The retinal origin: On the independence of retinal loci targeted by saccades, pursuits and fixation and their relationship to the retinal point of perceived fixation

Introduction. In representations of the visual system, the fovea is usually represented as the origin, simply because the fovea with its anatomical specializations would make it the retinal area that is mainly used to detect and discriminate the fine structure present in the visual world. Recently we have been given reason to question this assumption, It was shown by Putnam et al. (2005) that when subjects were asked to indicate that they were looking at the center of a stimulus, the center fell on a retinal location that is displaced from the foveal area with the highest cone density by an average of 10 arc minutes, indicating a degree of independence between retinal anatomy and the retinal substrate that is responsible for the perception of "I am looking at the target". This raise questions regarding the independence of other visual tasks and the underlying retinal architecture. In this study the following research questions will be addressed: (1) Are the retinal locations targeted by fixation, saccadic and pursuit eye movements independent of each other? (2) Are the retinal locations targeted by fixation independent of the retinal locations that are responsible for the perception of "I am looking at the target"? (3) Do the retinal locations targeted by saccades change with change in resolution demands? (4) Do local regions in the retina have unique saccadic targets in the central fovea?;Methods. We used a 1st Generation Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) in all our experiments to measure eye movements, present stimuli and capture videos of the retina concurrently. In our experiments the imaging field was set to 2 degrees. To extract eye movements from the captured videos we used a registration algorithm that was based on a cross-correlation engine. Four subjects were used in most of our experiments.;Results. The spread of targeted retinal locations by the fixation, saccadic and pursuits eye movement systems ranged from 4.6 to 7.8 arc minutes, 8.0 to 17.1 arc minutes, 5.9 to 13.4 arc minutes respectively. The pythagorean distances between the mean location targeted by the different eye movement systems ranged from 0.84 arc minutes to 17.34 arc minutes. All differences were statistically significant except for the distance between the mean location targeted by fixation and saccades for one subject. The spread of the retinal location targeted by fixation is smaller than the spread of retinal locations responsible for perception of "I am looking at the target" by approximately a factor of 0.4 on average, and the mean retinal locations are separated by an average of approximately 4 arc minutes. We also found the fixation locus shifts over a period of time but is always encompassed by the locus of perceived straight ahead. We did not find any statistically significant differences in mean retinal location targeted by saccades to targets that have different resolution requirements. Two of the four subjects tested displayed saccadic behavior that suggested that localized distortions were present in their saccadic targeting system.;Conclusions. (1) The targeting systems that serve the fixation, saccadic and pursuit eye movement systems are unique and independent of each other. (2) The retinal location targeted by the fixation system constantly shifts but is always maintained within the retinal region that is responsible for perception of "I am looking at the target". (3) Visual resolution demands do not alter saccadic targeting. 4. The retinotopy of saccadic targeting is idiosyncratic and most likely a reflection of past oculomotor experience. (Abstract shortened by UMI.).