Visual Neuroscience



PERIPHERAL VISUAL FIELD

The gap effect is exaggerated in parafovea


MARINA  DANILOVA  a1 c1 and JOHN  MOLLON  a2
a1 I.P. Pavlov Institute of Physiology, Laboratory of Visual Physiology, St. Petersburg, Russia
a2 Department of Experimental Psychology, Cambridge University, Cambridge, United Kingdom

Article author query
danilova m   [Google Scholar] 
mollon j   [Google Scholar] 
 

Abstract

In central vision, the discrimination of colors lying on a tritan line is improved if a small gap is introduced between the two stimulus fields. Boynton et al. (1977) called this a “positive gap effect.” They found that the effect was weak or absent for discriminations based on the ratio of the signals of long-wave and middle-wave cones; and even for tritan stimuli, the gap effect was weakened when forced choice or brief durations were used. We here describe measurements of the gap effect in the parafovea. The stimuli were 1 deg of visual angle in width and were centered on an imaginary circle of radius 5 deg. They were brief (100 ms), and thresholds were measured with a spatial two-alternative forced choice. Under these conditions we find a clear gap effect, which is of similar magnitude for both the cardinal chromatic axes. It may be a chromatic analog of the crowding effect observed for parafoveal perception of form.

(Received September 7 2005)
(Accepted January 27 2005)


Key Words: Color vision; Chromatic discrimination; Gap effect; Proximity factor; Periphery; Crowding.

Correspondence:
c1 Address correspondence and reprint requests to: Marina Danilova, Pavlov Institute of Physiology, Nab. Makarova 6, St. Petersburg 199034, Russia. E-mail: dan@pavlov.infran.ru