Visuotopic organization of projections from striate cortex to inferior and lateral pulvinar in rhesus monkey

Anatomical material from two series of monkeys (Macaca mulatta)was used to determine the full extent and visuotopic organization of striate projections to the pulvinar. One series was processed for degeneration by the Fink-Heimer procedure following unilateral lesions of lateral, posterior, or medial striate cortex (representing the central, peripheral, and far peripheral visual field, respectively); collectively, the lesions included all of area 17. The second series was processed for autoradiography following tritiated amino-acid injections into striate sites representing the center of gaze and eccentricities ranging from 0.5° to greater than 30° from fixation in both the upper and lower fields. The results indicate the existence of two separate striate projection zones within the pulvinar. One, the PI/PL zone, is located primarily within the inferiorpulvinar (PI) but extends into the adjacentlateral pulvinar (PL). The other, the PL zone, is located entirely within the lateral pulvinar and partially surrounds the first zone along its dorsal, lateral, and ventral aspects. Within the PI/PL zone, striate projections are topographically organized and represent the entire contralateral visual field. Central vision is represented laterally and posteriorly, with the fovea represented at the caudal pole of the nucleus; conversely, far peripheral vision is found medially and anteriorly, adjacent to the medial geniculate nucleus. The representation of the horizontal meridian runs obliquely across PI/PL, such that the upper visual field is located ventrolaterally and the lower visual field dorsomedially. The representation of the vertical meridian is located along the lateral margin of PI in anterior sections of the pulvinar, but within PL in posterior sections. Thus, the vertical meridian appears to form the border between the lateral margin of the PI/PL zone and the medial margin of the PL zone. At the lateral margin of the PL zone is the representation of its horizontal meridian. Striate projections to the PL zone, unlike those to the PI/PL zone, are limited to the representation of central vision. These results suggest that striate inputs contribute to the visual properties of neurons (Bender, 1981 a) throughout the PI/PL zone, but are insufficient to explain the visual properties of neurons outside of the central visual field representation in the PL zone.     

A double dissociation between striate and extrastriate visual cortex for pattern motion perception revealed using rTMS

The neural mechanisms underlying the integration and segregation of motion signals are often studied using plaid stimuli. These stimuli consist of two spatially coincident dynamic gratings of differing orientations, which are either perceived to move in two unique directions or are integrated by the visual system to elicit the percept of a checkerboard moving in a single direction. Computations pertaining to the motion of the individual component gratings are thought to take place in striate cortex (V1) whereas motion integration is thought to involve neurons in dorsal stream extrastriate visual areas, particularly V5/MT. By combining a psychophysical task that employed plaid stimuli with 1 Hz offline repetitive transcranial magnetic stimulation (rTMS), we demonstrated a double dissociation between striate and extrastriate visual cortex in terms of their contributions to motion integration. rTMS over striate cortex increased coherent motion percepts whereas rTMS over extrastriate cortex had the opposite effect. These effects were robust directly after the stimulation administration and gradually returned to baseline within 15 minutes. This double dissociation is consistent with previous patient data and the recent hypothesis that both coherent and transparent motion percepts are supported by the visual system simultaneously and compete for perceptual dominance. Hum Brain Mapp 2009. © 2009 Wiley‐Liss, Inc.     

Unidirectional visual motion adaptation induces reciprocal inhibition of human early visual cortex excitability

ObjectivesBehavioural observations provided by the waterfall illusion suggest that motion perception is mediated by a comparison of responsiveness of directional selective neurones. These are proposed to be optimally tuned for motion detection in different directions. Critically however, despite the behavioural observations, direct evidence of this relationship at a cortical level in humans is lacking. By utilising the state dependant properties of transcranial magnetic stimulation (TMS), one can probe the excitability of specific neuronal populations using the perceptual phenomenon of phosphenes.MethodWe exposed subjects to unidirectional visual motion adaptation and subsequently simultaneously measured early visual cortex (V1) excitability whilst viewing motion in the adapted and non-adapted direction.ResultFollowing adaptation, the probability of perceiving a phosphene whilst viewing motion in the adapted direction was diminished reflecting a reduction in V1 excitability. Conversely, V1 excitability was enhanced whilst viewing motion in the opposite direction to that used for adaptation.ConclusionOur results provide support that in humans a process of reciprocal inhibition between oppositely tuned directionally selective neurones in V1 facilitates motion perception.SignificanceThis paradigm affords a unique opportunity to investigate changes in cortical excitability following peripheral vestibular disorders.     

Response latencies of neurons in visual areas MT and MST of monkeys with striate cortex lesions

Cortical area, MT (middle temporal area) is specialized for the visual analysis of stimulus motion in the brain. It has been suggested [Brain 118 (1995) 1375] that motion signals reach area MT via two dissociable routes, namely a 'direct' route which bypasses primary visual cortex (area, striate cortex (V1)) and is specialized for processing 'fast' motion (defined as faster than 6 degrees/s) with a relatively short latency, and an 'indirect' route via area V1 for processing 'slow' motion (slower than 6 degrees/s) with a relatively long latency. We tested this proposal by measuring the effects of unilateral V1 lesions on the magnitudes and latencies of responses to fast- and slow-motion (depicted by random dot kinematograms (RDK) ) of single neurons in areas MT and medial superior temporal area (MST) of anaesthetized macaque monkeys. In the unlesioned hemisphere contralateral to a V1 lesion, response magnitudes and latencies of MT neurons were similar to those previously reported from MT neurons in normal monkeys, and there was no significant association between slow movement and long response latency (>100 ms), or between fast movement and short latency (< or =100 ms). V1 lesions led to diminished response magnitudes and increased latencies in area MT of the lesioned hemisphere, but did not selectively abolish MT responses to slow moving stimuli, or abolish long-latency responses to either slow- or fast-moving stimuli. Response magnitudes and latencies in area MST, which receives visual inputs directly from area MT and is also specialized for visual analysis of motion, were unaffected by V1 lesions (though we have shown elsewhere that directionally-selective responses in both areas were impaired by V1 lesions). Overall, the results are incompatible with the hypothesis that there are dissociable routes to MT specialized for processing separately fast and slow motion.     

Direction discrimination of moving gratings and plaids and coherence in dot displays without primary visual cortex (V1)

We present new experimental observations of G.Y., a well-tested patient with unilateral loss of primary visual cortex. We stimulated G.Y.'s blind hemifield using first- and second-order motion stimuli at velocities around psychophysical threshold. Using a dual response paradigm (awareness level of visual motion, motion direction discrimination) psychophysical performance improved with increasing velocity and dot coherence. We were also able to influence directly G.Y.'s performance for the better and at will, by placing the emphasis solely on direction discrimination. In the absence of V1, graduated detection and discrimination of stimuli known to activate both V1 and extrastriate motion areas MT/V5 and MST is still possible. These results are in line with residual visual processing but did not show evidence of unconscious processing of motion stimuli characteristic of ‘blindsight’.     

Relationships between global motion and global form processing, practice, cognitive and visual processing in adults with dyslexia or visual discomfort

The aim of the first of two experiments was to investigate the effect of practice on sensitivity to global motion and global form in a group of adults with dyslexia, a group of normal readers with visual discomfort, a group with dyslexia and visual discomfort, and a control group. In comparison to the control group, and regardless of the effect of practice, the group with dyslexia was significantly less sensitive to global motion. No differences in global motion sensitivity were found when individuals with dyslexia, with or without visual discomfort, were compared. The normal reading group with visual discomfort was less sensitive to global motion than the control group at baseline, but not when a second estimate of motion sensitivity was obtained. About 30% of the group with dyslexia had a global motion deficit on each threshold estimate. In contrast, there were no significant effects of practice or group on sensitivity to global form. In Experiment 2, performance on a number of cognitive and visual processing tasks was measured in four groups: two with dyslexia; one with and one without a global motion deficit, a normal reading group with visual discomfort, and a control group. The group with visual discomfort had reduced visual processing speed only. Regardless of whether a global motion processing deficit was present or absent in individuals with dyslexia, reduced accuracy was found on the language and visual processing measures, and on a rapid temporal sequencing task. Individuals with dyslexia and a global motion deficit had poorer accuracy than individuals with dyslexia and no motion deficit on the phonological processing and verbal short term memory tasks. We concluded that some adults with dyslexia have a persistent deficit when processing global motion but not global form. This is consistent with reports of a magnocellular pathway deficit in this group. Individuals with visual discomfort do not have a magnocellular processing deficit, but have perceptual difficulties when performing complex visual processing tasks.     

Distribution of neurons projecting to the superior colliculus correlates with thick cytochrome oxidase stripes in macaque visual area V2

In visual area V2 of macaque monkeys, cytochrome oxidase (CO) histochemistry reveals a pattern of alternating densely labeled thick and thin stripe compartments and lightly labeled interstripe compartments. This modular organization has been associated with functionally separate pathways in the visual system. We examined this idea further by comparing the pattern of CO stripes with the distribution of neurons in V2 that project to the superior colliculus. Visually evoked activity in the superior colliculus is known to be greatly reduced by blocking magnocellular but not parvocellular layers of the lateral geniculate nucleus (LGN). From previous evidence that V2 thick stripes are closely associated with the magnocellular LGN pathway, we predicted that a significant proportion of V2 neurons projecting to the superior colliculus would reside in the thick stripes. To test this prediction, the tangential distribution of retrogradely labeled corticotectal cells in V2 was compared with the pattern of CO stripes. We found that neurons projecting to the superior colliculus accumulated preferentially into band-like clusters that were in alignment with alternate CO dense stripes. These stripes were identified as thick stripes on the basis of their physical appearance and/or by their affinity to the monoclonal antibody Cat-301. A significantly smaller proportion of labeled cells was observed in thin and interstripe compartments. These data provide further evidence that the spatial distribution of subcortically projecting neurons can correlate with the internal modular organization of visual areas. Moreover, they support the notion that CO compartments in V2 are associated with functionally different pathways. J. Comp. Neurol. 377:313–323, 1997. © 1997 Wiley-Liss, Inc.     

Effects of visual cortex lesions on perceptual grouping in rats

Neural mechanisms mediating perceptual grouping serve to enhance associations among stimulus elements, thereby establishing unified forms. The goals of the present study were to identify cortical areas necessary to perceptually group spatially isolated elements, and to determine if these areas are distinct from regions necessary for the discrimination of simple, solid forms. Rats were trained to discriminate horizontal and vertical lines that were either solid or composed of disjunct elements in which discrimination required perceptual grouping by proximity. Psychophysical procedures established the limits at which proximity served as a cue for grouping. Following perceptual measurements, ablations were made to selective sites within visual cortex. Lesions within area 17 or area 18A, including their interface, produced nearly complete impairment of solid line discrimination as well as perceptual grouping at all levels of proximity, whereas lesions to areas 18 or the far lateral extent of area 18A no effect on these perceptual capacities. These results indicate that grouping by proximity requires early visual processing areas, and shares cortical areas necessary for simple pattern discrimination. These results suggest that mechanisms of grouping modify primary stimulus representations, constructing a pattern of activity functional similar to that elicited by solid forms.     

Auditory motion direction encoding in auditory cortex and high-level visual cortex

The aim of this functional magnetic resonance imaging (fMRI) study was to identify human brain areas that are sensitive to the direction of auditory motion. Such directional sensitivity was assessed in a hypothesis-free manner by analyzing fMRI response patterns across the entire brain volume using a spherical-searchlight approach. In addition, we assessed directional sensitivity in three predefined brain areas that have been associated with auditory motion perception in previous neuroimaging studies. These were the primary auditory cortex, the planum temporale and the visual motion complex (hMT/V5+). Our whole-brain analysis revealed that the direction of sound-source movement could be decoded from fMRI response patterns in the right auditory cortex and in a high-level visual area located in the right lateral occipital cortex. Our region-of-interest-based analysis showed that the decoding of the direction of auditory motion was most reliable with activation patterns of the left and right planum temporale. Auditory motion direction could not be decoded from activation patterns in hMT/V5+. These findings provide further evidence for the planum temporale playing a central role in supporting auditory motion perception. In addition, our findings suggest a cross-modal transfer of directional information to high-level visual cortex in healthy humans.     

Sensitivity of complex cells in cat striate cortex to relative motion

Sensitivity of 95 complex cells to relative motion between oriented bars and textured backgrounds was investigated monocularly in the striate cortex of lightly anesthetized, paralyzed cats. Cells were classified conventionally. Those in deep layers were either direction-selective, or strongly preferred one direction of motion, and responded well to background texture motion alone: backgrounds potentiated the response to the bar in the cell's preferred direction when moved in phase, or in the opposite direction when moved in antiphase; other combinations depressed the level of response compared with that for the bar alone. The majority of direction-selective or strongly direction-biased cells in superficial layers behaved similarly. The most interesting superficial-layer cells were bidirectional or weakly direction-biased, and recorded closer to the cortical surface than the direction-selective neurons. A majority showed preference for relative motion, some for antiphase, others for in-phase motion, regardless of the absolute direction of motion across the receptive field, which could not be accounted for on the basis of separate responses to bars and backgrounds alone. Three of the superficial-layer direction-selective cells also showed preference for antiphase relative motion. In a few complex cells from superficial laminae, backgrounds were either without influence on responses to oriented stimuli, or purely suppressive. Visual backgrounds against which objects are perceived are usually neither featureless nor motionless: the results suggest that most complex cells in striate cortex are sensitive to the context in which objects are seen and susceptible to relationships between objects and their backgrounds in relative motion.     

Effects of attention and arousal on early responses in striate cortex

Humans employ attention to facilitate perception of relevant stimuli. Visual attention can bias the selection of a location in the visual field, a whole visual object or any visual feature of an object. Attention draws on both current behavioral goals and/or the saliency of physical attributes of a stimulus, and it influences activity of different brain regions at different latencies. Attentional effect in the striate and extrastriate cortices has been the subject of intense research interest in many recent studies. The consensus emerging from them places the first attentional effects in extrastriate areas, which in turn modulate activity of V1 at later latencies. In this view attention influences activity in striate cortex some 150 ms after stimulus onset. Here we use magnetoencephalography to compare brain responses to foveally presented identical stimuli under the conditions of passive viewing, when the stimuli are irrelevant to the subject and under an active GO/NOGO task, when the stimuli are cues instructing the subject to make or inhibit movement of his/her left or right index finger. The earliest striate activity was identified 40-45 ms after stimulus onset, and it was identical in passive and active conditions. Later striate response starting at about 70 ms and reaching a peak at about 100 ms showed a strong attentional modulation. Even before the striate cortex, activity of the right inferior parietal lobule was modulated by attention, suggesting this region as a candidate for mediating attentional signals to the striate cortex.     

Prospective memory impairment in patients with white matter lesions

Objective: A vast majority of the episodic memory literature in white matter lesions (WML) had focused on “retrospective memory (RM)”, little was known about prospective memory (PM) in WML patients. The aim of our study was to investigate the effect of WML patients on event-based prospective memory (EBPM) and time-based prospective memory (TBPM). In addition, our study attempted to understand the possible mechanisms of PM damage in WML patients.

Methods: A total of 42 WML patients and 40 age and education level matched healthy controls were included. EBPM (an action whenever particular words were presented) and TBPM (an action at certain times) were performed to test the involvement of PM in WML. The extent of WML within cholinergic pathways were assessed using the cholinergic pathways hyperintensities scale (CHIPS).

Results: A significant difference was found in the performance of Montreal Cognitive Assessment (MOCA) (21.8?±?3.9 vs. 26.6?±?1.7, p?<?0.05) and TBPM (2.88?±?1.21 vs. 4.27?±?0.78, p?<?0.05), but not Mini-Mental State Examination (MMSE) (26.9?±?2.8 vs. 27.3?±?1.2, p?>?0.05) and EBPM (3.62?±?1.25 vs.4.47?±?1.11, p?>?0.05) in WML patients compared with the healthy controls. Moreover, TBPM and MOCA scores were negatively correlated with CHIPS scores.

Conclusions: WML patients were impaired in TBPM but not in EBPM, supporting that EBPM and TBPM have different neural mechanisms. Our results demonstrated that WML are involved in the TBPM probably by affecting the central cholinergic pathway.     

Seeing and extrapolating motion trajectories share common informative activation patterns in primary visual cortex

The natural environment is dynamic and moving objects become constantly occluded, engaging the brain in a challenging completion process to estimate where and when the object might reappear. Although motion extrapolation is critical in daily life—imagine crossing the street while an approaching car is occluded by a larger standing vehicle—its neural underpinnings are still not well understood. While the engagement of low‐level visual cortex during dynamic occlusion has been postulated, most of the previous group‐level fMRI‐studies failed to find evidence for an involvement of low‐level visual areas during occlusion. In this fMRI‐study, we therefore used individually defined retinotopic maps and multivariate pattern analysis to characterize the neural basis of visible and occluded changes in motion direction in humans. To this end, participants learned velocity‐direction change pairings (slow motion‐upwards; fast motion‐downwards or vice versa) during a training phase without occlusion and judged the change in stimulus direction, based on its velocity, during a following test phase with occlusion. We find that occluded motion direction can be predicted from the activity patterns during visible motion within low‐level visual areas, supporting the notion of a mental representation of motion trajectory in these regions during occlusion.     

Relative motion sensitivity in cat striate cortex as a function of stimulus direction

This paper presents the second stage of an investigation into the relative motion sensitivity of complex cells in the striate cortex of lightly anaesthetized adult cats. Relative motion between an oriented bar and a background of random visual texture was generated by moving both stimuli, in-phase or in antiphase, with dissimilar speed. Three configurations were compared: motion of both foreground bar and background texture in preferred directions for either (a) the bar or (b) the background, and also (c) where background motion was other than orthogonal to bar orientation. Providing foreground stimuli elicited substantial responses, sensitivity to relative motion was qualitatively but not quantitatively predictable from discrete responses to foreground or background alone, whatever the angular inclination between their respective directions, suggestive of strongly non-linear interactions between the two. Where the foreground evoked no response, or depressed firing, the pattern of sensitivity to relative motion could not be predicted.     

Phenomenology and neural correlates of implicit and emergent motor awareness in patients with anosognosia for hemiplegia

Anosognosia for hemiplegia (AH) is characterized by a lack of awareness of motor disorders and appears associated with fronto-temporal-parietal damage. Neuropsychological evidence indicates that behavioral indices of residual forms of motor awareness may co-exist with explicit denial of impairment. Here we explore whether the attempt by AH patients to perform an action may disclose residual forms of motor awareness and whether such forms are underpinned by different neural structures. Twelve hemiplegic patients affected by AH were tested in tasks assessing: (i) implicit awareness (IA), indexed by discrepancies between verbal reports and actual motor behavior; (ii) emergent awareness (EA), indexed by increased verbal awareness induced by the attempt to perform actions. IA and EA were found in five and three patients, respectively. Lesion analysis indicates that while the lack of IA is associated with damage to subcortical white matter anterior to the basal ganglia, lack of EA is linked to damage to cortical regions including insulo-frontal, temporal and parietal structures. Our results indicate that deficits in explicit and implicit awareness are associated with lesions involving different cortico-subcortical structures. Moreover, the results show that the attempt to perform an action may ameliorate body awareness deficits and have implications for rehabilitation.     

Neural selectivity for hue and saturation of colour in the primary visual cortex of the monkey

In the inferior temporal (IT) cortex of monkeys, which has been shown to play a critical role in colour discrimination, there are neurons sensitive to a narrow range of hues and saturation. By contrast, neurons in the retina and the parvocellular layer of the lateral geniculate nucleus (pLGN) encode colours in a way that does not provide explicit representation of hue or saturation, and the process by which hue- and saturation-selectivity is elaborated remains unknown. We therefore tested the colour-selectivity of neurons in the primary visual cortex (V1) and compared it with those of pLGN and IT neurons. Quantitative analysis was performed using a standard set of colours, systematically distributed within the CIE (Commission Internationale de l'Eclairage)-xy chromaticity diagram. Selectivity for hue and saturation was characterized by analysing response contours reflecting the overall distribution of responses across the chromaticity diagram. We found that the response contours of almost all pLGN neurons were linear and broadly tuned for hue. Many V1 neurons behaved similarly; nonetheless, a considerable number of V1 neurons had clearly curved response contours and were selective for a narrow range of hues or saturation. The relative frequencies of neurons exhibiting various selectivities for hue and saturation were remarkably similar in the V1 and IT cortex, but were clearly different in the pLGN. Thus, V1 apparently plays a very important role in the conversion of colour signals necessary for generating the elaborate colour selectivity observed in the IT cortex.     

Face processing among twins with and without autism: social correlates and twin concordance

Autism spectrum disorder (ASD) has a strong heritable basis, as evidenced by twin concordance rates. Within ASD, symptom domains may arise via independent genetic contributions, with varying heritabilities and genetic mechanisms. In this article, we explore social functioning in the form of (i) electrophysiological and behavioral measures of face processing (P1 and N170) and (ii) social behavior among child and adolescent twins with (N = 52) and without ASD (N = 66). Twins without ASD had better holistic face processing and face memory, faster P1 responses and greater sensitivity to the effects of facial inversion on P1. In contrast, N170 responses to faces were similar across diagnosis, with more negative amplitudes for faces vs non-face images. Across the sample, stronger social skills and fewer social difficulties were associated with faster P1 and N170 responses to upright faces, and better face memory. Twins were highly correlated within pairs across most measures, but correlations were significantly stronger for monozygotic vs dizygotic pairs on N170 latency and social problems. We suggest common developmental influences across twins for face processing and social behavior, but highlight (i) neural speed of face processing and (ii) social difficulties as important avenues in the search for genetic underpinnings in ASD.