It has been speculated that this might be due to a higher ecological relevance of the color red in natural environments ( Bartoli et al., 2020), and it was recently related to potentially faster adaptation of the M-cone pathway ( Peter et al., 2019). Interestingly, red stimuli have been reported to induce especially strong gamma-band responses in several of these reports ( Bartoli et al., 2019 Li et al., 2022 Perry et al., 2020 Rols et al., 2001 Shirhatti and Ray, 2018). Large homogeneous patches of color have also been shown to induce strong gamma oscillations, both in the early visual cortex of macaques ( Brunet et al., 2015 Peter et al., 2019 Rols et al., 2001 Shirhatti and Ray, 2018) and humans ( Bartoli et al., 2019 Li et al., 2022 Perry et al., 2020). Likely, this dependence exists because extensive and uniform stimuli synchronize larger populations of neurons into a common gamma rhythm, thereby generating a clearer oscillatory population signal ( Gray et al., 1989 Lowet et al., 2015 Lowet et al., 2017). Especially strong gamma oscillations have been observed for high-contrast black and white stimuli that extend beyond the size of single-neuron receptive fields ( Gieselmann and Thiele, 2008 Jia et al., 2011) and whose features are contiguous over space ( Brunet and Fries, 2019 Hermes et al., 2019 Lowet et al., 2017 Uran et al., 2022 Vinck and Bosman, 2016). In recent years, research on visually induced gamma has increasingly focused on the fundamental question of which visual stimulus features determine gamma amplitude. Gamma oscillations and their coherence between neuronal populations have been related to attentional selection ( Bauer et al., 2006 Bosman et al., 2012 Fries et al., 2001 Magazzini and Singh, 2018), directed information flow ( Bastos et al., 2015 Besserve et al., 2015 Michalareas et al., 2016 van Kerkoerle et al., 2014), stimulus learning ( Brunet et al., 2014 Peter et al., 2021 Stauch et al., 2021), and further cognitive functions (for reviews, see Fries, 2015 Singer, 2018). In early visual cortex of human and non-human primates, visual stimuli often induce rhythmic activity in the gamma band ( Hoogenboom et al., 2006 Jia et al., 2011 Kreiter and Singer, 1992). The strength of human color gamma responses for stimuli on the L-M axis could be well explained by L-M cone contrast and did not show a clear red bias when L-M cone contrast was properly equalized. Notably, blue stimuli with contrast exclusively on the S-cone axis induced very weak gamma responses, as well as smaller event-related fields and poorer change-detection performance. We found gamma oscillations in early visual cortex which, contrary to previous reports, did not differ between red and green stimuli of equal L-M cone contrast. With these stimuli, we recorded magnetoencephalography in 30 human participants. We presented stimuli that had equal luminance and cone contrast levels in a color coordinate system based on responses of the lateral geniculate nucleus, the main input source for area V1. Therefore, stronger responses to red might be due to differences in V1 input strength. However, precortical color processing and the resultant strength of input to V1 have often not been fully controlled for. Compared to other hues, particularly strong gamma oscillations have been reported for red stimuli. Strong gamma-band oscillations in primate early visual cortex can be induced by homogeneous color surfaces (Peter et al., 2019 Shirhatti and Ray, 2018).

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