SEARCH · PubMed Health

Results for “Disparities”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 127 records · Page 7Linked to original sources

PubMed2001

Spatial interactions minimize relative disparity between adjacent surfaces.

Computational models of stereopsis employ a number of algorithms that constrain stereo matches to produce the smallest absolute disparity and to minimize the relative disparity between nearby features. In some natural scenes, such as large slanted textured surfaces, these two constraints lead to different matching solutions. The current study utilized a stimulus in which there was a large discrepancy in both the magnitude and direction of matches that solved for minimum absolute and minimum relative disparity. This discrepancy revealed a dominance for the minimum relative disparity over the minimum absolute disparity matching solution that increased with spatial proximity, spatial frequency and width of adjacent features. The likelihood of a minimum-relative-disparity matching solution also increased when the difference between the amplitudes of the alternative relative disparities was large. When alternative relative disparity matching solutions had similar amplitudes but opposite signs (crossed vs. uncrossed), an idiosyncratic depth bias served as a tie-breaker. The present results show that absolute disparity matches are constrained to minimize relative disparity between adjacent features.

Depth Perception↗

PubMedDate not supplied

Depth perception and evoked brain activity: the influence of horizontal disparity and visual field location.

The perception of dynamic random-dot stereograms (RDS) depends on the physiological fusion of horizontally disparate binocular visual input. Thus, the use of RDS offers the possibility to study selectively cortical processing of visual information in man. We investigated the influence of horizontal disparity on the scalp topography of RDS evoked brain activity in 33 healthy subjects. Stereoscopic checkerboard patterns were presented in the center or lateralized in the left or right visual field with horizontal disparities changing at temporal frequencies of six or eight depth reversals/s using different disparity values ranging from 3.5 to 28 min of arc. In 11 subjects evoked potential fields were recorded from 16 electrodes, and 21 subjects participated in 30-channel recordings with electrodes located over the parietal and occipital brain areas. Stimulation frequency-related brain activity was obtained with all disparity values; however, with large or small disparities the potential field strength decreased significantly while largest responses were obtained with intermediate disparities. Significant differences were observed in RDS evoked brain activity when central and lateralized stimulus locations were compared. With lateral stimuli (extending from the fovea to 17.1-deg eccentricity) maximal amplitudes were obtained at larger disparities than with central stimuli. In addition there were pronounced differences between brain activity evoked with stimuli presented in the left or right visual field; however, there were very similar evoked potential signals recorded from electrodes located over the left and right hemispheres. Our findings indicate that the processing of disparity information with lateralized stimuli is different from the processing in the center of the visual field. In addition, lateralized stimulation yields a significant disparity tuning mainly with stereoscopic targets occurring to the right from the fixation point (but not with stimuli to the left) suggesting a functional difference between the visual half-fields.

Adult↗

PubMed1997

Spatial scale interactions in stereo sensitivity and the neural representation of binocular disparity.

How are binocular disparities encoded and represented in the human visual system? An 'encoding cube' diagram is introduced to visualise differences between competing models. To distinguish the models experimentally, the depth-increment-detection function (discriminating disparity d from d +/- delta d) was measured as a function of standing disparity (d) with spatially filtered random-dot stereograms of different centre spatial frequencies. Stereothresholds degraded more quickly as standing disparity was increased with stimuli defined by high rather than low centre spatial frequency. This is consistent with a close correlation between the spatial scale of detection mechanisms and the disparities they process. It is shown that a simple model, where discrimination is limited by the noisy ratio of outputs of three disparity-selective mechanisms at each spatial scale, can account for the data. It is not necessary to invoke a population code for disparity to model the depth-increment-detection function. This type of encoding scheme implies insensitivity to large interocular phase differences. Might the system have developed a strategy to disambiguate or shift the matches made at fine scales with those made at the coarse scales at large standing disparities? In agreement with Rohaly and Wilson, no evidence was found that this is so. Such a scheme would predict that stereothresholds determined with targets composed of compounds of high and low frequency should be superior to those of either component alone. Although a small stereoacuity benefit was found at small disparities, the more striking result was that stereothresholds for compound-frequency targets were actually degraded at large standing disparities. The results argue against neural shifting of the matching range of fine scales by coarse-scale matches posited by certain stereo models.

Computer Graphics↗

PubMed2000

Effects of nonius line and fusion lock parameters on fixation disparity.

PURPOSE: Clinical devices for fixation disparity vary in the size and position of the nonius test-lines and in the location of the binocular fusion lock, and published results of studies comparing fixation disparity devices also vary. This study examined how varying stimulus parameters affects the magnitude of horizontal fixation disparity and the precision of nonius alignment in normal observers. METHODS: Targets were bright vertical lines presented for 150 ms on an oscilloscope and viewed in the dark from 3.95 m through a mirror haploscope. The fusion stimulus was either central, peripheral, or central-plus-peripheral. Fixation disparity was measured for nonius lines with a fixed vertical separation and variable lengths, using the method of constant stimuli. In a second experiment, fixation disparity was assessed for vergence demands of 6delta base-in to 12delta base-out using nonius lines of fixed length and variable vertical separations. For comparison, binocular Vernier thresholds and constant errors were also assessed. RESULTS: Mean values of fixation disparity are invariant with the length and separation of the nonius lines and proximity to the fusion lock, over the range of values tested. When measured with a central fusion lock, the precision of binocular nonius alignment (vergence variability) worsens if the separation between the nonius lines increases beyond approximately 20 min arc. Vergence variability is larger with a peripheral fusion lock for small nonius-line separations and approximately the same as with the central lock for large line separations. Fixation disparity and precision were smallest with a central-plus-peripheral fusion-lock combination. Similar effects of line length and separation were found for binocular Vernier. CONCLUSIONS: The finding of invariance of fixation disparity with line length for nonius targets < 1degrees extends those of earlier findings for larger targets. Because the precision of binocular nonius alignment is poorer for large nonius-line separations or with a peripheral fusion lock, fixation disparity should be most repeatable when measured with a central fusion lock and nonius lines separated by 20 min arc or less. However, a peripheral fusion lock and small nonius-line separation may provide the most effective diagnostic combination for fixation disparity.

Convergence, Ocular↗

PubMed1999

Size-disparity correlation in human binocular depth perception.

To use the small horizontal disparities between images projected to the eyes for the recovery of three-dimensional information, our visual system must first identify which feature in one eye's image corresponds with which in the other. The earliest level of disparity processing in primates (V1) contains cells that are spatial-frequency tuned. If such cells have a disparity range that covers only a single period of their mean tuning frequency, there will always be exactly one potential match within this range. Here, this 'size-disparity' hypothesis was tested by measuring the contrast sensitivity of stereopsis as a function of disparity for single bandpass-filtered items. It was found that thresholds were low and relatively constant up to disparities an order of magnitude larger than is predicted by this constraint. Furthermore, peak sensitivity was relatively independent of spatial frequency. A control experiment showed that binocular correlation of the carrier is necessary for this task. In a third experiment, the maximum disparity that supports threshold performance was compared for an isolated bandpass item and bandpass-filtered noise. This limit was found to be five times larger for the isolated stimuli. In summary, these findings show that the initial stage of disparity detection is not limited by the size-disparity constraint. For stimuli with multiple false targets, however, processes subsequent to this stage reduce the disparity range over which the correspondence problem can be solved.

Depth Perception↗

PubMed2005

Disparity-tuning characteristics of neuronal responses to dynamic random-dot stereograms in macaque visual area V4.

Stereo processing begins in the striate cortex and involves several extrastriate visual areas. We quantitatively analyzed the disparity-tuning characteristics of neurons in area V4 of awake, fixating monkeys. Approximately half of the analyzed V4 cells were tuned for horizontal binocular disparities embedded in dynamic random-dot stereograms (RDSs). Their response preferences were strongly biased for crossed disparities. To characterize the disparity-tuning profile, we fitted a Gabor function to the disparity-tuning data. The distribution of V4 cells showed a single dense cluster in a joint parameter space of the center and the phase parameters of the fitted Gabor function; most V4 neurons were maximally sensitive to fine stereoscopic depth increments near zero disparity. Comparing single-cell responses with background multiunit responses at the same sites showed that disparity-sensitive cells were clustered within V4 and that nearby cells possessed similar preferred disparities. Consistent with a recent report by Hegdé and Van Essen, the disparity tuning for an RDS drastically differed from that for a solid-figure stereogram (SFS). Disparity-tuning curves were generally broader for SFSs than for RDSs, and there was no correlation between the fitted Gabor functions' amplitudes, widths, or peaks for the two types of stereograms. The differences were partially attributable to shifts in the monocular images of an SFS. Our results suggest that the representation of stereoscopic depth in V4 is suited for detecting fine structural features protruding from a background. The representation is not generic and differs when the stimulus is broad-band noise or a solid figure.

Action Potentials↗

PubMed2003

Ocular dominance predicts neither strength nor class of disparity selectivity with random-dot stimuli in primate V1.

We address two unresolved issues concerning the coding of binocular disparity in primary visual cortex. Experimental studies and theoretical models have suggested a relationship between a cell's ocular dominance, assessed with monocular stimuli, and its tuning to binocular disparity. First, the disparity energy model of disparity selectivity suggests that there should be a correlation between ocular dominance and the strength of disparity tuning. Second, several studies have reported a relationship between ocular dominance and the shape of the disparity tuning curve, with cells dominated by one eye more likely to have disparity tuning of the tuned-inhibitory type. We investigated both of these relationships in single neurons recorded from the primary visual cortex of awake fixating macaques, using dynamic random-dot patterns as a stimulus. To classify disparity tuning curves quantitatively, we develop a new measure of symmetry, which can be applied to any function. We find no evidence for any correlation between ocular dominance and the nature of disparity tuning. This places constraints on the circuitry underlying disparity tuning.

Algorithms↗

PubMed2004

Perceptual "read-out" of conjoined direction and disparity maps in extrastriate area MT.

Cortical neurons are frequently tuned to several stimulus dimensions, and many cortical areas contain intercalated maps of multiple variables. Relatively little is known about how information is "read out" of these multidimensional maps. For example, how does an organism extract information relevant to the task at hand from neurons that are also tuned to other, irrelevant stimulus dimensions? We addressed this question by employing microstimulation techniques to examine the contribution of disparity-tuned neurons in the middle temporal (MT) visual area to performance on a direction discrimination task. Most MT neurons are tuned to both binocular disparity and the direction of stimulus motion, and MT contains topographic maps of both parameters. We assessed the effect of microstimulation on direction judgments after first characterizing the disparity tuning of each stimulation site. Although the disparity of the stimulus was irrelevant to the required task, we found that microstimulation effects were strongly modulated by the disparity tuning of the stimulated neurons. For two of three monkeys, microstimulation of nondisparity-selective sites produced large biases in direction judgments, whereas stimulation of disparity-selective sites had little or no effect. The binocular disparity was optimized for each stimulation site, and our result could not be explained by variations in direction tuning, response strength, or any other tuning property that we examined. When microstimulation of a disparity-tuned site did affect direction judgments, the effects tended to be stronger at the preferred disparity of a stimulation site than at the nonpreferred disparity, indicating that monkeys can selectively monitor direction columns that are best tuned to an appropriate conjunction of parameters. We conclude that the contribution of neurons to behavior can depend strongly upon tuning to stimulus dimensions that appear to be irrelevant to the current task, and we suggest that these findings are best explained in terms of the strategy used by animals to perform the task.

Animals↗

PubMed2004

Receptive field size in V1 neurons limits acuity for perceiving disparity modulation.

Disparity selectivity in the striate cortex has generally been studied with uniform disparity fields covering the receptive field (RF). In four awake behaving monkeys, we quantitatively characterized the spatial three-dimensional structure of 55 V1 RFs using random dot stereograms in which disparity varied as a sinusoidal function of vertical position ("corrugations"). At low spatial frequencies, this produced a modulation in neuronal firing at the temporal frequency of the stimulus. As the spatial frequency increased, the modulation reduced. The mean response rate changed little and was close to that produced by a uniform stimulus at the mean disparity of the corrugation. In 48 of 55 (91%) neurons, the modulation strength was a lowpass function of spatial frequency. These results are compatible with a response determined only by the weighted mean of the disparities of the dots (the weights being set by the RF envelope) and suggest that there is no disparity-based surround inhibition or selectivity for disparity gradients. This simple weighting scheme predicts a relationship between RF size and the high-frequency cutoff. Comparison with independent measurements of RF size was compatible with this. All of this behavior closely matches the binocular energy model. The mean cutoff frequency, 0.5 cycles per degree, is similar to equivalent measures of decline in human psychophysical sensitivity for such depth corrugations as a function of frequency (Tyler, 1974; Prince and Rogers, 1998; Banks et al., 2004). This suggests that human cyclopean acuity for disparity modulations is limited by the selectivity of V1 neurons. This in turn is primarily limited by the RF size, because we find no sensitivity for disparity gradients or other disparity differences within the RFs.

Action Potentials↗

PubMed2001

Hierarchical processing of horizontal disparity information in the visual forebrain of behaving owls.

According to their restricted receptive fields and input-filter characteristics, disparity-sensitive neurons at early processing levels of the visual system perform rather ambiguous computations; they respond vigorously to disparity in false-matched images and show multiple response peaks in their disparity-tuning profiles. On the other hand, the perception of depth from binocular disparity is reliable, thus raising the question as to where and how in the brain additional processing is accomplished leading toward behaviorally relevant disparity detection. To address this issue, tuning data during stimulation with correlated and anticorrelated random-dot stereograms (a-RDS) were obtained from 52 disparity-sensitive visual Wulst neurons in three behaving owls. From the disparity-tuning curves, several quantitative measures were derived that allowed to determine the response ambiguity of a cell. A systematic decline of response ambiguities with increasing response latencies was observed. An increase in response latencies of neurons was correlated with a decrease of the strength of responses to a-RDS. Declining responses to a-RDS are expected for global detectors, because an owl was not able to discriminate depth in psychophysical tests with a-RDS. In addition, suppression of response side peaks was increased and disparity tuning was enhanced with growing response latencies. These results suggest a functional hierarchy of disparity processing in the owl's forebrain, leading from spatial filters to more global disparity detectors that may be able to solve the correspondence problem. Nonlinear threshold operations and inhibition are proposed as candidate mechanisms to resolve coding ambiguities.

Action Potentials↗

PubMed2001

Racial disparity in influenza vaccination: does managed care narrow the gap between African Americans and whites?

CONTEXT: Substantial racial disparities exist in use of some health services. Whether managed care could reduce racial disparities in the use of preventive services is not known. OBJECTIVE: To determine whether the magnitude of racial disparity in influenza vaccination is smaller among managed care enrollees than among those with fee-for-service insurance. DESIGN, SETTING, AND PARTICIPANTS: The 1996 Medicare Current Beneficiary Survey of a US cohort of 13 674 African American and white Medicare beneficiaries with managed care and fee-for-service insurance. MAIN OUTCOME MEASURES: Percentage of respondents (adjusted for sociodemographic characteristics, clinical comorbid conditions, and care-seeking attitudes) who received influenza vaccination and magnitude of racial disparity in influenza vaccination, compared among those with managed care and fee-for-service insurance. RESULTS: Eight percent of the beneficiaries were African American and 11% were enrolled in managed care. Overall, 65.8% received influenza vaccination. Whites were substantially more likely to be vaccinated than African Americans (67.7% vs 46.1%; absolute disparity, 21.6%; 95% confidence interval [CI], 18.2%-25.0%). Managed care enrollees were more likely than those with fee-for-service insurance to receive influenza vaccination (71.2% vs 65.4%; difference, 5.8%; 95% CI, 3.6%-8.3%). The adjusted racial disparity in fee-for-service was 24.9% (95% CI, 19.6%-30.1%) and in managed care was 18.6% (95% CI, 9.8%-27.4%). These adjusted racial disparities were both statistically significant, but the absolute percentage point difference in racial disparity between the 2 insurance groups (6.3%; 95% CI, -4.6% to 17.2%) was not. CONCLUSION: Managed care is associated with higher rates of influenza vaccination for both whites and African Americans, but racial disparity in vaccination is not reduced in managed care. Our results suggest that additional efforts are needed to adequately address this disparity.

Black or African American↗

PubMed1993

Binocular interaction and disparity coding in area 19 of visual cortex in normal and split-chiasm cats.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the principal cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: (1) the ganglion cells in the homonymous temporal and nasal hemiretinae and (2) the contralateral hemisphere via the corpus callosum (CC). The objectives of this study were, on one hand, to determine whether disparity-sensitive cells are present in a "higher order" area, namely area 19 of the visual cortex, of the cat and, on the other hand, to ascertain whether the CC contributes to the formation of these cells. As in areas 17-18, two types of disparity-sensitive neurons were found: one type, showing maximal interactive effects around zero disparity, responded with strong excitation or inhibition when the stimuli presented independently to the two eyes were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These are presumed to signal stimuli situated in front of or behind the fixation plane. Unlike areas 17-18, however, disparity-sensitive cells in area 19 of the normal cat were less finely tuned and their proportion was lower. In the split-chiasm animal, very few cells were sensitive to disparity. These results, when coupled with behavioral data obtained with destriate animals, indicate that (1) area 19 is probably less involved in the analysis of disparity information than area 17, (2) the disparity-sensitive neurons that are sensitive to disparity are not involved in the resolution of very fine three-dimensional spatial detail, and (3) the CC only determines a limited number of these cells in the absence of normal binocular input.

Animals↗

PubMed2005

Racial/ethnic disparities in injection drug use in large US metropolitan areas.

PURPOSE: Because blacks and Latinos bear a disproportionate burden of injection-related health problems compared with whites, we sought to describe black/white and Latino/white disparities in injecting drugs in 94 US metropolitan statistical areas (MSAs) in 1998. METHODS: Using US Census data and three databases documenting injectors' use of different healthcare services (drug treatment, HIV counseling and testing, and AIDS diagnoses), we calculated database-specific black/white and Latino/white disparities in injecting in each MSA and created an index of black/white and Latino/white disparities by averaging data across the three databases. RESULTS: The median black/white injecting disparity in the MSAs ranged from 1.4 to 3.7 across the three databases; corresponding median Latino/white injecting disparities ranged from 1.0 to 1.1. Median black/white and Latino/white index disparity values were 2.6 and 1.0, respectively. CONCLUSIONS: Although whites were the majority of injectors in most MSAs, database-specific and index black/white disparity scores indicate that blacks were more likely to inject than whites. While database-specific and index disparity scores indicate that Latinos and whites had similar injecting rates, they also revealed considerable variation in disparities across MSAs. Future research should investigate these disparities' causes, including racial/ethnic inequality and discrimination, and study their contributions to the disproportionate burden of injection-related health problems borne by blacks and Latinos.

Adult↗

PubMed1999

HLA-C disparity between patients and unrelated donors matched for HLA-A, -B, and -DRB1 alleles: impact of serological vs. DNA typing for HLA-A and -B loci.

High incidences of graft failure, graft-vs.-host disease (GVHD), and serious infections following unrelated donor (URD) marrow transplantation, despite apparent human leukocyte antigen (HLA) identity, may reflect the presence of molecular disparities, including those for HLA-C alleles between the patient and the URD. The level of these disparities could be significant, because as many as 42 alleles are currently known for HLA-C locus. We studied 84 patients and 251 potential URDs to evaluate 1) the extent of HLA-C disparity between the patient and the URD identified by serology for HLA-A and -B and by DNA typing for -DRB1 and 2) the level of HLA-C disparity between patients and URDs matched by high-resolution DNA typing for HLA-A, -B, and -DRB1. The DNA typing was performed at the Memorial Sloan Kettering Cancer Center, and the serotyping was provided by the registries. Of 251 URDs matched by HLA-A and -B serology and -DRB1 (sA_sB_dnaDRB1 ); 94, 75, and 82 were 6/6, 5/6, and 4/6 matches, respectively. Of 94 sA_sB_dnaDRB1 6/6 URDs, 51 (54.3%) were matched for both HLA-C alleles. In contrast, 31 (41.3%) 5/6 (p=0.12) and 15 (18.3%) 4/6 (p < 0.01) sA_sB_dnaDRB1 URDs were matched for both HLA-C alleles. Following DNA typing for HLA-A and -B, 52 (55.3%) of 94 6/6, 30 (40%) of 75 5/6, and 25 (30.5%) of 82 4/6 sA_sB_dnaDRB1 URDs remained 6/6, 5/6, and 4/6 matches at the DNA level (dnaA_B_DRB1). HLA-C disparities continued to exist in the dnaA_B_DRB1 URD group. Of 54 dnaA_B_DRB1 6/6 URDs, 41 (75.9%) were matched for both HLA-C alleles. Only 45.3% of the 5/6 (p=0.01) and 22.2% of the 4/6 (p < 0.01) dnaA_B_DRB1 URDs were matched for both HLA-C alleles. In the 6/6 category, the frequency of HLA-C matching improved (75.9 vs. 54.3%; p=0.01) following DNA matching for HLA-A and -B. In comparison to mismatching for HLA-B locus, mismatching for either HLA-DRB1 or -A resulted in a lower odds ratio for HLA-C disparity. The presence of a common haplotype in the sA_sB_dnaDRBl (p=0.06) URD category improved the level of HLA-C matching. We identified alleles that are associated with high (B*1501, B*4402, B*5101, DRB1*0101, A*0201, A*1101, A*2301, and A*3201) or low (B*0702, B*0801, B*1302, B*3502, DRB1*0301, DRB1*1104, A*0101, A*3001, and A*6801) probability of HLA-C disparity. Overall, sA_sB_dnaDRB1 as well as dnaA_B_DRB1 matched URDs for non-Caucasian patients were more likely to have HLA-C disparity in comparison to the matched URDs of Caucasian patients. However, a high incidence of HLA-C disparities was identified even in the URDs for Caucasian patients. Whether the disparities demonstrated by this study contribute to the higher immunological complications noted following URD bone marrow transplantation is unclear. Outcome analysis and studies aimed at understanding the functional role of HLA-C may provide an answer.

Bone Marrow Transplantation↗

PubMed1987

Parallel processing of binocular disparity in the cat's retinogeniculocortical pathways.

In the cat, parallel streams of information processing have been traced from X-, Y- and W-type retinal ganglion cells to visual cortical areas 17 (X-, Y- and W-type), 18 (Y-type) and 19 (W-type). In the present study we have examined, in the anaesthetized and paralysed adult cat, the role played by X-, Y- and W-subsystems, projecting to areas 17 and 19, in the processing of binocular retinal disparity. The tapetal reflection technique was used to monitor residual eye movements and to provide a map, for each eye, of the retinal blood vessels which could later be compared with retinal wholemounts stained with cresyl violet to reveal the area centralis. The receptive-field disparities of cells recorded from areas 17 and 19 were compared with each other and with reference to the visual axes defined by the area centralis of each eye. Cells of area 19 (receiving W-type input) had horizontal receptive-field disparities that were significantly more divergent than those of the cells in area 17 and 17-18 'border region'. Referred to the area centralis, the mean horizontal receptive-field disparity in area 19 was -0.5 degrees (+/- 0.8 degrees). The mean horizontal receptive-field disparity of area 17 (receiving X-, Y- and W-type input) was convergent with respect to the visual axis at +2 degrees (+/- 0.5 degrees). Finally, the mean horizontal receptive-field disparity of the cells in the 17-18 border region (which receive mainly Y-type input) was even more convergent (2.6 degrees +/- 1.5 degrees) than that of area 17. Binocular interactions of cortical neurons were tested with the Risley biprism technique. Area 19 cells had maximal responses to binocular stimulation when the receptive-field disparities were either close to zero or slightly divergent. In contrast, area 17 cells tended to respond optimally to disparities that were either slightly or strongly convergent. At the level of the lateral geniculate nucleus there were significant differences between the receptive-field disparities inferred from the comparison of receptive-field positions of adjacent neurons recorded on either side of the border between the A and A1 geniculate laminae and those inferred from a similar comparison at the C1-C2 border. The mean horizontal disparities inferred from the interlaminar comparison at the A-A1 border were +2.1 degrees (+/- 0.3 degrees); those inferred from the interlaminar comparison at the C1-C2 border -0.2 (+/- 0.2 degrees) were more divergent.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1972

Disparity processing of spatial frequencies in man.

1. Adaptation to a high-contrast sine-wave grating has been shown previously by Blakemore & Campbell (1969) to raise the modulation required to detect a low-contrast grating that has the same or similar spatial frequency as the adapting grating.2. A similar adaptation effect occurs when the adaptation and test gratings are seen binocularly and are presented off the plane of fixation. When the gratings are not located on the plane of fixation, however, the greatest rise in threshold following adaptation occurs for test gratings presented in the same plane as the adapting grating. Thus, the neural mechanisms adapted to the high contrast patterns must be processing disparity information.3. The spatial frequency response of the disparity adaptation effect has been measured by adapting to gratings of different spatial frequencies presented at a given disparity, and comparing threshold elevations for identical test gratings presented in the same (disparate) plane as the adapting grating or in the plane of fixation.4. The unbiased adaptation effect specific to disparity is greatest for gratings whose periods are twice the disparity.5. There is no adaptation effect specific to disparity for individuals possessing only convergent or only divergent disparity mechanisms.6. The results suggest that disparity mechanisms make bar by bar correlations as opposed to edge by edge correlations and that narrow bar detectors feed small disparity mechanisms whereas wide bar detectors feed large disparity mechanisms.

Adaptation, Physiological↗

PubMed1986

Human ocular vergence movements induced by changing size and disparity.

Human subjects viewed an electronically generated bright square. Horizontal movements of the two eyes were recorded with the scleral coil method. The dynamic properties of vergence movements induced by movement of the bright square were investigated for the following three kinds of stimulus motion: (a) both the size and the binocular disparity of the square changed together, in such a way as to exactly mimic the retinal image changes produced by a real object's motion in depth; (b) the changing-size component in (a) was present with no disparity component; (c) the changing-disparity component in (a) was present with no size component. The gain and phase of the ocular vergence responses to these three stimuli were computed. Ocular vergence movements were induced by changing size in all five subjects. Responses during binocular viewing were higher and less variable than responses during monocular viewing. Size oscillations induced ocular vergence oscillations with a phase lead of up to 65 deg relative to target size for frequencies of stimulation below 1.0 Hz. Vergence oscillation amplitudes were of the order of 10 min of arc and maximal for frequencies of 0.4-0.7 Hz. Ocular vergence movements were not induced by changes in target size in one dimension nor by flickering a stationary square. Ocular vergence movements induced by size changes were entirely transient with no sustained component: vergence responses to disparity were sustained. When the stimulus combined size change with disparity change in the ratio characteristic of a real moving object, vergence tracking was more accurate and less noisy than when the eyes were stimulated with the disparity component alone. The ocular vergence response induced by the combination of size change with disparity change was accurately predicted by linearly adding the vergence response produced by the size change alone to the vergence response produced by the disparity change alone: combined stimulation produced no evidence of non-linear interaction between responses to size change and to disparity change. The properties of vergence responses induced by changing size and by changing disparity showed several close correlations with the corresponding data on psychophysical sensitivity for motion-in-depth sensation. We suggest that responses to changing size contribute to the accuracy with which ocular vergence tracks real objects moving in depth.

Adult↗

PubMed2002

Disparity-selective neurons in area V4 of macaque monkeys.

Area V4 is an intermediate stage of the ventral visual pathway providing major input to the final stages in the inferior temporal cortex (IT). This pathway is involved in the processing of shape, color, and texture. IT neurons are also sensitive to horizontal binocular disparity, suggesting that binocular disparity is processed along the ventral visual pathway. In the present study, we examined the processing of binocular disparity information by V4 neurons. We recorded responses of V4 neurons to binocularly disparate stimuli. A population of V4 neurons modified their responses according to changes of stimulus disparity; neither monocular responses nor eye movements could account for this modulation. Disparity-tuning curves were similar for different locations within a neuron's receptive field. Neighboring neurons recorded using a single electrode displayed similar disparity-tuning properties. These findings indicate that a population of V4 neurons is selective for binocular disparity, invariant for the position of the stimulus within the receptive field. The finding that V4 neurons with similar disparity selectivity are clustered suggests the existence of functional modules for disparity processing in V4.

Animals↗