Simultaneity learning in vision, audition, tactile sense and their cross-modal combinations.

Latencies of sensory neurons vary depending on stimulus variables such as intensity, contrast, distance and adaptation. Therefore, different parts of an object and simultaneous environmental events could often elicit non-simultaneous neural representations. However, despite the neural discrepancies of timing, our actions and object perceptions are usually veridical. Recent results suggest that this temporal veridicality is assisted by the so-called simultaneity constancy which actively compensates for neural timing asynchronies. We studied whether a corresponding compensation by simultaneity constancy could be learned in natural interaction with the environment without explicit feedback. Brief stimuli, whose objective simultaneity/non-simultaneity was judged, consisted of flashes, clicks or touches, and their cross-modal combinations. The stimuli were presented as two concurrent trains. Twenty-eight adult participants practised unimodal (visual, auditory and tactile) and cross-modal (audiovisual, audiotactile and visuotactile) simultaneity judgement tasks in eight sessions, two sessions per week. Effects of practice were tested 7 months later. All tasks indicated improved judgements of simultaneity that were also long-lasting. This simultaneity learning did not affect relative temporal resolution (Weber fraction). Transfer of learning between practised tasks was minimal, which suggests that simultaneity learning mechanisms are not centralised but modally specific. Our results suggest that natural perceptual learning can generate simultaneity-constancy-like phenomena in a well-differentiated and long-lasting manner and concomitantly in several sensory systems. Hebbian learning can explain how experience with environmental simultaneity and non-simultaneity can develop the veridicality of perceived synchrony.