The College of Optometrists

           

Seeing involves making inferences about the visual world.  For example, we are not aware of the blind spot during monocular viewing.  This phenomenon is termed visual filling-in or completion.

  

Visual filling-in comprises a number of perceptual completion phenomena including amodal, modal, blind spot and scotomal completion.  Each of these phenomena demonstrates that higher cognitive processes strongly influence our visual perception.  In each case, subjects report that something is present in a particular region of visual space when it is actually absent from that region.   Our research is focused on determining the relationship between blind spot and scotomal completion and establishing the theoretical framework for the existence and application of these phenomena.  Stimuli were generated on a monochromatic high-resolution display (37° x 29°).                                                   

 

Three experiments were carried out:

 

Blind spot: The size, shape and position of the blind spot were determined using detection thresholds.  Visual completion of this region of the visual field was the examined using local (line) and global (noise) stimuli.  Secondary stimuli (distracters) were introduced to determine their effect on visual completion. 

                       

Artificial scotoma: A selective area of a global noise stimulus was removed and the time to completion was recorded both without and with distracters.

                       

Pathological scotoma: The size, shape and position of the scotomas were determined using detection thresholds.  Visual completion was examined using noise stimuli both without and with distracters. 

 

Conclusion:

 

We conclude that distracters strongly affect the perception of completion. This concept may explain why physiological (blind spot) and some pathological scotoma are not normally reported.  Practical implications for these phenomena need to be considered during multiple stimuli visual field screening.