Mesoscale Organization of Ventral and Dorsal Visual Pathways in Macaque Monkey Revealed by 7T fMRI

In the journal Progress in Neurobiology, a research team from the Interdisciplinary Institute of Neuroscience and Technology and the Department of Neurosurgery at the Second Hospital in Zhejiang University published a paper titled “Mesoscale organization of ventral and dorsal visual pathways in macaque monkey revealed by 7T fMRI”. This paper reveals, for the first time, the complex mesoscale organizational patterns in the ventral and dorsal visual pathways of macaque monkeys and elucidates the coordination of these functional domains when processing a single visual stimulus.

Background Introduction

The visual system is divided into the ventral pathway and the dorsal pathway, as defined by the early model proposed by Mishkin and his colleagues (Ungerleider & Mishkin, 1982; Mishkin et al., 1983). The ventral pathway primarily processes visual information necessary for object recognition, such as color, brightness, and shape, whereas the dorsal pathway deals with visual information related to self-navigation and self-motion, including the coordination of hand movements, such as how to catch an approaching ball at a specific direction and speed. Although both ventral and dorsal pathways are sensitive to visual cues such as motion and gaps, they distinguish themselves in different functions.

Research Origin

The main authors of this paper include Jianbao Wang, Xiao Du, Songping Yao, Lihui Li, Hisashi Tanigawa, Xiaotong Zhang, and Anna Wang Roe, all from Zhejiang University. They published this study in the Progress in Neurobiology journal, with the online publication date being February 1, 2024.

Research Methodology and Experimental Procedure

To explore the visual network of macaques, the research team developed an ultra-high-field (7T) fMRI method, enabling simultaneous mapping in single macaques to detect the responses of two sets of visual pathways to simple color and motion stimuli. The specific experimental procedure is as follows:

Experimental Steps

1. Technical Challenges and Animal Preparation: To enhance the signal-to-noise ratio (SNR) and achieve high spatial resolution, the team used a custom-made 16-channel radiofrequency (RF) coil in combination with a 7T magnet and multi-array RF coils. All animals (two adult female macaques) were anesthetized and intubated to a ventilator before the experiment to ensure stable scanning conditions.

2. Visual Stimulation and Data Acquisition: Low temporal frequency color drifting gratings and high temporal frequency achromatic drifting gratings were used to elicit specific responses. The complete visual stimulus was displayed on a projection screen, lasting 12 seconds with a 24-second interval of an equal brightness gray screen.

3. MRI Data Processing: Functional imaging was acquired using a gradient-echo EPI sequence, and whole-brain structural imaging was performed using a 3D T1-weighted MPRAGE sequence. For precise data analysis, FreeSurfer software was used to enhance the images, noise was canceled through radial smoothing, and spatial resolution was maintained.

Functional Domain Mapping at Mesoscale

By 7T fMRI, the team was able to identify and map functional domains in the two visual pathways, such as V2 stripes, V4 color and brightness processing domains, motion direction maps in MT and MST regions, and alternating color and motion direction domains in V3 and V3A regions.

V2 Stripes and Verification

The V2 region is known to contain “thin,” “pale,” and “thick” stripes arranged at periodic intervals. The fMRI mapping results showed the precise spatial location of these functional stripes, consistent with cytochrome oxidase staining images.

Mesoscale Organization of Motion Domains in the Dorsal Pathway

In the dorsal visual pathway, the study found that the functional domains in the MT and MST regions displayed response maps to motion direction, further supporting these regions’ ability to handle complex motion. Meanwhile, the V3d and V3A regions showed alternating color and motion domains, a spatial layout consistent with the organization of these regions in the human visual system.

Main Results and Significance

Scientific and Practical Value

This study not only reveals the complex functional organization of the macaque visual pathways but also enhances our understanding of how these pathways coordinate in processing visual information. It provides new insights into the functional basis of brain processes in visual cognition and action.

Highlights and Innovations

1. Innovative Method: By utilizing 7T fMRI combined with a 16-channel RF coil, the study achieved high spatial resolution and functional domain mapping across a wide field of view.
2. Mesoscale Organization Across Regions: This is the first time that the functional domain layout of multiple regions in the visual pathways of individual macaques has been demonstrated, offering new perspectives for understanding visual information processing.

Potential Applications

Future research can utilize these results to develop more precise brain stimulation technologies and brain-machine interface devices, further exploring and utilizing the neural basis of visual information processing. Additionally, the methods and findings demonstrated in this study have broad application potential for other neuroscience research, especially in studying complex brain networks and their functional connections. Overall, by revealing the detailed functional domain organization of the visual system, this study provides essential foundational data and research tools for scientists and clinicians.