Carol M. Cicerone

The relative numbers of long-wavelength-sensitive to middle-wavelength-sensitive cones in the human fovea centralis

The determination of the relative numbers of different cone types in the human retina is fundamental to our understanding of visual sensitivity and color vision; yet direct measurement which provide this basic information have not previously been made for all cone types. Here we present a model which links the detection of a test light of small dimension to the number of cones contributing to detection of the light. We selectively isolated either the long-wavelength-sensitive (L) or the middle-wavelength-sensitive (M) cones, by choosing combinations of wavelengths of adapting backgrounds and tests to favor detection by the cone class of interest. Our model was applied to the detection functions measured for six color normal observers to obtain estimates of the relative numbers of L to M cones. Our estimates ranged between 1.46 and 2.36 for our observers with a mean value near two L cones for every M cone in human fovea centralis.

OPPONENT-PROCESS ADDITIVITY-I: RED/GREEN EQUILIBRIA'

A red/green equilibrium light is one which appears neither reddish nor greenish (i.e. either uniquely yellow, uniquely blue, or achromatic). A subset of spectral and nonspectral red/green equilibria was determined for several luminance levels, in order to test whether the set of all such equilibria is closed under linear color-mixture operations. The spectral loci ofequilibrium yellow and blue showedeither no variation or visually insignificant varia- tion over a range of l-2 log,, unit. There were no trends that were repeatable across observers. We con- cluded that spectral red/green equilibria are closed under scalar multiplication; consequently they are in- variant hues relative to the Bezold-Briicke shift. The additive mixture of yellow and blue equilibrium wavelengths, in any luminance ratio, is also an equilibrium light. Small changes of the yellowish component of a mixture toward redness or greeness must be compensated by predictable changes of the bluish component of the mixture toward greenness or red- ness. We concluded that yellow and blue equilibria are complementary relative to an equilibrium white; that desaturation of a yellow or blue equilibrium light with such a white produces no Abney hue shift; and that the set of red/green equilibria is closed under general linear operations. One consequence is that the red/green chromatic-response function, measured by the Jameson-Hurvich technique of cancellation to equilibrium, is a linear function of the individuals color-matching coor- dinates. A second consequence of linear closure of equilibria is a strong constraint on the class of combina- tion rules by which receptor outputs are recoded into the red/green opponent process.

Cells in the pretectal olivary nucleus are in the pathway for the direct light reflex of the pupil in the rat

Extracellular microelectrode recordings from 148 single cells in the pretectum of the hooded rat were classified according to their temporal response properties to light stimulation of their retinal receptive fields. Fifty-six cells were classified as tonic-on cells, 22 cells were classified as tonic-off cells, and 53 cells were classified as phasic cells. Seventeen cells could not be assigned to one of these 3 groups. The diameters of the receptive field centers of the tonic-on pretectal cell were clustered about a mean of 31 degrees and the temporal response of these cells was sustained. Constriction of the contralateral pupil was produced by electrical stimulation through the recording electrode at sites containing tonic-on pretectal cells, but not at sites containing tonic-off pretectal cells or phasic pretectal cells. For this reason, we argue that tonic-on cells are likely to mediate constriction in the light reflex of the rats pupil. Receptive field maps together with electrolytic marking lesions at recording and stimulation sites showed that tonic-on pretectal cells are retinotopically organized and are aggregated in a strip running from the dorso-medial tip of the pretectum to the ventro-lateral boundary. The anatomical distribution of these cells is coextensive with the region known as the pretectal olivary nucleus (PO) in the rat. Using fine microelectrodes, recordings were obtained from 27 axons presumed to be of optic origin (fibers). Of these, 14 were tonic-on, 10 were phasic, 2 were tonic-off, and 2 were unclassified. Recordings from tonic-on fibers were obtained near tonic-on pretectal cells, typically in the most dorsal light-responsive region of the pretectum. These fibers were activated by single pulse electrical stimulation of the optic chiasm. The mean receptive field center diameter of 6 tonic-on fibers was 10.1 degrees, or about a factor of 3 less than that of pretectal tonic-on cells. The mean conduction velocity of 14 tonic-on fibers was 3.1 m/s. We argue that the tonic-on cells of the PO serve to integrate signals from tonic-on center retinal ganglion cells with adjacent receptive fields to provide signals for constriction of the pupil to neurons in the oculomotor nucleus.

Opponent process additivity--II. Yellow/blue equilibria and nonlinear models

Abstract A yellow/blue equilibrium light is one which appears neither yellowish nor bluish (i.e. uniquely red, uniquely green, or achromatic). The spectral locus of the monochromatic greenish equilibrium (around 500 nm) shows little, if any, variation over a luminance range of 2 log10 units. Reddish equilibria are extraspectral, involving mixtures of short- and long-wave light. Their wavelength composition is noninvariant with luminance: a reddish equilibrium light turns bluish-red if luminance is increased with wavelength composition constant. The additive mixture of the reddish and greenish equilibria is again a yellow/blue equilibrium light. We conclude that yellow/blue equilibrium can be described as the zeroing of a nonlinear functional, which is, however, approximately linear in the short-wavelength (“blue”) and middle-wavelength (“green”) cone responses and nonlinear only in the long-wavelength (“red”) cone response. The “red” cones contribute to yellowness, but via a compressive function of luminance. This effect works against the direction of the Bezold-Brucke hue shift. The Jameson-Hurvich yellow/blue chromatic-response function is only approximately correct: the relative values of yellow/blue chromatic response for an equal energy spectrum must vary somewhat with the energy level.

The ratio of L cones to M cones in the human parafoveal retina

We present psychophysically-based estimates of the relative numbers of long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones in the parafovea of three color-normal trichromats. Using methods previously applied to the fovea centralis, we obtained estimates of the relative numbers of L and M cones at retinal eccentricities of 1.0, 1.5, 2.0, 3.0, and 4.0 deg along the horizontal meridian of the temporal retina. Results for three observers indicate that the L to M cone ratio remains approximately invariant from the fovea to 4.0 deg eccentricity, with a mean ratio near 2:1.

Opponent-process additivity—III : Effect of moderate chromatic adaptation

Abstract The luminance invariance and additivity of opponent-color equilibria were tested under conditions of chromatic adaptation to unique blue, green, yellow and to (nonunique) 650-nm red lights, varying from about 40 to 2000 td. Although the adapting lights produced large shifts in the loci of the red/green equilibrium colors away from their dark-adapted values, hardly any nonadditivity was produced by blue or green adaptation. For yellow adapting lights of 900 td or more, the red/green equilibria are luminance dependent, hence nonadditive. A small luminance dependence was produced by 40 td red adaptation. As a first approximation, the effects of moderate chromatic adaptation on the red/green opponent code can be described by a coefficient law; however, the fact that unique hues do not shift under moderate self-adaptation implies that the coefficient for at least one receptor type depends on the adapting input to other receptor types. The fact that yellow and red adaptation make the dark determined unique blue wavelength look reddish implies that the short-wavelength cones contribute a red input to the red/green opponent system. The yellow/blue system exhibited nonlinearities under chromatic adaptation which were qualitatively similar to the nonlinearities found under dark adaptation.

The perception of color from motion

We introduce and explore a color phenomenon which requires the prior perception of motion to produce a spread of color over a region defined by motion. We call this motion-induced spread of colordynamic color spreading. The perception of dynamic color spreading is yoked to the perception of apparent motion: As the ratings of perceived motion increase, the ratings of color spreading increase. The effect is most pronounced if the region defined by motion is near 1° of visual angle. As the luminance

Retinal sensitivity measured by the pupillary light reflex in RCS and albino rats

The effects of retinal degeneration on the sensitivity of the retina were studied in the Royal College of Surgeons (RCS) rat by measuring the light reflex of the pupil in response to ganzfeld (full field) flashes. Light reflex thresholds were measured for animals from 32 to 683 days of age, and an age-related decrease in sensitivity of 5.2 log units (maximum) was measured. In contrast, thresholds for non-dystrophic albino controls increased only slightly during a comparable period. RCS rat thresholds increased more for short wavelength light than for long wavelength light. The end result was an altered action spectrum of the light reflex which largely, but not exclusively, reflected cone function. Even in cases of advanced degeneration the light reflex thresholds we measured showed significant input from rods. Pupiliary dark adaptation measured following ganzfeld bleaches (10%) with test stimuli of two different wavelengths revealed two mechanisms; a photopic mechanism (gamma max = 520) determined thresholds early in dark adaptation, but later a scotopic mechanism (gamma max = 500) participated in the light reflex.

Lateral spread of light adaptation in the rat retina.

Abstract Recordings from the rat optic tract fibers were used to assess changes in sensitivity under various conditions of adaptation. An adapting background which excites only a small fraction of the rods can yet cause a several-fold change in sensitivity. A small adapting spot much more effectively decreases the cells sensitivity to a superimposed test than to test spots in positions far from the adapting locus. Thus, adaptation spreads laterally but not uniformly throughout the ganglion cell center. Scattered light does not account for the spread, since a displaced adapting spot can be more effective than one superimposed on the test spot.

L and M cone relative numerosity and red-green opponency from fovea to midperiphery in the human retina.

The relative numerosity of the long-wavelength-sensitive (L) and middle-wavelength-sensitive (M) cones and the red-green color appearance, as assessed by means of unique yellow, are stable from fovea to midperiphery (+/- 28 deg nasotemporal). As foveal tests decrease in size, unique yellow progressively shifts toward longer wavelengths, favoring a model of red-green opponency carried by cells whose centers receive input from either L or M cones and whose surrounds receive mixed contributions from both. Individual differences in unique yellow over a 20-nm range and the relative numerosity of L and M cones can be linked by means of this model, suggesting that the relative number of L and M cones is a factor that regulates individual variations in red-green color appearance.