Impairment of the perception of second order motion but not first order motion in a patient with unilateral focal brain damage

Visual motion sensitivity as an indicator of diabetic retinopathy in type 2 diabetes mellitus

Objectives

This current study is based on a set of visual motion sensitivity tests, investigating the correlation between visual motion sensitivity and diabetic retinopathy (DR) in type 2 diabetes mellitus (T2DM), thereby furnishing a scientific rationale for preventing and controlling DR.

Methods

This research was conducted by a combination of questionnaire collection and on-site investigation that involved 542 T2DM recruited from a community. The visual motion sensitivity determined the visual motion perception of the participants across three spatial frequencies (low, medium, and high) for both the first- and second-order contrast. The logistic regression model was adopted to investigate the relationship between visual motion sensitivity and DR prevalence. Besides, the Pearson correlation analysis was used to analyze the factors influencing visual motion sensitivity and restricted cubic spline (RCS) functions to assess the dose-response relationship between visual motion sensitivity and glycated hemoglobin.

Results

Among 542 subjects, there are 162 cases of DR, with a prevalence rate of 29.89%. After adjusting factors of age, gender, glycated hemoglobin, duration of diabetes, BMI, and hypertension, we found that the decline in first- and second-order high spatial frequency sensitivity increased the risk for DR [odds ratio (OR): 1.519 (1.065, 2.168), 1.249 (1.068, 1.460)]. The decline in perceptual ability of second-order low, medium, and high spatial frequency sensitivity is a risk factor for moderate to severe DR [OR: 1.556 (1.116, 2.168), 1.388 (1.066, 1.806), 1.476 (1.139, 1.912)]. The first-order and the second-order high spatial frequency sensitivity are significantly positively correlated with glycated hemoglobin (r = 0.105, p = 0.015 and r = 0.119, p = 0.005, respectively).

Conclusion

Visual motion sensitivity especially for the second-order high spatial frequency stimuli emerges as a significant predictor of DR in T2DM, offering a sensitive diagnostic tool for early detection.

Motion perception and its disorders

As we live in a dynamic world, motion is a fundamental aspect of our visual experience. The advent of computerized stimuli has allowed controlled study of a wide array of motion phenomena, including global integration and segmentation, speed and direction discrimination, motion aftereffects, the optic flow that accompanies self-motion, perception of object form derived from motion cues, and point-light biological motion. Animal studies first revealed the existence of a motion-selective region, the middle temporal (MT) area, also known as V5, located in the lateral occipitotemporal cortex, followed by areas such as V5A (also known as MST, the middle superior temporal area), V6/V6A, the ventral intraparietal area, and others. In humans there are rare cases of bilateral lesions of the V5/V5A complex causing cerebral akinetopsia, a severe impairment of motion perception. Unilateral V5/V5A lesions are more common but cause milder asymptomatic deficits, often limited to the contralateral hemifield, while parietal lesions can impair perception of point-light biological motion or high-level motion tasks that are attentionally demanding. Impairments of motion perception have also been described in optic neuropathy, particularly glaucoma, as well as Alzheimer's disease, Parkinson's disease with dementia, and dementia with Lewy body disease. Prematurity with or without periventricular leukomalacia and developmental syndromes such as Williams' syndrome, autism, and dyslexia have also been associated with impaired motion perception, suggesting a developmental vulnerability of the dorsal pathway.

Differentiation of Theta Visual Motion from Fourier Motion Requires LC16 and R18C12 Neurons in Drosophila

Many animals perceive features of higher-order visual motion that are beyond the spatiotemporal correlations of luminance defined in first-order motion. Although the neural mechanisms of first-order motion detection have become understood in recent years, those underlying higher-order motion perception remain unclear. Here, we established a paradigm to assess the detection of theta motion—a type of higher-order motion—in freely walking Drosophila. Behavioral screening using this paradigm identified two clusters of neurons in the central brain, designated as R18C12, which were required for perception of theta motion but not for first-order motion. Furthermore, theta motion-activated R18C12 neurons were structurally and functionally located downstream of visual projection neurons in lobula, lobula columnar cells LC16, which activated R18C12 neurons via interactions of acetylcholine (ACh) and muscarinic acetylcholine receptors (mAChRs). The current study provides new insights into LC neurons and the neuronal mechanisms underlying visual information processing in complex natural scenes.

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Perception of theta motion requires LC16 and R18C12 neurons

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R18C12 neurons are activated by theta motion

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R18C12 neurons form synaptic connections with LC16 neurons

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LC16 neurons activate R18C12 neurons through ACh acting on mAChR

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.

Visual motion serves but is not under the purview of the dorsal pathway

Visual motion processing is often attributed to the dorsal visual pathway despite visual motion's involvement in almost all visual functions. Furthermore, some visual motion tasks critically depend on the structural integrity of regions outside the dorsal pathway. Here, based on numerous studies, I propose that visual motion signals are swiftly transmitted via multiple non-hierarchical routes to primary motion-dedicated processing regions (MT/V5 and MST) that are not part of the dorsal pathway, and then propagated to a multiplicity of brain areas according to task demands, reaching these regions earlier than the dorsal/ventral hierarchical flow. This not only places MT/V5 at the same or even earlier visual processing stage as that of V1, but can also elucidate many findings with implications to visual awareness. While the integrity of the non-hierarchical motion pathway is necessary for all visual motion perception, it is insufficient on its own, and the transfer of visual motion signals to additional brain areas is crucial to allow the different motion perception tasks (e.g. optic flow, visuo-vestibular balance, movement observation, dynamic form detection and perception, and even reading). I argue that this lateral visual motion pathway can be distinguished from the dorsal pathway not only based on faster response latencies and distinct anatomical connections, but also based on its full field representation. I also distinguish between this primary lateral visual motion pathway sensitive to all motion in the visual field, and a much less investigated optic flow sensitive medial processing pathway (from V1 to V6 and V6A) that appears to be part of the dorsal pathway. Multiple additional predictions are provided that allow testing this proposal and distinguishing between the visual pathways.

Visual mapping using blood oxygen level dependent functional magnetic resonance imaging

Functional magnetic resonance imaging (fMRI) is used clinically to map the visual cortex before brain surgery or other invasive treatments to achieve an optimal balance between therapeutic effect and the avoidance of postoperative vision deficits. Clinically optimized stimuli, behavioral task, analysis, and displays permit identification of cortical subregions supporting high-acuity central vision that is critical for reading and other essential visual functions. Emerging techniques such as resting-state fMRI may facilitate the use of fMRI-based vision mapping in a broader range of patients.

Luminance- and texture-defined information processing in school-aged children with autism

According to the complexity-specific hypothesis, the efficacy with which individuals with autism spectrum disorder (ASD) process visual information varies according to the extensiveness of the neural network required to process stimuli. Specifically, adults with ASD are less sensitive to texture-defined (or second-order) information, which necessitates the implication of several cortical visual areas. Conversely, the sensitivity to simple, luminance-defined (or first-order) information, which mainly relies on primary visual cortex (V1) activity, has been found to be either superior (static material) or intact (dynamic material) in ASD. It is currently unknown if these autistic perceptual alterations are present in childhood. In the present study, behavioural (threshold) and electrophysiological measures were obtained for static luminance- and texture-defined gratings presented to school-aged children with ASD and compared to those of typically developing children. Our behavioural and electrophysiological (P140) results indicate that luminance processing is likely unremarkable in autistic children. With respect to texture processing, there was no significant threshold difference between groups. However, unlike typical children, autistic children did not show reliable enhancements of brain activity (N230 and P340) in response to texture-defined gratings relative to luminance-defined gratings. This suggests reduced efficiency of neuro-integrative mechanisms operating at a perceptual level in autism. These results are in line with the idea that visual atypicalities mediated by intermediate-scale neural networks emerge before or during the school-age period in autism.

Dissociation of first- and second-order motion systems by perceptual learning

Previous studies investigating transfer of perceptual learning between luminance-defined (LD) motion and texture-contrast-defined (CD) motion tasks have found little or no transfer from LD to CD motion tasks but nearly perfect transfer from CD to LD motion tasks. Here, we introduce a paradigm that yields a clean double dissociation: LD training yields no transfer to the CD task, but more interestingly, CD training yields no transfer to the LD task. Participants were trained in two variants of a global motion task. In one (LD) variant, motion was defined by tokens that differed from the background in mean luminance. In the other (CD) variant, motion was defined by tokens that had mean luminance equal to the background but differed from the background in texture contrast. The task was to judge whether the signal tokens were moving to the right or to the left. Task difficulty was varied by manipulating the proportion of tokens that moved coherently across the four frames of the stimulus display. Performance in each of the LD and CD variants of the task was measured as training proceeded. In each task, training produced substantial improvement in performance in the trained task; however, in neither case did this improvement show any significant transfer to the nontrained task.

Lobula-specific visual projection neurons are involved in perception of motion-defined second-order motion in Drosophila

A wide variety of animal species including humans and fruit flies see second-order motion although they lack coherent spatiotemporal correlations in luminance. Recent electrophysiological recordings, together with intensive psychophysical studies, are bringing to light the neural underpinnings of second-order motion perception in mammals. However, where and how the higher-order motion signals are processed in the fly brain is poorly understood. Using the rich genetic tools available in Drosophila and examining optomotor responses in fruit flies to several stimuli, we revealed that two lobula-specific visual projection neurons, specifically connecting the lobula and the central brain, are involved in the perception of motion-defined second-order motion, independent of whether the second-order feature is moving perpendicular or opposite to the local first-order motion. By contrast, blocking these neurons has no effect on first-order and flicker-defined second-order stimuli in terms of response delay. Our results suggest that visual neuropils deep in the optic lobe and the central brain, whose functional roles in motion processing were previously unclear, may be specifically required for motion-defined motion processing.

Increasing stimulus size impairs first- but not second-order motion perception

As stimulus size increases, the direction of high-contrast moving stimuli becomes increasingly difficult to perceive. This counterintuitive effect, termed spatial suppression, is believed to reflect antagonistic center-surround interactions--mechanisms that play key roles in tasks requiring sensitivity to relative motion. It is unknown, however, whether second-order motion also exhibits spatial suppression. To test this hypothesis, we measured direction discrimination thresholds for first- and second-order stimuli of varying sizes. The results revealed increasing thresholds with increasing size for first-order stimuli but demonstrated no spatial suppression of second-order motion. This selective impairment of first-order motion predicts increasing predominance of second-order cues as stimulus size increases. We confirmed this prediction by utilizing compound stimuli that contain first- and second-order information moving in opposite directions. Specifically, we found that for large stimuli, motion perception becomes increasingly determined by the direction of second-order cues. Overall, our findings show a lack of spatial suppression for second-order stimuli, suggesting that the second-order system may have distinct functional roles, roles that do not require high sensitivity to relative motion.

Direction-selective patterns of activity in human visual cortex suggest common neural substrates for different types of motion

A sense of motion can be elicited by the movement of both luminance- and texture-defined patterns, what is commonly referred to as first- and second-order, respectively. Although there are differences in the perception of these two classes of motion stimuli, including differences in temporal and spatial sensitivity, it is debated whether common or separate direction-selective mechanisms are responsible for processing these two types of motion. Here, we measured direction-selective responses to luminance- and texture-defined motion in the human visual cortex by using functional MRI (fMRI) in conjunction with multivariate pattern analysis (MVPA). We found evidence of direction selectivity for both types of motion in all early visual areas (V1, V2, V3, V3A, V4, and MT+), implying that none of these early visual areas is specialized for processing a specific type of motion. More importantly, linear classifiers trained with cortical activity patterns to one type of motion (e.g., first-order motion) could reliably classify the direction of motion defined by the other type (e.g., second-order motion). Our results suggest that the direction-selective mechanisms that respond to these two types of motion share similar spatial distributions in the early visual cortex, consistent with the possibility that common mechanisms are responsible for processing both types of motion.

Sensitivity to first- and second-order drifting gratings in 3-month-old infants

In two experiments, we investigated 3-month-old infants' sensitivity to first- and second-order drifting gratings. In Experiment 1 we used forced-choice preferential looking with drifting versus stationary gratings to estimate depth modulation thresholds for 3-month-old infants and a similar task for a comparison group of adults. Thresholds for infants were more adult-like for second-order than first-order gratings. In Experiment 2, 3-month-olds dishabituated to a change in first-order orientation, but not to a change in direction of first- or second-order motion. Hence, results from Experiment 1 were likely driven by the perception of flicker rather than motion. Thus, infants' sensitivity to uniform motion is slow to develop and appears to be driven initially by flicker-sensitive mechanisms. The underlying mechanisms have more mature tuning for second-order than for first-order information.

Visual deficits in amblyopia constrain normal models of second-order motion processing

It is well established that amblyopes exhibit deficits in processing first-order (luminance-defined) patterns. This is readily manifest by measuring spatiotemporal sensitivity (i.e. the "window of visibility") to moving luminance gratings. However the window of visibility to moving second-order (texture-defined) patterns has not been systematically studied in amblyopia. To address this issue monocular modulation sensitivity (1/threshold) to first-order motion and four different varieties of second-order motion (modulations of either the contrast, flicker, size or orientation of visual noise) was measured over a five-octave range of spatial and temporal frequencies. Compared to normals amblyopes are not only impaired in the processing of first-order motion, but overall they exhibit both higher thresholds and a much narrower window of visibility to second-order images. However amblyopia can differentially impair the perception of some types of second-order motion much more than others and crucially the precise pattern of deficits varies markedly between individuals (even for those with the same conventional visual acuity measures). For the most severely impaired amblyopes certain second-order (texture) cues to movement in the environment are effectively invisible. These results place important constraints on the possible architecture of models of second-order motion perception in human vision.

Neuropsychological evidence for three distinct motion mechanisms

We describe psychophysical performance of two stroke patients with lesions in distinct cortical regions in the left hemisphere. Both patients were selectively impaired on direction discrimination in several local and global second-order but not first-order motion tasks. However, only patient FD was impaired on a specific bi-stable motion task where the direction of motion is biased by object similarity. We suggest that this bi-stable motion task may be mediated by a high-level attention or position based mechanism indicating a separate neurological substrate for a high-level attention or position-based mechanism. Therefore, these results provide evidence for the existence of at least three motion mechanisms in the human visual system: a low-level first- and second-order motion mechanism and a high-level attention or position-based mechanism.

Two mechanisms for optic flow and scale change processing of looming

The detection of looming, the motion of objects in depth, underlies many behavioral tasks, including the perception of self-motion and time-to-collision. A number of studies have demonstrated that one of the most important cues for looming detection is optic flow, the pattern of motion across the retina. Schrater et al. have suggested that changes in spatial frequency over time, or scale changes, may also support looming detection in the absence of optic flow (P. R. Schrater, D. C. Knill, & E. P. Simoncelli, 2001). Here we used an adaptation paradigm to determine whether the perception of looming from optic flow and scale changes is mediated by single or separate mechanisms. We show first that when the adaptation and test stimuli were the same (both optic flow or both scale change), observer performance was significantly impaired compared to a dynamic (non-motion, non-scale change) null adaptation control. Second, we found no evidence of cross-cue adaptation, either from optic flow to scale change, or vice versa. Taken together, our data suggest that optic flow and scale changes are processed by separate mechanisms, providing multiple pathways for the detection of looming.

No interaction of first- and second-order signals in the extraction of global-motion and optic-flow

Edwards and Badcock (Vision Research 35, 2589, 1995) argued for independent first-order (FO) and second-order (SO) motion systems up to and including the global-motion level. That study used luminance (which they called FO) and contrast (SO) modulated dots. They found that SO noise dots did not mask signal extraction with luminance increment dots while luminance increment dots did mask SO signal extraction. However, they argued this asymmetry was not due to a combined FO-SO pathway, but rather due to the fact that the luminance-modulated dots, being also local variations in contrast, are both FO and SO stimuli. We test their claim of FO and SO independence by using a stimulus that can generate pure FO and SO signals, specifically one consisting of multiple Gabors (the global-Gabor stimulus) in which the Gaussian envelopes are static and the carriers drift. The carrier can either be luminance-modulated (FO) or contrast-modulated (SO) and motion signals from the randomly-oriented local Gabors must be combined to detect the global-motion vector. Results show no cross-masking of FO and SO signals, thus supporting the hypothesis of independent FO and SO systems up to and including the level extracting optic-flow.

Consciousness of the first order in blindsight

At suprathreshold levels, detection and awareness of visual stimuli are typically synonymous in nonclinical populations. But following postgeniculate lesions, some patients may perform above chance in forced-choice detection paradigms, while reporting not to see the visual events presented within their blind field. This phenomenon, termed "blindsight," is intriguing because it demonstrates a dissociation between detection and perception. It is possible, however, for a blindsight patient to have some "feeling" of the occurrence of an event without seeing per se. This is termed blindsight type II to distinguish it from the type I, defined as discrimination capability in the total absence of any acknowledged awareness. Here we report on a well-studied patient, D.B., whose blindsight capabilities have been previously documented. We have found that D.B. is capable of detecting visual patterns defined by changes in luminance (first-order gratings) and those defined by contrast modulation of textured patterns (textured gratings; second-order stimuli) while being aware of the former but reporting no awareness of the latter. We have systematically investigated the parameters that could lead to visual awareness of the patterns and show that mechanisms underlying the subjective reports of visual awareness rely primarily on low spatial frequency, first-order spatial components of the image.

Perceptual plasticity in damaged adult visual systems

Plasticity appears to be a ubiquitous property of nervous systems, regardless of developmental stage or complexity. In the visual system of higher mammals, perceptual plasticity has been intensively studied, both during development and in adulthood. However, the last few years have seen some significant controversies arise about the existence and properties of visual plasticity after permanent damage to the adult visual system. The study of perceptual plasticity in damaged, adult visual systems is of interest for several reasons. First, it is an important means of unmasking the relative contribution of individual visual areas to visual learning, adaptation and priming, among other plastic phenomena. Second, it can provide knowledge that is essential for the development of effective therapies to rehabilitate the increasing number of people who suffer the functional consequences of damage at different levels of their visual hierarchy. This review summarizes the available evidence on the subject and proposes that visual plasticity may be just as ubiquitous after damage as it is in the intact visual system. However, damage may alter visual plasticity in ways that are still being defined.

Contrast detection in infants with fragile X syndrome

Studies have reported that a selective deficit in visual motion processing is present in certain developmental disorders, including Williams syndrome and autism. More recent evidence suggests a visual motion impairment is also present in adults with fragile X syndrome (FXS), the most common form of inherited mental retardation. The goal of the current study was to examine low-level cortical visual processing in infants diagnosed with FXS in order to explore the developmental origin of this putative deficit. We measured contrast detection of first-order (luminance-defined) and second-order (contrast-defined) gratings at two levels of temporal frequency, 0 Hz (static) and 4 Hz (moving). Results indicate that infants with FXS display significantly higher detection thresholds only for the second-order, moving stimuli compared to mental age-matched typically developing controls.

Eye Movements in Response to Dichoptic Motion: Evidence for a Parallel-Hierarchical Structure of Visual Motion Processing in Primates

Brief movements of a large-field visual stimulus elicit short-latency tracking eye movements termed “ocular following responses” (OFRs). To address the question of whether OFRs can be elicited by purely binocular motion signals in the absence of monocular motion cues, we measured OFRs from monkeys using dichoptic motion stimuli, the monocular inputs of which were flickering gratings in spatiotemporal quadrature, and compared them with OFRs to standard motion stimuli including monocular motion cues. Dichoptic motion did elicit OFRs, although with longer latencies and smaller amplitudes. In contrast to these findings, we observed that other types of motion stimuli categorized as non-first-order motion, which is undetectable by detectors for standard luminance-defined (first-order) motion, did not elicit OFRs, although they did evoke the sensation of motion. These results indicate that OFRs can be driven solely by cortical visual motion processing after binocular integration, which is distinct from the process incorporating non-first-order motion for elaborated motion perception. To explore the nature of dichoptic motion processing in terms of interaction with monocular motion processing, we further recorded OFRs from both humans and monkeys using our novel motion stimuli, the monocular and dichoptic motion signals of which move in opposite directions with a variable motion intensity ratio. We found that monocular and dichoptic motion signals are processed in parallel to elicit OFRs, rather than suppressing each other in a winner-take-all fashion, and the results were consistent across the species.

First and second-order motion perception after focal human brain lesions

Perception of visual motion includes a first-order mechanism sensitive to luminance changes and a second-order motion mechanism sensitive to contrast changes. We studied neural substrates for these motion types in 142 subjects with visual cortex lesions, 68 normal controls and 28 brain lesion controls. On first-order motion, the visual cortex lesion group performed significantly worse than normal controls overall and in each hemifield, but second-order motion did not differ. Only one individual showed a selective second-order motion deficit. Motion deficits were seen with lesions outside the small occipito-temporal region thought to contain a human homolog of motion processing area MT (V5), suggesting that many areas of human brain process visual motion.

Priming of first- and second-order motion: Mechanisms and neural substrates

Priming for luminance-modulated (first-order) motion has been shown to rely on the functional integrity of visual area V5/MT [Campana, G., Cowey, A., & Walsh, V. (2002). Priming of motion direction and area V5/MT: A test of perceptual memory. Cerebral Cortex, 12, 663-669; Campana, G., Cowey, A., & Walsh, V. (2006). Visual area V5/MT remembers "what" but not "where". Cerebral Cortex, 16, 1766-1770]. The high retinotopical organization of this area would predict that direction priming is sensitive to spatial position. In order to test this hypothesis, and to see whether a similar priming mechanism also exists with second-order motion, we tested motion direction priming and its interaction with spatial position with both first- and second-order motion. Indeed, whereas a number of studies have pinpointed the specific mechanisms and neural substrates for these two kinds of motion perception that appear to be (partially) non-overlapping (i.e., Lu, Z. L., & Sperling, G. (2001). Three-systems theory of human visual motion perception: Review and update. Journal of the Optical Society of America A, 18, 2331-2370; Vaina, L. M., & Soloviev, S. (2004). First-order and second-order motion: Neurological evidence for neuroanatomically distinct systems. Progress in Brain Research, 144, 197-212), the mechanisms and neural substrates mediating implicit memory for first- and second-order motion are still unknown. Our results indicate that priming for motion direction occurs not only with first-order but also with second-order motion. Priming for motion direction is position-sensitive both with first- and second-order motion, suggesting for both processes a locus of representation where retinotopicity is still maintained, that is within the V5/MT complex but earlier than MST. Cross-order motion priming also exists but is not sensitive to spatial position, suggesting that the locus where processing of first- and second-order motion converge is situated in MST or beyond.

Maturation of the sensitivity for luminance and contrast modulated patterns during development of normal and pathological human children

Any object may contain at least two spatio-temporal components referred to as first- and second-order, respectively, defined by spatial-temporal luminance modulation or by contrast, texture or depth modulation. This study investigates form sensitivity of infants, normals, premature or strabismic. A two-alternative forced-choice preferential looking procedure was used in monocular and binocular condition. Maturation profile for both stimuli was similar in the control group. Strabismic infants showed a vertical offset in maturation, which affected the second-order more severely. The pre-term group showed a lag of second-order sensitivity. Our results underline differences between first- and second-order processing.

Processing of first-order motion in marmoset visual cortex is influenced by second-order motion

We measured the responses of single neurons in marmoset visual cortex (V1, V2, and the third visual complex) to moving first-order stimuli and to combined first- and second-order stimuli in order to determine whether first-order motion processing was influenced by second-order motion. Beat stimuli were made by summing two gratings of similar spatial frequency, one of which was static and the other was moving. The beat is the product of a moving sinusoidal carrier (first-order motion) and a moving low-frequency contrast envelope (second-order motion). We compared responses to moving first-order gratings alone with responses to beat patterns with first-order and second-order motion in the same direction as each other, or in opposite directions to each other in order to distinguish first-order and second-order direction-selective responses. In the majority (72%, 67/93) of cells (V1 73%, 45/62; V2 70%, 16/23; third visual complex 75%, 6/8), responses to first-order motion were significantly influenced by the addition of a second-order signal. The second-order envelope was more influential when moving in the opposite direction to the first-order stimulus, reducing first-order direction sensitivity in V1, V2, and the third visual complex. We interpret these results as showing that first-order motion processing through early visual cortex is not separate from second-order motion processing; suggesting that both motion signals are processed by the same system.

Magnetoencephalography: In search of neural processes for visual motion information

Magnetoencephalography (MEG) has become a standard approach to the investigation of human brain functions. This review starts with a brief review of the human visual system and studies on visual motion detection mechanisms is followed by the presentation of MEG studies that have contributed to the field. Emphasis is placed on the fact that because the neural activities measured in functional magnetic resonance imaging (fMRI) differ substantially from those measured in MEG--fMRI data cannot be used directly to estimate MEG signal sources. The basic ideas behind the methods of signal processing and analyses generally used in MEG studies are described and theoretical considerations of the neural mechanisms determining MEG response latency and amplitude changes are discussed. Here, scalar fields theory is proposed to explain MEG responses to incoherent motions, and the ways in which detection of complex motions such as transparency, rotation and expansion can be explained by this theory are also presented. Relationships between human behavioral reaction time and MEG response latency suggest a new concept underlying the reasons why humans are late in detecting slow motion.

FMRI adaptation reveals separate mechanisms for first-order and second-order motion

A key unresolved debate in human vision concerns whether we have two different low-level mechanisms for encoding image motion. Separate neural mechanisms have been suggested for first-order (luminance modulation) and second-order (e.g., contrast modulation) motion in the retinal image but a single mechanism could handle both. Human functional magnetic resonance imaging (fMRI) has not so far convincingly revealed separate anatomical substrates. To examine whether two separate but co-localized mechanisms might exist, we used the technique of fast fMRI adaptation. We found direction-selective adaptation independently for each type of motion in the motion area V5/MT+ of the human brain. However, there was a total absence of cross-adaptation between first-order and second-order motion stimuli. This was true in both of the two subcomponents of MT+ (MT and MST) and similar results were found in V3A. This pattern of adaptation was consistent with psychophysical measurements of detection thresholds in similar stimulus sequences. The results provide strong evidence for separate neural populations that are responsible for detecting first- and second-order motion.

The role of human extra-striate visual areas V5/MT and V2/V3 in the perception of the direction of global motion: a transcranial magnetic stimulation study

Several published single case studies reveal a double dissociation between the effects of brain damage in separate extra-striate cortical visual areas on the perception of global visual motion defined by a difference in luminance (first-order motion) versus motion defined by a difference in contrast (second-order motion). In particular, the medial extrastriate cortical region V2/V3 seems to be crucial for the perception of first-order motion, but not for second-order, whereas a lateral and more anterior portion of the cortex close to the temporo-parieto-occipital junction (in the territory of the human motion area hV5/MT+) seems to be essential only for the perception of second-order motion. In order to test the hypothesis of a functional specialization of different visual areas for different types of motion, we applied repetitive transcranial magnetic stimulation (rTMS) unilaterally over areas V2/V3, V5/MT, or posterior parietal cortex (PPC) while subjects performed a 2AFC task with first- or second-order global motion displays in the contralateral visual field. Results showed a comparable disruption of the two types of motion, with both rTMS over V2/V3 or over MT/V5, and little or no effect with rTMS over PPC. The results suggest that either the previous psychophysical results with neurological patients are incorrect (highly unlikely) or that the lateral and medial regions are directly connected (as they are in macaque monkeys) such that stimulating one automatically affects the other, in this instance disruptively.

The temporal characteristics of motion processing in hMT/V5+: Combining fMRI and neuronavigated TMS

Functional imaging has demonstrated the specific involvement of the human middle-temporal complex (hMT/V5+) during processing of moving stimuli. Some studies applied transcranial magnetic stimulation (TMS) to investigate the causal relevance of hMT/V5+ for motion perception. Although the studies used similar visual stimuli and TMS parameters, the critical time point of functionally relevant hMT/V5+ activity differed by 100 ms and more. The present study aimed to elucidate further the temporal characteristics of motion processing in hMT/V5+ by investigating all critical time windows currently debated in the literature. In contrast to previous studies, we used TMS neuronavigation based on individual fMRI results of five participants to target hMT/V5+, applying single-pulse TMS at 24 different time windows (-50 till +200 ms relative to stimulus onset). We revealed that TMS significantly impaired motion perception when applied over hMT/V5+ at 40 to 30 ms before as well as 130 to 150 ms after onset of the moving stimuli. While the late effective time window conforms to results from previous experiments, we did not find evidence for an early time window around 0 ms that has been reported in other studies. Our neuronavigation approach enabled us to quantify the interindividual variance in the exact location of hMT/V5+ and the respective TMS target position on the skull of the participants. Considering that shifting the TMS coil position only by a few millimeters can already lead to a complete loss of TMS effects, our study clearly demonstrates the utility of neuronavigated TMS when investigating specific neuronal effects as in the case of motion processing.

Greater losses in sensitivity to second-order local motion than to first-order local motion after early visual deprivation in humans

We compared sensitivity to first-order versus second-order local motion in patients treated for dense central congenital cataracts in one or both eyes. Amplitude modulation thresholds were measured for discriminating the direction of motion of luminance-modulated (first-order) and contrast modulated (second-order) horizontal sine-wave gratings. Early visual deprivation, whether monocular or binocular, caused losses in sensitivity to both first- and second-order motion, with greater losses for second-order motion than for first-order motion. These findings are consistent with the hypothesis that the two types of motion are processed by different mechanisms and suggest that those mechanisms are differentially sensitive to early visual input.

Initial ocular following in humans: a response to first-order motion energy

Visual motion is sensed by low-level (energy-based) and high-level (feature-based) mechanisms. Ocular following responses (OFR) were elicited in humans by applying horizontal motion to vertical square-wave gratings lacking the fundamental ("missing fundamental stimulus"). Motion consisted of successive 1/4-wavelength steps, so the features and 4n+1 harmonics (where n=integer) shifted forwards, whereas the 4n-1 harmonics--including the strongest Fourier component (the 3rd harmonic)--shifted backwards (spatial aliasing). Initial OFR, recorded with the electromagnetic search coil technique, were always in the direction of the 3rd harmonic, e.g., leftward steps resulted in rightward OFR. Thus, the earliest OFR were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis.

The Ferrier Lecture 2004 what can transcranial magnetic stimulation tell us about how the brain works?

Transcranial magnetic stimulation (TMS) is a technique whereby parts of the cerebral cortex and underlying white matter can be excited by a brief electrical current induced by a similarly brief, rapidly fluctuating magnetic field which is itself produced by rapidly discharging a current through an insulated coil held against the scalp. When combined with magnetic resonance structural and functional images of the subject's brain, the stimulation can be directed at specific cortical areas. Over a period of only 15 years, TMS has revealed hitherto unsuspected aspects of brain function, such as the role of distant parts of the brain in recovery from stroke, and has helped to resolve several previously intractable disputes, such as the neuronal basis of conscious awareness. This article describes and discusses the origins and nature of TMS, its applications and limitations, and its especial usefulness in conjunction with other techniques of evaluating or imaging brain activity.

Deficits of motion integration and segregation in patients with unilateral extrastriate lesions

Functional neuroimaging in human subjects and single cell recordings in monkeys show that several extra-striate visual areas are activated by visual motion. However, the extent to which different types of motion are processed in different regions remains unclear, although neuropsychological studies of patients with circumscribed lesions hint at regional specialization. We, therefore, studied four patients with unilateral damage to different regions of extrastriate visual cortex on a series of visual discrimination tasks that required them, to a different extent, to integrate local motion signals in order to correctly perceive the direction of global motion. Performance was assessed psychophysically and compared with that of control subjects and with the patients' performance with stimuli presented in the visual field ipsilateral to the lesion. The results indicate considerable regional specialization in extra-striate regions for different aspects of motion processing, namely the largest displacement from frame to frame (D-max) that can sustain perception of coherent motion; perception of relative speed; the amount of coherent motion needed to sustain a percept of global motion in a particular direction; the detection of discontinuities within a moving display; the extraction of form from motion. It was also clear that a defect in local motion, i.e. D-max, can be overcome by integrating local motion signals over a longer period of time. Although no patient suffered from only one defect, the overall pattern of results strongly supports the notion of regional specialization for different aspects of motion processing.

Attention-driven discrete sampling of motion perception

In movies or on TV, a wheel can seem to rotate backwards, due to the temporal subsampling inherent in the recording process (the wagon wheel illusion). Surprisingly, this effect has also been reported under continuous light, suggesting that our visual system, too, might sample motion in discrete "snapshots." Recently, these results and their interpretation have been challenged. Here, we investigate the continuous wagon wheel illusion as a form of bistable percept. We observe a strong temporal frequency dependence: the illusion is maximal at alternation rates around 10 Hz but shows no spatial frequency dependence. We introduce an objective method, based on unbalanced counterphase gratings, for measuring this phenomenon and demonstrate that the effect critically depends on attention: the continuous wagon wheel illusion was almost abolished in the absence of focused attention. A motion-energy model, coupled with attention-dependent temporal subsampling of the perceptual stream at rates between 10 and 20 Hz, can quantitatively account for the observed data.

Human cortical responses to contrast modulations of visual noise

We studied visual evoked potentials (VEPs) elicited by second-order contrast modulations of binary dynamic noise and first-order luminance modulations. Using a 3-point Laplacian operator centred on Oz, we found that contrast modulations of both low and higher spatial frequencies elicited a negative component whose latency was about 200 ms. The latency of this component was significantly longer than that of the early Laplacian components to first-order luminance modulations. These findings could be due to slower first-stage linear filters and additional processing stages of the second-order pathway. The topographical analysis of scalp recorded VEPs to central and half-field stimulation has suggested that the responses to second-order patterns are likely to be generated by neuronal structures within the primary visual cortex which may have inputs from extrastriate neurons via feedback connections.

Deficits in the processing of local and global motion in very low birthweight children

This study evaluated the impact of premature birth on the development of local and global motion processing in a group of very low birthweight (<1500 g), 5- to 8-year-old children. Sensitivity to first- and second-order local motion stimuli and coherence thresholds for global motion in random dot kinematograms were measured. Relative to full-term controls, premature children showed deficits on all three aspects of motion processing. These problems could not be accounted for by stereo deficits, amblyopia, or attentional problems. A history of mild retinopathy of prematurity and/or intraventricular hemorrhage increased risk, but deficits were observed in some children with no apparent ocular or cerebral pathology. It is important to note that, despite the observed group differences, individual profiles of performance did vary; the results suggest that these three forms of motion processing may involve separate neural mechanisms. These findings serve to increase our understanding of the organization and functional development of motion-processing subsystems in humans, and of the impact of prematurity and associated complications on visual development.

Perceptual deficits in patients with impaired recognition of biological motion after temporal lobe lesions

"Biological motion" may be defined by the pattern of movement of a small number of lights attached to the major joints of a human performing simple actions. Normal observers watching such displays immediately recognize a person and his or her actions. In the present study, we investigated the effects of lesions of anterior cortical regions on the perception of biological motion. We measured the performance on psychophysical static and motion tasks and on object and action recognition tests in four stroke patients who presented with a disorder of recognition of biological motion. We relate our results to the finding that neurons in the rostral part of the superior temporal gyrus (the superior temporal polysensory area) respond selectively to biological motion, and to the idea that the superior temporal polysensory area integrates the late stages of the dorsal and ventral cortical visual streams, as well as to recent functional MRI studies on biological motion.

Texture segregation in traumatic brain injury––a VEP study

Visual evoked potentials (VEPs) were recorded to textures segregated by gradients in orientation or motion. Recordings were obtained in traumatic brain-injured (TBI) subjects and in normal controls. We analyzed both the low-level VEPs (llVEPs) evoked by homogenous stimuli, as well as the components associated with texture segregation (tsVEP) obtained through an appropriate linear combination. Our results suggest that the tsVEP, presumably higher up in the visual processing chain than the llVEP, is sensitive to TBI and can reveal further information as to the nature of possible information processing deficits after TBI. It could also help quantify cortical damage that is not revealed with more standard clinical tools.

Contrast-reversing global-motion stimuli reveal local interactions between first- and second-order motion signals

Motion perception appears to be mediated by, at least, two systems: a first-order and a second-order system. To investigate the degree of interaction between these systems, we used a contrast-reversing global-motion stimulus in which the signal dots reverse their contrast polarity as they move. In response to such a stimulus, fullwave-rectifying second-order units would signal motion in the displacement direction and first-order units would signal motion in the opposite direction (reverse-phi motion). If these signals were of equal strength, then any inhibitory interaction between them would lead to motion nulling. Such a situation would account for the failure to perceive coherent motion with such a stimulus in a previous study [Vis. Res. 34 (1994) 2849]. In order to test for this possibility we manipulated the stimulus in order to reduce the strength of the second-order response relative to the first-order response. This was achieved by: decreasing dot contrast; increasing stimulus eccentricity; and increasing dot speed. These manipulations resulted in an increase in the perception of (first-order mediated) reverse-phi motion. We conclude that interaction between first- and second-order motion signals occur at the local-motion-pooling level.

Investigating local network interactions underlying first- and second-order processing

We compared the spatial lateral interactions for first-order cues to those for second-order cues, and investigated spatial interactions between these two types of cues. We measured the apparent modulation depth of a target Gabor at fixation, in the presence and the absence of horizontally flanking Gabors. The Gabors' gratings were either added to (first-order) or multiplied with (second-order) binary 2-D noise. Apparent "contrast" or modulation depth (i.e., the perceived difference between the high and low luminance regions for the first-order stimulus, or between the high and low contrast regions for the second-order stimulus) was measured with a modulation depth-matching paradigm. For each observer, the first- and second-order Gabors were equated for apparent modulation depth without the flankers. Our results indicate that at the smallest inter-element spacing, the perceived reduction in modulation depth is significantly smaller for the second-order than for the first-order stimuli. Further, lateral interactions operate over shorter distances and the spatial frequency and orientation tuning of the suppression effect are broader for second- than first-order stimuli. Finally, first- and second-order information interact in an asymmetrical fashion; second-order flankers do not reduce the apparent modulation depth of the first-order target, whilst first-order flankers reduce the apparent modulation depth of the second-order target.

A higher order motion region in human inferior parietal lobule: evidence from fMRI

The proposal that motion is processed by multiple mechanisms in the human brain has received little anatomical support so far. Here, we compared higher- and lower-level motion processing in the human brain using functional magnetic resonance imaging. We observed activation of an inferior parietal lobule (IPL) motion region by isoluminant red-green gratings when saliency of one color was increased and by long-range apparent motion at 7 Hz but not 2 Hz. This higher order motion region represents the entire visual field, while traditional motion regions predominantly process contralateral motion. Our results suggest that there are two motion-processing systems in the human brain: a contralateral lower-level luminance-based system, extending from hMT/V5+ into dorsal IPS and STS, and a bilateral higher-level saliency-based system in IPL.

First-order and second-order motion: neurological evidence for neuroanatomically distinct systems

An unresolved issue in visual motion perception is how distinct are the processes underlying 'first-order' and 'second-order' motion. The former is defined by spatio-temporal variations of luminance and the latter by spatio-temporal variations in other image attributes such as contrast or depth, for example. Using neuroimaging and psychophysics we present data from four neurological patients with unilateral and mostly cortical infarcts, which strongly suggest that first- and second-order motion have a different neural substrate. We found that from the early stages of processing, these two types of motions are mediated by two distinct pathways: first-order motion is carried out by mechanisms along the dorsal pathway in the occipital lobe, while the second-order motion by mechanisms mostly along the ventral pathway. The data reported here also suggest that different cortical regions may be in charge of processing direction-discrimination in second-order motion defined by different second-order attributes.