Why the whole is more than the sum of its parts: Salience-driven overestimation in aggregated tactile sensations

Experimental psychology often studies perception analytically, reducing its focus to minimal sensory units, such as
thresholds or just noticeable differences in a single stimulus. Here, in contrast, we examine a synthetic aspect: how
multiple inputs to a sensory system are aggregated into an overall percept. Participants in three experiments judged the
total stimulus intensity for simultaneous electrical shocks to two digits. We tested whether the integration of component
somatosensory stimuli into a total percept occurs automatically, or rather depends on the ability to consciously
perceive discrepancy among components (Experiment 1), whether the discrepancy among these components influences
sensitivity or/and perceptual bias in judging totals (Experiment 2), and whether the salience of each individual component
stimulus affects perception of total intensity (Experiment 3). Perceptual aggregation of two simultaneous component
events occurred both when participants could perceptually discriminate the two intensities, and also when they could
not. Further, the actual discrepancy between the stimuli modulated both participants’ sensitivity and perceptual bias:
increasing discrepancies produced a systematic and progressive overestimation of total intensity. The degree of this
bias depended primarily on the salience of the stronger stimulus in the pair. Overall, our results suggest that important
nonlinear mechanisms contribute to sensory aggregation. The mind aggregates component inputs into a coherent and
synthetic perceptual experience in a salience-weighted fashion that is not based on simple summation of inputs.