The Effect of Spatial Attention on Contrast Response Functions in Human Visual Cortex

Giedrius T. Buracas and Geoffrey M. Boynton

Previous electrophysiology data suggests that the modulation of neuronal firing by spatial attention depends on stimulus contrast, which
has been described using either a multiplicative gain or a contrast-gain model. Herewemeasured the effect of spatial attention on contrast
responses in humans using functional MRI. To our surprise, we found that the modulation of blood oxygenation level-dependent (BOLD)
responses by spatial attention does not greatly depend on stimulus contrast in visual cortical areas tested [V1, V2, V3, andMT
(middle
temporal area)]. An additive model, rather than a multiplicative or contrast-gain model best describes the attentional modulations in V1.
This inconsistency with previous single-unit electrophysiological data has implications for the population-based neuronal source of the
BOLD signal.