When the sun prickles your nose: an EEG study identifying neural bases of photic sneezing

Abstract

Background: Exposure to bright light such as sunlight elicits a sneeze or prickling sensation in about one of every four individuals. This study presents the first scientific examination of this phenomenon, called 'the photic sneeze reflex'.

Methodology and principal findings: In the present experiment, 'photic sneezers' and controls were exposed to a standard checkerboard stimulus (block 1) and bright flashing lights (block 2) while their EEG (electro-encephalogram) was recorded. Remarkably, we found a generally enhanced excitability of the visual cortex (mainly in the cuneus) to visual stimuli in 'photic sneezers' compared with control subjects. In addition, a stronger prickling sensation in the nose of photic sneezers was found to be associated with activation in the insula and stronger activation in the secondary somatosensory cortex.

Conclusion: We propose that the photic sneeze phenomenon might be the consequence of higher sensitivity to visual stimuli in the visual cortex and of co-activation of somatosensory areas. The 'photic sneeze reflex' is therefore not a classical reflex that occurs only at a brainstem or spinal cord level but, in stark contrast to many theories, involves also specific cortical areas.

Figure 1. Comparing photic sneezers with control subjects.

(A) Visual event-related potential (global field power) 0–400 ms after stimulus onset of block1 (checkerboard-paradigm) for both groups (photic sneezers vs. controls). Two time segments at 56–68 ms and 200–212 ms after stimulus presentation survived the FDR-correction (p<0.05). These time segments are marked by transparent rectangles. (B) sLORETA-analysis of the FDR-corrected time segments revealed significantly increased activity of the photic sneezers compared with control subjects. Neural generators for the time segment 56–68 ms are located in the primary visual cortex. The increased activation for the time segment 200–212 ms was found in the secondary visual cortex. Cortical activation differences estimated with sLORETA are displayed in red. X, Y, Z MNI-coordinates of the local maximum of the activation difference.

Figure 2. Comparing cortical activations during strong vs. weak prickle sensations.

(A) Visual event-related potential (global field power) 0–400 ms after stimulus onset of block2 (flash-presentation) for both conditions (subjectively strong vs. weak prickle sensation) within the group of photic sneezers. FDR corrected significant differences in global field power were found at 204–238 ms after stimulus presentation. (B) sLORETA-analysis for this time segment revealed significantly (p<0.05) enhanced activity in the insula and secondary somatosensory cortex in the “strong prickle” condition. Cortical activation differences estimated with sLORETA are displayed in red. X, Y, Z MNI-coordinates of the local maximum of the activation difference.