Detection of the presence of bilateral symmetry was investigated at various retinal eccentricities for static and dynamic noise reflected around a vertical axis. At a low detection criterion (60% correct), peak duration sensitivities were high and varied little (<0.2 log units) from 0° eccentricity to 10° eccentricity for either static or dynamic targets. Duration thresholds for symmetry in dynamic noise fields were significantly higher (about 100 ms) than those for static symmetry detection (about 40 ms), despite the fact that the information was refreshed many times during the threshold presentation period. The spatial summation width for symmetry processing was evaluated with randomization around the axis of symmetry. The estimated summation width for static symmetry detection was approximately constant with eccentricity for short duration stimuli. For long duration stimuli, the summation width was substantially greater in central vision but *decreased* with eccentricity, the first known visual function to exhibit such reverse magnification behavior (Tyler, 1999). These findings suggest that static and dynamic symmetry detection are supported by different neural mechanisms and that these mechanisms are relatively invariant across the retina, unlike known mechanisms of spatial processing.

*A*versus eccentricity

*E*of the in degrees, where

*m*is the slope of the line (which is always positive) and

*E*

_{2}is the intercept, which is always negative for real resolution and positive

*m*. Levi, Klein, and Aitsebaomo (1985) pointed out that the characteristic value for such linear magnification is the constant

*E*

_{2}because

*m*just plays the role of a scaling parameter. For our purposes, note that

*E*

_{2}is so called because it has the property that angle

*A*doubles when the stimulus reaches that eccentricity, i.e., when

*E*=

*E*

_{2}.

*μ*is the logarithmic slope and

*E*

_{log2}is the eccentricity at which log

*A*increases by 0.3.

*p*< 0.05). There was no evidence for a secondary skirt to the function similar to that observed for static symmetry detection at 0° eccentricity and >1.5° noise gap (as found at the 60% level in Tyler et al., 1995).

*E*

_{log2}), just as for the linear fits. The negative intercepts are very shallow in comparison to those for grating acuity.

*E*

_{log2}values are given only in the decreasing direction because in the increasing direction the error could exceed infinity for the shallowest slopes. The decreasing direction is the one needed to distinguish the values from those for acuity.

*p*< 0.05).

Mean Y Intercept (ms) | Mean Slope (dl/deg) | Mean r^{2} | Doubling Eccentricity (E_{log2} | |
---|---|---|---|---|

Static 60% | 39 ± 2 | 0.11 (± 0.012) | 0.80 (± 0.07) | 41.8° (−21.6°) |

Dynamic 60% | 77 ± 6 | 0.18 (± 0.009) | 0.97 (± 0.03) | 30° (−10.1°) |

Static 90% | 62 ± 6 | 0.65 (± 0.035) | 0.95 (± 0.04) | 8.1° (−2.7°) |

Dynamic 90% | 129 ± 16 | 0.30 (± 0.004) | 0.97 (± 0.02) | 21.3° (−2.9°) |

*E*

_{log2}values of eccentricity at which duration sensitivity is doubled range from a high of 42° for the static symmetry at low sensitivity (60% correct) to a low of 8° for static symmetry at high sensitivity (90% correct), with the dynamic symmetry values falling between these extremes.

*E*

_{2s}for contrast detection tasks are in the range of 3° to 5° (Levi et al., 1985), so those for the symmetry sensitivities are all significantly flatter than predicted (taking the criterion of 2 σ,

*p*< 0.05) if this task was mediated by the local ganglion receptive field mechanisms implicated in contrast detection (Virsu & Rovamo, 1979).