Oblique Effect and the Organization of Visual Information

 

fMRI and behavioral measurements of an oblique effect in human striate cortex                                                                                                   (Reference: http://www.nature.com/neuro/journal/v3/n6/fig_tab/nn0600_535_F1.html)   

3. How can we use the oblique effect to study the organization of visual information? 

The oblique effect is a visual defect in which vertical and horizontal lines are more easily detected or identified quicker than lines that are oriented obliquely.  The oblique effect is seen in most humans; however, some experience the reverse of the oblique effect and still others fail to experience it at all.  Those who do not experience the oblique effect correctly usually have some degree of astigmatism.  In certain cases in which the oblique orientation is preferred for vision, this preference still continues even after the astigmatism is correct.  As a result, all orientations appear equally clear on the retina, a condition known as meridional amblyopia.  This shows that even prior to correction the brain was oriented in a way that it preferred oblique over horizontal or vertical lines.  Animal studies have shown that in the brains of animals that prefer oblique lines, that the brain was altered in a way to favor such a preference.  The cortical cells in these animals favor orientations that are centered around the one that is the clearly focused, while fewer cells respond to the unclear astigmatic orientation.  This shows that biased individual experience can change the orientation preferences of cortical cells and it is further theorized that people with meridional amblyopia have less cortical cells tuned to the formerly blurred orientation. 

            Furthermore, there are two theories to explain why vertical and horizontal orientation is more highly favored.  The first theory believes that we have developed this preference because of the carpenter environment we live in in which vertical and horizontal contours are abundant.  Because of this predisposed visual exposure, orientation preferences among cortical cells in the brain are influenced.  Another theory assumes that the neural basis for vertical and horizontal preference can be attributed to genetic factors that prefer the developmental of cortical cells to favor such orientations.