The Neural Mechanisms Underlying Flicker Fusion

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Every day, people worldwide operate under flickering artificial lighting conditions, or observe flickering computer or TV displays or cinematic pictures. Some people fuse these brief, still images into steady movement better than others. Given the widespread need for such fusion, advances leading to even modest gains of function will have a very significant impact. Moreover, some forms of flicker can lead to pathological activity states in the brain, such as photogenic epilepsy. The reasons are unknown, because the fundamental physiology underlying flicker perception is unknown. Although flickering stimuli are visible for only a fraction of the viewing time, they appear as continuous and stable because we perceptually integrate successive flashes in a process called flicker fusion. However, physiological evidence shows that flicker rates above the highest perceived rate of flicker nevertheless generates cortical and subcortical visual responses. Thus flicker fusion caused by integration at the retina cannot be the sole explanation of the phenomenon, and there must be flicker fusion mechanisms later in the visual system of the brain. Moreover, the process underlying flicker fusion may not be temporal integration whatsoever: the mechanistic processes behind flicker fusion are unknown. This gap in knowledge has made it difficult to optimize perceptual stability under flickering conditions. With support from NSF, Dr. Stephen Macknik of St. Joseph's Hospital and Medical Center hopes to discover the mechanistic processes that lead to flicker fusion. The central hypothesis of the study is that flicker fusion is caused by the suppressive interaction of transient neural responses generated by the flickering stimuli themselves, within a lateral inhibition network. By understanding the temporal dynamics of lateral inhibition, we will determine the parameters that maximize flicker fusion and perceptual stability under flickering conditions. The importance of central mechanisms will be assessed with a newly discovered illusion called "Temporal Fusion." In this, two flashes at an interval of 100 ms (typically easily seen as two flashes) appear to be one long flash if the correct masks are applied to the peripheral visual field. This highlights the importance both of lateral inhibition and of the need for explanations beyond flicker fusion.The results of the study potentially affect every person working under fluorescent and/or incandescent artificial lighting, or working with a TV, a monitor, or any cinematic viewing device. Thus the project impacts the entire modern workforce worldwide. These discoveries will serve to drive advances to be disseminated to monitor and lighting manufacturers, as well as to workforce health monitoring organizations such as OSHA, so as to maximize viewing comfort, and thus efficiency in the modern workforce.