Human area V5 and motion in the ipsilateral visual field

Investigating the effects of cortico-cortical paired associative stimulation in the human brain: A systematic review and meta-analysis

Recent decades have witnessed a rapid development of novel neuromodulation techniques that allow direct manipulation of cortical pathways in the human brain. These techniques, known as cortico-cortical paired stimulation (ccPAS), apply magnetic stimulation over two cortical regions altering interregional connectivity. This review evaluates ccPAS's effectiveness to induce plastic changes in cortical pathways in the healthy brain. A systematic database search identified 41 studies investigating the effect of ccPAS on neurophysiological or behavioural measures, and a subsequent multilevel meta-analysis focused on the standardized mean differences to assess ccPAS's efficacy. Most studies report significant neurophysiological and behavioural changes from ccPAS interventions across several brain networks, consistently showing medium effect sizes. Moderator analyses revealed limited influence of experimental manipulations on effect sizes. The multivariate approach and lack of small-study bias suggest reliable effect estimates. ccPAS is a promising tool to manipulate neuroplasticity in cortical pathways, showing reliable effects on brain cortical networks. Important areas for further research on the influence of experimental procedures and the potential of ccPAS for clinical interventions are highlighted.

The lateralized flash-lag illusion: A psychophysical and pupillometry study

The flash-lag illusion (FLI) is a visual phenomenon where a flashed object, either co-localized or in physical alignment with another continuously moving object, is perceived to lag behind the path of the moving object. In the present study, we reveal an anisotropy of the FLI between the lateral visual fields that was expressed psychophysically as different points of subjective equality, depending on the hemifield in which the stimuli appeared. Specifically, the study confirmed that, as seen in two previous studies, the FLI was significantly larger in the left visual field (LVF) than in the right (RVF). In addition, pupil dilations were larger in the RVF than in the LVF as well as returning to baseline levels more rapidly in the LVF. We interpret these findings as converging on revealing more efficient spatial and attentional processing and, in turn, extrapolation of motion in the LVF/right hemisphere than in the RVF/left hemisphere.

Increasing interhemispheric connectivity between human visual motion areas uncovers asymmetric sensitivity to horizontal motion

Our conscious perceptual experience relies on a hierarchical process involving integration of low-level sensory encoding and higher-order sensory selection.¹ This hierarchical process may scale at different levels of brain functioning, including integration of information between the hemispheres.^2-5 Here, we test this hypothesis for the perception of visual motion stimuli. Across 3 experiments, we manipulated the connectivity between the left and right visual motion complexes (V5/MT+) responsible for horizontal motion perception^2,3 by means of transcranial magnetic stimulation (TMS).^4,5 We found that enhancing the strength of connections from the left to the right V5/MT+, by inducing spike-timing-dependent plasticity⁶ in this pathway, increased sensitivity to horizontal motion. These changes were present immediately and lasted at least 90 min after intervention. Notably, little perceptual changes were observed when strengthening connections from the right to the left V5/MT+. Furthermore, we found that this asymmetric modulation was mirrored by an asymmetric perceptual bias in the direction of the horizontal motion. Overall, observers were biased toward leftward relative to rightward motion direction. Crucially, following the strengthening of the connections from right to left V5/MT+, this bias could be momentarily reversed. These results suggest that the projections connecting left and right V5/MT+ in the human visual cortex are asymmetrical, subtending a hierarchical role of hemispheric specialization^7-10 favoring left-to-right hemisphere processing for integrating local sensory input into coherent global motion perception.

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.

An investigation of the spatial selectivity of the duration after-effect

Adaptation to the duration of a visual stimulus causes the perceived duration of a subsequently presented stimulus with a slightly different duration to be skewed away from the adapted duration. This pattern of repulsion following adaptation is similar to that observed for other visual properties, such as orientation, and is considered evidence for the involvement of duration-selective mechanisms in duration encoding. Here, we investigated whether the encoding of duration - by duration-selective mechanisms - occurs early on in the visual processing hierarchy. To this end, we investigated the spatial specificity of the duration after-effect in two experiments. We measured the duration after-effect at adapter-test distances ranging between 0 and 15° of visual angle and for within- and between-hemifield presentations. We replicated the duration after-effect: the test stimulus was perceived to have a longer duration following adaptation to a shorter duration, and a shorter duration following adaptation to a longer duration. Importantly, this duration after-effect occurred at all measured distances, with no evidence for a decrease in the magnitude of the after-effect at larger distances or across hemifields. This shows that adaptation to duration does not result from adaptation occurring early on in the visual processing hierarchy. Instead, it seems likely that duration information is a high-level stimulus property that is encoded later on in the visual processing hierarchy.

Activation of the Human MT Complex by Motion in Depth Induced by a Moving Cast Shadow

A moving cast shadow is a powerful monocular depth cue for motion perception in depth. For example, when a cast shadow moves away from or toward an object in a two-dimensional plane, the object appears to move toward or away from the observer in depth, respectively, whereas the size and position of the object are constant. Although the cortical mechanisms underlying motion perception in depth by cast shadow are unknown, the human MT complex (hMT+) is likely involved in the process, as it is sensitive to motion in depth represented by binocular depth cues. In the present study, we examined this possibility by using a functional magnetic resonance imaging (fMRI) technique. First, we identified the cortical regions sensitive to the motion of a square in depth represented via binocular disparity. Consistent with previous studies, we observed significant activation in the bilateral hMT+, and defined functional regions of interest (ROIs) there. We then investigated the activity of the ROIs during observation of the following stimuli: 1) a central square that appeared to move back and forth via a moving cast shadow (mCS); 2) a segmented and scrambled cast shadow presented beside the square (sCS); and 3) no cast shadow (nCS). Participants perceived motion of the square in depth in the mCS condition only. The activity of the hMT+ was significantly higher in the mCS compared with the sCS and nCS conditions. Moreover, the hMT+ was activated equally in both hemispheres in the mCS condition, despite presentation of the cast shadow in the bottom-right quadrant of the stimulus. Perception of the square moving in depth across visual hemifields may be reflected in the bilateral activation of the hMT+. We concluded that the hMT+ is involved in motion perception in depth induced by moving cast shadow and by binocular disparity.

Altered recovery from inhibitory repetitive transcranial magnetic stimulation (rTMS) in subjects with photosensitive epilepsy

OBJECTIVE

To investigate functional changes underlying photosensitivity, we studied the response of the visual cortex to low-frequency, inhibitory repetitive transcranial magnetic stimulation (rTMS) in drug-free patients with photosensitive seizures and healthy volunteers.

METHODS

Visual evoked potentials (VEPs) triggered by grating stimuli of different contrasts were recorded in both hemispheres before and after transient functional inactivation of the occipital cortex of one side via low-frequency rTMS (0.5Hz for 20'). VEPs were recorded before (T0), immediately after (T1) and 45' following the completion of rTMS (T2).

RESULTS

Baseline amplitudes of the early VEP components (N1 and P1) were enhanced in photosensitive patients. At T1, rTMS produced an inhibitory effect on VEPs amplitudes at all contrasts in the targeted side and a concurrent facilitation of responses in the contralateral hemisphere. Compared with PSE subjects, VEP amplitudes remained persistently dampened in the stimulated hemisphere of controls (Holm-Sidak post-hoc method, p<0.05). In the contralateral hemisphere, we found a clear enhancement of VEP amplitude in photosensitive subjects but not controls at T2 (Holm-Sidak test, p<0.001).

CONCLUSIONS

Visual responses recovered more quickly in the stimulated hemisphere, and disinhibition persisted in the contralateral side of photosensitive subjects.

SIGNIFICANCE

The rapid recovery of excitability and the persistent transcallosal disinhibition following perturbation of cortical activity may play a role in the pathophysiology of photosensitive epilepsy.

Visualizing the blind brain: brain imaging of visual field defects from early recovery to rehabilitation techniques

Visual field defects (VFDs) are one of the most common consequences observed after brain injury, especially after a stroke in the posterior cerebral artery territory. Less frequently, tumors, traumatic brain injury, brain surgery or demyelination can also determine various visual disabilities, from a decrease in visual acuity to cerebral blindness. Visual field defects is a factor of bad functional prognosis as it compromises many daily life activities (e.g., obstacle avoidance, driving, and reading) and therefore the patient's quality of life. Spontaneous recovery seems to be limited and restricted to the first 6 months, with the best chance of improvement at 1 month. The possible mechanisms at work could be partly due to cortical reorganization in the visual areas (plasticity) and/or partly to the use of intact alternative visual routes, first identified in animal studies and possibly underlying the phenomenon of blindsight. Despite processes of early recovery, which is rarely complete, and learning of compensatory strategies, the patient's autonomy may still be compromised at more chronic stages. Therefore, various rehabilitation therapies based on neuroanatomical knowledge have been developed to improve VFDs. These use eye-movement training techniques (e.g., visual search, saccadic eye movements), reading training, visual field restitution (the Vision Restoration Therapy, VRT), or perceptual learning. In this review, we will focus on studies of human adults with acquired VFDs, which have used different imaging techniques (Positron Emission Tomography, PET; Diffusion Tensor Imaging, DTI; functional Magnetic Resonance Imaging, fMRI; Magneto Encephalography, MEG) or neurostimulation techniques (Transcranial Magnetic Stimulation, TMS; transcranial Direct Current Stimulation, tDCS) to show brain activations in the course of spontaneous recovery or after specific rehabilitation techniques.

Phosphene-guided transcranial magnetic stimulation of occipital but not parietal cortex suppresses stimulus visibility

Transcranial magnetic stimulation (TMS) applied over the occipital lobe approximately 100 ms after the onset of a stimulus decreases its visibility if it appears in the location of the phosphene. Because phosphenes can also be elicited by stimulation of the parietal regions, we asked if the same procedure that is used to reduce visibility of stimuli with occipital TMS will lead to decreased stimulus visibility when TMS is applied to parietal regions. TMS was randomly applied at 0-130 ms after the onset of the stimulus in steps of 10 ms in occipital and parietal regions. Participants responded to the orientation of the line stimulus and rated its visibility. We replicate previous reports of phosphenes from both occipital and parietal TMS. As previously reported, we also observed visual suppression around the classical 100 ms window both in the objective line orientation and subjective visibility responses with occipital TMS. Parietal stimulation, on the other hand, did not consistently reduce stimulus visibility in any time window.

An asymmetry of translational biological motion perception in schizophrenia

Background: Biological motion perception is served by a network of regions in the occipital, posterior temporal, and parietal lobe, overlapping areas of reduced cortical volume in schizophrenia. The atrophy in these regions is assumed to account for deficits in biological motion perception described in schizophrenia but it is unknown whether the asymmetry of atrophy found in previous studies has a perceptual correlate. Here we look for possible differences in sensitivity to leftward and rightward translation of point-light biological motion in data collected for a previous study and explore its underlying neurobiology using functional imaging.

Methods:n = 64 patients with schizophrenia and n = 64 controls performed a task requiring the detection of leftward or rightward biological motion using a standard psychophysical staircase procedure. six control subjects took part in the functional imaging experiment.

Results: We found a deficit of leftward but not rightward biological motion (leftward biological motion % accuracy patients = 57.9% ± 14.3; controls = 63.6% ± 11.3 p = 0.01; rightward biological motion patients = 62.7% ± 12.4; controls = 64.1% ± 11.7; p > 0.05). The deficit reflected differences in distribution of leftward and rightward accuracy bias in the two populations. Directional bias correlated with functional outcome as measured by the Role Functioning Scale in the patient group when co-varying for negative symptoms (r = -0.272, p = 0.016). Cortical regions with preferential activation for leftward or rightward translation were identified in both hemispheres suggesting the psychophysical findings could not be accounted for by selective atrophy or functional change in one hemisphere alone.

Conclusion: The findings point to translational direction as a novel functional probe to help understand the underlying neural mechanisms of wider cognitive dysfunction in schizophrenia.

Functional organisation of visual pathways in a patient with no optic chiasm

Congenital achiasma offers a rare opportunity to study reorganization and inter-hemispheric communication in the face of anomalous inputs to striate cortex. We report neuroimaging studies of a patient with seesaw nystagmus, achiasma, and full visual fields. The subject underwent structural magnetic resonance imaging (MRI), diffusion tensor imaging (DTI) studies, and functional MRI (fMRI) using monocular stimulation with checkerboards, motion, objects and faces, as well as retinotopic quadrantic mapping. Structural MRI confirmed the absence of an optic chiasm, which was corroborated by DTI tractography. Lack of a functioning decussation was confirmed by fMRI that showed activation of only ipsilateral medial occipital cortex by monocular stimulation. The corpus callosum was normal in size and anterior and posterior commissures were identifiable. In terms of the hierarchy of visual areas, V5 was the lowest level region to be activated binocularly, as were regions in the fusiform gyri responding to faces and objects. The retinotopic organization of striate cortex was studied with quadrantic stimulation. This showed that, in support of recent findings, rather than projecting to an ectopic location contiguous with the normal retinotopic map of the ipsilateral temporal hemi-retina, the nasal hemi-retina's representation overlapped that of the temporal hemi-retina. These findings show that congenital achiasma can be an isolated midline crossing defect, that information transfer does not occur in early occipital cortex but at intermediate and higher levels of the visual hierarchy, and that the functional reorganisation of striate cortex in this condition is consistent with normal axon guidance by a chemoaffinity gradient.

Intermixed structure of voxels with different hemispheric characteristics in the fusiform face area

The fusiform face area, a high-level visual area, is pivotal in processing facial information. This area receives inputs from the left and right visual fields unlike the primary visual area, which only receives inputs from its contralateral visual field. Response of the fusiform face area to ipsilateral stimulation depends on the signals crossing over at the corpus callosum. We investigated the distribution of voxel-wise activation to determine whether ipsilateral-dominant voxels exist in the fusiform face area using high spatial resolution functional MRI at 7 T. We further examined the possible functional differences between ipsilateral-dominant and contralateral-dominant voxels. By unilateral visual field stimulation, we detected ipsilateral-dominant voxels in the right fusiform face area. Their distribution was spatially heterogeneous. We tested upright and inverted facial stimulation confined to unilateral visual fields and found that these ipsilateral-dominant voxels had a different functional nature from contralateral-dominant voxels.

Interaction of cortical networks mediating object motion detection by moving observers

The task of parceling perceived visual motion into self- and object motion components is critical to safe and accurate visually guided navigation. In this paper, we used functional magnetic resonance imaging to determine the cortical areas functionally active in this task and the pattern connectivity among them to investigate the cortical regions of interest and networks that allow subjects to detect object motion separately from induced self-motion. Subjects were presented with nine textured objects during simulated forward self-motion and were asked to identify the target object, which had an additional, independent motion component toward or away from the observer. Cortical activation was distributed among occipital, intra-parietal and fronto-parietal areas. We performed a network analysis of connectivity data derived from partial correlation and multivariate Granger causality analyses among functionally active areas. This revealed four coarsely separated network clusters: bilateral V1 and V2; visually responsive occipito-temporal areas, including bilateral LO, V3A, KO (V3B) and hMT; bilateral VIP, DIPSM and right precuneus; and a cluster of higher, primarily left hemispheric regions, including the central sulcus, post-, pre- and sub-central sulci, pre-central gyrus, and FEF. We suggest that the visually responsive networks are involved in forming the representation of the visual stimulus, while the higher, left hemisphere cluster is involved in mediating the interpretation of the stimulus for action. Our main focus was on the relationships of activations during our task among the visually responsive areas. To determine the properties of the mechanism corresponding to the visual processing networks, we compared subjects' psychophysical performance to a model of object motion detection based solely on relative motion among objects and found that it was inconsistent with observer performance. Our results support the use of scene context (e.g., eccentricity, depth) in the detection of object motion. We suggest that the cortical activation and visually responsive networks provide a potential substrate for this computation.

Modulus and direction of the neural current vector identify distinct functional connectivity modes between human MT+ areas

Reconstruction of neural current sources from magnetoencephalography (MEG) data provides two independent estimates of the instantaneous current modulus and its direction. Here, we explore how different information on the modulus and direction affects the inter-hemisphere connectivity of the human medial temporal complex (hMT+). Connectivity was quantified by mutual information values of paired time series of current moduli or directions, with the joint probability distribution estimated with an optimized Gaussian kernel. These time series were obtained from tomographic analysis of single-trial MEG responses to a visual motion stimulus. With a high-contrast stimulus, connectivity measures based on the modulus were relatively strong in the prestimulus period, continuing until 100 ms after stimulus onset. The strongest modulus connectivity was produced with a long lag (19 ms) of the right hMT+ after the left hMT+. On the other hand, connectivity measures based on direction were relatively strong after 100 ms, with a short delay of less than 6 ms. These results suggest that nonspecific and probably indirect communication between the homologous areas is turned, by the stimulus arrival, into more precise and direct communication through the corpus callosum. The orientation of the estimated current vector for the strong connectivity can be explained by the curvature of the active cortical sheet. The temporal patterns of modulus and directional connectivity were different at low contrast, but similar to those at high contrast. We conclude that the modulus and direction indicate distinct functional connectivity modes.

Temporal characteristics of global motion processing revealed by transcranial magnetic stimulation

The ability to detect the motion of objects is critical to survival, and understanding the cortical mechanisms involved in this process remains a key challenge in sensory neuroscience. A relatively new approach to this problem is to temporarily disrupt processing at specific cortical sites and measure the behavioural consequences. Several previous studies have shown that transcranial magnetic stimulation (TMS) of human visual area V5/MT disrupts global motion perception, but reports vary widely in the timescale of this effect. To resolve this issue we employed psychophysical techniques to investigate how discrimination of translational, rotational and radial global motion is affected by TMS. Prior to applying TMS we established baseline coherence thresholds for global motion perception. Adopting each observer's coherence level at threshold we examined how TMS delivered to V5/MT modulated performance. Importantly, we measured the influence of single-pulse TMS over a broad temporal range to reveal the fine temporal structure of the disruption profile for global motion perception. Results show that the disruption profile consisted of two distinct epochs during which global direction judgments were reliably impaired, separated by an interval in which performance was unaffected. The bimodal nature of the distribution profiles is consistent with feedforward and feedback processing between visual areas mediating global motion processing. We present a novel quantitative model that characterizes the contribution of each process to visual motion perception.

Stimulus-contrast-induced biases in activation order reveal interaction between V1/V2 and human MT+

The luminance contrast of a visual stimulus is known to modulate the response properties of areas V1 and the human MT complex (hMT+), but has not been shown to modulate interactions between these two areas. We examined the direction of information transfer between V1/V2 and hMT+ at different stimulus contrasts by measuring magnetoencephalographic (MEG) responses to moving and stationary stimuli presented centrally or peripherally. To determine the direction of information flow, the different response latencies among stimuli and hemispheres in V1/V2 was compared with those of hMT+. At high contrast, responses to stimulus motion and position began in V1/V2, and were followed in hMT+ with a delay between 34 and 55 ms. However, at low contrast, lateralized responses in hMT+ came first, with those in V1/V2 lagging with a delay of 27 ms. Also, at high contrast, stationary stimuli produced greater responses than motion stimuli in V1/V2, while the reverse was true in hMT+, whose response lagged behind the initial response in V1/V2. The same activation order was found using Mutual Information Analysis of the response variances for each condition. Here, the response variances in hMT+ mimicked and trailed those of V1/V2 at high contrast, whereas the reverse was true at low contrast. Such consistent interactions found using two different methodologies strongly supports a processing link between these two areas. The results also suggest that feedback from hMT+ for low-contrast stimuli compensates for unresolved processing in V1/V2 when the input of a visual image is weak.

Visual trajectory perception in humans: is it lateralized? Clues from online rTMS of the middle-temporal complex (MT/V5)

Inconsistent observations have been reported in the literature regarding the asymmetrical contribution of higher visual areas of the left and right hemispheres to visual motion processing. In the present experiment, we tested for hemispheric asymmetry of the middle-temporal complex (V5/MT), which is a key-component of the visual motion network, by using rTMS applied over left or right V5/MT during a visual trajectory perception task. The results showed that the effect of rTMS was to enhance individual hemispheric asymmetries present when the test was performed without rTMS. The more general meaning of these results is that there are robust individual hemispheric asymmetries in motion perception but no general pattern of hemispheric differences.

Disentangling neural structures for processing of high- and low-speed visual motion

Human psychophysical and electrophysiological evidence suggests at least two separate visual motion pathways, one tuned to a lower and one tuned to a broader and partly overlapping range of higher speeds. It remains unclear whether these two different channels are represented by different cortical areas or by sub-populations within a single area. We recorded evoked potentials at 59 scalp locations to the onset of a slow (3.5 degrees /s) and fast (32 degrees /s) moving test pattern, preceded by either a slow or fast adapting pattern that moved in either the same direction or opposite to the test motion. Baseline potentials were recorded for slow and fast moving test patterns after adaptation to a static pattern. Comparison of adapted responses with baseline responses revealed that the N2 peak around 180 ms after test stimulus onset was modulated by the preceding adaptation. This modulation depended on both direction and speed. Source localization of baseline potentials as well as direction-independent motion adaptation revealed cortical areas activated by fast motion to be more dorsal, medial and posterior compared with neural structures underlying slow motion processing. For both speeds, the direction-dependent component of this adaptation modulation occurred in the same area, located significantly more dorsally compared with neural structures that were adapted in a direction-independent manner. These results demonstrate for the first time the cortical separation of more ventral areas selectively activated by visual motion at low speeds (and not high speeds) and dorsal motion-sensitive cortical areas that are activated by both high and low speeds.

Changes in connectivity after visual cortical brain damage underlie altered visual function

The full extent of the brain's ability to compensate for damage or changed experience is yet to be established. One question particularly important for evaluating and understanding rehabilitation following brain damage is whether recovery involves new and aberrant neural connections or whether any change in function is due to the functional recruitment of existing pathways, or both. Blindsight, a condition in which subjects with complete destruction of part of striate cortex (V1) retain extensive visual capacities within the clinically blind field, is an excellent example of altered visual function. Since the main pathway to the visual cortex is destroyed, the spared or recovered visual ability must arise from either an existing alternative pathway, or the formation of a new pathway. Using diffusion-weighted MRI, we show that both controls and blindsight subject GY, whose left V1 is destroyed, show an ipsilateral pathway between LGN (lateral geniculate nucleus) and human motion area MT+/V5 (bypassing V1). However, in addition, GY shows two major features absent in controls: (i) a contralateral pathway from right LGN to left MT+/V5, (ii) a substantial cortico-cortical connection between MT+/V5 bilaterally. Both observations are consistent with previous functional MRI data from GY showing enhanced ipsilateral activation in MT+/V5. There is also evidence for a pathway in GY from left LGN to right MT+/V5, although the lesion makes its quantification difficult. This suggests that employing alternative brain regions for processing of information following cortical damage in childhood may strengthen or establish specific connections.

Perception of structured optic flow and random visual motion in infants and adults: a high-density EEG study

Electroencephalogram (EEG) was used in 8-month-old infants and adults to study brain electrical activity as a function of perception of structured optic flow and random visual motion. A combination of visual evoked potential (VEP) analyses and analyses of temporal spectral evolution (TSE, time-dependent spectral power) was carried out. Significant differences were found for the N2 component of VEP for optic flow versus random visual motion within and between groups. Both adults and infants showed shorter latencies for structured optic flow than random visual motion, and infants showed longer latencies, particularly for random visual motion, and larger amplitudes than adults. Both groups also showed significant differences in induced activity when TSE of the two motion stimuli (optic flow and random visual motion) was compared with TSE of a static dot pattern. Infants showed an induced decrease in the amplitudes in theta-band frequency, while adults showed an induced increase in beta-band frequency. Differences in induced activity for the two motion stimuli could, however, not be observed. Brain activity related to motion stimuli is different for infants and adults and the differences are observed both in VEPs and in induced activity of the EEG. To investigate how changes in locomotor development are related to accompanying changes in brain activity associated with visual motion perception, more data of infants with different experiences in self-produced locomotion are required.

Cortical processing of visual motion in young infants

High-density EEG was used to investigate the cortical processing of a rotating visual pattern in 2-, 3-, and 5-month-old infants and in adults. Motion induced ERP in the parietal and the temporal-occipital border regions (OT) was elicited at all ages. The ERP was discernable in the 2-months-olds, significant and unilateral in the 3-month-olds and significantly bilateral in the 5-month-olds and adults. The motion induced ERP in the primary visual area was absent in the 2-month-olds and later than in the OT area for the 3-month-olds indicating that information to OT may be supplied by the V1 bypass at these ages. The results are in agreement with behavioural and psychophysical data in infants.

Making the blindsighted see

A lesion of striate cortex, area V1, produces blindness in the retinotopically corresponding part of the visual field, although in some cases visual abilities in the blind field remain that are paradoxically devoid of conscious visual percepts ("blindsight"). Here we demonstrate that the blindsight subject GY can experience visual sensations of phosphenes in his blind field induced by transcranial magnetic stimulation (TMS). Such blind field percepts could only be induced when stimulation was applied bilaterally, i.e. over GY's area V5/MT in both hemispheres. Consistent with an earlier report [Cowey, A., & Walsh, V. (2000). Magnetically induced phosphenes in sighted, blind and blindsighted observers. Neuroreport, 11, 3269-3273], GY never experienced phosphenes when stimulation was restricted to his ipsilesional V5/MT. To the best of our knowledge this is the first time GY has experienced visual qualia in his blind hemifield. The present report characterizes the necessary conditions for such conscious experience in his hemianopic visual field and interprets them as demonstrating that only via a contribution from GY's intact hemisphere can activation in the damaged hemisphere reach visual awareness.

Hemifield dependence of responses to colour in human fusiform gyrus

To investigate the hemifield dependence of visually evoked responses to colour in the human fusiform gyrus we recorded evoked potentials from subdural electrodes in a patient suffering from occipital epilepsy. The responses in the fusiform gyrus show a strong hemifield dependence and discriminate the onset from the offset of the stimulus. Additionally, we found responses to squares made of random dots, whereas no responses were found to squares with a homogeneous bright surface. Our findings further support the idea that the fusiform gyrus is related to colour and pattern perception. However, the hemifield dependence we found may indicate that further processing is required in order to combine information from both visual hemifields.

Temporal pattern of source activities evoked by different types of motion onset stimuli

The aim of this study was to compare the time course of motion-related source activities evoked by the onset of different kinds of visual motion stimuli in human subjects. Event-related potentials (ERP) were recorded from 64 scalp electrodes in ten healthy subjects while they were viewing four different types of motion stimuli (translation, rotation, expansion and contraction). Following a new approach combining a current density reconstruction with clustering algorithms, source maxima in the time range from 50 to 400 ms after the onset of the visual stimulus were localized and the time courses of activation were elaborated. Six regions contributed significantly to source activity, half originating in the occipital lobe and half in the right parietal and right temporal cortex. The comparison of their time courses led to the following conclusions: (i) the different kinds of motion stimuli activated about the same areas of the brain but with different temporal patterns. (ii) Mainly parietal and extrastriate areas, but not V1/V2, were significantly involved in the differentiation of different kinds of motion. (iii) Contrasting the different kinds of motion onsets, responses from parietal areas were found mainly before those from lateral occipital areas. (iv) The classically defined N2 and P2 components were significantly different among the four motion conditions, but not P1. The N2 motion-related component was elicited not only by lateral occipital areas and middle temporal areas but also by right parietal areas. (v) The rotation condition evoked a novel component P180, concomitant with an increased activity in the left middle temporal gyrus.

Visuo-motor pathways in humans revealed by event-related fMRI

Whether different brain networks are involved in generating unimanual responses to a simple visual stimulus presented in the ipsilateral versus contralateral hemifield remains a controversial issue. Visuo-motor routing was investigated with event-related functional magnetic resonance imaging (fMRI) using the Poffenberger reaction time task. A 2 hemifield x 2 response hand design generated the "crossed" and "uncrossed" conditions, describing the spatial relation between these factors. Both conditions, with responses executed by the left or right hand, showed a similar spatial pattern of activated areas, including striate and extrastriate areas bilaterally, SMA, and M1 contralateral to the responding hand. These results demonstrated that visual information is processed bilaterally in striate and extrastriate visual areas, even in the "uncrossed" condition. Additional analyses based on sorting data according to subjects' reaction times revealed differential crossed versus uncrossed activity only for the slowest trials, with response strength in infero-temporal cortices significantly correlating with crossed-uncrossed differences (CUD) in reaction times. Collectively, the data favor a parallel, distributed model of brain activation. The presence of interhemispheric interactions and its consequent bilateral activity is not determined by the crossed anatomic projections of the primary visual and motor pathways. Distinct visuo-motor networks need not be engaged to mediate behavioral responses for the crossed visual field/response hand condition. While anatomical connectivity heavily influences the spatial pattern of activated visuo-motor pathways, behavioral and functional parameters appear to also affect the strength and dynamics of responses within these pathways.

Transcranial magnetic stimulation of the left human frontal eye fields eliminates the cost of invalid endogenous cues

Humans are able to selectively attend to specific regions of space without moving their eyes. However, there is mounting evidence that these covert shifts of attention may employ many of the same brain regions involved when executing the eye movements. For example, functional magnetic resonance imaging (fMRI) studies show that the oculomotor region known as the frontal eye fields (FEF) are activated by the covert shifts of attention. However, it remains possible that the activations seen in these studies result from actively inhibiting eye movements rather than as a direct result of modulating perceptual processing. Here we provide direct evidence for the role of this region in endogenously driven spatial attention. We show that briefly disrupting the left FEFs with transcranial magnetic stimulation (TMS) eliminated the slow response times associated with the invalid strategic cues when the target appeared in the right visual field. At first glance, our findings appear incompatible with the results reported by Grosbras and Paus (Grosbras, M. -H., & Paus, T. (2002). Transcranial magnetic stimulation of the human frontal eye field: effects on visual perception and attention. Journal of Cognitive Neuroscience, 14(7), 1109--1120) and we suggest this is likely due to the design differences. Specifically, we disrupted the FEF at the time of cue onset, rather than target onset. Taken together with the findings of Grosbras and Paus, our findings suggest that the FEF plays an early role in the inhibition of perceptual information. Furthermore, our findings complement work by Ro et al. (Ro, T., Farne, F., & Chang, E. (2003). Inhibition of return and the frontal eye fields. Experimental Brain Research, 150, 290--296) who report that stimulation of the frontal eye fields disrupts the inhibitory consequences of reflexive attention shifts.

Motion-Sensitive Neurones in V5/MT Modulate Perceived Spatial Position

Until recently, it was widely believed that object position and object motion were represented independently in the visual cortex. However, several studies have shown that adaptation to motion produces substantial shifts in the perceived position of subsequently viewed stationary objects. Two stages of motion adaptation have been proposed: an initial stage at the level of V1 and a secondary stage thought to be located in V5/MT. Indeed, selective adaptation can be demonstrated at each of these levels of motion analysis. What remains unknown is which of these cortical sites are involved in modulating the positional representation of subsequently viewed objects. To answer this question directly, we disrupted cortical activity by using transcranial magnetic stimulation (TMS) immediately after motion adaptation. When TMS was delivered to V5/MT after motion adaptation, the perceived offset of the test stimulus was greatly reduced. In marked contrast, TMS of V1 had no effect on the changes that normally occur in perceived position after motion adaptation. This result demonstrates that the anatomical locus at which motion and positional information interact is area V5/MT rather than V1/V2.

Physiology of Human Photosensitivity

Human epileptic photosensitivity has been studied in several ways. (a) Visual stimulation that resembles the stimulation normally responsible for seizures, such as that from televisions or videogames, both of which typically use cathode ray tubes in which the display is created in a flickering pattern. Such stimulation is often rendered yet more epileptogenic by programmes with content that also involves flashing or patterned material. (b) Elementary visual stimuli that enable inferences to be drawn concerning the physiological trigger mechanisms. The topographic distribution of epileptiform EEG activity in response to such stimuli has complemented this approach, leading to the inference that the trigger is cortical and requires sychronised mass action of neurons. (c) Stimuli that avoid paroxysmal EEG activity and permit an investigation of the subepileptic response to visual stimuli, using the evoked potential. This has revealed abnormalities in the cortical mechanisms that control the response to strong visual stimulation.

Temporal and spatial congruence of components of motion-onset evoked responses investigated by whole-head magneto-electroencephalography

Motion-onset related components in averaged whole head co-recorded MEG and EEG responses of 24 adults to a low-contrast checkerboard pattern were studied. The aims were to identify these components, to characterize quantitatively their maps and to localize the underlying sources by equivalent-current-dipole (ECD) analyses with a spherical head model.After a weak P1, a large start-elicited negativity arises, comprising the novel N2a (occipital positive and parieto-central negative, peak-latency 141 ms) and the N2 like N2b (bilateral parieto-temporal, 175 ms) component. It is followed by a large positive stop-related component, P2 (156 ms after motion-offset). The corresponding MEG components N2am and N2bm showed bilateral dipole fields with considerable overlap. P1m has a single dipole field around the midline. N2a(m) and N2b(m) can be modelled with two bilateral ECDs with significant different locations. The study shows that accurate mapping and ECD analyses can distinguish two neighbouring areas of the visual cortex, 21+/-4 (SE) mm separated, which activities are reflected in both spatio-temporally closely related N2(m) components. N2a(m) and N2b(m) originate in the extrastriate cortex, possibly close to or in V3/V3A and MT/V5 respectively. Motion-evoked activity in (near) V3/V3A is novel on the basis of EEG data.

Human V5 demonstrated by magnetoencephalography using random dot kinematograms of different coherence levels

To investigate the cortical mechanisms for motion perception in human V5, we measured visual evoked magnetic fields in response to random dot kinematograms (RDKs) of three different coherence levels (50, 70 and 100%) using a 122-channel whole-head magnetometer. As the coherence level increased, the peak amplitude measured by the root mean square (RMS) of the local response increased significantly (7.4+/-1.0, 9.5+/-1.5 and 15.5+/-3.2 fT/cm on the right, 6.4+/-0.3, 7.8+/-0.7 and 12.5+/-0.9 fT/cm on the left; for the coherence level of 50, 70 and 100%, respectively). There was no significant difference between the hemispheres. As for the peak latency, there was no significant difference in terms of coherence levels or hemispheres. The response was localized posterior to the junction of the ascending limb of the inferior temporal and lateral occipital sulci (human V5). These findings indicate that processing of global motion in terms of the synchronized portion correlates well with the response amplitude but not with its latency. Thus, we could estimate the magnetic responses of human V5 non-invasively by presenting different coherence levels of the visual motion stimuli. Hemispheric laterality was recognized, although the dominant side varied among subjects.

Functional magnetic resonance imaging and its clinical utility in patients with visual disturbances

Functional magnetic resonance imaging (fMRI) is a powerful, non-invasive technique for mapping human brain function. Because of the robust signal intensity changes associated with visual stimuli, fMRI is particularly useful for studying visual cortex (including both striate and extrastriate cortex). Also, activation of the lateral geniculate nuclei has been successfully demonstrated by fMRI. Therefore, fMRI may be potentially useful in patients with visual deficits by providing a non-invasive method for assessing the afferent visual pathways and higher cortical areas. Although there have been several reviews on fMRI, few have highlighted its clinical applicability in patients with visual disturbances. Our article will review fMRI principles and methodology, then focus on the possible applications and limitations of this technique in clinical ophthalmology.

Brain representation of hemifield stimulation in poststroke visual field defects

BACKGROUND AND PURPOSE

Plasticity in extended, parallel, or reciprocal operating networks is well recognized. Changes in neuronal activity after lesions to distinct localized structures, such as the primary visual cortex, are less well characterized. We investigated the cortical reorganization in patients with poststroke visual field defects using blood oxygen level-dependent functional MRI.

METHODS

Brain activation was measured in 7 patients with a single occipital cortical lesion and partially recovered hemianopia and in 7 age-matched control subjects. Differences in activation between rest and visual hemifield stimulation were assessed with statistical parametric mapping (SPM'99).

RESULTS

In normal subjects, significant activation was found in the contralateral primary visual cortex and bilaterally in the extrastriate cortex. During hemifield stimulation of the unaffected side of stroke patients, a similar pattern was found compared with that seen in control subjects. During stimulation of the hemianopic side, bilateral activation was seen within the extrastriate cortex, stronger in the ipsilateral hemisphere. The primary visual cortex was not significantly activated in either hemisphere during stimulation of the hemianopic side.

CONCLUSIONS

Visual field defects after stroke are associated with bilateral activation of the extrastriate visual cortex. This pattern of activation indicates altered neuronal activity in the visual system. Further investigation is necessary to determine the relationship between functional reorganization and recovery of lost visual function after poststroke hemianopia.

Topography of evoked potentials associated with illusory motion perception as a motion aftereffect

Motion aftereffect (MAE) is a type of motion illusion. After visual focusing on an object moving in one direction, an illusory perception of motion in the opposite direction occurs while the object suddenly stops moving. In this study we explored components and distribution of evoked potentials related to this motion illusion using MAE caused by motion of concentric rings. When a single array of moving rings was placed to straddle right and left visual fields, a significant bilateral increase of a positive component at about 160 ms (P160) was observed in the occipitotemporal region at the time subjects perceived the motion illusion; this increase was most prominent in the right posterior temporal region. Thus, an early positive component P160 occurs in relation to motion illusion, in agreement with previous results concerning perception of actual motion. When stimuli were presented to produce MAE limited to either the right or left visual hemifield, we also observed a P160 distributed mainly in the right temporal and parietal region. A significant increase in this component was observed in the right posterior temporal region with left hemifield stimulation, while no significant increase was observed with right hemifield stimulation. The right hemispheric dominance of P160 seemed to result partly from functional specialization of the right hemisphere, but hemispheric differences in attentional mechanisms also might contribute to the asymmetric distribution of P160.

Functional magnetic resonance imaging of the visual system

Functional magnetic resonance imaging (fMRI), which is a technique useful for non-invasive mapping of brain function, is well suited for studying the visual system. This review highlights current clinical applications and research studies involving patients with visual deficits. Relevant reports regarding the investigation of the brain's role in visual processing and some newer fMRI techniques are also reviewed. Functional magnetic resonance imaging has been used for presurgical mapping of visual cortex in patients with brain lesions and for studying patients with amblyopia, optic neuritis, and residual vision in homonymous hemianopia. Retinotopic borders, motion processing, and visual attention have been the topics of several fMRI studies. These reports suggest that fMRI can be useful in clinical and research studies in patients with visual deficits.

Localization and globalization in conscious vision

The primate visual brain consists of many separate, functionally specialized processing systems, each consisting of several apparently hierarchical stages or nodes. The evidence reviewed here leads me to speculate (a) that the processing systems are autonomous with respect to one another, (b) that activity at each node reaches a perceptual end point at a different time, resulting in a perceptual asynchrony in vision, and (c) that, consequently, activity at each node generates a microconsciousness. Visual consciousness is therefore distributed in space and time, with the universal organizing principle of abstraction applied separately within each processing system. The consequence of spatially and temporally distributed microconsciousnesses is that their integration is a multistage, nonhierarchical process that may involve a neural "glue."

Imaging image processing in the human brain

Functional imaging in humans reveals the interplay of the many components of the human visual system: how they process the various types of information contained in the image to recover characteristics of the three-dimensional world surrounding us, but also how, in the course of this process, the retinal image is gradually integrated with non-retinal signals to provide information about the outside world in a format useful to other non-visual brain regions.