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Biomedical subjects

B Stabell

Publications and source records attributed to B Stabell.

At least 19 recordsLinked to original sources
PubMed1999

Rod-cone color mixture: effect of size and exposure time.

Chromaticity coordinates of monochromatic lights were obtained 8 deg extrafoveally at a retinal illumination of 50 photopic td during the cone-plateau period and also with the subject in a dark-adapted state, while size and exposure time of the test field were varied. Unexpectedly, we found that for the dark-adapted condition the points representing the chromaticity matches of the spectral lights generally moved away from the achromatic point in the chromaticity diagram when size or exposure time of the test stimulus was increased. Furthermore, the chromaticity shift toward the achromatic point, obtained between the cone-plateau period and the dark-adapted state (i.e., with rod intrusion), tended to decrease with these two test parameters. In fact, when size and exposure time both were at the maximum level investigated (7 deg, 500 ms), there was no measurable shift in chromaticity with rod intrusion. Our results suggest that the cone system may become progressively more effective in suppressing the rod system as an effect of both size and exposure time.

Color Perception↗
PubMed1998

Chromatic rod-cone interaction during dark adaptation.

Chromatic rod-cone interaction in mesopic vision was investigated by measuring chromaticity coordinates of spectral lights 3 deg extrafoveally during long-term dark adaptation, under conditions where the eye was chromatically adapted. It was concluded that (a) the chromaticity shift obtained between the cone-plateau period and the dark-adapted state is due, at least under some conditions, to rod signals elicited by the test stimulus, (b) the chromaticity points of cone-mediated colors may change in different directions in the chromaticity diagram as an effect of the rod intrusion, (c) cone-cone and rod-cone color-mixture processing may, at least under some conditions, be different, and (d) chroma-related processes of rods and cones tend to suppress each other, with rods dominating at low and cones at high mesopic intensities.

Color↗
PubMed1996

Peripheral colour vision: effects of rod intrusion at different eccentricities.

Chromaticities of monochromatic lights from different parts of the spectrum were measured both during the cone-plateau period of the long-term dark-adaptation curve and in a completely dark-adapted state. The measurements were obtained at 3, 8, 30 and 65 deg in the temporal field of view and at 1, 2, 3 and 4 log units above the cone-plateau level. The results show that cone-mediated colours obtained during the cone-plateau period in general are desaturated when rod signals intrude during long-term dark adaptation. The desaturation effect of rods obtained at high mesopic illumination levels was found to increase when the test field was moved from 3 to 30 deg but to reduce markedly between 30 and 65 deg. Surprisingly, the desaturation was clearly observable even at a retinal illumination of 20,000 ph td. The desaturation effect of rods is explained by the suggestion that differences in ongoing activity rates of the different types of spectrally opponent cells become levelled out to some extent when light signals from rods intrude during dark adaptation.

Color Perception↗
PubMed1996

Long-term rod dark adaptation in man. Threshold measurements, rhodopsin regeneration and allosteric sensitivity regulation. An evaluation.

Recent evidence strongly suggests that the relationship between threshold elevation (T) and fraction of bleached rhodopsin (B), obtained during a major, middle period of long-term rod dark adaptation in man, is well described by a power function, i.e., T = k.Bn, where k is a multiplicative constant and n is the exponent. Due primarily to the low reliability of measurements of rhodopsin regeneration, however, the exponent n of the power function cannot, at present, be given an exact value. Available information indicates that the value of the exponent ranges between 2.4 and 4. Implications of this uncertainty are discussed within the framework of the allosteric, tetrameric model of rod dark adaptation. It is concluded that this model in its simplest form may only offer a first approximation of the real system implicated in the process.

Allosteric Regulation↗
PubMed1994

Mechanisms of chromatic rod vision in scotopic illumination.

After viewing a coloured patch for 30 sec, successive contrast colours were triggered by stimulating either rods or cones. The conditions were arranged so that the rod and cone stimuli matched both with respect to chromaticness and brightness in a chromatically neutral state of adaptation. The results showed that the contrast colours triggered by rods were strikingly similar to those triggered by cones. Yet, the scotopic contrast colours, as compared with the photopic ones, were generally found to be somewhat displaced toward blue. This displacement was attributed to the difference in test conditions. Thus, it was suggested that, although rods may excite all the different types of spectrally opponent cells, they generally tend to prefer the short-wave cells. Moreover, it was concluded that the scotopic successive contrast colours are triggered by rod signals feeding into the primary rod pathway and therefore must originate centrally to the receptor level.

Color Perception↗
PubMed1993

Rod-cone interaction in form detection.

Using a Wright colorimeter, absolute threshold, absolute form threshold and specific form threshold were measured during long-term dark adaptation in the extrafoveal retina. The specific form threshold was found to fall markedly at about the cone-rod break but thereafter rose steeply. Furthermore, during the rod phase of the dark adaptation the form percept of the small, slender rectangular test field changed qualitatively from a line or rectangle to a circular field at all mesopic intensities. The results indicate that light signals from rods may both facilitate and suppress cone-mediated information about form, and that the rod system may completely dominate the perception of form several log units above the absolute dark-adapted cone threshold when the eye is dark adapted.

Dark Adaptation↗
PubMed1992

Mechanisms of long-term dark adaptation.

It has previously been suggested that long-term dark adaptation is controlled by bleaching signals that regulate the activity of an allosteric, positively cooperative protein (Stabell et al., 1986a, b). Recent biochemical evidence strongly supports this assumption, indicating that the primary regulator of the light-sensitive channels in the plasma membrane of the outer segments of the photoreceptors is a homo-oligomeric, allosteric, positively cooperative protein. In this report, we discuss the possibility that signals from bleached photopigments may control the dark-adaptation process through the allosteric protein of the plasma membrane. It is suggested that the concentrations of the bleached photopigment and of the allosteric effector are reciprocal quantities.

Animals↗
PubMed1991

Dark adaptation of the long-wave cones at different eccentricities.

Using a Wright colorimeter the ordinary long-term, long-wave cone dark-adaptation curve was measured at 0, 2, 4, 7, 17, 25, 40 and 49 degrees nasally in the visual field. In opposition to previous findings, the results show that the dark-adaptation function of the long-wave cones changes markedly when the test field is moved outward from the rod-free fovea. It is suggested that the kinetics of the long-wave cone photopigment change with eccentricity. Also, at variance with previous findings, the present curves at all eccentricities may reasonably well be interpreted as consisting of three different sections; a first section where the threshold decreases rapidly, followed by a major, approximately linear section and a terminating section that converges asymptotically towards the final level of sensitivity. This finding suggests that the dark-adaptation process of the cone system, under the given experimental conditions, is based on three somewhat different processes.

Color Perception↗
PubMed1990

Rod suppression of cone-mediated information about colour and form during dark adaptation.

Following substantial bleaches, the specific form and hue thresholds were measured during dark adaptation with a test stimulus of 1 x 2 degrees at 40 degrees extrafoveally. The wavelength of the test field was varied between runs. The results show that both thresholds started to rise at about the cone-rod break of the dark-adaptation curve, irrespective of wavelength used in the test. Furthermore, the specific threshold for form was found to rise when a scotopic stimulus was superimposed on a photopic test flash. On the other hand, both thresholds remained at the cone-plateau level when the test flash was confined within the rod-free fovea. In order to explain the rise in the specific thresholds, it is suggested that signals from rods generated directly in response to the test stimulus may suppress both cone-mediated form and colour. It is also suggested that this type of rod-cone interaction represents a general characteristic involved in several kinds of visual information processing.

Attention↗
PubMed1990

Dark adaptation of foveal cones during the cone-plateau period.

Following substantial bleaching by "white" light, absolute threshold, relative spectral sensitivity and sensation of hue of monochromatic lights were measured at the central fovea during the cone-plateau period. The absolute-threshold level was found to increase and then decrease markedly, the relative spectral sensitivity remained invariant, while the sensation of hues of monochromatic lights from the long- and middle-wave regions of the spectrum changed toward hues of shorter wavelengths.

Dark Adaptation↗
PubMed1989

Dark-adaptation mechanisms of the long-wave foveal cones.

The ordinary long-term rod and cone dark-adaptation curves have generally been assumed to follow a single exponential rate of recovery. However, in two previous papers on rod dark-adaptation (Stabell et al., 1986a, b), the recovery curve was found to consist of three different sections. The results of the present paper show the same type of recovery function with three different sections for the long-term dark-adaptation curve of the long-wave cone system. During the major, middle section log cone threshold, like log rod threshold, is linearly related to the logarithm of the concentration of bleached photopigment. Presupposing that the bleached cone photopigment acts as a ligand, the change in threshold level obtained during the middle section of the dark-adaptation curve is well described by the change in activity rate of an allosteric, postively cooperative enzyme built as a dimer.

Dark Adaptation↗
PubMed1984

Color-vision mechanisms of the extrafoveal retina.

Wavelength discrimination, spectral sensitivity as well as color-matching performance were measured at the fovea and at different eccentricities in the peripheral retina. The results show that the underlying mechanisms of color vision in the normal peripheral retina are different from those of the classic forms of congenital color blindness. On the other hand, a close correspondence was found between color-vision characteristics obtained in the extrafoveal retina and in patients with acquired color-vision defects due to diseases of the optic nerve, suggesting that the loss of color discrimination with eccentricity and during progression of these diseases has a common underlying basis.

Color Perception↗
PubMed1984

Dark-adaptation of the human rod system.

Following substantial bleaching, dark-adaptation thresholds of a complete rod monochromat and of a subject with normal colour vision were measured using a Wright colorimeter. When precautions were taken to ensure that the fixation point fell on the same retinal area during the threshold measurements as during the bleaching period, the dark-adaptation threshold curves of the rod monochromat followed exactly the same course as those of the normal subject subsequent to the cone-rod break of the long-term, normal dark-adaptation curve; irrespective of the intensity and the duration of the bleaching and the wavelength of the test stimulation. In contrast to the normal subject, however, the dark-adaptation curves of the rod monochromat showed no evidence of any cone function at photopic intensities. Furthermore, as opposed to previous measurements which show a simple linear relationship between fraction of bleached rhodopsin and log threshold, the present results show that there is a close linearity between log fraction of bleached rhodopsin and log threshold. This linear relationship is obtained despite varying extents of bleaching and subsequent dark-adaptation periods.

Adaptation, Ocular↗
PubMed1982

Color vision in the peripheral retina under photopic conditions.

Chromaticities of spectral colors were measured during the cone-plateau period at 17 degrees, 25 degrees, 40 degrees and 60 degrees in the nasal field of view and at 40 degrees and 70 degrees in the temporal field. The results obtained in the nasal field show a progressive contraction of the color gamut with distance from the fovea with maximum shrinkage in the middle-wave region. Color discrimination in the temporal field of view was found to be much better developed than in the nasal field. Thus, all the primary hues were clearly observable at 70 degrees temporally. The exceptionally good color discrimination obtained in the present study is explained on the assumption that only cones are effectively excited upon stimulation during the cone-plateau period.

Adaptation, Ocular↗
PubMed1982

Bezold-Brücke phenomenon of the far peripheral retina.

The Bezold-Brücke phenomenon was measured during the cone-plateau period of the long-term dark-adaptation curve at 25 degrees, 40 degrees and 60 degrees in the nasal field of view, and at 40 degrees and 70 degrees in the temporal field. In striking contrast to previous measurements of the B-B phenomenon the present results generally show that an increase of the luminance level in the middle- and long-wave regions of the spectrum produces, respectively, a trend toward green and red instead of toward yellow. The present results are explained on neural rather than on photochemical mechanisms.

Color Perception↗
PubMed1981

Absolute spectral sensitivity at different eccentricities.

Absolute spectral-threshold functions were measured during the cone-plateau period and in a dark-adapted state at 0, 6, 17, 28, 45, and 65 degrees temporally to the fovea. It was found that, when the photopic functions were brought together at 660 nm, they closely coincided in the 520-700-nm region of the spectrum, irrespective of location, suggesting that the relative spectral sensitivity and the weighted contributions of the middle-and long-wave cone photopigments remain invariant across the retina. On the other hand, the results suggest that the relative contribution of the short-wave cone mechanism increases between fovea and 17 degrees, stays essentially constant between 17 and 28 degrees, and decreases between 28 and 65 degrees. Furthermore, the results suggest that the absolute sensitivity of the middle- and long-wave cones decreases between fovea and 65 degrees, whereas the absolute sensitivity of the rods increased form fovea to 17 degrees and decreases between 17 and 65 degrees degrees. Finally, the log difference between the absolute dark-adapted cone and the rod threshold was found to increase between fovea and 45 degrees and to decrease between 45 and 65 degrees.

Color Perception↗
PubMed1981

Spectral sensitivity of the dark-adapted extrafoveal retina at photopic intensities.

By using a heterochromatic brightness-matching technique, in which the test and comparison field were presented in succession, spectral equal-brightness functions were measured in a dark-adapted state at a retinal illumination of 1000 photopic trolands at 6, 28, 45, and 65 degrees temporally to the fovea. In addition, the spectral equal-brightness functions were measured at 10, 100, 1000, and 6400 photopic trolands at 17 degrees temporally. In striking contrast with previous results, all the spectral brightness functions obtained were found to be basically scotopic in form, with peak sensitivities at about 500 nm. The difference in results between the present study and previous studies could be ascribed to the difference in method employed. Thus it was found that simultaneous, relative to successive, presentation of test and comparison fields depresses rod activity in the test field to a considerable extent. It was concluded that rods may function and influence the brightness response in extrafoveal vision at much higher intensity levels than was previously assumed.

Color Perception↗