Differential attentional modulation of cortical responses to S-cone and luminance stimuli.

Neural signals driven by short-wave-sensitive (S) cones are, to a large degree, anatomically and functionally separate from the achromatic luminance pathway until at least one synapse into V1. Attentional mechanisms that act at an anatomically early stage in V1 may, therefore, affect S-cone and luminance signals differently. Here, we used a steady-state visually evoked potential (SSVEP) paradigm combined with electrical source imaging to study the effects of contrast and attention on neural responses to both chromatic S-cone isolating and achromatic stimuli in five human visual areas including V1. The responses to these gratings were affected very differently by changes in contrast and attention. Increasing cone contrast increased the response amplitude for both types of stimulus. For the S-cone-defined stimuli, we also observed a systematic decrease in the response phase of the first harmonic with increasing stimulus contrast, but there was no corresponding change in phase for the first harmonic of the luminance probes. Attending to the contrast of the grating increased the amplitude and phase of luminance-driven responses but had no effect on S-cone-driven responses. We conclude that while attentional modulation can be observed in achromatic pathways as early as V1, attention may not affect SSVEP signals generated by S-cone stimuli.