Flexible Retinotopy: Motion-Dependent Position Coding in the Visual Cortex

1. Flexible Retinotopy: Motion-Dependent Position Coding in the Visual Cortex David Whitney, Herbert C. Goltz, Christopher G. Thomas, Joseph S. Gati, Ravi S. Menon, Melvyn A. Goodale Science, 2003, Vol 302, 878-881.

2. Say something first • Background of Retinotopy • A anti-tradition fMRI result • How the authors exclude other possible explanations • Seeking help from behavioral designs • A bit of my experiments (Possible Connection) • Discussion

3. fMRI for Dummies Visual areas subdivision • The occipital lobe contains many maps of the visual world Visual maps in the macaque monkey brain derived from single neuron electrophysiology Visual maps in the human brain derived from fMRI “fMRI for Dummies” are Slides from Jody Culham’s PPT file.

4. fMRI for Dummies Macaque Retinotopy • Retinotopy: Adjacent points of the visual world fall on adjacent points on the retina and are represented in adjacent points in the visual cortex Source: Tootell et al., 1982

5. fMRI for Dummies Transforming the Map to Polar Coordinates N x direction Polar Angle

6. fMRI for Dummies Taking Care of All the Flips in V1 Left Occipital Lobe Right Occipital Lobe • One visual area has one complete map of visual space • Here is the map in primary visual cortex, V1 N N

7. fMRI for Dummies V2 is a mirror image of V1 Left Occipital Lobe Right Occipital Lobe

8. ROTATING WEDGES fMRI for Dummies Retintopy: Flickering Checkerboard • 8 Hz flicker (checks reverse contrast 8X/sec) • good stimulus for driving visual areas • subjects must maintain fixation (on red dot)

9. fMRI for Dummies Retinotopy Source: Sereno et al., 1995, Science

10. A anti-tradition fMRI result Fig 2 E Traditional Retinotopy works in Flickering Gabor stimulus with different physical position Flickering patterns used, its physical position is displaced by the amount of the visual illusion (from Fig. 1E).

11. A anti-tradition fMRI result Moving Gabor stimulus with same physical position Fig 1 E Vision Illusion (not a new finding) The perceived position of the patterns is shifted in the direction of the moving texture. (E) The psychophysically measured magnitude of the illusion. Will it create the same Retinotopy as flickering one? Will EXP activation be more far away?

12. A anti-tradition fMRI result Fig 2 A-DNone-Traditional (flexible) Retinotopy Cortical surface, Right hemisphere, Occipital region. (Increasing eccentricity in the direction of the yellow arrow. (C) inward - outward =orange (significantly activated). Lower panel, event related average for orange voxels (D) (ourward - inward =blue).

13. Excluding other possible explanations A: Is this strange retinotopy caused by that Illusion Perception (a blurry edge is needed)? Fig 4-A Patterns containing motion with sharply defined borders. Although no illusory position shift, the same pattern of activation was observed.

14. Excluding other possible explanations B: Is spatially localized attention responsible for the results? No, details please see Figure 7S. Controlling spatially localized attention. Subjects need performed a difficult attentionally demanding task at the fixation point (Counting how many red and blue flashes were presented and whether there were more red or blue flashes). The pattern of activation was identical to that in the first experiment. This rules out the possibility that the results are attentionally modulated.

15. Seeking help from behavioral designs Trailing edge mechanism as Whitney’s explanation True reason underlying flexible Retinotopy: Peak activity always occurs at the trailing edges, which is the product of a mechanism that operates more strongly on the trailing edge. But what mechanism? It seems no Clear answer Provided . Three behavioral experiments: An imbalance in motion processing between the trailing and leading edges. Behavioral designs did not directly explain fMRI result, it seems parallel but not explanatory. • Figure 4S. The position shift at the trailing edge is more larger than that of the leading edge. • 2Figure 5S. The luminance contrast of the trailing edge is not just reduced, but also appears distorted. (if contrast reduced, how it creates peak activation?) • 3 Figure 8S. The illusion of induced motion was stronger near the trailing edge of the moving pattern.

16. Seeking help from behavioral designs Figure 4S. Figure 5S. Figure 8S.

17. Seeking help from behavioral designs Trailing edge mechanism as Whitney’s explanation Link between position shift and contrast decrease Partially Explain the illusion in Experiment 1: Because the contrast of the trailing edge is perceptually reduced, the midpoint of the pattern as a whole appears displaced toward the leading edge, causing Illusion. Links between Trailing effect and Flexible Retinotopy: The peak activation always occurs closer to the trailing edge of moving patterns (or origin of the motion), regardless of perceived position (Illusion independent). This is verified by previous and following fMRI results.

18. Excluding another possible explanation and further verification C: Is it possible that optic flow in previous stimuli responsible for the results? So, how about no exp/con optic flow, only trailing edge? Patterns did not contain exp/con optic flow. Motion originated from either the vertical or the horizontal meridian. Upper Stimulus: Trailing Edge-Vertical Leading Edge-Horizontal Lower Stimulus: Trailing Edge-Horizontal Leading Edge-Vertical

19. Excluding another possible explanation and further verification So, how about no optic flow, only trailing edge? Fig 4-B V1/V2 border represents the motion along vertical angle, while motion along horizontal angle should appear within V1.

20. Implication of this study: • the results demonstrated a clear dissociation: • fMRI activation did not correlate with what subjects perceived, showing that the BOLD response is not a necessary correlateof perception. • Did not provide the answer (?) why the trailing edge creates peak activation. • It seems correlative? but not causal?

21. A bit of my experiments Possible Connection? What we are interested? • What will happen if we use moving dot pattern instead of Gabor or grating? • 2 What will happen if we use two trailing edges or two leading edges together? • Will the trailing effect be strengthened or neutralized? • 3 Any other possible explanations that could account for fMRI results besides trailing effect?

22. A bit of my experiments Possible Connection? Figure 4S. Three behavioral experiments: E1: o ← Vs o → CP Vs CF E2: o ← -- Vs o → -- CP+S Vs CF+S E3: o ← → Vs o → ← EXP (CP-CF) Vs CON (CF-CP) Task: discriminating the motion-defined border. The design is similar to Whitley et. al of Figure 4S.

23. A bit of my experiments Possible Connection? In order to match expansion /CP with contraction /CF, we created the contraction sequence first, then get the expansion simply by reversing the frame order of contraction. This guarantees that the two patterns are completely matched in properties, such as dot density near the perceived border line. E1: o ← Vs o → CP Vs CF Unsignificant result of perceived border shift.

24. A bit of my experiments Possible Connection? E2: o ← -- Vs o → -- CP+S Vs CF+S Significant result in the direction of forward movement. It seems adding stationery point facilitates the forward movement.

25. A bit of my experiments Possible Connection? E3: o ← → Vs o → ← EXP (CP-CF) Vs CON(CF-CP) Significant result, but opposite to the direction of forward movement. There must exist a very strong mechanism who can reverse the whole pattern, which favor forward movement direciton. What is it ?

26. A bit of my experiments Possible Connection? E3: o ← → Vs o → ← EXP Vs CON According to Whitney’s finding, the trailing effect should be in the same direction with the forward movement. Unexpected Result: the trailing effect NOT been strengthened (increasing the Forward Movement Effect) or neutralized (only Forward Movement Effect). Section Width of left/right to border line: E3: ←─ > → Vs ─→ > ← Left S > Right S in width (Forward Movement Effect)

27. A bit of my experiments Other Possible Explanation? E3: o ← → Vs o → ← EXP Vs CON Noisiness hypothesis: the perceived border line of CON seems more noisy than that of EXP. Maybe not, if so, E2 should has similar effect. Adding stationery points seems facilitate the forward movement instead of reducing it. Contrast reduction hypothesis: the contrast of CON border line is reduced compared with that of EXP. Maybe not, if so, the contrast reduction should facilitate the forward movement direction (according to Whitney’s result) or make the threshold different between EXP/CON (but not).

28. A bit of my experiments E3: o ← → Vs o → ← EXP Vs CON Other Possible Explanation? Anisotropic sensitivity to motion direction: Vision system is more sensitive to CentriPetal rather than CentriFugal motion direction. CP > CF Experimental evidences: By measuring coherence threshold using a signal-to-noise technique, Raymond, 1994 found that sensitivity to CP moving stimuli was significantly better than to CF moving stimuli. Dyre & Andersen, 1997,found that perceived heading would drift toward the relatively weaker hemi-field when there is an imbalance of image velocity between the two hemi-fields of an optic flow pattern. o ← → (CP > CF, drift to Right) o → ← (CF < CP, drift to Left)

29. Discussion Any other possible explanation to the fMRI result besides trailing effect? Patterns did not contain exp/conoptic flow, but it does contain Centripetal – Centrifugal motion dimension. Trailing edge= CP Leading edge= CF Maybe CP>CF be possible candidate?

30. Discussion Sub regions’ preference to different motion type ? Fig 3 (D) Activation produced by the flickering patterns alone. The activity in (A) overlapped in the flickering condition. (E) Event-related average for the entire ROI. Although no difference in the event-related activity for the three conditions (inward motion, outward motion, and flickering) for this ROI as a whole, but it did not ruin out the possibility that sub-regions in this area have preference to CP/CF motion.