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Interneuronal projections to identified cilia-activating pedal neurons in Hermissenda.

Neural networks have been shown to support the generation of more than one behavioral motor act. In the nudibranch mollusk Hermissenda, Pavlovian conditioning results in light, the conditioned stimulus (CS), evoking both inhibition of locomotion and foot contraction. The synaptic organization of the eyes and optic ganglion is well documented; however, the characterization of the neural network mediating visually modulated behaviors is incomplete. We have now characterized synaptic connections between identified photoreceptors and a newly identified interneuron (II(b)), identified synaptic projections from type I and type II interneurons to an inhibitory interneuron (III(i)) and to two newly identified pedal neurons, VP1 and VP2. Here we show that VP1 activates ciliary movement on the anterior foot and VP2 innervates the anterior foot and ventral tentacle. Stimulation of the photoreceptors with light produced two effects on the activity of VP1 and VP2. First, light inhibits type I(i) and II(i) interneurons and disinhibits VP1 and VP2. Depolarization of type II(e) interneurons also disinhibits VP1 and VP2. Second, the light-elicited depolarization and increased tonic activity of VP1 and VP2 is produced by excitatory synaptic input from ipsilateral and contralateral type II(b) interneurons. Pedal neurons VP1 and VP2 receive similar synaptic input from type I, II, and III(i) interneurons; this is in agreement with previous research showing that the visual pathway influences both ciliary locomotion and foot movement. The organization of the visual system in Hermissenda provides for the expression of cellular and synaptic plasticity supporting learning without altering the networks ability to carry out the requirements for normal visual processing.

Animals↗

Williams syndrome: neuronal size and neuronal-packing density in primary visual cortex.

BACKGROUND: Williams syndrome (WMS) is a rare, genetically based syndrome associated with a hemideletion in chromosome 7 (7q11.22-23) and characterized by a unique constellation of somatic, brain, and cognitive features. Individuals with WMS demonstrate an unusual and uneven neuropsychological profile showing cognitive and visual spatial deficits juxtaposed with relative language preservation and excellent facial recognition. OBJECTIVES: A neuroanatomical hypothesis for these behavioral findings suggests predominant involvement of the dorsal portions of the hemispheres relative to the ventral portions, including preferential involvement of peripheral visual field cortical representations over central representation. Predominant involvement of magnocellular visual pathways, as opposed to parvocellular pathways, is also suggested by this hypothesis. SUBJECTS: We examined primary visual cortical area 17 in the right and left hemispheres in 6 age- and sex-matched autopsy specimens from 3 WMS-affected brains (1 male and 2 females; mean [SD] age, 44 [14] years) and 3 control brains (1 male and 2 females; mean age, 43 [11] years). DESIGN: Neurons in layers II, III, IVA, IVB, IVCalpha, IVCbeta, V, and VI were measured using an optical dissector method to determine possible differences between WMS-affected and control brains in cell-packing density, neuronal size, and neuronal size distribution. RESULTS: We found abnormalities in peripheral visual cortex in WMS-affected brains, but not in magnocellular subdivisions. There was a hemisphere by layer IV interaction and a layer IV left hemisphere and diagnosis interaction in cell-packing density. Williams syndrome-affected brains showed increased cell-packing density in left sublayer IVCbeta and an excess of small neurons in left layers IVA, IVCalpha, IVCbeta, V, and VI. CONCLUSIONS: Cell measurements differ in peripheral visual cortical fields of WMS, with significantly smaller, more closely packed cells in some layers on the left side. These cell-packing density and neuronal size differences may be related to visuospatial deficits in this population.

Adult↗

Fibre organization of the monkey's optic tract: I. Segregation of functionally distinct optic axons.

The fibre organization of the monkey's optic tract was examined by implanting pellets of horseradish peroxidase into different locations within the tract, or into the superior colliculus and pretectum. Retinae were examined for the distribution, size, and morphological types of retrogradely labelled ganglion cells; optic tracts were examined for the distribution of anterogradely and retrogradely labelled axonal profiles; and lateral geniculate nuclei were examined for the distribution of anterogradely labelled processes within distinct geniculate laminae. Localized implants in the optic tract produced retrograde labelling of ganglion cells across wide regions of the retinal surface. The maximum density of labelled cells was always substantially less than the total ganglion cell density known to be present at those retinal loci. Distinct retinal ganglion cell types were labelled from differing regions within the optic tract: implants into the deep (dorsal) portion of the tract, far removed from the outer, pial, surface, retrogradely labelled predominantly P beta retinal ganglion cells, whereas implants into the superficial (ventral), subpial, part of the tract retrogradely labelled primarily the other retinal ganglion cell types, i.e., the P alpha, P gamma, and P epsilon cells. Within any given class of axon, there is a mapping of the centroperipheral retinal axis across the deep-to-superficial dimension of the tract, but this retinotopy is extremely coarse. Anterograde labelling of axonal terminations within the lateral geniculate nucleus showed a corresponding specificity for distinct geniculate laminae, the deep implants labelling the parvocellular laminae, superficial implants labelling the magnocellular laminae. Implants into the visual centres of the midbrain produced retrograde axonal labelling rostral to the lateral geniculate nucleus only in the superficial part of the optic tract. These results demonstrate that the monkey's optic tract is not a simple topographic mapping of retinal eccentricity. Rather, the primary organizational principle is that of a segregation of functionally distinct optic axon classes. As fibre order in the mammalian optic tract is also a chronological index of axonal arrival during development, the present results provide specific predictions about the temporal order of ganglion call genesis and axonal addition within the visual pathway. They also provide an anatomical basis for the functionally selective visual impairments that may arise following local damage to the optic tract in humans.

Animals↗

Correlation between optical coherence tomography, pattern electroretinogram, and visual evoked potentials in open-angle glaucoma patients.

OBJECTIVE: [corrected] To correlate the nerve fiber layer (NFL) thickness and the visual function evaluated by electrophysiologic retinal and cortical responses assessed in open-angle glaucoma (OAG) eyes. DESIGN: Prospective case-control study. PARTICIPANTS: Thirty glaucoma patients (mean age, 47.1 +/- 7.15 years; refractive error range, +/- 2 spherical equivalent) with a mean deviation of computerized static perimetry (24/2 Humphrey, Dublin, CA) from -5 to -28 dB and intraocular pressure less than 21 mmHg on pharmacologic treatment and 14 age-matched control participants. METHODS: Nerve fiber layer thickness was measured by optical coherence tomography. Retinal and visual pathway function was assessed by simultaneously recording pattern electroretinograms (PERGs) and visual evoked potentials (VEPs) using high-contrast (80%) checkerboard stimuli (the single check edges subtend 15 minutes of the visual arc) reversed at the rate of two reversals per second. Linear regression analyses were adopted to establish the correlation between NFL thickness and PERG and VEP parameters. MAIN OUTCOME MEASURES: Nerve fiber layer thickness measurements in each quadrant (superior, inferior, nasal, and temporal) were taken and then averaged (12 values averaged) and identified as NFL overall, whereas the data obtained in the temporal quadrant only (three values averaged) were identified as NFL temporal. PERG P50 implicit time and P50-N95 amplitude and VEP P100 implicit time and N75-P100 amplitude were also measured. RESULTS: In OAG eyes, we found a significant (P < 0.01) reduction in NFL thickness in both NFL overall and NFL temporal evaluations with respect to the values observed in control eyes. PERG and VEP parameters showed a significant (P < 0.01) delay in implicit time and a reduction in peak-to-peak amplitude. In OAG eyes, the NFL overall and NFL temporal values were significantly correlated (P < 0.01) with the PERG P50 implicit time and P50-95 peak-to-peak amplitude. No correlations (P > 0.01) between NFL values and VEP parameters were found. CONCLUSIONS: There is a correlation between PERG changes and NFL thickness, but there is no correlation between VEP changes and NFL thickness in patients affected by OAG.

Adult↗

Computational modeling of orientation tuning dynamics in monkey primary visual cortex.

In the primate visual pathway, orientation tuning of neurons is first observed in the primary visual cortex. The LGN cells that comprise the thalamic input to V1 are not orientation tuned, but some V1 neurons are quite selective. Two main classes of theoretical models have been offered to explain orientation selectivity: feedforward models, in which inputs from spatially aligned LGN cells are summed together by one cortical neuron; and feedback models, in which an initial weak orientation bias due to convergent LGN input is sharpened and amplified by intracortical feedback. Recent data on the dynamics of orientation tuning, obtained by a cross-correlation technique, may help to distinguish between these classes of models. To test this possibility, we simulated the measurement of orientation tuning dynamics on various receptive field models, including a simple Hubel-Wiesel type feedforward model: a linear spatiotemporal filter followed by an integrate-and-fire spike generator. The computational study reveals that simple feedforward models may account for some aspects of the experimental data but fail to explain many salient features of orientation tuning dynamics in V1 cells. A simple feedback model of interacting cells is also considered. This model is successful in explaining the appearance of Mexican-hat orientation profiles, but other features of the data continue to be unexplained.

Action Potentials↗

Visual disorders in children with brain lesions: 2. Visual impairment associated with cerebral palsy.

Disorders of visual function are a common finding in children with cerebral palsy. In some cases they are secondary to ophthalmologic abnormalities such as cataract or retinopathy, but more often they are due to damage of the central visual pathway. We review the literature on the prevalence and distribution of visual abnormalities in children with cerebral palsy and their relation to cognitive, motor and emotional development.

Cerebral Palsy↗

The neurology of visual acuity.

A series of patients with well defined lesions of various parts of the visual pathways was studied in an attempt to iluminate the neuropathophysiology of visual acuity. Acuity was found to remain normal in all cases with unilateral retrochiasmal lesions, including those of the optic tract. Bilateral retrochiasmal lesions involving the foveal nerve fibres on both sides impaired acuity to the same degree in both eyes. Lateral chiasmal lesions regularly produced impaired acuity in the ipsilateral eye. Midchiasmal lesions commonly led to an impairment of visual acuity in both eyes, usually asymmetrically, and roughly proportionate to the severity of the visual field defect. Compression optic neuropathy was found to reduce acuity in rough proportion to the severity of compression. It was concluded that acuity remains normal as long as either the crossing or the non-crossing neural outflow from the retinal fovea remains intact: acuity fails only when both sets of nerve fibres are compromised. A properly executed acuity test seems to be a powerful tool for detecting such conditions. The lower limit of normal acuity should never be set below 1.0 or 20/20: even this level is clearly subnormal in many subjects.

Adult↗

Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia.

Several behavioral studies have shown that developmental dyslexics do poorly in tests requiring rapid visual processing. In primates fast, low-contrast visual information is carried by the magnocellular subdivision of the visual pathway, and slow, high-contrast information is carried by the parvocellular division. In this study, we found that dyslexic subjects showed diminished visually evoked potentials to rapid, low-contrast stimuli but normal responses to slow or high-contrast stimuli. The abnormalities in the dyslexic subjects' evoked potentials were consistent with a defect in the magnocellular pathway at the level of visual area 1 or earlier. We then compared the lateral geniculate nuclei from five dyslexic brains to five control brains and found abnormalities in the magnocellular, but not the parvocellular, layers. Studies using auditory and somatosensory tests have shown that dyslexics do poorly in these modalities only when the tests require rapid discriminations. We therefore hypothesize that many cortical systems are similarly divided into a fast and a slow subdivision and that dyslexia specifically affects the fast subdivisions.

Adult↗

The superior colliculus and visual neglect in rat and hamster. II. Possible mechanisms.

The observations that removal of the superior colliculus in rats and hamsters produces a striking visual neglect, whereas damage to striate cortex does not, require explanation in terms of the anatomy and physiology of visual pathways in these animals. One proposal is that neglect is produced because the superior colliculus is the only visual structure directly concerned with the production of orienting movements. However, recent behavioural evidence indicates that this mechanism on its own is insufficient: collicular neglect is not confined to orienting movements, but is in part an inability to register particular kinds of visual stimulus. Two additional mechanisms are considered. The superior colliculus receives visual information that the geniculostriate pathway does not. A variety of anatomical and electrophysiological evidence suggests that this is the case. Although the exact nature of the information is unclear, there is some suggestion that the superior colliculus has a stronger representation of the peripheral field than the geniculostriate pathway, and may be more concerned with small transient stimuli throughout the visual field. An intact superior colliculus is essential for normal functioning of the geniculostriate system. Anatomical evidence indicates that there are pathways whereby: (a) visual cortex could use the superior colliculus as an output station; and (b) the superior colliculus could control signals entering or leaving the geniculostriate system, although the nature of the information carried by the pathways is not yet understood. The precise contribution of these two additional mechanisms to collicular neglect remains to be determined. However, it may be conjectured that the first would underly the deficit in stimulus detection that appears to be one component of collicular neglect, whereas the second mechanism might underly an attentional component.

Animals↗

The relationship between static perimetry and the relative afferent pupillary defect.

This study was undertaken to understand better how damage to the anterior visual pathway may affect the relationship between the visual and pupillomotor systems. The relative afferent pupillary defect and the interocular difference in visual field mean deviation (determined by the Humphrey Field Analyzer Statpac program) were correlated in 137 patients. A moderate linear correlation (r = .66) was found. In 25 patients tested by both static and kinetic perimetry, the correlation could not be significantly improved by considering field loss outside of 30 degrees. The correlation was further studied in four subcategories of diagnosis: optic neuritis (n = 36), idiopathic intracranial hypertension (n = 26), compressive optic neuropathy (n = 14), and anterior ischemic optic neuropathy (n = 7). In compressive optic neuropathy and idiopathic intracranial hypertension, the difference in mean deviation between the two eyes was associated with a larger relative afferent pupillary defect than in optic neuritis and anterior ischemic optic neuropathy. In optic neuritis, the correlation was the poorest. These results indicate that diseases of the afferent visual system may not necessarily affect visual threshold (as tested by static perimetry) and the pupillary light reflex (a suprathreshold test) in the same way.

Adolescent↗

[Retino-hypothalamic pathways in vertebrates].

Forty years ago Hollwich (1948) introduced the conception of an "energetic portion of the visual pathway". Contributions to this conception of a direct connection of the retina with the hypothalamus accumulated since then and summarized here in tabular form give rise to the following conclusions: In fish the main hypothalamic termination of retinofugal axons is the nucleus hypothalamicus opticus. It may pass for the suprachiasmatic nucleus of fishes. In amphibians retino-hypothalamic fibres project to the area praeoptica. In reptiles retinofugal fibres innervate hypothalamic neuronal populations called either Nucleus suprachiasmaticus or Nucleus praeopticus. In birds retinohypothalamic axons project to a circumscribed anterior hypothalamic area termed "suprachiasmatic nucleus" by some authors. In mammals at last the main part of the retinohypothalamic tract terminates in the suprachiasmatic nuclei, especially favouring their caudal and ventrolateral parts. The interneuronal connections are axo-dendritic synapses of the Gray Types-I and II. Connections of retinal neurons, especially with suprachiasmatic hypothalamic nuclei or their homologues, are by now well established facts. They represent relatively constant and phylogenically stable components of the centripetal retinal projection. These projections are probably in all, certainly in most of the vertebrates bilateral. Some former but also newer methods of research (Stumpf and Sar, 1975) also depicted optic fibers which terminate in hypothalamic sites apart from the nucleus suprachiasmaticus and the area hypothalamica anterior (Conrad and Stumpf, 1975). A review of the literature on the existence of nerve fibers directly connecting the retina with the hypothalamus is tabulated.

Animals↗

Discrimination of mirror-image stimuli after lesions of the visual system in pigeons.

Nine pigeons were trained to perform a simultaneous discrimination task with stimuli that were lateral mirror images, vertical mirror images and nonmirror images. All subjects acquired the discriminations rapidly and at approximately equal rates. Following training, bilateral stereotaxic lesions were made in either the visual wulst or ectostriatum, which are telencephalic components of the thalamofugal and tectofugal visual pathways, respectively. After surgery both groups were retrained to their preoperative performance levels or for a maximum of 140 sessions. The performance of the four subjects that received visual wulst lesions was only mildly and transiently impaired and was equally disrupted on each pattern discrimination. The performance of the five subjects that received ectostriatal lesions, however, was markedly and persistently impaired on all pattern discriminations. The impairment was most severe and sustained on the lateral mirror-image discrimination problem in all subjects. Only three of the five subjects with ectostriatal lesions reached their preoperative performance levels on the lateral mirror-image problem, whereas all subjects returned to their preoperative performance levels on the other problems. Possible reasons for this selective deficit in lateral mirror-image pattern discrimination are discussed in relation to interhemispheric pathways and the relative importance of the thalamofugal and tectofugal visual pathways in birds and in mammals.

Animals↗

MT neurons in the macaque exhibited two types of bimodal direction tuning as predicted by a model for visual motion detection.

We previously proposed a model for detecting local image velocity on the magnocellular visual pathway (Kawakami & Okamoto (1996) Vision Research, 36, 117-147). The model detects visual motion in two stages using the hierarchical network that includes component and pattern cells in area MT. To validate the model, we predicted two types of bimodal direction tuning for MT neurons. The first type is characteristic of component cells. The tuning is bimodal when stimulated with high-speed spots, but unimodal for low-speed spots or for bars. The interval between the two peaks widens as the spot's speed increases. The second type is characteristic of pattern cells. The tuning is bimodal when stimulated with low-speed bars, but unimodal for high-speed bars or for spots. The interval widens as the bar's speed decreases. To confirm this prediction, we studied the change of direction tuning curves for moving spots and bars in area MT of macaque monkeys. Out of 35 neurons measured at various speeds, six component cells and four pattern cells revealed the predicted bimodal tunings. This result provided neurophysiological support for the validity of the model. We believe ours is the first systematic study that records the two types of bimodality in MT neurons.

Animals↗

A bedside test to determine motion stereopsis using the Pulfrich phenomenon.

OBJECTIVE: Many diseases induce asymmetric delays in the visual pathway, resulting in a spontaneous Pulfrich phenomenon (PP). The PP is a visual stereoillusion that may cause difficulties in persons when traveling in cars, crossing the road, or playing ball games. The authors developed and tested a simple new bedside procedure to detect PP. DESIGN: A case series. PARTICIPANTS: Disease simulation in 2 normal subjects and 18 patients with optic neuritis (ON) was examined. Ninety normal subjects were studied to determine normal range of PP. INTERVENTION: The new test, called swinging pen test (SPT), is performed by oscillating a pen by hand. The SPT was compared to a gold standard, a mechanical pendulum (MP). MAIN OUTCOME MEASURES: The authors measured simulated PP in two normal subjects and PP in 18 patients with ON and 90 normal control subjects. The Pearson product-moment correlation (r) and the Spearman rank correlation (rs) between SPT and MP were calculated. RESULTS: The magnitudes of simulated PP determined with the SPT and the MP correlated well (r = 0.92, P < 0.005, and r = 0.96, P < 0.001). Correlation also was good in patients with ON (rs = 0.90, P < 0.05). The positive predictive value of the SPT was 100%, and the negative predictive value was 92%. The PP was absent in all control subjects testing with either pendulum. The normal range for PP varied from -1.40 to 1.52 msec. For the SPT, the intraobserver variability coefficient was 8.2%, and the interobserver variability coefficient was 10.5%. CONCLUSIONS: The authors believe that SPT will be of value to clinicians on bedside evaluation of motion stereopsis dysfunctions. The normal range of PP was approximately +/- -1.5 msec (approximately +/- -1.5 cm), corresponding to a 0.3-log unit neutral density filter).

Acute Disease↗

[Anatomical differences in optic nerve, chiasma and tractus opticus in human albinism as demonstrated by standardised clinical and MRI evaluation].

BACKGROUND: There has been long-standing clinical and electrophysiological evidence that in patients with albinism the visual pathways cross atypically: most fibres from one eye cross to the contralateral visual cortex. PURPOSE OF THE STUDY: to determine whether the size and configuration of the optic chiasm in human albinos is different from normally pigmented controls. PATIENTS AND METHODS: 17 patients (11 female, mean age 35.8 years) with oculocutaneous albinism underwent a standardised graded morphological and functional evaluation. Magnetic resonance images were reformatted to the region of the optic chiasm and analysed using observer-independent morphometry. In addition, fMRI of the visual cortex was performed during VEP analysis (1.5 Tesla Siemens Vision). Morphological and fMRT results were compared to an age-correlated group of n = 16 normally pigmented healthy volunteers with normal visual acuity and stereopsis. RESULTS: 65 % of the patients (n = 10) showed signs of dysplasia of the optic nerve head. Statistical morphometry showed distinct differences in chiasmal morphology between albinos and normally pigmented probands (smaller optic nerves, different angles of optical entry into the chiasm and of the beginning of the Tractus optici leaving the chiasm, overall chiasmal width and height). CONCLUSIONS: Optic nerve head anomalies are frequent in albinism and influence visual outcome. Size and configuration of the optic chiasm in human albinos is distinctly different from normally pigmented control persons and reflects the atypical crossing of optic fibres.

Adult↗

Specificity of neural connections in the retinotectal system.

The existence of a retinotopic map raises the question of how such a map is formed during development. Since little is known about molecular mechanisms which may be involved in the formation of retinotopy, the structural aspects of this developmental process were studied to a greater detail in the chick. In the retina the following patterns of growth can be observed: (1) Ganglion cell formation starts in the centre and spreads towards the periphery. Thus, each cell is labelled by both its position and its time of origin. (2) Axons tend to grow towards the optic fissure by the shortest possible route, and more peripheral axons whose perikarya are born later than that of central ones tend to lie next to the inner limiting membrane. A regular change in the pattern of fibres, which we call transformation, occurs at the optic fissure. The incremental rings are transformed into crescents lying ventrally in the optic stalk at early stages of development. In the middle of the optic nerve retinotopy is re-established. Another transformation occurs shortly behind the chiasm. Except for these transformations, fibres run parallel to each other even across the chiasm. Thus, fibres arriving at the optic tectum are well organized. It is concluded that a multiplicity of factors contribute to the formation of the retinotectal map at any site of the visual pathway. The analysis of these factors may indicate how topographically organized projections develop.

Animals↗

M pathway and areas 44 and 45 are involved in stereoscopic recognition based on binocular disparity.

We characterized the visual pathways involved in the stereoscopic recognition of the random dot stereogram based on the binocular disparity employing a functional magnetic resonance imaging (fMRI). The V2, V3, V4, V5, intraparietal sulcus (IPS) and the superior temporal sulcus (STS) were significantly activated during the binocular stereopsis, but the inferotemporal gyrus (ITG) was not activated. Thus a human M pathway may be part of a network involved in the stereoscopic processing based on the binocular disparity. It is intriguing that areas 44 (Broca's area) and 45 in the left hemisphere were also active during the binocular stereopsis. However, it was reported that these regions were inactive during the monocular stereopsis. To separate the specific responses directly caused by the stereoscopic recognition process from the nonspecific ones caused by the memory load or the intention, we designed a novel frequency labeled tasks (FLT) sequence. The functional MRI using the FLT indicated that the activation of areas 44 and 45 is correlated with the stereoscopic recognition based on the binocular disparity but not with the intention artifacts, suggesting that areas 44 and 45 play an essential role in the binocular disparity.

Adult↗

Phased-array surface coil MR of the orbits and optic nerves.

PURPOSE: To devise a practical technique for high-resolution evaluation of the anterior optic apparatus using a phased-array surface coil system, and to evaluate this system in patients with suspected optic pathway abnormalities. METHODS: A four-element phased-array coil pair was placed on each side of the head, and signal-to-noise measurements were obtained using a head phantom. Comparison between the phased-array coil, the quadrature coil, and a single-turn 12.7-cm (5-in) surface coil was done. T1 spin-echo and T2 fast spin-echo sequences were obtained in the oblique axial and oblique sagittal planes, to approximate the long axis of the optic nerves and the nonoblique coronal plane. RESULTS: The phantom signal-to-noise measurements at simulated locations of the optic nerve head, optic canal, and optic chiasm revealed an improvement of at least 30% using the phased-array system. Of 24 imaged cases, 9 had trauma, 5 had suspected neoplasms, and 2 had optic neuritis. In 3 patients, an unexpected diagnosis of optic pathway contusion or infarction was made. The remaining 8 patients had various suspected visual pathway lesions. CONCLUSION: Phased-array surface coils allow rapid, thin-section imaging of the entire anterior optic pathway, with improved signal-to-noise ratio. This may improve evaluation of optic pathway lesions over conventional techniques.

Adolescent↗