Spatial resolution underlies the set size effect in conjunction search

Most theories of visual search have attributed the presence of a set size effect - decreased performance with increasing set size - to the involvement of attentional mechanisms. According to these theories, conjunction search of two features has been considered a serial process requiring attention. However, this seriality may be due to the fact that at larger set sizes targets are more often presented at farther retinal eccentricities where spatial resolution is worse, thus leading to an overall decrease in performance as set size increases (e.g., Carrasco & Frieder, 1997). The present visual search experiment teased out the spatial resolution vs. attentional factors involved in conjunction search. We employed a conjunction search of tilted, low-frequency targets among distracters that shared either the same orientation or spatial frequency. Stimuli were presented in a square array subtending 3° to 14° eccentricity. All stimulus spatial dimensions - size, orientation, and spatial frequenc - were held constant either at: the retinal level (control condition) or the cortical level (magnified condition). Analysis of accuracy performance for present targets revealed a set size effect in the control but not in the magnified condition. However, when the set size effect in the control condition was measured separately at each eccentricity, a confound emerged: the set size effect changed as a function of target eccentricity. Finally, analysis according to the location at which the target appeared revealed an eccentricity effect - performance gradually worsened with increasing eccentricity - in the control condition. This eccentricity effect was completely eliminated in the magnified condition. These results indicate that in this conjunction search performance was limited by spatial rather than by attentional constraints.