The use of environmental geometry as a spatial cue is well established for a range of species. Previous research has focused largely on the use of global geometry (e.g., the shape of a room). Thus, comparatively less is known about how local geometry (e.g., corner angles within a room) is encoded. The purpose of the research presented in this thesis was to examine how angular information is encoded and to determine whether angle size influences encoding, using a discrimination task and a spatial array task. Chapter 2 presents a study during which pigeons were trained to discriminate between a small (60°) and large (120°) angle. Once the birds were accurately choosing the angle associated with reward, they were tested on their ability to discriminate between their training angle and one of a series of novel angles. The pigeons showed an absolute learning pattern for the small training angle, but not the large angle. The significance of this result is that the small angle may have been perceived as more distinctive compared to the large angle. Adopting a comparative approach, Chapter 3 presents a study during which adult humans were trained and tested using a similar paradigm but with different training angles (25°, 50° and 75°). The results of this study also support an absolute learning pattern for the small training angle but not the large. These results are significant in that they suggest that angle size may be an important local geometric cue that is encoded in a similar way by both pigeons and humans. To understand how angular information may be processed during a spatial task, Chapter 4 presents a study during which adult humans were trained and tested on their ability to use local angles (either 50° or 75°) to find a goal location within an object array. The results showed that the smaller angle was used more effectively as a spatial cue than the larger angle. Overall, these results are important as they suggest that small and large angles are encoded differently by pigeons and humans, with small angles perceived as more distinctive than large angles.

The use of environmental geometry as a spatial cue is well established for a range of species. Previous research has focused largely on the use of global geometry (e.g., the shape of a room). Thus, comparatively less is known about how local geometry (e.g., corner angles within a room) is encoded. The purpose of the research presented in this thesis was to examine how angular information is encoded and to determine whether angle size influences encoding, using a discrimination task and a spatial array task. Chapter 2 presents a study during which pigeons were trained to discriminate between a small (60°) and large (120°) angle. Once the birds were accurately choosing the angle associated with reward, they were tested on their ability to discriminate between their training angle and one of a series of novel angles. The pigeons showed an absolute learning pattern for the small training angle, but not the large angle. The significance of this result is that the small angle may have been perceived as more distinctive compared to the large angle. Adopting a comparative approach, Chapter 3 presents a study during which adult humans were trained and tested using a similar paradigm but with different training angles (25°, 50° and 75°). The results of this study also support an absolute learning pattern for the small training angle but not the large. These results are significant in that they suggest that angle size may be an important local geometric cue that is encoded in a similar way by both pigeons and humans. To understand how angular information may be processed during a spatial task, Chapter 4 presents a study during which adult humans were trained and tested on their ability to use local angles (either 50° or 75°) to find a goal location within an object array. The results showed that the smaller angle was used more effectively as a spatial cue than the larger angle. Overall, these results are important as they suggest that small and large angles are encoded differently by pigeons and humans, with small angles perceived as more distinctive than large angles.

No / Amblyopia results in a severe loss of positional information and in the ability to accurately enumerate objects (V. Sharma, D. M. Levi, & S. A. Klein, 2000). In this study, we asked whether amblyopia also disrupts the ability to track a near-threshold change in the trajectory of a single target amongst multiple similar potential targets. In the first experiment, we examined the precision for detecting a deviation in the linear motion trajectory of a dot by measuring deviation thresholds as a function of the number of moving trajectories (T). As in normal observers, we found that in both eyes of amblyopes, threshold increases steeply as T increases from 1 to 4. Surprisingly, for T = 1-4, thresholds were essentially identical in both eyes of the amblyopes and were similar to those of normal observers. In a second experiment, we measured the precision for detecting a deviation in the orientation of a static, bilinear "trajectory" by again measuring deviation thresholds (i.e., angle discrimination) as a function of the number of oriented line "trajectories" (T). Relative to the nonamblyopic eye, amblyopes show a marked threshold elevation for a static target when T = 1. However, thresholds increased with T with approximately the same slope as in their preferred eye and in the eyes of the normal controls. We conclude that while amblyopia disrupts static angle discrimination, amblyopic dynamic deviation detection thresholds are normal or very nearly so.

no / In normal vision, detecting a kink (a change in orientation) in a line is scale invariant: it depends solely on the length/width ratio of the line (D. Whitaker, D. M. Levi, & G. J. Kennedy, 2008). Here we measure detection of a change in the orientation of lines of different length and blur and show that strabismic amblyopia is qualitatively different from normal foveal vision, in that: 1) stimulus blur has little effect on performance in the amblyopic eye, and 2) integration of orientation information follows a different rule. In normal foveal vision, performance improves in proportion to the square root of the ratio of line length to blur (L: B). In strabismic amblyopia improvement is proportional to line length. Our results are consistent with a substantial degree of internal neural blur in first-order cortical filters. This internal blur results in a loss of scale invariance in the amblyopic visual system. Peripheral vision also shows much less effect of stimulus blur and a failure of scale invariance, similar to the central vision of strabismic amblyopes. Our results suggest that both peripheral vision and strabismic amblyopia share a common bottleneck in having a truncated range of spatial mechanisms-a range that becomes more restricted with increasing eccentricity and depth of amblyopia. / Leverhulme Trust, Wellcome Trust, NIH