Grey matter volume in early human visual cortex predicts proneness to the sound-induced flash illusion

Lauri Jalkanen,Benjamin De Haas,Geraint Rees,Ryota Kanai

doi:10.1098/rspb.2012.2132

Abstract

Visual perception can be modulated by sounds. A drastic example of this is the sound-induced flash illusion: when a single flash is accompanied by two bleeps, it is sometimes perceived in an illusory fashion as two consecutive flashes. However, there are strong individual differences in proneness to this illusion. Some participants experience the illusion on almost every trial, whereas others almost never do. We investigated whether such individual differences in proneness to the sound-induced flash illusion were reflected in structural differences in brain regions whose activity is modulated by the illusion. We found that individual differences in proneness to the illusion were strongly and significantly correlated with local grey matter volume in early retinotopic visual cortex. Participants with smaller early visual cortices were more prone to the illusion. We propose that strength of auditory influences on visual perception is determined by individual differences in recurrent connections, cross-modal attention and/or optimal weighting of sensory channels.

Highlights

When a single flash is accompanied by a rapid series of two or more bleeps, a perceptual ‘fission’ of the flash sometimes occurs and it is incorrectly perceived as multiple flashes [1]
No significant correlation between local grey matter (GM) volume and fission score’ (FiS) was found for the primary auditory cortex, posterior superior temporal sulcus (STS) and superior colliculus (SC) region of interest (ROI)
We found a strong correlation between proneness to the illusion and local GM volume in early visual cortex

Summary

INTRODUCTION

When a single flash is accompanied by a rapid series of two or more bleeps, a perceptual ‘fission’ of the flash sometimes occurs and it is incorrectly perceived as multiple flashes [1]. We tested whether proneness to the sound-induced flash illusion was correlated with differences in regional grey matter (GM) volume as measured using voxel-based morphometry (VBM; [13]). (c) Procedure Participants sat on a chair in front of the monitor at 65 cm distance They were asked to indicate whether they saw one or two flashes after each trial pressing either ‘1’ or ‘2’ on a numerical keypad with the index and middle finger of their right hand (in a time-window lasting 1800 ms after the stimulus presentation). (f) Voxel-based morphometry: statistical analysis To test for correlations between GM volume and illusion strength, multiple regression analyses were performed on the smoothed GM images.

FiS above

Findings

DISCUSSION