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

Abstract

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 second-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 second-order spatial frequency of the stimulus. At parafoveal locations, attention enhanced second-order contrast sensitivity to relatively high, but not to low second-order spatial frequencies. In the present study we investigated whether endogenous attention-the more voluntary, conceptually-driven component of spatial attention--affects second-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 second-order, orientation-defined, texture patterns of either high or low second-order spatial frequencies. The results show that, like exogenous attention, endogenous attention enhances second-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 second-order spatial frequency content, endogenous attention affected second-order contrast sensitivity independent of the second-order spatial frequency content. This finding supports the notion that both exogenous and endogenous attention can affect second-order contrast sensitivity, but that endogenous attention is more flexible, benefitting performance under different conditions.

Figure 1

Natural scene containing boundaries signaled by differences in luminance (first-order information) and/or by differences in textural properties (second-order information). Picture obtained from http://www.pdphoto.org.

Figure 2.

(A) Stimulus construction. Texture patterns were computed by modulating two orthogonal luminance gratings (the carriers, with ± 45° orientations) with a third vertical or horizontal modulator grating of lower spatial frequency. The contrast-modulated carrier patterns were summed, and the result was multiplied by a circular window with raised-cosine edges. Mean luminance was constant across the stimulus. (B) Stimulus conditions. Two spatial frequencies were used for the second-order modulator (‘low': 0.5 cycle/deg; ‘high': 1 cycle/deg), while the first-order carrier was of higher spatial frequency (4 cycle/deg).

Figure 3.

Trial sequence. The trial sequence was identical for the exogenous and endogenous attention conditions except for the location and timing of the peripheral and central cues, which differed to maximize the effects of each type of attention.

Figure 4

Cue validity for exogenous and endogenous attention.

Figure 5.

(A,B) Effects of exogenous attention on performance (d′) as a function of second-order modulator contrast for the low (A) and high (B) second-order spatial frequency conditions (Figure 2). Each panel shows psychometric functions and parameter estimates (c₅₀: second-order contrast yielding half-maximum performance; d′_max: asymptotic performance) for each cueing condition (valid, neutral and invalid). Each data point represents the mean across observers (N = 8). Error bars correspond to ±1 SEM for data points and 68%-confidence intervals obtained by bootstrapping for parameter estimates. All r² > .98. (C, D) Effects of exogenous attention on individual observers' parameter estimates in the low (C) and high (D) second-order spatial frequency conditions (c₅₀: open circles; d′_max: open squares). Each plot displays individual observers' parameter estimates in the valid (red symbols) and invalid (blue symbols) cue conditions normalized by the corresponding values in the neutral-cue condition. Filled symbols indicate mean across observers.

Figure 6

Effects of endogenous attention on performance (d′) as a function of second-order modulator contrast for the low (A) and high (B) second-order spatial frequency conditions (N = 7). All r² > .98. All plotting conventions as in Figure 5.