Suprathreshold stereo-depth matches as a function of contrast and spatial frequency

Thresholds for stereoscopic-depth perception increase with decreasing spatial frequency below 2.5 cycles deg-1. Despite this variation of stereo threshold, suprathreshold stereoscopic-depth perception is independent of spatial frequency down to 0.5 cycle deg-1. Below this frequency the perceived depth of crossed disparities is less than that stimulated by higher spatial frequencies which subtend the same disparities. We have investigated the effects of contrast fading upon this breakdown of stereo-depth invariance at low spatial frequencies. Suprathreshold stereopsis was investigated with spatially filtered vertical bars (difference of Gaussian luminance distribution, or DOG functions) tuned narrowly over a broad range of spatial frequencies (0.15-9.6 cycles deg-1). Disparity subtended by variable width DOGs whose physical contrast ranged from 10-100% was adjusted to match the perceived depth of a standard suprathreshold disparity (5 min visual angle) subtended by a thin black line. Greater amounts of crossed disparity were required to match broad than narrow DOGs to the apparent depth of the standard black line. The matched disparity was greater at low than at high contrast levels. When perceived contrast of all the DOGs was matched to standard contrasts ranging from 5-72%, disparity for depth matches became similar for narrow and broad DOGs. 200 ms pulsed presentations of DOGs with equal perceived contrast further reduced the disparity of low-contrast broad DOGs needed to match the standard depth. A perceived-depth bias in the uncrossed direction at low spatial frequencies was noted in these experiments. This was most pronounced for low-contrast low-spatial-frequency targets, which actually needed crossed disparities to make a depth match to an uncrossed standard. This bias was investigated further by making depth matches to a zero-disparity standard (ie the apparent fronto-parallel plane). Broad DOGs, which are composed of low spatial frequencies, were perceived behind the fixation plane when they actually subtended zero disparity. The magnitude of this low-frequency depth bias increased as contrast was reduced. The distal depth bias was also perceived monocularly, however, it was always greater when viewed binocularly. This investigation indicates that contrast fading of low-spatial-frequency stimuli changes their perceived depth and enhances a depth bias in the uncrossed direction. The depth bias has both a monocular and a binocular component.     

An attentional mechanism in the analysis of spatial frequency

Sinusoidal gratings of various spatial frequencies were used as masking stimuli in a detection task and a vernier acuity task. The test stimuli were 1 cycle/deg square-wave gratings. The spatial frequency of the most effective mask was 1 cycle/deg for the detection task but 3 cycles/deg for the vernier acuity task. The different masking functions for the two tasks show that the visual system analyzes the square-wave stimulus into its various spatial-frequency components. Since the test stimulus was the same for both tasks, the different masking functions may be the result of an attentional mechanism that weighs the importance of the output from various spatial-frequency analyzers. Whether the information from a particular spatial-frequency analyzer is attended or not depends upon the task the visual system must perform.     

The precision of velocity discrimination across spatial frequency

The precision of velocity coding for moving stimuli of different spatial frequencies was assessed by measuring velocity discrimination thresholds for a 1-c/deg grating paired with a grating whose spatial frequency ranged from 0.25 to 4 c/deg and for grating pairs of the same spatial frequency (0.25, 1, and 4 c/deg). The gratings always moved upward, with velocities ranging from 0.5 to 16 deg/sec, Velocity discrimination was as precise for stimuli that varied in spatial frequency by: ±2 octaves (0.25 vs. 1 c/deg and 4 vs. 1 c/deg) as for stimuli of the same spatial frequency, for specific ranges of velocity that depended on the spatial and, therefore, the temporal frequencies of the stimuli. Compared with a 1-c/deg grating, the perceived velocity of 4-c/deg gratings was about 1.3 times faster and that of 0.25-c/deg gratings was about 1.3 times slower. Although these perceived velocity biases imply variation of velocity-signal processing among spatial frequency channels, the discrimination results indicate that the motion-sensing system can compare signals across different spatial frequency channels to make fine velocity discrimination within appropriate temporal frequency limits.     

The processing of spatial frequency and orientation information

Three identification experiments were completed to disambiguate the associations between spatial frequency and orientation information at the sensory, decisional, and response levels. The stimuli were gratings generated by crossing four levels each of spatial frequency and orientation. In Experiment 1, the subjects made a single identification response to the stimuli. In Experiment 2, two identification responses were made, one for the spatial frequency component and the other for the orientation component. In Experiment 3, the subjects identified either the spatial frequency or the orientation component in any block of trials. The data were confusion matrices, and an information-transmission approach was used to investigate the interactions in the system. The results show that although there were sensory associations, there were no interactions at the decisional level. Performance parity was found: there was no significant difference between the single- and double-judgment paradigms in terms of information transmitted. Overall, the results suggest that although spatial frequency and orientation information is coded jointly at the sensory level, subsequent processing is independent, with each dimension drawing upon different attentional resources.     

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.     

Visual search: spatial frequency and orientation

Observers searched for a Gaussian-windowed patch of sinewave grating (Gabor pattern) through displays containing varying numbers of other such patterns (distractors). When the spatial frequencies of target and distractors differed by +/- 2 octaves and their orientations by +/- 60 degrees, the search proceeded spatially in parallel irrespective of whether the target could be discriminated in terms of spatial frequency differences alone, orientation differences alone, or their combination. However, when target and distractors differed by only +/- .5 octave in spatial frequency and by +/- 15 degrees in orientation, the search was serial and self-terminating, again irrespective of the nature of the target-distractor differences. These findings show that, contrary to some suggestions, the preattentive detection of targets defined by conjunctions of spatial frequency and orientation may occur, but only when the spectral distance between target and distractors allows their encoding by independent mechanisms.     

The dependence of monocular rivalry on spatial frequency: some interaction variables

The effect of spatial frequency upon the rate of monocular rivalry has been investigated on several occasions, with conflicting results. With increases in spatial frequency the rivalry rate has been variously reported as declining monotonically, increasing slightly and briefly before declining sharply, or showing a sharp increase before a sharp decrease. Experiments are reported which show that all three functions may be obtained depending on whether the number of cycles of the gratings is held constant across spatial frequency, or the field size is held constant (and, if the latter, what field size is used). The point at which any high-frequency decline occurs is also shown to increase with increasing luminance. All functions obtained correspond to visual sensitivity functions obtained under comparable conditions.     

Task-unrelated-thought frequency as a function of age: a laboratory study

Using retrospective reports, Giambra (1977-1978, 1979-1980) found an inverse relation between age and daydreaming/mind wandering. To deal with an alternate explanation of these results based on age-dependent memorial deficiencies and attitudes toward daydreaming/mind wandering and to provide independent convergent validity, five experiments were carried out. Task-unrelated thoughts (TUTs) were taken as the operational definition of daydreams/mind wanderings and their frequency recorded in vigilance tasks. All five experiments found an inverse relation between age (17-92 years, N = 471) and TUTs, confirming the reliability and validity of the earlier studies. The age-dependent reduction in TUTs was considered as evidence of reduced nonconscious information processing with increased age. The results of this study were incompatible with a recent theory that predicts for older individuals an increased input of irrelevant thoughts into working memory due to the older individual's reduced inhibitory control.     

The effects of spatial frequency overlap on face recognition

The effects of spatial frequency overlap between pairs of low-pass versus high-pass images on face recognition and matching were examined in 6 experiments. Overlap was defined as the range of spatial frequencies shared by a pair of filtered images. This factor was manipulated by processing image pairs with high-pass/low-pass filter pairs whose 50% cutoff points varied in their separation from one another. The effects of the center frequency of filter pairs were also investigated. In general, performance improved with greater overlap and higher center frequency. In control conditions, the image pairs were processed with identical filters and thus had complete overlap. Even severely filtered low-pass or high-pass images in these conditions produced superior performance. These results suggest that face recognition is more strongly affected by spatial frequency overlap than by the frequency content of the images.     

The development of spatial frequency biases in face recognition

Previous research has suggested that a mid-band of spatial frequencies is critical to face recognition in adults, but few studies have explored the development of this bias in children. We present a paradigm adapted from the adult literature to test spatial frequency biases throughout development. Faces were presented on a screen with particular spatial frequencies blocked out by noise masks. A mid-band bias was found in adults and 9- and 10-year-olds for upright faces but not for inverted faces, suggesting a face-sensitive effect. However, 7- and 8-year-olds did not demonstrate the mid-band bias for upright faces but rather processed upright and inverted faces similarly. This suggests that specialization toward the mid-band for upright face recognition develops gradually during childhood and may relate to an advanced level of face expertise.     

The role of spatial frequency in emotional face classification

Previous studies with emotional face stimuli have revealed that our ability to identify different emotional states is dependent on the faces’ spatial frequency content. However, these studies typically only tested a limited number of emotional states. In the present study, we measured the consistency with which 24 different emotional states are classified when the faces are unfiltered, high-, or low-pass filtered, using a novel rating method that simultaneously measures perceived arousal (high to low) and valence (pleasant to unpleasant). The data reveal that consistent ratings are made for every emotional state independent of spatial frequency content. We conclude that emotional faces possess both high- and low-frequency information that can be relied on to facilitate classification.     

The neural code for written words: a proposal

How is reading, a cultural invention, coded by neural populations in the human brain? The neural code for written words must be abstract, because we can recognize words regardless of their location, font and size. Yet it must also be exquisitely sensitive to letter identity and letter order. Most existing coding schemes are insufficiently invariant or incompatible with the constraints of the visual system. We propose a tentative neuronal model according to which part of the occipito-temporal 'what' pathway is tuned to writing and forms a hierarchy of local combination detectors sensitive to increasingly larger fragments of words. Our proposal can explain why the detection of 'open bigrams' (ordered pairs of letters) constitutes an important stage in visual word recognition.     

Hemispheric specialization for spatial frequency processing in the analysis of natural scenes

Experimental data coming from visual cognitive sciences suggest that visual analysis starts with a parallel extraction of different visual attributes at different scales/frequencies. Neuropsychological and functional imagery data have suggested that each hemisphere (at the level of temporo-parietal junctions-TPJ) could play a key role in spatial frequency processing: The right TPJ should predominantly be involved in low spatial frequency (LFs) analysis and the left TPJ in high spatial frequency (HFs) analysis. Nevertheless, this functional hypothesis had been inferred from data obtained when using the hierarchical form paradigm, without any explicit spatial frequency manipulation per se. The aims of this research are (i) to investigate, in healthy subjects, the hemispheric asymmetry hypothesis with an explicit manipulation of spatial frequencies of natural scenes and (ii) to examine whether the 'precedence effect' (the relative rapidity of LFs and HFs processing) depends on the visual field of scene presentation or not. For this purpose, participants were to identify either non-filtered or LFs and HFs filtered target scene displayed either in the left, central, or right visual field. Results showed a hemispheric specialization for spatial frequency processing and different 'precedence effects' depending on the visual field of presentation.     

Recency judgments as a function of word frequency: A framing effect and frequency misattributions

The effects of word frequency on judgments of recency of item presentation were examined in two experiments. Subjects in Experiment 1 were presented two mixed lists of high- and low-frequency words followed by a list assignment task for recognized items. It was found that subjects were biased toward assigning low-frequency words to the more recently presented list. Subjects in Experiment 2 were presented a single mixed list of high- and low-frequency words followed by either a relative recency of presentation judgment task or a relative primacy of presentation judgment task. Each word pair on the tests contained one high-frequency word and one low-frequency word. It was found that, for the recency judgment task, subjects were biased to select the low-frequency item as having been presented more recently. However, on the parallel primacy judgment task, there were no effects of word frequency; moreover, overall accuracy levels were higher with primacy than with recency instructions. We interpret the effects of word frequency on recency judgments in Experiments 1 and 2 in terms of a misattribution of frequency-related differences in recollection-based recognition. The finding that recency and primacy instructions produced different patterns of results provides further evidence (Flexser & Bower, 1974) for an effect on performance of the way in which the temporal judgment task was framed.     

Visual discomfort: the influence of spatial frequency.

The response of different visual discomfort groups to a range of spatial frequencies at threshold and suprathreshold was investigated. In experiment 1, a paired-comparison task was conducted. The high visual discomfort group judged a spatial frequency of 4 cycles deg-1 as the most perceptually distorted and somatically unpleasant to view. The moderate and low visual discomfort groups judged 8 and 12 cycles deg-1 as more perceptually and somatically unpleasant to view than lower spatial frequencies. In experiment 2, the spatial contrast-sensitivity function (CSF) for the high visual discomfort group was depressed for spatial frequencies between 1 and 12 cycles deg-1 in comparison with the moderate and low visual discomfort groups. When these same spatial frequencies were modulated at 6 Hz, CSFs were the same for all groups. These results are discussed in relation to a failure of inhibition across spatial-frequency channels in the high visual discomfort group. This may be explained by a more generalised parvocellular system processing deficit. Possible similarities between some forms of migraine and visual discomfort are highlighted.     

Pattern alternation: Effects of spatial frequency and orientation

Campbell and Howell (1972) reported an effect called “monocular pattern alternation.” They found that a pattern composed of two orthogonal sinusoidal gratings, one horizontal and the other vertical, underwent rivalry when viewed monocularly for a period of time. In the present study, it has been shown that monocular pattern alternation depends upon the orientation of the pattern and the spatial frequency of its components. Fewer reversals were found for an obliquely oriented pattern than for a pattern with components in the horizontal and vertical meridians. Alternation rate was higher when the gratings were similar in frequency but differed in orientation than when the components of the pattern differed in both dimensions. It was concluded that pattern alternation reflects an antagonistic interaction between interdependent channels in the human visual system that respond to orientation and spatial frequency.