Jyrki Rovamo

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.

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.

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 Riccos area at each eccentricity, CFF became independent of visual field location.

Critical flicker frequency as a function of stimulus area and luminance at various eccentricities in human cone vision: A revision of granit-harper and ferry-porter laws

When the photopic luminous flux collected by ganglion cells was kept constant at all retinal locations by reducing average stimulus luminance in inverse proportion to photopic Riccos area (F-scaling), critical flicker frequency to stimuli of 1.2-88 deg2 in area, presented at various eccentricities along the temporal meridian of the visual field, increased as a single logarithmic function of the number of retinal ganglion cells stimulated. Their number was calculated by multiplying stimulus area by the ganglion cell receptive field density of the human retina. When the number of ganglion cells stimulated was kept constant by enlarging the stimulus area in inverse proportion to the ganglion cell density (M-scaling), the logarithm of CFF to green, yellow, orange and red cone-targets increased as parallel linear functions of logarithmic flux, calculated by multiplying retinal illuminance by photopic Riccos area.

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 resonably 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.

Perimetry of critical flicker frequency in human rod and cone vision

Photopic critical flicker frequency (CFF) to green and yellow-red targets became independent of visual field location when the decrease in the density of retinal ganglion cells and increase in their receptive-field size towards the retinal periphery were compensated for by increasing stimulus area in inverse proportion to the human cortical magnification factor squared (M-scaling) and by reducing stimulus luminance in inverse proportion to Riccos area (F-scaling). In mesopic and scotopic vision CFF to green targets increased monotonically with eccentricity despite MF-scaling. Instead, CFF to MF-scaled yellow-red targets that predominantly stimulated cones was independent of eccentricity at all luminance levels tested.

Texture discrimination at different eccentricities

Differences in preattentive texture discrimination between central vision and peripheral vision were studied with textures composed of random dots. The subject had to discriminate between two textures whose first-order statistics were kept identical but whose second-order statistics were different. For textures of constant retinal size the discrimination was easy in central vision, but the decrease of visual acuity with increasing eccentricity made the textures unresolvable in peripheral vision. When the textures were scaled by the cortical magnification factor derived from the frequency of retinal ganglion cells so that the calculated neural representations of the textures became similar at different eccentricities, texture discrimination became independent of visual field location. This indicates that preattentive texture discrimination based on differences in second-order statistics of random dots operates similarly in central vision and peripheral vision.

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 patients performance in different visual and visuomotor tasks.

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 retinotopical 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.

Retinal ganglion-cell density and receptive-field size as determinants of photopic flicker sensitivity across the human visual field

At 1, 10, and 50 Hz, photopic flicker sensitivity to a nonpatterned stimulus of constant area and luminance with a small equiluminous surround tended to decrease when eccentricity increased from 0 to 70 deg. The decrease was steeper for lower flicker frequencies. When the stimulus and surround were M scaled by magnifying them in inverse proportion to retinal ganglion-cell sampling density, flicker sensitivity tended to increase with eccentricity. The increase was steeper for higher flicker frequencies. When the stimulus and surround were F scaled by reducing their average luminance in inverse proportion to Riccos area, flicker sensitivity again decreased with increasing eccentricity, but now the decrease was steeper for higher flicker frequencies. When the stimulus and surround were MF scaled, flicker sensitivity became independent of eccentricity at all flicker rates tested.