Contrast Increment Thresholds Research Articles

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.

Short-term monocular deprivation reduces inter-ocular suppression of the deprived eye

The adult visual system was traditionally thought to be relatively hard-wired, but recent studies have challenged this view by demonstrating plasticity following short-term monocular deprivation. Depriving one eye of spatial information for 2–3 h increased subsequent sensory dominance of that eye. However, the mechanism underlying this phenomenon is unclear. The present study sought to address this issue and determine the consequences of short-term monocular deprivation on inter-ocular suppression of each eye. Sensory eye dominance was examined before and after depriving an eye of all input using an opaque patch for 2.5 h, in six adult participants with normal binocular vision. We used a percept tracking task during binocular rivalry (BR) to assess the relative eye dominance, and an objective probe detection task under continuous flash suppression (CFS) to quantify each eye’s susceptibility to inter-ocular suppression. The monocular contrast increment threshold of each eye was also measured using the probe task to ascertain if the altered eye dominance is accompanied by changes in monocular perception. Our BR results replicated previous findings of a shift of relative dominance towards the eye that has been deprived of form information. More crucially, using CFS we demonstrated reduced inter-ocular suppression of the deprived eye with no complementary changes in the other eye, and no monocular changes in increment threshold. These findings imply that short-term monocular deprivation alters binocular interactions. The differential effect on inter-ocular suppression between eyes may have important implications for the use of patching as a therapy to recover visual function in amblyopia.

The impact of higher-order aberrations on the strength of directional signals produced by accommodative microfluctuations.

It has been proposed that the accommodation system could perform contrast discrimination between the two dioptric extremes of accommodative microfluctuations to extract directional signals for reflex accommodation. Higher-order aberrations (HOAs) may have a significant influence on the strength of these contrast signals. Our goal was to compute the effect HOAs may have on contrast signals for stimuli within the upper defocus limit by comparing computed microcontrast fluctuations with psychophysical contrast increment thresholds (Bradley & Ohzawa, 1986). Wavefront aberrations were measured while subjects viewed a Maltese spoke stimulus monocularly. Computations were performed for accommodation or disaccommodation stimuli from a 3 Diopter (D) baseline. Microfluctuations were estimated from the standard deviation of the wavefronts over time at baseline. Through-focus Modulation Transfer, optical contrast increments (ΔC), and Weber fractions (ΔC/C) were derived from point spread functions computed from the wavefronts at baseline for 2 and 4 cycles per degree (cpd) components, with and without HOAs. The ΔCs thus computed from the wavefronts were compared with psychophysical contrast increment threshold data. Microfluctuations are potentially useful for extracting directional information for defocus values within 3 D, where contrast increments for the 2 or 4 cpd components exceed psychophysical thresholds. HOAs largely reduce contrast signals produced by microfluctuations, depending on the mean focus error, and their magnitude in individual subjects, and they may shrink the effective stimulus range for reflex accommodation. The upper defocus limit could therefore be constrained by discrimination of microcontrast fluctuations.

Marked selective impairment in autism on an index of magnocellular function

Atypical high-level vision in autism is sometimes attributed to a core deficit in the function of lateral geniculate nucleus magnocells or their retinal drives. While some physiological measures provide indirect, suggestive evidence for such a deficit, support from behavioural measures is lacking and contradictory. We assessed luminance contrast increment thresholds on pulsed- and steady- pedestals in 17 children with autism spectrum conditions (ASC) compared to 17 typically developing children; these two conditions correspond to widely-used indices of magnocellular and parvocellular function. As a group, children with ASC had strikingly elevated thresholds on the steady pedestal-paradigm, yet performed similarly to controls on the pulsed pedestal paradigm, a finding that would typically be interpreted to reflect impaired magnocellular function. The effect size of the impairment was large and a substantial minority (41.2%) of the ASC group showed significantly impaired performance on an individual basis. This finding is consistent with a selective magnocellular deficit. It directly contradicts previous claims that such deficits are confined to ‘complex’ visual stimuli and likely does not reflect atypical attention, adaptation or high-level vision. The pattern of results is not clearly predicted by notions of imbalance of excitation versus inhibition, atypical lateral connectivity or enhanced perceptual function that account for a range of other findings associated with perception in autism. It may be amenable to explanation in terms of decreased endogenous neural noise, a novel alternative we outline here.

The locus of flicker adaptation in the migraine visual system: A dichoptic study

Flickering light has been shown to sensitize the migraine visual system at high stimulus contrast while elevating thresholds at low contrast. The present study employs a dichoptic psychophysical paradigm to ask whether the abnormal adaptation to flicker in migraine occurs before or after the binocular combination of inputs from the two eyes in the visual cortex. Following adaptation to high contrast flicker presented to one eye only, flicker contrast increment thresholds were measured in each eye separately using dichoptic viewing. Modest interocular transfer of adaptation was seen in both migraine and control groups at low contrast. Sensitization at high contrast in migraine relative to control participants was seen in the adapted eye only, and an unanticipated threshold elevation occurred in the non-adapted eye. Migraineurs also showed significantly lower aversion thresholds to full field flicker than control participants, but aversion scores and increment thresholds were not correlated. The results are simulated with a three-stage neural model of adaptation that points to strong adaptation at monocular sites prior to binocular combination, and weaker adaptation at the level of cortical binocular neurons. The sensitization at high contrast in migraine is proposed to result from stronger adaptation of inhibitory neurons, which act as a monocular normalization pool.

Detecting contrast changes in invisible patterns during binocular rivalry

When dissimilar images are presented to the two eyes, the human visual system lapses into binocular rivalry, a unique perceptual state characterized by stochastic alternations in dominance of one of the two source images over the other. Probe targets delivered to an eye during suppression phases are more difficult to detect than probes delivered during dominance phases. Nearly all probe studies have involved presenting new stimulation (e.g., a spot of light) either superimposed on or replacing the suppressed stimulus. Here, we ask whether observers can detect a reduction in the contrast of the suppressed stimulus itself. In other words, can observers detect a probe that should make an already invisible stimulus even weaker? Specifically, we compared observers’ ability to detect contrast increments and contrast decrements introduced within a rival pattern during dominance and suppression. Contrast increment thresholds were elevated across all pedestal contrasts when the increment was introduced during suppression compared to during dominance, replicating previous results. Contrast decrement thresholds measured during suppression were elevated to an even greater extent, but the fact that they were obtained at all establishes that observers were able to detect probes that should make an already invisible target even more difficult to perceive. In a second experiment, we found a similar pattern of results for contrast change detection in complex images of faces as well. Based on the resulting threshold-vs.-contrast functions, we suggest that, regardless of the complexity of the image, rivalry suppression modulates the neural contrast response function through a mixture of reduced overall response gain and a shift in the contrast gain.

Attentional modulation of visual motion perception using novel wavelet stimuli

We have previously characterized the effects of withdrawing attention on detection and discrimination of static visual stimuli (Lee et al. Nature Neuroscience 1998). Here we report attentional modulation of motion perception. A novel motion stimulus comprising spatiotemporally contrast-modulated Gabor wavelets was used to distinguish attentional effects on mechanisms sensitive to component motion from those sensitive to pattern motion (Schrater et al Nature Neuroscience 2000). Methods: Multiple moving wavelets were presented as a composite patch (1degree diameter) 4degrees away from fixation. Each wavelet had a random orientation and phase, and moved orthogonally to its orientation. A local wavelet mask of different orientation and contrast was superimposed on each target wavelet. This composite stimulus is expected to selectively activate component motion mechanisms located in primary visual cortex. A concurrent letter discrimination task presented at fixation was used to withdraw attention from the peripheral motion stimulus. 3 observers performed a 2AFC target detection task on the peripheral stimulus, while contrast and local direction of the mask wavelets was systematically manipulated. Results: Novel moving wavelet stimuli can be successfully used to characterize the effects of withdrawing attention on component-motion sensitive mechanisms. 1) Contrast increment thresholds for motion detection showed a 'dipper' function 2) Withdrawing attention led to elevation of motion detection thresholds that depended systematically on mask contrast but had little dependence on mask direction. 3) Motion detection thresholds with full attention (or without attention) were surprisingly similar whether the mask wavelets moved locally in either the same or opposite directions as the target.

Local binocular fusion is involved in global binocular rivalry

We examined whether interocular inhibition in binocular rivalry could occur at the interocular intersection of horizontal and vertical rectangular patches which are locally fusible but globally rivalrous between the two eyes. We measured contrast increment (and decrement) thresholds of a monocularly presented probe which was presented on the horizontal patch corresponding to the intersection. We found that the threshold was higher when the horizontal patch was perceptually suppressed than when it was dominant. In addition, threshold elevation did not occur when both patches were dominant, or when the horizontal patch was viewed in isolation. These results indicate that interocular inhibition occurs at the potentially fusible region, and the determination of binocular fusion or binocular rivalry does not depend on physical stimulus but rather perceptual state at the time.

Contextual modulation involves suppression and facilitation from the center and the surround

In psychophysics, cross-orientation suppression (XOS) and cross-orientation facilitation (XOF) have been measured by investigating mask configuration on the detection threshold of a centrally placed patch of sine-wave grating. Much of the evidence for XOS and XOF comes from studies using low and high spatial frequencies, respectively, where the interactions are thought to arise from within (XOS) and outside (XOF) the footprint of the classical receptive field. We address the relation between these processes here by measuring the effects of various sizes of superimposed and annular cross-oriented masks on detection thresholds at two spatial scales (1 and 7 c/deg) and on contrast increment thresholds at 7 c/deg. A functional model of our results indicates the following (1) XOS and XOF both occur for superimposed and annular masks. (2) XOS declines with spatial frequency but XOF does not. (3) The spatial extent of the interactions does not scale with spatial frequency, meaning that surround-effects are seen primarily at high spatial frequencies. (4) There are two distinct processes involved in XOS: direct divisive suppression and modulation of self-suppression. (5) Whether XOS or XOF wins out depends upon their relative weights and mask contrast. These results prompt enquiry into the effect of spatial frequency at the single-cell level and place new constraints on image-processing models of early visual processing.

Neuronal Mechanisms for Illusory Brightness Perception in Humans

Biological visual systems are extraordinarily capable of recovering the shape and brightness of objects from sparse and fragmentary information. Using functional magnetic imaging, we show that two associative areas of the dorsal pathway--in the caudal region of the intrapariatal sulcus and in the lateral occipital sulcus--respond specifically to the Craik-O'Brien-Cornsweet illusion generated by high-pass filtered edges. Other visual areas, including primary visual cortex, also respond strongly to the retinotopic location of the edge, but these areas respond equally well to a line of matched contrast and detectability, rather than specifically to the brightness illusion. The reconstruction of surface and/or its brightness seems to be achieved by associative areas from the information about visual features provided by the primary visual cortices, even where there is no physical difference in luminance.

Preserved gain control for luminance contrast during binocular rivalry suppression

Binocular rivalry elevates contrast increment thresholds for the detection of a transient stimulus presented to the suppressed eye, while thresholds measured during dominance are identical to those during monocular viewing (e.g. [Wales, R., & Fox, R. (1970). Increment detection thresholds during binocular rivalry suppression. Perception and Psychophysics, 8, 90–94]). It is well established that contrast increment thresholds depend on reference (pedestal) contrast. With high contrasts, increment thresholds increase with pedestal contrast, reflecting a gain control with sigmoidal non-linearity. We examined how this gain control mechanism operates during binocular rivalry (i.e., with and without perception of a pedestal mask). Subjects viewed a horizontal sine-wave grating (steady pedestal) and a radial checkerboard dichoptically. When the grating achieved a pre-specified phenomenal state (dominance or suppressed), subjects initiated the transient presentation (500-ms Gaussian pulse) of a contrast increment of the same spatial frequency. The pulse appeared in either the upper or lower half of the pedestal. Subjects indicated which half of the pedestal contained the pulse. Contrast increment thresholds were measured using a staircase method with various pedestal contrasts, which yielded threshold versus contrast (TvC) functions during dominance and suppression. The measured thresholds were reliably higher during suppression, but the rising slopes of TvC functions did not differ significantly between dominance and suppression (i.e., constant upward shift of TvC function). A control experiment demonstrated that the TvC function during dominance was identical to that during non-rivalry, monocular viewing. Evidently, the contrast gain control for transient luminance increment does not require the perception of pedestal contrast.

Preserved gain control for luminance contrast during binocular rivalry suppression

Binocular rivalry elevates contrast increment thresholds for the detection of a transient stimulus presented to the suppressed eye, while thresholds measured during dominance are identical to those during monocular viewing (e.g. [Wales, R., & Fox, R. (1970). Increment detection thresholds during binocular rivalry suppression. Perception and Psychophysics, 8, 90-94]). It is well established that contrast increment thresholds depend on reference (pedestal) contrast. With high contrasts, increment thresholds increase with pedestal contrast, reflecting a gain control with sigmoidal non-linearity. We examined how this gain control mechanism operates during binocular rivalry (i.e., with and without perception of a pedestal mask). Subjects viewed a horizontal sine-wave grating (steady pedestal) and a radial checkerboard dichoptically. When the grating achieved a pre-specified phenomenal state (dominance or suppressed), subjects initiated the transient presentation (500-ms Gaussian pulse) of a contrast increment of the same spatial frequency. The pulse appeared in either the upper or lower half of the pedestal. Subjects indicated which half of the pedestal contained the pulse. Contrast increment thresholds were measured using a staircase method with various pedestal contrasts, which yielded threshold versus contrast (TvC) functions during dominance and suppression. The measured thresholds were reliably higher during suppression, but the rising slopes of TvC functions did not differ significantly between dominance and suppression (i.e., constant upward shift of TvC function). A control experiment demonstrated that the TvC function during dominance was identical to that during non-rivalry, monocular viewing. Evidently, the contrast gain control for transient luminance increment does not require the perception of pedestal contrast.

The lingering effects of artificial scotomata

Prolonged fixation produces artificial scotomata in dynamic random noise: peripheral, zero-contrast regions disappear. When the noise's contrast is suddently zeroed, dynamic afterimages are seen where the scotomata had been (Ramachandran & Gregory, Nature, 1991). We measured contrast increment thresholds for dynamic noise coextensive with these afterimages, using various pedestal and adapting contrasts. When the adapting contrast was zero, increment thresholds formed a dipper-shaped function of pedestal contrast. When the adapting contrast was high, the dip disappeared, but there was no change to the left or right side of the function. Inverting popular theories for dipper-shaped increment-threshold functions, we infer linear transduction and/or intrinsic certainty from the lack of dipper. However, changes in transduction or certainty should also be reflected in absolute threshold. To explain adaptation's lack of effect on detection without a pedestal, we are forced to believe in a lingering effect of the sensitivity loss responsible for the artificial scotomata (Morgan & Solomon, ECVP, 2003), one that perfectly balances the effect of linear transduction and/or uncertainty reduction on absolute threshold.

Contrast discrimination with sinusoidal gratings of different spatial frequency.

The detectability of contrast increments was measured as a function of the contrast of a masking or "pedestal" grating at a number of different spatial frequencies ranging from 2 to 16 cycles per degree of visual angle. The pedestal grating always had the same orientation, spatial frequency, and phase as the signal. The shape of the contrast-increment threshold versus pedestal contrast (TvC) functions depends on the performance level used to define the "threshold," but when both axes are normalized by the contrast corresponding to 75% correct detection at each frequency, the TvC functions at a given performance level are identical. Confidence intervals on the slope of the rising part of the TvC functions are so wide that it is not possible with our data to reject Weber's law.

Adaptation and gain pool summation: alternative models and masking data

Foley [J. Opt. Soc. Am. A 11 (1994) 1710] has proposed an influential psychophysical model of masking in which mask components in a contrast gain pool are raised to an exponent before summation and divisive inhibition. We tested this summation rule in experiments in which contrast detection thresholds were measured for a vertical 1 c/deg (or 2 c/deg) sine-wave component in the presence of a 3 c/deg (or 6 c/deg) mask that had either a single component oriented at −45° or a pair of components oriented at ±45°. Contrary to the predictions of Foley's model 3, we found that for masks of moderate contrast and above, threshold elevation was predicted by linear summation of the mask components in the inhibitory stage of the contrast gain pool. We built this feature into two new models, referred to as the early adaptation model and the hybrid model. In the early adaptation model, contrast adaptation controls a threshold-like nonlinearity on the output of otherwise linear pathways that provide the excitatory and inhibitory inputs to a gain control stage. The hybrid model involves nonlinear and nonadaptable routes to excitatory and inhibitory stages as well as an adaptable linear route. With only six free parameters, both models provide excellent fits to the masking and adaptation data of Foley and Chen [Vision Res. 37 (1997) 2779] but unlike Foley and Chen's model, are able to do so with only one adaptation parameter. However, only the hybrid model is able to capture the features of Foley's (1994) pedestal plus orthogonal fixed mask data. We conclude that (1) linear summation of inhibitory components is a feature of contrast masking, and (2) that the main aftereffect of spatial adaptation on contrast increment thresholds can be assigned to a single site.

Contrast discrimination and choice reaction times at near-threshold pedestals.

Choice reaction times (CRTs) to contrast differences were measured and compared with contrast increment thresholds obtained from concurrently measured psychometric functions at pedestal contrasts in the vicinity of detection threshold. Contrast discrimination functions had a classical dipper shape. The main finding was that CRTs were shorter at low pedestal contrasts but longer at higher pedestal contrasts compared to detection, reflecting the behaviour of increment thresholds. Even when equalized for response accuracy, CRTs varied with pedestal contrast in a similar manner to the contrast increment thresholds. The finding that CRTs and contrast increment thresholds depended on pedestal contrast in a similar manner suggests that both share a common origin. This common origin is proposed to lie in the variability of the sensory effect which determines the variability of the information accumulation process, which in turn affects the response criterion and contrast increment thresholds. At low pedestals, a decrease in variability lowers thresholds and results in a lower response criterion, thereby accelerating reaction times. At high pedestals, increasing signal-dependent noise inflates variability and thus raises thresholds and the response criterion, which results in slower CRTs.

Contrast discrimination under temporally varying contrast conditions.

Psychophysical contrast discrimination of a 0.8-cpd vertical grating was tested using a paradigm that alternated test and masking gratings at 8 Hz. Masking contrasts were lower than, equal to, or higher than the test contrasts. Six test contrasts were combined factorially with six masking contrasts to generate a series of six contrast increment threshold versus test contrast curves (tvc curves). A particularly simple relationship existed between these curves. The curves could be brought into alignment by shifting them diagonally by the ratio of their masking contrasts. It is shown that this behavior is predicted by a model in which contrast gain is set by the average of the test and masking contrasts coupled with a simple model of contrast discrimination. Contrast gain control integrates contrast over a period of at least 125 msec, and contrast discrimination is a function of this time-averaged contrast.

Neuronal basis of contrast discrimination

Psychophysical contrast increment thresholds were compared with neuronal responses, inferred from functional magnetic resonance imaging (fMRI) to test the hypothesis that contrast discrimination judgements are limited by neuronal signals in early visual cortical areas. FMRI was used to measure human brain activity as a function of stimulus contrast, in each of several identifiable visual cortical areas. Contrast increment thresholds were measured for the same stimuli across a range of baseline contrasts using a temporal 2AFC paradigm. FMRI responses and psychophysical measurements were compared by assuming that: (1) fMRI responses are proportional to local average neuronal activity; (2) subjects choose the stimulus interval that evoked the greater average neuronal activity; and (3) variability in the observer’s psychophysical judgements was due to additive (IID) noise. With these assumptions, FMRI responses in visual areas V1, V2d, V3d and V3A were found to be consistent with the psychophysical judgements, i.e. a contrast increment was detected when the fMRI responses in each of these brain areas increased by a criterion amount. Thus, the pooled activity of large numbers of neurons can reasonably well predict behavioral performance. The data also suggest that contrast gain in early visual cortex depends systematically on spatial frequency.

Contrast discrimination, non-uniform patterns and change blindness.

Change blindness--our inability to detect large changes in natural scenes when saccades, blinks and other transients interrupt visual input--seems to contradict psychophysical evidence for our exquisite sensitivity to contrast changes. Can the type of effects described as 'change blindness' be observed with simple, multi-element stimuli, amenable to psychophysical analysis? Such stimuli, composed of five mixed contrast elements, elicited a striking increase in contrast increment thresholds compared to those for an isolated element. Cue presentation prior to the stimulus substantially reduced thresholds, as for change blindness with natural scenes. On one hand, explanations for change blindness based on abstract and sketchy representations in short-term visual memory seem inappropriate for this low-level image property of contrast where there is ample evidence for exquisite performance on memory tasks. On the other hand, the highly increased thresholds for mixed contrast elements, and the decreased thresholds when a cue is present, argue against any simple early attentional or sensory explanation for change blindness. Thus, psychophysical results for very simple patterns cannot straightforwardly predict results even for the slightly more complicated patterns studied here.

Suprathreshold Contrast Increment Detection is not Affected by Grating Edge-Type

We have shown that the spatial frequency components introduced by sharply truncating the edge of a sinusoidal grating do not affect detection thresholds (Kukkonen et al, 1996 Perception25 Supplement, 117). This could be due to the fact that contrast energy at these spatial frequencies is much lower than at the nominal frequency. Any effect upon detection attributable to the broadening of the Fourier spectrum might therefore exert itself most strongly at suprathreshold rather than threshold contrast levels. To establish whether these extra components interfere with performance at higher contrasts, we measured contrast increment thresholds of circular-aperture sinusoidal gratings with a pedestal contrast of 0.5. Gratings were either sharply truncated or Gaussian-edged having a half-contrast diameter equal to that of its corresponding sharp-edged stimulus. Gratings of 0.125 and 0.5 cycle deg−1 were used, each tested with 1 – 8 cycles. There was no significant difference between contrast increment thresholds for the sharp-edged and Gaussian-edged gratings, which were spectrally narrower. The frequency components introduced into the Fourier spectrum by the abrupt edge of the truncated gratings appear to have no effect upon subject performance even in suprathreshold contrast increment detection.