Object discrimination using peripheral vision is signigicantly poorer than for foveal vision. This is especially the case when objects are surrounded by irrelevant visual structure (“clutter”), which can have a profound effect on recognition. Such
crowding limits peripheral vision. Crowding is not attributable to known decreases in acuity and occurs when a flanking element falls within an area surrounding the target known as its interference zone, hindering discrimination of the target's features. This zone is primarily defined by the critical distance between the target and flanker element(s), which is thought to be roughly half the eccentricity of the target (Bouma,
1970). It is important to note that while crowding affects discrimination of element features, detection of the element remains unaffected.
Crowding has been demonstrated with many features such as color, shape, contrast polarity (Kooi, Toet, Tripathy, & Levi,
1994), spatial frequency (Chung, Levi, & Legge,
2001), position (Greenwood, Bex, & Dakin,
2009) and orientation (Hariharan, Levi, & Klein,
2005). While most early crowding research emphasized the role of local interactions (between adjacent elements) it is now clear that the global arrangement of features is important (Livne & Sagi,
2007; Saarela, Sayim, Westheimer, & Herzog,
2009). Crowding is particularly powerful when distractor and target co-align to form an extended visual contour (Livne & Sagi,
2007). Conversely, where flankers are arranged to promote perceptual grouping with one another a release from crowding is seen, as the target “jumps out.”
As to the mechanism of crowding, it is now widely acknowledged that visual crowding involves inappropriate pooling of constituent element features (Dakin, Cass, Greenwood, & Bex,
2010; Greenwood et al.,
2009; Parkes, Lund, Angelucci, Solomon, & Morgan,
2001). For example, discrimination of the orientation of a near-vertical central target is impaired when the element is surrounded by vertical distractor elements because the observer bases their judgment on what is effectively the average orientation of the target and distractors (Parkes et al.,
2001). Thus in crowded displays, target and flanker orientation signals are not lost completely but are pooled together. Dakin et al. (
2010) presented evidence that this averaging process has a probabilistic component: Stimulus separation defines not only the strength of crowding but the probability that it occurs. Furthermore the model as described had elongated averaging zones promoting stronger averaging within contours. This is in accord with work described above suggesting that crowding and
contour-integration are related phenomena (Livne & Sagi,
2007; May & Hess,
2007). We return to this point below.
Crowding is not the only phenomenon where surrounding structure influences orientation-processing. In the
tilt illusion (Gibson,
1937), perceived target-orientation is repelled from the orientation of a surrounding/abutting grating. However, by increasing eccentricity it is possible to change repulsion to assimilation so that the orientation of the target is drawn towards that of the interfering stimulus (Mareschal, Morgan, & Solomon,
2010). The critical eccentricity for this switch to arise depends on stimulus separation: The switch arises at greater eccentricities for wider separations (Mareschal et al.,
2010).
Although both crowding and the tilt illusion lead to a distortion of target orientation, it is currently uncertain the extent to which these phenomena share an underlying mechanism. Assimilative tilt effects cause target orientations to appear more like those of the flanking stimuli; because it is known that crowding occurs between similar elements (Chung et al.,
2001; Hariharan et al.,
2005; Kooi et al.,
1994), the conditions that lead to assimilation will also lead to increased crowding (Mareschal et al.,
2010). Furthermore, one of the candidate theories of crowding is
averaging (Parkes et al.,
2001; Greenwood et al.,
2009), whereby the identity of a crowded feature is not lost (e.g., by being swamped by noise) but rather is diluted by averaging with features of the flankers. Clearly such an averaging mechanism could also be used to explain assimilative tilt-illusions (e.g., Dakin, Williams, & Hess,
1999); if the orientations of the target and flanking stimuli are averaged, the observer's perception is of the target being drawn towards the orientation of the flanking stimuli as seen with assimilation.
Focusing on oriented stimuli, it has been noted previously that strong crowding arises not only when target and flanker have similar orientations but also when they are positioned and oriented to form a
contour (Livne & Sagi,
2007; May & Hess,
2007; Chakravarthi & Pelli,
2011). Specifically, Livne and Sagi (
2007) showed a release from crowding when flankers grouped into contours, while Chakravarthi and Pelli (
2011) showed that such effets depended mainly on contour-grouping between target and flankers. May and Hess (
2007) went so far as to suggest that the integration fields supporting contour detection might explain interference zones for crowding. Beyond this little is known about the nature of “contour-crowding” and specifically what impact it has on local orientation coding (of constituent features). This is critical if we are to better understand the role of contour integration in crowding and relate its mechanism to other phenomena involving a disruptive influence of oriented-context, such as the tilt illusion. Here we investigate how being located within a crowded contour influences orientation-processing of the constituent elements and note strong similarities between the influence of global contour-orientation and the influence of oriented surrounds (i.e., as in the tilt illusion) on perceived orientation of a Gabor target.