Study on Orientation-Tuned Gain Control Mechanisms in the Visual Cortex

Academic Background

Visual perception is a complex neural process influenced by contextual environments. Surround suppression is an important phenomenon that describes how the perceived contrast of a stimulus is attenuated when surrounded by other stimuli. The neural mechanisms of this phenomenon have been extensively studied in animal electrophysiological recordings, showing that the neural response in the central receptive field is reduced when the central stimulus is surrounded by other stimuli. However, although it is known that the strength of surround suppression is influenced by the feature similarity between the center and surround stimuli, how this orientation-tuned suppression affects the gain control mechanisms in the visual cortex remains poorly understood.

This study aims to explore how orientation-tuned surround suppression affects contrast response functions (CRFs) in early visual cortex. Using functional magnetic resonance imaging (fMRI), the researchers measured the contrast responses in early visual cortex (V1-V3) when viewing full-field center-surround grating stimuli, further revealing the distribution of orientation-tuned suppression across the visual field and its impact on neural gain.

Source of the Paper

This paper was co-authored by Michaela Klímová, Ilona M. Bloem, and Sam Ling. Michaela Klímová is from the Department of Psychological and Brain Sciences and the Center for Systems Neuroscience at Boston University, Ilona M. Bloem is from the Department of Computational Cognitive Neuroscience and Neuroimaging at the Netherlands Institute for Neuroscience, and Sam Ling is from the Department of Psychological and Brain Sciences at Boston University. The paper was first published on January 7, 2025, in the Journal of Neurophysiology.

Research Process

1. Experimental Design and Stimulus Presentation

Researchers recruited 10 observers (8 female) aged between 18 and 35, all of whom reported normal or corrected-to-normal vision. In the experiment, participants were asked to view full-field center-surround grating stimuli. The contrast of the center stimulus varied between 2.67% and 96%, while the surround grating contrast remained at 100%. The orientation of the surround grating was either the same as the center stimulus (collinear) or orthogonal to it.

2. fMRI Data Acquisition

All fMRI data were collected at the Center for Cognitive Neuroimaging at Boston University using a Siemens 3T Prisma scanner with a 64-channel head coil. fMRI data were acquired using multiband acceleration technology (multiband acceleration factor 5) with a spatial resolution of 2 mm. Prior to the experiment, each participant also underwent a separate population receptive field mapping (pRF mapping) scan to determine the spatial preferences of their early visual cortex.

3. Data Analysis

The researchers first preprocessed the fMRI data, including motion correction, slice timing correction, and boundary-based registration. Next, the BOLD responses under each stimulus condition were estimated using a finite impulse response (FIR) model. To quantify the contrast response functions, the researchers fitted the CRFs in V1-V3 using the Naka-Rushton equation. This model describes the relationship between BOLD responses and stimulus contrast, allowing the researchers to compare CRF parameter changes under different conditions.

Key Findings

1. Contrast Response Functions Under Orientation-Tuned Suppression

The study found that when the surround grating was collinear with the center grating, the BOLD responses in early visual cortex were significantly weaker than when the surround grating was orthogonal to the center grating. This suppressive effect was most prominent in voxels near the center-surround boundary, while it was almost absent in voxels far from the boundary. This suggests that the orientation-tuned suppressive effect is localized in the visual field, primarily acting at the boundary between the center and surround stimuli.

2. Spatial Distribution of Contrast Response Functions

The researchers divided the visual field into multiple eccentricity bins and found that the strength of suppression gradually increased as it approached the center-surround boundary. In V1, voxels near the boundary showed the strongest suppressive effects, while in V2 and V3, this effect was even more pronounced. These results indicate that the orientation-tuned suppressive effects differ across visual areas, with V2 and V3 being more sensitive to suppression by collinear stimuli.

3. Naka-Rushton Model Parameter Analysis

Through the analysis of Naka-Rushton model parameters, the researchers found that in the collinear condition, the semi-saturation constant (c50) of the CRF was higher, indicating that the center stimulus required higher contrast to achieve the same level of neural response. However, these parameter changes did not reach statistical significance, likely due to the large variability in CRFs across individuals.

Conclusions and Significance

This study demonstrates that orientation-tuned surround suppression exhibits a localized gain control effect in early visual cortex, primarily affecting the boundary between the center and surround stimuli. This finding deepens our understanding of how the visual cortex processes complex visual scenes, particularly how interactions between stimuli with similar features optimize information processing. Additionally, the study highlights the special role of V2 and V3 in orientation-tuned suppression, providing new clues for further research on the functions of higher-order visual cortex.

Research Highlights

1. Localized Effects of Orientation-Tuned Suppression: The study found that the suppressive effects of orientation-tuned suppression primarily act at the boundary between the center and surround stimuli, rather than uniformly across the entire center stimulus area. This finding challenges the traditional assumption that surround suppression is uniformly distributed within the center stimulus area.

2. Special Role of Higher-Order Visual Cortex: The study suggests that V2 and V3 are more sensitive to suppression by collinear stimuli, indicating that higher-order visual cortex may play a more important role in feature-tuned gain control.

3. Application of the Naka-Rushton Model: By using the Naka-Rushton model, the researchers were able to quantify changes in contrast response functions, providing a quantitative tool for understanding gain control mechanisms in the visual cortex.

Additional Valuable Information

The researchers also discussed the role of contrast adaptation in modulating contrast response functions and suggested that future studies could further explore the interactions between adaptation mechanisms and orientation-tuned suppression. Additionally, the research data has been made publicly available, facilitating further cross-laboratory verification and analysis.

Summary

This study, using fMRI technology, revealed the spatial distribution of orientation-tuned surround suppression in early visual cortex and its modulation of contrast response functions. The findings not only deepen our understanding of visual gain control mechanisms but also provide new directions and tools for future visual neuroscience research.