Biomedical subjects

K T Mullen

Publications and source records attributed to K T Mullen.

13 recordsLinked to original sources

PubMed1992

Absence of smooth motion perception in color vision.

We have tested the behavioral evidence for a separation of the processing of color contrast from motion in the human visual system. Two different aspects of motion perception are examined; the identification of the direction of movement of a chromatic grating and the perception of smooth motion. The results show that color vision is at no great disadvantage in the identification of direction of movement, since this can be done at color contrasts quite close to detection threshold over a wide range of spatial and temporal frequencies. However, we find that subjects can identify direction without having the genuine perception of smooth motion. Smooth motion perception is revealed to be highly impaired since it is detected only at very high color contrasts and over a narrow range of spatial temporal conditions.

Color Perception↗

PubMed1992

Mutual rod-cone suppression within the central visual field.

Under mesopic conditions the contrast sensitivity of the central visual field is reduced as the result of a non-linear interaction between rod- and cone-mediated signals, each of which is capable of higher sensitivity in isolation. The interaction is produced only when the rod-mediated system is driven at flicker rates above 6 Hz. This finding bears upon how rod and cone signals are combined and therefore affects our interpretation of the significance of the relationship between retinal illuminance and both contrast sensitivity and temporal resolution.

Contrast Sensitivity↗

PubMed1992

Interactions between colour and luminance contrast in the perception of motion.

It has been demonstrated widely that at isoluminance moving chromatic stimuli are seen to be stationary or moving more slowly than their luminance counterparts. We have examined the effect on perceived velocity of adding luminance contrast to an isoluminant chromatic stimulus. We show that moving luminance contrast 'captures' colour so that a combined colour and luminance stimulus is seen moving as a unified percept. However, in the presence of colour contrast, significantly higher levels of luminance contrast are required to achieve a veridical velocity than for monochromatic stimuli with only luminance contrast. We show that this interactive effect between colour and luminance contrast cannot be fully explained by a threshold masking of luminance by colour contrast. The effect suggests that a breakdown in the veridical perception of velocity should be expected for colours with a wide range of associated luminance contrasts and not just for those at the point of isoluminance.

Color Perception↗

PubMed1991

Colour vision as a post-receptoral specialization of the central visual field.

The experiments address the question whether there is evidence that the central visual field is any more specialized for colour than it is for luminance contrast detection. The decline in contrast sensitivity across the visual field for colour-only (red-green) gratings is compared to that for monochromatic luminance gratings at a range of spatial frequencies in the nasal and temporal fields. Measurements are made of the chromatic spatial summation area and the relevant parts of the chromatic temporal contrast sensitivity function at different eccentricities in order to control for their influence on the decline in contrast sensitivity. Results show that at each spatial frequency colour contrast sensitivity declines with eccentricity approximately twice as steeply as luminance contrast sensitivity. The more rapid decline in colour contrast sensitivity than luminance contrast sensitivity across the visual field reveals that chromatic mechanisms are more confined than luminance mechanisms to the central field.

Color Perception↗

PubMed1991

Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors.

1. The aim of this study was to determine whether optical, receptoral or higher-order neural properties limit spatial resolution (acuity) in human vision, especially in the peripheral regions of the visual field. 2. Both achromatic and chromatic stimuli were used, and measures were taken to ensure that the resolution estimates were not contaminated by the detection of spatial sampling artifacts. Spatial contrast sensitivity functions were measured at retinal locations from 0 to 55 deg along the naso-temporal meridian for: (i) discriminating the direction of drift of luminance-modulated (black-white) sinusoidal stimuli drifting at 8 Hz (achromatic task); and (ii) for detecting isoluminant red-green sinusoidal stimuli drifting at 0.4 Hz (chromatic task). Achromatic contrast sensitivity functions were also measured along the vertical meridian for eccentricities of 8 and 40 deg. Each achromatic function was extrapolated to a contrast sensitivity of one (100% contrast) to estimate achromatic acuity. Chromatic acuities were obtained by expressing chromatic contrast in terms of cone contrasts and using the same method of extrapolation. We compared the results with recent data on human optical properties and retinal anatomy. 3. Both achromatic and chromatic acuity decline with distance from the fovea, but at a faster rate than that dictated by the known optical and/or receptoral properties of the human eye. We conclude that, for stimuli of either achromatic or chromatic contrast, peripheral spatial resolution is limited by post-receptoral mechanisms. Also, chromatic acuity declines more steeply than luminance acuity with eccentricity suggesting that there are additional post-receptoral limitations on colour resolution in the periphery. 4. A clear naso-temporal asymmetry is seen in the resolution whose dependence is qualitatively, but not quantitatively, similar to the Nyquist limits imposed by the asymmetric density of human retinal ganglion cells. We discuss the possibility that in peripheral vision (beyond the optic nerve head) the spacing of ganglion cells may pose a fundamental limit on the resolution of achromatic stimuli, but not chromatic stimuli.

Color Perception↗

PubMed1990

Wavelength discrimination at detection threshold.

The experiments that we report aim to elucidate the linkage between cone outputs and color sensation. This is investigated by measuring wavelength discrimination between stimuli at threshold levels of detection. Stimuli are large spots (0.75 deg) presented on a white background. A 2 x 2 alternate forced choice method is used to measure simultaneously the detection of different wavelengths and discrimination between them. This method reveals at least four distinguishable colors, indicating the presence of four different sets of mechanisms at threshold. These are associated with the color sensations of orange, pale yellow, green, and blue. There is also evidence for a fifth imperfectly distinguished color (violet) in the shortest wavelength region. Results show that the boundaries between the distinguishable colors have little variation in their spectral positions. This is compatible with the presence of fixed perceptual boundaries in the spectrum dividing the different types of detection mechanism. The correspondence of the spectral locations of the distinguishable colors to the cone opponent responses revealed in the spectral sensitivity function suggests that these color sensations are postreceptoral in origin, arising from different combinations of the three cone outputs.

Calibration↗

PubMed1989

The photoreceptors in atypical achromatopsia.

1. The receptoral mechanisms underlying the vision of two atypical achromats of the complete variety were studied with standard psychophysical procedures. 2. Under scotopic conditions the spectral sensitivity of each achromat was well described by the CIE (Commission Internationale de l'Eclairage) scotopic sensitivity function and the recovery of sensitivity after a retinal bleach showed characteristic duplex behaviour with the time constant of recovery of the slower phase matching that of normal rod vision for both foveal and peripheral stimulation. 3. Their spectral sensitivity was measured under conditions of chromatic adaptation in order to reveal any residual middle or long wavelength cone activity. Only one photopic spectral responses was found and this was adequately described by the spectral sensitivity function of Stiles pi 3 mechanism of normal vision. 4. Increment threshold measurements as a function of background intensity revealed a double-branched function in the fovea. The lower branch was found to have the spectral sensitivity of the rods; the upper branch that of Stiles' pi 3 mechanism. Stiles-Crawford measurements of directional sensitivity confirmed that the branch with the rhodopsin action spectrum had the directional sensitivity of rods and that the branch with the action spectrum of pi 3 had the directional sensitivity of cones. 5. These was no evidence for hue discrimination under photopic conditions. Regions of apparently normal performance on hue discrimination tests on more careful examination could be explained by luminosity judgements mediated by short wavelength-absorbing receptors. 6. We reject the notion of there being rhodopsin-filled cones in the fovea of these subjects. The foveal and peripheral vision of each of these achromats can be adequately described in terms of the participation of only two types of receptor, namely normally functional rods under scotopic conditions and normally functioning short wavelength-absorbing cones under photopic conditions. They are therefore functional blue mono-cone monochromats, an explanation which was originally proposed by Blackwell & Blackwell (1957) over thirty years ago.

Color Vision Defects↗

PubMed1989

Human photopic vision with only short wavelength cones: post-receptoral properties.

1. Spatial and temporal contrast sensitivities were investigated in two subjects whose photopic vision has been previously shown to be subserved by only short wavelength cones. 2. Spatial contrast sensitivity was uniformly reduced compared with that of the normal trichromatic observer. Peak contrast sensitivity reached 40 which is a factor of 2-3 better than previous estimates and extrapolated acuity was around 15 cycles deg-1. Central, non-aliased grating acuity was between 6-9 cycles deg-1. This declined with eccentricity such that at 20 deg it was around 1 cycle deg-1. 3. The variation in contrast sensitivity across the visual field was measured for a range of different spatial frequencies. It was found to be of the same form as that for the normal trichromat but reduced in overall sensitivity. 4. Temporal contrast sensitivity was measured for two different spatial frequencies and found to exhibit the spatio-temporal covariation which is typical of normal trichromatic vision. Temporal acuity exhibited a strong dependence on illuminance and reached asymptotic values of around 40-45 Hz. While this is more than a factor of two above most previous estimates for the short wavelength receptors of normal vision it agrees with some more recent estimates obtained using a different technique. Temporal resolution was found to be evenly distributed across the visual field. 5. Similarities were found between the post-receptoral properties of these achromats and the properties of the isolated blue mechanism of normal vision and also the properties of normal luminance contrast processing in general. The present results provide an upper bound on the contribution of the short wavelength mechanism to normal vision and also provide a suitable model of its possible contribution to the processing of luminance contrast in the normal visual system.

Adaptation, Ocular↗

PubMed1987

Spatial influences on colour opponent contributions to pattern detection.

The contribution of colour opponent mechanisms to detection thresholds is investigated at different spatial frequencies by presenting monochromatic, sinusoidal gratings on a uniform white background. Colour opponent mechanisms, characterised by a triple peaked spectral sensitivity function, determine threshold at low spatial frequencies (below 1 c/deg) and their contribution flattens the Weber function. They display low pass spatial frequency characteristics, becoming relatively more sensitive than non-opponent mechanisms as spatial frequency decreases. Colour opponent contributions are not revealed when the test grating and background are presented dichoptically.

Color Perception↗

PubMed1987

Tolerance to visual defocus.

Low-resolution optical systems are more tolerant to defocus than are high-resolution systems. We wished to determine whether this principle applies to human vision. We used psychophysical methods to measure the effects of defocus in normal eyes under low-resolution conditions. Modulation transfer of sine-wave gratings was measured as a function of dioptric defocus at low and medium spatial frequencies. We defined the depth of focus at a given spatial frequency to be the dioptric range for which the modulation transfer exceeds 50% of its peak value. For dilated pupils, depth of focus increased from about 2.5 diopters (D) at 3.5 cycles/deg to about 17 D at 0.25 cycles/deg. From our results we predicted that tasks requiring only low spatial frequencies will be more tolerant to defocus than tasks requiring higher spatial frequencies. This prediction was confirmed in a letter-recognition experiment. The increasing tolerance to defocus at low spatial frequencies also implies that individuals with low acuity will be more tolerant to defocus than people with normal vision. We confirmed this prediction by measuring tolerance to defocus in 30 low-vision eyes.

Humans↗

PubMed1986

Colour and luminance vision in human optic neuritis.

A comparison of sensitivities to chromatic and luminance stimuli has been carried out in patients with a past history of optic neuritis. Patients were selected with differing degrees of stable residual visual deficits, and with marked interocular differences in sensitivity. Threshold contrast sensitivity was measured to sinusoidal luminance gratings and to chromatic red/green and blue/yellow gratings, all with the same spatial frequency of 1 cycle per degree. A psychophysical criterion was used to ensure that detection of the chromatic grating was determined only by its colour differences. When the difference between the sensitivity to luminance and chromatic gratings was compared between the more and less severely affected eyes of each subject, it was found that, overall, chromatic sensitivity was more severely impaired than luminance sensitivity in the disorder. Sensitivities to the red/green and the blue/yellow stimuli were found to be affected equally.

Color Perception↗

PubMed1985

A motion aftereffect from an isoluminant stimulus.

We investigate whether a motion aftereffect (MAE) can be induced by an isoluminant stimulus which contains colour contrast but no luminance contrast. We created a red/green chromatic stimulus, composed of red and green monochromatic grating added in antiphase, and corrected for the chromatic aberrations of the eye. We varied the ratio of the red to green luminances in the stimulus and found that there is no luminance ratio at which the MAE disappears. The results suggest that isoluminant stimuli can induce a MAE which is as great and sometimes greater than that induced by luminance contrast.

Color↗

PubMed1985

The contrast sensitivity of human colour vision to red-green and blue-yellow chromatic gratings.

A method of producing red-green and blue-yellow sinusoidal chromatic gratings is used which permits the correction of all chromatic aberrations. A quantitative criterion is adopted to choose the intensity match of the two colours in the stimulus: this is the intensity ratio at which contrast sensitivity for the chromatic grating differs most from the contrast sensitivity for a monochromatic luminance grating. Results show that this intensity match varies with spatial frequency and does not necessarily correspond to a luminance match between the colours. Contrast sensitivities to the chromatic gratings at the criterion intensity match are measured as a function of spatial frequency, using field sizes ranging from 2 to 23 deg. Both blue-yellow and red-green contrast sensitivity functions have similar low-pass characteristics, with no low-frequency attenuation even at low frequencies below 0.1 cycles/deg. These functions indicate that the limiting acuities based on red-green and blue-yellow colour discriminations are similar at 11 or 12 cycles/deg. Comparisons between contrast sensitivity functions for the chromatic and monochromatic gratings are made at the same mean luminances. Results show that, at low spatial frequencies below 0.5 cycles/deg, contrast sensitivity is greater to the chromatic gratings, consisting of two monochromatic gratings added in antiphase, than to either monochromatic grating alone. Above 0.5 cycles/deg, contrast sensitivity is greater to monochromatic than to chromatic gratings.

Color Perception↗