Although neuroimaging studies have shown that exogenous and endogenous attention are dissociable, only a few behavioural studies have explored their differential effects on visual sensitivity, and none have directly focused on visual appearance. Here, we show that exogenous and endogenous attention produces contrasting effects on apparent size. Participants performed a spatial pre-cueing comparative judgement task that had been frequently used to test the attentional effects on visual perception. The results showed that a smaller stimulus within the focus of exogenous attention was perceived to be equal in size as a larger unattended stimulus, whereas a larger stimulus within the focus of endogenous attention was perceived to be equal in size as a smaller unattended stimulus. In other words, exogenous attention increased the perceived size while endogenous attention decreased the perceived size. The contrasting effects may be attributed to the mechanism that exogenous attention favours parvocellular processing for which more neurons with smaller receptive fields (RFs) are activated for a given size, whereas endogenous attention favours magnocellular processing for which fewer neurons with larger RFs are activated. This finding shows that exogenous and endogenous attention acts differentially on size perception, and provides supportive evidence for the distinct mechanisms underlying the two types of attentional processing.

A series of CeO2-SiO2composite nanoparticles with different cerium and silica (Ce/Si) mole ratios were synthesized via a coprecipitation method using cerium nitrate, tetraethylorthosilicate and ammonia as raw materials. X-ray diffraction (XRD), fourier transform infrared spectrology (FT-IR), thermogravimetry-differential scanning calorimetry (TG/DSC), transmission electron microscopy (TEM) and energy dispersive analysis of X-ray (EDAX) were used to characterize the CeO2-SiO2nanoparticles. With the increase of CeO2content, the crystal of CeO2grew up gradually and the average crystallite size of the CeO2decreased. There are Ce-O-Si bonds in the CeO2-SiO2composite nanoparticles. The particle size of the CeO2-SiO2composite nanoparticles with a Ce/Si mole ratio of 1 is about 20–30 nm. The dispersion stability of the CeO2-SiO2composite nanoparticles with a Ce/Si mole ratio of 1 was studied. It was found that cationic and nonionic surfactant could obviously affect the dispersion stability of suspension, but not the anion surfactant. A suspension with CeO2-SiO2composite nanoparticles at 0.1 wt% was added in 0.1 g/L cationic surfactant CTAB, and its pH value was adjusted within the range of 10-10.5. In this condition, the stable suspension without sedimentation time was successfully obtained.