Biomedical subjects

J Rovamo

Publications and source records attributed to J Rovamo.

At least 19 recordsLinked to original sources

PubMed1995

Detection of blue under chromatic adaptation: the effects of stimulus size and eccentricity.

We measured thresholds for the perception of blue under chromatic adaptation to white, green, yellow or red at the eccentricities of 0-70 deg in the temporal visual field of four subjects. We used a series of stimulus sizes at each eccentricity, without a prior assumption of any peripheral size-scaling factor. The CIE 1976 UCS (u',v') chromaticity coordinates corresponding to blue perception were subtracted from the chromaticity coordinates of the adaptation field in order to obtain the threshold differences (du',dv') in chromaticity coordinates. Spatial scaling factors for the perception of blue were obtained by non-linear regression (E2 + 5 deg) refers to the eccentricity at which stimulus diameter had to be doubled in order to maintain performance found at the eccentricity of 2.5 deg. E2 for the perception of blue tint varied from 1.2 to 36 deg depending on the state of chromatic adaptation and subject. For the perception of blue tint in yellow three subjects and for the perception of blue tint in red one subject had no spatial scaling factor that would make performance independent of eccentricity. Thus, spatial scaling does not always work.

Adaptation, Ocular↗

PubMed1995

Neural modulation transfer function of the human visual system at various eccentricities.

We measured r.m.s. contrast sensitivity as a function of retinal illuminance at various spatial frequencies within 3-37 deg of eccentricity in the nasal visual field. In dim light contrast sensitivity increased in proportion to the square root of retinal illuminance obeying the DeVries-Rose law but in bright light contrast sensitivity was independent of luminance following Weber's law. Critical retinal illuminance (Ic) marking the transition between the laws was found to be independent of grating area but proportional to the spatial frequency squared at all eccentricities, in agreement with the Van Nes-Bouman law of foveal vision. In addition, the proportionality constant was found to be independent of eccentricity and similar to that of the fovea. According to our contrast detection model of human vision the modulation transfer function (PMTF) of the neural visual pathways squared is directly proportional to the critical retinal illuminance. On this basis our result means that PMTF is similar, i.e. equal to spatial frequency across the visual field, thus attenuating low spatial frequencies relatively more than high spatial frequencies. Hence, up to the spatial cut-off frequency determined by the lowest neural sampling density of each retinal location the neural modulation transfer function is independent of visual location.

Adult↗

PubMed1995

Modelling the increase of contrast sensitivity with grating area and exposure time.

We extended the contrast detection model of human vision to temporal integration by taking into account the effect of exposure duration on contrast sensitivity for stationary gratings. The extended model thus comprised: (i) low-pass filtering due to the optical modulation transfer function of the eye; (ii) high-pass filtering (lateral inhibition) due to the neural modulation transfer function of the visual pathways; (iii) addition of internal neural noise; and (iv) detection by a local matched filter whose efficiency for gratings decreased with increasing area and exposure duration. To test the model we measured binocular contrast sensitivity in foveal photopic vision as a function of exposure duration and area for sinusoidal gratings with equiluminous surround at spatial frequencies of 0.25-16 c/deg. In agreement with the model, contrast sensitivity at all grating areas first increased in proportion to square root of t when exposure duration (t) was shorter than critical duration. Thereafter the increase saturated and contrast sensitivity became independent of exposure time. Critical exposure duration was found to be independent of grating area but increased with spatial frequency. Similarly, at all exposure durations contrast sensitivity first increased in proportion to square root of A when grating area (A) was smaller than critical area. Thereafter the increase saturated and contrast sensitivity became independent of area. Critical area was found to be independent of exposure duration but decreased with increasing spatial frequency. The extended model explained 95-97% of the total variance of our contrast sensitivity data at the spatial frequencies studied. Our results also mean that spatial and temporal integration processes are mutually independent and thus area and time are separable variables in the detection of stationary gratings.

Adult↗

PubMed1995

Effect of location and orientation uncertainty on r.m.s. contrast sensitivity with and without spatial noise in peripheral and foveal vision.

PURPOSE: We studied the effect of spatial location and orientation uncertainty on r.m.s. contrast sensitivity with and without external spatial noise in peripheral and foveal vision. METHODS: In the first experiment we used a small circular cosine grating with randomized location embedded in spatial noise and exposed for 33 to 533 ms in peripheral vision. In the second experiment, performed with and without noise, we varied the randomization range of stimulus location in a foveal search task with free eye movements and an unlimited exposure time. In the third experiment we used a vertical cosine grating exposed for 500 ms and varied the randomization range of aperture orientation in the fovea with and without noise. RESULTS: Uncertainty of spatial location had no effect on r.m.s. contrast sensitivity in the periphery or fovea. However, sensitivity decreased with increasing randomization range of aperture orientation in the fovea. CONCLUSIONS: Uncertainty of spatial location had no effect because the accuracy of positional information is inherently poor in peripheral vision, whereas in the fovea the effect of location uncertainty was compensated for by searching eye movements. Randomization of aperture orientation reduced contrast sensitivity in the fovea because in this case the effect of randomization could not be compensated for.

Adult↗

PubMed1995

Masking potency and whiteness of noise at various noise check sizes.

PURPOSE: The masking effect of spatial noise can be increased by increasing either the rms contrast or check size of noise. In this study, the authors investigated the largest noise check size that still mimics the effect of white noise in grating detection and how it depends on the bandwidth and spatial frequency of a grating. METHODS: The authors measured contrast energy thresholds, E, for vertical cosine gratings at various spatial frequencies and bandwidths. Gratings were embedded in two-dimensional spatial noise. The side length of the square noise checks was varied in the experiments. The spectral density, N(0,0), of white spatial noise at zero frequency was calculated by multiplying the noise check area by the rms contrast of noise squared. RESULTS: The physical signal-to-noise ratio at threshold [E/N(0,0)]0.5 was initially constant but then started to decrease. The largest noise check that still produced a constant physical signal-to-noise ratio at threshold was directly proportional to the spatial frequency. When expressed as a fraction of grating cycle, the largest noise check size depended only on stimulus bandwidth. The smallest number of noise checks per grating cycle needed to mimic the effect of white noise decreased from 4.2 to 2.6 when the number of grating cycles increased from 1 to 64. CONCLUSION: Spatial noise can be regarded as white in grating detection if there are at least four square noise checks per grating cycle at all spatial frequencies.

Adult↗

PubMed1994

Effects of luminance and external temporal noise on flicker sensitivity as a function of stimulus size at various eccentricities.

We studied how the dependence of flicker sensitivity on stimulus size was affected by the eccentricity of the stimulus at high luminance, at low luminance (with quantal noise), and at high luminance with the addition of pure white temporal noise. Flicker sensitivity was measured as a function of stimulus size for temporal frequencies of 1-30 Hz with uniform sinusoidally flickering spots. Sensitivity first increased with increasing stimulus size but then the increase saturated. At high luminance the saturation took place at larger stimulus sizes with increasing eccentricity. Without externally added temporal noise the maximum sensitivity was higher at the fovea than in the periphery at temporal frequencies of 1-10 Hz, but at 30 Hz this situation reversed. Therefore only the ascending parts of the spatial integration curves from various eccentricities could be superimposed by size scaling. E2, the eccentricity at which the spatial scale doubles, was found to be 2.2-2.7 deg for 1-10 Hz but 4.4 deg for 30 Hz. When enough temporal noise was added, performance at all stimulus sizes studied could be made independent of eccentricity by spatial scaling, since noise reduced maximum sensitivities to a constant level at all eccentricities. E2 was found to be 4.1 deg for 3 Hz and 7.2 deg for 30 Hz. When light level was reduced by 3 log10 units, foveal and peripheral flicker sensitivity functions almost superimposed at all stimulus sizes studied. Hence, at 1 and 3 Hz E2 was very large, about 70 and 22 deg, respectively. At 10 and 30 Hz no size scaling was needed and E2 was therefore infinite.

Adult↗

PubMed1994

Relationship between spatial integration and spatial spread of contrast energy in detection.

Detection efficiencies were measured for two kinds of grating stimuli. The stimuli of the first kind were uniform square shaped cosine gratings of various sizes but of constant spatial frequency. The stimuli of the second kind were composed of nine small grating patches of the same spatial frequency arranged into a square array. The array size was varied by changing the inter-patch distance. The efficiencies for the two kinds of stimuli obeyed the same decreasing function of area defined by the respective outlines of the grating patch array and the uniform grating. The result means that detection efficiency is not determined by the retinal area stimulated, but by the distances between different parts of stimulus, i.e. the spatial spread of contrast energy, which can be described by a radial moment measure computed from the image.

Contrast Sensitivity↗

PubMed1994

Illusory perception of gratings stimulating a small number of neurones.

We studied pattern perceptions caused by drifting gratings presented monocularly in the nasal and temporal visual fields at various suprathreshold contrasts. The grating and its surround and background were matched in luminance. Small grating produced illusions and reduced perceptions. When grating area or contrast increased from a subthreshold value, the gratings were first seen as mere flashes. Then each grating was sometimes perceived as a single small bright spot or point. Next each grating was seen as a single dark or bright line. Finally the stimuli were perceived as gratings consisting of several bars. Orientation or direction of movement were perceived correctly, but velocity, colour and number of bars were often perceived as illusions. Thus, in spite of the illusions, some features of the stimuli could have allowed correct discriminations. The area and contrast limits of illusory perception depended on eccentricity. Irrespective of retinal size, the stimuli were not perceived correctly as gratings at any eccentricities when the gratings were smaller than about 1 x 1 mm in their calculated cortical area and stimulated a small constant number of retinal ganglion cells. Relations between the results and retinal aliasing, cortical columns and phase locking of neuronal oscillations are discussed.

Contrast Sensitivity↗

PubMed1994

Modelling contrast sensitivity as a function of retinal illuminance and grating area.

We extended the contrast detection model of human vision [Rovamo, Luntinen & Näsänen (1993b) Vision Research, 33, 2773-2788] to low light levels by taking into account the effect of light-dependent quantal noise. The extended model comprises (i) low-pass filtering due to the optical modulation transfer function of the eye, (ii) addition of light-dependent noise at the event of quantal absorption, (iii) high-pass filtering of neural origin (lateral inhibition), (iv) addition of internal neural noise, and (v) detection by a local matched filter whose efficiency decreases with increasing grating area. To test the model we measured foveal contrast sensitivity as a function of retinal illuminance and grating area at spatial frequencies of 0.125-32 c/deg. In agreement with the model, monocular contrast sensitivity at all grating areas increased in proportion to I when retinal illuminance (I) was smaller than critical illuminance. Thereafter the increase saturated and contrast sensitivity became independent of retinal illuminance. Similarly, at all levels of retinal illuminance contrast sensitivity increased in proportion to A when grating area (A) was smaller than critical area. Thereafter the increase saturated and contrast sensitivity became independent of area. Critical level of retinal illuminance increased in proportion to the spatial frequency squared. Critical area marking the saturation of spatial integration was constant at low spatial frequencies but decreased in inverse proportion to spatial frequency squared at medium and high spatial frequencies. The maximum contrast sensitivity obtainable by spatial integration in bright light increased at low spatial frequencies in proportion to spatial frequency, was constant at medium spatial frequencies, and decreased in inverse proportion to spatial frequency cubed at high spatial frequencies. The increase was due to the neural modulation transfer function of the visual pathways whereas the decrease was due to the optical modulation transfer function of the eye. The model explained 91-99% of the total variance of our contrast sensitivity data at various spatial frequencies.

Adult↗

PubMed1994

Two simple psychophysical methods for determining the optical modulation transfer function of the human eye.

The foveal optical modulation transfer function (OMTF) of the human eye for a pupil 8 mm in diameter was determined at 1-23 c/deg by using two new methods. The first method was based on the comparison of the effects of quantal and added spatial noise on grating contrast sensitivity at each spatial frequency. The rationale behind the comparison is the fact that quantal noise is not affected by the OMTF of the human eye, because individual quanta cannot be blurred by the point spread function of ocular optics. In the second method we measured spatial contrast sensitivity for gratings whose area decreased but illuminance increased in proportion to spatial frequency squared. For such gratings the variation of contrast sensitivity with spatial frequency (f > or = 0.5 c/deg) is solely due to the OMTF of the human eye, because the effects of grating area and retinal illuminance (quantal noise) on contrast sensitivity are independent of spatial frequency. The two methods provided identical OMTFs. The foveal optical modulation transfer function of the human eye for an 8 mm pupil was found to be OMTF = [1 + (f/11.1)2.5]-1, where f refers to spatial frequency in c/deg. The equation means that the OMTF is constant at low spatial frequencies, becomes reduced to 50% at 11.1 c/deg and decreases thereafter with a slope of -2.5 when plotted in double logarithmic coordinates. At 1-16 c/deg our OMTF showed less image degradation than any foveal OMTF of the human eye for 6-8 mm pupils reported so far in the literature.

Adult↗

PubMed1994

Contrast matching of two-dimensional compound gratings.

The contrast of a two-dimensional compound grating consisting of 2, 3 or 4 components with the same spatial frequency but different orientations was matched to the contrast of a simple cosine grating. At all spatial frequencies and contrast levels studied the stimuli which matched in apparent contrast had nearly equal r.m.s. (root-mean-square) but different Michelson contrasts. Thus, matching was based on r.m.s. contrast which takes into account the distribution of luminance levels in the stimulus. A matched filter provides a simple model to explain the contrast matching results.

Contrast Sensitivity↗

PubMed1994

Three-dimensional illusory objects produced by rotation in depth.

Rotation of a Kanizsa triangle in depth around its vertical axis causes a perception of a three-dimensional object with a flat, rigid illusory triangle between the inducing discs. When the inducing discs of a Kanizsa triangle were made thicker, the illusory triangle between the discs also became thicker. In the experiments both computer animation and real inducers made of plastic were used. The method promoted border perception in a three-dimensional illusory figure. We suggest that the perception of three-dimensional illusory objects is due to a process which is also used in the perception of real three-dimensional objects.

Adult↗

PubMed1994

Perception of green and red under chromatic adaptation: the effects of stimulus size and eccentricity.

We measured thresholds for the perception of green and red light added to white, blue, green, yellow, or red adaptation fields presented at eccentricities of 0 to 70 degrees in the temporal visual field of two subjects. A series of stimulus sizes was used at each eccentricity to determine size-scaling factor E2--the eccentricity at which stimulus diameter has to be doubled in order to maintain foveal performance. For green light added to white or yellow, E2 varied from 3.1 to 41.5 degrees. When red light was added to white, yellow, or blue, E2 varied from 0.1 to 11.8 degrees. For red color in green and green color in blue or red there was no spatial scaling factor that would make performance independent of eccentricity. Our results suggest that E2 value for green and red perception depends on the state of chromatic adaptation and also shows marked interindividual differences.

Adaptation, Ocular↗

PubMed1994

Effects of colour adaptation and stimulus size on the detection of chromatic deviations from achromatic as a function of eccentricity in man.

By using constant size and M-scaled stimuli (the stimulus size was magnified towards the visual field periphery in inverse proportion to the lowest local sampling density of the human retina) we measured the thresholds for perceiving the complementary colours of blue, green and red (i.e. yellow, purple or blue-green) under chromatic adaptation at the eccentricities of 0-15 degrees in the nasal visual field. The CIE 1931 (x, y) chromaticity coordinates corresponding to complementary hue perception were subtracted from the chromaticity coordinates of achromatic threshold. The difference was found to be constant irrespective of stimulus size and eccentricity. This means that the perception of chromatic deviation from achromatic under chromatic adaptation is independent of stimulus size and eccentricity.

Adaptation, Ocular↗

PubMed1994

Spatial integration of compound gratings with various numbers of orientation components.

PURPOSE: The human foveal visual system in a detection task was recently modeled as a simple image processor comprising low-pass filtering due to the optical modulation transfer function of the eye, high-pass filtering (lateral inhibition) due to the neural modulation transfer function of visual pathways, addition of internal neural noise, and detection by a local matched filter, the efficiency of which decreases with increasing grating area. The applicability of this model was now tested by studying spatial integration for sums of various numbers of cosine gratings with different orientations. METHODS: Binocular root-mean-square contrast sensitivity was measured as a function of area for sums of cosine gratings (n = 1 to 16) with the same contrast, phase, and spatial frequency but with an orientation difference of 180/n between the components. RESULTS: In agreement with the model, contrast sensitivity increased in proportion to the square root of grating area at small areas. When grating area exceeded its critical value, the increase saturated, and contrast sensitivity then became independent of area. The critical area and maximum contrast sensitivity of spatial integration first decreased with an increasing number of components, reaching minima at n = 5 to 6, but increased thereafter. A plausible explanation for the variation of critical area and maximum sensitivity could be the variation of the amount of contour and detail per unit area in the sums of cosine gratings. Critical area divided by maximum sensitivity squared refers to the contrast energy threshold at small grating areas. It was independent of the number of components but, because of lateral inhibition, decreased in inverse proportion to spatial frequency squared. Contrast energy threshold as a function of normalized grating area (grating area divided by critical area) also was independent of the number of components and decreased in inverse proportion to spatial frequency squared. CONCLUSIONS: Within the framework of the local matched filter model, the dependency of contrast sensitivity on the grating area and number of orientation components resulted from the decrease in the efficiency of contrast energy collection, which was probably due to the increasing amount of contour and detail in the stimulus to be detected.

Adult↗

PubMed1994

Detection efficiency of circular gratings and bandpass filtered points with randomized phase spectra.

PURPOSE: In the studies of spatial integration an increase in the spatial extent of the stimulus usually results in a decrease in the spatial frequency bandwidth of the stimulus. The authors investigated separately the effects of these two factors on contrast detectability. METHODS: Efficiencies were measured for a circular grating at 4 c/deg and for bandpass-filtered point stimuli having a constant center frequency at 4 c/deg and bandwidths of 0.25, 0.5, 1, and 2 octaves. The phase range of these two-dimensional stimuli was increased from zero to 90, 180, 270, and 360 degrees by replacing the original zero phase at each spatial frequency component by a random number with zero mean. This procedure left the spatial frequency bandwidth unaffected. RESULTS: The increase in phase range and decrease in spatial frequency bandwidth caused a progressively larger proportion of the contrast energy of the point stimuli to spread into their surroundings. As a result, detection efficiency decreased with increasing bandwidth and phase range for all point stimuli. However, a change in the stimulus bandwidth affected efficiency only when it altered stimulus area. The area of the circular grating and its detection efficiency remained almost constant irrespective of the phase range. When efficiency was plotted in semi-logarithmic coordinates as a function of stimulus area expressed in terms of the spatial spread of contrast energy, the line of least squares explained 85% of the total variance. CONCLUSION: The primary determinant of detection efficiency for stimuli with constant center spatial frequency is not stimulus bandwidth but stimulus area expressed in terms of the spatial spread of contrast energy.

Contrast Sensitivity↗

PubMed1993

The effects of grating area and spatial frequency on contrast sensitivity as a function of light level.

Contrast sensitivity was measured as a function of retinal illuminance (I) for vertical cosine gratings of various circular areas (A) and spatial frequencies (f < or = 4 c/deg). Spatial frequency and grating diameter varied in inverse proportion to each other in order to keep the relative grating area (A x f2) constant at either 3.14, 12.6, 50.3, or 201 square cycles. At all grating areas and spatial frequencies contrast sensitivity in dim light first increased in proportion to the square root of retinal illuminance. Then the increase saturated and contrast sensitivity became independent of luminance level in bright light. For gratings with constant relative area contrast sensitivity functions were similar in shape and had the same maximum sensitivity but were shifted horizontally towards lower illuminances with decreasing spatial frequency. However, when replotted as a function of retinal illuminance divided by spatial frequency squared, contrast sensitivity functions fell on a common curve at all levels of relative retinal illuminance (I/f2).

Adult↗

PubMed1993

Michelson contrast, RMS contrast and energy of various spatial stimuli at threshold.

Contrast sensitivity was measured as a function of median spatial frequency for vertical cosine gratings, narrow-band noise stimuli, and spots with luminance increment or decrement. Contrast sensitivity was expressed in terms of Michelson contrast, RMS contrast, and contrast energy in order to demonstrate the characteristics of various contrast measures. Gratings and noise stimuli had either constant stimulus area or constant number of cycles. Michelson contrast sensitivity was better for gratings than for noise stimuli or spots, whereas RMS contrast sensitivity was almost equal for gratings and noise stimuli but lowest for spots. Contrast energy sensitivity takes into account the stimulus area and was therefore best for spots.

Contrast Sensitivity↗