- (Strasburder, 2005) ⇒ Hans Strasburger. (2005). “Unfocussed Spatial Attention Underlies the Crowding Effect in Indirect Form Vision.” In: Journal of Vision, 5(11):8. doi:10.1167/5.11.8
We studied mechanisms underlying the crowding effect in indirect form vision by measuring recognition contrast sensitivity of a character with flankers to the left and right. Attentional and featural contributions to the effect can be separated by a new paradigm that distinguishes pattern location errors from pattern recognition errors and further by manipulating the focusing of spatial attention through a positional cue, appearing 150 ms before the target. Measurements were on the horizontal meridian, at 1, 2, and 4 deg eccentricity, and a range of flankers' distances were used. Our results show that in normal indirect viewing, the impairment of character recognition by crowding is — in particular at intermediate flanker distances — caused to a large part by spatially imprecise focusing of attention. In contrast, the enhancement of performance by a transient positional cue seems mediated through a separate attentional mechanism such that attentional locus and focus are controlled independently. Our results furthermore lend psychophysical support to a separate coding of the what and where in pattern recognition.
Theoretical accounts: focusing attention and the field of feature integration
In a comprehensive analysis, Pelli et al. (2004) have characterized crowding as a process of impaired feature integration occurring in the visual periphery, in contradistinction to (lateral) masking as occurring from impaired feature detection anywhere in the visual field. We have ourselves characterized the visual periphery — where the interesting cases of crowding occur (Strasburger et al., 1991) — as differing from the fovea by the architecture of feature integration (Strasburger & Rentschler, 1996). That argument was based on the differing dependence-on-eccentricity functions of contrast sensitivity for grating detection and for character recognition (Strasburger, 2003b; Strasburger, Gothe, & Lutz, 2000; Strasburger, Rentschler, & Harvey, 1994) and by showing that the difference between the two cannot be explained by a spatial scaling concept (M scaling, cortical-magnification scaling). We concluded that there must be architectural differences across the visual field — in particular between the fovea and the rest of the field — that concern feature integration not feature detection. In a hierarchy of task complexity ranging from
- (1) pattern detection (present/nonpresent),
- (2) coarse grating discrimination (horizontal/vertical),
- (3) fine grating discrimination (orientation threshold), and
- (4) character recognition or identification,
Tasks 1 and 2 can be treated as more or less equivalent and as different from Task 4. The distinctive characteristic of the highest-level perceptual tasks–recognition or identification–as compared to the lower level (discrimination) we there suggested to be the dimensionality of the decision space, i.e. the requirement for the observer to chose his or her response from a comparatively large number of alternatives.
- The term “discrimination task” is sometimes used in a different meaning, implying the judgement of a quantity being larger or smaller than another (the corresponding psychometric function then goes from −1 to 1). This is not implied here, the intended meaning being that the observer can discriminate between two broadly different stimuli and thereby identify each. The term “identification task” is sometimes used for that case but is avoided here to reserve the concept of identification for those tasks where discrimination between a few cases will not solve the identification.
|2005 UnfocussedSpatialAttention||Hans Strasburger||Unfocussed Spatial Attention Underlies the Crowding Effect in Indirect Form Vision||Journal of Vision||http://jwww.journalofvision.org/content/5/11/8.full||10.1167/5.11.8||2005|