Lateral modulation, divisive inhibition, and neural mechanism of perceptual filling-in

Lateral modulation refers to the phenomenon that the percept of a test stimulus can be modified by a surrounding pattern. Lateral modulation is ubiquitous throughout the visual system. Thus, understanding the underlying mechanism of lateral modulation can not only unveil the fundamental properties of visual system but also have the potential to increase our understanding of how eye diseases such as macular degeneration that leads to scotoma impact on visual perception. The purpose of this project is to study lateral modulation by investigating visual phenomena such as perceptual filling-in and orientation-specific lateral inhibition by means of psychophysics, computational modeling, and neuroimaging techniques. To address the missing pieces of this puzzling phenomenon, this project defines three goals: 1) To study multiple lateral modulation effects such as center-surround interaction and perceptual filling-in with new paradigms; 2) to analyze the observed lateral modulation effects with a computational model; and 3) to understand the neural mechanism underlying perceptual filling-in. Three studies have been conducted. Within the first two studies, we established an orientation adaption paradigm in which center-surround sinusoidal gratings are used as adapters to estimate the amount of tilt-aftereffect (TAE) induced onto the percept of the subsequent target. In Study 1, we selectively adapted the center, the surround, and both the center and surround regions and measured the tilt-aftereffect on the subsequently presented target. The TAE was the strongest in the center-only condition, intermediate in the center plus surround condition, and the weakest in the surround only condition. The difference between the center and both center and surround conditions indicated a lateral inhibition effect from the surround. Perceptual filling-in arose in the surround-only condition thus allowed us to investigate the filling-in effect. The TAE occurred even when no physical stimulus was presented at the target location during adaptation, and the TAE was more pronounced when filling-in was reported. In Study 2, we independently manipulated the adapter center and surround orientations and measured the TAE. We discovered that the lateral inhibition we found in Study 1 was orientation specific. We developed a divisive inhibition model that could explain both the adaptation effect and the lateral modulation effects in the empirical data of Study 1 and 2. In the third study, we implemented functional magnetic resonance imaging to study lateral modulation by presenting radial sinusoidal gratings that activates either the center, the surround, or both the center and surround regions in both left and right visual fields. When the surround pattern was added to the central pattern, the blood-oxygen level-dependent signal decreased in V1 to V3 regions, suggesting a lateral inhibition effect. The multivariate pattern analysis revealed that trained linear classifiers could differentiate between filling-in and non-filling-in trials, indicating that the neural activation pattern was different between the two percepts although the stimuli were the same. The current PhD project demonstrated effective paradigms that provided new evidence in lateral modulation in human vision. Our computational model captured both the adaptation and lateral modulation aspects in the data. The empirical findings and modeling results provide new evidence in the neural mechanism of perceptual filling-in. These paradigms and model have the potential to improve the understanding of how the brain adapts to eye diseases that could potentially lead to better detection techniques and rehabilitation programs

The role of lateral modulation in orientation-specific adaptation effect

Center-surround modulation in visual processing reflects a normalization process of contrast gain control in the responsive neurons. Prior adaptation to a clockwise (CW) tilted grating, for example, leads to the percept of counterclockwise tilt in a vertical grating, referred to as the tilt-aftereffect (TAE). We previously reported that the magnitude of the TAE is modulated by adding a same-orientation annular surround to an adapter, suggesting inhibitory lateral modulation. To further examine the property of this lateral modulation effect on the perception of a central target, we here used center-surround sinusoidal patterns as adapters and varied the adapter surround and center orientations independently. The target had the same spatial extent as the adapter center with no physical overlap with the adapter surround. Participants were asked to judge the target orientation as tilted either CW or counterclockwise from vertical after adaptation. Results showed that, when the surround orientation was held constant, the TAE magnitude was determined by the adapter center, peaking between 10° and 20° of tilt. More important, the adapter surround orientation modulated the adaptation effect such that the TAE magnitude first decreased and then increased as the surround orientation became increasingly more different from that of the center, suggesting that the surround modulation effect was indeed orientation specific. Our data can be accounted for by a divisive inhibition model, in which (1) the adaptation effect is represented by increasing the normalizing constant and (2) the surround modulation is captured by two multiplicative sensitivity parameters determined by the adapter surround orientation

Neural correlates of lateral modulation and perceptual filling-in in center-surround radial sinusoidal gratings: an fMRI study

We investigated lateral modulation effects with functional magnetic resonance imaging. We presented radial sinusoidal gratings in random sequence: a scotoma grating with two arc-shaped blank regions (scotomata) in the periphery, one in the left and one in the right visual field, a center grating containing pattern only in the scotoma regions, and a full-field grating where the pattern occupied the whole screen. On each trial, one of the three gratings flickered in counterphase for 10 s, followed by a blank period. Observers were instructed to perform a fixation task and report whether filling-in was experienced during the scotoma condition. The results showed that the blood-oxygen-level-dependent signal was reduced in areas corresponding to the scotoma regions in the full-field compared to the center condition in V1 to V3 areas, indicating a lateral inhibition effect when the surround was added to the center pattern. The univariate analysis results showed no difference between the filling-in and no-filling-in trials. However, multivariate pattern analysis results showed that classifiers trained on activation pattern in V1 to V3 could differentiate between filling-in and no-filling-in trials, suggesting that the neural activation pattern in visual cortex correlated with the subjective percept

Training-induced changes in population receptive field properties in visual cortex: Impact of eccentric vision training on population receptive field properties and the crowding effect

This study aimed to investigate the impact of eccentric-vision training on population receptive field (pRF) estimates to provide insights into brain plasticity processes driven by practice. Fifteen participants underwent functional magnetic resonance imaging (fMRI) measurements before and after behavioral training on a visual crowding task, where the relative orientation of the opening (gap position: up/down, left/right) in a Landolt C optotype had to be discriminated in the presence of flanking ring stimuli. Drifting checkerboard bar stimuli were used for pRF size estimation in multiple regions of interest (ROIs): dorsal-V1 (dV1), dorsal-V2 (dV2), ventral-V1 (vV1), and ventral-V2 (vV2), including the visual cortex region corresponding to the trained retinal location. pRF estimates in V1 and V2 were obtained along eccentricities from 0.5° to 9°. Statistical analyses revealed a significant decrease of the crowding anisotropy index (p = 0.009) after training, indicating improvement on crowding task performance following training. Notably, pRF sizes at and near the trained location decreased significantly (p = 0.005). Dorsal and ventral V2 exhibited significant pRF size reductions, especially at eccentricities where the training stimuli were presented (p < 0.001). In contrast, no significant changes in pRF estimates were found in either vV1 (p = 0.181) or dV1 (p = 0.055) voxels. These findings suggest that practice on a crowding task can lead to a reduction of pRF sizes in trained visual cortex, particularly in V2, highlighting the plasticity and adaptability of the adult visual system induced by prolonged training

Contour Erasure Filling-in

Here in our demos, you will see several examples of the fascinating contour erasure effect: objects completely disappear into the background or merge together after only a short adaptation period on their contours. Contour erasure, first discovered by Stuart Anstis in 2013, describes the phenomenon that low-contrast objects could completely disappear or be filled in by the background texture after being adapted to flickering contour outlines that match the objects edges. Such contour adaptation drastically speeds up the filling-in process, leading to an instantaneous filling-in percept. Such contour erasure effect reveals the importance of edge information in perceiving objects and surfaces as well as the mechanism underlying perceptual filling-in. Contact me: [email protected]

The role of color contrast gain control in global form perception

葛拉斯(Glass)圖形是由隨機散佈的「雙點 (dipole)」部件所組成，而這些雙點之間的排列遵照特定的幾何規則，不同的規則將決定該圖形的整體知覺。知覺一個葛拉斯圖形仰賴兩階段的處理機制，局部處理—將兩個點組合成雙點部件—以及整體處理—將這些局部的雙點部件整合成整體的圖形。有別於傳統的葛拉斯圖形，在我們的研究中我們使用局部由「三點 (tripole)」部件組成的葛拉斯圖形，以之研究色彩對比 (color contrast) 對於人類群聚視知覺 (visual grouping) 的影響。每個三點部件包含一個定錨點 (anchor dot) 與兩個周邊點 (context dot)，將定錨點與其中一個周邊點整合則葛拉斯圖形整體看起來會呈現為一個逆時針螺旋，與另一個周邊點整合則會看成順時針螺旋。在我們的色彩操弄中，每個三點葛拉斯圖形中的所有點色調 (hue) 一致，只有色彩對比有所不同。我們使用四種色調的刺激，對比根據+/-(L-M)以及+/-S主軸軸線變化。實驗參與者必須在看完每個三點葛拉斯圖形之後判斷該圖形是順時針還是逆時針的螺旋並按下相對應的反應按鍵。結果發現判斷圖形為順時針或逆時針螺旋的機率會隨著其中一個周邊點的色彩對比上升，直到超過特定的對比值後該機率又會下降；而整體的機率又會隨著另外一個周邊點的對比上升而下降。這樣的結果無法用過去提出的理論模型來解釋，而必須使用我們提出的「除法抑制模型(divisive inhibition model)」才能解釋資料變異。本研究得到的結果與之前操弄明暗對比(luminance contrast)類似，都有呈現倒U(inverted-U)的趨勢，相異之處在於色彩對比模型中的抑制部件較明暗對比來的弱。A Glass pattern consists of randomly distributed dot pairs, or dipoles, whose orientation is determined by a geometric transform, which defines the global percept perceived by an observer. The perception of Glass patterns involves a local process that associates dot pairs into dipoles and a global process that groups the dipoles into a global structure. In the present study, we used a variant of Glass patterns, which was composed of randomly distributed tripoles instead of dipoles, to estimate the influence of color contrast on perceptual grouping. Each tripole contained an anchor dot and two context dots. Grouping the anchor dot with one of the context dot would result in a global percept of a clockwise (CW) spiral while grouping with the other dot, a counterclockwise (CCW) spiral. All dots in each pattern were modulated in the same color direction but different contrasts. Four types of patterns were involved, namely modulating in +/-(L-M), and +/-S cardinal directions. The observers were to determine whether the spiral in each trial was CW or CCW. The probability of a context dot being grouped with the anchoring dot increased along with its color contrast to certain level before the probability started to drop. Our result cannot be explained by the existing models for perceptual grouping but a divisive inhibition model. The isoluminance contrast result observed is similar to the inverted U-shaped function for luminance contrast result previously reported (by us); except that color contrast model comprises a weaker self-inhibition component

The role of color contrast gain control in global form perception

A Glass pattern consists of randomly distributed dot pairs, or dipoles, whose orientation is determined by a geometric transform, which defines the global percept perceived by an observer. The perception of Glass patterns involves a local process that associates dot pairs into dipoles and a global process that groups the dipoles into a global structure. In the present study, we used a variant of Glass patterns, which was composed of randomly distributed tripoles instead of dipoles, to estimate the influence of color contrast on perceptual grouping. Each tripole contained an anchor dot and two context dots. Grouping the anchor dot with one of the context dots resulted in a global percept of a clockwise spiral, while grouping with the other dot, a counter-clockwise spiral. All dots in each pattern were modulated in the same color direction but different contrasts. Four colors were involved, namely, red, green, blue, and yellow. The observers were to determine whether the spiral in each trial was clockwise or counter-clockwise. The probability of a context dot being grouped with the anchoring dot increased with its color contrast to a certain level, then decreased when the contrast continued to increase. Such probability decreased as the contrast of the other context dot increased. Our result cannot be explained by existing models in the literature, but with a divisive inhibition model. The equiluminance contrast result observed here is similar to the inverted U-shaped function for luminance contrast result previously reported by us, except that the color contrast model comprises a weaker self-inhibition component