Neural locus of color afterimages Qasim Zaidi, Robert Ennis, Ding Cao, Barry Lee.
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Transcript of Neural locus of color afterimages Qasim Zaidi, Robert Ennis, Ding Cao, Barry Lee.
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Neural locus of color afterimages
Qasim Zaidi, Robert Ennis, Ding Cao, Barry Lee
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Psychophysical Procedure
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Used by Al-Haytham to disprove the theory that visual rays emanated from the eye.
Used by Newton to first demonstrate inter-ocular interactions.
Used by Wheatstone to show that binocular vision does not require eye-movements.
Historical Critical Experiments with Afterimages
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Photoreceptors (Williams & MacLeod).
Post-receptoral (Loomis).
Cortical (Shimojo et al, Shevell et al).
Direct measurements of the neural locus have never been reported.
Proposed Neural Loci of Color Afterimages
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Psychophysical Procedure
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Psychophysical Procedure
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Psychophysical Procedure
• 3 cardinal directions at maximum monitor contrasts.
• Each stimulus was repeated 100 times divided into 10 sessions. The starting point of the clock was randomly jittered on each trial.
• 5 color-normal observers
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Identity Points (Stimulus at 1/32Hz)(Means of 100 repeats per 5 observers)
• Adaptation is lower for Δ(L+M+S) than Δ(L-M) despite 10 times greater cone modulation by Δ(L+M+S), indicating post-receptoral adaptation.
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The clock can be used for timing judgments without an attentional load.
Point of identity is much more definite than deciding when the afterimage has faded.
Linking Hypothesis: When the difference between the two halves becomes imperceptible, cell responses at some visual stage, should be back to baseline.
Can be used to make Class A measures of adaptation (Brindley,1970) as a function of spatial frequency, temporal frequency, etc.
Advantages of the psychophysical method
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Recording from Retinal Ganglion Cells
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Physiological Stimulus
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Physiological Stimulus
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+M-Center Parvo-Ganglion Cell
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Ganglion Cell Adaptation Model
R(t) = Response at time t; R(0) = Baseline response Q(t) = combination of cone inputs to ganglion cell τ = time constant of adaptation κ = subtractive constant ν = adaptation strength constant (always 3) Estimated parameters = R(0), κ, τ, ω, ν
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Ganglion Cell Adaptation Model
No Adaptation Fast Adaptation Slow Adaptation
Photoreceptor adaptation time constant = 0.01s (Smith, Pokorny, Lee, & Dacey, Vis. Research (2008))
Stimulus value at response baseline should correspond to psychophysical nulled contrast.
“Rebound” response after the stimulus has returned to mean gray provides the afterimage signal to later stages.
τ = 10000000s τ = 0.01s τ = 5s
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+S-Center Konio-Ganglion Cell
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ON-Center Magno-Ganglion Cell
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Virtues of Physiological Method
• Measures adaptation state of neurons without using a probe that could disturb adaptation.
• Provides a simple way to compare adaptation time constants of the neuron for different stimulus time-courses.
• Estimated time constants were in the range of 4-10s.
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Response Maximum vs. Rebound Maximum
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Time-constants vs. Identity Points
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Conclusions
We identify a retinal locus for generation of color afterimage signals.
A subtractive adaptation model fits responses of Parvo-, Konio- and Magno- RGCs.
The slow time constants are consistent with post-receptoral adaptation following the combination of cone signals.