Luke E. Mahon

Spatial and temporal receptive fields of geniculate and cortical cells and directional selectivity

The spatio-temporal receptive fields (RFs) of cells in the macaque monkey lateral geniculate nucleus (LGN) and striate cortex (V1) have been examined and two distinct sub-populations of non-directional V1 cells have been found: those with a slow largely monophasic temporal RF, and those with a fast very biphasic temporal response. These two sub-populations are in temporal quadrature, the fast biphasic cells crossing over from one response phase to the reverse just as the slow monophasic cells reach their peak response. The two sub-populations also differ in the spatial phases of their RFs. A principal components analysis of the spatio-temporal RFs of directional V1 cells shows that their RFs could be constructed by a linear combination of two components, one of which has the temporal and spatial characteristics of a fast biphasic cell, and the other the temporal and spatial characteristics of a slow monophasic cell. Magnocellular LGN cells are fast and biphasic and lead the fast-biphasic V1 subpopulation by 7 ms; parvocellular LGN cells are slow and largely monophasic and lead the slow monophasic V1 sub-population by 12 ms. We suggest that directional V1 cells get inputs in the approximate temporal and spatial quadrature required for motion detection by combining signals from the two non-directional cortical sub-populations which have been identified, and that these sub-populations have their origins in magno and parvo LGN cells, respectively.

Hue Scaling of Isoluminant and Cone-specific Lights

Using a hue scaling technique, we have examined the appearance of colored spots produced by shifts from white to isoluminant stimuli along various color vectors in order to examine color appearance without the complications of the combined luminance and chromatic stimulation involved in most previous hue scaling studies, which have used flashes of monochromatic light. We also used spots lying along cone-isolating vectors in order to determine what hues would be reported with a change in activation of only single cone types or of only single geniculate opponent-cell types, an issue of direct relevance to any model of color vision. We find that: 1. Unique hues do not correspond either to the change in activation of single cone types or of single geniculate opponent-cell types. This is well known to be the case for yellow and blue, but we find it to be true for red and green as well. 2. These conclusions are not limited to the particular white (Illuminant C) used as an adapting background in most of the experiments. Shifts along the same cone-contrast vectors relative to different backgrounds lead to much the same hue percepts, independent of the starting white used. 3. The shifts of the perceptual colors from the geniculate axes are in the directions, and close to the absolute amounts, predicted by our [De Valois & De Valois (1993). Vision Research, 33, 1053-1065] multi-stage color model in which we postulate that the S-opponent cells are added to or subtracted from the M- and L-opponent cells to form the four perceptual color systems. 4. There are distinct asymmetries with respect to the extent to which various hues within each perceptual opponent system deviate from the geniculate opponent-cell axes. Blue is shifted more from the S-LM axis than is yellow; green is shifted more from the L-M axis than is red. There are also asymmetries in the angular extent of opponent color regions. Blue is seen over a larger range of color vectors than is yellow, and red over a slightly larger range than green. 5. Such asymmetries are not accounted for by any model that treats red-green and yellow-blue each as unitary, mirror-image opponent-color systems. Although red and green are perceptually opponent, the red and green perceptual systems do not appear to be constructed in a mirror-image fashion with respect to input from different cone types or from different geniculate opponent-cell types. The same is true for yellow and blue.

Normal Saturation Processing Provides a Model for Understanding the Effects of Disease on Color Perception

Saturation discrimination has been reported to be affected early in the course of a disease. Our empirical data show a compressive curvi-linear relationship between opponent/nonopponent channel activity and saturation thresholds in normal trichromatic observers. This relationship can be explained by a model based on the Hurvich and Jameson saturation coefficient (1957), Psychology Reviews, 64, 384-404. The model considers effects of both selective and nonselective channel losses on saturation processing based on the assumption that disease produces elevated thresholds while maintaining normal psychometric response functions. Both the model and data support clinical observations of saturation losses occurring early in disease. However, the results also indicate that saturation may not be the best modality for monitoring long-term progression of such conditions. We suggest that the different processing characteristics for blue-yellow thresholds may yield added information for saturation testing under some circumstances and that saturation processing occurs at a higher cortical level.

Optimal step sizes for colour and luminance staircasing

We investigated the best step size for measuring luminous and isoluminant coloured (red or blue/tritan) detection thresholds. Stimuli were generated on a colour TV monitor as 1°, 600 msec spots at one of four eccentric (1.6°) locations. Thresholds were expressed as contrasts (%) in cardinal colour space (RG, L or BY) for incremental probes from a white background (CIE 1931 x = 0.30, y = 0.31; L=33 cd/m2). One observer (aged 28) was extensively tested, and trends were confirmed on three others (aged 25-33). Staircase step sizes varied from 0.2 to 4.8 deci-log units (dLog) with starting points chosen to he within 2-3 steps of threshold. A logarithmic staircase comprising 13 reversals was performed at each step size, with threshold being taken as the geometric mean of the last 12 reversals of four randomly interleaved staircases. Both threshold and variance increased with larger step sizes. Thresholds show a V-shaped relationship with step size; minimum thresholds were found with 1.25 dLog steps. Variance showed a broken-line relationship with step size, increasing for step sizes >1.0 dLog. No differences were found among the three coloured stimuli. We conclude that small steps (0.4-1.0 dLog) enhance precision at the expense of efficiency (time), whereas large steps (1.5-3 dLog) improve efficiency but reduce precision. In a clinical setting, 2-3 dLog step sizes should be adequate for all colours.

Evidence for non‐selective colour channel involvement in diabetic eyes especially after laser treatment

Purpose: We consider the hypothesis that proliferative diabetes produces selective loss of colour channels. We also consider the possibility that laser treatment for this condition does not affect macula function.

Scoring the Farnsworth-Munsell 100-hue for vocational guidance.

Background. We considered whether the color discrimination of mild color defectives scoring <100 is the same as that of normals. Methods. We analyzed the FM 100-hue results of 126 normals and 94 congenital color defectives retrospectively by considering the Total Error Score (TES) and individual cap errors (error profiles). Results. A TES of 100 passes 95% of normals and 24% of congenital color defectives. The error profiles of some of the mild defectives who pass show abnormal peaks along a red-green axis. An error >5 in these regions is a good indicator of abnormal color discrimination. Conclusions. Some 30% of mild defectives (TES <100) have limited hue discrimination in the red-green domain, so both the TES and error profiles need to be considered when providing vocational guidance.