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J Rovamo

Publications and source records attributed to J Rovamo.

71 records · Page 4Linked to original sources
PubMed1984

Critical flicker frequency and M-scaling of stimulus size and retinal illuminance.

Using various stimulus areas and luminances we measured monocular critical flicker frequency (CFF) as a function of eccentricity in the temporal visual field. With constant stimulus area and luminance, CFF was not independent of visual field location. When stimulus area was scaled by the magnification factor of the human striate cortex to produce equal cortical stimulus areas from different retinal locations, CFF increased monotonically with increasing eccentricity. Hence, CFF cannot be made independent of visual field location by spatial M-scaling. However, when also retinal illuminance was M-scaled by reducing stimulus luminance in inverse proportion to Ricco's area at each eccentricity, CFF became independent of visual field location.

Flicker Fusion↗
PubMed1984

Temporal integration and contrast sensitivity in foveal and peripheral vision.

Spatial contrast sensitivity functions and temporal integration functions for gratings with dark surrounds were measured at various eccentricities in photopic vision. Contrast sensitivity decreased with increasing eccentricity at all exposure durations and spatial frequencies tested. The decrease was faster at high than at low spatial frequencies, but similar at different exposure durations. When cortically similar stimulus conditions were produced at different eccentricities by M-scaling, contrast sensitivity became independent of visual field location at all exposure durations tested. The results support the view that in photopic vision spatiotemporal information processing is qualitatively similar across the visual field, and that quantitative differences result from retino-topical differences in ganglion cell sampling. For gratings of constant retinal area temporal integration (improvement of contrast sensitivity with increasing exposure duration) was more extensive at high than at low retinal spatial frequencies but independent of cortical spatial frequency and eccentricity. For M-scaled gratings temporal integration was more extensive at high than at low cortical spatial frequencies but independent of retinal spatial frequency and eccentricity. The results suggest that the primary determinant of temporal integration is not spatial frequency but grating value that is calculated as AF2 square cycles (cycle2), where A is grating area and F spatial frequency.

Fixation, Ocular↗
PubMed1983

Cortical magnification factor and contrast sensitivity to luminance-modulated chromatic gratings.

Contrast sensitivity to luminance-modulated blue, green, red, and neutral gratings was measured at different spatial frequencies and eccentricities within 0-86 deg in the temporal visual field. Contrast sensitivity to gratings of constant area and spatial frequency was independent of wavelength composition but decreased with increasing eccentricity. When the gratings were scaled by the magnification factor of the human striate cortex to produce cortically similar stimulus conditions at different eccentricities (M-scaling), contrast sensitivities became independent of visual field location irrespective of grating colour. Using colour naming we found, in accordance with previous studies, that hue changed and saturation decreased when the eccentricity of a constant-size grating field increased. In contrast, the hue and saturation of M-scaled grating fields were independent of eccentricity. The results suggest that the effects of eccentricity on photopic colour vision can largely be counteracted by M-scaling which adjusts the spatial aspects of stimuli with respect to the decrease in ganglion cell density and increase in receptive field size towards the periphery of the visual field.

Color Perception↗
PubMed1983

Contrast sensitivity in evaluation of visual impairment due to diabetes.

Spatial contrast sensitivity of 19 diabetics with different degrees of visual impairment was studied. It was found that contrast sensitivity at intermediate and low spatial frequencies may decrease without corresponding loss of visual acuity. In advanced cases of diabetes the opposite may be true: contrast sensitivity was better than expected on the basis of visual acuity. Thus both visual acuity and contrast sensitivity measurements are useful in the evaluation of the nature of visual impairment due to diabetic eye disease.

Color Perception Tests↗
PubMed1983

Contrast sensitivity in evaluation of visual impairment due to macular degeneration and optic nerve lesions.

Spatial contrast sensitivity was investigated in a group of 17 patients with macular degeneration or optic atrophy. It has been reported earlier that patients with optic atrophy may have reduced contrast sensitivity at low spatial frequencies without decrease of visual acuity, a phenomenon called 'hidden visual loss'. The present study shows that also the opposite may be true: patients with macular degeneration or optic atrophy may have greatly decreased visual acuity and yet normal or nearly normal contrast sensitivity at low spatial frequencies and reasonably good sensitivity at intermediate spatial frequencies. This phenomenon could be called 'hidden vision'. In normal individuals and many visually impaired patients the spatial contrast sensitivity at high and intermediate spatial frequencies is equal when measured using grating fields of different size. In patients with central scotoma due to macular degeneration or optic atrophy contrast sensitivity is dependent on the size of the grating field: the maximal contrast sensitivity is higher when larger grating fields are used. The size of the smallest grating field that is needed for normal contrast sensitivity values at low spatial frequencies is one measure of visual impairment in this group of patients. Another measure of the function of the eccentric viewing area is the grating acuity which may also vary as a function of the grating field size.

Humans↗
PubMed1983

Contrast sensitivity in monocular glaucoma.

The results of this selected group of glaucoma patients demonstrate that the effect of glaucoma on the contrast sensitivity can be assessed only after careful exclusion of all other causes for decrease in contrast sensitivity. Further development of methods is necessary.

Aged↗
PubMed1982

Temporal contrast sensitivity and cortical magnification.

We measured temporal and spatial contrast sensitivity functions of foveal and peripheral photopic vision at various locations in the nasal visual field. Sensitivity decreased monotonically with increasing eccentricity when it was measured by using the same test gratings at different eccentricities. When the gratings were normalized in area, spatial frequency, and translation velocity by means of the cortical magnification factor M so that the calculated cortical representations of the gratings became equivalent at different eccentricities, the temporal contrast sensitivity functions became similar at all eccentricities. The normalization was effective under all experimental conditions that included various kinds of temporal modulation from 0 to 25 Hz (movement, counterphase flicker and on-off flicker) and different threshold tasks (detection, orientation discrimination, and discrimination of movement direction), independently of the subjective appearances of the gratings at threshold. We conclude that central and peripheral vision are qualitatively similar in spatiotemporal visual performance. The quantitative differences observed without normalization seem to be caused by the spatial sampling properties of retinal ganglion cells that are directly related to the values of M used in the normalization.

Humans↗
PubMed1982

Resolution of gratings oriented along and across meridians in peripheral vision.

Grating resolution was measured at various locations of the visual field for four grating orientations. As an instance of the oblique effect, vertical and horizontal gratings produced the highest resolution values in the central area. At eccentricities larger than about 20 deg, the oblique effect was replaced by a meridional resolution effect, in which resolution was systematically best for meridionally oriented grating bars and worst for grating bars perpendicular to the visual-field meridians. The origin of the effect seems to be neural because it was not caused by peripheral refractive errors or optical distortion of the peripheral retinal image.

Humans↗
PubMed1981

Contrast sensitivity function in evaluation of visual impairment due to retinitis pigmentosa.

Spatial contrast sensitivity functions of 11 retinitis pigmentosa patients were studied. The patients represented 3 different stages of the disease: 1: in the very severely impaired patients the contrast sensitivity and grating resolution had decreased to a fraction of normal. 2: in severely impaired patients, who had lost peripheral field but had subjectively satisfactory vision in the central field, contrast sensitivity differed considerably from one patient to another, 3: some of the moderately impaired patients, who still had useful peripheral vision, had nearly normal contrast sensitivity in the central vision while they already had large ring scotomas. The severity of reduction in contrast sensitivity was poorly correlated with visual acuity, the size of the visual field and the age of the patient. Because contrast sensitivity function cannot be predicted by means of other clinical measurements, it should be included in evaluation of visual impairment due to retinitis pigmentosa. We also measured contrast sensitivity at low luminance levels: this procedure provided useful information for evaluation of vision in retinitis pigmentosa. The contrast sensitivity function of each patient agreed with the subjective view of the patient about his visual impairment and it also corresponded with the examiners' evaluation of the patient's performance in different visual and visuomotor tasks.

Adolescent↗
PubMed1980

Perceived spatial frequency of flickering gratings: a local spatial distortion.

Perceived spatial frequency of a low-frequency grating increases if the grating contrast is temporally modulated at about 4 to 16 Hz. The amount of increase can be up to 50% as estimated by a grating-match technique. It is shown here that the illusion is not a simple spatial-frequency shift phenomenon but also involves local spatial distortions. This was discovered by using the method of distance match, in which the perceived length of a spatial period or its multiples was estimated in a flickering grating. The results indicate that the illusory increase in perceived spatial frequency does not predict the perceived period length of a grating in a simple reciprocal manner. The decrease in the apparent period length is restricted to distances covering only one or two spatial cycles. This points out that care should be taken in interpreting various illusory changes in spatial frequency simply as shifts of frequency. The results are interpreted as demonstrating the operation of a size-analyzing mechanism which has a limited spatial bandwidth and which is sensitive to flicker-induced disturbances.

Humans↗
PubMed1979

Visual resolution, contrast sensitivity, and the cortical magnification factor.

This study shows that photopic contrast sensitivity and resolution can be predicted by means of simple functions derived by using the cortical magnification factor M as a scale factor of mapping from the visual field into the striate cortex. We measured the minimum contrast required for discriminating the direction of movement or orientation of sinusoidal gratings, or for detecting them in central and peripheral vision. No qualitative differences were found between central and peripheral vision, and almost all quantitative differences observed could be removed by means of a size compensation derived from M. The results indicated specifically that (1) visual patterns can be made equally visible if they are scaled so that their calculated cortical representations become equivalent; (2) contrast sensitivity follows the same power function of the cortical area stimulated by a grating at any eccentricity; (3) area and squared spatial frequency are reciprocally related as determinants of contrast sensitivity; and (4) acuity and resolution are directly proportional to M, and the minimum angle of resolution is directly proportional to M-1. The power law of spatial summation expressed in (2) and (3) suggests the existence of a central integrator that pools the activity of cortical neurons. This summation mechanism makes the number of potentially activated visual cells the most important determinant of visibility and contrast sensitivity. The functional homogeneity of image processing across the visual field observed here agrees with the assumed anatomical and physiological uniformity of the visual cortex.

Brain Mapping↗
PubMed1979

An estimation and application of the human cortical magnification factor.

Comparisons of the published data on the density D of receptive fields of retinal ganglion cells and on the cortical magnification factor M indicated that M2 is directly proportional to D in primates. Therefore, the human M can be estimated for the principal meridians of the visual field from the density-distribution of retinal ganglion cells and from the density of the centralmost cones. Using the previously published empirical data, we estimated the values of the human M and express the values in four simple equations that can be used for finding the value of M for any location of the visual field. The monocular values of M are not radially symmetric. These analytically expressed values of M make it possible to predict contrast sensitivity and resolution for any location of the visual field. We measured contrast sensitivity functions at 25 different locations and found that the functions could be made similar by scaling the retinal dimensions of test gratings by the inverse values of M. Visual acuity and resolution could be predicted accurately for all retinal locations by means of a single constant multiplier of the estimated M. The results indicate that the functional and structural properties of the visual system are very closely and similarly related across the whole retina. Visual acuity, e.g., bears the same optimal relation to the density of sampling executed by retinal ganglion cells at all locations of the visual fields.

Brain Mapping↗
PubMed1978

Receptive field density of retinal ganglion cells and cortical magnification factor in man.

A mathematical relationship is derived between the density or retinal ganglion cell receptive fields and cortical magnification factor. The derivation is based on an assumed constant density of visual fibres entering the striate cortex and on a continuous retinotopic mapping of the visual field thereto. After considering the effect of cortical ocular-dominance columns the human cortical magnification factor along the principal meridians of the visual field is calculated from the empirical estimates of receptive field density of ganglion cells in human retina. The computed estimates of cortical magnification factor agree with experimental data obtained from studies of cortically induced phosphenes, visual migraine hallucinations, and contrast sensitivity functions.

Computers↗
PubMedDate not supplied

Modelling of orientation discrimination across the visual field.

In many visual tasks thresholds can be made equal across the visual field by increasing the size of peripherally presented stimuli by appropriate magnification. The parameter E2 represents the eccentricity at which foveal stimulus size must double in order to maintain performance equivalent to that at the fovea. We measured orientation discrimination for a set of line sizes at various eccentricities. Thresholds were successfully scaled by applying a scaling function estimated by a technique that requires no prior assumptions regarding the size of peripherally presented stimuli. E2 of 1.95 deg was found for orientation discrimination. From our results we were able to develop a single equation which predicts orientation discrimination thresholds for an average observer over a range of different eccentricities and line lengths.

Discrimination, Psychological↗