The visibility of an object is determined by its contrast against the background. But there is another critical factor in contrast perception that is less well understood: the object's duration. Eye movements, motion of the object in the world, and lighting that changes over time, such as flicker in artificial lighting, all act to vary the amount of time that the object shines photons onto the photoreceptors of the retina. Psychological experiments showed that the apparent contrast, the salience of the object against its background, is highest when an object is presented for ~60-~90 milliseconds and reduced if the object is presented for too short or too long a duration. Dr. Stephen Macknik and his colleagues at the Barrow Neurological Institute will investigate the neural physiological processes underlying these temporal effects by recording electrical activity of single neurons from the visual cortex. This project will provide the empirical data leading to a mechanistic account of how these temporal effects occur. Understanding the neural mechanisms that determine optimal duration of object presentation may allow the discovery of the optimal flickering rate in the artificial lighting. It is possible that temporal dynamics in lighting that is different from what is used in today's flickering lighting can support optimal perception and at the same time save power. That is, if visual lighting is optimized to human perception, less electrical current could be used in lighting devices to achieve the same amount of apparent contrast as in current "unoptimized" artificial lighting dynamics.
To investigate the neural mechanisms associated with optimal stimulus durations, the investigators will record from the ON versus OFF neurons in the primary visual cortex (area V1) that increase and decrease their firing rates in response to light increments and decrements respectively. They will test the hypothesis that the interplay between these two types of neurons will explain the variations in perceived contrast associated with different stimulus durations. To ensure that the findings are applicable to real world scenarios such as lighting in the office and home, the investigators will use not only carefully controlled visual stimuli (Objective 1) but also visual stimuli from natural scenes (Objective 2). Finally, the investigators will determine how higher level cognitive functions, specifically attention, modulates the interactions between both single ON and OFF neurons and populations of ON and OFF neurons (Objective 3). Contrast perception is fundamental to all vision, normal or diseased. Yet the large body of scientific literature has mainly focused on contrast across visual space and little is known about how contrast perception vary with stimulus duration. The current project fills this major knowledge gap by taking the temporal aspect of contrast perception into consideration and suggesting new roles for the well studied ON and OFF neurons.
