The transition from scotopic to photopic vision in 3-month-old infants and adults: an evaluation of the rod dominance hypothesis

The use of chromatic information for motion segmentation: differences between psychophysical and eye-movement measures

Previous psychophysical studies have shown that chromatic (red/green) information can be used as a segmentation cue for motion integration. We investigated the mechanisms mediating this phenomenon by comparing chromatic effects (and, for comparison, luminance effects) on motion integration between two measures: (i) directional eye movements with the notion that these responses are mediated mainly by low-level motion mechanisms, and (ii) psychophysical reports, with the notion that subjects' reports should employ higher-level (attention-based) mechanisms if available. To quantify chromatic (and luminance) effects on motion integration, coherent motion thresholds were obtained for two conditions, one in which the signal and noise dots were the same 'red' or 'green' chromaticity (or the same 'bright' or 'dark' luminance), referred to as homogeneous, and the other in which the signal and noise dots were of different chromaticities (or luminances), referred to as heterogeneous. 'Benefit ratios' (theta(HOM)/theta(HET)) were then computed, with values significantly greater than 1.0 indicating that chromatic (or luminance) information serves as a segmentation cue for motion integration. The results revealed a high and significant chromatic benefit ratio when the measure was based on psychophysical report, but not when it was based on an eye-movement measure. By contrast, luminance benefit ratios were roughly the same (and significant) for both measures. For comparison to adults, eye-movement data were also obtained from 3-month-old infants. Infants showed marginally significant benefit ratios in the luminance, but not in the chromatic, condition. In total, these results suggest that the use of chromatic information as a segmentation cue for motion integration relies on higher-level mechanisms, whereas luminance information works mainly through low-level motion mechanisms.

Light-dependent magnetoreception in birds: increasing intensity of monochromatic light changes the nature of the response

Background

The Radical Pair model proposes that magnetoreception is a light-dependent process. Under low monochromatic light from the short-wavelength part of the visual spectrum, migratory birds show orientation in their migratory direction. Under monochromatic light of higher intensity, however, they showed unusual preferences for other directions or axial preferences. To determine whether or not these responses are still controlled by the respective light regimes, European robins, Erithacus rubecula, were tested under UV, Blue, Turquoise and Green light at increasing intensities, with orientation in migratory direction serving as a criterion whether or not magnetoreception works in the normal way.

Results

The birds were well oriented in their seasonally appropriate migratory direction under 424 nm Blue, 502 nm Turquoise and 565 nm Green light of low intensity with a quantal flux of 8·10¹⁵ quanta s^-1 m^-2, indicating unimpaired magnetoreception. Under 373 nm UV of the same quantal flux, they were not oriented in migratory direction, showing a preference for the east-west axis instead, but they were well oriented in migratory direction under UV of lower intensity. Intensities of above 36·10¹⁵ quanta s^-1 m^-2 of Blue, Turquoise and Green light elicited a variety of responses: disorientation, headings along the east-west axis, headings along the north-south axis or 'fixed' direction tendencies. These responses changed as the intensity was increased from 36·10¹⁵ quanta s^-1 m^-2 to 54 and 72·10¹⁵ quanta s^-1 m^-2.

Conclusion

The specific manifestation of responses in directions other than the migratory direction clearly depends on the ambient light regime. This implies that even when the mechanisms normally providing magnetic compass information seem disrupted, processes that are activated by light still control the behavior. It suggests complex interactions between different types of receptors, magnetic and visual. The nature of the receptors involved and details of their connections are not yet known; however, a role of the color cones in the processes mediating magnetic input is suggested.

Infant color vision: prediction of infants' spontaneous color preferences

Infants show spontaneous looking preferences among isoluminant chromatic stimuli [Adams, R. J. (1987). An evaluation of color preferences in early infancy. Infant Behavior and Development, 10, 143-150; Bornstein, M. H. (1975). Qualities of color vision in infancy. Journal of Experimental Social Psychology, 19 (3), 401-419.]. These differences in preference have often been called "hue" or "color" preferences, and attributed to differences in hue, but there are alternative explanations. Spontaneous preference variations remain after stimuli are equated for adult brightness, and thus cannot be attributed to adult-like brightness differences [Teller, D. Y., Civan, A., & Bronson-Castain, K. (2004). Infants' spontaneous color preferences are not due to adult-like brightness variations. Visual Neuroscience, 21 (3), 397-401]. In the present paper, we address three more alternative explanations: colorimetric purity; infant detection thresholds; and adult-like variations in saturation. Three experiments were conducted. In Experiment 1 we measured infants' spontaneous preferences for each of 22 different chromatic stimuli of varying dominant wavelength and colorimetric purity, each paired against the same white standard. In Experiment 2, we measured infants' chromatic detection thresholds. In Experiment 3, adult subjects made saturation matches between a blue-green standard and each of five other chromatic stimuli. Infant detection thresholds accounted for 34% of the variance in infant "hue" preferences, much more than colorimetric purity (2.4%) or adult saturation judgments (3%), but none of the three variables accounted for the majority of the variance. In our view, the most likely remaining option is that infants' spontaneous "hue" preferences indeed arise from preferences for the hues of stimuli that adults see as blue, purple and red.

The Purkinje rod-cone shift as a function of luminance and retinal eccentricity

In the Purkinje shift, the dark adapted eye becomes more sensitive to blue than to red as the retinal rods take over from the cones. A striking demonstration of the Purkinje shift, suitable for classroom use, is described in which a small change in viewing distance can reverse the perceived direction of a rotating annulus. We measured this shift with a minimum-motion stimulus (Anstis & Cavanagh, Color Vision: Physiology & Psychophysics, Academic Press, London, 1983) that converts apparent lightness of blue versus red into apparent motion. We filled an iso-eccentric annulus with radial red/blue sectors, and arranged that if the blue sectors looked darker (lighter) than the red sectors, the annulus would appear to rotate to the left (right). At equiluminance the motion appeared to vanish. Our observers established these motion null points while viewing the pattern at various retinal eccentricities through various neutral density filters.

RESULTS

The luminous efficiency of blue (relative to red) increased linearly with eccentricity at all adaptation levels, and the more the dark-adaptation, the steeper the slope of the eccentricity function. Thus blue sensitivity was a linear function of eccentricity and an exponential function of filter factor. Blue sensitivity increased linearly with eccentricity, and each additional log(10) unit of dark adaptation changed the slope threefold.

Infant photometry: are mean adult isoluminance values a sufficient approximation to individual infant values?

Individual differences in isoluminance values were studied in infants and adults using a motion nulling paradigm. Two luminance-modulated sinusoidal grating components (spatial frequency=0.25 cpd, temporal frequency=5.6 Hz, speed=22.4 deg/s) were superimposed and moved in opposite directions across a color video screen. The contrasts of the two components were traded off to determine motion nulls. Two conditions were used: red/black vs. green/black, and red/black vs. blue/black grating components. An eye movement based response measure was used for infant subjects, and an average of 308 trials per infant were obtained. As observed in earlier studies, the mean motion null values for infants and adults were highly similar in each condition. The standard errors of motion null values for individual subjects were very small. Individual differences among infants were also small, and were clearly measurable only in the red/black vs. blue/black condition. The close similarity of mean null values, combined with the small individual differences among infants, supports the idea that under the right circumstances mean adult isoluminance values can be used as a sufficient approximation to individual infant isoluminance values in studies of infant color vision. These circumstances are discussed and evaluated in detail.

Development of psychophysically-derived detection contours in L- and M-cone contrast space

In order to investigate the development of color mechanisms in infants we fitted elliptical detection contours to psychophysically-derived contrast thresholds plotted in L- and M-cone contrast space. Detection ellipses were obtained for 47 infants (ages 2-5 months of age), and were compared to those of six adults tested under nearly identical conditions. The parameters of the fitted ellipses allowed us to address several aspects of color development. First, the lengths and widths were used to assess the relative development of chromatic, with respect to luminance, sensitivity. The results of these analyses revealed a sharp increase in chromatic sensitivity between 3 and 4 months of age, suggesting an accelerated development of chromatic mechanisms around this time. Second, the angles of the ellipses provided estimates of individual red/green isoluminance points. In line with previous reports, we found that isoluminance points do not vary significantly with age. Finally, our ellipse-fitting procedures were used to assess whether color sensitivity is best described by a model that assumes independence between post-receptoral chromatic and luminance mechanisms. Similar to previous results of Kelly and Chang [Kelly, J. P. & Chang, S. (2000). Vision Research 40, 1887-1906] obtained using steady-state visually evoked potentials, only a proportion (approximately half) of our infants exhibited detection contours that were consistent with independent mechanisms, a finding that most likely results from statistical noise in the infant data sets.

Infant light adaptation shows Weber's law at photopic illuminances

In the present study we investigate the dependence of photopic contrast thresholds on retinal illuminance in infants and adults. Contrast thresholds were measured at five retinal illuminances between about 6 and about 20,000 Td in subjects in both age groups. The forced-choice preferential looking technique was used in 3-month-old infants, and standard forced-choice techniques were used in adults. The stimulus was a 0.25 cy/deg squarewave grating phase alternated at 6 Hz. Infants' contrast thresholds were more than two log units higher than those of adults at all retinal illuminances. Contrast thresholds had a similar dependence on retinal illuminance in both infants and adults. For both age groups, contrast thresholds initially decreased with increasing retinal illuminance. However, at both ages, above a critical illuminance of about 200 Td, contrast thresholds remained constant, following Weber's law. Thus a vertical shift was sufficient to bring the two data sets into correspondence. In the context of a two-site model of light adaptation, our results imply that infants' elevated contrast thresholds cannot be explained solely on the basis of photoreceptoral immaturities. Later physiological immaturities must also limit infants' photopic contrast thresholds.