as modest improvements in tactile sensitivity (Lloydet al., 2008; McKenzieet al., 2010, 2012). Anecdotally, the reported intensity of the bi-modal stimuli was also enhanced — an effect that was investigated in the current studies. In Experiment 1 participants discriminated between ‘weak’ and ‘strong’ tactile stimuli, which were accompanied by a light flash on 50% of trials. We observed a bias towards classifying light-present vibrations as ‘strong’ regardless of signal strength, as well as some evidence for improved accuracy in discrimination. In Experiment 2, participants gave a subjective intensity rating on a 6-point scale, which showed an increase in magnitude ratings for both ‘weak’ and ‘strong’ vibrotactile stimuli when the light was present. This shows that a simultaneous light can both improve tactile discrimination and lead to both types of tactile stimulus being rated as stronger. Primary somatosensory cortex may underlie these effects, as it receives direct projections from visual cortex and indirect projections from association areas of the brain. However, it is as yet unclear whether or not these effects are due to the same underlying mechanism, or at which processing stage they occur. Therefore, we are currently exploring the neural underpinnings of this amplification effect using both fMRI and EEG.