A retrospective, ex post facto study was performed using a repeated measures design. All participants were subjected to the same randomly presented spatial frequencies. The design was formed by two independent variables (spatial frequencies and group) and one dependent variable (threshold or contrast sensitivity).
Fifty volunteers participated in this study. These volunteers were selected for their accessibility and age (between 40 and 65
years old). Forty of the volunteer patients had suffered a stroke (the Experimental Group, EG, (M
0.65)), and 10 were healthy volunteers (the Control Group, CG, (M
0.66)). The patients were divided into four subgroups, each with 10 participants, according to their level of functional independence, which was determined based on the Barthel Index: Experimental Group A (EGA), 20–35 points, serious dependence; Experimental Group B (EGB), 40–55 points, moderate dependence; Experimental Group C (EGC), 60–95 points, mild dependence; and Experimental Group D (EGD), 100 points, independence
]. There were 5 women and 5 men in each group (CG, EGA, EGB, EGC, and EGD). Table
presents the demographic and clinical data of the participants.
Personal data and neuropsychological assessment (Mean±SD) arranged by group
The inclusion criteria adopted in the study included diagnosis of non-recurring unilateral ischemic stroke, acute stage (occurring at least one month after the vascular event), having injuries to the middle cerebral artery and normal or corrected visual acuity. Data were obtained from medical records and Functional Resonance Magnetic Imaging (fMRI). Participants in the Control Group were healthy individuals who were accompanying the patients or working in the institution where the experiment was performed. The pathology diagnosis was performed based on ICD-10 (International Statistical Classification of Diseases and Related Health Problems).
Exclusion criteria were hemorrhagic stroke, recurring, extensive cerebral lesion, incapability of completing the interview and assessment due to serious aphasia, psychiatric dysfunctions, ocular diseases, unconsciousness or use of drugs that modulate activity of the central nervous system, serious functional independence, with total dependence corresponding to a score lower than 15 on the Barthel Index.
All participants underwent prior ophthalmologic assessment, which included anamnesis and ophthalmological examination. The examination included: determination of visual acuity, external examination, anterior segment biomicroscopy, tonometry, gonioscopy, fundus examination and determination of biometric eye data, such as corneal curvature (keratometry), axial diameter (biometry), corneal thickness (pachymetry) and corneal diameter.
Only participants with visual acuity or corrected acuity, as analyzed by the Snellen chart, and who were not suffering from eye diseases were admitted to the study. This exclusion reduces the possibilty of creating bias in the comparison, because variables such as degree of stromal hydration, presence of cataracts, glaucoma, or even changes in intraocular pressure can have a very significant effect on visual function and may affect interpretation of the data. There were no changes in the visual field of the participants. The presence of hemineglect was evaluated using the Letter Cancellation Test (p>0.05).
presents the average ocular biometric data; based on t-tests, there were no significant differences between healthy subjects and patients.
Differences between the means of groups (Control and Experimental) in relation to axial diameter, corneal diameter, central corneal thickness and keratometry
Functional dependence was assessed based on the Barthel Index. This index measures the level of assistance demanded by an individual in 10 items of Activities of Daily Living (ADL), which involve mobility and personal care. The levels of measurement range from complete independence to the need for assistance. Each item of performance is assessed on an ordinal scale, and a specific number of points is marked for each level or classification. Different measures were established for each item, based on clinical judgment and other implicit criteria. Example scores include: 0 (dependent), 5 (need for help or supervision), 10 (partially dependent) and 15 (independent). The latter score is considered only for chair and bed transference or ambulation. In total, there are 100 possible points that define the level of dependence of a person (0 – 15
total dependence; 20 – 35
serious dependence; 40–55
moderate dependence; 60 – 95
mild dependence; and 100
independent). This scale, which was developed for assessing the level of dependence in elderly patients or those who have serious sequels, for instance stroke, has been used since 1965, and its use is recommended by the World Health Organization (WHO) because it is easy to apply and can be adapted to different cultures
For the Control Group, a Cumulative Illness Research Scale (CIRS) was applied to guarantee the participation of healthy individuals in this group. This scale investigates the presence of 14 disease sets (cardiac, vascular, hematological, respiratory, ocular, upper and lower gastrointestinal tract, hepatic and pancreatic, renal, genitourinary, musculoskeletal and integumental, neurologic, endocrine-metabolic, breast and psychiatric), taking into consideration situations in which each set of diseases is absent, mild, moderate, severe or extremely severe, with scores ranging from 0 to 4, respectively
The participants were informed about the study protocol and the objective of the experiment. Subjects signed informed consent forms according to Resolution n° 196/96 of the Brazilian National Health Council (Health Ministry, Brazil), which determines guidelines for research involving human beings in compliance with the Declaration of Helsinki. The local Ethics Committee approved this research (Protocol # 249/09).
Equipment and stimuli
The stimuli were set to appear in the center of a 19-inch video monitor (LG) CRT (Cathode Ray Tube) with high resolution (1024 x 768) and a 70-Hz frame rate. Input was controlled by a microcomputer through a video board with VGA and DVI connectors. The voltage luminance of the monitor was expanded from 8 to 14 bits using BITS++ (Cambridge Research Systems, Rochester, Kent, England, 2002), allowing the use of visual stimuli with lower contrast gradations. LightScan software, equipped with OptiCAL Photometry (Cambridge Research Systems, Rochester, Kent, England, 2002), was used to measure screen luminance and gamma correct the monitor using 48 index values ranging from 0 to 255 (gamma
1.8) as a sample. The lowest and highest luminance values of the screen were 0.20
). The room was 2.5 x 2.0
m in size and was illuminated by a fluorescent 20
W bulb (Philips). The walls of the room were gray, which allowed for better control of the room lightning conditions during the experiment. A C++ computer program, developed by the responsible lab, was used to run the experiment (generating the stimuli, controlling stimuli presentation and registering contrast thresholds).
Achromatic and vertical static sine-wave grating stimuli with spatial frequencies of 0.6, 2.5 and 10.0
cycles per degree (cpd) of visual angle were used in this work (Figure
Figure 1 Example of a pair of stimuli. On the left, a sine-wave grating with the spatial frequency of 0.6cpd, and on the right, a neutral stimulus. The stimuli were originally calibrated for viewing at a distance of 150cm.
All of the stimuli had a diameter of approximately 7.2 degrees of visual angle and were designed to be presented in the middle of the monitor at a distance of 150
cm from the observer.
Estimates were obtained using the psychophysical method of forced choice between two temporal alternatives
]. This method is based on the probability of consecutive correct choices being made by participants; that is, in 100 presentations of a choice between two stimuli, the spatial frequency (test stimulus) is perceived by the participant in 7
9% of the presentations. The procedure for measuring the threshold for each frequency was successive presentation of pairs of stimuli, from which the participant had to choose which had the spatial frequency of interest. The other stimulus was always a homogeneous pattern of mean luminance 40.1
cd/m2. The criterion adopted to vary the contrast of each tested spatial frequency was three consecutive correct choices for decreasing by unity and an incorrect choice for increasing by the same unity (20%)
An experimental session began with a brief beep followed by the presentation of a 2
s stimulus (test or neutral) with an inter-stimulus interval of 1
s and presentation of the second stimulus for 2
s, followed by the volunteers’ response. The interval between stimuli pairs and trials was 3
s. A different beep gave feedback to the volunteer for each correct choice. The order of presentation of the stimuli was random.
The participants received the following instructions: “pairs of stimuli, one with clear-dark stripes and another totally gray, will be presented. You must always choose the stimulus that contains the stripes, pressing the button on the left (button number 1) of the mouse when the stimulus with stripes is presented first, and the button on the right (button number 2) when it is presented in the second place (after the gray stimulus)”. Each session began with the test stimulus contrast at supra-threshold level, and the experiments began only when the investigator was convinced that the participant understood the directions and responded as instructed.
The experimental session varied in duration (with an average time ranging from 10 to 15
minutes) depending on the correct and incorrect choices made by the volunteer, until a total of six reversals (or six contrast values, three peaks and three valleys) were obtained. Each one of the points (or frequencies) of the contrast sensitivity curve was estimated at least twice (two experimental sessions), on different days, for each of the participants. All of the estimates were measured with binocular vision and natural pupil.
Descriptive statistics (Mean and SD) were used to analyze the clinical and demographic groups. A two-tailed Student t-test for paired samples was used to verify differences between the ocular conditions of control subjects and affected individuals. We used Analysis of Variance (ANOVA) to determine whether there were significant differences between the groups. Simple differences between treatments were adjusted using a post hoc Tukey HSD test, and the level of significance was p