Differential effects of exogenous and endogenous attention on 2nd-order texture contrast sensitivity

The visual system can use a rich variety of contours to segment visual scenes into distinct perceptually coherent regions. However, successfully segmenting an image is a computationally expensive process. Previously we have shown that exogenous attention—the more automatic, stimulus-driven component of spatial attention—helps extract contours by enhancing contrast sensitivity for 2nd-order, texture-defined patterns at the attended location, while reducing sensitivity at unattended locations, relative to a neutral condition. Interestingly, the effects of exogenous attention depended on the 2nd-order spatial frequency of the stimulus. At parafoveal locations, attention enhanced 2nd-order contrast sensitivity to relatively high, but not to low 2nd-order spatial frequencies. In the present study we investigated whether endogenous attention—the more voluntary, conceptually-driven component of spatial attention—affects 2nd-order contrast sensitivity, and if so whether its effects are similar to those of exogenous attention. To that end we compared the effects of exogenous and endogenous attention on the sensitivity to 2nd-order, orientation-defined, texture patterns of either high or low 2nd-order spatial frequencies. The results show that, like exogenous attention, endogenous attention enhances 2nd-order contrast sensitivity at the attended location and reduces it at unattended locations. However, whereas the effects of exogenous attention are a function of the 2nd-order spatial frequency content, endogenous attention affected 2nd-order contrast sensitivity independent of the 2nd-order spatial frequency content. This finding supports the notion that both exogenous and endogenous attention can affect 2nd-order contrast sensitivity, but that endogenous attention is more flexible, benefitting performance under different conditions.