The oblique effect has been characterized by Appelle (1972) as a small but consistent superiority in performance when visual stimuli are horizontal or vertical, as opposed to oblique (p. 266). The oblique effect shows itself perceptually in lowered acuity, reduced contrast sensitivity, reduced increment sensitivity (Rentschler & Fiorentini, 1974) in response to oblique stimuli. These behavioral studies are coincided by evidence that cortical orientation detectors are less narrowly tuned to oblique lines than to vertical and horizontal lines (Rose & Blakemore, 1974). As shown by Petersik and Pantle (1982), participants take longer to mirror-trace a line when using an oblique stimulus than when using a horizontal/vertical stimulus. The goal of this study is to see if the oblique effect occurs in reversed right-left range of motions as compared to reversed up-down range of motions. The hypothesis is that the oblique effect will induce the participants to trace at a slower rate in the reversed right-left range of motions experimental group as compared to the reversed up-down range of motions group.
Figure 1. The horizontal-vertical stimulus on the left and the oblique stimulus to the right of the it demonstrates what the subjects have to trace.
Methods
Subjects were 32 undergraduate students enrolled at Alma College. Their ages ranged from 19 to 22 years. There was an even ratio of males to females. The first group (HV-UD) had the up-down directions reversed and were tested on the horizontal-vertical (HV) stimulus for the experiment. The second group (HV RL) had the right-left directions reversed and were tested on the HV stimulus. The third group (OB-UD) had the up-down directions reversed, but were tested on the oblique (OB) stimulus. The fourth and final group (OB-RL) had the right-left directions reversed, and were tested on the OB stimulus. Then the participants were instructed to trace a line as quickly as they were capable of between the boxes (with the mouse tool) from the starting point to the finish. Participants ran through the experiment in pairs. Each participant ran through 8 trials, alternating with the subject he/she was paired with.
Results
Figure 2 shows the means of all four stimuli. As predicted by our hypothesis, the HV-UD displayed the fastest means. The second fastest means were shown by the HV-RL. The third fastest means were the OB UD, but it intertwined with the HV-RL stimuli. And lastly, the slowest timed means were the OB-RL stimulus.
Figure 2. The mean times of all four stimuli with the OB-RL stimulus showing the greatest improvement.
Also, Figure 3 shows the mean of the means for each stimuli. As predicted, the HV-UD proved to be the fastest mean (26.8 seconds). The second fastest was the HV-RL (41.7 seconds). The third fastest was the OB-UD (43.8 seconds) and the slowest mean was the OB-RL (60.1 seconds). Also, the mean of the mean for the OB-RL stimulus showed the greatest variance.
Figure 3. The mean of the mean times for all four stimuli. The OB-RL stimulus showed the greatest variance.
Discussion
The present experiment has shown the perceptual-motor consequences of the oblique effect to be especially crucial in mirror drawing tasks. This is especially true when tracing the oblique stimuli. However, as the data shows, tracing performances improves with practice. It took more time for the participants to trace oblique stimuli than horizontal-vertical stimuli.
The right-left reversed movements seemed to be more difficult than the up-down reversed movements. The oblique right-left stimulus was the most difficult to trace because as the participant traced the reversed right-left stimulus, he/she had to trace without the up-down direction reversed.
A small number of our participants seemed to be uneffected by the oblique effect, especially the obique up-down stimulus. Many of these participants in doing the oblique up-down stimulus, matched the times with the horizontal right-left stimulus. Also, because of this immunity to the oblique effect combined with negation of the up-down reversed direction while tracing the oblique right-left stimulus, subjects showed the greatest variability in times recorded. This should inspire future research.
References
Appelle, S. Percepttion and discrimination as a function of stimulus orientation: The oblique effect in man and animals. Psychological Bulletin, 1972, 78, 266-278. As cited in Petersik, J. & Pantle, A. The oblique effect in a mirror-tracing task. Bulletin of the Psychonomic Society, 1982, 20(2), 69-71.
Rentschler, I., & Fiorentini, A. Meridional anisotropy of psychophysical spatial interactions. Vision Research, 1974, 14, 1467-1473. As cited in Petersik, J. & Pantle, A. The oblique effect in a mirror-tracing task. Bulletin of the Psychonomic Society, 1982, 20(2), 69-71.
Rose, D., & Blakemore, C. An analysis of orientation selectivity in the catŐs visual cortex. Experimental Brain Research, 1974, 20, 1- 17. As cited in Petersik, J. & Pantle, A. The oblique effect in a mirror-tracing task. Bulletin of the Psychonomic Society, 1982, 20(2), 69-71.
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