Abstract
The human brain processes different aspects of the surrounding environment through multiple sensory modalities (e.g., vision, audition, touch), and each modality can be subdivided into multiple attribute-specific channels (e.g., color, motion, form). When the brain re-binds sensory signals across different channels, temporal coincidence, along with spatial coincidence, provides a critical binding clue. It however remains unknown whether neural mechanisms for binding synchronous attributes are specific to each attribute combination, or universal and central. In a series of human psychophysical experiments, we examined how combinations of visual, auditory, and tactile attributes affect the temporal binding of attribute-value combinations. Observers discriminated the phase relationship of the two repetitive sequences. Each sequence was an alternation of two attribute values (e.g., red/green, high/low pitches, left/right finger vibrations). The alternation was always synchronized between the two sequences, but the paring of attribute values was changed between the two phase conditions (in-phase or 180-deg out-of-phase). We measured the upper temporal-frequency limit to perform this binding discrimination task. The results indicated that the temporal limits of cross-attribute binding were relatively low in comparison with those of within-attribute binding. Furthermore, they were surprisingly similar (2-3 Hz) for any combination of visual, auditory and tactile attributes. The cross-attribute binding limits remained low and invariant even when we increased the stimulus intensity or adjusted the relative timing of the two sequences. They are unlikely to reflect the limits for judging synchrony, since the temporal limits of a comparable cross-attribute synchrony task (Fujisaki & Nishida, 2005; 2009) were higher and more variable with the modality combination (4-9 Hz). These findings suggest that cross-attribute temporal binding is mediated by a slow central process universal to any attribute combinations. We conjecture that ‘what’ and ‘when’ properties of a single event are once separately processed, and then combined in this slow central universal process.