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 3. Various studies performed in the field of ophthalmological surgery, especially studies involving surgeries of the anterior eye segment, have attempted to lessen induced cornea astigmatism 4567. Furthermore, both regular astigmatism 891011 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 1516, analysis of the form of the whole cornea by corneal videokeratoscopy 171819, and measurement of the total ocular profile of refraction by wavefront analysis 202122. 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.

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.

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 parametric mapping (SPM99; Wellcome Department of Cognitive Neurology, London, UK). After uniting the position of each image, all volumes were realigned to the first volume. Spatial smoothing of the functional signal within each slice was performed prior to the analysis. We then defined a design matrix that consisted of contrast modeling of the alternating periods of ‘baseline’ and ‘activation’ when using a delayed boxcar reference vector 27 that accounted for the delayed cerebral hemodynamic response function after the stimulus presentation. Data were first analyzed for neural activations that were common to the experimental conditions relative to the baseline. To determine the statistical significance, we used a corrected significance level of the individual voxel of P < 0.005 and a cluster size of 26, with an AlphaSim-corrected cluster threshold of P < 0.05.

Several studies have investigated both the influence of astigmatism on vision, and the evaluation methods used to examine astigmatism 2829. However, these studies only examined the influence and the evaluation methods in the anterior and posterior eye segments. In the current study, we investigated astigmatism influence on the visual pathway located beyond the posterior eye segment.

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 2526. This method calculates brain activity, by measuring the induction of regional cerebral blood flow changes. Although there are several different ways that the fMRI measurements can be made 303132, we used the blood oxygenation level dependent (BOLD) method 30 in our study. This method determines the hemodynamic changes associated with the variations in the amount of blood flow that accompany the brain activity, which is partly based on the phenomenon of magnetic resonance. The amount or speed of the inflow of blood to a peripheral blood vessel floor increases in conjunction with the activity of cerebral cortex is anaerobic, there is a fall in the ratio of the deoxy hemoglobin / oxy hemoglobin relative to the blood vessel floor 33. In the human body, deoxy hemoglobin is a magnetic substance, and thus, is capable of creating a detectable magnetic resonance signal. The BOLD fMRI method measures the rise and fall of this magnetic resonance signal, and thus, does not require the injection of a contrast media.. Since this method can be quickly performed and used to simultaneously measure the entire brain, there has been a rapid increase in the use of fMRI in many different types of studies

When a seeing on subject with WTR astigmatism views an object at a long distance, the image will be distorted perpendicularly, while a subject with ATR astigmatism will find the image to be distorted horizontally 34. In the current study, stimulation 1 was designed so that the subject would experience WTR and ATR astigmatism. we used stimulation 1. To achieve this stimulation, subjects were simultaneously presented with an image that contained the same quantity of vertical and horizontal lines. In a previous study that examined the developmental origins of visual perception, visual circuity involved with the processing of vertical and horizontal contours was found to be important in the enhanced visual capabilities of humans 35. Furthermore, since these contours and conditions are found throughout an image, our stimulations were designed to be circular in shape. The stimulation used in condition 1 resulted in a higher amplitude of vascular response as compared to that seen in conditions 2 and 3. It has been suggested that both WTR and ATR astigmatism can adversely influence the primary visual cortex. In conditions 2 and 3, activation led to a response in dorsal visual pathway. The dorsal visual pathway carries vision information going to the parietal association area from area V1. This pathway is roughly divided into two, with one section going to area 7 via the medial superior temporal area (MST) 36,37, ventral intraparietal area (VIP) 38, middle temporal area (MT) 39,40, etc. This pathway is involved in the analysis of visual motion. The other pathway goes to the lateral intraparietal area (LIP) or the caudal intraparietal sulcus area (cIPS) via the V3A and PO. areas. It is thought that this pathway is related to the objective position or the information processing of the three-dimensional form 4142. However, the exact nature of this pathway has yet to be fully understood. The activation response in the dorsal visual pathway observed for conditions 2 and 3 suggests that astigmatism can influence activation whithin the dorsal visual pathway. However, the question of what the exact nature of this influence really is remains to be answered. If the direction of the line of the stimuli and the axis of the astigmatism are in agreement, then the line will be extended in this direction as it is projected on the retina. In contrast, when the directions of the line of the stimuli and the axis of the astigmatism differ, the line fades as it is projected on the retina. Thus, this suggests that this phenomenon is the visual factor responsible for generating noise on the retina can be considered. When stimulation 1 was presented through the range that resulted in activation, the response for condition 3 was larger than that for condition 2. However, there was no difference between stimulation 2 for condition 2 and stimulation 3 for condition 3. Moreover, the large difference seen between conditions 2 and 3 was not observed between stimulation 2 with condition 3 and stimulation 3 with condition 2. This suggests that ATR astigmatism may have a greater influence on the visual noise as compared to the WTR astigmatism.

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 47. 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 444546 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 474849. Irregular astigmatism is commonly treated by laser in situ keratomileusis (LASIK), using the wavefront technique 1214. 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.