![]() Indeed, anatomical studies have documented projections in the top-down direction that are as numerous as bottom-up projections. These neurophysiological consequences of top-down control must be mediated by corresponding anatomical projections. Correspondingly, neurons in higher visual areas show enhanced firing when processing attended stimuli. For example, when top-down influences pre-allocate attention to specific spatial locations, stimulus processing is more accurate and/or faster. Many cognitive effects in vision can only be explained by invoking the concept of top-down influences ( Gilbert and Sigman, 2007). These results show that for top-down processes such as spatial attention, elevated top-down beta-band influences directly enhance feedforward stimulus-induced gamma-band processing, leading to enhancement of the selected stimulus. We evaluate this hypothesis determining that beta-band top-down influences from parietal area 7a to visual area V1 are correlated with bottom-up gamma frequency influences from V1 to area V4, in a spatially specific manner, and that this correlation is maximal when top-down activity precedes bottom-up activity. This arrangement inspires an attractive hypothesis, which posits that top-down beta-band influences directly modulate bottom-up gamma band influences via cross-frequency interaction. SIGNIFICANCE STATEMENT Contemporary research indicates that the alpha-beta frequency band underlies top-down control, whereas the gamma-band mediates bottom-up stimulus processing. We propose that this cross-frequency interaction mechanistically subserves the attentional control of stimulus selection. This enhancement was spatially specific and largest when beta-band activity preceded gamma-band activity by ∼0.1 s, suggesting a causal effect of top-down processes on bottom-up processes. Top-down 7a-to-V1 beta-band influences enhanced visually driven V1-to-V4 gamma-band influences. ![]() To test this possibility, we investigated Granger-causal influences among awake macaque primary visual area V1, higher visual area V4, and parietal control area 7a during attentional task performance. These processes may implement top-down control of stimulus processing if top-down and bottom-up mediating rhythms are coupled via cross-frequency interaction. Hence, gamma value of 2.2 has become the golden standard of digital display for a proper calibration.Several recent studies have demonstrated that the bottom-up signaling of a visual stimulus is subserved by interareal gamma-band synchronization, whereas top-down influences are mediated by alpha-beta band synchronization. The exponent of 0.43 is approximately 2.33, quite close to gamma 2.2. This phenomenon was also found in Ebner and Fairchild’s study in 1998, where they found using an exponent of 0.43 to convert linear intensity into lightness for neutrals can provide an optimal perceptual encoding of grays. Looking at the overall grays in visual encoding row, the perceived differences between each gray patch are almost identical. ![]() And from 0.9 to 1.0 in linear intensity, the difference is subtle, whereas it is more obvious in visual encoding. Note that between 0.0 and 0.1, there is a big visual gap in linear intensity, with a smaller visual gap between 0.0 and 0.1 in visual encoding. The top row, visual encoding, represents the increased intensity from black to white in a low power fashion. The bottom row represents the intensity increasing from black to white in a linear fashion. Impact on luminance impacts the visual system, but the effects are not identical. There is a power law relationship between the output luminance and the input voltage or digital value. You may have heard that gamma 2.2 is standard and wondered why.
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