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PubMed1991

Visual-field map in the transcallosal sending zone of area 17 in the cat.

The representation of the visual field in the part of area 17 containing neurons that project axons across the corpus callosum to the contralateral hemisphere was defined in the cat. Of 1424 sites sampled along 77 electrode tracks, 768 proved to be in the callosal sending zone, which was identified by retrograde transport of horseradish peroxidase that had been deposited in the opposite hemisphere. The results show that the callosal sending zone has a fairly constant width of between 3 and 4 mm at most levels in area 17. However, the representation of the contralateral field at the different elevations of the visual field is not equal in this zone. The zone represents positions within 4 deg of the midline at the 0-deg horizontal meridian, and positions out to 15-deg azimuths in the upper hemifield and out to positions of 25-deg azimuth in the lower hemifield. The shape of the representation is approximately mirror-symmetric about the horizontal meridian, although there is a greater extent in the lower hemifield, which can be accounted for by the greater range of elevations (greater than 60 deg) represented there compared with the upper hemifield (approximately 40 deg). The representation in the sending zone of one hemisphere matches that present in the area 17/18 transition zone, which receives the bulk of transcallosal projections, in the opposite hemisphere. The observations on the sending zone show that callosal connections of area 17 are concerned with a vertical hour-glass-shaped region of the visual field centered on the midline. The observations suggest that in addition to interactions between neurons concerned with positions immediately adjacent to the midline, there are positions, especially high and low in the visual field, where interactions can occur between neurons that have receptive fields displaced some distance from the midline.

Animals↗
PubMed1992

Topographic organization, number, and laminar distribution of callosal cells connecting visual cortical areas 17 and 18 of normally pigmented and Siamese cats.

The callosal connections between visual cortical areas 17 and 18 in adult normally pigmented and "Boston" Siamese cats were studied using degeneration methods, and by transport of WGA-HRP combined with electrophysiological mapping. In normal cats, over 90% of callosal neurons were located in the supragranular layers. The supragranular callosal cell zone spanned the area 17/18 border and extended, on average, some 2-3 mm into both areas to occupy a territory which was roughly co-extensive with the distribution of callosal terminations in these areas. The region of the visual field adjoining the vertical meridian that was represented by neurons in the supragranular callosal cell zone was shown to increase systematically with decreasing visual elevation. Thus, close to the area centralis, receptive-field centers recorded from within this zone extended only up to 5 deg into the contralateral hemifield but at elevations of -10 deg and -40 deg they extended as far as 8 deg and 14 deg, respectively, into this hemifield. This suggests an element of visual non-correspondence in the callosal pathway between these cortical areas, which may be an essential substrate for "coarse" stereopsis at the visual midline. In the Siamese cats, the callosal cell and termination zones in areas 17 and 18 were expanded in width compared to the normal animals, but the major components were less robust. The area 17/18 border was often devoid of callosal axons and, in particular, the number of supragranular layer neurons participating in the pathway were drastically reduced, to only about 25% of those found in the normally pigmented adults. The callosal zones contained representations of the contralateral and ipsilateral hemifields that were roughly mirror-symmetric about the vertical meridian, and both hemifield representations increased with decreasing visual elevation. The extent and severity of the anomalies observed were similar across individual cats, regardless of whether a strabismus was also present. The callosal pathway between these visual cortical areas in the Siamese cat has been considered "silent," since nearly all neurons within its territory are activated only by the contralateral eye. The paucity of supragranular pyramidal neurons involved in the pathway may explain this silence.

Albinism↗
PubMed1991

Visuotopic organization of the lateral suprasylvian area and of an adjacent area of the ectosylvian gyrus of cat cortex: a physiological and connectional study.

We have explored the visuotopic organization of the territory surrounding the middle suprasylvian sulcus (MSS) of cat cerebral cortex by electrophysiological mapping, and by tracing the topography of its cortical and subcortical connections using wheatgerm-agglutinin horseradish peroxidase (WGA-HRP). Observations from the two approaches were concordant, and confirmed the presence of two separate visual areas in the MSS that approximate, but do not exactly correspond, to the location and internal organization of the posterior medial and posterior lateral lateral suprasylvian (PMLS, PLLS) areas of Palmer et al. (1978). We define as part of the lateral suprasylvian (LS) area the territory on the medial bank and caudal end of the lateral bank of the MSS that receives a topographically organized projection from the region of area 17 representing the lower visual quadrant. This territory is connected with other structures that are themselves striate-recipient (cortical areas 18 and 19, and the lateral division of the lateral posterior (LPl) nucleus), and with a variety of nuclei that receive direct retinal input, such as the C-laminae of the LGd, the medial interlaminar nucleus (MIN), and the superficial layers of the superior colliculus (SC). Its connections with the LPl, LGd, MIN, and SC correspond topographically with the input from area 17. Revised maps of area LS were produced from the physiological and connectional data: its rostral border is formed by a representation of lower visual elevations with the horizontal meridian represented caudally, and its lateral border is formed by the vertical meridian; area LS shares a representation of the center of gaze with the visual area of the lateral bank at its caudal end. The adjacent lateral bank area has larger receptive fields than area LS, and very different connectivity. It receives no input from area 17 and little input from striate-recipient structures, including area LS, but instead is connected to more remote extrastriate visual areas, such as the anterior ectosylvian visual (AEV) area in insular cortex, and to zones of the thalamus in receipt of tectal input (LPm and the lateromedial-suprageniculate nuclear complex). According to both mapping approaches, the lateral bank area contains representations of both the upper and lower visual quadrants but a rather limited degree of visuotopic order. We refer to it as the posterior ectosylvian visual (PEV) area, because it appears to be functionally and connectionally dissociated from area LS, but is possibly a functional antecedent of area AEV.

Animals↗
PubMed1992

Packing geometry of human cone photoreceptors: variation with eccentricity and evidence for local anisotropy.

Disorder in the packing geometry of the human cone mosaic is believed to help alleviate spatial aliasing effects. To characterize cone packing geometry, we gathered positions of cone inner segments at seven locations along four primary and two oblique meridians in an adult human retina. We generated statistical descriptors based on the distribution of distances and angles to Voronoi neighbors. Parameters of a compressed-jittered model were fit to the actual mosaic. Local anisotropies were investigated using correlograms. We find that (1) median distance between Voronoi neighbors increases with eccentricity, but the minimum distance is constant (6-8 microns) across peripheral retina; (2) the cone mosaic is least compressed and jittered at the edge of the foveal rod-free zone; (3) disorder in the foveal center resembles that described by Pum et al. (1990); (4) cone spacing is 10-15% less in one direction than in the orthogonal direction; and (5) cone spacing is greater in the radial direction (along meridians) than in the tangential direction (along lines of isoeccentricity). The nearly constant minimum distance implies that high spatial frequencies may be sampled even in peripheral retina. Local anisotropy of the cone mosaic is discussed in relation to the growth of the primate retina during development and to the orientation biases of retinal ganglion cells.

Adult↗
PubMed1991

Metastable motion anisotropy.

The phenomenon of apparent motion can arise when two spatially separated visual tokens are presented in temporal sequence. If tokens at opposite corners of a hypothetical square are presented simultaneously followed by simultaneous presentation of tokens at the remaining two corners, an apparent motion percept may occur along either the vertical or horizontal axis. The display is perceptually metastable since most observers will perceive motion along only one axis at a time. The metastable display, however, produces anisotropic results, in that with central fixation, vertical motion is seen more frequently than horizontal motion. The ratio of the vertical to horizontal length of the sides of a rectangle needed to achieve equal frequencies of motion judgments along the respective axes falls in the range of 1.18-1.92 for different observers in our experiments. It appears that signal transmission across the vertical midline is a major determinant of the vertical bias, since the anisotropic effects disappear when the fixation point is sufficiently offset along the horizontal meridian so as to cause a fully homonymous representation of all of the metastable tokens. One of the factors may be signal degradation or delay in callosal transmission which could reduce the strength of the motion signal along the horizontal axis. In addition, there appears to be a strip along the vertical midline with a width of 30-50 min arc within which reduced levels of anisotropy are found. The possibility that this strip is a consequence of a zone of naso-temporal overlap in the projection of the retina to the brain along the vertical meridian will be discussed.

Anisotropy↗
PubMed1991

Spatial vision of the cat: variation with eccentricity.

We examined the grating acuity and contrast sensitivity of cats whose eye position was monitored with a scleral search-coil technique. During each trial, the cat was required to maintain fixation on a laser spot and respond to the presence or the absence of a grating by pressing a right or left pedal. With this procedure, acuity was measured along the horizontal and vertical meridian over a range of eccentricities in the nasal, temporal, superior, and inferior retina. In addition, contrast sensitivity for stationary and drifting gratings was measured for the temporal retina along the horizontal meridian. Acuity in area centralis reached about 3.5 cycle/deg and declined by 0.5 octaves at 4 deg and by about 1.3 octaves at 16-deg eccentricity in the nasal retina. The acuity was higher in the nasal than temporal retina. At all eccentricities, spatial resolution exceeded the resolution limit derived from Y (alpha)-cell properties. Contrast sensitivity also decreased as the eccentricity increased when the target size was held constant. The slope of sensitivity-eccentricity function was relatively shallow for a low spatial frequency (0.30 cycle/deg) with sensitivity decreasing by a factor of 1.5-2 at 8-deg eccentricity. The slope of the sensitivity falloff for high spatial-frequency gratings (1.2 cycle/deg) was steeper, with a 5-10-fold difference in sensitivity between 0 and 8 deg. By varying the target size, we determined that the summation area in the cat is about a factor of 3 smaller in area centralis than a 16-deg eccentricity.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMedDate not supplied

Tyrosine hydroxylase expression in the Cebus monkey retina.

Tyrosine hydroxylase (TH) expression was used as a marker to study the dopaminergic cells in the Cebus monkey retina. Two types of dopaminergic cells were identified by cell body size and location, level of arborization in the inner plexiform layer, and amount of immunolabeling. Type 1 cells displayed intense immunoreactivity and larger somata (12-24 microns) located in the inner nuclear layer or ganglion cell layer, whereas type 2 had smaller cell bodies (8-14 microns) found either in the inner plexiform layer or ganglion cell layer and were more faintly labeled. Interplexiform cells were characterized as type 1 dopaminergic cells. Immunoreactive axon-like processes were seen in the nerve fiber layer, and a net of fibers was visible in the foveal pit and in the extreme periphery of the retina. The population of TH+ cells was most numerous in the temporal superior quadrant and its density peaked at 1-2 mm from the fovea. Type 1 TH+ cells were more numerous than type 2 cells at any eccentricity. Along the horizontal meridian, type 1 cell density was slightly higher in temporal (29 cells/mm2) than in nasal (25 cells/mm2) retina, while type 2 cells had a homogeneous distribution (4.5 cells/mm2). Along the vertical meridian, type 1 cells reached lower peak density (average 17.7 cells/mm2) in the inferior retina (central 4 mm), compared to the superior portion (23.7 cells/mm2). Type 2 cell density varied from 4.5 cells/mm2 in the superior region to 9.4 cells/mm2 in the inferior region. The spatial density of the two cell types varied approximately inversely while the total density of TH+ cells was virtually constant across the retina. No correlation between dopaminergic cells and rod distribution was found. However, we suggest that dopaminergic cells could have a role in mesopic and/or photopic vision in this species, since TH+ fibers are present in cone-dominated regions like the foveola and extreme nasal periphery.

Animals↗
PubMedDate not supplied

The functional organization of area V2, II: the impact of stripes on visual topography.

We have examined the visuotopic organization of area V2 of macaque monkeys in relation to its modular construction, comprising repetitive cycles of stripes running perpendicular to the border with area V1. Receptive fields were plotted in anesthetised animals, mainly using long penetrations parallel to the V1 border crossing several stripes in dorsal V2 within the representation of paracentral, inferior visual field. We confirm that each set of modules (thick, thin, and interstripes) mounts an unbroken coverage of the visual field, since there is almost invariably some overlap between the aggregate fields recorded in successive stripes of the same class, at intervals of one cycle. Also as expected, penetrations perpendicular to the stripes record changes in eccentricity along an isopolar visual meridian. We measured the size of the point image along such an isopolar meridian in nine cases, and showed that on average it exceeds the length of a typical cycle; again, this implies that no point in space escapes analysis by any of the functional modules. The representation of eccentricity across a cycle of stripes resembles a "ratchet" model, in which the gradient of eccentricity across a single stripe exceeds the gradient across the full cycle, leading to discontinuities ("switchbacks") at the borders between stripes. The shift in eccentricity across the width of a stripe is sufficient to maintain a virtually continuous map across successive stripes of the same class; when coupled to receptive field scatter about the mean trend, this creates the overlap of aggregate fields. The presence of topographic discontinuities, in addition to functional ones, at stripe boundaries, reflects autonomy of function at the local level. At a global level, however, the organization of V2 may promote integration across stripes, since the range of visual overlap, and intrinsic connections, exceeds a single cycle. Furthermore, the reduplication of modules along the isopolar axis has the effect of increasing the isopolar magnification factor (mm/deg); the ratio of isopolar to iso-eccentric magnification is approximately 1.5:1, and this elongation abets the external configuration of V2 that, stretching concentrically around V1, has long been hypothesized to facilitate functional interaction between the two areas.

Animals↗
PubMedDate not supplied

An albino-like decussation error in the optic chiasm revealed by anomalous ocular dominance columns.

We report a unique anomaly in the ocular dominance column pattern of a single, normally pigmented macaque monkey. The column pattern contained large monocular areas inserted between the normal columns and the dorsal V1 border. These monocular regions received transneuronal input from the contralateral eye, indicating that a small population of temporal ganglion cells erroneously decussated at the optic chiasm. Projection of the column pattern back onto the visual field showed that the monocular wedges represented a approximately 5-deg sector of ipsilateral field. This corresponded to the extent of naso-temporal overlap of ganglion cells in the normal retina, suggesting an error in axon guidance affecting cells close to the vertical midline of the retina. The consequences of the crossing error in this animal were threefold: it produced an anomalous monocular zone near the V1 border, the vertical meridian was not represented at the V1 border, and points near the vertical meridian were represented twice in the brain, once in each hemisphere.

Albinism↗
PubMedDate not supplied

Gradients of cone differentiation and FGF expression during development of the foveal depression in macaque retina.

Cones in the foveola of adult primate retina are narrower and more elongated than cones on the foveal rim, which in turn, are narrower and more elongated than those located more eccentric. This gradient of cone morphology is directly correlated with cone density and acuity. Here we investigate the hypothesis that fibroblast growth factor (FGF) signaling mediates the morphological differentiation of foveal cones--in particular, the mechanism regulating the elongation of foveal cones. We used immunoreactivity to FGF receptor (R) 4, and quantitative analysis to study cone elongation on the horizontal meridian of macaque retinae, aged between foetal day (Fd) 95 and 2.5 years postnatal (P 2.5 y). We also used in situ hybridization and immunohistochemistry to investigate the expression patterns of FGF2 and FGFR1-4 at the developing fovea, and three other sample locations on the horizontal meridian. Labeled RNA was detected using the fluorescent marker "Fast Red" (Roche) and levels of expression in cone inner segments and in the ganglion cell layer (GCL) were compared using confocal microscopy, optical densitometry, and tested for statistical significance. Our results show that morphological differentiation of cones begins near the optic disc around Fd 95, progressing toward the developing fovea up until birth, approximately. Levels of FGF2 and FGFR4 mRNAs expression are low in foveal cones, compared with cones closer to the optic disc, during this period. There is no similar gradient of FGF2 mRNA expression in the ganglion cell layer of the same sections. Maturation of foveal cones is delayed until the postnatal period. The results suggest that a wave of cone differentiation spreads from the disc region toward the developing fovea during the second half of gestation in the macaque. A gradient of expression of FGFR4 and FGF2 associated with the wave of differentiation suggests that FGF signalling mediates cone narrowing and elongation.

Animals↗
PubMed1986

Structure of the crystalline bilayer in the subgel phase of dipalmitoylphosphatidylglycerol.

The structure of the subgel phase of dipalmitoylphosphatidylglycerol (DPPG) has been analyzed by X-ray diffraction techniques. Diffraction recorded from highly oriented DPPG specimens in the subgel phase extends to 2-A resolution. There are sharp lamellar reflections on the meridian, and other reflections lie on a series of wide-angle lattice lines parallel to the meridian and crossing the equator in the range of 8-2 A. The wide-angle lattice lines consist of radially sharp reflections centered on the equator of the X-ray film and also a series of broader, off-equatorial maxima. The lattice lines indicate that the DPPG molecules in each bilayer crystallize in a two-dimensional oblique lattice with dimensions a = 5.50 A, b = 7.96 A, and gamma = 100.5 degrees. These oblique lattices are not regularly aligned from bilayer to bilayer. Analysis of the lamellar diffraction shows that the bilayer has about the same thickness in the subgel and gel (L beta') phases. In the direction normal to the hydrocarbon chains, the chains are significantly closer together in the subgel phase as compared to the normal L beta' gel phase but have about the same separation as the chains in polyethylene and the crystalline n-alkanes. The bilayer thickness, area per lipid molecule, and intensity distribution along the lattice lines all indicate that in the subgel phase the hydrocarbon chains are tilted between 30 and 35 degrees from the normal to the bilayer plane.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization↗
PubMed2003

Structure of reciprocal connections of visual cortical fields 17 and 18 in the cat.

Microiontophoretic application of horseradish peroxidase to individual columns in fields 17 and 18 of the cat cortex was used to identify the distribution by area and layer of retrograde labeled cells in both fields. After application of marker to fields 17 or 18, the area of labeled cells in field 17 (in the tangential plane) was extended and orientated along the projection of the horizontal meridian of the field of vision. The area of labeled cells in field 18 in these cases was orientated along the projections of the vertical meridian. Similar differences in the organization of the connections of fields 17 and 18 were seen in the projection zone of the central 10 degrees of the field of vision at different elevations. Thus, the spatial distributions of internal and external connections in each field coincide and their orientations in fields 17 and 18 are mutually perpendicular. It is suggested that field 17 performs the more detailed analysis of information on the horizontal components of an image and communicates this to field 18, while field 18 is responsible for the more detailed analysis of information about the vertical components of the same image, communicating this to field 17.

Animals↗
PubMed2001

Biometric measurements of the eyes in teenagers and young adults with Down syndrome.

PURPOSE: To examine ocular biometric variables in subjects with Down syndrome. METHODS: In a population-based study we have compared ocular biometric variables in a group of 47 individuals with Down syndrome (20.0+/-3.9 years) with 51 control subjects (21.0+/-4.6 years). RESULTS: A thinner cornea (0.48+/-0.04 mm vs. 0.55+/-0.03 mm, p<0.001) and higher keratometry values (46.39+/-1.95 D vs. 43.41+/-1.40 D, p<0.001) were found in the Down syndrome group than in the control group. Oblique astigmatism was commonly found in the Down syndrome individuals, showing a strong right-left specificity (right eyes' axes in the 135 degrees -meridian, left eyes' axes in the 45 degrees -meridian). The lens was thinner (3.27+/-0.29 mm vs. 3.49+/-0.20 mm) and the calculated lens power was weaker (17.70+/-2.36 D vs. 19.48+/-1.24 D) in the Down syndrome group than in the control group (p<0.001 in both cases). CONCLUSIONS: Thinning of the corneal stroma may account for the steeper cornea and the high frequency of astigmatism in Down syndrome due to lower corneal rigidity. It may also be of etiological importance to the increased incidence of keratoconus in Down syndrome.

Accommodation, Ocular↗
PubMed1993

Visual offsets facilitate saccadic latency: does predisengagement of visuospatial attention mediate this gap effect?

Saccadic reaction time (RT) is reduced when a fixation stimulus is extinguished 200 ms before a target appears. An attentional predisengagement theory (APT) may explain this gap effect: When covert attention is engaged (e.g., on fixation), the saccadic system is inhibited and RT is delayed; when the attended stimulus is extinguished, attention is disengaged, the inhibition is removed, and RT is facilitated. In 3 experiments covert attention was endogenously or exogenously cued to an object on the vertical meridian. Onset of a saccadic target on the horizontal meridian could be preceded by the offset of an attended or unattended object. Contrary to APT, RTs were identical after attended and unattended offsets. Results suggest that the gap effect has 2 components, and covert visual attention plays no role. One component is motor system preparation; the other is a fixation offset effect specific to the oculomotor system.

Attention↗
PubMed1993

The management of post-keratoplasty astigmatism by post-operative adjustment of a single continuous suture.

Penetrating keratoplasty was performed in 39 consecutive patients. A 16 bite 10-0 nylon continuous suture was used. Post-operatively, the suture was adjusted to reduce astigmatism, as determined by refraction, keratoscopy and keratometry. Suture adjustment was performed if astigmatism was over 4 dioptres (D). The suture was tightened in the flatter meridian and loosened in the steeper meridian. Twenty-one patients had suture adjustment performed. Prior to adjustment, mean (+/- standard deviation) astigmatism measured 6.33 +/- 1.38 D. Following adjustment, mean astigmatism was significantly reduced to 2.69 +/- 1.14 D (p < 0.0001). The average final astigmatism for the entire group was 2.66 +/- 1.12 D. The use of a continuous adjustable suture in penetrating keratoplasty significantly reduced the post-operative astigmatism in a group of patients undergoing penetrating keratoplasty.

Adolescent↗
PubMed1997

Variation in astigmatism following the single-step, self-sealing clear corneal section for phacoemulsification.

PURPOSE: A single-step, self-sealing, 3.2 mm clear corneal section is described and the incidence and variation of surgically induced astigmatism following phacoemulsification over a period of 3 months is determined. METHODS: Twenty-two patients who underwent uncomplicated 3.2 mm clear corneal phacoemulsification with foldable Allergan silicone intraocular lens implantation were autorefracted pre-operatively and on day 1, week 1, week 6 and at 3 months. The variation in induced astigmatism was analysed using the subtraction method and vector analysis. The change in direction of the cylindrical axis was examined. RESULTS: The induced astigmatism represented by the total vector on day 1 was 1.17 D and vector decomposition ratio, ATR:WTR (against-the-rule:with-the-rule), was 21.05:78.95. The total vector increased by 0.5 D in the first week and then stabilised. Vector decomposition showed an against-the-rule astigmatic drift so that by 3 months the ATR:WTR was 40.49:59.91. There was a tendency of the axis of the negative cylinder to swing towards the corneal section meridian on day 1 and to oscillate around that meridian at week 1 and week 6. By 3 months the direction reverted to that pre-operatively, in most cases. CONCLUSIONS: The single-step, self-sealing clear corneal section is mechanically stable and, though there is some variability in the measured astigmatism, there is an acceptable functional result throughout the post-operative period.

Aged↗
PubMed2002

Corneal topography and Schirmer testing in eyes with the Hudson-Stahli line.

PURPOSE: To evaluate corneal topographic features and tear secretion in eyes with the Hudson-Stahli line. METHODS: Keratometry, computerized videokeratography and Schirmer testing were performed in 50 cases with bilateral Hudson-Stahli line, and 55 controls without the Hudson-Stahli line. Similar tests were performed in 21 subjects with unilateral Hudson-Stahli line. RESULTS: Corneal topographic features and keratometry in the horizontal meridian were similar in cases and controls, and in fellow eyes of subjects with unilateral Hudson-Stahli line. Keratometry in the vertical meridian in cases (43.01 +/- 2.01) was significantly lesser than in controls (43.94 +/- 1.77) (P = 0.01). This value was not different in fellow eyes of patients with unilateral Hudson-Stahli line. Schirmer testing revealed significantly greater tear secretion in cases (16.72 +/- 4.99 mm) compared to controls (12.57 +/- 3.62 mm) (P < 0.01). In subjects with unilateral Hudson-Stahli line, mean Schirmer values in the eye with the line (17.52 +/- 6.86 mm) were significantly greater than in eyes without (13.67 +/- 4.64 mm) (P = 0.04). CONCLUSION: Formation of the Hudson-Stahli line may be dependent on the presence of normal tear secretion in the eye.

Adult↗
PubMed2003

Effects of fibrovascular traction and pooling of tears on corneal topographic changes induced by pterygium.

AIM: To investigate the effects of fibrovascular traction and the pooling of tears at the pterygium apex on the corneal topographic changes induced by pterygium. METHODS: A total of 16 eyes of 14 cases with primary pterygium were included in the study. A computerized corneal topography system was used for corneal topography examinations. Baseline keratographs were taken two times at straight gaze. A repeat corneoscope photograph was immediately obtained in temporal gaze. Then the tears at the pterygium apex were dried with a cellulose sponge, and a new corneoscope photograph was immediately obtained without allowing one to blink. Corneal topographic maps (numeric maps) were divided into 301 fields in 24 meridians. One colour was allocated to each field, representing its mean refractive power for all groups. In all eyes, keratometric astigmatism at the 3 mm central cornea and total mean corneal refractive power were found. Data were compared using paired-samples two-tailed t-tests. RESULTS: Keratometric astigmatism at the 3 mm central cornea was significantly reduced at the temporal gaze (3.10+/-2.34 D, t=3.40, P=0.027) and dried eyes (2.12+/-1.01 D, t=4.74, P=0.001) according to the first baseline measurement (4.31+/-1.91 D) of the total mean corneal refractive power was found to be 43.45+/-1.28 D (39.29-45.87) at the first baseline measurement. There was no change at the temporal gaze (43.54+/-1.06 D, P>0.05). However, the total corneal refractive power was significantly higher in dried eyes (44.26+/-0.93 D, t=34.92, P<0.001). The steepest region of corneal topography was a superior quadrant, and the flattest area was a nasal quadrant at the baseline. At the temporal gaze, the cornea was significantly flatter in the superior and inferior sides of the pterygium meridian. After dried pooling of tears, topographic abnormalities returned, and the cornea became more uniform and symmetric. CONCLUSION: We conclude that the pooling of tears at the pterygium apex plays an important role, but fibrovascular traction has no effect on the corneal topographical changes induced by pterygium.

Adult↗