New insights into color processing in the primate visual cortex
30 Apr 2018
By Dr. Supratim Ray, Intermediate Fellow
Indian Institute of Science, Bengaluru
To understand brain function, electrical activity can be recorded using a variety of techniques, such as using microelectrodes in monkeys that provide information at a very local scale (one or a few neurons), to diffuse population measures such as electroencephalography (EEG) in humans that provide information at a much larger scale (millions of neurons). At such scales, brain signals often show oscillations at different frequencies, whose magnitude or frequency may depend on the cognitive state. Signals recorded from the brain often show rhythmic patterns at different frequencies. One such rhythm is called “gamma”, which occurs between 30-80 Hz, and has been linked with high-level cognition such as meditation and attention. Interestingly, gamma can also be induced in the visual cortex (part of the brain that processes visual information) by viewing certain stimuli, even when no other task is involved. One stimulus that induces strong gamma is called a grating, which consists of black-and-white alternating stripes, but whether natural stimuli (which are typically not black-and-white) also produce strong gamma is not well understood.
In our lab, we showed monkeys a variety of natural stimuli while recording gamma rhythm from their primary visual cortex. Surprisingly, we found that whenever the monkeys saw a red colored object, their brain generated an intensely large gamma wave, almost ~10-fold larger than grating-induced gamma. The magnitude of gamma was dependent only on the purity of the color, but largely invariant to overall brightness. This gamma appeared to be correlated with a specific type of color computation done in the brain.
These results provide new insights about the generation of gamma rhythm as well as processing of color along the visual pathway.
Long-wavelength (reddish) hues induce unusually large gamma oscillations in the primate primary visual cortex. Vinay Shirhatti and Supratim Ray (2018). PNAS, April 9, 2018
Banne image credit: Wellcome Images