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A Valberg

Publications and source records attributed to A Valberg.

At least 37 records · Page 2Linked to original sources
PubMed1986

Colour and brightness signals of parvocellular lateral geniculate neurons.

We recorded from single neurons in the parvocellular layers of the lateral geniculate body of anesthetized monkeys. Spectral response curves of parvocellular neurons depended on the luminance ratio between the chromatic stimuli and achromatic background. From response/intensity curves, we determined the relative luminance between a coloured and an achromatic (white) light at which a given cell became non-responsive (critical luminance ratio, CLR). The spectral dependence of the CLRs of narrow (N) and wide band (W) cells with opponent receptor input showed characteristic differences. The activity of W-cells increased with luminance increase of a white light and of a coloured light in the specific spectral region of the cell (yellow-red for the long wave length sensitive WL-, and yellow-green-blue for the short wave length sensitive WS-cells), while N-cells were activated by their specific spectral light (blue for NS-cells, red for NL-cells) and by a luminance decrease of achromatic white. N-cells discriminate best between their characteristic colour and white at luminance ratios below their respective CLR, while W-cells distinguish best between a light of their characteristic colour and white at chromatic/achromatic luminance ratios above their respective CLR. Yellow sensitive W-cells with a narrow spectral sensitivity peaking around 570 nm and with only a small or no response to white light, could enable distinction between white and yellow of similar luminance. The findings are consistent with the opponency model of spectrally sensitive cells in the LGB. We discuss their implications for colour coding by parvocellular cells. N- and W-cells appear to behave complementary with respect to luminance information (N-cells may be compared to the cat's off-cells, W-cells to on-cells). S- and L-cells are complementary with respect to colour. The yellow sensitive WM-cells are critical for the discrimination of yellow and white, while cells with excitatory cone input from blue and red cones (W-SL-cells) may aid the perception of purple. The fact that, at different relative luminance ratios between a chromatic stimulus and a white background, the whole family of parvocellular cells is involved differently in coding for colour, may explain the different appearance of colours against a white background at different luminance ratios and the perception of induced colours.

Animals↗
PubMed1986

Neurones with strong inhibitory S-cone inputs in the macaque lateral geniculate nucleus.

Neurons with strong inhibitory short-wavelength sensitive cone (S-cone) inputs have been identified in the macaque geniculate using a tritanopic confusion line test, i.e. by stimulation with equiluminous stimuli which leave excitations of long- and middle-wavelength sensitive cones (L- and M-cones) constant while differentially exciting S-cones. Mathematical simulation of the responses of these cells, using known spectral sensitivities of the cone receptors, demonstrates that they receive excitation from M-cones, inhibition from S-cones, and little or no inhibition from L-cones. Excitatory and inhibitory pools are largely spatially coextensive.

Animals↗
PubMed1986

Reconstruction of equidistant color space from responses of visual neurones of macaques.

We demonstrate that a combination of responses of various types of spectrally opponent sustained cells of the macaque lateral geniculate nucleus (LGN) may be related to equidistant color space. Response curves of such cells to stimuli of different luminance ratios and wavelengths are similar to the first stage opponent coordinate functions of the new SVF color-difference formula [T. Seim and A. Valberg, Color Res. Appl. 11, 11 (1986)]. Mathematical simulation of the responses of these cells to a variety of color stimuli is possible through computation of cone excitations and subsequent sums and differences of cone signals. When the response functions thus obtained for cells are used to replace the corresponding coordinates of the SVF diagram, the distributions of equiluminous stimuli with the same sensory differences from an achromatic stimulus approximate ellipses about the white point, and loci of constant hue approximate straight lines. Improved uniformity may be obtained by linear combinations of these cells' outputs or by including more cell types with best responsiveness to other directions of color space. This indicates possible roles of LGN cell types for color scaling in primates, in that color scaling observed psychophysically is an implicit property of these cells' responses.

Animals↗
PubMed1985

A simultaneous contrast effect of steady remote surrounds on responses of cells in macaque lateral geniculate nucleus.

Steadily illuminated surrounds, remote from the receptive field centre, are shown to affect the responses of primate visual cells. Intensity-response curves of cells of the macaque lateral geniculate nucleus were measured using a successive contrast paradigm where chromatic or achromatic stimuli were presented in alternation with a white adaptation field of constant luminance. Adding white surround annuli around stimuli and adaptation field shifted the intensity-response curves to higher intensity ranges. Since response curves can be nonmonotonic, this remote surround effect can result in an increase or decrease in responsiveness (facilitation or suppression) dependent on stimulus intensity. Steady surrounds, remote from the receptive field centre, thus control cell sensitivity and responses by means of simultaneous contrast.

Adaptation, Physiological↗
PubMed1983

Chromatic induction: responses of neurophysiological double opponent units?

Equations have been derived that improve the quantification of sensory equidistant colour and lightness differences. This has been achieved by a physiological approach involving non-linear responses of cone mechanisms and two subsequent stages of linear opponent transformation to describe the Munsell System (Seim and Valberg, 1980). Using the formulation for the first opponent stage, colours induced into an achromatic center field by a chromatic surround varying in purity, are shown to follow the same power function of the opponent coordinates for all hues. By analogy, a physiological model for colour coding and colour induction is offered. Double opponent neurones with spatially antagonistic, spectrally opponent and symmetric receptive fields constitute the units of the model. Colour induction is related to lateral excitation and colour differences to response differences of these units.

Color Perception↗
PubMed1982

Foveal inhibition and facilitation caused by remote grating jerks: interaction between long-range and short-range effects.

Periodic oscillation of a luminance grating imaged upon the peripheral retina reduces the threshold visibility of a foveally presented test spot. This new effect has been named the "jerk effect". The present investigation is concerned with the effect of a single jerk of the remote grating on threshold sensitivity. Foveal sensitivity changes were measured for different delays between grating jerk and test spot presentation. For 0.38 degrees, 100 ms test spot, long-range transient inhibition was found for all delays, with a maximal effect between 0 and 30 ms delay. By combining the jerk effect with the Westheimer paradigm, both facilitatory and inhibitory long-range effects could be demonstrated. For facilitation to occur, it was necessary that the steady background extended into the sensitization zone of the Westheimer area. Inhibition was the only result for smaller backgrounds. Reduced visibility is consistent with the hypothesis that peripheral transient mechanisms inhibit foveal sustained mechanisms. Enhanced visibility indicates that thresholds depend on an interaction between foveal-sustained and foveal-transient units. Transient peripheral stimulation and steady backgrounds of increasing diameter change the balance of inhibitory and facilitatory processes between these units.

Evoked Potentials, Visual↗
PubMed1982

Remote pattern reversal reduces the proximal negative response of the goldfish retina.

1. Using the eyecup preparation, proximal negative responses (PNR) to small test spots of different irradiance were recorded with (a) a stationary peripheral black and white grating surrounding the test spot, and (b) with contrast reversal of the same grating. In the latter case, the PNR-amplitude was reduced by a magnitude that was dependent on the frequency of contrast reversal. The reduction was maximum (approximately 50%) for a frequency of 8-10 Hz. 2. The attenuation was constant for PNR-amplitudes greater than half the maximum value, but increased for smaller responses. The fact that the intensity-response curve was not merely shifted towards higher values on the log intensity axis, indicates that the suppression was an effect neither of stray light nor of adaptive processes in the distal retina. 3. The effect of a single shift of the grating (by half a cycle) on the PNR was studied at different delays between grating shift and test spot presentation. Strong suppression of the PNR was found for delays between 100 ms (shift preceding test spot) and -50 ms (test spot preceding grating shift), with a maximum at about 30 ms. 4. This long-range effect of peripheral transient stimulation is of inhibitory nature, and probably related to Werblin's windmill effect.

Animals↗
PubMed1979

Local foveal inhibitory effects of global peripheral excitation.

Global excitation produced by oscillating a peripheral square-wave grating back and forth through one-half cycle inhibits the visibility of an incremental test flash only when the flash is presented in the foveal region of the visual field. This finding is discussed in the context of the neurophysiological periphery effect and shift-effect and their possible role in saccadic suppression.

Eye Movements↗
PubMed1979

Chromatic border distinctness: not an index of hue or saturation differences.

Some investigators have suggested that the distinctness of chromatic borders (i.e., borders visible in photic arrays of uniform luminance) can be used as an index of hue and saturation differences between lights. However, recent evidence indicates that only two types of cones in the trichromatic eye contribute to chromatic border perception. A series of experiments are reported that were designed to discriminate between these alternatives, utilizing mainly the short-wavelength visible spectrum. The results support the notion that only R and G cones in the trichromatic eye mediate the perception of chromatic borders; thus the distinctness of such borders alone cannot be used as an index of either hue or saturation differences, because both of these aspects of color involve contributions from B cones.

Color Perception↗
PubMed1977

Tritanopic purity-difference function to describe the properties of minimally distinct borders.

Tansley and Boynton have recently demonstrated that color stimuli whose chromaticities all fall on a particular triptanopic confusion line in the CIE (x,y) diagram do not form distinct borders with each other. A tritanopic purity-difference function, involving only r- and g-cone contributions, is demonstrated to provide (i) a prediction of which chromatic stimuli have equivalent border-forming properties, and (ii) a description of the distinctness of minimally distinct borders (MDB) in terms of an equivalent luminance contrast. The tritanopic purity-difference concept is demonstrated to account for all available data on the assessment of the distinctness of borders at the MDB point.

Color Perception↗
PubMedDate not supplied

Possible contributions of magnocellular- and parvocellular-pathway cells to transient VEPs.

We have measured transient visual evoked potentials (VEPs) to low-contrast luminance stimuli favoring responses of magnocellular pathway cells and to low-contrast red-green stimuli favoring parvocellular cells. Stimuli were square-wave alternating, 3-deg homogeneous disks. Low-contrast stimuli modulated in luminance elicited relatively simple responses. For some observers, a negativity was present that saturated at low contrast. This may be the signature of inputs from magnocellular channels to the visual cortex. The slope of the contrast-response curve for low-contrast stimuli was about the same for all subjects. For medium contrasts, these contrast-response curves displayed an abrupt increase of slope. The shallower slope may reflect the responsivity of magnocellular-pathway inputs to the cortex, whereas the steeper slope may be caused by additional parvocellular activation. Contrast-response curves for the most sensitive waveforms of the isoluminant green-red modulation also showed two branches, although not as clearly as for luminance. This may indicate parvocellular-mediated activity for small chromatic differences, and a combination of parvocellular and magnocellular inputs for larger contrasts. Curves of time-to-peak response as a function of contrast often changed their monotonous behavior near the kink of the corresponding contrast-response curve, thus supporting the notion of a contribution from several mechanisms to the main waveforms.

Color Perception↗