The Relationship between Search Efficiency and Crowding

The role of target salience in crowding has remained controversial largely because salience usually escapes objective measurement. Here we address this problem using search efficiency as a measure of target salience. In separate experiments observers determined whether parafoveal arrays of vertical Gabor patterns contained targets having a unique colour, a unique direction of motion, and a unique temporal frequency. We analysed search efficiency in the conventional manner using reaction-time gradients (in seconds per item). We also considered accuracy gradients (in percent-correct per item). Crowding is typically quantified by comparing the acuity for a target within an array to the acuity for a target presented alone. We measured orientation acuity for determining whether a slightly tilted target was clockwise or anticlockwise of vertical. Targets with a unique colour or direction of motion were found to pop out, ie (with one exception) reaction-time and accuracy gradients were insignificantly different from zero. Acuity for these targets was significantly greater than acuity for targets whose neighbours had the same colour and direction of motion. Manipulation of temporal frequency produced a wide range of search efficiencies. For three of four observers we found a linear relationship between acuity and the accuracy gradient, shallow gradients being associated with high acuity. In general, we find that crowding is weakened when observers can find a parafoveally presented target quickly and accurately.