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.
Natural images were shown for 800ms each to monkeys on a screen. Leftmost: 6 example images (tomatoes in different hues or colours) shown to a monkey. Signals recorded from one of the recording sites during a presentation of these images are shown in the respective colour (to the right of the eye and brain depictions), for the period 250-750ms after onset of the image. The signals showed strong oscillations in the gamma range (~30-80 Hz) for the red coloured tomato image (3rd from the top), stronger than for tomatoes of other colours. At the extreme right are the respective time-frequency power spectra showing the average change in signal power at different frequencies across multiple presentations of these images. The power increased maximally in the gamma range when the red coloured tomato was shown (the case highlighted by the encapsulating black box). This picture illustrates a key finding of this study that strong gamma oscillations are observed in signals recorded from a visual area of the brain while viewing coloured visual stimuli and the magnitude of gamma depends on the colour.
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; https://doi.org/10.1073/pnas.1717334115
Lab website URL: http://www.cns.iisc.ac.in/~sray/