Spatial Frequency Channels Research Articles

Integration of monocular motion signals and the analysis of interocular velocity differences for the perception of motion-in-depth

We investigated how the mechanism for perceiving motion-in-depth based on interocular velocity differences (IOVDs) integrates signals from the motion spatial frequency (SF) channels. We focused on the question whether this integration is implemented before or after the comparison of the velocity signals from the two eyes. We measured spatial frequency selectivity of the MAE of motion in depth (3D MAE). The 3D MAE showed little spatial frequency selectivity, whereas the 2D lateral MAE showed clear spatial frequency selectivity in the same condition. This indicates that the outputs of the monocular motion SF channels are combined before analyzing the IOVD. The presumption was confirmed by the disappearance of the 3D MAE after exposure to superimposed gratings with different spatial frequencies moving in opposite directions. The direction of the 2D MAE depended on the test spatial frequency in the same condition. These results suggest that the IOVD is calculated at a relatively later stage of the motion analysis, and that some monocular information is preserved even after the integration of the motion SF channel outputs.

Dissociation of Neural Mechanisms Underlying Orientation Processing in Humans

Orientation selectivity is a fundamental, emergent property of neurons in early visual cortex, and the discovery of that property has dramatically shaped how we conceptualize visual processing. However, much remains unknown about the neural substrates of this basic building block of perception, and what is known primarily stems from animal physiology studies. To probe the neural concomitants of orientation processing in humans, we employed repetitive transcranial magnetic stimulation (rTMS), which can significantly attenuate neuronal spiking activity, hemodynamic responses, and local field potentials within a focused cortical region. Using rTMS to suppress neural responses evoked by stimuli falling within a local region of the visual field, we were able to dissociate two distinct components of the neural circuitry underlying orientation processing: selectivity and contextual effects. Orientation selectivity gauged by masking was unchanged by rTMS, whereas an otherwise robust orientation repulsion illusion was weakened after rTMS. This dissociation implies that orientation processing in humans relies on distinct mechanisms, only one of which was impacted by rTMS. These results are consistent with models positing that orientation selectivity is governed by patterns of convergence of thalamic afferents onto cortical neurons, with intracortical activity then shaping population responses amongst those cortical neurons.

Scale dependence and channel switching in letter identification

Letters are broadband visual stimuli with information useful for discrimination over a wide range of spatial frequencies. Yet, recent evidence suggests that observers use only a single, fixed spatial-frequency channel to identify letters and that the scale of that channel, in units of letter size, is determined by the size of the letter (scale dependence). We report two letter-identification experiments using critical-band masking. With sufficiently high-amplitude, low- or high-pass masking noise, observers switched to a different range of spatial frequencies for the task. Thus, letter channels are not fixed for a given letter size. When an additional white-noise masker was added to the stimulus to flatten the contrast-sensitivity function, the letter channel used by the observer still depended on letter size, further supporting the hypothesis that letter identification is scale dependent.

Measurement of Passband of Spatial-Frequency Channels of the Visual System

A method for measurement of the passband of spatial-frequency channels of a visual system is suggested. The spatial-frequency parameters were measured using differential threshold of flicker frequency. The results of experimental tests are considered.

Phenotyping parallel visual pathways in autosomal dominant optic atrophy

Abstract Purpose To characterize different phenotypes of Kjer optic atrophy along different visual pathways. Methods Novel computerized psychophysical assessment methods (CCT ‐ Cambridge Colour Test and CSF ‐ Metropsis Contrast Sensitivity Function Test) were used to evaluate visual function in a population of 13 subjects (26 eyes) from 8 families with Autosomal Dominant Optic Atrophy (ADOA). This evaluation was completed with electrophysiological assessment (Pattern ERG, Pattern and Multifocal VEP) and Automated Static Perimetry (ASP). Results CCT shows evidence for severe damage of all cone populations (p<<0.0001), and of similar magnitude, implying concomitant damage of parvo and koniocellular pathways. Achromatic contrast sensitivity is severely impaired for all six spatial frequency channels studied (p<0.002), suggesting mixed magno/parvocellular impairment. A decrease of both P‐50 and N‐95 amplitudes of PERG is found (p<<0.001), while implicit times are normal. MfVEP results show significant impairment in amplitudes of the most central rings (1, 2 ‐ p<0.001), with concomitant local increased implicit times, no significant changes being found at most eccentric rings. Pattern VEP impairment is consistent with these results. Amplitude values of MfVEP in visual quadrants are significantly correlated with the decreased retinal sensitivity obtained in the 4 quadrants by ASP. Conclusion Our results suggest that all functional types of nerve fibers are damaged in ADOA, with a predominance of the parvocellular ones. Multimodal psychophysical and electrophysiological methods are good quantitative markers to understand the pathophysiology of damage of central and peripheral pathways in this condition.

Multiresolution wavelet framework models brightness induction effects

A new multiresolution wavelet model is presented here, which accounts for brightness assimilation and contrast effects in a unified framework, and includes known psychophysical and physiological attributes of the primate visual system (such as spatial frequency channels, oriented receptive fields, contrast sensitivity function, contrast non-linearities, and a unified set of parameters). Like other low-level models, such as the ODOG model [Blakeslee, B., & McCourt, M. E. (1999). A multiscale spatial filtering account of the white effect, simultaneous brightness contrast and grating induction. Vision Research, 39, 4361–4377], this formulation reproduces visual effects such as simultaneous contrast, the White effect, grating induction, the Todorović effect, Mach bands, the Chevreul effect and the Adelson–Logvinenko tile effects, but it also reproduces other previously unexplained effects such as the dungeon illusion, all using a single set of parameters.

The aesthetic experience of 'contour binding'

To find the diagnostic spatial frequency information in different painting styles (cubism, impressionism and realism), we have compared sensitivity (d') in distinguishing signal (subject of the painting) from noise with normal, high-pass and low-pass filtered images at long (150 ms) and short (30 ms) exposure. We found that for cubist-style images, d' increases with high-pass filtering compared with normal and low-pass filtered images, but decreases with low-pass filtering compared with normal images. These results indicate that channels with high spatial resolution provide the diagnostic information to solve the binding problem. Sensitivity for images in impressionist style was instead reduced by both low- and high-pass filtering. This indicates that both high and low spatial frequency channels play a role in solving the binding problem, suggesting the involvement of large collator units that group the response of small channels tuned to the same orientation. The difference between realism, which shows higher sensitivity for low-frequency filtering at short durations and cubism in which the binding problem is solved by high spatial frequency channels, has a corresponding difference in aesthetic judgment: the probability of judging a painting as 'intriguing' is larger with low-pass filtering than with high-pass filtering in realism, while the opposite is true for cubism. This suggests that the aesthetic experience is available during early processing of an image, and could preferentially influence high-level categorization of the subject of a painting.

Effect of Spatial-Frequency Distribution on Brightness Perception in Light Environment

We conducted a psychophysical experiment using pictures of a light environment as image stimuli. Brightness perception in light environment depends on not only the illumination or mean luminance but also the luminance distribution of the scene. We analyzed the data from the viewpoint of spatial-frequency information processing to estimate the effects of non-uniform luminance distribution on brightness perception. We modified still pictures of scenes of a room (reference images) using two-dimensional frequency analysis. Lowpass-filtering and contrast-gain control operations in the spatial-frequency dimension provided the test images. By adjusting the brightness of the reference images to that of the test images, we derived equi-brightness pairs of the reference and test images. The results showed that even though the mean luminance values of the two images (reference and test stimuli) were equal, the brightness of the lower contrast image was substantially lower and that of the lowpass-filtered image was slightly lower. This suggests that all spatial-frequency channels affect brightness perception and that low spatial-frequency components of non-uniform images are critical factors for increasing brightness.

Retinal and cortical patterns of spatial anisotropy in contrast sensitivity tasks

It has often been postulated that asymmetries in performance within the visual field (VF) are not characteristic of early visual processing. Here, human retinal (naso/temporal), cortical (left/right) and superior/inferior patterns of asymmetry were explored with achromatic contrast sensitivity (CS) tasks, that probed distinct spatiotemporal frequency channels. Low spatial, high temporal frequency stimuli (illusory frequency-doubling (FD)) yielded superior and temporal field disadvantage. Independent right and nasal visual hemifield patterns of disadvantage were found when probing an intermediate spatial frequency (ISF) channel, with stationary sinusoidal gratings. These findings show that asymmetries in spatial vision are explained by independent retinal and cortical mechanisms.

Spatial Frequency Integration for Binocular Correspondence in Macaque Area V4

Neurons in the primary visual cortex (V1) detect binocular disparity by computing the local disparity energy of stereo images. The representation of binocular disparity in V1 contradicts the global correspondence when the image is binocularly anticorrelated. To solve the stereo correspondence problem, this rudimentary representation of stereoscopic depth needs to be further processed in the extrastriate cortex. Integrating signals over multiple spatial frequency channels is one possible mechanism supported by theoretical and psychophysical studies. We examined selectivities of single V4 neurons for both binocular disparity and spatial frequency in two awake, fixating monkeys. Disparity tuning was examined with a binocularly correlated random-dot stereogram (RDS) as well as its anticorrelated counterpart, whereas spatial frequency tuning was examined with a sine wave grating or a narrowband noise. Neurons with broader spatial frequency tuning exhibited more attenuated disparity tuning for the anticorrelated RDS. Additional rectification at the output of the energy model does not likely account for this attenuation because the degree of attenuation does not differ among the various types of disparity-tuned neurons. The results suggest that disparity energy signals are integrated across spatial frequency channels for generating a representation of stereoscopic depth in V4.

The lowest spatial frequency channel determines brightness perception

This study investigates the role played by individual spatial scales in determining the apparent brightness of greyscale patterns. We measured the perceived difference in brightness across an edge in the presence of notch filtering and high-pass filtering for two stimulus configurations, one that elicits the perception of transparency and one that appears opaque. For both stimulus configurations, the apparent brightness of the surfaces delimited by the border decreased monotonically with progressive (ideal) high-pass filtering, with a critical cut-off at 1 c/deg. Using two octave ideal notch filtering, the maximum detrimental effect on apparent brightness was observed at about 1 c/deg. Critical frequencies for apparent brightness did not vary with contrast, viewing distance, or surface size, suggesting that apparent brightness is determined by the channel tuned at 1 c/deg. Modelling the data with the local energy model [Morrone, M. C., & Burr, D. C. (1988). Feature detection in human vision: a phase dependent energy model. Proceedings of the Royal Society (London), B235, 221–245] at 1 c/deg confirmed the suggestion that this channel mediates apparent brightness for both opaque and transparent borders, with no need for pooling or integration across spatial channels.

Shift in spatial scale in identifying crowded letters

Crowding refers to the increased difficulty in identifying a letter flanked by other letters. The purpose of this study was to determine if the peak sensitivity of the human visual system shifts to a different spatial frequency when identifying crowded letters, compared with single letters. We measured contrast thresholds for identifying the middle target letters in trigrams, for a range of spatial frequencies, letter separations and letter sizes, at the fovea and 5° eccentricity. Plots of contrast sensitivity vs. letter frequency exhibit spatial tuning, for all letter sizes and letter separations tested. The peak tuning frequency grows as the 0.6–0.7 power of the letter size, independent of letter separation. At the smallest letter separation, peak tuning frequency occurs at a frequency that is 0.17 octaves higher for flanked than for unflanked letters at the fovea, and 0.19 octaves at 5° eccentricity. This finding suggests that the human visual system shifts its sensitivity toward a higher spatial-frequency channel when identifying letters in the presence of nearby letters. However, the size of the shift is insufficient to account for the large effect of crowding in the periphery.

Finite Discrete, Shift-Invariant, Directional Filterbanks for Visual Information Processing, I : Construction

As is well known in neuroscience, simple cells of the mammalian’s striate cortex possess both orientation and spatial-frequency selectivity, and are similar to the Gabor filters or Gaussian derivative filters in shape. The purpose of this paper is to propose a method of designing perfect reconstruction 2D filterbanks which act on finite dimensional linear spaces consisting of 2D signals of a certain size, and have several analogous features to simple cells: (1) the filterbanks consist of several spatial-frequency channels with orientation selectivity, (2) the filterbanks have shift-invariant multiresolution (multiscale) structures, (3) filters contained in them are FIR, and are similar in appearance to not only Gaussian derivatives of 1st and 2nd order, but also ones of higher order. Moreover, they are constructed by finite linear combinations of separable filters. As is described in the text, by virtue of these properties, our 2D filterbanks can become bases of constructing computational nonlinear models of visual information processing. In this paper we construct the 2D filterbanks, and discuss them from the viewpoint of vision science. For example we disclose a possible role of “Gaussian-derivative-like” filters of higher order in our filterbanks. Practical applications of our 2D filterbanks to vision science and image processing will be given in our subsequent papers.

Channel surfing in the visual brain

Vision provides us with an ever-changing neural representation of the world from which we must extract stable object categorizations. We argue that visual analysis involves a fundamental interaction between the observer's top-down categorization goals and the incoming stimulation. Specifically, we discuss the information available for categorization from an analysis of different spatial scales by a bank of flexible, interacting spatial-frequency (SF) channels. We contend that the activity of these channels is not determined simply bottom-up by the stimulus. Instead, we argue that, following perceptual learning a specification of the diagnostic, object-based, SF information dynamically influences the top-down processing of retina-based SF information by these channels. Our analysis of SF processing provides a case study that emphasizes the continuity between higher-level cognition and lower-level perception.

Second-order spatial frequency and orientation channels in human vision

We compared the number of spatial frequency and orientation mechanisms underlying first- versus second-order processing by measuring discrimination at detection threshold for first- and second-order Gabors to determine the smallest difference in spatial frequency and orientation that permits accurate discrimination at threshold. For second-order gratings, the number of channels is the same as for first-order gratings for spatial frequencies up to about 2 cpd; however, there are fewer second-order channels at higher spatial frequencies. In contrast, the number of labeled channels for orientation is the same for first- and second-order gratings. In conclusion, our findings provide evidence for distinct spatial frequency and orientation labeled detectors in second-order visual processing. We also show that, relative to first-order, there are fewer second-order channels processing higher spatial frequencies. This is consistent with a filter-rectify-filter scheme for second-order in which the second stage of filtering is at lower spatial frequencies.

Category-specific visual recognition as affected by aging and expertise

The influence of aging and expertise on visual identification as function of semantic category (animals and tools) and spatial-frequency content of stimuli was investigated. Three-age groups of experts (whose profession required the use of manipulable tools) and non-experts, participated to the experiment. Data showed a different involvement of low and high spatial-frequency channels in processing objects semantically different: animals were identified at a coarse spatial scale (low spatial frequencies) and tools at a finer spatial scale (higher spatial frequencies). A gradual age-related decline in the identification for both categories (more physical information was required as the age increased) was found in non-experts, while in experts a similar trend was found only for animals and not for tools. In fact, independently on the age, experts could identify tools without requiring more visual details, probably for the benefits from their long-term experience with tools. Expertise, represented at higher levels of cognitive processing, might compensate the loss of lower levels of information processing due to aging.

A standard model for foveal detection of spatial contrast

The ModelFest data set was created to provide a public source of data to test and calibrate models of foveal spatial contrast detection. It consists of contrast thresholds for 43 foveal achromatic contrast stimuli collected from each of 16 observers. We have fit these data with a variety of simple models that include one of several contrast sensitivity functions, an oblique effect, a spatial sensitivity aperture, spatial frequency channels, and nonlinear Minkowski summation. While we are able to identify one model, with particular parameters, as providing the lowest overall residual error, we also note that the differences among several good-fitting models are small. We find a strong reciprocity between the size of the spatial aperture and the value of the summation exponent: both are effective means of limiting the extent of spatial summation. The results demonstrate the power of simple models to account for the visibility of a wide variety of spatial stimuli and suggest that special mechanisms to deal with special classes of stimuli are not needed. But the results also illustrate the limited power of even this large data set to distinguish among similar competing models. We identify one model as a possible standard, suitable for simple theoretical and applied predictions.

Noise masking reveals channels for second-order letters

We investigate the channels underlying identification of second-order letters using a critical-band masking paradigm. We find that observers use a single 1–1.5 octave-wide channel for this task. This channel’s best spatial frequency (c/letter) did not change across different noise conditions (indicating the inability of observers to switch channels to improve signal-to-noise ratio) or across different letter sizes (indicating scale invariance), for a fixed carrier frequency (c/letter). However, the channel’s best spatial frequency does change with stimulus carrier frequency (both in c/letter); one is proportional to the other. Following Majaj et al. (Majaj, N. J., Pelli, D. G., Kurshan, P., & Palomares, M. (2002). The role of spatial frequency channels in letter identification. Vision Research, 42, 1165–1184), we define “stroke frequency” as the line frequency (strokes/deg) in the luminance image. That is, for luminance-defined letters, stroke frequency is the number of lines (strokes) across each letter divided by letter width. For second-order letters, letter texture stroke frequency is the number of carrier cycles (luminance lines) within the letter ink area divided by the letter width. Unlike the nonlinear dependence found for first-order letters (implying scale-dependent processing), for second-order letters the channel frequency is half the letter texture stroke frequency (suggesting scale-invariant processing).

The effects of amplitude-spectrum statistics on foveal and peripheral discrimination of changes in natural images, and a multi-resolution model

Psychophysical thresholds were measured for discriminating small changes in spatial features of naturalistic scenes (morph sequences), for foveal and peripheral vision, and under M-scaling. Sensitivity was greatest for scenes with near natural Fourier amplitude slope, perhaps implying that human vision is optimised for natural scene statistics. A low-level model calculated differences in local contrast between pairs of images within a few spatial frequency channels with bandwidth like neurons in V1. The model was “customised” to each observer’s contrast sensitivity function for sinusoidal gratings, and it could replicate the “U-shaped” relationships between discrimination threshold and spectral slope, and many differences between picture sets and observers. A single-channel model and an ideal-observer analysis both failed to capture the U-shape.

Retinotopic sensitisation to spatial scale: Evidence for flexible spatial frequency processing in scene perception

Observers can use spatial scale information flexibly depending on categorisation task and on their prior sensitisation. Here, we explore whether attentional modulation of spatial frequency processing at early stages of visual analysis may be responsible. In three experiments, we find that observers’ perception of spatial frequency (SF) band-limited scene stimuli is determined by the SF content of images previously experienced at that location during a sensitisation phase. We conclude that these findings are consistent with the involvement of relatively early, retinotopically mapped, stages of visual analysis, supporting the attentional modulation of spatial frequency channels account of sensitisation effects.