Binocular vision enhances phase discrimination by filtering the background

Previous studies have shown that the detectability of a noise-masked target can be enhanced under stereoscopic viewing when the target’s interocular disparity differs from that of the noise. This enhanced detectability can be accounted for by a model postulating that the binocular system linearly sums the left-eye and right-eye views of a visual scene. This model also predicts enhanced phase discrimination under specifiable interocular disparities of target and noise. Two experiments were conducted in which subjects were asked to discriminate between two luminance patterns (target and foil) that differed only in phase. The target patterns were constructed by summating two vertical sinusoidal gratings in which the phase difference between the higher and the lower spatial frequency gratings was 45°. The foils contained the same two component frequencies, with a phase difference of ?45°. Thus, targets and foils were mirror images of one another. The ability of subjects to Discriminate between these stereoscopically viewed mirror-image patterns was investigated under two sets of interocular disparities: those that, according to our model, would unmask one or both spatial frequency components, and those that would leave both components masked by the noise. Phase discrimination was enhanced only when both component frequencies of the target and foil were unmasked. The implications of these findings for template-matching and phase-discrimination models of pattern discrimination are considered.     

Binocular unmasking with unequal interocular contrast: the case for multiple Cyclopean eyes.

Under certain conditions, the detection threshold for a sinusoidal grating embedded in a noisy background may be an order of magnitude lower when binocular cues are available than when monocular cues only are present. Such binocular unmasking occurs only when the degree of interocular disparity for the target differs from that of the background. Two classes of models have been advanced to account for such unmasking. The first assumes that orientation-specific, spatial frequency channels in each eye encode the amplitude and phase of the spatial frequency component of the pattern the channel is tuned to detect. Thus, a difference in interocular disparity between target and background could result in interocular amplitude and/or phase differences in left- and right-eye spatial frequency channels. When, however, there are no disparity differences between target and background, there will be no interocular differences in amplitude and phase in the left- and right-eye channels. In this model, then, binocular unmasking reflects the binocular system's ability to respond to interocular amplitude and/or phase differences in the patterns presented to the two eyes. In the second class of models, it is assumed that the left- and right-eye patterns are first summed to form a "Cyclopean" eye. In these models, detection depends on the effect this summation process has on the power spectrum of the summated patterns. To decide between these two classes of models, we observed the occurrence of binocular unmasking when (1) the contrast of masker and signal was varied identically in both eyes and (2) the contrast of masker and signal was varied in one eye only. Consistent with our previous research, we found that the results can be accounted for in terms of a linear summation model of binocular unmasking; the alternative interocular phase detection model was disproved. The implications of these findings for binocular contrast summation in the absence of visual noise are discussed.     

Binocular unmasking with unequal interocular contrast: The case for multiple Cyclopean eyes

Under certain conditions, the detection threshold for a sinusoidal grating embedded in a noisy background may be an order of magnitude lower when binocular cues are available than when monocular cues only are present. Such binocular unmasking occurs only when the degree of interocular disparity for the target differs from that of the background. Two classes of models have been advanced to account for such unmasking. The first assumes that orientation-specific, spatial frequency channels in each eye encode the amplitude and phase of the spatial frequency component of the pattern the channel is tuned to detect. Thus, a difference in interocular disparity between target and background could result in interocular amplitude and/or phase differences in left- and right-eye spatial frequency channels. When, however, there are no disparity differences between target and background, there will be no interocular differences in amplitude and phase in the left- and right-eye channels. In this model, then, binocular unmasking reflects the binocular system’s ability to respond to interocular amplitude and/or phase differences in the patterns presented to the two eyes. In the second class of models, it is assumed that the left and right-eye patterns are first summed to form a “Cyclopean” eye. In these models, detection depends on the effect this summation process has on the power spectrum of the summated patterns. To decide between these two classes of models, we observed the occurrence of binocular unmasking when (1) the contrast of masker and signal was varied identically in both eyes and (2) the contrast of masker and signal was varied in one eye only. Consistent with our previous research, we found that the results can be accounted for in terms of a linear summation model of binocular unmasking; the alternative interocular phase detection model was disproved. The implications of these findings for binocular contrast summation in the absence of visual noise are discussed.     

Frequency discrimination between and within line gratings by dynamic touch

Line gratings were used to investigate the tactual discrimination thresholds for line frequency. In Experiment 1, participants were asked to discriminate between two gratings, each with a different line frequency. We used four standard frequencies in the eightfold range from 0.5 to 4 lines/cm. Thresholds were found to be constant at about 10.6%. In this experiment, we also measured hand speed and contact force. Hand speed was roughly in the range between 0.12 and 0.44 m/sec; contact force ranged from 0.62 to 2.76 N. In Experiment 2, we determined discrimination thresholds for line frequency transitions within a single grating. We used two frequencies and three transition lengths. The transition length had no effect on the threshold. In a third experiment, line frequency was modulated periodically. Varying the standard frequency and the size of the modulation period was found to have no effect on the discrimination thresholds. We conclude three things. First, Weber fractions for line frequency discrimination decrease as a function of line frequency within the experimental range. Second, discrimination thresholds are not altered by the length of the transition between two adjacent gratings with different line frequencies. And finally, the size of a modulation period in periodically modulated gratings is of no influence on the modulation detection threshold.     

Spatial frequency and selective attention to spatial location

The effect of spatial attention on the detectability of gratings of different spatial frequency was measured using a probe technique. Three experiments are reported in which the detectability of full-field probe gratings was measured while subjects analyzed stimuli presented in either the central or the peripheral visual field. Selective attention to peripheral stimuli produced a facilitation at low frequencies and a decrement at high frequencies. These effects disappeared under forced-choice presentation.     

The temporal course of suppression during binocular rivalry

Orthogonally oriented sinusoidal luminance gratings were dichoptically presented to the observers' left and right eyes. During the subsequent binocular rivalry, a small target was briefly presented (4AFC) to probe the strength of interocular suppression at various temporal latencies. Both stationary and moving rivalrous patterns were investigated. The purpose of experiment 1 was to compare the temporal characteristics of stationary and motion rivalry (0 and 1.2 deg s-1), while that of experiment 2 was to examine rivalry suppression for higher speeds (2 and 4 deg s-1). In all cases, it was found that the strength of suppression remained essentially constant throughout a single phase of binocular rivalry. The results of the investigation also revealed that moving rivalrous patterns lead to greater magnitudes of interocular suppression than static patterns. Despite these differences in the strength of suppression, the results of both experiments show that the temporal characteristics of motion and static rivalry are essentially identical.     

Binocular unmasking with vertical disparity

We recently found (Schneider, Moraglia, & Jepson, 1989) that the contrast threshold for the detection of a visual signal in a noisy background can be considerably lower when binocular cues are available then when monocular cues only are present. Here, we investigated the occurrence of binocular unmasking with vertical interocular disparities. Subjects reported about the presence of Gabor signals in fields of two-dimensional broadband Gaussian noise surrounded by a frame of uniform noise. They saw these stimuli through a stereoscope; in all cases, the right-eye noise field was vertically displaced relative to the left one in either an upward or a downward direction, by up to 67.6'. In one condition, the right-eye signal was displaced by an amount equal to that of the noise, so that no opportunities for binocular unmasking existed; in the other, it appeared in exactly corresponding locations in the two fields--here, binocular disparities could be used to unmask the signal. Enhanced signal detectability, by up to 12.7 dB, was observed in the latter case for both directions of displacement, but only for displacements of 13.52' and only when the signal's orientation was horizontal. We argue that these effects result from the summation of monocular inputs carried out by linear binocular mechanisms.     

Rapid discrimination of McCollough effects.

The McCollough effect is a colour aftereffect that is contingent on pattern orientation. Three experiments were conducted to establish whether such aftereffect colours could serve as a basis for discrimination in several rapid discrimination tasks. In the first experiment it was investigated whether aftereffect colours could act like a simple 'feature' in a visual search task involving a difficult orientation discrimination. Without McCollough adaptation, the time taken to detect a 'target' among 'distractors' increased substantially as the number of distractors increased. With adaptation, detection time was essentially independent of the number of distractors, indicating that the nature of the task changed from a difficult orientation discrimination to a simple discrimination based on differences in aftereffect colours. The second and third experiments employed a difficult four-alternative forced-choice procedure in which subjects were required to discriminate a monochromatic patch of square-wave grating oriented at 45 degrees from three others oriented at 135 degrees (and vice versa). The gratings were presented very briefly (67-333 ms) followed by a 500 ms mask. Subjects performed the task with and without McCollough adaptation. Performance was strikingly better after adaptation: colour aftereffects could be used to make the discrimination even at exposure durations as short as 67 ms. The third experiment demonstrated that this enhanced performance was indeed due to perceived colour differences (rather than a possible contrast difference). The results of the three experiments are discussed in relation to proposals about the locus of the McCollough effect.     

Right-hemisphere superiority in the discrimination of spatial phase

Visual field differences have been investigated in various detection and discrimination tasks for simple sinusoidal gratings or for complex gratings composed of two sinusoids of spatial frequencies f and 3f. Sinusoidal gratings were employed to evaluate contrast sensitivity, subthreshold summation effects, aftereffects of adaptation to a high-contrast grating, and spatial-frequency discrimination. The tasks with complex gratings were detection of the 3f component in the presence of a high-contrast f component and spatial-phase discrimination. The stimuli were presented either in the left or in the right visual hemifield. The results indicate a lack of lateralization for detection and spatial-frequency discrimination of sinusoidal gratings, and for the bandwidth of subthreshold summation effects and adaptation aftereffects, whereas the detection of the 3f component in the presence of a high-contrast f component, as well as spatial-phase discrimination of f +3f gratings, show a left-field advantage. This suggests a right-hemisphere superiority in the processing of spatial phase.     

On utrocular discrimination

Observers with good stereoacuity judged which eye received sine-wave grating patterns in a two-category forced-choice procedure. Large individual differences were found, but for most observers reliable discrimination was achieved at low spatial frequencies. No observer could perform the task above chance levels at high spatial frequencies. Discrimination was unaffected by retinal location, grating orientation, grating contrast, stimulus duration, or practice with feedback. Among observers who could perform the task, the following results were obtained: (1) Introduction of high spatial frequency components did not interfere with performance so long as a low spatial frequency component was present. (2) When gratings of low equal spatial frequency were presented to both eyes simultaneously at different contrast levels, observers could identify which eye received the higher contrast. (3) At low spatial frequencies, observers could distinguish monocular from binocular presentation. (4) Temporal frequency variations (counterphase flicker) influenced performance for some observers. Binocular summation and interocular transfer were unaffected by the spatial frequency variations which modulate utrocular discrimination. A new procedure for measuring stereopsis was developed which made possible comparison of utrocular discrimination with stereopsis at specific spatial frequencies. Stereopsis appeared mildly affected by spatial frequency.     

The development of basic mechanisms of pattern vision: spatial frequency channels

The mature visual system possesses mechanisms that analyze visual inputs into bands of spatial frequency. This analysis appears to be important to several visual capabilities. We have investigated the development of these spatial-frequency channels in young infants. Experiment 1 used a masking paradigm to test 6-week-olds, 12-week-olds, and adults. The detectability of sine wave gratings of different spatial frequencies was measured in the presence and the absence of a narrowband noise masker. The 12-week data showed that at least two spatial-frequency channels with adultlike specificity are present at 12 weeks. The 6-week data did not reveal the presence of narrowband spatial-frequency channels. Experiment 2 used a different paradigm to investigate the same issue. The detectability of gratings composed of two sine wave components was measured in 6-week-olds and adults. The results were entirely consistent with those of experiment 1. The 12-week and adult data indicated the presence of narrowband spatial-frequency channels. The 6-week data did not. The results of these experiments suggest that the manner in which pattern information is processed changes fundamentally between 6 and 12 weeks of age.     

Spatial frequency and selective attention to local and global information

Probe methods were used to investigate whether the distribution of attention to the local or the global structure of a stimulus affects the detectability of different spatial frequencies. Four experiments are reported in which the detectability of threshold probe gratings of different spatial frequencies was measured while subjects analyzed either the local or the global information from a display. A relative shift in the detectability of low and high frequencies was observed. Low frequencies were facilitated during global processing and/or high frequencies were facilitated during local processing.     

Panum's fusional area estimated with a criterion-free technique

It has been reported that criterion-free estimates of the upper disparity limits for fusion of line targets are small enough to be accounted for by monocular vernier sensitivity. However, targets such as lines, which contain high spatial frequencies, may ensure small fusion limits, since fusion limits obtained with criterion-dependent methods for narrow-band targets, such as sinusoids or difference-of-Gaussian luminance profiles, are proportional to target spatial periods. Experiment 1 therefore explored whether criterion-free methods give fusion limits for narrow-band targets that can be accounted for by vernier sensitivity. Vertical fusion limits were estimated by a method that forced observers to discriminate a disparate sinusoidal grating from an immediately adjacent zero-disparity grating. Fusion limits were too large to be explained by monocular vernier thresholds obtained for the same targets. In addition, fusion limits were not affected by large changes in target contrast, whereas vernier thresholds increased as contrast was decreased. The results of Experiment 1 also argued against interocular suppression as the cause of single vision, since vernier offsets that were visible when viewed monocularly were invisible under binocular viewing conditions. In Experiment 2, manual adjustment of disparities yielded fusion limits little different from those obtained with the forced-choice method of Experiment 1, demonstrating that it is possible to design adjustment methods for assessing fusion limits that are as sensitive as forced-choice methods. In Experiment 3, large reductions in target contrast, which have the effect of decreasing disparity sensitivity, did not alter fusion limits, disconfirming the idea that fusion limits estimated with discriminative procedures represent disparity-detection thresholds. In Experiment 4, disparities were adjusted until a just noticeable difference in grating contrast appeared. These disparities were larger than fusion limits, indicating that fusion limits did not represent a change in apparent contrast arising from disparity limitations of binocular summation. Together, the four experiments support the existence of binocular fusion as a unique category of sensory performance, disconfirm several nonfusional explanations of single vision, and support the use of criterion-free as well as adjustment methods in measuring fusion limits.     

Panum’s fusional area estimated with a criterion-free technique

It has been reported that criterion-free estimates of the upper disparity limits for fusion of line targets are small enough to be accounted for by monocular vernier sensitivity. However, targets such as lines, which contain high spatial frequencies, may ensure small fusion limits, since fusion limits obtained with criterion-dependent methods for narrow-band targets, such as sinusoids or difference-of-Gaussian luminance profiles, are proportional to target spatial periods. Experiment 1 therefore explored whether criterion-free methods give fusion limits for narrow-band targets that can be accounted for by vernier sensitivity. Vertical fusion limits were estimated by a method that forced observers to discriminate a disparate sinusoidal grating from an immediately adjacent zero-disparity grating. Fusion limits were too large to be explained by monocular vernier thresholds obtained for the same targets. In addition, fusion limits were not affected by large changes in target contrast, whereas vernier thresholds increased as contrast was decreased. The results of Experiment 1 also argued against interocular suppression as the cause of single vision, since vernier offsets that were visible when viewed monocularly were invisible under binocular viewing conditions. In Experiment 2, manual adjustment of disparities yielded fusion limits little different from those obtained with the forced-choice method of Experiment 1, demonstrating that it is possible to design adjustment methods for assessing fusion limits that are as sensitive as forced-choice methods. In Experiment 3, large reductions in target contrast, which have the effect of decreasing disparity sensitivity, did not alter fusion limits, disconfirming the idea that fusion limits estimated with discriminative procedures represent disparity-detection thresholds. In Experiment 4, disparities were adjusted until a just noticeable difference in grating contrast appeared. These disparities were larger than fusion limits, indicating that fusion limits did not represent a change in apparent contrast arising from disparity limitations of binocular summation. Together, the four experiments support the existence of binocular fusion as a unique category of sensory performance, disconfirm several nonfusional explanations of single vision, and support the use of criterion-free as well as adjustment methods in measuring fusion limits.     

Inferior temporal lesions do not impair discrimination of rotated patterns in monkeys.

Ablation of inferior temporal cortex in the rhesus monkey produces a visual discrimination learning deficit. The severity of this deficit has often been found to be a function of task difficulty. This report concerns a type of visual discrimination problem that, although difficult, is not sensitive to inferior temporal lesions. Monkeys with anterior, posterior, and complete inferior temporal lesions were repeatedly unimpaired or only slightly impaired in learning to discriminate a pattern from the same pattern rotated 90 degrees or 180 degrees; yet they were very severely impaired in learning equally or more difficult discriminations of two different patterns. This demonstration that discrimination of orientation of patterns is relatively spared after inferior temporal lesions helps specify the pattern-recognition processes that require inferior temporal cortex.     

Acuity of sound localisation: a topography of auditory space. III. Monaural hearing conditions

A study is reported in which the acuity of azimuth and elevation discrimination under monaural listening conditions was measured. Six subjects localised a sound source (white noise through a speaker) which varied in position over a range of elevations (-40 degrees to +40 degrees) and azimuths (0 degrees to 180 degrees), at 10 degrees intervals, on the left side of the head. Monaural listening conditions were established by the fitting of an ear defender and one earmuff to the right ear. The absolute and algebraic, azimuth and elevation errors were measured for all subjects at each position of the source. The results indicate that all subjects suffered a marked reduction of azimuth acuity under monaural conditions, although a coarse capacity to discriminate azimuth still remained. Considerable between-subject variability was observed. Front/back discrimination was retained, although it was slightly impaired compared to that observed under normal listening conditions. Elevation discrimination was, on the whole, quite good under monaural conditions. However, a comparison of the performance of these subjects under monaural conditions with that observed under normal listening conditions indicated that some reduction in elevation localisation acuity occurred in the frontal quadrants in the median plane and in the upper quadrants of more lateral source positions. The reduction in acuity seen in these regions is attributed to the loss of information from the pinna of the occluded ear rather than to the observed reduction in azimuth error. The results provide partial support for the binaural pinna disparity model.     

Monocular unmasking of noise-embedded patterns.

Binocular disparity cues may help an observer "unmask" a target in a background, thereby enhancing its detectability (e.g., Moraglia & Schneider, 1992). Here, we sought to determine whether similar effects could be produced by monocular displacement cues resulting from a two-frame sequential presentation of a Gabor pattern (a sinusoidal modulation of luminance combined with a Gaussian modulation of local contrast) embedded in an unvarying field of two-dimensional Gaussian noise. The Gabor in the second frame was spatially displaced relative to its location in the first frame; the horizontal displacement corresponded to a phase shift of the peak spatial frequency of the Gabor of 0 degree, 90 degrees, 180 degrees, 360 degrees, or 540 degrees. Monocular detection thresholds for the Gabor were appreciably lower for the 90 degrees, 180 degrees, and 540 degrees shift, than for the 0 degree and 360 degrees values. We explain these findings in terms of a model that constitutes the monocular analog of our summation model of binocular unmasking.     

Spatial frequency discrimination in schizophrenia

Pathways within the visual system can be distinguished on the basis of selectivity for low or high spatial frequencies. Spatial frequency discrimination was evaluated in 17 medicated male patients with schizophrenia and 19 male control subjects. Subjects were required to discriminate whether pairs of high contrast, sinusoidally modulated gratings were the same or different in spatial frequency. Accuracy performance was compared at high, medium, and low spatial frequencies on tasks matched for control performance. Patients showed a greater performance decrement of 12% on low as compared with 4% on high spatial frequencies. These findings suggest a disturbance of right hemisphere mechanisms involved in spatial perception and attention in schizophrenia.     

Individual differences in forced-choice recognition memory: Partitioning contributions of recollection and familiarity

In forced-choice recognition memory, two different testing formats are possible under conditions of high target–foil similarity: Each target can be presented alongside foils similar to itself (forced-choice corresponding; FCC), or alongside foils similar to other targets (forced-choice noncorresponding; FCNC). Recent behavioural and neuropsychological studies suggest that FCC performance can be supported by familiarity whereas FCNC performance is supported primarily by recollection. In this paper, we corroborate this finding from an individual differences perspective. A group of older adults were given a test of FCC and FCNC recognition for object pictures, as well as standardized tests of recall, recognition, and IQ. Recall measures were found to predict FCNC, but not FCC performance, consistent with a critical role for recollection in FCNC only. After the common influence of recall was removed, standardized tests of recognition predicted FCC, but not FCNC performance. This is consistent with a contribution of only familiarity in FCC. Simulations show that a two-process model, where familiarity and recollection make separate contributions to recognition, is 10 times more likely to give these results than a single-process model. This evidence highlights the importance of recognition memory test design when examining the involvement of recollection and familiarity.     

Pattern vision of the honeybee (Apis mellifera): blue and green receptors in the discrimination of translocation

The visual discrimination of horizontal gratings by the honeybee (Apis mellifera) was studied in a Y-choice apparatus with fixed patterns presented vertically at a set range. Translocation in this context is the exchange of the positions of two different colored or black areas. This paper investigates what cues the bees have learned in this task. The patterns, made from combinations of calibrated colored papers, are designed to explore the parts played by the blue and green receptors when the boundary between the two colors provides contrast to only one receptor type. Horizontal translocation is not discriminated without contrast to the green receptors, but up/down translocation can be discriminated whatever the contrast at the boundary. The trained bees were tested on the same patterns made with different papers that included extreme changes in contrast. The results show that discrimination of up/down translocation involves green receptors and also blue receptors. When bees discriminate a translocation that shows contrast to only one type of receptor, they do not use the apparent brightness or the direction of the contrast to that receptor type acting alone. Instead, they discriminate the locations of colored areas irrespective of intensity differences or directions of contrasts. They use some measure of the photon flux at both receptor types and remember the difference between the colors and their locations.