Influence of Regular Astigmatism on the Human Visual Cortex . A Functional Magnetic Resonance Imaging Study

Purpose: To describe a new functional magnetic resonance imaging (fMRI) method for measuring the influence of regular astigmatism, both against-the-rule (ATR) and with-the-rule (WTR), on the human visual cortex. Setting: Department of Ophthalmology, Jikei University School of Medicine, Tokyo, Japan. Design: Experimental study. Methods: Images were acquired in two healthy volunteers using a 1.5 T scanner equipped for echo planar imaging. Horizontal and vertical sine wave grating flickering at a frequency of 8 Hz were simultaneously presented during the 20-second stimulation period. During the control period , subjects fixated on a control target. Stimulations were performed under three different conditions that included with hard contact lenses that were equal to emmetropia without astigmatism (condition 1); with hard contact lenses and cylindrical glasses of +6.00D at 0°, imitating WTR (condition 2); and with hard contact lenses and cylindrical glasses of +6. 00 D at 90°, imitating ATR (condition 3). Raw data were processed using in-house software with the significance of activation determined by Statistical Parametric Mapping (SPM 99). Results: Although higher activation was found in the primary visual cortex for condition 1 versus conditions 2 and 3, activation in the dorsal pathway was higher in conditions 2 and 3 compared to condition 1. Dorsal pathway activation was also higher in condition 3 versus condition 2. Conclusions: Study findings showed the potential influence of ATR and WTR on the human visual cortex , with fMRI able to detect the influence of regular astigmatism on the visual cortex. Our current results suggest that fMRI may be useful in exploring the influence of astigmatism on vision. DOI : 10.14302/issn.2470-0436.jos-14-571 Corresponding author: Shiba Takuya, MD, PhD, 3-19-18 Nishishinbashi, Minato-ku, Tokyo, 105-8461, Japan. Phone: +81 3 3433 1111, Fax +81 3 3433 1936, E-mail: shiba@jikei.ac.jp


Introduction
Unlike in a normal eye that focuses rays of light on a single point, the astigmatic eye refracts two focal lines separated from each other by a focal interval (1).
Furthermore, astigmatism is a typical refractive anomaly that can be roughly divided into regular astigmatism and irregular astigmatism. While regular astigmatism can be fully corrected by using a spectacle lens (2), irregular astigmatism is interrupted by a variety of alterations of the refractive corneal surface, thereby presenting different degrees of the astigmatism Furthermore, both regular astigmatism (8)(9)(10)(11) and irregular astigmatism have recently become more treatable due to the progress that has been made in refractive surgery (12)(13)(14). The better medical treatments have also greatly contributed to achieving better diagnoses. For example, analysis of the radius of curvature and refractive power can be determined by kerato-refractometer (15)(16), analysis of the form of the whole cornea by corneal videokeratoscopy (17)(18)(19), and measurement of the total ocular profile of refraction by wavefront analysis (20)(21)(22). However, the influence of astigmatism on the visual pathway has yet to be fully investigated. Several studies have recently reported using functional magnetic resonance imaging (fMRI) to diagnose the visual pathway (23)(24). fMRI is a noninvasive method that can be used for human brain activity imaging (25)(26). We used this methodology to investigate the influence of regular astigmatism on the visual cortex in addition to trying to determine the influence of astigmatism on the visual pathway.

Subjects
Two normal healthy male volunteers (aged 27, 34 years) gave informed consent prior enrollment in the study. Both participants were right-handed, and had no history, past or current, of ophthalmologic, neurological, or psychiatric illness. Measurements were performed 6 times in each subject.

Device for Refractive Correction and Simulation
The left eye of both subjects was covered with an eyepatch. In the right eye, each subject wore a hard contact lens, which was set to emmetropia. Moreover, +6.00 diopter (D) glasses were positioned at two different angles, one at 0 degrees and the other at 90 degrees. These three different conditions were used to create emmetropia (condition 1), with-the-rule (WTR) astigmatism (condition 2), and against-the-rule (ATR) astigmatism (condition 3), respectively.

Visual Stimulation
Visual stimuli were back-projected onto a translucent screen placed at the subjects' feet. Subjects viewed the visual stimuli using the built-in mirror of a standard head coil. Stimuli were created using in-house software.
The three types of visual stimulation used during the experiment are described below.

Stimulation 1
During the 20 second stimulation period, horizontal and  1). This phase was defined as 'activation'.
During the control period, the subjects fixated on the control target. This target was a 50% grey circle of 9 degrees, with the fixation point located at the center (FIGURE 1). This phase was defined as the 'baseline'.

Data Analysis
After acquisition, images were converted from the native GE or Siemens format to the format used to analyze the data in the current study. After the image processing steps, data analysis was performed using statistical

Stimulation 1
Remarkably consistent activations were detected bilaterally for each condition in the primary visual cortex Recent studies have reported using the fMRI cerebral functional appraisal method to evaluate the influence of regular astigmatism on the visual cortex. For more than ten years, fMRI has been used to evaluate cerebral function (25)(26). This method calculates brain activity, by measuring the induction of regional cerebral blood In studies of ophthalmological surgeries, especially surgeries of the anterior eye segment, there have been several attempts to find ways to lessen induced cornea astigmatism if possible have been attempted (4-7). One study reported that a superior incision induced ATR astigmatism (43). Although the position of the incision during cataract surgery can be due to a variety of reasons (44)(45)(46), the overall outcomes seen in these cases have led to the creation of a defined protocol when performing these surgeries in the general population. Moreover, although there have been many reported cases of WTR astigmatism in young men, there have been other studies that have found that aging can cause a large number of people to develop ATR astigmatism (47)(48)(49). Irregular astigmatism is commonly treated by laser in situ keratomileusis (LASIK), using the wavefront technique (12)(13)(14). After this medical treatment, the cornea temporarily becomes a perfect surface of a sphere. However, a large number of corneas could change to ATR astigmatism with aging.
Thus, it will be necessary to closely follow these subjects in order to determine the validity of this hypothesis in the future.
Ophthalmologists need to pay close attention to basic fundamental abnormalities, such as WTR and ATR astigmatism, in order to ensure patients receive the best possible ophthalmological care.