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Luminance modulation
Contrast modulated static noise Contrast modulated dynamic noise
Drift temporal frequency (Hz)
Figure 4.10: Contrast thresholds for direction detection plotted in terms of relative sensitivity as a function of temporal frequency. Data re-plotted from (a) Lu & Sperling,
order motion is also supported by Jamar & Koenderink (1985) who showed that modulation detection thresholds for contrast modulated noise gratings showed a lowpass response over spatial frequency. Detection thresholds for a sine wave grating showed a bandpass response with the lowest contrast thresholds at ~3 cycles per degree.
In terms of the patterns of results described in this chapter, there is no evidence for any difference over spatial and temporal parameters between the first- and second-order motion stimuli. In order to support the two channel model one would have to suppose that the two channels are alike enough so that there appear to be no differences in terms of the pattern of their SDTs, but that the channels may possibly differ from one another at some stage that precedes velocity extraction. This latter point is necessary to account for any differences in response to other measures of the sensitivity of the systems that may occur, particularly any degree of independence between speed discrimination thresholds and contrast thresholds for direction discrimination that may occur. For example Cropper (1994), examining the perception of motion in beat stimuli, found that, with increases in beat contrast, speed discrimination thresholds dropped to a level compatible with those found for luminance gratings. He notes that this pattern is not seen with direction discrimination.
Of course the lack of a difference in the patterns of response also supports the hypothesis that first- and second-order velocity are extracted by the same mechanism. If first- and second-order motion are detected by a single mechanism then the differences in magnitudes of the speed discrimination thresholds between the first- and second-order stimuli must adequately be accounted for. SDTs for second-order stimuli were higher than those for first- order stimuli. This difference was more pronounced with Gaussian bar stimuli than with the integral of Gaussian edge stimuli. Given the results of work presented in Chapter 3 it may be profitable to suppose that the larger first-order/second-order differences for bar stimuli compared to edge stimuli may be due to the different carrier types. The second-order bar is a contrast modulation of dynamic noise, the second-order edge stimulus is a modulation of static noise.
Figure 4.11 shows Fourier spectra for the contrast-defined stimuli. These were created by adding the Fourier amplitude spectra of 100 instantiations of each of the respective images. It is clear that, in the case of the second-order edge, far more of the energy in the image lies
on the line signifying components with a temporal frequency of zero than in the case of the second-order bar. Therefore in the latter, a greater proportion of the expected first-order energy in the image will contain motion information than in the case of the second-order edge. It would seem reasonable to suppose that an increased proportion of random first- order motion energy would lead to increased thresholds for speed discrimination. This assertion is supported by evidence from Bowne, McKee & Glaser (1989) who showed that, in a two-dot apparent motion task, speed discrimination thresholds were raised by the inclusion of elements that added additional motion signals into the local velocity field.
The data presented in this chapter therefore supports two hypotheses. Firstly, that first- and second-order velocity are measured by different but very similar systems. It is not clear how this hypothesis can account for any differences that do exist in the perception of first- and second-order stimuli. Secondly; that first- and second-order velocity are extracted through the same mechanism. In the latter, any differences that do arise between the perception of motion elicited by the two stimulus types must be explained by appealing to differences in the form of the stimuli rather than upon the operations of separate mechanisms.
4.5. Conclusion.
There were no systematic differences in the patterns of results between the first- and second- order stimuli over velocity and width. This implies that either separate but very similar mechanisms measure velocity in both types of stimulus or that first- and second-order velocity are extracted by the same mechanism. The latter of these two options presents the simplest explanation of the results. There were systematic differences in the magnitudes of the speed discrimination thresholds. Those for the second-order stimuli were higher than those for the equivalent first-order stimuli. This was more pronounced with first- and second-order bars than with first- and second-order edges. It is possible that these differences may be accounted for by appealing to the presence of the different carriers in the second-order stimuli.
Figure 4.11: Fourier spectra created from the addition of 100 instantiations of a second- order Gaussian bar (top) and 100 instantiations of a second-order Gaussian edge (bottom).