Luminance Pedestal Research Articles

Enhanced luminance sensitivity on color and luminance pedestals: Threshold measurements and a model of parvocellular luminance processing.

Psychophysical interactions between chromatic and achromatic stimuli may inform our understanding of the cortical processing of signals of parvocellular origin, which carry both luminance and color information. We measured observers’ sensitivity in discriminating the luminance of circular patch stimuli with a range of baseline (“pedestal”) luminance and chromaticity. Pedestal stimuli were defined along vectors in cone-contrast space in a plane spanned by the red-green cone-opponent (L-M) and achromatic (L + M + S) axes. For a range of pedestal directions and intensities within this plane, we measured thresholds for discriminating increments from decrements along the achromatic axis. Low-contrast pedestals lowered luminance thresholds for every pedestal type. Thresholds began to increase with higher pedestal contrasts, forming a “dipper”-shaped function. Dipper functions varied systematically with pedestal chromaticity: Compared to the achromatic case, chromatic pedestals were effective at lower contrast. We suggest that the enhancement of luminance sensitivity caused by both achromatic and chromatic pedestals stems from a single mechanism, which is maximally sensitive to chromatic stimuli. We fit our data with a computational model of such a mechanism, in which luminance is computed from the rectified output of cone-opponent mechanisms similar to parvocellular neurons.

Efficiently Measuring Magnocellular and Parvocellular Function in Human Clinical Studies.

Pokorny and Smith (J Opt Soc Am A Opt Image Sci Vis. 1997;14:2477-2486) described a laboratory method to behaviorally measure magnocellular and parvocellular pathway sensitivity. We investigated whether their method may be more efficiently applied to clinical populations by reducing adaptation times. We measured contrast detection thresholds to a 30-ms increment on a 30 cd/m2 background every 2 seconds after a 1-minute preadaptation to either a bright (90 cd/m2) or dim (3 cd/m2) luminance, in four observers. We also measured increment thresholds atop a steady 60 cd/m2 luminous pedestal (30 cd/m2 above the background) that remained on for 80 seconds, and tracked thresholds for 60 seconds after pedestal offset. We also assessed the minimum number of stimulus presentations required to reliably estimate thresholds using our four alternative forced choice (4-AFC) zippy estimation by sequential testing (ZEST) procedure. Detection thresholds between the bright and dim preadaptation conditions were identical within seconds after the offset of the preadaptation luminance. Thresholds on the steady luminance pedestal reached stable values within approximately 10 seconds from pedestal onset, and recovered within 2 seconds of pedestal offset. Analysis of the 4-AFC ZEST procedure found little decrease in threshold variability after approximately 14 stimulus presentations. Preadaptation and stimulus adaptation times may be reduced dramatically from those described by Pokorny and Smith, without altering thresholds. Experimental time with clinical populations often is limited. Increasing the efficiency of the method of Pokorny and Smith allows for either shorter test sessions, or for a more extensive range of experimental parameters to be explored in disease.

Crowding is reduced by onset transients in the target object (but not in the flankers).

In peripheral vision, objects that are visible in isolation become difficult to identify in clutter. This crowding effect is typically strong when objects are similar in a given dimension (e.g., color) and weak when they differ. Here we examine the selectivity of crowding for temporal differences-namely, the transient signals associated with object onsets and offsets. Observers judged the orientation of a peripheral Gabor target surrounded by four flankers. Midway through each trial, selected elements "blinked" off and on again. Performance was poor (crowding was strong) when all Gabors blinked simultaneously or when only the flankers blinked. In contrast, performance improved dramatically when the target alone blinked despite the continued presence of the flankers. This asymmetric release from crowding occurs across a range of blink durations and target-flanker separations. A similar release was found when the target onset was delayed relative to the flanker onsets, though varying the target offset had little effect. This suggests that blinks (composed of offset and onset events) reduce crowding specifically because they separate target and flanker onsets. Finally, with luminance pedestals added to the Gabors, crowding was reduced by blinks in the target pedestal only when the target Gabor was present; pedestal blinks before/after the stimulus Gabors (as precues/postcues) had no effect. That is, transients do not simply cue the target location. The asymmetry of this effect (reduced crowding with target transients, no effect with flanker transients) also precludes explanations based on similarity or grouping. We attribute our findings to the isolation of the target in transient (vs. sustained) visual channels.

Progression of flicker perimetry in clinical application

Flicker perimetry measures the function of retina by stimulating it using flickering light. It has three test modes:contrast modulation flicker, critical flicker fusion frequency and luminance pedestal flicker. These 3 modes have their own characteristics. Flicker perimetry shows a higher sensitivity than white-white standard automated perimetry in early diagnosis of glaucoma. It also has advantages in the diagnosis for age-related macular degeneration, diabetic retinopathy and other retinal diseases. With further understanding of the flicker perimetry, its clinical application will gradually expand. In this paper, the basic principle and clinical application are reviewed.

A magnocellular contribution to conscious perception via temporal object segmentation.

The human visual system is continuously confronted with dynamic visual input. One challenge that the visual system must solve, therefore, is recognizing when two distinct objects have appeared at a given location despite their brief presentation and rapid succession, that is, temporal object segmentation. Here we examined the role of magnocellular neurons in this process. We measured temporal object segmentation via object substitution masking (OSM), which reflects the failure to distinguish the target and mask as distinct objects through time. We isolated the selective role of magnocellular neurons by comparing performance under conditions of pulsed luminance pedestals, which are designed to saturate the magnocellular response, with that in a steady-pedestal condition that leaves both magnocellular and parvocellular channels available to process the target. Across two experiments, we found that OSM magnitude was enhanced under pulsed-pedestal conditions, in which the magnocellular response was impaired. This indicates that magnocellular neurons contribute to temporal object segmentation. Given that temporal object segmentation has consequences for which stimuli are consciously perceived, this demonstrates a functional mechanism via which magnocellular neurons contribute to determining the contents conscious perception. Implications for models of specialization of dorsal and ventral cortical streams are discussed.

Do different ‘magnocellular tasks’ probe the same neural substrate?

The sensory abnormalities associated with disorders such as dyslexia, autism and schizophrenia have often been attributed to a generalized deficit in the visual magnocellular-dorsal stream and its auditory homologue. To probe magnocellular function, various psychophysical tasks are often employed that require the processing of rapidly changing stimuli. But is performance on these several tasks supported by a common substrate? To answer this question, we tested a cohort of 1060 individuals on four 'magnocellular tasks': detection of low-spatial-frequency gratings reversing in contrast at a high temporal frequency (so-called frequency-doubled gratings); detection of pulsed low-spatial-frequency gratings on a steady luminance pedestal; detection of coherent motion; and auditory discrimination of temporal order. Although all tasks showed test-retest reliability, only one pair shared more than 4 per cent of variance. Correlations within the set of 'magnocellular tasks' were similar to the correlations between those tasks and a 'non-magnocellular task', and there was little consistency between 'magnocellular deficit' groups comprising individuals with the lowest sensitivity for each task. Our results suggest that different 'magnocellular tasks' reflect different sources of variance, and thus are not general measures of 'magnocellular function'.

Detection of colour signals in objects defined by luminance contrast

AbstractPurpose The aim was to compare chromatic detection thresholds using two methods that are often employed to isolate the use of colour signals: a. Colour thresholds measured in dynamic luminance contrast (LC) noise when a spatially structured object is defined only by colour signals b. Thresholds for detection of colour changes in objects defined by luminance contrast (i.e., the luminance pedestal (LP) technique that is often used in visual psychophysics to isolate the use of colour signals) Earlier findings in patients with cerebral achromatopsia (Proc.R.Soc.Lond.B. 258 :327‐334 (1994)) suggest that different neural mechanisms handle the use of colour signals in the two conditions. It is therefore of interest to establish whether the corresponding colour thresholds in (a) and (b) are also different in normal subjects.Methods Colour detection thresholds were measured using both LP and LC noise methods for discrete LP contrast levels and background luminances. The same four‐alternative, forced‐choice staircase procedure was used to measure thresholds for every combination of target and background luminance.Results The amplitude of dynamic LC noise does not affect colour thresholds. In general, technique (a) yields smaller thresholds when compared to the LP technique (b) for all pedestal LC levels and background luminances. Negative luminance contrast pedestals yield increased colour thresholds for both the red/green and yellow/blue axes.Conclusion When chromatic signals are added to objects defined by negative luminance contrast, the corresponding colour thresholds are no longer processed independently and increase monotonically with luminance contrast.

Differential effects of transient attention on inferred parvocellular and magnocellular processing

The pulsed-pedestal paradigm consists of the simultaneous brief presentation of a test stimulus and luminance pedestals. Processing with this paradigm is thought to be mediated by the parvocellular pathway. The steady-pedestal paradigm consists of the brief presentation of a test stimulus against a continuously presented luminance pedestals. Processing with this paradigm is thought to be mediated by the magnocellular pathway. To test the prediction that transient attention should have a differential effect on performance with these two paradigms, we added to their typical procedures peripheral precues that trigger transient attention. As expected, we have found that the attraction of transient attention to the target location improved performance with the pulsed-pedestal paradigm, but had no reliable effect on performance with the steady-pedestal paradigm. These findings support the hypothesis that transient attention favors parvocellular over magnocellular processing.

Do “Magnocellular Tasks” Measure “Magnocellular Function”?

Magnocellular deficit theories propose that certain cognitive disorders, such as developmental dyslexia, arise from a generalised deficit in the visual dorsal–magnocellular system and its auditory homologue. Psychophysical tasks requiring rapid temporal processing are often used to probe magnocellular function in this context. However, it is not clear whether performance on these various tasks is actually supported by a common substrate. This study investigated whether different putative measures of magnocellular function produce mutually consistent results, and thus evaluated the extent to which they target the same neural mechanisms. Within the PERGENIC project, 1060 participants completed four psychophysical tasks: detection of ‘frequency-doubled’ gratings (FD); detection of pulsed gratings of low spatial frequency on a steady luminance pedestal (SP); detection of coherent motion (CM); and auditory discrimination of temporal order (TO). Although all measures exhibited good test–retest reliability, only...

Psychophysical analysis of contrast processing segregated into magnocellular and parvocellular systems in asymptomatic carriers of 11778 Leber's hereditary optic neuropathy

We examined achromatic contrast discrimination in asymptomatic carriers of 11778 Leber's hereditary optic neuropathy (LHON 18 controls) and 18 age-match were also tested. To evaluate magnocellular (MC) and Parvocellular (PC) contrast discrimination, we used a version of Pokorny and Smith's (1997) pulsed/steady-pedestal paradigms (PPP/SPP) thought to be detected via PC and MC pathways, respectively. A luminance pedestal (four 1 degree x 1 degree squares) was presented on a 12 cd/m2 surround. The luminance of one of the squares (trial square, TS) was randomly incremented for either 17 or 133 ms. Observers had to detect the TS, in a forced-choice task, at each duration, for three pedestal levels: 7, 12, 19 cd/m2. In the SPP, the pedestal was fixed, and the TS was modulated. For the PPP, all four pedestal squares pulsed for 17 or 133 ms, and the TS was simultaneously incremented or decremented. We found that contrast discrimination thresholds of LHON carriers were significantly higher than controls' in the condition with the highest luminance of both paradigms, implying impaired contrast processing with no evidence of differential sensitivity losses between the two systems. Carriers' thresholds manifested significantly longer temporal integration than controls in the SPP, consistent with slowed MC responses. The SPP and PPP paradigms can identify contrast and temporal processing deficits in asymptomatic LHON carriers, and thus provide an additional tool for early detection and characterization of the disease.

Chromatic discrimination in the presence of incremental and decremental rod pedestals

Signals from rods can alter chromatic discrimination. Here, chromatic discrimination ellipses were determined in the presence of rod incremental and decremental pedestals at mesopic light levels. The data were represented in a relative cone Troland space, normalized by discrimination thresholds measured along the cardinal axes without a rod pedestal. In the quadrant of cone space where L-cone relative to M-cone excitation increased, and S-cone excitation decreased, rod incremental pedestals degraded chromatic discrimination, and rod decremental pedestals improved chromatic discrimination. Discrimination in the other three quadrants of cone space was unaffected by the incremental or decremental rod pedestals. A second experiment measured chromatic discrimination under conditions where cone pedestals were matched to the appearances of the incremental and decremental rod pedestals. Based on the matching pedestal data, discrimination then could be measured independently along the cardinal axes using either chromatic [L/(L + M); S/(L + M)] or luminance (L + M) pedestal components. The discrimination data altered by the rod pedestals were similar to chromatic cone pedestals for L/M increment discrimination, but similar to luminance cone pedestals for S decrement discrimination. The results indicated that the rod and cone signals combined differently in determining chromatic discrimination for different post-receptoral pathways.

Contrast Sensitivity Deficits in Inferred Magnocellular and Parvocellular Pathways in Retinitis Pigmentosa

To define the contrast sensitivity deficits of patients with retinitis pigmentosa (RP) under testing conditions designed to emphasize threshold mediation by either the magnocellular (MC) or parvocellular (PC) pathway. Contrast sensitivity was measured with spatially localized, narrow-band test patterns at peak spatial frequencies ranging from 0.25 to 8 cycles per degree (cpd), using a steady-pedestal paradigm (brief presentation of the test stimulus against a continuously presented luminance pedestal) and a pulsed-pedestal paradigm (simultaneous brief presentation of the test stimulus and luminance pedestal) to favor the MC and PC pathways, respectively. The contrast sensitivity functions of 12 patients with RP who had visual acuities ranging between 20/12.5 and 20/40 were compared to those of 10 visually normal, age-equivalent control observers. Five of the patients with RP who had Snellen visual acuities better than 20/25 had contrast sensitivity functions that were within the normal limits at all spatial frequencies for both testing paradigms. The other seven patients with RP had reduced contrast sensitivities for both paradigms, with the greatest reduction in sensitivity occurring at the highest spatial frequency. Their contrast sensitivity deficits were equivalent for the steady- and pulsed-pedestal paradigms. As observed in previous studies, the degree of contrast sensitivity loss shown by the patients with RP was greatest at the highest stimulus spatial frequency. However, in comparison to prior studies of contrast discrimination in patients with RP, there was no evidence of a preferential contrast sensitivity loss within the MC pathway. This apparent discrepancy is attributed to differences in the test targets and psychophysical judgments that were used in the studies, which emphasizes the importance of task characteristics in evaluating relative deficits within the MC and PC processing streams in visual disorders.

Spatial frequency processing in inferred PC- and MC-pathways

The goal of this study was to investigate the role of inferred parvocellular (PC) and magnocellular (MC) pathways in spatial contrast sensitivity. Localized, spatially narrow-band patterns (sixth derivatives of Gaussians, D6s) were presented at various peak spatial frequencies. When the D6 appeared on a pulsed luminance pedestal (Pulsed-Pedestal Paradigm), the spatial contrast sensitivity showed a band-pass shape with good contrast sensitivity at medium spatial frequencies. When the D6 appeared on a steady luminance pedestal (Steady-Pedestal Paradigm), the spatial contrast sensitivity showed a low-pass shape with decreased sensitivity at high spatial frequencies. The band-pass CSF was interpreted as reflecting PC-pathway mediation; the lower spatial frequency region of the low-pass CSF as reflecting MC-pathway mediation.

Effect of eccentricity on luminance-pedestal flicker thresholds

We investigated the effect that spatially coincident luminance increments ( luminance pedestals) have on flicker thresholds at several eccentricities and target sizes. Luminance pedestals elevated flicker amplitude-thresholds more when stimuli were presented eccentrically, both at low (4 Hz) and high (20 Hz) temporal frequencies. Altering the size of the eccentric stimulus failed to equate central and eccentric thresholds at all pedestal amplitudes. Comparisons with flicker thresholds at various background luminances suggests that the increase in luminance-pedestal flicker thresholds peripherally is due to increased suppressive rod–cone interactions, increased effectiveness of luminous contrast on edge-sensitive flicker mechanisms, as well as increased gain in the light adaptation response.

Interactions between flicker thresholds and luminance pedestals

We investigated the interactions between flicker thresholds and luminance pedestals using threshold versus contrast (TvC) and method of constant stimuli paradigms. High amplitude luminance pedestals were found to elevate flicker thresholds, but low amplitude luminance pedestals were unable to reduce flicker thresholds. Luminance pedestals elevated flicker thresholds more at low temporal frequencies. A simple model based on local light adaptation was able to capture the general form of the TvC functions. Our results suggest that flicker thresholds derived in the presence of a luminance pedestal (luminance-pedestal flicker) may vary from those obtained by modulating about a mean luminance (mean-modulated flicker).

Effect of stimulus duration in flicker perimetry.

We investigated the relationship between stimulus duration and flicker thresholds when flickering stimuli were presented on luminance pedestals. Mean-modulated flicker thresholds remained constant with changes in stimulus duration. However, flicker presented on a luminance pedestal gave masking at short durations, decaying exponentially to stable thresholds with time. These stable thresholds were elevated when compared with the mean-modulated condition. The lowest threshold in a stimulus onset asynchrony function predicted the threshold obtained from a luminance-pedestal flicker stimulus of the same duration. This suggests that luminance-pedestal flicker thresholds are determined by the most detectable cycle in a multiple cycle stimulus. We find that the 800 ms stimulus duration used in the Medmont M600 perimeter (Medmont, Camberwell, Australia) is suitable to determine stable flicker thresholds.

Attention and luminance detection: effects of cues, masks, and pedestals.

Three experiments investigated the effects of attention on the detection of luminance targets in a spatial-cuing paradigm. Gabor patch stimuli of 30, 60, or 90 ms duration, equated for detectability, were presented (a) against a uniform field with a checkerboard backward mask, (b) atop a 15% luminance pedestal with a backward mask, or (c) atop a luminance pedestal with no backward mask. Signal detection analysis showed that detection sensitivity was significantly enhanced at attended locations for all observers when backward masks were used, both when targets were presented against a uniform field and when a pedestal was used. However, when no masks were used there was no cuing advantage of any kind. The results show that the cuing effect in simple detection depends on the use of backward masks, a finding that resolves the inconsistencies previously associated with studies of this type.

Attention and luminance detection: Effects of cues, masks, and pedestals.

Three experiments investigated the effects of attention on the detection of luminance targets in a spatial-cuing paradigm. Gabor patch stimuli of 30, 60, or 90 ms duration, equated for detectability, were presented (a) against a uniform field with a checkerboard backward mask, (b) atop a 15% luminance pedestal with a backward mask, or (c) atop a luminance pedestal with no backward mask. Signal detection analysis showed that detection sensitivity was significantly enhanced at attended locations for all observers when backward masks were used, both when targets were presented against a uniform field and when a pedestal was used. However, when no masks were used there was no cuing advantage of any kind. The results show that the cuing effect in simple detection depends on the use of backward masks, a finding that resolves the inconsistencies previously associated with studies of this type.

Facilitation between the luminance and red–green detection mechanisms: enhancing contrast differences across edges

Previous studies have shown that detection of a red–green test pattern, such as a spot or grating, may be facilitated two to three times by a suprathreshold luminance pedestal of the same shape. We measured facilitation between the red–green (RG) and luminance (LUM) detection mechanisms using sine and square-wave gratings. Facilitation of RG by luminance pedestals was 3-fold for in phase sine-wave gratings of 0.8 cpd and a remarkable 7-fold for square-wave gratings. The latter facilitation was greatly reduced at intermediate relative phases and was generally reduced at higher spatial frequencies. We show that on a uniform field, the red or green regions of low spatial frequency test patterns are detected approximately independently, but in the presence of the LUM pedestal RG becomes sensitive to the red–green difference across the luminance edges. Under optimal conditions (with the low-frequency, square-wave luminance pedestal) this increased red–green sensitivity corresponds to a wavelength discrimination threshold as small as ∼0.04 nm. This conversion of RG into an ‘edge detector’ may explain why facilitation is twice as large for square-wave gratings (bipolar patterns) than spots (unipolar patterns). The reverse facilitation, that of LUM by the red–green pedestal, is weaker and the results suggest that this is because LUM is initially sensitive to the light–dark difference across luminance edges even in the absence of the red–green pedestal.

The peripheral flicker effect: desensitization of the luminance pathway by static and modulated light.

Previous studies have shown that luminance flicker, presented peripheral to a foveal test target, increases thresholds for target detection: the peripheral flicker (PF) effect. These studies have also shown that thresholds are elevated more for luminance targets, relative to chromatic targets. In the present study we examined the specificity of the PF effect on the luminance mechanism and assessed the contribution of modulated stray-light to the test field, as well as longer range spatial interactions. We found that the presence of a foveal luminance pedestal, as well as PF, caused a notch to appear in the spectral sensitivity function around 570 nm. This result confirms the hypothesis that the PF effect decreases the sensitivity of the luminance pathway. To assess the contribution of stray-light to the PF effect, we modulated a luminance pedestal without the presence of PF in order to simulate the stray-light effect in isolation. A decrease in sensitivity for wavelengths around 570 nm occurred with modulated stray-light, suggesting that modulated stray-light contributes substantially to this effect. We then minimized the modulated stray-light by phase-reversing a checkerboard pattern in the periphery. A significant, though smaller, threshold elevation to mid-spectrum stimuli was obtained, suggesting that long range spatial effects are also active in the PF effect. We conclude that the PF effect causes a desensitization of foveal luminance pathways via local and more long range spatial interactions. Our results are consistent with previous data which suggest that the PF effect is due to selective adaptation of cells in the magnocellular pathway (M-cells). Our data imply that local network adaptation may be a property of the magnocellular pathway.