Fine-tuning exogenous attention at the foveal scale: effects on different spatial frequencies

We previously showed that exogenous attention can be selectively shifted within the 1-deg foveola, enhancing visual discrimination at the attended location only arcminutes away from distractors. In this study, we investigated how fine-tuning exogenous attention in the foveola affects contrast thresholds for different spatial frequencies. While observers (N = 5) maintained fixation on a central marker, an exogenous cue (100% validity) was presented at 0.75 deg eccentricity to the left/right of fixation. Shortly after the cue disappeared, two small gabor patches (0.5 deg visible area), tilted +/- 45 degrees, were shown (50 ms) on the left/right side at 0.5 deg eccentricity. The spatial frequency of the Gabors was either 4 or 12 CPD. Observers reported the orientation of the gabor that was presented at the location indicated by a response cue. Contrast thresholds for each spatial frequency were assessed in the presence (valid trials) and absence (neutral trials) of the attentional cue. Tight fixation was ensured by monitoring gaze position with a high-precision eye-tracker. In the neutral condition, contrast sensitivity peaked at 4CPD when tested over a wider range of spatial frequencies. In the attended condition, we observed a decrease in contrast thresholds with both 4 CPD (8%+/-2% v.s. 6%+/-2% contrast, P = 0.0082) and 12 CPD (24%+/-5% v.s. 20%+/-4% contrast, P = 0.0086) stimuli. The cueing benefit, the ratio between the contrast threshold in neutral and valid conditions, for high and low spatial frequencies was comparable. Further testing at intermediate spatial frequencies will determine whether these results are the outcome of either a broad attentional enhancement across spatial frequencies or a narrowly tuned attention enhancement peaking at frequencies slightly higher than the frequency at which contrast sensitivity peaks.