Retinal locus for fixation and identification

Retinal locus for fixation/identification are defined as the retinal locations used to fixate/identify temporally or spatially different objects. I determined the characteristics of the retinal locus for fixation/identification by using a scanning laser ophthalmoscope (Rodenstock) to display objects on the retina with a 5 × 5 grid (16 × 15). To determine retinal locus for fixation, each object (0.25 circle, 1 circle, 1 crosshair with 6 ft. bars) was first presented in the center grid position and then randomly presented (without replacement) in an eccentric grid position (giving 24 central and 24 eccentric fixations for each object). To determine retinal locus for identification, a randomly rotated C (0.25) was randomly presented (without replacement) at eccentric grid locations and observers determined the orientation (up, down, right, or left). Data were obtained from three experienced psychophysical observers and three naive observers. The results show that the retinal locus for fixation is nearly constant for different shapes and sizes of fixation objects. However, the retinal locus for fixation is larger for eccentric grid locations compared to central grid locations. The retinal locus for identification is larger than either the central or eccentric retinal locus for fixation. The implications for eye tracker calibration and perimetry will be discussed.