Duration thresholds for chromatic stimuli.

The duration necessary to detect chromatic stimuli was measured for wavelengths between 463 and 620 nm. Stimuli were presented either in hue substitution (replacement of white by a chromatic stimulus of matched luminance) or as increments. Two observers viewed a 1 degrees 45' homogeneous white field. A trial consisted of replacement of the central 40' of the field by a chromatic stimulus. In substitution mode the white field was 2.4 cd/m2., the chromatic replacement was of matched luminance using heterochromatic flicker photometry (HFP). In increment mode, the white field was decreased to 1.2 cd/m2., the chromatic replacement remained at 2.4 cd/m2. In substitution mode, duration threshold varied from approximately 3--4 ms for the spectral extremes to 45--66 ms at 570 nm. Detected stimuli were always seen as a change in chromaticity. In increment mode, thresholds were in the 2--4 ms range with no dependence upon spectral composition. Detected stimuli were seen either as changes in chromaticity or brightness. A control experiment indicated that HFP did establish equivalent luminance for the hue substitution mode. We conclude that duration thresholds in substitution mode reveal chromatic processing channels; duration thresholds in increment mode are mediated by chromatic and/or achromatic processing channels.     

Simultaneous masking interactions between chromatic and luminance gratings

Simultaneous masking using test and mask gratings composed of isochromatic luminance variations and isoluminant chromatic variations was studied. Masking of chromatic gratings by chromatic gratings shows less spatial-frequency specificity than does masking of luminance gratings by luminance gratings. Luminance gratings mask chromatic gratings of identical space-average luminance and chromaticity little and only when the spatial frequencies of the test and mask gratings are similar. Chromatic gratings, however, profoundly mask luminance gratings with a degree of spatial-frequency specificity akin to that of luminance-luminance masking. The insensitivity of the luminance-color masking results to the relative phase of the chromatic and luminance gratings indicates that the observed asymmetry is not due to local interactions.     

Psychophysical and physiological responses to gratings with luminance and chromatic components of different spatial frequencies

Gratings that contain luminance and chromatic components of different spatial frequencies were used to study the segregation of signals in luminance and chromatic pathways. Psychophysical detection and discrimination thresholds to these compound gratings, with luminance and chromatic components of the one either half or double the spatial frequency of the other, were measured in human observers. Spatial frequency tuning curves for detection of compound gratings followed the envelope of those for luminance and chromatic gratings. Different grating types were discriminable at detection threshold. Fourier analysis of physiological responses of macaque retinal ganglion cells to compound waveforms showed chromatic information to be restricted to the parvocellular pathway and luminance information to the magnocellular pathway. Taken together, the human psychophysical and macaque physiological data support the strict segregation of luminance and chromatic information in independent channels, with the magnocellular and parvocellular pathways, respectively, serving as likely the physiological substrates.     

Sensory recoding via neural synchronization: integrating hue and luminance into chromatic brightness and saturation

If neural spike trains carry information in the frequency and timing of the spikes, then neural interactions--such as oscillatory synchronization--that alter spike frequency and timing can alter the encoded information. Using coupled oscillator theory, we show that synchronization-based processing can be used to integrate sensory information, resulting in new second-order sensory percepts signaled by the compromise frequency of the coupled system. If the signals to be coupled are nonlinearly compressed, the coupled system behaves as if it signals the product or ratio of the uncoupled signals, e.g., chromatic brightness can be signaled by the compromise frequency of coupled neurons responding to hue and luminance, and chromatic saturation can be signaled by the coupled frequency of neurons responding to hue and brightness, with a power- (Stevens's) law scaling like that observed psychophysically. These emergent properties of coupled sensory systems are intriguing because multiplicative processing and power-law scaling are fundamental aspects of sensory processing.     

Chromaticity diagram showing cone excitation by stimuli of equal luminance.

In a space where Cartesian coordinates represent the excitations of the three cone types involved in color vision, a plane of constant luminance provides a chromaticity diagram in which excitation of each cone type (at constant luminance) is represented by a linear scale (horizontal or vertical), and in which the center-of-gravity rule applies with weights proportional to luminance.     

Perceived shifts in saturation and hue of chromatic stimuli in the near peripheral retina

Using an asymmetric color matching technique, we measured the perceived changes that occur in the saturation and hue of colored stimuli at different eccentricities within the central 25 degrees of the human retina in nine color-normal subjects. A cone-opponent-based vector model was used to compute the activity of the L-M and S-(L+M) channels. The results show that a large proportion of the shifts in saturation and hue that occur with increasing retinal eccentricity are mirrored by decreased activity of the L-M channel. In comparison, the contribution of the S cone-opponent system undergoes relatively little change within the central 20 degrees . In addition, we also found that changes in saturation and hue are different from each other in terms of their variation across color space and their variation with stimulus size. Our findings suggest that perceived shifts in saturation and hue are mediated largely via the reduction in activation of the L-M cone-opponent channel but that saturation and hue might be subject to different retinal and/or cortical influences that contribute to their differing size dependencies in the peripheral retina.     

Luminace, not brightness, determines temporal brightness enhancement with chromatic stimuli.

Brightness-duration relations for chromatic stimuli were studied using three pulse-to-background luminace relations: chromatic equal-luminance pulses (3.2 cd/m2) were presented as increments of 0.3 or 1.0 log units above a lower luminance achromatic background, or were presented in hue substitution, equated in luminance to the achromatic background, so that no spatio-temporal luminance transients occurred during stimulus presentation. Incremental pulses produced temporal brightness enhancement (the Broca-Sulzer phenomenon), but hue substitution pulses did not. Temporal brightness enhancement thus depends upon the occurrence of luminance transients and cannot be produced by pulsed-to-background brightness differences associated solely with chromaticity differences.     

Artifacts in spatiochromatic stimuli due to variations in preretinal absorption and axial chromatic aberration: implications for color physiology

The spatiochromatic receptive-field structure of neurons in the macaque visual system has been studied almost exclusively with stimuli based on the human foveal cone fundamentals of Smith and Pokorny [Vision Res. 15, 161 (1975)] and generated on cathode ray tube displays. In the current study the artifacts evoked by cone-isolating, spatially structured stimuli due to variations in the eye's preretinal absorption characteristics and axial chromatic aberration are quantified. In addition, the luminance artifacts evoked by nominally isoluminant sinusoidal grating stimuli due to the same factors are quantified. The results indicate that the spatiochromatic stimuli commonly employed to map receptive fields of neurons at eccentricities > 10 deg are especially prone to artifacts and that these artifacts are maximal for the high-contrast S-cone-isolating stimuli that are often used. On the basis of these simulations, a method is introduced that improves spatiochromatic receptive-field estimates by compensating for response contributions from the incompletely silenced cone mosaics during cone-isolating stimulation.     

Effective contrast of colored stimuli in the mesopic range: a metric for perceived contrast based on achromatic luminance contrast

Little is known about how color signals and cone- and rod-based luminance signals contribute to perceived contrast in the mesopic range. In this study the perceived contrast of colored, mesopic stimuli was matched with that of spatially equivalent achromatic stimuli. The objective was to develop a metric for perceived contrast in the mesopic range in terms of an equivalent achromatic luminance contrast, referred to here as effective contrast. Stimulus photopic luminance contrast, scotopic luminance contrast, and chromatic difference from the background all contributed to effective contrast over the mid-mesopic range, but their contributions were not independent and varied markedly with background luminance. Surprisingly, color made a significant contribution to effective contrast from 10 to approximately 0.003 cd m(-2). A model describing this relationship is introduced (R2 = 0.89) and compared with predictions of mesopic luminance contrast obtained from a number of models proposed as systems of mesopic photometry.     

Microcontrast and Blur in Imaging Systems

Experience in measuring television impairments suggests that for many imaging systems a practical objective test concerned with the definition or sharpness component of picture quality might be based on the concept of “microcontrast”. This term, as the opposite of “macrocontrast”, is taken to refer to the relative contrast of picture details as a function of their size. Some preliminary experiments on photographic films and lenses have been made with targets in which "details" are represented by sequences of isolated graduated-width “slits”. The impairment, termed “blur”, caused by loss of contrast in fine details can be assessed by a general-purpose method of subjective testing.     

Evidence for an independent luminance channel

There is a discrepancy between several studies that have shown the human luminous-efficiency function to vary with surround color and a recent study that failed to find this dependence. Data are presented that show that this discrepancy can be explained by differences in the matching techniques. Luminous efficiency measured by direct heterochromatic brightness matching does depend on surround color, whereas luminous efficiency measured by the flicker method does not. The independence of luminous efficiency as measured by flicker is evidence for an independent luminance channel.     

New formulas for computing luminance and color temperatures

Conclusions 1. On the basis of Planck's radiation law precise formulas have been obtained for computing the true temperature of a nonblack body from its luminance temperature and emissivity factor. 2) Precise formulas have also been obtained for calculating the color temperature of a nonblack body from its luminance temperature and color emissivity factor. 3) Approximate formulas have been obtained which relate the color and true temperatures of an actual body and provide the computation of the color and true temperatures with the required precision. 4) Approximate formulas have been obtained which provide the computation of the color temperature of a nonblack body with the required precision from two measured luminance temperatures of this body.Translated from Izmeritel'naya Tekhnika, No. 8, pp. 35–37, August, 1967.