Neural Mechanism for Coding Depth from Motion Parallax in Area MT: Gain Modulation or Tuning Shifts?

There are two distinct sources of retinal image motion: objects moving in the world and observer movement. When the eyes move to track a target of interest, the retinal velocity of some object in the scene will depend on both eye velocity and that object's motion in the world. Thus, to compute the object's velocity relative to the head, a coordinate transformation must be performed by vectorially adding eye velocity and retinal velocity. In contrast, a very different interaction between retinal and eye velocity signals has been proposed to underlie estimation of depth from motion parallax, which involves computing the ratio of retinal and eye velocities. We examined how neurons in the middle temporal (MT) area of male macaques combine eye velocity and retinal velocity, to test whether this interaction is more consistent with a partial coordinate transformation (for computing head-centered object motion) or a multiplicative gain interaction (for computing depth from motion parallax). We find that some MT neurons show clear signatures of a partial coordinate transformation for computing head-centered velocity. Even a small shift toward head-centered velocity tuning can account for the observed depth-sign selectivity of MT neurons, including a strong dependence on speed preference that was previously unexplained. A formal model comparison reveals that the data from many MT neurons are equally well explained by a multiplicative gain interaction or a partial transformation toward head-centered tuning, although some responses can only be well fit by the coordinate transform model. Our findings shed new light on the neural computations performed in area MT, and raise the possibility that depth-sign selectivity emerges from a partial coordinate transformation toward representing head-centered velocity.SIGNIFICANCE STATEMENT Eye velocity signals modulate the responses of neurons in the middle temporal (MT) area to retinal image motion. Two different types of interactions between retinal and eye velocities have previously been considered: a vector addition computation for computing head-centered velocity, and a multiplicative gain interaction for computing depth from motion parallax. Whereas previous work favored a multiplicative gain interaction in MT, we demonstrate that some MT neurons show clear evidence of a partial shift toward coding head-centered velocity. Moreover, we demonstrate that even a small shift toward head coordinates is sufficient to account for the depth-sign selectivity observed previously in area MT, thus raising the possibility that a partial coordinate transformation may also provide a mechanism for computing depth from motion parallax.

Integration of Motion and Form Cues for the Perception of Self-Motion in the Human Brain.

When moving around in the world, the human visual system uses both motion and form information to estimate the direction of self-motion (i.e., heading). However, little is known about cortical areas in charge of this task. This brain-imaging study addressed this question by using visual stimuli consisting of randomly distributed dot pairs oriented toward a locus on a screen (the form-defined focus of expansion [FoE]) but moved away from a different locus (the motion-defined FoE) to simulate observer translation. We first fixed the motion-defined FoE location and shifted the form-defined FoE location. We then made the locations of the motion- and the form-defined FoEs either congruent (at the same location in the display) or incongruent (on the opposite sides of the display). The motion- or the form-defined FoE shift was the same in the two types of stimuli, but the perceived heading direction shifted for the congruent, but not for the incongruent stimuli. Participants (both sexes) made a task-irrelevant (contrast discrimination) judgment during scanning. Searchlight and ROI-based multivoxel pattern analysis revealed that early visual areas V1, V2, and V3 responded to either the motion- or the form-defined FoE shift. After V3, only the dorsal areas V3a and V3B/KO responded to such shifts. Furthermore, area V3B/KO shows a significantly higher decoding accuracy for the congruent than the incongruent stimuli. Our results provide direct evidence showing that area V3B/KO does not simply respond to motion and form cues but integrates these two cues for the perception of heading.SIGNIFICANCE STATEMENT Human survival relies on accurate perception of self-motion. The visual system uses both motion (optic flow) and form cues for the perception of the direction of self-motion (heading). Although human brain areas for processing optic flow and form structure are well identified, the areas responsible for integrating these two cues for the perception of self-motion remain unknown. We conducted fMRI experiments and used multivoxel pattern analysis technique to find human brain areas that can decode the shift in heading specified by each cue alone and the two cues combined. We found that motion and form cues are first processed in the early visual areas and then are likely integrated in the higher dorsal area V3B/KO for the final estimation of heading.

First report of Xanthomonas albilineans causing leaf scald on two chewing cane clones in Zhejiang province, China.

In recent years, outbreaks of leaf scald have been reported in two chewing cane clones "Guangdong Huangpi" and "Taoshang Guozhe" in Zhejiang province, China. From May to July 2019, we collected 11 and 13 leaf or stalk samples from symptomatic "Guangdong Huangpi" from four farms in Wenling and "Taoshang Guozhe" clones from three farms in Ruian, Zhejiang province, respectively. Leaves in young plants exhibited white pencil-line streaks (Supplement Fig. 1A & 1D) as well as partial or complete chlorosis of the leaf blade (Supplement Fig. 1B & 1E). Internal symptoms included an orange-red discoloration of the vascular bundles at the basal nodes of the stalk and discoloration extension into the internodes (Supplement Fig. 1C & 1F). Leaf and stalk tissues were used for bacterial isolation and purification on XAS medium, which is selective for Xanthomonas albilineans (Davis et al. 1994), using the streak plate method to obtain 24 isolates (Lin et al. 2018). Circular, convex, smooth, shiny and yellow colonies were isolated from all the samples. The pathogenicity of two isolates, XaCN30 from "Guangdong Huangpi" and XaCN43 from "Taoshang Guozhe", was confirmed with Koch's postulates according to the protocol reported by Lin et al. (2018). The incidences of diseased plants (56% and 63%) were observed in individual host clones at 28 d post-inoculation with isolates XaCN30 and XaCN43, respectively. Furthermore, all isolates were confirmed as X. albilineans via molecular methods. PCR amplification was conducted for all 24 isolates using the primer pairs XgyrB1F/XgyrB1R2 (Ntambo et al. 2019) and XAF1/XAR1 (Wang et al. 1999), which targeting the gyrB (encoding the b subunit of the DNA gyrase) and abc (encoding an ABC transporter) genes, and generating 904 bp and 608 bp amplicons, respectively. The PCR fragments were cloned into the pMD19-T vector (TaKaRa, Dalian, China). For each isolate, three single colonies of transformed Escherichia coli DH5α carrying targeted fragment were sequenced. These sequences were deposited into the GenBank with accession no. MT776053-MT776059 and MT776061-MT776077 for gyrB gene and MT776098-MT776104 and MT776106-MT776122 for abc gene. Based on the two concatenated DNA sequences of our 24 isolates, compared with 27 previously reported X. albilineans isolates obtained from the GenBank database, pairwise sequence identity analysis revealed that all 24 isolates from Zhejiang province had 99.4-100% identity with each other, 99.6-100% identity with 14 published domestic isolates, and 98.3-100% identity with 13 foreign isolates. Furthermore, phylogenetic analysis with MEGA 7.0 (Kumar et al. 2016) showed that the isolates from Zhejiang province clustered into two distinct groups (Supplement Fig. 2). One group consisted of 25 Chinese isolates (including all 11 isolates from Wenling) along with four isolates from the French West Indies (GPE PC73, GPE PC17, GPE PC86, and MTQ032), and one isolate from the USA (XaFL07-1), which were assigned to pulsed-field gel electrophoresis (PFGE) group B (Davis et al. 1997; Pieretti et al. 2012). A putative group was also proposed, which included all 13 isolates from Ruian, indicating that isolates from Ruian are distinct from the isolates isolated from other Chinese sugarcane-planting areas, including Wenling. We conclude that leaf scald disease in local clones of chewing cane are caused by X. albilineans in Zhejiang province in China, which will be helpful for leaf scald management in chewing cane, a cash crop.

The human visual system estimates angle features in an internal reference frame: A computational and psychophysical study.

Angle perception is an important middle-level visual process, combining line features to generate an integrated shape percept. Previous studies have proposed two theories of angle perception-a combination of lines and a holistic feature following Weber's law. However, both theories failed to explain the dual-peak fluctuations of the just-noticeable difference (JND) across angle sizes. In this study, we found that the human visual system processes the angle feature in two stages: first, by encoding the orientation of the bounding lines and combining them into an angle feature; and second, by estimating the angle in an orthogonal internal reference frame (IRF). The IRF model fits well with the dual-peak fluctuations of the JND that neither the theory of line combinations nor Weber's law can explain. A statistical image analysis of natural images revealed that the IRF was in alignment with the distribution of the angle features in the natural environment, suggesting that the IRF reflects human prior knowledge of angles in the real world. This study provides a new computational framework for angle discrimination, thereby resolving a long-standing debate on angle perception.