Frieder, KS & Carrasco, M (2002,
May). Spatial resolution underlies the set size effect in conjunction
search. Vision ScienceS Society, Sarasota, FL. 541-B4.104.
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 frequency - 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.