Sensation and Perception Presentation

1. Sensation and Perception Presentation “Human Orientation Discrimination Tested with Long Stimuli” Guy A. Orban, Erik Vandenbussche, and Rufin Vogels 1984

2. Question Are S cells of area 17 (V1) involved in orientation discrimination?

3. Alternatives YES, S cells are involved in orientation discrimination. NO, S cells are not involved in orientation discrimination.

4. Logic Prediction 1 Long stimuli should have better discrimination for orientations within 15° from a principal meridian. Equally worse outside of the 15° range. Why? Because the there are more S cells tuned for those orientations than for the others.

5. Logic Prediction 2 Because of the length summation of S cells, the meridional variations should be much stronger for long lines than for short lines.

6. Methods Stimuli Subjects were presented with an oriented fluorescent rod inside an otherwise dark box at a viewing distance of the standard 57 cm. Stimuli were presented briefly and successively with a short gap between them. Subjects had to respond within an allotted time. Subjects Some male and female students between 20 and 25 years of age. Some of the subjects were naïve about the experiment and some were not. They all had normal or corrected vision.

7. Methods Method of Constant Stimuli Used to determine the JND in orientation at a given reference orientation. Subjects were supposed to press a button to indicate whether the stimulus was clockwise or counterclockwise from the reference orientation. Each orientation was presented randomly 64 times. JND was based on 320 responses.

8. Results JNDs in orientation are much smaller at principal orientations (horizontal or vertical) than at oblique orientations. (Fig. 1) (Replication) JND in orientation increases as a function of obliquity from the principal orientation up to 20° obliquity and then levels off. (Fig. 3) (Prediction 1) Long stimuli give a strong oblique effect in orientation acuity whereas short stimuli give a weaker oblique effect. In other words, the oblique effect increases as line length increases. (Fig. 4) (Prediction 2)

9. Figure 1 Distribution of JNDs in orientation at the four main reference orientations (horizontal, vertical, left oblique, right oblique).

10. Results JNDs in orientation are much smaller at principal orientations (horizontal or vertical) than at oblique orientations. (Fig. 1) (Replication) JND in orientation increases as a function of obliquity from the principal orientation up to 20° obliquity and then levels off. (Fig. 3) (Prediction 1) Long stimuli give a strong oblique effect in orientation acuity whereas short stimuli give a weaker oblique effect. In other words, the oblique effect increases as line length increases. (Fig. 4) (Prediction 2)

11. Figure 3 JNDs in orientations plotted as a function of obliquity. (A) 3 subjects (B) 5 subjects

12. Results JNDs in orientation are much smaller at principal orientations (horizontal or vertical) than at oblique orientations. (Fig. 1) (Replication) JND in orientation increases as a function of obliquity from the principal orientation up to 20° obliquity and then levels off. (Fig. 3) (Prediction 1) Long stimuli give a strong oblique effect in orientation acuity whereas short stimuli give a weaker oblique effect. In other words, the oblique effect increases as line length increases. (Fig. 4) (Prediction 2)

13. Figure 4 (A) JNDs in orientations plotted as a function of stimulus length. (B) Mean oblique effect index plotted as a function of stimulus length for 4 subjects.

14. Other explanations for these results? Orientations correspond to different endpoints. Could it be a positional instead of an orientational oblique effect?  Replicate the experiment by just showing the endpoints. Is the effect retinal or gravitational? Replicate the experiment where subjects are tilted 20°.

15. Results of Control Studies Position discrimination is much worse than orientation discrimination and shows no meridional variation. (Fig. 5) The meridional variation in JNDs in orientation follows retinal and not gravitational coordinates. (Fig. 6)

16. Figure 5 JNDs in orientations for lines (open symbols) and equivalent differential thresholds for dots placed at the end of an invisible rod of the same length (solid symbols) at the 4 main orientations.

17. Results of Control Studies Position discrimination is much worse than orientation discrimination and shows no meridional variation. (Fig. 5) The meridional variation in JNDs in orientation follows retinal and not gravitational coordinates. (Fig. 6)

18. Figure 6 JNDs in orientations plotted as a function of gravitational reference orientation for: (A) subject R.V. in an untilted position (continuous line) and a tilted 20° clockwise position (stippled lines). (B) subject M.D.W. in an untilted position and a tilted 20° counterclockwise position.

19. Interpretation Both predictions came true, both control experiments did not threaten the conclusions.  Cells similar to area 17 cells of the cat and monkey are involved in orientation discrimination for long lines. Of course, such a coarse correspondence between prediction and results does not rule out other alternative explanations.

20. Problems Logic flawed (psychophysics in people, physiology in animals). Not much contrast between the rod and the background, so subjects may have had a difficult time seeing the stimulus. Only 6 subjects. Some of them were the authors. Memory effects cloud clarity of conclusions. No control of eye movements.