Continued stimulation with an auditory and visual stream of input, in which the inter-modal asynchrony is kept constant, is known to alter perceptual functions in humans, like the perceived temporal relationship between events in the two sensory modalities (e.g. Fujisaki et al, 2004, Nature Neuroscience; Vroomen et al, 2004, Cognitive Brain Research).
In order to explore potential neuronal mechanisms that mediate such phenomena, we have subjected rodents (Mongolian gerbils, Meriones unguiculatus) to a similar protocol and have analyzed coherence between oscillatory components of local field potential (LFP) activity measured in auditory and visual cortex. Oscillatory coherence in brain dynamics can be an expression of its self-sustained behavior and is also considered to reflect integration of sensory input. For example, coherence is discussed as a substrate for binding near-synchronous, but distinct, sensory features into the perception of whole objects. On longer time scales, neither the perceptual role nor the neuronal mechanisms of integration are well understood.
Here, we study coherence, between auditory and visual local field potentials, before (only tone stimulus), during and after (only tone stimulus) exposure to streams of paired tone-flash stimuli (50 ms stimulus durations, fixed 200 ms stimulus onset asynchrony, randomized intervals between the audiovisual stimuli ranging from 1 to 2 s, stimulation for 10 daily sessions with approx. 750 repetitions). Figure 1 shows the three types (in-phase, anti-phase and stationary-shift phase) of coherence considered in this study. We analyzed interaction dynamics of auditory and visual cortical local field potentials in the broad band from 7 to 200 Hz. We introduced a Phase Coherence Metric (PCM) to calculate the phase coherence distance (derivative of phase difference) between auditory and visual cortex activities. Effective modulation of neuronal activities and evolution of phase-coherent states of LFP signals in both areas involving multiple band frequencies within 50 ms of the tone stimulus, have been observed in a continuous enhancement over multiple recording sessions.
Our data indicate, first, that continued presentation of sequentially paired audiovisual stimuli created a dynamical process of phase coherence as a potential physiological correlate of the altered cortical processing during adaptation. Secondly, we found that after the audiovisual adaptation sessions responses of visual cortex to auditory stimuli were increased relative to pre-adaptation sessions. We observed a significant enhancement of Global PCM averaged across all frequencies bands (Fig. 2, PCM+95% confidence intervals). Figure 3 shows a representative example (for the frequency band 82-93 Hz) of increased phase coherence between auditory (continuous) and visual (dashed) LFP after audiovisual adaptation (b) as compared to before adaptation (a). Top panels depict the LFPs in the respective filter band, bottom panels the corresponding phases.