Freeman, E. D., Ipser, A., Palmbaha, A., Paunoiu, D., Brown, P., Lambert, C., Leff, A. & Driver, J. (2013). Sight and sound out of synch: Fragmentation and renormalisation of audiovisual integration and subjective timing. Cerebral Cortex, 49(10), pp. 2875-2887. doi: 10.1016/j.cortex.2013.03.006
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The sight and sound of a person speaking or a ball bouncing may seem simultaneous, but their corresponding neural signals are spread out over time as they arrive at different multisensory brain sites. How subjective timing relates to such neural timing remains a fundamental neuroscientific and philosophical puzzle. A dominant assumption is that temporal coherence is achieved by sensory resynchronisation or recalibration across asynchronous brain events. This assumption is easily confirmed by estimating subjective audiovisual timing for groups of subjects, which is on average similar across different measures and stimuli, and approximately veridical. But few studies have examined normal and pathological individual differences in such measures.
Case PH, with lesions in pons and basal ganglia, hears people speak before seeing their lips move. Temporal order judgements (TOJ) confirmed this: voices had to lag lip-movements (by ~200ms) to seem synchronous to PH. Curiously, voices had to lead lips (also by ~200ms) to maximise the McGurk illusion (a measure of audiovisual speech integration). Thus PH’s timing was still veridical on average across measures. Similar kinds of discrepancies were also found in age-matched control participants. Most surprisingly, normal individual differences in TOJ and McGurk timing correlated negatively: subjects needing an auditory lag for subjective simultaneity needed an auditory lead for maximal McGurk, and vice versa. This generalised to the Stream-Bounce illusion. Such antagonism seems opposed to good sensory resynchronisation, yet average timing across tasks was still near-veridical.
Our findings reveal surprising disunity of subjective timing within and between subjects. To account for this we propose that the neural timing within different mechanisms is perceived relative to the average timing across mechanisms. Such renormalisation fully explains the curious antagonistic relationship between disparate timing estimates in PH and healthy participants, and how they can still perceive the timing of external events correctly, on average.
|Uncontrolled Keywords:||Audiovisual integration; Psychophysics; Individual differences; Illusions; Sensory timing|
|Subjects:||B Philosophy. Psychology. Religion > BF Psychology
R Medicine > RC Internal medicine > RC0321 Neuroscience. Biological psychiatry. Neuropsychiatry
|Divisions:||School of Social Sciences > Department of Psychology|
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