The functional benefits of tilt adaptation

Many have argued that effects of adaptation, such as aftereffects from motion or tilt, reflect that the visual system hones its responses in on the characteristics of the adapting stimulus. This view entails that on average, the discrimination of the characteristics of an adapting stimulus should become easier as viewing time increases since the variation in the response gradually adapts to the range and variation in the stimulus. Here this was tested for adaptation to tilt. Observers viewed a Gabor patch which varied in contrast from 0 to 74% at a rate of 0.6 Hz, for 4, 8, 16 or 32 s, after which the Gabor patch changed orientation (at the point when contrast was 0). The results show that the longer the observers adapt to the dynamic Gabor, the better they become at discriminating between clockwise (CW) or counterclockwise (CCW) changes in tilt (orientation) of the same patch. Experiment 2 confirms that both the direct and indirect tilt aftereffects are seen with this contrast varying Gabor patch and Experiment 3 shows that the aftereffects are only slightly smaller than in other studies with stimuli such as lines and sinusoidal gratings. These results show that adaptation to tilt leads to better discrimination around the orientation of the adapting stimulus itself, and that discrimination performance improves steadily with increased adaptation time. The results support proposals that the visual system adjusts its response characteristics to the properties of the visual input at a given time.     

Lateral interactions: size does matter

Usually a high-contrast, co-local mask increases contrast threshold (inhibition). Interestingly, a laterally displaced mask (flanker) can facilitate contrast detection (Vision Research 33 (1993) 993; 34 (1994) 73). When spatial scaling of these flanker effects was implied, stimulus bandwidth was confounded with spatial frequency (lambda(-1)). Under conditions where at lower spatial frequencies, the size (standard deviation, sigma) of the Gabor patch was smaller (sigma相似文献   

Contrast adaptation in striate cortex of macaque

We have characterized the contrast-response relationships for simple and complex cells in striate cortex of macaque monkey, before and during adaptation to high-contrast sinusoidal gratings of the optimal spatial-frequency and orientation. Adaptation brings about systematic changes in the steepness of contrast-response curves and in the effective contrast of stimuli. Adaptation reduces the detectability of low-contrast gratings by almost a factor of three, but by extending the operating range of most cells it appears to improve the discriminability of high-contrast stimuli that previously gave rise to responses of saturating amplitude.     

Some observations on contrast detection in noise

The standard psychophysical model of our early visual system consists of a linear filter stage, followed by a nonlinearity and an internal noise source. If a rectification mechanism is introduced at the output of the linear filter stage, as has been suggested on some occasions, this model actually predicts that human performance in a classical contrast detection task might benefit from the addition of weak levels of noise. Here, this prediction was tested and confirmed in two contrast detection tasks. In Experiment 1, observers had to discriminate a low-contrast Gabor pattern from a blank. In Experiment 2, observers had to discriminate two low-contrast Gabor patterns identical on all dimensions, except for orientation (-45 degrees vs. +45 degrees). In both experiments, weak-to-modest levels of 2-D, white noise were added to the stimuli. Detection thresholds vary nonmonotonically with noise power, i.e., some noise levels improve contrast detection performance. Both simple uncertainty reduction and an energy discrimination strategy can be excluded as possible explanations for this effect. We present a quantitative model consistent with the effects and discuss the implications.     

Two expressions of "surround suppression" in V1 that arise independent of cortical mechanisms of suppression

The preferred stimulus size of a V1 neuron decreases with increases in stimulus contrast. It has been supposed that stimulus contrast is the primary determinant of such spatial summation in V1 cells, though the extent to which it depends on other stimulus attributes such as orientation and spatial frequency remains untested. We investigated this by recording from single cells in V1 of anaesthetized cats and monkeys, measuring size-tuning curves for high-contrast drifting gratings of optimal spatial configuration, and comparing these curves with those obtained at lower contrast or at sub-optimal orientations or spatial frequencies. For drifting gratings of optimal spatial configuration, lower contrasts produced less surround suppression resulting in increases in preferred size. High contrast gratings of sub-optimal spatial configuration produced more surround suppression than optimal low-contrast gratings, and as much or more surround suppression than optimal high-contrast gratings. For sub-optimal spatial frequencies, preferred size was similar to that for the optimal high-contrast stimulus, whereas for sub-optimal orientations, preferred size was smaller than that for the optimal high-contrast stimulus. These results indicate that, while contrast is an important determinant of spatial summation in V1, it is not the only determinant. Simulation of these experiments on a cortical receptive field modeled as a Gabor revealed that the small preferred sizes observed for non-preferred stimuli could result simply from linear filtering by the classical receptive field. Further simulations show that surround suppression in retinal ganglion cells and LGN cells can be propagated to neurons in V1, though certain properties of the surround seen in cortex indicate that it is not solely inherited from earlier stages of processing.     

Effects on grating detection of vertically displaced peripheral gratings

Contrast threshold for a sinusoidal target in the simultaneous presence of the vertically displaced peripheral gratings was measured as a function of the peripheral contrast and the separation. When the signal and peripheral gratings were in phase, the low-contrast peripheral gratings produced the improvement in signal threshold, while the high-contrast gratings produced threshold elevation. The facilitation effect was extended to the separation equating to 10 grating cycles, whereas the inhibition effect was restricted to the region near the border. When the two gratings were out of phase, the low-contrast peripheral gratings produced threshold elevation, while the high-contrast gratings exerted little effect on signal threshold. The results were explained in terms of spatial summation and lateral inhibition.     

Contrast amplification in global texture orientation discrimination

We show that adding a low-contrast texture stimulus that is far below its own detection threshold to an ambiguously oriented high-contrast texture can produce an easily perceived global orientation. When such a low-contrast (e.g., 0.1%) test texture and a high-contrast (e.g., 2%) amplifier texture are interleaved, the effective strength for global orientation detection closely approximates the product of the two contrasts. Therefore, adding two ambiguous textures, an amplifier texture at 5x its threshold contrast for global orientation discrimination and a test texture at 1/5x its threshold contrast, produces threshold global orientation discrimination, that is, 5x amplification of the below-threshold test texture. The observed 5x amplification factors are larger than facilitation effects reported in other pattern tasks. Amplification is 11x when orientation discrimination thresholds are compared to absolute detection thresholds. For second-order textures, maximum contrast amplification is about 2.5x. A contrast gain control model is presented that accounts for 90% of the variance in observed d' for texture patterns of differing geometries, exposure durations, and component contrasts. In the model, very low-contrast orientations are represented by power functions of their contrasts, with an exponent greater than two. As the contrast of an amplifier texture increases beyond about 4%, feed-forward gain control exerted by the amplifier ultimately nullifies the amplification effect and produces masking.     

Low-contrast visual acuity test for pediatric use

Single-letter flash cards were made at contrast values of 96%, 7% and 4% to compare high-, intermediate- and low-contrast visual performance in amblyopic children young enough to benefit from occlusion therapy. The single-letter format was intended to differentiate the effect of contrast on reading performance from any effects of nearby contours. Two groups of patients were tested: 37 children aged 3 to 8 years who had completed occlusion therapy and 15 children aged 4 to 8 years who were still receiving occlusion therapy. Their results were compared with the results of 45 control subjects aged 3 to 8 years. Three patterns of visual loss were identified in the patients: predominantly for high-contrast acuity, fairly uniform at high-, intermediate- and low-contrast levels, and, in two patients, loss at low- and intermediate-contrast levels with relative sparing at the high-contrast level. The incidence of high-contrast acuity loss was not significantly different between the two groups, but the proportion of children with no acuity loss at any contrast level was 54% in the group that had completed treatment, compared with 17% of those who were still receiving treatment.     

Visual performance after photorefractive keratectomy with a 6-mm ablation zone.

PURPOSE: To prospectively examine the effect of photorefractive keratectomy with a 6-mm ablation zone on best-spectacle-corrected visual performance. METHODS: A prospective study was conducted of 164 eyes of 164 patients with an average (+/-SD) of -4.02 +/- 1.74 diopters (range, -0.63 to -8.38 diopters spherical equivalent). Best-spectacle-corrected high-contrast and low-contrast visual acuity (18% Weber contrast) was measured with both natural and dilated pupils. Patients were tested preoperatively and at 3, 6, and 12 months after photorefractive keratectomy. Photorefractive keratectomy was performed with an argon fluoride excimer laser. Fifty-five eyes of 55 patients also underwent astigmatic keratotomy. RESULTS: Twelve months after photorefractive keratectomy, best-spectacle-corrected high-contrast visual acuity with natural pupils showed no significant change from preoperative values; mean (+/-SD) change was 0.004 +/- 0.10 logMAR (t = 0.45, P = .65). Best-spectacle-corrected low-contrast visual acuity with natural pupils was significantly reduced compared to baseline; mean (+/-SD) change was 0.04 +/- 0.13 logMAR (t = 3.3, P = .001). The low-contrast loss was larger (1.5 lines) with dilated pupils; mean (+/-SD) change was 0.13 +/- 0.15 logMAR (t = 9.31, P < .001). Greater losses in dilated low-contrast visual acuity were associated with concurrent astigmatic ketatotomy (t = 2.28, P = .025) and corneal haze of grade 1 or greater (t = 2.71, P = .005). CONCLUSIONS: Reductions in visual performance occur after photorefractive keratectomy with a 6-mm zone. These changes are greatest for low-contrast visual acuity with dilated pupils. Corneal haze and concurrent astigmatic keratotomy are associated with greater losses in low-contrast visual acuity. Best-spectacle-corrected low-contrast visual acuity is a sensitive measure for evaluating visual performance after refractive surgery.     

Flanker effects in peripheral contrast discrimination--psychophysics and modeling.

We studied lateral interactions in the periphery by measuring how contrast discrimination of a peripheral Gabor patch is affected by flankers. In the psychophysical experiments, two Gabor targets appeared simultaneously to the left and right of fixation (4 degrees eccentricity). Observers reported which contrast was higher (spatial 2-alternative-forced-choice). In different conditions, Gabor flankers of different orientation, phase, and contrast were present above and below the two targets, at a distance of three times the spatial Gabor period. The data show that collinear flanks impair discrimination performance for low pedestal contrasts but have no effect for high pedestal contrasts. The transition between these two result patterns occurs typically at a pedestal contrast which is similar to the flanker contrast. For orthogonal flanks, we find facilitation at low pedestal contrasts, and suppression at intermediate contrasts. We account for this complex interaction pattern by a model that assumes that flankers can provide additive input to the target unit, and that they further contribute to the target's gain control, but only in a limited range of pedestal contrasts; once the target contrast exceeds a critical value, inhibition becomes subtractive rather than divisive. We further make specific propositions on how this model could be implemented at the neuronal level and show that a simple integrate and fire unit that receives time-modulated inhibition behaves in a fashion strikingly similar to the model inferred from the psychophysical data.     

Facilitation of contrast detection in near-peripheral vision

Foveal detection of a Gabor patch (target) is facilitated by collinear, displaced high-contrast flankers. Polat and Sagi reported that the same phenomenon occurred in the periphery, but no data were presented [Proc. Natl. Acad. Sci. 91 (1994) 1206]. Others have found no facilitation in a limited number of conditions tested. To resolve this apparent conflict, we measured lateral facilitation in the near-periphery using a range of stimulus parameters. We found facilitation for a range of target-flanker distances for peripheral eccentricities up to 6 degrees , but the magnitude of the effect was less than found in central vision. Facilitation varied across subjects and with spatial frequency. Flanker contrast had no effect over the range evaluated (10-80%). Equal facilitation was found for two global arrangements of the stimulus pattern. Facilitation was found using a temporal, but not a spatial two-alternative forced-choice paradigm, accounting for the different results among previous studies. This finding supports previous indications of the role of attention in altering such facilitation. The value of facilitation from lateral interactions for persons with central vision impairment, who have to shift their attention to a peripheral locus constantly, needs to be examined.     

The effect of posterior capsule opacification on visual function

PURPOSE: To investigate how posterior capsule opacification (PCO) affects visual function in pseudophakic eyes. METHODS: One hundred and six eyes that had undergone uncomplicated phacoemulsification were recruited sequentially. Patients with surgical complications or other ocular disease were excluded. PCO was assessed by a digital retroillumination camera using a software program based on the analysis of texture in the image, and the percentage area within the central 3-mm zone of the posterior capsule was calculated. Visual function assessment included Early Treatment Diabetic Retinopathy Study (ETDRS) high- and low-contrast visual acuity, contrast sensitivity with the Pelli-Robson and CSV-1000 grating charts, and forward light-scatter by the direct-compensation method (van den Berg). RESULTS: The percentage PCO required for decline in high-contrast ETDRS was 78%; for low-contrast acuity and Pelli-Robson, 46%; for CSV-1000 contrast sensitivity, 38% to 51%; and for forward light-scatter, less than 1% PCO. CONCLUSIONS: Central PCO affects psychophysical test results with differing degrees of sensitivity. Forward light-scatter is the most sensitive, followed by contrast sensitivity and visual acuity.     

Is contrast just another feature for visual selective attention?

The biased-competition theory of attention [Annual Review of Neuroscience 18 (1995) 193] suggests that attention and stimulus contrast trade off, and implies that high-contrast stimuli should be easy to attend to and hard to ignore. To test this, observers searched displays for a target digit. Observers were well able to exclude high-contrast distractors when attempting to search only among low-contrast stimuli (Experiment 1). In Experiments 2 and 3, location determined which stimuli were relevant. When contrast of relevant and irrelevant stimuli was uncertain (due to contrast varying between trials, Experiment 2), increasing the contrast of distractors impaired performance. However, when contrast was certain (due to blocking of trials, Experiment 3) and targets were of low contrast, high contrast distractors produced less interference than low contrast distractors. The ability of subjects to attend selectively to low vs. high contrast items in Experiments 1 and 3 suggests that selectivity for stimulus contrast might be similar to other types of feature selectivity (e.g., color and location). Such findings are inconsistent with the biased competition theory regarding the interplay of contrast and attention. However, results from Experiment 2 suggest that, when target contrast varies, the default tendency is to attend to high-contrast items.     

Contrast integration across space.

Contrast integration across space was studied in respect to stimulus extent and the spatial layout, using high-contrast stimuli. Contrast discrimination thresholds were measured (2AFC) by either increasing the size of a peripheral (2.4 degrees) Gabor signal (GS: lambda = 0.08 degree) or by increasing the number of GS elements in a circular arrangement. The supra-threshold mask (pedestal) was either increased with the target or fixed at maximal size and had 30% contrast. For stimuli with an increasing size of both the pedestal and the increment target, we find approximately constant discrimination thresholds. Contrast discrimination improved linearly on a log-log scale with slopes average of -1/4 (fourth-root summation) when the size of the Gabor target was increased but the mask was kept at maximal size, indicating contrast integration across space. Taken together, these results indicate balanced spatial integration of both contrast increment and pedestal, resulting object-size invariant contrast discrimination. Contrast discrimination was found to improve as well when the number of aligned Gabor elements was increased (both pedestal and increment), pointing to independent contrast normalization for disconnected (sparsely positioned) stimuli. The results indicate a complex pattern of spatial integration involved in contrast discrimination, possibly depending on image segmentation.     

Hyperchromatic lenses as potential aids for the presbyope

It has been suggested that a 'hyperchromatic' lens which enhanced the longitudinal chromatic aberration (LCA) of the eye would increase its depth of focus (DOF) and hence might prove a useful aid to presbyopes who have lost their natural ability to accommodate for different distances. Experiments are described in which high- and low-contrast visual acuity, and contrast sensitivity were measured as a function of object vergence using the naked eye and various levels of effective lens-eye LCA. It was found that doubling the effective lens-eye LCA increased total DOF in the high-contrast acuity task by about 0.5 D, while correcting the ocular LCA reduced DOF by about 0.3 D. On the other hand, enhanced LCA led to some loss in performance in the low-contrast acuity task and in contrast sensitivity.     

Visual evoked potentials and magnocellular and parvocellular segregation

We have measured visual evoked potentials (VEPs) to luminance-modulated, square-wave alternating, 3-deg homogeneous disks for stimulus frequencies ranging from 1 Hz to 16.7 Hz. The aim of the study was to determine the range of frequencies at which we could reproduce the two-branched contrast-response (C-R) curves we had seen at 1 Hz (Valberg & Rudvin, 1997) and which we interpreted as magnocellular (MC) and parvocellular (PC) segregation. Low-contrast stimuli elicited relatively simple responses to luminance increments resulting in waveforms that may be the signatures of inputs from magnocellular channels to the visual cortex. At all frequencies, the C-R curves of the main waveforms were characterized by a steep slope at low contrasts and a leveling off at 10%-20% Michelson contrast. This was typically followed by an abrupt increase in slope at higher contrasts, giving a distinctive two-branched C-R curve. On the assumption that the low-contrast, high-gain branch reflects the responsivity of magnocellular-pathway inputs to the cortex, the high-contrast branch may be attributed to additional parvocellular activation. While a two-branched curve was maintained for frequencies up to 8 Hz, the high-contrast response was significantly compromised at 16.7 Hz, revealing a differential low-pass filtering. A model decomposing the measured VEP response into two separate C-R curves yielded a difference in sensitivity of the putative MC- and PC-mediated response that, when plotted as a function of frequency, followed a trend similar to that found for single cells. Due to temporal overlap of responses, the MC and PC contributions to the waveforms were hard to distinguish in the transient VEP. However, curves of time-to-peak (delay) as a function of contrast often went through a minimum before the high-contrast gain increase of the corresponding C-R curve, supporting the notion of a recruitment of new cell ensembles in the transition from low to high contrasts.     

Visual acuity in contact lens wearers.

PURPOSE: The difference between high- and low-contrast visual acuity provides a sensitive indicator of vision loss in ocular disease; however, the effect of refractive error correction on this difference is still debated. METHODS: High- and low-contrast visual acuity was measured in 116 rigid gas permeable contact lens wearers, 51 spectacle wearers, and 50 soft contact lens wearers with habitual and best correction. Twenty-nine of the soft contact lens wearers reported that they wore disposable contact lenses (discarded on a monthly or more frequent basis), whereas the other 21 soft contact lens wearers wore traditional soft contact lenses. RESULTS: Rigid gas permeable contact lens wearers had statistically worse high-contrast habitual visual acuity than spectacle wearers (Tukey-Kramer, p = 0.0075). Traditional soft contact lens wearers had significantly worse low-contrast visual acuity compared with all other groups (Tukey-Kramer, p < 0.02 for each comparison). Traditional soft contact lens wearers had a significantly larger difference between high- and low-contrast visual acuity with best correction compared with rigid gas permeable wearers (Tukey-Kramer, p = 0.0099). CONCLUSIONS: Rigid gas permeable contact lens wearers had statistically worse habitual high-contrast visual acuity compared with spectacle wearers, but no difference was present under best-corrected conditions. We hypothesize that rigid gas permeable contact lens wearers were not wearing their optimal correction habitually. Traditional soft contact lens wearers had significantly worse low-contrast visual acuity. They also had a larger difference between their best-corrected high- and low-contrast visual acuity scores compared with rigid gas permeable contact lens wearers.     

Measurement of glare sensitivity in cataract patients using low-contrast letter charts*

The Committee on Ophthalmic Procedures Assessment of the American Academy of Ophthalmology suggested using low-contrast visual acuity measured before and after adding a glare source as a test for assessing overall visual disability from immature cataracts. We have developed a test that follows the Committee's three principles of design, and we report that the effect of glare on visual acuity is considerably greater for recognizing low-contrast letters than for recognizing high-contrast letters. The effect of glare on visual acuity increases progressively as letter contrast is reduced in the stages 96%, 50%, 25%, 11% and 4%, The 25% chart (and possibly the 11% chart) gives the most suitable sensitivity for eyes with immature cataracts. Age-related brunescence and aging itself do not necessarily produce high sensitivity to glare. Sensitivity to glare was markedly different in eyes with different kinds of cataract.     

Is binocular contrast sensitivity at distance compromised with multifocal soft contact lenses used to correct presbyopia?

PURPOSE: The purpose of this investigation was to determine whether new experimental multifocal optical designs incorporating diffractive/refractive optics for correction of presbyopia in a soft contact lens would compromise binocular contrast sensitivity at distance while achieving 20/20 binocular visual acuity at near. METHODS: Thirty presbyopic volunteers were fitted with Acuvue Bifocal soft lenses, two (FO1 and 3B1) experimental diffractive/refractive multifocal soft lenses separately and in combination as a pair and soft spherical lenses. Visual performance was evaluated via binocular contrast sensitivity and binocular distance high- and low-contrast visual acuity after 1 week of lens wear. RESULTS: The experimental lenses FO1 and the combination FO1/3B1 performed as well for binocular contrast sensitivity, binocular distance high-contrast visual acuity, and binocular distance low-contrast visual acuity as the habitual presbyopic correction and the spherical soft distance correction. A strong correlation (r = 0.73 and 0.53, respectively) was found between binocular contrast sensitivity and binocular distance low-contrast visual acuity with experimental FO1 and FO1/3B1 lenses. However, the correlation (r = 0.37 and 0.60, respectively) between binocular contrast sensitivity and binocular distance high- and low-contrast visual acuity with FO1 was weaker than that with the combination FO1/3B1 lenses. Subjective responses support the objective data. DISCUSSION: The data show that experimental FO1 lens and the experimental combination of the FO1/3B1 lenses can be prescribed to not compromise distance binocular contrast sensitivity.     

Attentional effects on contrast detection in the presence of surround masks

We studied how attention affects contrast detection performance when the target is surrounded by mask elements. In each display quadrant we presented a hexagon of six vertical Gabor patches (the 'surround'). Only one of the hexagons contained a central Gabor patch (the 'target') and the task was to report that quadrant (spatial four-alternative-forced choice). Attention was manipulated by means of a double-task paradigm: in one condition observers had to perform concurrently a central letter-discrimination task, and the contrast-detection task was then only poorly attended, while attention was fully available in the other condition. We find that under poorly attended conditions targets can be detected only when the target contrast exceeds the surround contrast (contrast popout) or when the target orientation differs from the surround orientation by more than 10-15 degrees (orientation popout). When the target orientation is similar to the surround orientation, attention can reduce the contrast detection thresholds in some cases more than four-fold, demonstrating a very strong attentional effect.