The relatively small decline in orientation acuity as stimulus size decreases

Masking, crowding, and grouping: Connecting low and mid-level vision

An important task for vision science is to build a unitary framework of low- and mid-level vision. As a step on this way, our study examined differences and commonalities between masking, crowding and grouping—three processes that occur through spatial interactions between neighbouring elements. We measured contrast thresholds as functions of inter-element spacing and eccentricity for Gabor detection, discrimination and contour integration, using a common stimulus grid consisting of nine Gabor elements. From these thresholds, we derived a) the baseline contrast necessary to perform each task and b) the spatial extent over which task performance was stable. This spatial window can be taken as an indicator of field size, where elements that fall within a putative field are readily combined. We found that contrast thresholds were universally modulated by inter-element distance, with a shallower and inverted effect for grouping compared with masking and crowding. Baseline contrasts for detecting stimuli and discriminating their properties were positively linked across the tested retinal locations (parafovea and near periphery), whereas those for integrating elements and discriminating their properties were negatively linked. Meanwhile, masking and crowding spatial windows remained uncorrelated across eccentricity, although they were correlated across participants. This suggests that the computation performed by each type of visual field operates over different distances that co-varies across observers, but not across retinal locations. Contrast-processing units may thus lie at the core of the shared idiosyncrasies across tasks reported in many previous studies, despite the fundamental differences in the extent of their spatial windows.

Frequency ratio determines discrimination of concentric radial frequency patterns in the peripheral visual field

Using a radial frequency discrimination task that has not been tested in many previous studies, we examined the dependence of the pattern radius (4 to 16 deg) on the radial frequency thresholds of two different types of concentric radial frequency (RF) patterns: constant circular contour frequency (CCF) RF patterns with different radii, which have the constant physical length of modulation cycle in external real-world space, and constant radial frequency magnified RF patterns with different radii, which have the constant cortical length of modulation cycles. These two types RF patterns used as the reference stimuli had an equal maximum orientation difference from circularity regardless of change in radius. The discrimination threshold expressed by the frequency ratio between RF patterns of different frequencies vs. radius functions for the constant CCF RF patterns indicated different functional forms dependent on the modulation amplitude of the RF patterns. The thresholds increased with increasing pattern radius for small modulation amplitude RF patterns but were relatively flattened for large-amplitude RF patterns. This dependence was ascribed to the eccentricity effect wherein the deformation thresholds for discriminating the RF pattern from a circle increase with increasing stimulus eccentricity (Feng et al. 2020). The discrimination thresholds vs. radius functions for the magnified RF patterns were also flattened for different modulation amplitudes and frequencies. The thresholds (frequency ratio) were similar at all eccentricities. Cortical magnification neutralized the eccentricity effect observed for the constant CCF patterns.

Detection of small orientation changes and the precision of visual working memory

We investigated the precision of orientation representations with two tasks, change detection and recall. Previously change detection has been measured only with relatively large orientation changes compared to psychophysical thresholds. In the first experiment, we measured the observers' ability (d') to detect small changes in orientation (5-30°) with 1-4 Gabor items. With one item even a 10° change was well detected (average d'=2.5). As the amount of change increased to 30°, the d' increased to 5.2. When the number of items was increased, the d's gradually decreased. In the second experiment, we used a recall task and the observers adjusted the orientation of a probe Gabor to match the orientation of a Gabor held in the memory. The standard deviation (s.d.) of errors was calculated from the Gaussian distribution fitted to the data. As the number of items increased from 1 to 6, the s.d. increased from 8.6° to 19.6°. Even with six items, the observers did not make any random adjustments. The results show a square root relation between the d'/s.d. and the number of items. The d' in change detection is directly proportional to the square root of (1/n) and the orientation change. The increase of the s.d. in recall task is inversely proportional to square root of (1/n). The results suggest that limited resources and precision of representations, without additional assumptions, determine the memory performance.

Relationship between orientation sensitivity and spatiotemporal receptive field structures of neurons in the cat lateral geniculate nucleus

Although it is thought that orientation selectivity first emerges in the primary visual cortex, several studies have reported that neurons in the cat lateral geniculate nucleus (LGN) are sensitive to stimulus orientation, especially for high spatial frequency (SF) stimuli. To understand how this orientation sensitivity emerges, we investigated the spatiotemporal structures of linear receptive fields (RFs) of LGN neurons. Orientation tunings at several SFs were measured using sinusoidal drifting grating stimuli. Fine spatiotemporal structures of the linear RFs were measured using a reverse correlation technique and two-dimensional dynamic Gaussian white noise stimuli. A non-linear response modulation function was estimated by comparing measured responses with responses predicted from a linear RF structure. Although we found that a population of LGN neurons exhibited significantly elongated linear RF centers and that the angles of the long axes corresponded well to the preferred orientations, the orientation tunings predicted from the linear RFs were significantly broader than those measured. These results suggest that orientation-tuned non-linear response modulation induced by stimulation outside the classical RF contributes to the sharp orientation tuning seen in LGN neurons.

Contrast sensitivity of MT receptive field centers and surrounds

Neurons throughout the visual system have receptive fields with both excitatory and suppressive components. The latter are responsible for a phenomenon known as surround suppression, in which responses decrease as a stimulus is extended beyond a certain size. Previous work has shown that surround suppression in the primary visual cortex depends strongly on stimulus contrast. Such complex center-surround interactions are thought to relate to a variety of functions, although little is known about how they affect responses in the extrastriate visual cortex. We have therefore examined the interaction of center and surround in the middle temporal (MT) area of the macaque ( Macaca mulatta) extrastriate cortex by recording neuronal responses to stimuli of different sizes and contrasts. Our findings indicate that surround suppression in MT is highly contrast dependent, with the strongest suppression emerging unexpectedly at intermediate stimulus contrasts. These results can be explained by a simple model that takes into account the nonlinear contrast sensitivity of the neurons that provide input to MT. The model also provides a qualitative link to previous reports of a topographic organization of area MT based on clusters of neurons with differing surround suppression strength. We show that this organization can be detected in the gamma-band local field potentials (LFPs) and that the model parameters can predict the contrast sensitivity of these LFP responses. Overall our results show that surround suppression in area MT is far more common than previously suspected, highlighting the potential functional importance of the accumulation of nonlinearities along the dorsal visual pathway.

Patterns and trajectories in Williams Syndrome: the case of visual orientation discrimination

Williams Syndrome (WS) is a developmental disorder typified by deficits in visuospatial cognition. To understand the nature of this deficit, we characterized how people with WS perceive visual orientation, a fundamental ability related to object identification. We compared WS participants to typically developing children (3-6 years of age) and typical adults in an orientation discrimination task with four stimulus types (small circular, large circular, collinear elongated and parallel elongated gratings). We measured orientation discrimination thresholds and the proportion of orthogonal errors (i.e., mirror-image reversal errors). We evaluated how these metrics (1) are modulated by stimulus condition, and (2) vary with chronological or mental age. We found that orientation perception in WS is comparable to that of typically developing children. Orientation discrimination thresholds were better for elongated gratings than circular gratings across all participant groups. For large circular gratings, the proportion of orthogonal errors was disproportionately greater in WS participants and typically developing 3-6 year old children than in typical adults. Moreover, we found that the ability to judge orientation in WS improves with increasing mental age, but not chronological age. These results suggest that orientation discrimination in WS is developmentally arrested, as opposed to abnormal or delayed.

Orientation disparity: a cue for 3D orientation?

Orientation disparity, the difference in orientation that results when a texture element on a slanted surface is projected to the two eyes, has been proposed as a binocular cue for 3D orientation. Since orientation disparity is confounded with position disparity, neither behavioral nor neurophysiological experiments have successfully isolated its contribution to slant estimates or established whether the visual system uses it. Using a modified disparity energy model, we simulated a population of binocular visual cortical neurons tuned to orientation disparity and measured the amount of Fisher information contained in the activity patterns. We evaluated the potential contribution of orientation disparity to 3D orientation estimation and delimited the stimulus conditions under which it is a reliable cue. Our results suggest that orientation disparity is an efficient source of information about 3D orientation and that it is plausible that the visual system could have mechanisms that are sensitive to it. Although orientation disparity is neither necessary nor sufficient for estimating slant, it appears that it could be useful when combined with estimates from position disparity gradients and monocular perspective cues.

Orientation discrimination in human vision: Psychophysics and modeling

We evaluated orientation discrimination thresholds using an external noise paradigm. Stimuli were spatiotemporal Gaussian patches of 2D orientation noise band-pass filtered in Fourier domain. Orientation acuity was measured for various combinations of stimulus spatial bandwidth, spatial frequency, and size as a function of orientation bandwidths of the stimuli. Stimulus contrast was matched in multiples of detection threshold. Consistent with the idea that stimulus orientation bandwidth acts as a source of external noise, orientation discrimination thresholds increased monotonically in all conditions with stimulus bandwidth. To interpret these results quantitatively, we first fitted a variance summation model to the data and derived the internal orientation noise, relative sampling efficiency, and orientation tuning of the mechanism underlying orientation discrimination. Due to the equivocal biological nature of these parameters for orientation discrimination, we investigated, with a modeling approach, how neural detectors characterized by a broad orientation tuning may support orientation discrimination. We demonstrated using the ideal-observer theory that while linear models, based on either unimodal filtering or center-surround opponency, predict the monotonic relationship between orientation discrimination threshold and orientation noise, a nonlinear model incorporating a broadband divisive inhibition in the orientation domain is a better candidate due to its contrast invariance. This model, using broad and similar orientation tuning for its excitatory and inhibitory inputs, accounts for the acute orientation acuity of human vision and proves to be robust despite the variance found in natural stimuli.

Stimulation of non-classical receptive field enhances orientation selectivity in the cat

We have investigated how the nonclassical receptive field (nCRF) affects dynamic orientation selectivity of cells in the primary visual cortex (V1) in anaesthetized and paralysed cats using the reverse correlation method. We found that tuning to the orientation of the test stimulus depends on the size of the stimulation area. A significant sharpening of orientation tuning was induced by nCRF stimulation, with the magnitude of the effect increasing with the size of stimulation. The effect of the nCRF on the temporal dynamics of orientation tuning was also investigated by examining the tuning over a range of delays from stimulus onset. We found small but detectable changes in both the preferred orientation and the bandwidth of tuning over time when the classical receptive field (CRF) was stimulated alone. Stimulation in nCRF significantly increased the magnitude of these temporal changes. Thus, nCRF stimulation not only enhances the overall orientation selectivity, but also enriches the temporal dynamics of cortical neurones, which may increase the computational power of the visual cortex in information processing.

Effects of contrast and size on orientation discrimination

Motivated by the recent physiological finding that a neuron's receptive field can increase in size by a factor of 2-4-fold at low contrast [Nat. Neurosci. 2 (1999) 733, Proc. Natl. Acad. Sci. USA 96 (1999) 12073], we sought to examine whether a psychophysical task might reflect the contrast dependent changes in the size/structure of a receptive field. We postulate that since spatial summation is not contrast invariant, a task that relies on the spatial structure of a receptive field, such as orientation discrimination, should also be affected by changes in contrast. Previously, orientation discrimination thresholds have been reported to be roughly independent of the contrast of a stimulus for most of the visible range of contrasts [i.e. J. Neurophysiol. 57 (1987) 773, J. Opt. Soc. Am. 6 (1989) 713, Vis. Res. 30 (1990) 449, Vis. Res. 39 (1999) 1631]. Here, we found large improvements in orientation discrimination with contrast that were dependent on stimulus area. Furthermore, the apparent constancy of orientation discrimination for large area stimuli is possibly a result of a floor effect on the threshold. Therefore we conclude that there is not strong evidence for contrast invariant orientation discrimination. We interpret these results in the context of recent neurophysiological results about the expansion of cortical cells' receptive fields at low contrast.

Orientation discrimination in visual noise using global and local stimuli

We have investigated orientation discrimination in visual noise using two types of high contrast, broadband stimuli. Discrimination thresholds are better for Local stimuli, in which the orientation signal is spatially limited, than for Global stimuli, in which the orientation signal extends across the entire stimulus. Performance improves with increasing stimulus area, reaching an optimum threshold of about 11% orientation signal. Thresholds were not influenced by brief presentation times or practice. These results, along with results from a simple computational model, suggest that human orientation discrimination for this kind of pattern is mediated by pooling local responses of low-level neural mechanisms and is limited by two stages of intrinsic neural noise.

A psychophysical correlate of contrast dependent changes in receptive field properties

Recent physiological investigations have demonstrated that a neuron's area of spatial summation can vary depending on stimulus contrast. Specifically, when the same stimulus is presented to a neuron at a low contrast, the area of summation (or neuron's receptive field) can increase by at least a factor of two, compared to that estimated with a high contrast stimulus. We sought to examine this phenomenon psychophysically by using an orientation discrimination task carried out in the presence of contextual stimuli. We have found previously that orientation discrimination thresholds for a sine-wave grating are elevated by the presence of a surround pattern of similar orientation (with an offset) and spatial frequency. However, when these patterns were separated by a gap of mean luminance exceeding roughly 1 deg, thresholds dropped to the level measured using the center pattern alone. Here, we examined the surround pattern's effect on orientation thresholds as a function of the contrast of the center and surround. We find that when both are presented at a low contrast, the detrimental influence of the surround on orientation thresholds is maintained over larger gap separations. We also find that the spatial frequency and orientation selectivity of the surround's masking effect on orientation thresholds is broader at low contrast than at high contrast. Although the results support the idea of a spatial reorganization of the mechanisms involved in the task at low contrast, these changes are insufficient, in and of themselves, to account for the data. We suggest that additional influences possibly reflecting image segmentation also affect performance.