Motion-dependent representation of space in area MT+

Abstract

How is visual space represented in cortical area MT+? At a relatively coarse scale, the organization of MT+ is debated; retinotopic, spatiotopic, or mixed representations have all been proposed. However, none of these representations entirely explain the perceptual localization of objects at a fine spatial scale--a scale relevant for tasks like navigating or manipulating objects. For example, perceived positions of objects are strongly modulated by visual motion; stationary flashes appear shifted in the direction of nearby motion. Does spatial coding in MT+ reflect these shifts in perceived position? We performed an fMRI experiment employing this "flash-drag" effect and found that flashes presented near motion produced patterns of activity similar to physically shifted flashes in the absence of motion. This reveals a motion-dependent change in the neural representation of object position in human MT+, a process that could help compensate for perceptual and motor delays in localizing objects in dynamic scenes.

Figure 1. The flash-drag effect

Visual motion can change the perceived position of brief flashes presented nearby: they appear “dragged” in the direction of motion.

Figure 2. Stimulus and conditions in the fMRI experiment

(a) Gratings along the visual field meridians were oscillating between inward and outward motion, flashes were presented once per cycle in the space between the gratings. (b) IM and OM conditions both contained inward and outward motion, only the timing of the flashes relative to the phase of the oscillating motion changed. (c) In IS and OS conditions the physical position of flashes was shifted inward or outward by 1 bar width. See also Figure S1.

Figure 3. Multi-voxel pattern analysis strategy

(a) We calculated differences between flashes presented during inward and outward motion (IM – OM) and flashes in physical positions shifted inward and outward (IS – OS). Insets show maps for MT+ in one subject (color code in b). (b) These difference maps were correlated with each other for a ROI responding to the flashes within area MT+ (one subject shown; each point on the correlation plot is one voxel within MT+). A positive correlation signifies a similar representation of illusorily and physically shifted flashes. (c) Histogram of resulting correlation values for the same voxels after random shuffling of condition labels. See also Figure S2.

Figure 4. Results of the multi-voxel pattern analysis

Mean correlation scores (n = 6) are shown for ROIs in V1-V3A and MT+. Errorbars are bootstrapped 95%-confidence intervals. See also Figure S3.