The influence of attention on visual asymmetries in the foveola

We have previously shown that fine visual discrimination varies over different directions in the foveola; in particular, it is better along the horizontal compared to the vertical meridian. Here we examine whether fine-tuning attention in the foveola could compensate for these asymmetries by enhancing perception to a greater degree at those locations characterized by lower performance. Participants (n= 5) performed a 2AFC orientation discrimination task while maintaining fixation on a central marker. Stimuli, small bars (7x2 arcminutes) titled ±45 deg, could be presented at four cardinal locations, 20 arcminutes from the preferred locus of fixation. Stimuli’s contrast was changed adaptively using a PEST procedure. In half of the trials subjects were centrally cued to deploy their attention to one of the four locations where the target would briefly appear (100% cue validity). In the rest of the trials a neutral cue pointed in all four directions. The same task was replicated at 4.5 degrees eccentricity (n=3) with stimuli size adjusted to account for cortical magnification. Consistent with our previous work, when subjects were not cued to attend to a specific location, contrast thresholds along the horizontal meridian were more than double than along the vertical meridian in the foveola (p<0.01). However, when attention was engaged, contrast thresholds across the vertical (25%±9% contrast) and horizontal (21%±7% contrast) meridian did not differ statistically. Interestingly, a different trend was reported in the parafovea, where the horizontal-vertical meridian asymmetry was still present even when attention was engaged. These findings show that fine tuning of endogenous attention in the foveola can overcome visual asymmetries at this scale by enhancing visual discrimination to a greater extent in regions with lower baseline sensitivity.