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T Seim

Publications and source records attributed to T Seim.

8 recordsLinked to original sources
PubMed1991

Colour changes as a function of luminance contrast.

When spectral light increases in luminance, the hues change. Normally, long-wavelength light becomes increasingly yellow, and short-wavelength light turns blue or blue-green. This is known as the Bezold-Brücke hue shift. Less notice has been paid to the change in relative chromatic content (saturation or chromatic strength) that accompanies these shifts in hue. As luminance contrast increases from zero, chromatic strength increases to reach a maximum at a luminance that is wavelength dependent. Short-wavelength blueish light reaches this maximum at low relative luminances, whereas midspectral yellowish stimuli need several log units higher luminance. Red and green are somewhere in between. For luminances above this maximum, the chromatic content usually diminishes, and most light becomes more whitish in appearance. In this study it is demonstrated how the combined chromatic appearance of hue and chromatic strength change with intensity. Both phenomena find a common physiological interpretation in the nonlinear and nonmonotonic responses of colour-opponent P cells in the retina and lateral geniculate nucleus of the primate. A model that combines the outputs of six P-cell types accounts for observers' estimates of hue and chromatic strength.

Attention↗
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↗
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

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↗
PubMed1978

Calibration of the Goldmann perimeter and accessories used in specific quantitative perimetry.

With known radiometric data of the components of the projection perimeter and its accessories it is possible to perform spectral sensitivity measurements which are essential for the performance of quantitative colour perimetry. The authors describes modifications made the Goldmann perimeter for the performance of such measurements. The calibration data of the perimeter and accessories are given. A good fit between the CIE scotopic spectral sensitivity curve (V' lambda) and our experimental results for a dark adapted eye serves as a control of the calibration.

Calibration↗