Disruption of MT impairs motion processing

Facilitation and inhibition effects of anodal and cathodal tDCS over areas MT+ on the flash-lag effect

The perceived position of a moving object in vision entails an accumulation of neural signals over space and time. Due to neural signal transmission delays, the visual system can not acquire immediate information about the moving object's position. Although physiological and psychophysical studies on the flash-lag effect (FLE), a moving object is perceived ahead of a flash even they are aligned at the same location, have shown that the visual system develops the mechanisms of predicting the object's location to compensate for the neural delays, the neural mechanisms of motion-induced location prediction are not still understood well. Here, we investigated the role of neural activity changes in areas MT+ (specialized for motion processing) and the potential contralateral processing preference of MT+ in modulating the FLE. Using transcranial direct current stimulations (tDCS) over the left and right MT+ between pre-and post-tests of the FLE in different motion directions, we measured the effects of tDCS on the FLE. The results found that anodal and cathodal tDCS enhanced and reduced the FLE with the moving object heading to but not deviating from the side of the brain stimulated, respectively, compared to sham tDCS. These findings suggest a causal role of area MT+ in motion-induced location prediction, which may involve the integration of position information.

V5/MT+ modulates spatio-temporal integration differently across and within hemifields: Causal evidence from TMS

It is unclear how the brain reaches the correct balance between temporal and spatial processing necessary to perceive motion across space. Here, we tested whether visual motion area V5/MT + plays a causal role in Ternus illusion. Ternus displays can be perceived as showing either group motion or element motion and are empirically useful for dissociating temporal and spatial grouping across visual fields. Online single-pulse TMS was applied to observers during the presentation of Ternus displays, either within or across hemifields, over left V5/MT + or, respectively, a control site in the left somatosensory cortex, or an additional 'Sham' control condition. In the cross-hemifields condition, observers perceived more element motion with TMS over left V5/MT + than in either control condition. By contrast, in the within-hemifield condition, observers reported more group motion after left V5/MT + TMS. Our findings demonstrate a causal role of left V5/MT+ in the spatio-temporal grouping of Ternus apparent motion, and in maintaining the balance of spatio-temporal processing both within and across individual hemifields.

An enhanced role for right hV5/MT+ in the analysis of motion in the contra- and ipsi-lateral visual hemi-fields

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TMS applied to MT/TO-1 and MST/TO-2 disrupts translational motion.

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In the right hemisphere, disruption affects contra-and ipsi-lateral hemi-fields.

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In the left hemisphere, disruption is restricted to the contra-lateral hemi-field.

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Suggests enhanced role for right hemisphere in full-field motion perception.

Previous experiments have demonstrated that transcranial magnetic stimulation (TMS) of human V5/MT+, in either the left or right cerebral hemisphere, can induce deficits in visual motion perception in their respective contra- and ipsi-lateral visual hemi-fields. However, motion deficits in the ipsi-lateral hemi-field are greater when TMS is applied to V5/MT + in the right hemisphere relative to the left hemisphere. One possible explanation for this asymmetry might lie in differential stimulation of sub-divisions within V5/MT + across the two hemispheres. V5/MT + has two major sub-divisions; MT/TO-1 and MST/TO-2, the latter area contains neurons with large receptive fields (RFs) that extend up to 15° further into the ipsi-lateral hemi-field than the former. We wanted to examine whether applying TMS to MT/TO-1 and MST/TO-2 separately could explain the previously reported functional asymmetries for ipsi-lateral motion processing in V5/MT + across right and left cerebral hemispheres. MT/TO-1 and MST/TO-2 were identified in seven subjects using fMRI localisers. In psychophysical experiments subjects identified the translational direction (up/down) of coherently moving dots presented in either the left or right visual field whilst repetitive TMS (25 Hz; 70%) was applied synchronously with stimulus presentation. Application of TMS to MT/TO-1 and MST/TO-2 in the right hemisphere affected translational direction discrimination in both contra-lateral and ipsi-lateral visual fields. In contrast, deficits of motion perception following application of TMS to MT/TO-1 and MST/TO-2 in the left hemisphere were restricted to the contra-lateral visual field. This result suggests an enhanced role for the right hemisphere in processing translational motion across the full visual field.

Reevaluating hMT+ and hV4 functional specialization for motion and static contrast using fMRI-guided repetitive transcranial magnetic stimulation

Although visual areas hMT+ and hV4 are considered to have segregated functions for the processing of motion and form within dorsal and ventral streams, respectively, more recent evidence favors some functional overlap. Here we use fMRI-guided online repetitive transcranial magnetic stimulation (rTMS) to test two associated hypotheses: that area hV4 is causally involved in the perception of motion and hMT+ in the perception of static form. We use variations of a common global stimulus to test two dynamic motion-based tasks and two static form-based tasks in ipsilateral and contralateral visual fields. We find that rTMS to both hMT+ and hV4 significantly impairs direction discrimination and causes a perceptual slowing of motion, implicating hV4 in motion perception. Stimulation of hMT+ impairs motion in both visual fields, implying that disruption to one hMT+ disrupts the other with both needed for optimal performance. For the second hypothesis, we find the novel result that hV4 stimulation markedly reduces perceived contrast of a static stimulus. hMT+ stimulation also produces an effect, implicating it in static contrast perception. Our findings are the first to show that rTMS of hV4 can produce a large perceptual effect and, taken together, suggest a less rigid functional segregation between hMT+ and hV4 than previously thought.

Stochastic resonance enhances the rate of evidence accumulation during combined brain stimulation and perceptual decision-making

Perceptual decision-making relies on the gradual accumulation of noisy sensory evidence. It is often assumed that such decisions are degraded by adding noise to a stimulus, or to the neural systems involved in the decision making process itself. But it has been suggested that adding an optimal amount of noise can, under appropriate conditions, enhance the quality of subthreshold signals in nonlinear systems, a phenomenon known as stochastic resonance. Here we asked whether perceptual decisions made by human observers obey these stochastic resonance principles, by adding noise directly to the visual cortex using transcranial random noise stimulation (tRNS) while participants judged the direction of coherent motion in random-dot kinematograms presented at the fovea. We found that adding tRNS bilaterally to visual cortex enhanced decision-making when stimuli were just below perceptual threshold, but not when they were well below or above threshold. We modelled the data under a drift diffusion framework, and showed that bilateral tRNS selectively increased the drift rate parameter, which indexes the rate of evidence accumulation. Our study is the first to provide causal evidence that perceptual decision-making is susceptible to a stochastic resonance effect induced by tRNS, and to show that this effect arises from selective enhancement of the rate of evidence accumulation for sub-threshold sensory events.

A Role for the Left Angular Gyrus in Episodic Simulation and Memory

Functional magnetic resonance imaging (fMRI) studies indicate that episodic simulation (i.e., imagining specific future experiences) and episodic memory (i.e., remembering specific past experiences) are associated with enhanced activity in a common set of neural regions referred to as the core network. This network comprises the hippocampus, medial prefrontal cortex, and left angular gyrus, among other regions. Because fMRI data are correlational, it is unknown whether activity increases in core network regions are critical for episodic simulation and episodic memory. In the current study, we used MRI-guided transcranial magnetic stimulation (TMS) to assess whether temporary disruption of the left angular gyrus would impair both episodic simulation and memory (16 participants, 10 females). Relative to TMS to a control site (vertex), disruption of the left angular gyrus significantly reduced the number of internal (i.e., episodic) details produced during the simulation and memory tasks, with a concomitant increase in external detail production (i.e., semantic, repetitive, or off-topic information), reflected by a significant detail by TMS site interaction. Difficulty in the simulation and memory tasks also increased after TMS to the left angular gyrus relative to the vertex. In contrast, performance in a nonepisodic control task did not differ statistically as a function of TMS site (i.e., number of free associates produced or difficulty in performing the free associate task). Together, these results are the first to demonstrate that the left angular gyrus is critical for both episodic simulation and episodic memory.SIGNIFICANCE STATEMENT Humans have the ability to imagine future episodes (i.e., episodic simulation) and remember episodes from the past (i.e., episodic memory). A wealth of neuroimaging studies have revealed that these abilities are associated with enhanced activity in a core network of neural regions, including the hippocampus, medial prefrontal cortex, and left angular gyrus. However, neuroimaging data are correlational and do not tell us whether core regions support critical processes for simulation and memory. In the current study, we used transcranial magnetic stimulation and demonstrated that temporary disruption of the left angular gyrus leads to impairments in simulation and memory. The present study provides the first causal evidence to indicate that this region is critical for these fundamental abilities.

Global processing in amblyopia: a review

Amblyopia is a neurodevelopmental disorder of the visual system that is associated with disrupted binocular vision during early childhood. There is evidence that the effects of amblyopia extend beyond the primary visual cortex to regions of the dorsal and ventral extra-striate visual cortex involved in visual integration. Here, we review the current literature on global processing deficits in observers with either strabismic, anisometropic, or deprivation amblyopia. A range of global processing tasks have been used to investigate the extent of the cortical deficit in amblyopia including: global motion perception, global form perception, face perception, and biological motion. These tasks appear to be differentially affected by amblyopia. In general, observers with unilateral amblyopia appear to show deficits for local spatial processing and global tasks that require the segregation of signal from noise. In bilateral cases, the global processing deficits are exaggerated, and appear to extend to specialized perceptual systems such as those involved in face processing.

Global motion perception in 2-year-old children: a method for psychophysical assessment and relationships with clinical measures of visual function

PURPOSE

We developed and validated a technique for measuring global motion perception in 2-year-old children, and assessed the relationship between global motion perception and other measures of visual function.

METHODS

Random dot kinematogram (RDK) stimuli were used to measure motion coherence thresholds in 366 children at risk of neurodevelopmental problems at 24 ± 1 months of age. RDKs of variable coherence were presented and eye movements were analyzed offline to grade the direction of the optokinetic reflex (OKR) for each trial. Motion coherence thresholds were calculated by fitting psychometric functions to the resulting datasets. Test-retest reliability was assessed in 15 children, and motion coherence thresholds were measured in a group of 10 adults using OKR and behavioral responses. Standard age-appropriate optometric tests also were performed.

RESULTS

Motion coherence thresholds were measured successfully in 336 (91.8%) children using the OKR technique, but only 31 (8.5%) using behavioral responses. The mean threshold was 41.7 ± 13.5% for 2-year-old children and 3.3 ± 1.2% for adults. Within-assessor reliability and test-retest reliability were high in children. Children's motion coherence thresholds were significantly correlated with stereoacuity (LANG I & II test, ρ = 0.29, P < 0.001; Frisby, ρ = 0.17, P = 0.022), but not with binocular visual acuity (ρ = 0.11, P = 0.07). In adults OKR and behavioral motion coherence thresholds were highly correlated (intraclass correlation = 0.81, P = 0.001).

CONCLUSIONS

Global motion perception can be measured in 2-year-old children using the OKR. This technique is reliable and data from adults suggest that motion coherence thresholds based on the OKR are related to motion perception. Global motion perception was related to stereoacuity in children.

Why do adults with dyslexia have poor global motion sensitivity?

Two experiments aimed to determine why adults with dyslexia have higher global motion thresholds than typically reading controls. In Experiment 1, the dot density and number of animation frames presented in the dot stimulus were manipulated because of findings that use of a high dot density can normalize coherence thresholds in individuals with dyslexia. Dot densities were 14.15 and 3.54 dots/deg². These were presented for five (84 ms) or eight (134 ms) frames. The dyslexia group had higher coherence thresholds in all conditions than controls. However, in the high dot density, long duration condition, both reader groups had the lowest thresholds indicating normal temporal recruitment. These results indicated that the dyslexia group could sample the additional signals dots over space and then integrate these with the same efficiency as controls. In Experiment 2, we determined whether briefly presenting a fully coherent prime moving in either the same or opposite direction of motion to a partially coherent test stimulus would systematically increase and decrease global motion thresholds in the reader groups. When the direction of motion in the prime and test was the same, global motion thresholds increased for both reader groups. The increase in coherence thresholds was significantly greater for the dyslexia group. When the motion of the prime and test were presented in opposite directions, coherence thresholds were reduced in both groups. No group threshold differences were found. We concluded that the global motion processing deficit found in adults with dyslexia can be explained by undersampling of the target motion signals. This might occur because of difficulties directing attention to the relevant motion signals in the random dot pattern, and not a specific difficulty integrating global motion signals. These effects are most likely to occur in the group with dyslexia when more complex computational processes are required to process global motion.

Memory for motion and spatial location is mediated by contralateral and ipsilateral motion processing cortex

Memory and perception have been associated with common sensory cortical activity. However, previous studies have only investigated memory and perception effects associated with a single feature (i.e., spatial location or color). The aim of the present functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) study was to assess whether memory for multiple (two) features would produce sensory cortical activity that mirrored perceptual processing of the same features. During encoding, moving or stationary abstract shapes were presented to the right or left of fixation. During retrieval, shapes were presented at fixation and participants classified each item as previously in motion or stationary within the right or left visual field. Memory for items in motion, regardless of spatial location, produced fMRI activity in perceptual motion processing region MT+. Memory for motion and spatial location produced contralateral and ipsilateral fMRI activity in perceptual motion processing sub-region MT. Following TMS to MT, memory for motion was impaired, but performance did not differ between the contralateral and ipsilateral visual fields. The present results are consistent with previous findings in that memory for motion produced fMRI activity in MT+ and was impaired following TMS to MT. However, memory for motion and spatial location produced contralateral and ipsilateral fMRI and TMS effects, deviating from the primarily contralateral perceptual processing organization of MT. The present evidence suggests that during memory for motion and spatial location only motion information is coded in motion processing cortex, while previous findings suggest spatial location information is coded in earlier extrastriate cortex.