# Parafoveal and Perifoveal Accommodation Response to Defocus Changes Induced by a Tunable Lens
## 1 Introduction  
Accommodation is the process of change in the optical power of the eye to compensate for defocus induced by different viewing distances. The amplitude of accommodation reduces with age and becomes almost zero at around 52 years of age [1], although some studies have found that this reduction can be extended up to an age of 60 or 70 years [1; 2; 3]. The accommodative response also depends on stimulus properties, such as spatial frequency content [4] and the location in the visual field [5]. In this study, we investigate the ability of the human eye to accommodate to annular stimuli presented in the parafoveal and perifoveal visual field.  
[PERSON] concluded that foveal cones are responsible for accommodation, i.e., only when the luminance equals the cone threshold of visibility will the accommodative reflex be triggered [6]. In the case of parafoveal vision, [PERSON] reported that observers were unable to accommodate to a white spot of light when its width exceeded 10 arcmin and the monocular viewing angle was more than 10 arcmin away from the spot [7]. Meanwhile, [PERSON] and [PERSON] measured parafoveal accommodative convergence response while viewing a 6 arcmin dark target at eccentricities up to 6\({}^{\circ}\) and found an approximately linear drop-off in response with eccentricity [8]. Similarly, [PERSON] and [PERSON] measured parafoveal accommodation by varying the size of a dark disc on a bright background and instructing observers to view the contour of the circle as sharp as possible at eccentricities of 1\({}^{\circ}\), 7\({}^{\circ}\), 15\({}^{\circ}\) and 30\({}^{\circ}\)[9]. They concluded that peripheral vision evokes accommodative responses, albeit with reduced magnitude. The same conclusion was reached by [PERSON], [PERSON] and [PERSON] when measuring the peripheral accommodative response in myopesand emmetropes [10]. When both foveal and parafoveal accommodative cues are present, a compromise in response has been reported [11; 12].  
The question of parafoveal accommodation is not only of fundamental interest for research but it is also directly relevant for subjects affected by central vision loss who rely partially or entirely on parafoveal vision and vision aids [13; 14]. Furthermore, peripheral optics are believed to play a role in emmetropization. The image on the peripheral retina depends on the refractive errors, the state of accommodation, and environmental factors, since the surroundings are not dioptrically uniform [15; 16]. One such environmental factor is an increase in indoor activities, which has been found to be a major reason for myopia prevalence in the young population [17]. So, understanding the sensitivity to defocus at different retinal eccentricities, and the corresponding effect on accommodation in various settings, may provide important insights into the mechanisms which stimulate emmetropization and myopia [18].  
We recently introduced an automated method to study foveal-driven accommodation using a current-driven tunable lens (TL) in a conjugated pupil plane to introduce a random sequence of defocus step changes within the accommodative range of each observer [19]. Randomness is used to prevent subjects from learning the required response that can otherwise mask the desired signal [20]. We expand on our earlier study and report here on measurements of the accommodative response across the macula from the fovea and parafovea to the perifovea for young observers to green-colored annular stimuli with visual angle radii from 1\({}^{\circ}\) to 8\({}^{\circ}\). Other studies have used other smaller targets to examine the peripheral retina [9; 12]. They have the advantage of including higher spatial frequencies but are localized to just a small area of the retina at any time. In turn, the use of annular targets benefits from including all equidistant points from the fovea simultaneously.