I Chapter 3: Spatial interference with relative localisation
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flanking distance (min arc)
Figure 3.16: Gaussian bars, space constant 1.0 min arc: vernier thresholds shown for two subjects (filled circles: GLC, open circles: IRP). There is no evidence of any increase in vernier thresholds for the smaller flanking distances. Hence the mean shifts obtained in the graph above represent changes in the perceived relative location of the target bars without accompanying changes in the precision of localisation of the target bars.
Chapter 3: Spatial interference with relative localisation
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flanking distance (min arc)
Figure 3,17: Location biases for one subject (IRP) for Gaussian bars with space constant of 1.5 min arc in the presence of flanking bars. Attraction is indicated by a positive value on the ordinate. Repulsion is observed at all but the smallest of flanking distances. At flanking distances smaller than those tested, it becomes increasingly more difficult to resolve the two bars.
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fianking distance (min arc)
Figure 3.18: Vernier thresholds for localising Gaussian bars with space constant of 1.5 min arc in the presence of flanking Gaussian bars are shown for subject IRP.
Chapter 3: Spatial interference with relative localisation
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flanking distance (min arc)
Figure 3.19: Flanked Gaussian bars with space constant 2.0 min arc; location biases are shown for two subjects (IRP; open circles, and GLC; filled circles). Attraction is only weakly present for both subjects at the smallest flanking distances tested.
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fianking distance (min arc)
Figure 3.20: Vernier thresholds are shown for flanked Gaussian bars with space constants of 2.0 min arc. Data from two subjects is shown (IRP; open circles, and GLC; filled circles). There is no clear effect of flanking distance on precision of localisation in this data.
Chapter 3: Spatial interference with relative localisation
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flanking distance (mln arc)
Figure 321: Gaussian bars with space constant of 2.5 min arc; location biases are shown for two subjects (IRP; open circles, and GLC; filled circles). Attraction is represented by a positive value on the ordinate, and is not evident at all for either subjects at any of the flanking distances tested.
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flanking distance (mln arc)
Figure 3.22: Vernier thresholds are shown above for localisation of flanked Gaussian bars with space constants of 2.5 min arc. Data is shown for two subjects (IRP; open circles, and GLC; filled circles). There is some suggestion of elevated thresholds at the smallest flanking distances tested, but this was not evident in the data from the smaller blurs, and is probably illusory and due to noise in the data.
Chapter 3: Spatial interference with relative localisation
Mean shift values and vernier thresholds obtained are shown in figures 3.15-3.22 as a function of the separation of the flanking bar and the target bar. Results for the observers are closely similar. Note that as in the previous experiment, the perceived shifts determined represent the combined effects of two flanking bars on opposite sides of the upper and lower target bars. These mean shifts were obtained without any noticeable trends in the vernier thresholds for the different flanking distances.
The results of this experiment confirm and extend those of Badcock and Westheimer (1985a). Attraction between resolved bars (bars which could be distinctly perceived as spatially separated objects: the stimulus intensity distribution of the two bars should possess a sufficiently large ‘dip’ between the luminance maxima for the bars to be resolved, of the order of 10% of the maximum amplitude of the luminance excursion) disappears if the bar is blurred beyond 2-2.5 min arc space constant, and is obtained at flanking distances of less than 6-7 min arc. This was verified for subject IRP and another observer with bars of 4.0 min arc; with bars of this blur, if they could be resolved, then attraction could not be observed.
In the 1st experiment involving a direction of displacement discrimination task, attraction was obtained at greater flanking distances, and with slightly more blurred bars than in the vernier acuity task. This indicates that the mean size of spatial filters mediating direction of displacement discrimination (ie. perception of apparent motion) may be somewhat larger than those subserving vernier acuity.
As a control condition, vernier thresholds and means were collected at different exposure durations for one subject, to discover the effect of exposure duration; since it did not appear to affect the basic pattern of results, this was not pursued further. These experiments were conducted prior to publication of Watt's (1987) results indicating coarse-to-fine processing in early vision, and it did not appear at the time theoretically interesting to pursue these results further, although in retrospect it may have been interesting to have done so, since they conflict with the basic notion of a coarse-to-fine analysis of spatial relations. The data is reported here for completeness in figures 3.23-3.26, but since it is based on only data from the author (IRP), only tentative conclusions should be drawn.
Chapter 3: Spatial interference with relative localisation
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flanking distance (mln arc)
Figure 3.23: Mean shifts obtained under two exposure durations from subject IRP; filled circles are for a duration of 100 msec, and open circles are for a duration of2000 msec. There is a remarkable correspondence between the two sets of data. Space constants of the target and flanking bars was 1.5 min arc.
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flanking distance (mln arc)
Figure 3.24: Localisation thresholds for two different exposure durations for subject IRP. Open circles are for the 2 sec exposure duration, filled circles represent data from the 100 msec condition. The space constant for the bars was 1.5 min arc. Thresholds were somewhat elevated by shortening the exposure duration, but neither function has any obvious structure to it.
Chapter 3; Spatial interference with relative localisation
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flanking distance (mln arc)
Figure 3.25: Mean shifts from subject IRP, for two different exposure durations. Both target and flanking bars had Gaussian contrast profiles with space constants of 2.0 min arc. Open circles are for the 2 sec exposure duration, filled circles for the 60 msec exposure duration, 100 S k 10 - Q . X> 4 6 8 1 0 1 2 14 16
flanking distance (min arc)
Figure 3.26: Vernier thresholds from subject IRP, for two different exposure durations. Both target and flanking bars had Gaussian contrast profiles with space constants of 2.0 min arc. Open circles are for the 1 sec exposure duration, filled circles for the 60 msec exposure duration condition. Note that there is some indication of decreased precision of localisation for the 60 msec exposure duration condition at smaller flanking distances.
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Chapter 3: Spatial interference with relative localisation
3.4.3: Experiment 3: Vernier acuity for edges flanked bv bars
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Figure 327: Mean shifts are shown for edge targets, with a single Gaussian bar flank, which varied between flanking the upper and lower edge. Data is shown for two subjects (IRP; open circles aiü GLC; filled circles). The flanking bar had a contrast 50% that of the edge and had a negative polarity- i.e. the appearance of the target was of a dark bar beside a bright edge. Attraction is evident at small flanking distances for both subjects despite the polarity of the target feature, which is the reverse of the results when the target is a bar (eg. Badcock & Westheimer, 1985a,b).
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flanking distance (min arc)
Figure 3.28: Localisation thresholds are shown for edge targets, with a single Gaussian bar flank, which has a negative contrast polarity (and is 50% the contrast of the edge). Data is shown for two subjects (IRP; open circles, and GLC; filled circles). There is no clear dependency of precision of localisation on the separation of the target edges and the flanking bar. The edges and bar had space constants of 1.0 min arc.
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Chapter 3: Spatial interference with relative localisation