Revealing Neural Loci of Color Afterimages: Psychophysical and Physiological Experiments

1. Neural locus of color afterimages Qasim Zaidi, Robert Ennis, Ding Cao, Barry Lee

2. Psychophysical Procedure

3. Historical Critical Experiments with Afterimages • 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.

4. Proposed Neural Loci of Color Afterimages • Photoreceptors (Williams & MacLeod). • Post-receptoral (Loomis). • Cortical (Shimojo et al, Shevell et al). • Direct measurements of the neural locus have never been reported.

5. Psychophysical Procedure

6. Psychophysical Procedure

7. Psychophysical Procedure • 3 cardinal directions at maximum monitor contrasts. • Eachstimulus was repeated 100 times divided into 10 sessions. The starting point of the clock was randomly jittered on each trial. • 5 color-normal observers

8. 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.

9. Advantages of the psychophysical method • 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.

10. Recording from Retinal Ganglion Cells

11. Physiological Stimulus

12. Physiological Stimulus

13. +M-Center Parvo-Ganglion Cell

14. 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

15. 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

16. +S-Center Konio-Ganglion Cell

17. ON-Center Magno-Ganglion Cell

18. 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.

19. Response Maximum vs. Rebound Maximum

20. Time-constants vs. Identity Points

21. 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.