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PubMed1989

Human macular pigment assessed by imaging fundus reflectometry.

A computerized, television-based, imaging fundus reflectometer was used to obtain estimates of the spatial distribution of macular pigment (xanthophylls) from seven normal subjects. Digitized images of the bleached macula of each subject were acquired at illuminating wavelengths from 462 to 697 nm. An analysis of spectral reflectances indicated that differences in short-wavelength reflectance between the foveal center and parafovea were influenced by spatial variations in melanin and oxyhemoglobin absorption as well as by the distribution of macular pigment. To provide an estimate of the spatial distribution of macular pigment alone, we have corrected fundus images obtained at 462 nm for the effect of melanin and oxyhemoglobin absorption. The spatial variation in macular pigment double density across the horizontal and vertical meridians of the retina was well described by Gaussian functions. The peak double densities for the individual subjects ranged from 0.22 to 0.45 and the standard deviations of the Gaussian functions averaged approx. 1 degree.

Adult↗
PubMed1989

Computer methods for sampling, reconstruction, display and analysis of retinal whole mounts.

We are quantifying the distribution of photoreceptors and ganglion cells in human retina with the goal of establishing a reliable anatomical database which may be compared to information about visual function. We required a representation of retinal cell distributions which facilitated collection, analysis, and display of morphometric data from the entire retina of a large number of eyes. We report computer methods to (1) reconstruct the original retinal sphere from a three-piece whole mount preparation; (2) sample the retina in a manner which allowed description of approximately radially symmetrical cell distributions and avoided both undersampling (which produces interpolation artifacts) and oversampling (which wastes time); (3) interpolate between data points in order to produce plots of cell density along arbitrary meridians and maps of average cell density from several eyes; (4) specify locations on the retinal surface using a spherical coordinate system with its primary axis through the fovea; and (5) produce color-coded maps of cell distributions in a standard perimetric projection.

Cell Count↗
PubMed1989

The spatial resolution capacity of human foveal retina.

An image on the retina of a human eye enters the visual system through an array of photoreceptors that sets the boundaries on the spatial detail available for neural representation. In order to investigate the extent to which the input spatial detail is preserved by the human neural system, we compare the anatomical spatial limits as determined by the Nyquist frequency, the highest spatial frequency reconstructable from the cone array, and measures of human acuity, the minimum angle resolvable. We find that the anatomical Nyquist limits determined along the temporal horizontal meridian of a well-studied human retina (Curcio, Sloan, Packer, Hendrickson & Kalina, 1987b) offer a reasonable prediction of human acuity within the retinal region extending from slightly off the exact foveal center to about 2.0 deg of retinal eccentricity. However, we find a narrow peak of anatomical resolution at the foveal center where the acuity appears to be overestimated by cone spacing.

Adult↗
PubMed1989

Retinal ganglion cell distribution in the cebus monkey: a comparison with the cortical magnification factors.

The distribution of ganglion cells was determined in whole-mounted Cebus monkey retinae. Ganglion cell density along the horizontal meridian was asymmetric, being 1.2-4.3 higher in the nasal retinal region when compared to temporal retina at the same eccentricities. The total number of ganglion cells varied from 1.34 to 1.4 million. Ganglion cell density peaked at 49,000/mm2 about 0.5 mm nasal to the fovea. Comparison between ganglion cell density and areal cortical magnification factors for V1 and V2 reveals that the relative representation of the fovea increases in the visual cortex. This effect seems to be a general feature of the visual system of primates.

Animals↗
PubMed1990

Recognition of stimulus orientation.

The threshold for detecting a deviation in the orientation of a test stimulus from vertical and from horizontal was measured with a single test interval. The test stimuli were step edges, thin lines, and sine wave gratings of 2.5, 5.0 and 10.0 c/deg. Recognition thresholds were found to be independent of the frequency content and nature of the test stimulus. The average threshold value on the principal meridians was found to be approx. 0.6 deg. Additional testing was conducted with two interval forced choice designs, both with and without random perturbation of the orientation of the stimuli. Random perturbation of orientation reduces acuity estimates. The results are compared with those obtained by testing on the oblique meridia. We conclude that the meridional anisotropy of orientation discrimination, which favours the vertical and horizontal is a result, in part, of the influence of non-visual mechanisms.

Adolescent↗
PubMed1990

Evaluation of retinal orientation and gaze direction in the perception of the vertical.

The orientation of the median plane of the eye with respect to the head varies with gaze direction according to Listing's Law. The subjective vertical (SV), however, is known to be only partially affected by these involuntary variations of eye orientation. In order to learn more about the compensatory process underlying this finding, six normal-sighted young subjects were tested monocularly and binocularly in eight directions of gaze. The results show that: (1) the SVs, determined monocularly for both eyes, fall on corresponding retinal meridians, the binocular SV-settings generally lie between the monocular ones; (2) the tilt of the SV is not linearly related to the tilt angle of the median plane of the eye, as hitherto assumed. On theoretical considerations, the dependence of ocular tilt on gaze direction may be decomposed into three components, which are all treated differently in the compensation process. We interpret these results in the following way: the SV is determined from the sensorially fused image of both eyes and the tilts of the eyes are accounted for by an extra-retinal signal which is common to both eyes. The characteristics of the compensation mechanism may be explained by an extra-retinal signal which relies on information about gaze direction and Listing's Law. Such a signal might be derived from an efference copy of gaze direction commands.

Eye Movements↗
PubMed1990

Psychophysical measurement of the blur on the retina due to optical aberrations of the eye.

The blur on the retina in the horizontal meridian due to monochromatic and chromatic aberrations has been measured using a novel psychophysical technique. Longitudinal chromatic aberration gives the dominant blur for pupil sizes of 4-5 mm, followed by monochromatic aberrations, and blur due to optical transverse chromatic aberration. In some eyes, coma was present as a result of a displacement of the axis of symmetry from the centre of the pupil, but in three eyes, coma was present without spherical aberration. The technique also allows a measurement of the effective pupil centre relative to the geometric centre and a partial analysis of the relative positions of the reference axes of the eye.

Adolescent↗
PubMed1991

The time course of adaptation to spatial contrast.

We explored the buildup and decay of threshold elevation during and after adaptation to sinewave gratings in a series of experiments investigating the effects of adapting time, adapting contrast, spatial frequency and retinal eccentricity. Contrast thresholds for vertical sinewave gratings truncated in space by a one-dimensional Gaussian envelope were measured before and after adaptation to a full-field suprathreshold grating of the same spatial frequency and orientation. Thresholds were measured intermittently after adaptation in a "seen/not-seen" single presentation procedure until these thresholds returned to baseline values. The first test grating was presented 300 msec after the offset of the adapting stimulus, and thereafter at regular intervals. At different times after adaptation, contrast thresholds were estimated by off-line analysis of the data using the QUEST algorithm. Adapting time was either 1, 10, 100 or 1000 sec and adapting contrast was either 9, 19, 29 or 39 dB (re. 1%). The test gratings were presented centered either at the fixation point or at 5 and 10 deg eccentricity along the horizontal meridian. The results suggest that up to the saturation level the buildup and the decay of adaptation to contrast is well described by a power function of time. The slope of the best fitting line on log-log axes is fairly constant for the adaptation times tested. As reported earlier, thresholds increased with adapting contrast and these contrast-dependent differences were evident 300 msec after the termination of adaptation. Adaptation at 10 deg eccentricity yielded slightly higher threshold elevations than for central vision. Based on these results, a description is given of the dynamic response of the underlying neural mechanisms.

Adaptation, Ocular↗
PubMed1991

Vernier acuity with stationary moving Gabors.

We examined the ability of observers to determine the vertical alignment of three Gabor patches (cosine gratings tapered in X and Y by Gaussians) when the grating within the middle patch was moving right or left. The comparison patches were flickered in counterphase, as was the test patch in a control condition. In all conditions, the Gabor patch itself (the envelope) was stationary. Vernier acuity (i.e. sensitivity) was almost as good with the moving as with the flickering Gabors, but there was a very pronounced positional bias in the case of the patterns in which the internal gratings were moving. The (stationary) patches appeared to be displaced in the direction of the grating movement. Thus if the grating were drifting rightwards, the observer would see the patches as being aligned only when the test patch position in fact was shifted far over to the left. This movement-related bias increased rapidly with retinal eccentricity, reaching 15 min at 8 deg eccentricity. The bias was greatest at 4-8 Hz temporal frequency, and at low spatial frequencies. Whether the patterns were on the horizontal or the vertical meridian was largely irrelevant, but larger biases were found with patterns moving towards or away from the fovea than with those moving in a tangential direction.

Fixation, Ocular↗
PubMed1991

Spatial-interval discrimination in two-dimensions.

This study is concerned with the precision of spatial-interval discrimination when the stimulus is varied in two-dimensions rather than one-dimension. For the two-dimensional task, observers were required to judge the centrality of a dot within a circle. Similar measurements were made in a one-dimensional task, i.e. the test dot was displaced only along one meridian, and the results were compared. This study shows that thresholds for the two-dimensional task are approx. square root of 2 times the threshold of the one-dimensional task when the observer's task is to simply detect an offset in both one- and two-dimensional conditions. However, thresholds are about a factor of two higher for the two-dimensional task if the observer is required to label the direction of offset.

Discrimination, Psychological↗
PubMed1991

Short-term nonconjugate adaptation of human saccades to anisometropic spectacles.

It has been demonstrated before that the long-term wearing of anisometropic spectacles may induce nonconjugate adaptations of saccades. Saccades then become different in size in the two eyes. We examined the time-course and the limits of such adaptations of horizontal and vertical saccades during the short-term (1-6 hr) wearing of anisometropic spectacles. After only 1 hr of conditioning to 2 D of anisometropia, the nonconjugate size-adaptations were almost complete along the horizontal meridian. For progressively larger anisometropias (up to 8 D) the adaptative nonconjugacies after 1 hr became also systematically larger. An anisometropia larger than 6 D did not further increase the rate of adaptation during the first 6 hr of conditioning, which suggests that about 6 D of difference in spectacle correction, causing size differences of about 12%, may be the upper limit of the nonconjugate adaptive range of the saccadic subsystem. Post-saccadic drift of horizontal saccades was also adequately changed. In addition, nonconjugate adaptations had developed in smooth-pursuit eye movements. All of these plastic changes persisted during monocular viewing, indicating that the basic programming of these eye movements was changed.

Adaptation, Ocular↗
PubMed1992

The ratio of L cones to M cones in the human parafoveal retina.

We present psychophysically-based estimates of the relative numbers of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones in the parafovea of three color-normal trichromats. Using methods previously applied to the fovea centralis, we obtained estimates of the relative numbers of L and M cones at retinal eccentricities of 1.0, 1.5, 2.0, 3.0, and 4.0 deg along the horizontal meridian of the temporal retina. Results for three observers indicate that the L to M cone ratio remains approximately invariant from the fovea to 4.0 deg eccentricity, with a mean ratio near 2:1.

Cell Count↗
PubMed1992

Red-green and yellow-blue opponent-color responses as a function of retinal eccentricity.

Using the hue cancellation technique, I measured the spectral sensitivities of red-green (r-g) and yellow-blue (y-b) opponent-color responses of two color-normal observers at 0 (fovea), 3.3, 10, 21, and 31 degrees along the horizontal meridian of the temporal retina. The stimulus size as a function of retinal eccentricity was varied according to the cortical magnification factor. I found that the net spectral sensitivity (i.e. corrected for macular pigment) of the r-g channel was approximately constant as a function of retinal eccentricity. On the other hand, the net spectral sensitivity of the y-b channel was not constant. For the y-b channel I found a constant spectral sensitivity as a function of eccentricity when no macular correction was made. This means that the y-b system changes its yellow and blue responses with eccentricity to counterbalance the effects of macular pigment.

Adaptation, Ocular↗
PubMed1992

Spatial frequency discrimination of band-limited periodic targets: effects of stimulus contrast, bandwidth and retinal eccentricity.

Two experiments were conducted to explore the ability of human observers to discriminate the spatial frequency of briefly-presented, Gaussian-truncated sinewave gratings. In the first experiment, the influence of stimulus contrast and stimulus bandwidth on discrimination thresholds was measured after removing any position cues by randomizing the spatial phase of the gratings for each presentation. In a second experiment, the influence of retinal eccentricity on discrimination thresholds was explored for Gaussian-truncated gratings of constant spatial frequency bandwidth (0.5 octave) and suprathreshold contrast value (5 x detection threshold). The spatial frequency of the reference gratings varied from 1 to 8 c/deg. The gratings were positioned centered at the fixation point or 1-20 deg eccentric of the point of fixation along the horizontal meridian. Two observers responded in a two-interval forced-choice paradigm, which of two gratings had a higher spatial frequency. A difference frequency was randomly added to or subtracted from the spatial frequency of either the first or second grating. Using a maximum-likelihood algorithm, the spatial-frequency discrimination threshold delta f was computed from 40 trials, at which the observer responded with 75% accuracy. The results indicate that discrimination thresholds increase with (1) decreasing stimulus contrast, (2) increasing stimulus bandwidth, and (3) increasing retinal eccentricity. It is shown that spatial-frequency discrimination thresholds are only independent of contrast for narrow bandwidth stimuli having a contrast greater than 0.02. The eccentricity-dependent increase in discrimination thresholds varies with reference spatial frequency: with increasing retinal eccentricity delta f/f increases gradually for low spatial frequencies but rapidly for high spatial frequencies.

Contrast Sensitivity↗
PubMed1993

Spatial aspects of vertical phoria adaptation.

Vergence adaptation to vertical disparity spreads to unadapted directions of gaze. The spatial spread function for prism adaptation was estimated from aftereffects of a vertical disparity presented at a single position. Constraints limiting the spatial spread of adaptation were investigated with two stimuli of opposite disparity (hyper and hypo), presented at two different eye positions with a separation that varied from 6 to 18 deg in either the horizontal or vertical meridian. On average, phoria adaptation to the single point paradigm spread uniformly across the entire 18 deg test field. A resolution limit for adaptation to the two point paradigm was demonstrated by a reduction of phoria aftereffects with decreasing target separation (crowding). Vertical phoria aftereffects were reduced more by horizontal than by vertical crowding. A disparity gradient limit was demonstrated for a fixed target separation by a reduction of gain (phoria change/stimulus disparity) with increasing stimulus disparity.

Adaptation, Ocular↗
PubMed1993

Immunocytochemical localization of GABA and glycine in amacrine and displaced amacrine cells of macaque monkey retina.

Recent studies have varied widely in the percentages of GABA- and glycine-immunoreactive (GABA+, GLY+) amacrines reported for primate retina. We compared the distributions of GABA+ and GLY+ amacrines and displaced amacrines at seven locations along the horizontal meridian of macaque retina using postembedding immunogold labeling with silver intensification. The percentage of GABA+ amacrine profiles was higher in central retina (50-55%) than peripheral retina (30-40%), whereas the percentage of GLY+ amacrine profiles did not vary much with eccentricity (52-57%). GABA and glycine were colocalized in 5-20% of amacrines, depending on the eccentricity, whereas 5-30% of amacrines were not immunoreactive for either neurotransmitter. GABA+ amacrines were slightly larger than GLY+ amacrines or Müller cells. In the ganglion cell layer, 5-20% of neurons were labeled for either GABA or glycine and were identified as displaced amacrines. Of these, 53% were GABA+ only, 11% were GLY+ only, and 37% were double-labeled. A few large, very lightly labeled GABA+ cells were identified as ganglion cells. Other features that varied with eccentricity included the linear density of GABA+ and GLY+ amacrines, and the ratio of amacrines to Müller cells.

Animals↗
PubMed1993

An oblique effect in parafoveal motion perception.

An observer's ability to discriminate the angular direction of a moving grating depends on the grating orientation. Observers can more accurately judge the angular direction of vertical or horizontal gratings than oblique gratings. We discovered that this oblique effect becomes very large at high spatial frequencies in the parafovea. Perceived direction was quantified with a direction matching task at spatial frequencies ranging from 7.6 to 22.6 c/deg. As spatial frequency increased, direction matches of oblique gratings deviated away from the diagonal and towards vertical or horizontal axes. Subjects reported that the higher spatial frequency gratings appeared as grainy noise, particularly at oblique orientations. Our results indicate that, in the parafovea, subjects perceive movement of high spatial frequencies mainly along principal meridians. One possible explanation for this effect is that the high frequency patterns are aliased by the irregular mosaic of parafoveal cones. The aliasing noise generated by irregular sampling contains spatial energy at all orientations, but perhaps only vertical and horizontal components of the noise are visible to the observer.

Adult↗
PubMed1995

Refraction, aliasing, and the absence of motion reversals in peripheral vision.

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd.m-2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial.(ABSTRACT TRUNCATED AT 250 WORDS)

Discrimination, Psychological↗