What changes inside the brain when one sees a colourful flower as opposed to a grayscale version of it? How do the brain signals change when one sees a green jackfruit versus a red tomato? Does the redness of the tomato matter? We studied such questions by recording signals from the primary visual cortex (an area of brain involved in visual processing) of monkeys while they were shown various natural images. To our surprise, we found that there were strong oscillations in the recorded signals at frequencies in the range 30-80 Hz whenever reddish images were shown. Oscillations in this range are traditionally known as gamma oscillations and have been previously linked to functions such as attention, working memory and meditation. To investigate this further, we presented uniform colour stimuli of different hues and found that gamma was indeed sensitive to the hue of the colour, with reddish hues generating the strongest gamma. In the visual cortex, gamma has been known to be induced strongly by gratings (alternating black and white stripes), but the gamma generated by colour stimuli was even stronger, almost 10-fold in some cases. The magnitude of gamma depended on the purity of the colour but not so much on the overall brightness. Importantly, it was related to a particular mechanism by which colour signals received by the retinal cone receptors are processed in the brain. These findings provide new insights about the generation of gamma oscillations and processing of colour in the brain

Faculty: Supratim Ray, CNS

Publication: Vinay Shirhatti and Supratim Ray (2018). Long-wavelength (reddish) hues induce unusually large gamma oscillations in the primate primary visual cortex. PNAS, April 9, 2018. 201717334