Multiple Spatial Frequency Channels Research Articles

A quasi-comprehensive exploration of the mechanisms of spatial working memory.

Why are some spatial patterns remembered more easily than others? There are many possible mechanisms underlying spatial working memory function. Here, the author explores different mechanisms simultaneously in a single conceptual model. He conducts a large-scale experiment (35.4 million responses used to measure human observers' spatial working memory across 80,000 patterns) and builds a convolutional neural network as a benchmark for what is expected to be explainable. The author then creates a quasi-comprehensive exploration model of spatial working memory based on classic concepts, as well as new notions, including spatial uncertainty, Bayesian integration, out-of-range responses, averaging, grouping, categorical memory, line detection, gap detection, blurring, lateral inhibition, chunking, multiple spatial-frequency channels, redundancy, response bias and random guess. This model provides a tentative overarching framework for the mechanisms of spatial working memory.

Retina inspired no-reference image quality assessment for blur and noise

A blind image quality assessment technique with no-training is proposed in this paper. The proposed technique considers two important types of distortions viz. noise and blur for quality estimation of an image. The technique is motivated by two significant phenomena of perception in the retina of an eye. First being the center-surround retinal receptive field and second, existence of multiple spatial frequency channels. Center-surround retinal receptive field in the proposed technique is modeled with the help of Difference of Gaussians (DoG). In retina, multiple spatial frequencies have been found and due to this, signals generated from center and surround fields exist at different frequencies. In order to mimic center-surround receptive field at multiple frequencies, we compute multiple DoG images at multiple standard deviation values generated for different frequencies. Further, two significant features based on entropy and edges are extracted from the obtained DoG images and are subsequently used to compute the quality of the image. The proposed technique does not require any training with distorted or pristine images; or subjective human score to predict quality of the image. We evaluate the proposed technique on LIVE, CSIQ and TID08 databases and observe that the obtained results match very well with human subjective opinions. The proposed technique outperforms the latest no-training, no-reference (NR) based image quality assessment techniques.

Lithium-Ion Battery Capacity Estimation: A Method Based on Visual Cognition

This study introduces visual cognition into Lithium-ion battery capacity estimation. The proposed method consists of four steps. First, the acquired charging current or discharge voltage data in each cycle are arranged to form a two-dimensional image. Second, the generated image is decomposed into multiple spatial-frequency channels with a set of orientation subbands by using non-subsampled contourlet transform (NSCT). NSCT imitates the multichannel characteristic of the human visual system (HVS) that provides multiresolution, localization, directionality, and shift invariance. Third, several time-domain indicators of the NSCT coefficients are extracted to form an initial high-dimensional feature vector. Similarly, inspired by the HVS manifold sensing characteristic, the Laplacian eigenmap manifold learning method, which is considered to reveal the evolutionary law of battery performance degradation within a low-dimensional intrinsic manifold, is used to further obtain a low-dimensional feature vector. Finally, battery capacity degradation is estimated using the geodesic distance on the manifold between the initial and the most recent features. Verification experiments were conducted using data obtained under different operating and aging conditions. Results suggest that the proposed visual cognition approach provides a highly accurate means of estimating battery capacity and thus offers a promising method derived from the emerging field of cognitive computing.

Integration of Multiple Spatial Frequency Channels in Disparity-Sensitive Neurons in the Primary Visual Cortex.

Our visual world is broadband, containing features with a wide range of object scales. On the other hand, single neurons in the primary visual cortex are narrow-band, being tuned narrowly for a specific scale. For robust visual perception, narrow-band information of single neurons must be integrated eventually at some stage. We have examined whether such an integration process begins in the primary visual cortex with respect to binocular processing. The results suggest that a subset of cells appear to combine binocular information across multiple scales. Furthermore, for the majority of these neurons, an optimal parameter of binocular tuning is matched across multiple scales, suggesting the presence of a highly specific neural integration mechanism.

Multiple Spatial Frequency Channels in Human Visual Perceptual Memory

Current models of short-term visual perceptual memory invoke mechanisms that are closely allied to low-level perceptual discrimination mechanisms. The purpose of this study was to investigate the extent to which human visual perceptual memory for spatial frequency is based upon multiple, spatially tuned channels similar to those found in the earliest stages of visual processing. To this end we measured how performance on a delayed spatial frequency discrimination paradigm was affected by the introduction of interfering or ‘memory masking’ stimuli of variable spatial frequency during the delay period. Masking stimuli were shown to induce shifts in the points of subjective equality (PSE) when their spatial frequencies were within a bandwidth of 1.2 octaves of the reference spatial frequency. When mask spatial frequencies differed by more than this value, there was no change in the PSE from baseline levels. This selective pattern of masking was observed for different spatial frequencies and demonstrates the existence of multiple, spatially tuned mechanisms in visual perceptual memory. Memory masking effects were also found to occur for horizontal separations of up to 6 deg between the masking and test stimuli and lacked any orientation selectivity. These findings add further support to the view that low-level sensory processing mechanisms form the basis for the retention of spatial frequency information in perceptual memory. However, the broad range of transfer of memory masking effects across spatial location and other dimensions indicates more long range, long duration interactions between spatial frequency channels that are likely to rely contributions from neural processes located in higher visual areas.

Blur clarified: A review and synthesis of blur discrimination

Blur is an important attribute of human spatial vision, and sensitivity to blur has been the subject of considerable experimental research and theoretical modeling. Often, these models have invoked specialized concepts or mechanisms, such as intrinsic blur, multiple spatial frequency channels, or blur estimation units. In this paper, we review the several experimental studies of blur discrimination and find that they are in broad empirical agreement. However, contrary to previous modeling efforts, we find that specialized mechanisms are not required and that the essential features of blur discrimination are fully accounted for by a visible contrast energy (ViCE) model, in which two spatial patterns are distinguished when the integrated difference between their masked local visible contrast energy responses reaches a threshold value. In the ViCE model, intrinsic blur is represented by the high-frequency limb of the contrast sensitivity function, but the low-frequency limb also contributes to the predictions for large reference blurs, and the model includes masking, which improves predictions for high-contrast stimuli.

Multiple spatial frequency channels in human visual perceptual memory

Current models of short-term visual perceptual memory invoke mechanisms that are closely allied to low-level perceptual discrimination mechanisms. The purpose of this study was to investigate the extent to which human visual perceptual memory for spatial frequency is based upon multiple, spatially tuned channels similar to those found in the earliest stages of visual processing. To this end we measured how performance on a delayed spatial frequency discrimination paradigm was affected by the introduction of interfering or ‘memory masking’ stimuli of variable spatial frequency during the delay period. Masking stimuli were shown to induce shifts in the points of subjective equality (PSE) when their spatial frequencies were within a bandwidth of 1.2 octaves of the reference spatial frequency. When mask spatial frequencies differed by more than this value, there was no change in the PSE from baseline levels. This selective pattern of masking was observed for different spatial frequencies and demonstrates the existence of multiple, spatially tuned mechanisms in visual perceptual memory. Memory masking effects were also found to occur for horizontal separations of up to 6 deg between the masking and test stimuli and lacked any orientation selectivity. These findings add further support to the view that low-level sensory processing mechanisms form the basis for the retention of spatial frequency information in perceptual memory. However, the broad range of transfer of memory masking effects across spatial location and other dimensions indicates more long range, long duration interactions between spatial frequency channels that are likely to rely contributions from neural processes located in higher visual areas.

Multiple channels for horizontal, but only one for vertical corrugations? A new look at the stereo anisotropy

Stereo vision displays a well-known anisotropy: disparity-defined slant is easier to detect for rotations about a horizontal axis than about a vertical axis, and low-frequency sinusoidal depth corrugations are easier to detect when the corrugations are horizontal than when they are vertical. Here, we determined disparity thresholds for vertically and horizontally oriented depth corrugations with both sinusoidal and square-wave profiles. We found that the orientation anisotropy for square waves is much weaker than for sine waves and is almost independent of frequency. This weaker anisotropy for square waves can be explained by considering the Fourier harmonics present in the stimulus. Using linear models imported from the luminance and texture perception domain, the disparity thresholds for square waves can be very well predicted from those for sine waves, for both horizontally and vertically oriented corrugations. For horizontally oriented corrugations, models based on the root mean square of the output of a single linear channel or the output of multiple linear channels worked equally well. This is consistent with previous evidence suggesting that stereo vision has multiple channels tuned to different spatial frequencies of horizontally oriented disparity modulations. However, for vertically oriented corrugations, only the root mean squared output of a single linear channel explained the data. We suggest that the stereo anisotropy may arise because the stereo system possesses multiple spatial frequency channels for detecting horizontally oriented modulations in horizontal disparity, but only one for vertically oriented modulations.

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.

Independent processing across spatial frequency in moving broadband patterns.

The aim of the experiments was to discover whether the visual system has independent access to motion information at different spatial scales when presented with a broadband stimulus. Subjects were required to discriminate between a pair of two-frame motion sequences, one containing a coherently displacing pattern and the other containing a pattern with high-frequency noise. The stimuli were either narrowband (1 octave) or broadband (6 octaves spanning 0.23-15.0 cycles deg-1) and their power spectra were either flat or followed a 1/f2 function. For the broadband stimuli, noise was introduced cumulatively into increasingly lower frequencies. For the narrowband stimuli, noise was introduced into the same frequency band as the signal. All stimuli could be defined by the lowest noise frequency (nl) they contained. For each stimulus, the largest spatial displacement across the two frames at which the task could be performed was measured (dmax). For the narrowband stimuli, dmax increased as nl was lowered. This was true over the entire frequency range for the 1/f2 stimuli, though the task became impossible for the flat-spectrum stimuli at the lowest frequencies. This is attributed to the very low contrast of these latter stimuli. The dmax values for the broadband stimuli tended to shadow those of the narrowband stimuli with the equivalent values of nl being around 25% lower. The data were modelled by spatiotemporally filtering the stimuli and considering the amount of directional power in the signal and noise sequences. The results suggest that there must be multiple spatial-frequency channels in operation, and that for broadband patterns the visual system has perceptual access to these individual channel outputs, utilising different filters depending on the task requirements.

Motion Transparency in Superimposed Dense Random-Dot Patterns: Psychophysics and Simulation

To determine the mechanism underlying motion transparency, representative examples of motion transparency are listed and possible mechanisms are suggested. Those are feature tracking, multiple spatial-frequency channels, luminance-based transparency rules, and motion energy. Next, an interesting stimulus for motion transparency is introduced, namely superimposed dense random-dot patterns, which is not explained by feature tracking or multiple spatial-frequency channels. A psychophysical experiment reveals that the occurrence of motion transparency in this stimulus depends on three luminance levels assigned to three possible combinations of component dots: (1) white dots superimposed upon white dots, (2) white dots upon black dots, and (3) black dots upon black dots. However, physical rules of luminance-based transparency fail to explain the results. Finally, a computer simulation reveals that a computational model based on motion energy quantitatively predicts the human psychophysical performance. All the results support the idea that motion-energy detection followed by spatial integration is a likely candidate for the mechanism underlying motion transparency.

Effects of Contrast on Perceived Size of Visual Patterns: Theory, Data, and Implications

The perceived spatial frequency of a visual pattern can vary with changes in contrast. Because size is inversely related to spatial frequency and because perceived size is an important distance cue, this has implications for task performance in a variable contrast environment. These environments are common in everyday situations, such as driving in the fog, and in the use of night vision devices. Understanding the underlying visual mechanisms of this effect would help us design systems that compensate for the effect. This understanding also could further develop models of the human low-level visual processes. However, most testing of perceived size and contrast has been done at relatively high contrast levels. This research is conducted at contrast levels near detection threshold. This range allows a more thorough testing of different models of contrast detection. We tested two versions of a multiple spatial-frequency channels model of contrast perception. One model assumes a single set of channels functioning throughout the dynamic range of contrasts used here, the other assumes two sets of channels based on the parvo and magnocellular systems. Our results support the hypothesis that there is a single set of channels at work in the contrast range tested.

The effect of luminance decrements upon the detection of sinusoidal gratings.

Observers detected 20-msec sinusoidal gratings presented at various asynchronies relative to a decrement in the mean luminance of a uniform field. Brief luminance decrements have no effect upon contrast thresholds determined with high-spatial-frequency gratings. With low-spatial-frequency gratings, transient threshold elevations were found at decrement onset and offset. For longer decrements, threshold changes were found for both high- and low-spatial-frequency gratings. However, the time courses of these changes differed. With high-spatial-frequency gratings, thresholds changed more gradually, whereas with low-spatial-frequency gratings, the time courses contained marked transients at the onset and offset of the decrement. These results are discussed in terms of contribution of multiple spatial-frequency channels to temporal resolution.

Temporal sensitivities to square-wave gratings, sawtooth-wave gratings and their fundamentals: More evidence for multiple spatial frequency channels in human vision

New evidence is presented for the multiple spatial-frequency channel theory of the human visual system. Temporal sensitivities instead of the usual contrast sensitivities were measured to squarewave gratings, sawtooth-wave gratings and their fundamentals. No differences were found between them except between the temporal sensitivities to sawtooth-wave gratings and those to their fundamentals in a low spatial frequency range. The results can be explained by the multiple channel theory.

Allocation of attention: uncertainty effects when monitoring one or two visual gratings of noncontiguous spatial frequencies.

When an observer is visually presented with a sinusoidal grating, he will often do worse in detecting a given grating when he is uncertain about its spatial frequency than when he is certain. Theoretical explanations of such uncertainty effects assume that the observer has attentional control over multiple spatial-frequency channels. This attentional control can be selectively allocated. If one grating is presented on most of the trials, randomly intermixed with trials of gratings of other spatial frequencies, an experienced observer will use a stationary single-band attention strategy. If two gratings, separated in spatial frequency by four octaves, are randomly presented on most of the intermixed trials, an experienced observer will use a more complex attention strategy; he can monitor the spatial frequencies of the two extreme stimuli with little or no monitoring of intermediate spatial frequencies.

Spatial frequency uncertainty effects in the detection of sinusoidal gratings

A sinusoidal grating is less detectable when it is randomly intermixed in a block of trials with gratings of other spatial frequencies than when it is the only grating presented in a block of trials. This spatial-frequency uncertainty effect is expected if observers have attentional control over multiple spatial-frequency channels. When intermixed blocks contain a preponderance of one spatial frequency, the primary, the uncertainty effect is smaller for frequencies near the primary than for frequencies further away. Both the tuning of the uncertainty effect and the analysis of sequential conditional probabilities suggest that observers can employ different attention strategies.