Galina V. Paramei

Impairments of Gestalt perception in the intact hemifield of hemianopic patients are reflected in gamma-band EEG activity

Gamma-band responses (GBRs) are associated with Gestalt perception processes. In the present EEG study, we investigated the effects of perceptual grouping on the visual GBR in the perimetrically intact visual field of patients with homonymous hemianopia and compared them to healthy participants. All observers were presented either random arrays of Gabor elements or arrays with an embedded circular arrangement. For the hemianopic patients, the circle was presented in their intact hemifield only. For controls, the hemifield for the circle presentation was counterbalanced across subjects. The participants were instructed to detect the circle by pressing a corresponding button. A wavelet transform based on Morlet wavelets was employed for the calculation of oscillatory GBRs. The early evoked GBR exhibited a larger amplitude and shorter latency for the healthy group compared to hemianopic patients and was associated with behavioral measures. The late total GBR between 200 and 400ms after stimulus onset was significantly increased for Gestalt-like patterns in healthy participants. This effect was not manifested in patients. The present findings indicate deficits in the early and late visual processing of Gestalt patterns even in the intact hemifield of hemianopic patients compared to healthy participants.

Contour-integration deficits on the intact side of the visual field in hemianopia patients.

OBJECTIVEnVisual impairments in hemianopia are thought to be exclusively caused by the reduced visual field size. However, the primary lesion may affect the contralateral hemisphere through damage of interhemispheric projections. The question therefore arises if the presumed intact hemifield is perceptually impaired.nnnMETHODSnThree hemianopia patients and three matched controls carried out a Yes/No figure detection task with their intact side of the visual field. The figure (square) contours were composed of non-contiguous Gabor patches embedded in a random patch array of different background densities (low, delta=2; high, delta=1). Response accuracy and reaction times were recorded.nnnRESULTSnA temporal-parietal patient revealed figure detection impairments, with accuracy rate, 77% (delta=2) and 53% (delta=1), below compared control values. An occipital patient was comparable to his match: 99% (delta=2); 84% (delta=1). Both patients exhibited frequent false alarms to random patterns and required longer presentation times to perform the task. In the third patient, with optic tract lesion, figure detection was nearly normal at low density (92%, delta=2) but impaired at noisy background (62%, delta=1).nnnCONCLUSIONnThe intact visual field in hemianopes is impaired in detection of incomplete figures embedded in a noisy background. This deficit may be caused by damage to higher visual centers and/or loss of interhemispheric interactions.

Bezold-Brücke effect in normal trichromats and protanopes

Luminance-dependent change in color appearance--the Bezold-Brücke effect--was investigated in protanopes and related to that in normal trichromats. Spectral lights were presented at six luminance levels covering mesopic, low, and high photopic vision-across three log steps from 0.76 to 760 Td. To judge color appearance, a variant of the color-naming method was used with four primary basic color terms and a White response. This modification enabled us to examine apparent saturation changes along with the Bezold-Brücke hue shift. Color-naming frequency functions were acquired across ten presentations of each stimulus. Since protanopes name colors idiosyncratically, changes in color appearance cannot be quantified directly from the color-naming functions. To circumvent the difficulty, these functions were transformed into color similarity measures for analysis with multidimensional scaling purported to reconstruct individual color spaces. In these, luminance-dependent shifts in color appearance were represented by means of geometric displacements. We found that for normal trichromats, shifts measured in this way agreed with those derived in our study directly, and with the hue shifts reported in earlier studies. For protanopes, contrary to some models of dichromatic vision, changes in color appearance are significant and indicate superimposed shifts in hue and saturation. The results obtained for normal trichromats, especially for protanopes, imply that nonlinearity in the yellow-blue opponent system is insufficient to explain the Bezold-Brücke effect, given the nature of the saturation shift and the demonstrated divergence between unique hues and invariant hues.

Colour perception in twins: individual variation beyond common genetic inheritance

Background: The twin method was used to examine the genotype/phenotype relationship in colour vision, by determining concordance in colour perception within pairs of monozygotic (MZ) twins and dizygotic (DZ) twins. For MZ twins, whose photopigments are genetically identical, higher concordance in colour perception was expected; conversely, differences within each MZ pair would indicate a non‐genetic contribution.

VECTOR CODING UNDERLYING INDIVIDUAL TRANSFORMATIONS OF A COLOR SPACE

A process for producing a hydroxyfatty acid polyhydric alcohol ester, which comprises; adding at least one polyhydric alcohol represented by the formula (III) or (IV), (III) or (IV) wherein R5 represents a hydrogen atom or a methyl group, R6 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and m and n represent integers from 1 to 6, to hydrated Sophorolipid which is a fermentation product of Torulopsis bombicola; distilling off water under reduced pressure; and subjecting the resulting mixture to an alcoholysis reaction by adding an acid catalyst to the mother liquor.

Luminance-dependent hue shift in protanopes

For normal trichromats, the hue of a light can change as its luminance varies. This Bezold-Brücke (B-B) hue shift is commonly attributed to nonlinearity in the blue-yellow opponent system. In the present study, we questioned whether protanopes experience analogous changes. Two protanopes (Ps) viewed spectral lights at six luminance levels across three log steps. Two normal trichromats (NTs) were tested for comparison. A variant of the color-naming method was used, with an additional white term. To overcome the difficulty of Ps idiosyncratic color naming, we converted color-naming functions into individual color spaces, by way of interstimulus similarities and multidimensional scaling (MDS). The color spaces describe each stimulus in terms of spatial coordinates, so that hue shifts are measured geometrically, as displacements along specific dimensions. For the NTs, a B-B shift derived through MDS agreed well with values obtained directly by matching color-naming functions. A change in color appearance was also observed for the Ps, distinct from that in perceived brightness. This change was about twice as large as the B-B shift for NTs and combined what the latter would distinguish as hue and saturation shifts. The protanopic analogue of the B-B shift indicates that the blue-yellow nonlinearity persists in the absence of a red-green signal. In addition, at mesopic levels (< or = 38 td), the Ps MDS solution was two dimensional at longer wavelengths, suggesting rod input. Conversely, at higher luminance levels (76 td-760 td) the MDS solution was essentially one dimensional, placing a lower limit on S-cone input at longer wavelengths.