1.1 PLANTEAMIENTO DEL PROBLEMA
1.6.2 INTERPRETACIÓN DEL PATRIMONIO
So far, all the spatial discrimination tasks performed have been conducted at frequencies close to the peak sensitivity of the DSF. The purpose of Experiment 7 was to compare corrugation frequency discrimination and cyclopean vernier acuity with the analogous tasks in the luminance domain, across a wider range of spatial and corrugation frequencies^^ The similarity of the results for vernier and corrugation frequency tasks in Experiment 6 offers some suggestion that a single mechanism of localization might be a limiting factor in both cases. Whether the two tasks show the same dependency on corrugation frequency may shed further light on this possibility.
It was also hypothesized that the slight superiority of corrugation frequency discrimination over spatial frequency discrimination, found for the well-practiced observer in Experiment 6, depends on local fluctuations in frequency sensitivity. Although it is generally the case that, with extensive practice, spatial frequency discrimination is approximately independent of the reference frequency (Mayer and Kim, 1986), there have previously been reports of Huctuations of discrimination performance with frequency, presumably resulting from sampling effects across space or frequency. Another possibility is that perfoimance for corrugation frequency discrimination depends on an extra cue which exists in the three-dimensional stimulus (a shape cue, perhaps). If this is the case then one would predict a continuing superiority o f the task at frequencies away from 0.5 c/deg.
M ethods
The methods employed in Experiment 7 were identical to the vernier acuity and corrugation frequency discrimination tasks of Experiment 6, except that the following 4 frequencies were tested, in both the luminance and cyclopean cases: 0.3, 0.6, 1.2 and 2.4 c/deg. The frequency of 0.3 c/deg was the lowest which could be employed while ensuring that at least two full cycles of the waveform were presented. The peak-trough disparity of the corrugations was held constant at 4 min arc. At disparities of greater than this value the highest frequency cyclopean stimuli could not be fused because the disparity gradient became too steep. The contrast of the luminance gratings was again 50%.
Runs of 64 were organized into blocks of 3. Any given block contained both luminance and cyclopean tasks and both frequency and vernier tasks. Within this constraint, conditions were randomized and counter-balanced across blocks.
An examination o f cyclopean orientation discrimination at a range of frequencies for vertical, horizontal and oblique stimuli is provided in Experiment 9.
Two observers participated in this experiment: PDL and PMV. PMV has normal, uncorrected vision, was entirely naive as to the purpose of the experiment and had no previous experience of either RDS’s or psychophysical experiments. PMV was tested prior to the experiment with a range of RDS stimuli. He had no difficulty in determining the cyclopean forms o f the shapes which they contained.
0.25
P M V
0.2- (4 -r < -#— 50% contrast ■©— 4' disparity 0.05 - Frequency (c/deg) 0.06P D L
0.05 - 0.04 - 0.03 4 <+- < 3 0.02 - — 50% contrast g — 4' disparity 0.01 - Frequency (c/deg)F ig u re 5.9. C o rru g a tio n f r e q u e n c y a n d s p a tia l fre q u e n c y discrim ina tid n thresholds f o r two observers, p l o tt e d a s a function o f fre q u e n c y . F o r the w e ll- p r a c tic e d o b se rv e r, PD L, the su p e r io r ity o f co rru g a tio n f r e q u e n c y discrim ina tio n is only evid en t a t 0 .6 c/deg. In general, the da ta s u g g e s t that o v e r at lea st a 2 - 3 o c ta ve range, c y clo p ea n spa tia l discrim ination is co m p a ra b le with that fo u n d in the luminance domain.
Results
Again, there were surprisingly few effects of learning observed. As was the case for DD in Experiment 6, the first two measurements of corrugation frequency discrimination were elevated with respect to the runs which followed (16.2% at 0.6 c/deg, 10.45% at 0.3 c/deg). No noticeable learning effects were found with the cyclopean vernier task, nor with either of the luminance tasks.
PMV
9 - > < — 50% con trast ^ — 4' disparity Frequency (c/deg) ■s 'S Oh ><
5PDL
4 3 2 -# — 50% contrast -0 — 4' disp arity 0 Frequency (c/deg)F i g u r e 5 . 1 0 V ernier tl ires ho Ids f o r 2 o b s e r v e r s w ith c y c l o p e a n a n d luminance-defined stimuli, p lo tte d a g a in st frequency. F or PD L, cyclopean a n d lum in a n ce th re s h o ld s a r e c o m p a r a b l e . F o r PM V , s e n s it iv i ty is re d u c e d in the cyclopean case.
Corrugation frequency and spatial frequency discrimination thresholds (Af/f) for both observers are plotted in figure 5.9 as a function of frequency. For PMV, performance is between 5% and 7% for both cyclopean and luminance versions o f the task at all frequencies except 2.4 c/deg, where the cyclopean task falls off considerably. For PDL, discrimination is slightly better in the cyclopean task at 0.6 c/deg, but this trend is not found for any other frequency and is reversed at 2.4 c/deg. These data therefore provide evidence that the superiority of corrugation discrimination over spatial frequency discrimination for well-practiced observers (found in this experiment and in Experiment 7) is due to local fluctuations in performance with frequency, rather than to the use of, for example, a shape cue which appears only in the three-dimensional stimulus. For PDL, there is also a general tendency for thresholds to rise with spatial and corrugation frequency. This trend is not so apparent in the data of PMV. As previously stated, small variations in spatial frequency discrimination with reference frequency have been reported previously (Hirsch and Hylton, 1982), although at higher frequencies than those employed in the present experiment. Despite these slight differences, for both observers there is a 2 - 3 octave range over which performance for the cyclopean and luminance- defined stimuli is approximately the same.
400 n 350 : 300 4 250 Î
2004
150 100-^ 50^0
C o n sia n i p h a se P M V lu m in a n ce P M V c y c lo p e a n P D L lu m in a n c e P D L c y c lo p e a nT
C o n sia n i d is p la c c m c n i1
Frequency (c/deg)F ig u r e 5 . I I . Vernier thresholds f o r both o b se rv e rs r e -p lo tte d a s absolute d isplacem ents. The data conform neither to a constan t d isplacem ent nor a constan t phase.
There is less agreement between the observers in the vernier acuity task. Figure 5.10 plots the thresholds (Av) as a function of frequency for both observers. Thresholds are given in terms of the percentage of the | period of the sinewave. For PDL, thresholds are again equivalent for the cyclopean and luminance-defined tasks across a 2
- 3 octave region, but acuity drops off for the cyclopean task at 2.4 c/deg There is no superiority of the cyclopean task at 0.6 c/deg in this case. For PMV, there is a significant drop in performance for the cyclopean stimuli relative to the luminance stimuli at all frequencies. But performance is still in the region of 2 - 3 % of the phase angle. For both observers there is a rise in threshold with increased frequency. This trend has been previously reported by Bradley and Skottun (1987) and may reflect the increased use of the luminance edge at the border between the gratings.
Vernier thresholds are re-plotted as absolute displacements in figure 5.11. The lowest absolute value for cyclopean vernier acuity in this case was 47 sec arc for PMV at 2.4 c/deg, which corresponds to 31 sec arc for 75% frequency-of-seeing. Again this is consistent with the results of Morgan (1986). Thresholds conform to neither a constant displacement nor a constant phase angle. The latter would be predicted by a purely statistical theory, because the localization of a central tendency should be inversely proportional to blur and hence also inversely proportional to frequency.