August 2016
Volume 16, Issue 12
Open Access
Vision Sciences Society Annual Meeting Abstract  |   September 2016
The hidden spatial dimension of alpha: occipital EEG channels encode contralateral and ipsilateral visual space at distinct phases of the alpha cycle
Author Affiliations
  • Diego Lozano-Soldevilla
    Universite Paul Sabatier, Toulouse, France
  • Rufin VanRullen
    Universite Paul Sabatier, Toulouse, France
Journal of Vision September 2016, Vol.16, 1226. doi:
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      Diego Lozano-Soldevilla, Rufin VanRullen; The hidden spatial dimension of alpha: occipital EEG channels encode contralateral and ipsilateral visual space at distinct phases of the alpha cycle. Journal of Vision 2016;16(12):1226.

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      © ARVO (1962-2015); The Authors (2016-present)

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Visual information is encoded periodically at alpha frequency (~10Hz): for example, a robust alpha component was found by cross-correlating visually presented white-noise (random) luminance fluctuations with corresponding EEG responses over posterior sensors (VanRullen & Macdonald, 2012). This oscillatory impulse response function can be interpreted as a ~10Hz perceptual echo that reverberates the input sequence periodically. Here, we explored the spatial dimension of these perceptual echoes. Two independent random luminance white-noise sequences were simultaneously displayed in two discs on the left and right of fixation. For each EEG channel, two echo functions could thus be computed, by cross-correlating each random luminance sequence (left and right discs) with the simultaneously acquired EEG activity, and subsequently averaging these location-specific visual responses across trials. Multiple posterior sites gave rise to two sizeable echo functions. Interestingly, the echo phase for a given screen location was about 10-12ms earlier on contralateral than ipsilateral electrodes. This travelling wave propagating across the scalp was highly consistent across participants (n=10). Likewise, for each posterior sensor, we found systematic phase differences between locations, such that the echo in response to the ipsilateral disc always lagged by 10–12ms relative to the echo in response to the contralateral disc. In other words, echo functions also behaved like travelling waves across the visual field, sequentially propagating from contra- to ipsi-lateral screen locations. This reveals that occipital cortex, beyond its standard encoding of retinotopic spatial dimensions, also encodes at least one more spatial dimension –but in the temporal domain, i.e. in the phase of alpha reverberations. These results constitute the first direct experimental evidence for Pitts & McCulloch's (1947) scanning hypothesis: "the alpha rhythm performs a temporal 'scanning' of the cortex which thereby gains, at the cost of time, the equivalent of another spatial dimension in its neural manifold".

Meeting abstract presented at VSS 2016


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