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

Visual Short-Term Memory for Coherent and Sequential Motion: A rTMS Investigation

We investigated the role of the human medio-temporal complex (hMT+) in the memory encoding and storage of a sequence of four coherently moving random dot kinematograms (RDKs), by applying repetitive transcranial magnetic stimulation (rTMS) during an early or late phase of the retention interval. Moreover, in a second experiment, we also tested whether disrupting the functional integrity of hMT+ during the early phase impaired the precision of the encoded motion directions. Overall, results showed that both recognition accuracy and precision were worse in middle serial positions, suggesting the occurrence of primacy and recency effects. We found that rTMS delivered during the early (but not the late) phase of the retention interval was able to impair not only recognition of RDKs, but also the precision of the retained motion direction. However, such impairment occurred only for RDKs presented in middle positions along the presented sequence, where performance was already closer to chance level. Altogether these findings suggest an involvement of hMT+ in the memory encoding of visual motion direction. Given that both position sequence and rTMS modulated not only recognition but also the precision of the stored information, these findings are in support of a model of visual short-term memory with a variable resolution of each stored item, consistent with the assigned amount of memory resources, and that such item-specific memory resolution is supported by the functional integrity of area hMT+.

Visual Neuropsychology in Development: Anatomo-Functional Brain Mechanisms of Action/Perception Binding in Health and Disease

Vision is the main entrance for environmental input to the human brain. Even if vision is our most used sensory modality, its importance is not limited to environmental exploration. Rather it has strong links to motor competences, further extending to cognitive and social aspects of human life. These multifaceted relationships are particularly important in developmental age and become dramatically evident in presence of complex deficits originating from visual aberrancies. The present review summarizes the available neuropsychological evidence on the development of visual competences, with a particular focus on the associated visuo-motor integration skills in health and disease. With the aim of supporting future research and interventional settings, the goal of the present review is to constitute a solid base to help the translation of neuropsychological hypotheses into straightforward empirical investigations and rehabilitation/training protocols. This approach will further increase the impact, ameliorate the acceptance, and ease the use and implementation of lab-derived intervention protocols in real-life situations.

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.

Motion perception at mesopic light levels: effects of physiological ageing and eccentricity

PURPOSE

To explore the differential effects of age and eccentricity on the perception of motion at photopic and mesopic light levels.

METHODS

Thirty-six visually normal participants (18 younger; mean age 25 years, range: 20-31) and (18 older; mean age 70 years, range: 60-79) underwent two testing sessions, one at photopic and one at mesopic light levels. In each session, motion perception was tested binocularly at two eccentricities (centrally, and peripherally at 15° rightwards and 5° superior to the horizontal) for four motion tasks: minimum contrast of a drifting Gabor to identify motion direction (motion contrast); translational global motion coherence; biological motion embedded in noise and the minimum duration of a high-contrast Gabor to determine the direction of motion, using two Gabor sizes to measure spatial surround suppression of motion.

RESULTS

There was a significant main effect of light condition (higher thresholds in mesopic) for motion contrast (p < 0.001), translational global motion (p = 0.001) and biological motion (p < 0.001); a significant main effect of age (higher thresholds in older adults) for motion contrast (p < 0.001) and biological motion (p = 0.04) and a significant main effect of eccentricity (higher thresholds peripherally) for motion contrast (p < 0.001) and biological motion (p < 0.001). Additionally, we found a significant three-way interaction between light levels, age and eccentricity for translational global motion (similar increase in mesopic thresholds centrally for both groups, but a much larger deterioration in older adult's peripheral mesopic thresholds, p = 0.02). Finally, we found a two-way interaction between light condition and eccentricity for translational global motion (higher values in central mesopic relative to peripheral photopic, p = 0.001) and for biological motion (higher values in peripheral mesopic relative to central photopic, p < 0.001).

CONCLUSIONS

For the majority of tasks assessed, motion perception was reduced in mesopic relative to photopic conditions, to a similar extent in both age groups. However, because some older adults exhibited elevated thresholds even under photopic conditions, particularly in the periphery, the ability to detect mesopic moving stimuli even at high contrast was markedly impaired in some individuals. Our results imply age-related differences in the detection of peripheral moving stimuli at night that might impact hazard avoidance and night driving ability.

Effect of Repetitive Transcranial Magnetic Stimulation on a Target Moving in Front of a Static or Random Dynamic Visual Noise

Observers report seeing as slower a target disk moving in front of a static visual noise (SVN) background than the same object moving in front of a random dynamic visual noise (rDVN) background when the speed is the same. To investigate in which brain region (lower vs. higher visual areas) the background and the target signals might be combined to elicit this misperception, the transcranial magnetic stimulation (TMS) was delivered over the early visual cortex (V1/V2), middle temporal area (MT) and Cz (control site) while participants performed a speed discrimination task with targets moving in front of an SVN or an rDVN. Results showed that the TMS over MT reduced the perceived speed of the target moving in front of an SVN, but not when the target was moving in front of an rDVN background. Moreover, the TMS do not seem to interfere with encoding processing but more likely affected decoding processing in conditions of high uncertainty (i.e., when targets have similar speed).

Spatial Frequency Tuning and Transfer of Perceptual Learning for Motion Coherence Reflects the Tuning Properties of Global Motion Processing

Perceptual learning is typically highly specific to the stimuli and task used during training. However, recently, it has been shown that training on global motion can transfer to untrained tasks, reflecting the generalising properties of mechanisms at this level of processing. We investigated (i) if feedback was required for learning in a motion coherence task, (ii) the transfer across the spatial frequency of training on a global motion coherence task and (iii) the transfer of this training to a measure of contrast sensitivity. For our first experiment, two groups, with and without feedback, trained for ten days on a broadband motion coherence task. Results indicated that feedback was a requirement for robust learning. For the second experiment, training consisted of five days of direction discrimination using one of three motion coherence stimuli (where individual elements were comprised of either broadband Gaussian blobs or low- or high-frequency random-dot Gabor patches), with trial-by-trial auditory feedback. A pre- and post-training assessment was conducted for each of the three types of global motion coherence conditions and high and low spatial frequency contrast sensitivity (both without feedback). Our training paradigm was successful at eliciting improvement in the trained tasks over the five days. Post-training assessments found evidence of transfer for the motion coherence task exclusively for the group trained on low spatial frequency elements. For the contrast sensitivity tasks, improved performance was observed for low- and high-frequency stimuli, following motion coherence training with broadband stimuli, and for low-frequency stimuli, following low-frequency training. Our findings are consistent with perceptual learning, which depends on the global stage of motion processing in higher cortical areas, which is broadly tuned for spatial frequency, with a preference for low frequencies.

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.

A Direct Demonstration of Functional Differences between Subdivisions of Human V5/MT

Two subdivisions of human V5/MT+: one located posteriorly (MT/TO-1) and the other more anteriorly (MST/TO-2) were identified in human participants using functional magnetic resonance imaging on the basis of their representations of the ipsilateral versus contralateral visual field. These subdivisions were then targeted for disruption by the application of repetitive transcranial magnetic stimulation (rTMS). The rTMS was delivered to cortical areas while participants performed direction discrimination tasks involving 3 different types of moving stimuli defined by the translational, radial, or rotational motion of dot patterns. For translational motion, performance was significantly reduced relative to baseline when rTMS was applied to both MT/TO-1 and MST/TO-2. For radial motion, there was a differential effect between MT/TO-1 and MST/TO-2, with only disruption of the latter area affecting performance. The rTMS failed to reveal a complete dissociation between MT/TO-1 and MST/TO-2 in terms of their contribution to the perception of rotational motion. On the basis of these results, MT/TO-1 and MST/TO-2 appear to be functionally distinct subdivisions of hV5/MT+. While both areas appear to be implicated in the processing of translational motion, only the anterior region (MST/TO-2) makes a causal contribution to the perception of radial motion.

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.

Psychophysical and rTMS Evidence for the Presence of Motion Opponency in Human V5

BACKGROUND

Motion sensitive cells within macaque V5, but not V1, exhibit motion opponency whereby their firing is suppressed by motion in their anti-preferred direction. fMRI studies indicate the presence of motion opponent mechanisms in human V5.

OBJECTIVE/HYPOTHESIS

We tested two hypotheses. 1) Performance of a motion discrimination task would be poorer when stimuli were constructed from pairs of dots that moved in counter-phase vs. in-phase, because counter-phase dots would activate motion opponent mechanisms in V5. 2) Offline 1 Hz rTMS of V5 would impair discrimination performance for in-phase stimuli but not counter-phase stimuli, and the opposite effect would be found for rTMS of V1.

METHODS

Stimuli were constructed from 100 dot pairs. Paired dots moved along a fixed motion axis either in counter-phase (motion opponent stimulus) or in-phase (non-opponent motion stimulus). Motion axis orientation discrimination thresholds were measured for each stimulus. Blocks of 300 trials were then presented at 85% correct threshold and discrimination accuracy was measured before and after 1 Hz offline rTMS of either V1 or V5. Subjects were 8 healthy adults.

RESULTS

Discrimination thresholds were significantly larger (worse) for counter-phase than in-phase stimuli (p = 0.02). V5 rTMS mildly impaired discrimination accuracy for the in-phase dot stimuli (p = 0.02) but not the counter-phase dot stimuli. The opposite effect occurred for V1 rTMS (p = 0.05).

CONCLUSIONS

Opponent motion mechanisms are present within human V5 and activation of these mechanisms impairs motion discrimination. In addition, perception of the motion axis within opponent motion stimuli involves processing within V1.

TMS reveals flexible use of form and motion cues in biological motion perception

The perception of human movement is a key component of daily social interactions. Although extrastriate area MT+/V5 is closely associated with motion processing, its role in the processing of sparse 'biological motion' displays is still unclear. We developed two closed matched psychophysical tasks to assess simple coherent motion perception and biological motion perception, and measured changes in performance caused by application of TMS over MT+/V5. Performance of the simple motion discrimination task was significantly depressed by TMS stimulation, and highly correlated within observers in TMS conditions, but there was no significant decrement in performance of the biological motion task, despite low intra-observer correlations across TMS conditions. We conclude that extrastriate area MT+/V5 is an obligatory waypoint in the neural processing of simple coherent motion, but is not obligatory for the processing of biological motion. Results are consistent with a dual neural processing route for biological motion processing.

Progressive thinning of visual cortex in primary open-angle glaucoma of varying severity

The aim of this study was to investigate possible changes of cortical thickness in the visual cortex in primary open-angle glaucoma (POAG) of varying severity. Twenty normal controls (NC), 20 mild (MP) and 17 severe (SP) POAG patients were recruited and scanned using magnetic resonance imaging. Cortical thickness analyses with regions of interest (V1, V2, ventral V3, V4 and V5/MT+) were used to assess the cortical changes among the three groups. Furthermore, the associations of cortical thickness with retinal nerve fiber layer (RNFL) thickness and mean deviation of visual field were analyzed. Compared with the NC group, decreased cortical thickness was detected in the bilateral V5/MT+ areas in the MP group and the left V1, bilateral V2 and V5/MT+ areas in the SP group. Cortical thinning of the bilateral V2 areas was detected in the SP group compared with the MP group. In addition, cortical thinning of these visual areas was related to the ophthalmologic measurements. In conclusion, POAG patients exhibit cortical thinning in the bilateral V5/MT+ in the early stage of disease. The cortical degeneration in visual areas is discrepant with disease progressing and the dorsal pathway might be selectively damaged in POAG. Therefore, the cortical thinning of these visual areas may play a key role in the progression of POAG and can serve as a novel biomarker for accurately evaluating the severity of POAG.

TMS over V5 disrupts motion prediction

Given the vast amount of sensory information the brain has to deal with, predicting some of this information based on the current context is a resource-efficient strategy. The framework of predictive coding states that higher-level brain areas generate a predictive model to be communicated via feedback connections to early sensory areas. Here, we directly tested the necessity of a higher-level visual area, V5, in this predictive processing in the context of an apparent motion paradigm. We flashed targets on the apparent motion trace in-time or out-of-time with the predicted illusory motion token. As in previous studies, we found that predictable in-time targets were better detected than unpredictable out-of-time targets. However, when we applied functional magnetic resonance imaging-guided, double-pulse transcranial magnetic stimulation (TMS) over left V5 at 13–53 ms before target onset, the detection advantage of in-time targets was eliminated; this was not the case when TMS was applied over the vertex. Our results are causal evidence that V5 is necessary for a prediction effect, which has been shown to modulate V1 activity (Alink et al. 2010). Thus, our findings suggest that information processing between V5 and V1 is crucial for visual motion prediction, providing experimental support for the predictive coding framework.

Neural correlates of visual motion prediction

Predicting the trajectories of moving objects in our surroundings is important for many life scenarios, such as driving, walking, reaching, hunting and combat. We determined human subjects’ performance and task-related brain activity in a motion trajectory prediction task. The task required spatial and motion working memory as well as the ability to extrapolate motion information in time to predict future object locations. We showed that the neural circuits associated with motion prediction included frontal, parietal and insular cortex, as well as the thalamus and the visual cortex. Interestingly, deactivation of many of these regions seemed to be more closely related to task performance. The differential activity during motion prediction vs. direct observation was also correlated with task performance. The neural networks involved in our visual motion prediction task are significantly different from those that underlie visual motion memory and imagery. Our results set the stage for the examination of the effects of deficiencies in these networks, such as those caused by aging and mental disorders, on visual motion prediction and its consequences on mobility related daily activities.

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.

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.

Improved discrimination of visual stimuli following repetitive transcranial magnetic stimulation

Background

Repetitive transcranial magnetic stimulation (rTMS) at certain frequencies increases thresholds for motor-evoked potentials and phosphenes following stimulation of cortex. Consequently rTMS is often assumed to introduce a “virtual lesion” in stimulated brain regions, with correspondingly diminished behavioral performance.

Methodology/Principal Findings

Here we investigated the effects of rTMS to visual cortex on subjects' ability to perform visual psychophysical tasks. Contrary to expectations of a visual deficit, we find that rTMS often improves the discrimination of visual features. For coarse orientation tasks, discrimination of a static stimulus improved consistently following theta-burst stimulation of the occipital lobe. Using a reaction-time task, we found that these improvements occurred throughout the visual field and lasted beyond one hour post-rTMS. Low-frequency (1 Hz) stimulation yielded similar improvements. In contrast, we did not find consistent effects of rTMS on performance in a fine orientation discrimination task.

Conclusions/Significance

Overall our results suggest that rTMS generally improves or has no effect on visual acuity, with the nature of the effect depending on the type of stimulation and the task. We interpret our results in the context of an ideal-observer model of visual perception.

Differential and co-involvement of areas of the temporal and parietal streams in visual tasks

Transcranial magnetic stimulation (TMS) is particularly useful in teasing apart the contrasting contributions of different anatomical and functional systems in particular aspects of behaviour, for example the involvement of the dorsal and ventral visual streams in tasks involving the perception of distance, shape and colour. In order to investigate the dual involvement of two areas, namely right posterior parietal cortex (PPC) and lateral occipital cortex (LO), in a distance discrimination task, neural processing in both areas was concurrently disrupted using dual site TMS. Although there was no change in error rates, reaction time was significantly lengthened over that seen with TMS over either site alone. This additive effect indicates that both PPC and LO are concurrently active and essential for efficient processing of this task. The second experiment investigated the specificity of function within the ventral stream. Performance was assessed for distance and shape discrimination when TMS was applied to our original LO site and an area rostral to V5 but still part of the lateral occipital complex (rostral LOC) that is activated in form and colour discrimination. Performance deficits were restricted to TMS over LO; no significant impairment for either task followed TMS at the rostral LOC site.

Induced deficits in speed perception by transcranial magnetic stimulation of human cortical areas V5/MT+ and V3A

In this report, we evaluate the role of visual areas responsive to motion in the human brain in the perception of stimulus speed. We first identified and localized V1, V3A, and V5/MT+ in individual participants on the basis of blood oxygenation level-dependent responses obtained in retinotopic mapping experiments and responses to moving gratings. Repetitive transcranial magnetic stimulation (rTMS) was then used to disrupt the normal functioning of the previously localized visual areas in each participant. During the rTMS application, participants were required to perform delayed discrimination of the speed of drifting or spatial frequency of static gratings. The application of rTMS to areas V5/MT and V3A induced a subjective slowing of visual stimuli and (often) caused increases in speed discrimination thresholds. Deficits in spatial frequency discrimination were not observed for applications of rTMS to V3A or V5/MT+. The induced deficits in speed perception were also specific to the cortical site of TMS delivery. The application of TMS to regions of the cortex adjacent to V5/MT and V3A, as well as to area V1, produced no deficits in speed perception. These results suggest that, in addition to area V5/MT+, V3A plays an important role in a cortical network that underpins the perception of stimulus speed in the human brain.

Distinct position assignment mechanisms revealed by cross-order motion

Motion perception influences perceived position. It has been shown that first-order (luminance defined) motion shifts perceived position across a wide range of spatial and temporal frequencies. On the other hand, second-order (contrast defined) motion shifts perceived position over a narrow range of temporal frequencies, regardless of spatial frequency [Bressler, D. W., & Whitney, D. (2006). Second-order motion shifts perceived position. Vision Research, 46(6-7), 1120-1128]. These results suggest the presence of distinct position assignment mechanisms for first- and second-order motion. We investigated whether the first- and second-order systems independently encode and assign the position of a moving stimulus. To measure motion induced position shift we presented two horizontally offset Gabors placed above and below a central fixation point, with sine wave carriers drifting in opposite directions. Subjects judged the position of the top Gabor relative to the bottom one. We used both first-order Gabors (sinusoidal luminance modulation of a dynamic noise carrier enveloped by a static Gaussian) and second-order Gabors (sinusoidal contrast modulation of a dynamic noise carrier enveloped by a static Gaussian). Results showed a strong position shift in the direction of the carrier motion when both Gabors were first-order, a weak position shift when both Gabors were second-order, and no appreciable position shift when one Gabor was first-order and the other was second-order (cross-order motion). The absence of a position shift using cross-order motion supports the hypothesis that the two motion systems independently encode and assign the position of a moving object. These results are consistent with those of experiments investigating global spatial interactions between static first-order and second-order Gabor patches, indicating a commonality in the underlying spatial integration processes.

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.

Distinct causal influences of parietal versus frontal areas on human visual cortex: evidence from concurrent TMS-fMRI

It has often been proposed that regions of the human parietal and/or frontal lobe may modulate activity in visual cortex, for example, during selective attention or saccade preparation. However, direct evidence for such causal claims is largely missing in human studies, and it remains unclear to what degree the putative roles of parietal and frontal regions in modulating visual cortex may differ. Here we used transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) concurrently, to show that stimulating right human intraparietal sulcus (IPS, at a site previously implicated in attention) elicits a pattern of activity changes in visual cortex that strongly depends on current visual context. Increased intensity of IPS TMS affected the blood oxygen level-dependent (BOLD) signal in V5/MT+ only when moving stimuli were present to drive this visual region, whereas TMS-elicited BOLD signal changes were observed in areas V1-V4 only during the absence of visual input. These influences of IPS TMS upon remote visual cortex differed significantly from corresponding effects of frontal (eye field) TMS, in terms of how they related to current visual input and their spatial topography for retinotopic areas V1-V4. Our results show directly that parietal and frontal regions can indeed have distinct patterns of causal influence upon functional activity in human visual cortex.