Behavioral/Systems/Cognitive Separable Codes for Attention and Luminance Contrast in the Primary Visual Cortex

Thevisual systemencodes the features of visual stimuli aswell as their behavioral relevance. Stimuliwith ahigh luminance contrast evoke more activity in the visual cortex than stimuli with a low contrast. At the same time, attended stimuli evoke more activity than nonat-tended stimuli. There is a debate about how visual features and attention jointly determine neuronal activity in the visual cortex. Some studies suggested that attention increases apparent contrast (Reynolds et al., 2000), others that attention amplifies responses by a constant factor (Williford and Maunsell, 2006), and yet others that attention and contrast have largely additive effects (Buracas and Boynton, 2007; Thiele et al., 2009). The influence of attention on contrast sensitivity differs between neurons, raising the possibility that attention and contrast could be coded conjointly in a population of neurons. Here we investigate this possibility by recording neuronal activity atmultiple sites in the primary visual cortex ofmacaquemonkeys usingmultielectrode recording techniques and support vector machines to decode attended stimuli as well as stimulus contrast.We find thatmany, but not all, V1 neurons are influenced by attention and that the effects of attention and contrast are additive on average. Stimulus contrast can be decoded from neuronal responses not stronglymodulated by attention, whereas the attended stimulus can be decoded as the difference in activity of cells that are influenced by attentionandcells that arenot. The successof the approach suggests that visual attentionandstimulus contrast are representedby largely separable codes