Contrast Increment Research Articles

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.

The effect of inducer polarity and contrast on the perception of illusory figures.

A study designed to determine how inducer-surround contrast and inducer polarity affect the contour clarity and the lightness of illusory figures is reported. Using magnitude estimation procedures, ten naive subjects rated both the contour clarity and the lightness of Kanizsa squares. The magnitude of the inducer-surround contrast and the inducer polarity (all-black, all-white, or black-and-white) were varied randomly on each trial. The data indicate that contour clarity increases with contrast at the same rate across polarity conditions but that contour clarity at any given contrast level depends significantly on polarity. Contour clarity judgments were significantly lower when the inducers were all-white than when the inducers were all-black or black-and-white, and significantly greater in the 'mixed' polarity case (black-and-white inducers) than in the 'same' polarity case (the average of the all-black and all-white inducer conditions). Inducer contrast and polarity significantly affected the lightness of the illusory figure in a manner consistent with simultaneous spatial contrast. Also, for a given increment in contrast, contour clarity altered significantly more than surface lightness, regardless of inducer polarity. The findings suggest that the mechanism which mediates boundary formation is sensitive to the direction of contrast, and that the boundary formation mechanism is more sensitive than the surface lightness mechanism to changes in contrast magnitude. The results are considered within the context of neural network models of form perception.

The Effect of Pedestal Contrast on Choice Reaction Times to Contrast Increments

Two-alternative forced-choice reaction times (RTs) were measured and psychometric functions constructed for ten contrast increments at seven pedestal contrasts ranging from 0% to 4.8%. Two sine gratings at 2 cycles deg−1 differing only in contrast were presented on a computer screen, and the subject's task was to indicate as quickly as possible whether the stimulus with higher contrast appeared to the left or to the right of a fixation point. There were 100 trials per stimulus pair from which the percentage of correct responses and the median correct RTs were calculated. As the contrast increment increased, the percentage of correct responses increased and RTs decreased reaching a minimum with large increments. However, RTs continued to decrease even when performance reached 100% correct. Contrast increment thresholds calculated at 82% correct level formed a classical dipper-shaped function when plotted as a function of pedestal contrast. Response variability, reflected in standard errors of increment thresholds, was greater at high pedestal contrasts. When RTs corresponding to threshold increments were interpolated and plotted against pedestal contrast, the functions were also dipper-shaped. Discriminative RTs were on average faster at low and slower at high pedestal contrasts compared to detection. These findings show that equalising the percentage of correct responses did not equalise RTs, and that processing time increased with pedestal contrast. The increase of both increment thresholds and RTs with pedestal contrast may be due to an increase in signal-dependent noise which increases response variability and slows down the decision process.

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.

Contrast discrimination: a model and a hypothesis concerning the role of cholinergic modulation in contrast perception.

A model of contrast discrimination performance in human observers is developed and then extended to cover effects on performance of anticholinergic drugs. It is shown that it is necessary to assume that neural noise increases at high spatial frequencies in order to provide a satisfactory model of variations in discrimination performance with spatial frequency. The model results are compared with the results of empirical studies in which the effects of the muscarinic antagonist scopolamine (hyoscine) on contrast discrimination performance in human observers are examined. The purpose of the pharmacological work is to test the hypothesis that the differential contrast gain found psychophysically at different spatial frequencies might reflect differential facilitation by extrinsic cholinergic neurons. Contrast discrimination and contrast increment detection are found to be impaired by scopolamine in a manner that depends on both spatial frequency and base contrast. By comparing the empirical data with the predictions of the model, it is concluded that contrast constancy may reflect differential cholinergic modulation.

Bloch's Law for Contrast Increment Detection: The Effects of Pedestal Contrast and Light Level

The duration over which contrast detection improves (Bloch's regime) decreases with increasing light level and is often thought to reflect the temporal characteristics of the visual system. There is also some evidence to suggest that the temporal characteristics of the visual system might also change with increasing contrast level (M A Georgeson, 1987 Vision Research27 765 – 780). Here we compare temporal summation for stimuli presented on a blank field or on a high contrast background. On each trial a test grating was presented for X ms with the use of a spatial-alternate forced-choice procedure. The test grating (2 cycles deg−1) was presented superimposed on a similar pedestal grating which was also present for 500 ms prior to and after the test grating. Pedestal contrasts of 0% and 32% were tested at mean luminance levels of 150 cd m−2 and 1.5 cd m−2. The results show that both increasing light level and increasing contrast level resulted in smaller temporal summation times. In the current conditions both these effects approximately halve the summation time such that for a stimulus of low light level and of low pedestal contrast the summation time was ∼60 ms; low light, high contrast ∼30 ms; high light, low contrast ∼30 ms; and high light, high contrast ∼15 ms. The results imply that the temporal response of the visual system quickens with increasing contrast.

Discrimination of compound gratings: Spatial-frequency channels or local features?

Models based on spatial-frequency channels and local features provide alternative explanations for suprathreshold pattern discrimination. We compared psychophysical discrimination data with the predictions of the Wilson and Gelb channel model and three local-feature models. The features were peak-valley local contrast, peak-peak local contrast, and luminance gradients. We measured visual sensitivity for discriminating compound gratings ( F + 3 F or F + 5 F, in peaks-add or peaks-subtract phases) whose component contrasts were yoked together so that a contrast increment in one component was accompanied by an equal decrement in the other. The Wilson and Gelb model accounted for the results with peaks-add gratings, but failed to predict those with peaks-subtract gratings. None of the local-feature models explained the results by themselves. Most of the data fell close to an envelope composed of the lowest thresholds of the three feature-detector models, although there were important exceptions. Our findings are consistent with the view that suprathreshold pattern discrimination is mediated by mechanisms responsive to spatially localized features and that more than one type of feature is used.

Isolation and interaction of ON and OFF pathways in human vision: Pattern-polarity effects on contrast discrimination

To activate selectively cortical ON and OFF pathways, I measured pattern contrast discrimination functions and manipulated contrast polarity (positive and negative) of base contrast ( C ) and added contrast ( ΔC ). C was a large, long-duration cosine mask and ΔC was a brief, localized, spatially narrow-band “D6” pattern. For same polarity C and ΔC , contrast discrimination followed a “dipper” pattern threshold facilitation at low C and a power relation (exponent < 1.0) at high C . The facilitation is predicted from the low-contrast response of cortical neurons and seems to represent isolation of an ON or OFF pathway. Opposite polarity C and ΔC give a monotonic function. ΔC increases at low base C and remaining higher than the same-polarity function at higher C values. This represents interaction between ON and OFF pathways. Pathway isolation also occurs: a positive test is detected as a contrast increment if masked by negative contrast and a negative test is detected as a contrast decrement if masked by positive contrast. Quantitative aspects of the data suggest a subtractive interaction at low C values and a divisive interaction between pathways at high C values. Test contrast thresholds upon uniform fields of varying luminance show that both the dipper effect and most of the rise in ΔC with C are mediated in pattern-selective pathways rather than at a site of luminance adaptation. The pattern-polarity effects on contrast discrimination rule out the “channel uncertainty” explanation for the facilitation dipper. My results suggest that parallel ON and OFF pathways evolved because stimulus-produced decreases in the response of a single pathway are potentially confounded with the effects of contrast adaptation. Thus transient decreases in response in either pathway are not processed and both decreases and increases in contrast are expressed as response increases in separate pathways.

Isolation and interaction of ON and OFF pathways in human vision: Pattern-polarity effects on contrast discrimination

To activate selectively cortical ON and OFF pathways, I measured pattern contrast discrimination functions and manipulated contrast polarity (positive and negative) of base contrast ( C) and added contrast ( ΔC). C was a large, long-duration cosine mask and ΔC was a brief, localized, spatially narrow-band “D6” pattern. For same polarity C and ΔC, contrast discrimination followed a “dipper” pattern threshold facilitation at low C and a power relation (exponent < 1.0) at high C. The facilitation is predicted from the low-contrast response of cortical neurons and seems to represent isolation of an ON or OFF pathway. Opposite polarity C and ΔC give a monotonic function. ΔC increases at low base C and remaining higher than the same-polarity function at higher C values. This represents interaction between ON and OFF pathways. Pathway isolation also occurs: a positive test is detected as a contrast increment if masked by negative contrast and a negative test is detected as a contrast decrement if masked by positive contrast. Quantitative aspects of the data suggest a subtractive interaction at low C values and a divisive interaction between pathways at high C values. Test contrast thresholds upon uniform fields of varying luminance show that both the dipper effect and most of the rise in ΔC with C are mediated in pattern-selective pathways rather than at a site of luminance adaptation. The pattern-polarity effects on contrast discrimination rule out the “channel uncertainty” explanation for the facilitation dipper. My results suggest that parallel ON and OFF pathways evolved because stimulus-produced decreases in the response of a single pathway are potentially confounded with the effects of contrast adaptation. Thus transient decreases in response in either pathway are not processed and both decreases and increases in contrast are expressed as response increases in separate pathways.

Attention in Visual Search: Distinguishing Four Causes of a Set-Size Effect

Visual search is a common task both in naturalistic settings and in the laboratory. Outside the labora tory, one might look for a car in a parking lot, a name in text, or a nav igation marker on the horizon. In the laboratory, search is simplified in several ways; commonly, the sub ject views a set of distinct objects and is asked to detect the presence of a particular object (the target) among a set of distractors. Two ex amples are shown in Figure 1. The top two panels illustrate the contrast increment task, in which the target is a disk of high luminance and the dis tractors are disks of lower lumi nance; the bottom two panels illus trate the line bisection task, in which the target is a rotated L and the dis tractors are rotated Ts. One of the most studied aspects of visual search is the effect on per formance of the number of objects, here referred to as the display set size. Display set sizes of 2 and 24 are illustrated in Figure 1. In the top panels, the target pops out?even for a large display set size. More pre cisely, display set size has little or no effect on search time or accuracy when the target is much brighter than the distractors. In contrast, in the bottom panels, finding the target requires scrutiny for the large set size. Display set size has a large ef feet on both search time and accu

Stimulus manipulations that reduce the square-wave illusion.

When observers view a triangle-wave luminance profile, they often report a square-wave illusion with a depth component. Alternate bars appear to be in different depth planes and the surface appears corrugated; illuminated from either the right or the left. These perspectives alternate with continuous viewing. One explanation for this illusion stems from a local energy model of feature detection proposed by Morrone and Burr (Proc. Roy. Soc. Lond. B235, 221-245, 1988). This model assumes two phase-sensitive mechanisms that process lines and edges in the visual image. It is suggested that the square-wave illusion derives from rivalry between these two mechanisms. In experiment 1, the aim was to determine whether phase alternation of the triangle wave would lead to differences in the duration and number of perceptual reversals of the illusion. The results indicate a decline in illusion duration and frequency of reversal rate with increased alternation rate. With the addition of some assumptions about the temporal resolution of the line and edge detectors, the results support the proposed explanation. In Experiment 2, the effects of high spatial-frequency contrast increments and decrements were explored. Increments did not lead to significant increases in the duration of the square-wave illusion or reversal rate, but decrements resulted in a substantial reduction in the illusion duration and reversal rate. The results indicate that manipulations which alter the phase relationships of the triangle wave decrease the illusion, but manipulations which maintain them do not.

The effects of adaptation and masking on incremental thresholds for contrast

Using a temporal two-alternative forced-choice procedure, we measured thresholds for detecting increments in contrast of a 2 c/deg vertical grating at a wide range of pedestal contrasts, (1) before and after adapting to a grating of the same orientation and spatial frequency, and (2) in the presence of superimposed masks that varied in either orientation or spatial frequency. The adapting grating and all masks were of fixed 40% contrast. The results show that prior adaptation and concurrent masking have qualitatively similar effects on incremental thresholds; both raise threshold at low pedestal contrasts and leave them unaltered at higher contrasts. But masks have greater effects than adaptors, the effect of an orthogonal mask, or one two octaves higher in spatial frequency, being about the same as a parallel adaptor of the same spatial frequency as the pedestal grating. The results are explained by a model of Ross and Speed [(1991) Proceedings of the Royal Society of London B, 246, 61–69] that assumes that masks and adaptors both reposition the transducer function of contrast sensitive mechanisms and that masks, but not adaptors, also stimulate the detecting mechanism.

Can sinusoidal vernier acuity be predicted by contrast discrimination?

A test-pedestal approach, with a test grating superimposed on a masking pedestal, was used to compare sinusoidal grating vernier acuity and contrast discrimination thresholds. The goal is to develop a simple, model for vernier acuity without assumptions about underlying mechanisms. In the contrast discrimination task, subjects were asked to detect contrast increments in the presence of a base pedestal. In the vernier task, a test grating shifted by 90 deg relative to the pedestal grating was added to one-half of the pedestal grating to produce a vernier offset. When expressed in the same contrast units and compared under optimal conditions, vernier and contrast discrimination thresholds agree well at spatial frequencies between 2 and 20c/deg and at pedestal contrasts above 10 times detection threshold. Thus, under these conditions, contrast discrimination predicts grating vernier acuity. To account for the discrepancies between vernier thresholds and contrast just noticeable difference (JND) when conditions deviate from optimal, one needs to make assumptions about the underlying mechanisms.

Visual effects of damage to P ganglion cells in macaques.

Four indices of visual performance were measured in control macaques and in macaques that had been exposed to monomeric acrylamide, a neurotoxicant that preferentially damages P retinal ganglion cells. Morphological examination of the retina and visual pathways of these monkeys showed virtually complete loss of P ganglion cells over a region extending to at least 40 deg from the fovea, and relative sparing of M ganglion cells. The four tests examined visual functions for which the visual pathway from P ganglion cells might be of great importance: visual acuity, contrast discrimination, hyperacuity, and shape discrimination. In the acrylamide-dosed monkeys, visual acuity was reduced slightly more than fourfold, a somewhat larger reduction than that seen previously after ibotenic-acid lesions of the P pathway in the geniculate. The residual acuity was in good agreement with the Nyquist frequency calculated from the density of ON or OFF M ganglion cells. Contrast increment thresholds were elevated for the dosed monkeys only in one of the two conditions tested. The elevation was found only under those spatiotemporal conditions for which we have previously shown that contrast thresholds are increased by acrylamide exposure, and was most marked at low background contrasts. Vernier acuity was elevated in one dosed monkey, but not affected in a second monkey that also had severe loss of P ganglion cells. Finally, we found no effect of acrylamide exposure on the number of training trials required to learn simple or complex shape discriminations. These results support previous findings in showing that the P pathway mediates visual acuity, and they show that several other important aspects of visual perception are not exclusively dependent on the P pathway.

Visual evoked potential responses of the anesthetized cat to contrast modulation of grating patterns

Contrast modulation affords independent control of static contrast ( C) and changes in contrast (Δ C). We found that in anesthetized, paralyzed cats, the visual evoked potential (VHP) was dependent only on ¦Δ C ¦ at each pattern transition, and was independent of the starting or ending contrast level. Increasing modulation frequency to above 2 Hz reduced the VEP monotonically, implying that the time constant for differentiation by the VEP is of the order of 250 msec. The essentially perfect a.c. coupling suppresses standing contrast completely, permitting the full dynamic range of the VEP response system to be used for detection of contrast increments (which results in a decreasing Weber fraction). The difference between our results and those of behavioral studies using contrast modulation can be explained by eye movements present in the behavioral studies which refresh the retinal image of the static contrast in a way uncorrelated to temporal modulation of the stimulus, thus introducing a masking effect.

The subjective sensitivity to small changes in the contrast of a suprathreshold grating

Small periodic (0.25–40 Hz) changes in the contrast of suprathreshold, spatially sinusoidal (0.8–10.5 c/deg) grating test objects were generated on an oscilloscope screen. It was found that the minimum temporal modulation in spatial contrast which could be detected subjectively was approximately proportional to the mean contrast of the grating during the change, provided that the minimal spatial modulation of the grating was above threshold. The contrast increment threshold was dependent on both the grating spatial frequency and the temporal frequency of the modulation: for any fixed temporal frequency in the range 2–8 Hz it increased approximately linearly with spatial frequency; for fixed spatial frequency the threshold was minimal at temporal frequencies of a few Hz. The relevance of these results to the possible role of fast fluctuations in the control of accommodation response is considered.

The pattern visual evoked cortical response in human ageing.

The N1, P1, N2 and P2 components of the pattern visual evoked response (PVER) have been recorded to the onset of presentation of a sinusoidal grating pattern of 3 and 8 cycles/deg in groups of young (15-34 years) and old (53-94 years) subjects. The negative components were taken to arise from striate cortex and the positive components from extrastriate cortex. For grating contrasts of 3-40%, the time-to-peak of the N1, P1 and N2 components, but not the P2 component, increased with age while the amplitude and rise time showed no consistent changes. Comparisons between 3 and 8 cycles/deg revealed a longer time-to-peak of N1 and P1 at 8 cycles/deg within the young group though not within the old group, i.e. an additional increment in the time-to-peak had occurred at 3 cycles/deg with ageing. This was also reflected for grating contrasts which were multiples of each individual's contrast threshold, irrespective of age. Now, the time-to-peak of N1, P1 and N2 was similar in young and old groups at 8 cycles/deg. At 3 cycles/deg, however, the time-to-peak was still longer in the old group, though the intervals between components were similar to those of the young group. This suggested the requirement for a further increment in contrast to make the time-to-peak similar, due to a selective loss of sensitivity in ageing within the low-spatial-frequency channels which are additionally sensitive to temporal modulation. Thus, it appeared that, by incrementing the stimulus contrast by the appropriate amount, the PVER of the old group could be made to resemble that of the young group. These results are consistent with the occurrence of neural changes during ageing in the retino-geniculate pathway prior to the visual cortex.

Binocular reaction times to contrast increments

Binocular and monocular reaction times were measured in response to an abrupt increment in the standing contrast of a grating. For near threshold contrast increments the advantage of binocular over monocular viewing was substantial when the standing contrast was low, but this advantage was reduced at higher standing contrasts. With high contrast increments the advantage of binocular over monocular viewing was uninfluenced by standing contrast and exceeded the level expected from probability summation.

CRYSTAL STRUCTURES AND DEFECTS IN PHASES EXISTING IN Ba3Y2Cu5O15−x

Crsytal structures and defects of the phases existing in Ba3Y2Cu5O15−x have been investigated with SAED, CBED and HREM. Sample A was furnace cooled and is a high Tc superconductor whereas sample B is air cooled and not superconducting at 77.35K. Both samples contain predominantly the so called superconducting phase with the space group Pmmm (a=0.386, b=0.389 and c=1.16 nm) and a minor phase, called phase-II with the space group Pbam (a=1.23, b=0.740 and c=0.571 nm). In sample B, the other three minor phases are: phase-III: orthorhombic, a=0.892, b=0.639 and c=0.535 nm; phase-IV: BCC, a=1.82 nm and phase-V: hexagonal, a=b=0.526, c=0.635 nm. The positions of the Ba, Y and Cu atoms can clearly be seen in the structural image viewed along the [100] direction of the superconducting phase. The contrast increment of the bright spots on the basal plane was perhaps related to the oxygen vacancies. Lattice distortion, (010) and [Formula: see text] domain boundaries in the superconducting phase, microdomains and twins in phase-III have been observed. High resolution electron micrographs of both phase-II and phase-IV were also presented. The amount of the superconducting phase and defects in the crystal structure may affect the superconductivity of the Ba3Y2Cu5O15−x oxide significantly, the former being bene-ficial and the latter harmful.