Automatic gain control contrast mechanisms are modulated by attention in humans: evidence from visual evoked potentials

This study investigated the effect of attention on the contrast response cu rves of steady-state visual evoked potentials (VEPs) to counter-phased sinu soidal gratings. The I cyc/deg gratings were modulated either in luminance or chromaticity (equiluminant red-green). The luminance grating counter-pha sed at 9 Hz (to favour activation of the magno-cellular system), and the ch romatic grating at 2.5 Hz (to favour activation of the parvo-cellular syste m). Attention was directed towards the gratings (displayed in the left visu al field) by requiring subjects to detect and respond to randomly occurring changes in contrast. In the control condition, attention towards the grati ng was minimised by requiring subjects to detect a target letter amongst di stracters briefly flashed in the contra-lateral visual field. Attention inc reased VEP amplitudes for both luminance and chromatic stimuli, more so at high than at low contrasts, increasing the slope of the contrast amplitude curves (over the non-saturating range of contrasts). The estimates of contr ast threshold from extrapolation of amplitudes were unaffected by attention . Attention also changed the VEP phases, but only for luminance gratings. w here it acted to reduce the magnitude of phase advance with contrast. Atten tion had no effect on the average phases for chromatic gratings. The result s are consistent with the notion that attention acts on cortical gain contr ol mechanisms, which are known to be different for the magno- and parvo-cel lular systems. (C) 2001 Elsevier Science Ltd. All rights reserved.