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Biomedical subjects

J Rovamo

Publications and source records attributed to J Rovamo.

At least 55 records · Page 3Linked to original sources
PubMed1993

Spatial and temporal properties of illusory figures.

The best known example of illusory figures is the Kanizsa triangle consisting of three disks with a sector removed. The disks and sectors are arranged so that they form the corners of a triangle. Although the sides of the triangle are not physically present, they are clearly visible to the observer. In this study the effect of sequential presentation of the inducing disks on an illusory (Kanizsa) triangle was investigated. The task of each subject was to find, by the method of adjustment, the longest critical duration that allowed perception of the illusory triangle produced by presenting the three inducing disks sequentially. We varied the sizes and/or separations of disks. An illusory triangle produced by flashing the three inducing disks simultaneously for 33 msec served as a comparison stimulus. Our experiments showed that increasing the inducing disk size or reducing the inter-disk distance increased critical duration. The result means that the shorter the illusory contour to be induced the longer the critical duration and vice versa. Thus, if the inductive disks are less separated in space they can be more separated in time and vice versa. These findings seem to agree with the suggestion that illusory contours emerge from the synchronization of gamma-waves emitted by the neurons in the visual cortex.

Adult↗
PubMed1993

Spatial integration of band-pass filtered patterns in noise.

Contrast energy thresholds were measured for noisy band-pass filtered Sloan letters, K, H, O, and a plus symbol (+) at various centre frequencies and bandwidths. For all patterns the efficiency and energy sensitivity of detection increased with filter bandwidth but decreased with increasing filter centre frequency. Hence, neither parameter alone can explain the changes in efficiency and energy sensitivity. Irrespective of centre frequency and bandwidth, efficiency and energy sensitivity were found to decrease as a single power function of image complexity (Z) defined as the product of the square of the median spatial frequency (fm) of the image energy spectrum and image area (A95) comprising 95% of total contrast energy of the stimulus. The product Z = fm2A95 describes image complexity in the sense that when f2m or A95 increase the number of details increase, too. The decrease of the efficiency of detection with increasing image complexity suggests that the proportion of the integrated image area decreases as the complexity of an image increases.

Adult↗
PubMed1993

Detection and discrimination of curvature in foveal and peripheral vision.

Curvature detection and discrimination were measured across the visual field in order to determine whether performance at a given eccentricity could be related to that at another by a simple change of magnification. Both types of task obeyed this concept, the critical variable being the rate at which magnification must increase as a function of eccentricity. The situation can be expressed in terms of the eccentricity (E2) at which magnification must be doubled to retain performance equivalent to that at the fovea. E2 values were 1.84-1.96 deg for the detection task and 1.42-2.27 deg for discrimination. These are similar to E2 values for vernier acuity and orientation discrimination, suggesting that all of these tasks share a common determining feature across the visual field, perhaps the distribution of neural elements selective for stimulus orientation.

Discrimination, Psychological↗
PubMed1993

Modelling the dependence of contrast sensitivity on grating area and spatial frequency.

We modelled the human foveal visual system in a detection task as a simple image processor comprising (i) low-pass filtering due to the optical transfer function of the eye, (ii) high-pass filtering of neural origin, (iii) addition of internal neural noise, and (iv) detection by a local matched filter. Its detection efficiency for gratings was constant up to a critical area but then decreased with increasing area. To test the model we measured Michelson contrast sensitivity as a function of grating area at spatial frequencies of 0.125-32 c/deg for simple vertical and circular cosine gratings. In circular gratings luminance was sinusoidally modulated as a function of the radius of the grating field. In agreement with the model, contrast sensitivity at all spatial frequencies increased in proportion to the square-root of grating area at small areas. When grating area exceeded critical area, the increase saturated and contrast sensitivity became independent of area at large grating areas. Spatial integration thus obeyed Piper's law at small grating areas. The critical area of spatial integration, marking the cessation of Piper's law, was constant in solid degrees at low spatial frequencies but inversely proportional to spatial frequency squared at medium and high spatial frequencies. At low spatial frequencies the maximum contrast sensitivity obtainable by spatial integration increased in proportion to spatial frequency but at high spatial frequencies it decreased in proportion to the cube of the increasing spatial frequency. The increase was due to high-pass filtering of neural origin (lateral inhibition) and the decrease was mainly due to the optical transfer function of the eye. Our model explained 95% of the total variance of the contrast sensitivity data.

Computer Simulation↗
PubMed1993

Effects of luminance and exposure time on contrast sensitivity in spatial noise.

Using gratings with and without spatial noise we measured r.m.s. contrast sensitivity as a function of (i) retinal illuminance, (ii) amount of light added onto the screen, and (iii) exposure time. Our experiments showed that contrast sensitivity in external spatial noise was independent of decreasing retinal illuminance and exposure time as long as contrast sensitivity was lower with noise than without. Thereafter the decrease of contrast sensitivity was identical with and without noise. Although contrast sensitivity without external spatial noise was independent of the amount of added light, contrast sensitivity in external spatial noise was found to increase with the amount of added light until it reached the sensitivity measured without noise. Thereafter contrast sensitivities with and without noise were identical. Our results (i) provide experimental evidence for the validity of the generally accepted hypothesis that in the human contrast detection mechanism signal-to-noise ratio is constant at threshold and (ii) indicate that external spatial noise is the principal source of noise when it reduces contrast sensitivity.

Adult↗
PubMed1992

The effects of colour adaptation and stimulus size on white perception as a function of eccentricity in man.

We studied how much desaturating complementary colour had to be added to blue, green or red after homochromatic adaptation of short duration in order to perceive white or grey at eccentricities of 0-15 deg in the nasal visual field. The CIE 1931 (x,y) chromaticity coordinates corresponding to achromatic perception were subtracted from the chromaticity coordinates of blue, green and red in order to obtain the threshold differences (dx, dy) in chromaticity coordinates. When the stimulus size was constant at all visual field locations, the absolute values of dx and dy decreased with increasing eccentricity, which means that the desaturation threshold for white or grey perception was higher at the fovea than in the periphery. However, when the stimulus size was M-scaled magnifying its size with increasing eccentricity in inverse proportion to the lowest local sampling density of the human retina (cones and ganglion cells at eccentricities 0-10 and above 10 deg, respectively), dx and dy with blue became independent of visual field location except for the very centre of the fovea. However, M-scaling was unsuccessful with green and red. Although M-scaling with green reduced the change in dx and dy as a function of eccentricity, it had no effect on dx and dy with red colour.

Adaptation, Ocular↗
PubMed1992

The influence of eccentricity on position and movement acuities as revealed by spatial scaling.

The rate of decline with increasing eccentricity of several position and movement acuities was measured using a method of spatial scaling. In this method all stimuli at each visual field location are simply magnified versions of each other. The influence of separation and eccentricity were dissociated by presenting stimuli on an iso-eccentric arc. For each task, the rate of decline in performance was quantified by the parameter E2 which represents the eccentricity at which stimulus size must double in order to maintain performance equivalent to that at the fovea. All tasks were found to obey the concept of spatial scaling in that performance at a given field location could be equated with performance at any other location simply by a change of scale. However, the rate at which performance deteriorated with eccentricity varied over an enormous range (over 100-fold) depending on the task itself. The advantage of such diverse peripheral gradients is clear; the goal is to establish the physiological mechanisms which underlie this phenomenon.

Discrimination, Psychological↗
PubMed1992

Contrast sensitivity as a function of spatial frequency, viewing distance and eccentricity with and without spatial noise.

Using computer graphics and a two-alternative forced-choice method we measured threshold contrast as a function of viewing distance, spatial frequency, and eccentricity for gratings with and without added, white two-dimensional spatial noise. Our experiments showed that in spatial noise contrast sensitivity was independent of viewing distance as long as contrast sensitivity was lower with noise than without. With increasing spatial frequency (f) the grating area (A) was reduced in order to keep the relative grating size (Af2) constant. At all spatial frequencies the test gratings thus had the same amount of detail and contour. Noise spectral density was reduced in direct proportion to grating area in order to keep the physical signal-to-noise ratio constant. An increase in spatial frequency was thus accompanied with reductions in grating area and noise spectral density similar to those produced by a corresponding increase in viewing distance. In agreement, contrast detection in spatial noise was found to be independent of spatial frequency as long as contrast sensitivity was lower with noise than without. The effect of increasing eccentricity on visual performance can be compensated for by reducing the viewing distance (M-scaling). Hence, without M-scaling the effect of increasing eccentricity is similar to that of increasing viewing distance. In agreement, we found that contrast sensitivity in spatial noise was independent of eccentricity as long as contrast sensitivity was lower with noise than without.

Adult↗
PubMed1992

Spatial scaling of vernier acuity tasks.

Vernier acuity thresholds for two abutting lines and for a two-dot stimulus were measured as a function of stimulus magnification at eccentricities of 0, 5, 10 and 15 deg using a spatial scaling technique in which all stimuli are simply magnified versions of each other. The advantage of such a technique is that no prior knowledge of a suitable magnification factor with which to increase the size of peripheral stimuli is required. Thresholds for the line stimulus could be successfully scaled by the application of a magnification factor with an E2 value of 1.23-1.78 deg. Further, provided that the effects of dot separation and eccentricity were dissociated, spatial scaling with an E2 value of 1.06-1.96 deg was also successful in removing eccentricity dependence for two-dot vernier thresholds.

Fovea Centralis↗
PubMed1991

Detection of chromatic deviations from white across the human visual field.

We studied how much blue, green, or red light had to be added to or subtracted from white to obtain veridical hue perception (blue, green, red, or their complementary colours) at various locations in the temporal visual field. The CIE 1931 (x, y) chromaticity coordinates corresponding to a veridical hue perception were subtracted from the chromaticity coordinates of the white (0.35, 0.35) in order to obtain the threshold differences (dx, dy) in chromaticity coordinates. When stimulus size was constant at all visual field locations, dx and dy changed with eccentricity. However, when the stimulus was M-scaled by magnifying its size with increasing eccentricity in inverse proportion to the lowest local sampling density across the human retina (cones and ganglion cells at eccentricities 0-10 and above 10 deg, respectively), dx and dy remained constant at all eccentricities.

Adult↗
PubMed1991

Critical flicker frequency to red targets as a function of luminance and flux across the human visual field.

When the number of cells (cones at eccentricities 0-10 deg and ganglion cells above 10 deg) stimulated at various retinal locations was kept constant by enlarging the stimulus area with increasing eccentricity in the temporal visual field (M-scaling), CFF to red stimuli with dark surround increased as a single function of photopic luminous flux, collected by ganglion-cell receptive-field centres and calculated by multiplying Ricco's area with retinal illuminance at each eccentricity studied. The increase of CFF with the logarithm of photopic flux could be best explained by the Collins logarithmic law, the Kelly square-root law was almost equally good and the Ferry-Porter law was poorest. Adopting the general formulation of Corwin and Dunlap (Vision Research, 27, 2119-2123, 1987) the exponent of CFF is 0, 0.5, and 1 for the Collins, Kelly and Ferry-Porter laws, respectively. The exponent that best explained our results was found to be 0-0.3.

Adult↗
PubMed1990

Cortical acuity and the luminous flux collected by retinal ganglion cells at various eccentricities in human rod and cone vision.

Using areally M-scaled, luminance-modulated orange-red and black-and-white gratings, we measured monocular resolution as a function of luminance at various eccentricities in the temporal visual field. In cone vision the increase of grating acuity with luminance became similar at all eccentricities when (1) acuity values were divided by the human cortical magnification factor to express grating resolution in cortical terms (c mm-1) and (2) retinal illuminance was multiplied by Ricco's area to express luminance in terms of photopic luminous flux. The same MF-scaling procedure also applied to scotopic acuity except that the amount of luminous flux collected by retinal ganglion-cell receptive-fields was in rod vision found to increase with eccentricity faster than photopic Ricco's area.

Adult↗
PubMed1989

Analysis of spatial structure in eccentric vision.

The analysis of spatial structure, ie, the encoding of relative positions between pattern elements, was studied in central and eccentric vision. In a two-alternative forced-choice task the observer had to discriminate between two patterns consisting of short line segments. At each trial the two patterns were flashed for 140 msec and the observer indicated whether the patterns were identical or mirror symmetric. Psychometric functions were measured by changing pattern size at each eccentricity in order to find the threshold size allowing 75% of correct responses. The scaling factor, required for discriminating between mirror symmetric and identical patterns independent of eccentricity, was found to be similar to the size-scaling proposed by Levi et al (Vision Res 25:963, 1985) for vernier acuity tasks.

Differential Threshold↗
PubMed1988

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 Ricco's 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 Ricco's area.

Adult↗
PubMed1987

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 Ricco's 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.

Adult↗
PubMed1987

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.

Discrimination, Psychological↗
PubMed1986

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 Ricco's 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.

Adult↗