Asymmetric pedestal masking of S-cone increments and decrements: Does sawtooth polarity matter? Scott H. Gabree & Rhea T. Eskew Jr. Department of Psychology, Northeastern University S-cone Tests on S-cone Pedestals - S+ pedestals (right half of each panel) mask more than S- pedestal (left half) regardless of test polarity (S+ and S-, top and bottom respectively)
S-cone Tests on (L+M) Pedestals Note: Signs of the (L+M) pedestals have been reversed 6
1.5
Threshold S-cone S+ in threshold ratio
S-
2
4
2
0
-2
-4
-6 -20
-15
-10
-5
0
- 5 - 10 - 15 - 20
(L+M) Pedestal Intensity (L+M) Pedestal Intensity
1 0.5 0 -0.5 -1 -1.5 -2
-3
-2
-1
0
-1
-2
-3
(L+M) Pedestal Intensity (L+M) Pedestal Intensity
- All points on graphs are displayed in units of their own threshold; up to 4 different scale factors area used for each graph - Same asymmetry would be found if plotted in cone contrast units
A. Same masking asymmetry is found with the polarity of the (L+M) pedestal, again with both S+ and S- tests B. Approximate subthreshold summation at low pedestal contrast for both pedestal chromaticities C. A and B suggest that S and (L+M) are being processed in the same pathway
Test Interval I
Ped
Contrast
Interval II
Which interval contained the pedestal + test?
Contrast
Position (deg)
Position (deg)
S- test
S+ test
SInput
S- test
∆R ∆R
Contrast (+)
- 2 AFC - Well-practiced observers - All test chromaticities were S-cone isolating (S+ or S-)
Differential Contrast Gain Control
ON
∆R ∆R
Time
Time
Contrast (-)
Ped
Output
OFF
Output
- Colored points and lines represent rapid start sawtooth stimuli and black points and lines represent rapid stop stimuli.
S+ test
S+ Input
- Flatter transducer (more contrast gain) in the ON pathway than the OFF - Higher test contrast required to reach threshold in the ON pathway (at high contrast pedestals) - Asymmetry is associated with the pedestal polarity, not the test polarity, because the local curvature around the pedestal is relatively small - Solomon and Lennie (2005) report greater contrast gain control in S-ON pathways than S-OFF in LGN
- Facilitation and masking were found for L+M pedestals (displayed above) - No facilitation or masking was found with L- or M-cone pedestals or (L-M) pedestals (data not shown) - Therefore the L and M signals to the S-cone pathways must be combined in a nonlinear fashion
8 6 4 2 0 -2 -4 -6
15
6
10
4
5
0
-5
-10
RTE
SHG
-8 -20
-15
-10
-5
0
5
10
15
S-cone test on (L+M) Pedestal S-coneThreshold Threshold S-cone
- “Purple” pedestals produce more masking with S-cone increment (S+) and decrement (S-) tests than do “yellow” pedestals - This suggests more contrast gain control in S-cone increment pathway (likely the S-ON pathway) - Assuming contrast polarity separates ON and OFF pathways, will the temporal polarity of the sawtooth pulse modulate this effect?
- Contrast polarity separates signals into ON and OFF pathways - What about the polarity of the sawtooth? - We expect no effect except possibly at high pedestal contrasts
- Rapid Stop waveform produced consistently less masking, but the differences from the Rapid Start waveform are quite small - The slightly lower masking could be consistent with some adaptation to the strong pedestal - More masking is found for “purple” pedestals using both polarities of the sawtooth.
Conclusions - The polarity of the sawtooth makes little difference even for strong pedestal conditions - Contrast polarity seems to determine which pathway (ON or OFF) is processing the stimulus - The temporal polarity has little effect, even at high contrast levels
Asymmetric pedestal masking of S-cone increments ...
The polarity of the sawtooth makes little difference even for strong pedestal conditions. - Contrast polarity seems to determine which pathway (ON or OFF) is processing the stimulus. - The temporal polarity has little effect, even at high contrast levels. - Colored points and lines represent rapid start sawtooth stimuli and black ...
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