Pedestal Contrast Research Articles

Further Examination of the Pulsed- and Steady-Pedestal Paradigms under Hypothetical Parvocellular- and Magnocellular-Biased Conditions.

The pulsed- and steady-pedestal paradigms were designed to track increment thresholds (ΔC) as a function of pedestal contrast (C) for the parvocellular (P) and magnocellular (M) systems, respectively. These paradigms produce contrasting results: linear relationships between ΔC and C are observed in the pulsed-pedestal paradigm, indicative of the P system's processing, while the steady-pedestal paradigm reveals nonlinear functions, characteristic of the M system's response. However, we recently found the P model fits better than the M model for both paradigms, using Gabor stimuli biased towards the M or P systems based on their sensitivity to color and spatial frequency. Here, we used two-square pedestals under green vs. red light in the lower-left vs. upper-right visual fields to bias processing towards the M vs. P system, respectively. Based on our previous findings, we predicted the following: (1) steeper ΔC vs. C functions with the pulsed than the steady pedestal due to different task demands; (2) lower ΔCs in the upper-right vs. lower-left quadrant due to its bias towards P-system processing there; (3) no effect of color, since both paradigms track the P-system; and, most importantly (4) contrast gain should not be higher for the steady than for the pulsed pedestal. In general, our predictions were confirmed, replicating our previous findings and providing further evidence questioning the general validity of using the pulsed- and steady-pedestal paradigms to differentiate the P and M systems.

Asymmetries between achromatic increments and decrements: Perceptual scales and discrimination thresholds.

The perceptual response to achromatic incremental (A+) and decremental (A-) visual stimuli is known to be asymmetrical, due most likely to differences between ON and OFF channels. In the current study, we further investigated this asymmetry psychophysically. In Experiment 1, maximum likelihood difference scaling (MLDS) was used to estimate separately observers' perceptual scales for A+ and A-. In Experiment 2, observers performed two spatial alternative forced choice (2SAFC) pedestal discrimination on multiple pedestal contrast levels, using all combinations of A+ and A- pedestals and tests. Both experiments showed the well-known asymmetry. The perceptual scale curves of A+ follow a modified Naka-Rushton equation, whereas those of A- follow a cubic function. Correspondingly, the discrimination thresholds for the A+ pedestal increased monotonically with pedestal contrast, whereas the thresholds of the A- pedestal first increased as the pedestal contrast increased, then decreased as the contrast became higher. We propose a model that links the results of the two experiments, in which the pedestal discrimination threshold is inversely related to the derivative of the perceptual scale curve. Our findings generally agree with Whittle's previous findings (Whittle, 1986, 1992), which also included strong asymmetry between A+ and A-. We suggest that the perception of achromatic balanced incremental and decremental (bipolar) stimuli, such as gratings or flicker, might be dominated by one polarity due to this asymmetry under some conditions.

Examining Increment thresholds as a function of pedestal contrast under hypothetical parvo- and magnocellular-biased conditions.

Theoretically, the pulsed- and steady-pedestal paradigms are thought to track contrast-increment thresholds (ΔC) as a function of pedestal contrast (C) for the parvocellular (P) and magnocellular (M) systems, respectively, yielding linear ΔC versus C functions for the pulsed- and nonlinear functions for the steady-pedestal paradigm. A recent study utilizing these paradigms to isolate the P and M systems reported no evidence of the M system being suppressed by red light, contrary to previous physiological and psychophysical findings. Curious as to why this may have occurred, we examined how ΔC varies with C for the P and M systems using the pulsed- and steady-pedestal paradigms and stimuli biased towards the P or M systems based on their sensitivity to spatial frequency (SF) and color. We found no effect of color and little influence of SF. To explain this lack of color effects, we used a quantitative model of ΔC (as it changes with C) to obtain Csat and contrast-gain values. The contrast-gain values (i) contradicted the hypothesis that the steady-pedestal paradigm tracks the M-system response, and (ii) our obtained Csat values indicated strongly that both pulsed- and steady-pedestal paradigms track primarily the P-system response.

Poster Session: Achromatic increments and decrements are different: the relationship between scaling and discrimination.

A basic problem in psychophysics is to relate the internal representation of a stimulus to its physical intensity. In this study, we measured perceptual scales for achromatic contrast with Maximum Likelihood Difference Scaling (MLDS), using squares against a mid-grey background. Observers compared two stimulus pairs and chose the more different pair. All four squares were either achromatic increments (A+), or achromatic decrements (A-). The MLDS result was then compared with 2AFC achromatic pedestal discrimination, with pedestals and tests that were all combinations of A+ and A-. The main result is not novel: A+ and A- obey different rules. A Naka-Rushton saturating function describes the A+ MLDS result well, and the derivative of that function is proportional to the A+ pedestal discrimination for some (but not all) observers. A- MLDS and discrimination results are more complicated and are reminiscent of the classic findings of Whittle (1986, 1992). The sensitivity of A- is a cubic polynomial function of pedestal contrast. These findings will be compared with a similar study of S-cone contrast (reported at VSS 2022), which found a different type of asymmetry between S+ and S-. Presumably these increment/decrement asymmetries are due to underlying differences between ON and OFF neural pathways. One implication is that using stimuli that include both contrast signs, such as gratings and flicker, may obscure important asymmetries in the processing of contrast.

Examining increment thresholds as a function of pedestal contrast under hypothetical parvocellular- and magnocellular-biased conditions.

Examining increment thresholds as a function of pedestal contrast under hypothetical parvocellular- and magnocellular-biased conditions.

Examining Increment Thresholds as a Function of Pedestal Contrast Under Hypothetical Parvocellular- And Magnocellular-Biased Conditions

Examining Increment Thresholds as a Function of Pedestal Contrast Under Hypothetical Parvocellular- And Magnocellular-Biased Conditions

Color discrimination in anomalous trichromacy: Experiment and theory

In anomalous trichromacy, the color signals available from comparing the activities of the two classes of cone sensitive in the medium and long wavelength parts of the spectrum are much reduced from those available in normal trichromacy, and color discrimination thresholds along the red-green axis are correspondingly elevated. Yet there is evidence that suprathreshold color perception is relatively preserved; this has led to the suggestion that anomalous trichromats post-receptorally amplify their impoverished red-green signals. To test this idea, we measured chromatic discrimination from white and from saturated red and green pedestals. If there is no post-receptoral compensation, the anomalous trichromat’s loss of chromatic contrast will apply equally to the pedestal and to the test color. Coupled with a compressively nonlinear neural representation of saturation, this means that a given pedestal contrast will cause a smaller than normal modulation of discrimination sensitivity. We examined cases where chromatic pedestals impair the color discrimination of normal trichromatic observers. As predicted, anomalous observers experienced less impairment than normal trichromats, though they remained less sensitive than normal trichromats. Although the effectiveness of chromatic pedestals in impairing color discrimination was less for anomalous than for normal trichromats, the chromatic pedestals were more effective for anomalous observers than would be expected if the anomalous post-receptoral visual system were the same as in normal trichromacy; the hypothesis of zero compensation can be rejected. This might suggest that the effective contrast of the pedestal is post-receptorally amplified. But on closer analysis, the results do not support candidate simple models involving post-receptoral compensation either.

Magnocellular and Parvocellular Mediated Luminance Contrast Discrimination in Severe Alcohol Use Disorder

Severe alcohol use disorder (SAUD) is associated with widespread cognitive impairments, including low-level visual processing deficits that persist after prolonged abstinence. However, the extent and characteristics of these visual deficits remain largely undetermined, impeding the identification of their underlying mechanisms and influence on higher-order processing. In particular, little work has been conducted to assess the integrity of the magnocellular (MC) and parvocellular (PC) visual pathways, namely the 2 main visual streams that convey information from the retina up to striate, extrastriate, and dorsal/ventral cerebral regions. We investigated achromatic luminance contrast processing mediated by inferred MC and PC pathways in 33 patients with SAUD and 32 matched healthy controls using 2 psychophysical pedestal contrast discrimination tasks that promote responses of inferred MC or PC pathways. We relied on a staircase procedure to assess participants' ability to detect small changes in luminance within an array of 4 gray squares that were either continuously presented (steady pedestal, MC-biased) or briefly flashed (pulsed pedestal, PC-biased). We replicated the expected pattern of MC and PC contrast responses in healthy controls. We found preserved dissociation of MC and PC contrast signatures in SAUD but higher MC-mediated mean contrast discrimination thresholds combined with a steeper PC-mediated contrast discrimination slope compared with healthy controls. These findings indicate altered MC-mediated contrast sensitivity and PC-mediated contrast gain, confirming the presence of early sensory disturbances in individuals with SAUD. Such low-level deficits, while usually overlooked, might influence higher-order abilities (e.g., memory, executive functions) in SAUD by disturbing the "coarse-to-fine" tuning of the visual system, which relies on the distinct functional properties of MC and PC pathways and ensures proper and efficient monitoring of the environment.

Enhancement of pre-pulse and picosecond pedestal contrast of the petawatt J-KAREN-P laser

Abstract We have experimentally improved the temporal contrast of the petawatt J-KAREN-P laser facility. We have investigated how the generation of pre-pulses by post-pulses changes due to the temporal overlap between the stretched pulse and the post-pulse in a chirped-pulse amplification system. We have shown that the time at which the pre-pulse is generated by the post-pulse and its shape are related to the time difference between the stretched main pulse and the post-pulse. With this investigation, we have found and identified the origins of the pre-pulses and have demonstrated the removal of most pre-pulses by eliminating the post-pulse with wedged optics. We have also demonstrated the impact of stretcher optics on the picosecond pedestal. We have realized orders of magnitude enhancement of the pedestal by improving the optical quality of a key component in the stretcher.

Contrast discrimination under task-induced mental load

Task-induced mental load can potentially degrade visual performance, which could be important during tasks such as driving. In the natural world, most objects have visual contrasts that are supra-threshold, and so the ability to reliably distinguish the borders between these objects is related to contrast discrimination, rather than absolute contrast detection performance. This study investigated the effects of auditory task-induced mental load on contrast discrimination threshold across several spatial frequencies. Binocular contrast discrimination thresholds were measured in 14 participants at spatial frequencies of 0.25, 1, 4, 8, 14 cpd across pedestal contrast levels of 10, 16, 24, 38 and 60%. The task was repeated with an auditory 0-back task and 2-back task to impose a mental load, the magnitude of which was measured through subjective ratings and heart rate recording. A concurrent 2-back task significantly increased contrast discrimination thresholds across all spatial frequencies when compared to both the baseline and 0-back conditions. There was no significant effect of mental load on the slope of the Threshold vs Contrast function once potential ceiling effects in our staircase procedure were removed. We conclude that mental load induced by concurrent cognitive tasks can increase contrast discrimination thresholds. When evaluating an individuals’ performance under natural-world condition such as driving, the effects of task-induced mental load on contrast discrimination thresholds should be considered.

Scattering pulse-induced temporal contrast degradation in chirped-pulse amplification lasers.

Herein, a theory for modeling the problem of scattering pulse-induced temporal contrast degradation in chirped-pulse amplification (CPA) lasers is introduced. Using this model, the temporal evolutions of the scattering and signal pulses were simulated, the temporal contrasts for different cases were compared, and finally the theoretical prediction was verified by an experimental demonstration. The result shows that the picosecond and the nanosecond temporal contrast is mainly determined by the scattering pulses generated in the stretcher and the compressor, respectively. In addition, the B-integral accumulation will further degrade the temporal contrast, especially the picosecond temporal contrast. We believe it is helpful for solving the problem of the picosecond pedestal contrast (i.e., noise limit). With reference to these results, some suggestions for the temporal contrast improvement are presented.

Contrast Gain Control in Plaid Pattern Detection.

A plaid is a combination of two gratings whose orientations are orthogonal to each other with the same or similar contrasts. We used plaid patterns as stimuli to investigate the mechanisms underlying the detection of a plaid to understand how the visual system combines information from orientation-selective channels. We used a masking paradigm in which an observer was required to detect a target (either a spiral or a plaid) superimposed on a pedestal. We measured the target threshold versus pedestal contrast (TvC) functions at 7 pedestal contrasts for various target-pedestal combinations with a temporal 2AFC paradigm and a staircase procedure. All TvC functions, except the one with an orthogonal spiral pedestal, showed a dipper shape, although the position of the dip and the slope varied across conditions. The result can be explained by a multiple-mechanism divisive inhibition model, which contains several orientation-selective mechanisms. The response of each mechanism is the excitation of a linear filter divided by a broadband inhibitory input. The threshold is determined by a nonlinear combination of the responses of those mechanisms. Alternative models with mechanism(s) specific for plaid did not provide a better description of the data. Thus, a plaid pattern is mediated by a combination of orientation-selective mechanisms. An early plaid-specific mechanism is not necessary for plaid detection.

An Image-Based Model for Early Visual Processing

Figure 2: Contrast discrimination. Increment threshold vs. pedestal contrast for gratigs of 8.37 cyc/deg and 1497ms presentation time. Early spatial vision was explored extensively over several decades in many psychophysical detection and discrimination experiments. Thus a large body of such data is available. Goris et al. (2013) integrated this psychophysical literature and proposed a model based on maximumlikelihood decoding of a neurophysiologically inspired population of model neurons. Their neural population model (NPM) is unique in its ability to predict several data sets simultaneously, using a single set of parameters. However, the NPM is only one-dimensional, operating on the activity of abstract spatial frequency channels. Thus it cannot be applied to arbitrary images as a generic front-end to explore the influence of early visual processing on midor high-level vision. Bradley et al. (2014), on the other hand, recently presented an early vision model operating on images. Their model is thus able to make predictions for arbitrary images. However, compared to the NPM, their model lacks in nonlinear processing, which is replaced by an effective masking contrast depending on the detection target. Thus while Bradley et al. fit a range of detection data they do not fit nonlinear aspects of early vision like the dipper function. Here we combine both approaches into a model which includes nonlinear processing and statistically efficient decoding and operates on images explicitly calculating channel activities over the image. The model applies optical degradation and retinal processing to the image before the image is passed to a spatial frequency and orientation decomposition followed by divisive inhibition. We tested the predictions of our model against a broad range of early psychophysical experiments and found it predicts some hallmarks of early visual processing like the contrast sensitivity function (Figure 1), the dipper function for contrast discrimination (Figure 2) and oblique masking data (Figure 3). The measured data we plot against stems from standard 2AFC contrast detection and discrimination experiments run for Wichmann’s DPhil thesis (1999). Early vision was extensively explored empirically and there are plenty of models which explain different parts of the empirical findings. These results provide detailed information about the first processing steps in visual perception thought to happen in the retina, LGN and V1, which stimuli can be differentiated or detected using their output and in what format the output is available to any higher visual processing. However, research on higher visual processing rarely employs the information available from early vision psychophysics. The main reasons may be that the models described in the early spatial vision literature typically do not predict behaviour on arbitrary images, because they often do not process images, but only predict the (putative) responses of the early visual system to Gabor patches or simple grating stimuli. We try to overcome these limitations by explaining many psychophysical observations within a model taking images as input.

Alterations of the contrast gain during normal aging: a dissociation between the Magnocellular and Parvocellular signatures for old and very-old groups

Functional changes in normal aging for both Magnocellular (M) and Parvocellular (P) pathways have been reported, with a greater loss for the P system (Elliot & Werner, 2010, JOV). The aim of the study was to investigate the late development of the two systems in old age by comparing two aged groups. A short version of the original paradigm (Pokorny and Smith, 1997, JOSA) has been used in 18 young (19-34), 25 old (61-74) and 11very old (75-80) observers. They had to discriminate the location of the higher luminance square with in a 33-msec four-square-array. In the steady-pedestal condition (M-bias), the array was preceded and followed by a four identical squares pedestal whereas, in the pulse pedestal condition (P-bias), the array was presented alone on a gray background. Three target luminance discrimination thresholds were collected for each of the 6 experimental conditions (order counterbalanced): 3 pedestal-contrasts (63%; 70%; 75%) x 2 pedestal-conditions (pulse and steady) using an adaptive staircase procedure. The results showed a higher increase of threshold when pedestal contrast increased in the pulse-pedestal than in the steady-pedestal condition, consistently with the original results in young adults. The main result was a double interaction between group, pedestal contrast and pedestal-condition : There was a huge increase of threshold with aging in the pulse-pedestal condition, with no differences between the two aged groups, whereas in the steady-pedestal, there was a slight increase in threshold in old age and a larger increase in threshold in the very-old aged group. These results are consistent with a dissociation in the evolution of the two systems with aging. Under the conditions tested, the functional loss for the P pathway, tough larger than the M pathway, seems to be stable from 60-year-old, although the M pathways still deteriorated after 75-year-old. Meeting abstract presented at VSS 2012

Optimal decisions for contrast discrimination

Contrast discrimination functions for simple gratings famously look like a dipper. Discrimination thresholds are lower than detection thresholds for moderate pedestal contrasts, and the rate of growth of thresholds as the pedestal contrast gets larger typically lies between the values implied by two popular treatments of noise. Here, we suggest a new normative treatment of the dipper, showing how it emerges from Bayesian inference based on the responses of a population of orientation-tuned units. Our central assumption concerns the noise corrupting the outputs of these units as a function of the contrast: We suggest that it has the shape of a hinge. We show the match to the psychophysical data and discuss the neurobiological and statistical rationales for this form of noise. Finally, we relate our model to other major accounts of contrast discrimination.

Mechanisms of Selective Attention: Response Enhancement, Noise Reduction, and Efficient Pooling of Sensory Responses

In this issue of Neuron, Pestilli and coworkers provide evidence that response gain and noise reduction are insufficient to account for attention-induced changes in perception. Instead, selection may critically depend on the biased pooling of sensory signals during decision making.

Direction discrimination thresholds in binocular, monocular, and dichoptic viewing: Motion opponency and contrast gain control

We studied the binocular organization of motion opponency and its relationship to contrast gain control. Luminance contrast thresholds for discriminating direction of motion were measured for drifting Gabor patterns (target) presented on counterphase flickering Gabor patterns (pedestal). There were four presentation conditions: binocular, monocular, dichoptic, and half-binocular. For the half-binocular presentation, the target was presented to one eye while pedestals were presented to both eyes. In addition, to test for motion opponency, we studied two increment and decrement conditions, in which the target increased contrast for one direction of movement but decreased it for the opposite moving component of the pedestal. Threshold versus pedestal contrast functions showed a dipper shape, and there was a strong interaction between pedestal contrast and test condition. Binocular thresholds were lower than monocular thresholds but only at low pedestal contrasts. Monocular and half-binocular thresholds were similar at low pedestal contrasts, but half-binocular thresholds became higher and closer to dichoptic thresholds as pedestal contrast increased. Adding the decremental target reduced thresholds by a factor of two or more-a strong sign of opponency-when the decrement was in the same eye as the increment or the opposite eye. We compared several computational models fitted to the data. Converging evidence from the present and previous studies (Gorea, Conway, & Blake, 2001) suggests that motion opponency is most likely to be monocular, occurring before direction-specific binocular summation and before divisive, binocular gain control.

Direction discrimination thresholds in binocular, monocular, and dichoptic viewing: Motion opponency and contrast gain control

Purpose: The present study investigated the binocular organization of motion opponency and its relation to contrast gain control. Method: We measured luminance contrast thresholds for discriminating direction of motion for drifting Gabor patterns (target) presented on counter-phase flickering Gabor patterns (pedestal; equivalent to the superposition of two Gabors drifting in opposite directions). There were four presentation conditions. (i) Binocular: all stimuli were presented to both eyes, (ii) Monocular: all stimuli were presented to the same one eye, (iii) Dichoptic: the target was presented to one eye while the pedestal was presented to the other eye, (iv) Half-binocular: the target was presented to one eye while pedestals were presented to both eyes. In addition, we tested an increment-and-decrement condition, in which the target increased contrast for one direction of movement, but decreased it by the same amount for the opposite moving component of the pedestal. The decrement was either in the same eye as the increment, or in the other eye. Results and Discussion: Threshold-versus-pedestal-contrast (TvC) functions showed a dipper shape: thresholds decreased and then increased with pedestal contrast. At low pedestal contrasts, there was binocular summation: binocular thresholds were lower than monocular. But at high pedestal contrasts there was little difference between them. The ‘dip’ was smaller for dichoptic presentation than for other presentations. Thresholds were similar for monocular and half-binocular presentations at low pedestal contrasts, but half-binocular thresholds became higher and closer to dichoptic thresholds as pedestal contrast increased. The added decremental target lowered thresholds by about a factor of 2, compared with half-binocular, when the decrement was in the same eye as the increment, or the opposite eye. Several models fit to the data strongly suggest that motion opponency and contrast gain control operate at a binocular level of processing.

A comparison of the pedestal effects in the 1st- and 2nd-order patterns

Purpose. Human visual system is sensitive to both luminance (the first-order) modulations and contrast (the second-order) modulations in an image. A linear-nonlinear-linear (LNL) model is commonly used to explain the visual process of the 2nd-order stimuli. Here we used pattern masking paradigm to compare the 1st-order and the 2nd-order visual mechanisms and to characterize the nonlinear properties of their underlying mechanisms. Methods. The stimuli were a high frequency horizontal grating (8 cyc/d) either added to (1st-order stimuli) or multiplied with (2nd-order stimuli) a vertical low frequency (2 cyc/d) Gabor function. The discrimination threshold of the target was measured with pedestals whose spatial properties as that of the target except contrast (1st-order pedestal) or modulation depth (2nd-order pedestal) of either the low or the high frequency components. Results. The threshold function showed a typical dipper shape for both the 1st- and the 2nd-order stimuli: the threshold first decreased (facilitation) and then increased (suppression) with pedestal contrast or modulation depth. The results for the 1st-order stimuli were well explained by divisive inhibition model in which the facilitatory input was divided by the sum of broadband inhibitory inputs. The results for the 2nd-order stimuli were also well explained by a modified that operated on modulation depth rather than contrast in the input images. Conclusion. Our result suggests that divisive inhibition is required to explain visual discrimination in both the first- and the second-order patterns. However, the source and the nonlinearity of the divisive inhibition may be different for these two types of patterns.

Iso- and cross-orientation surround modulation of contrast detection and discrimination

Background: Neurophysiological and psychophysical evidence reveals strong modulation of contextual stimuli on visual performance. Here we studied how contrast detection and discrimination in foveal vision are affected by surround stimuli at iso- or cross-orientations. Methods: The target was a small D6 grating (8 cpd, vertical) centered on a slightly larger sinusoid grating pedestal (8 cpd, vertical, contrast = 0–40%) with an abutting sinusoid grating surround (8 cpd, contrast = 0–80%, horizontal or vertical). Thresholds were measured with a temporal 2AFC staircase procedure. Results: (1) Iso-oriented surrounds have little effect on detection except at high contrast where they become suppressive. For discrimination, iso-oriented surrounds facilitate when the surround/center contrast ratio is <= 1 and suppress when the s/c ratio is >1. (2) Surprisingly, cross-oriented surrounds can facilitate detection and discrimination by a factor of up to 2, even in the dipper region of the tvc function. This effect is not due to reduced uncertainty, since the slope of the psychometric function is unchanged by the surround, and facilitation is not impaired by jittering the surround contrast. Cross-orientation facilitation is largely unaffected by the s/c ratio. Under each surround contrast condition, facilitation dips when the pedestal contrast is around 5–20%, and becomes stronger at lower and higher pedestal contrasts. The data also indicate that cross-oriented surrounds may facilitate detection and near-threshold discrimination (pedestal contrast <10%) via direct exCitation, and facilitate suprathreshold discrimination (pedestal contrast >=10%) via indirect disinhibition of masking. Conclusion: Our results suggest that surround effects participate in complex gain control processes in spatial filters, which are not easily explained by the standard spatial filter models, but are supported by recent neurophysiological findings.