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PubMed1996

Lens induced glaucomas--visual results and risk factors for final visual acuity.

Lens induced glaucomas are a common occurrence in India. An attempt was made to understand the clinical modes of presentation and post operative visual results in 93 patients with lens induced glaucoma, 49 phacomorphic and 44 phacolytic, attending our institute during 1994. All these patients were subjected to a planned extracapsular cataract extraction. Forty four percent had a posterior chamber intraocular lens implantation following surgery. Fifty seven percent eyes with phacomorphic glaucoma and 61% with phacolytic glaucoma recovered visual acuity of 6/12 or better. There was no significant difference in the final visual acuity between those patients who had an intraocular lens implanted and those who did not (P = 0.18). Univariate analysis was performed for selected risk factors such as age, sex and duration of glaucomatous process as predictors of final visual acuity and odds ratios with 95% confidence intervals were calculated. Patients with age more than 60 years (OR = 2.7, 95% CI = 1.04-6.93) and in whom the glaucoma was present for more than 5 days (OR = 3.1, 95% CI = 1.21-8.13) had a significantly higher risk of poor visual outcome post-operatively.

Adult↗

PubMed1978

Subcortical projections of the dorsomedial visual area (DM) of visual association cortex in the owl monkey, Aotus trivirgatus.

The efferent subcortical connections of the dorsomedial cortical visual area (DM) in the owl monkey were determined by tracing degenerating axons following lesions or by tracing axonal pathways following injection of radioactively labeled proline. Areas of termination included structures which are known to receive input from many other regions of cortex such as the claustrum, putamen, caudate nucleus, reticular nucleus of the thalamus and pontine nuclei. Other terminations were in subcortical structures which primarily receive input from visual area: these included the pregeniculate nucleus, the medial and central divisions of the inferior pulvinar nucleus, two loci in the superior pulvinar complex, the pretectum and the superior colliculus. Terminations were also seen in the lateral posterior nucleus, a part of the thalamus associated with the somatosensory system. These results further identify Area DM as an integral part of the visual system, suggest functional subdivisions of the pulvinar complex, and implicate the lateral posterior nucleus in the mediation of visual, as well as somatosensory, behavior.

Animals↗

PubMed1989

Binocular depth perception, visual acuity and visual fields in cats following neonatal section of the optic chiasm.

We studied the role of the transcallosal pathway in stereopsis by measuring binocular and monocular depth perception in two cats that had undergone section of the optic chiasm at the age of 21 d. To ensure that the surgery did not impair vision to the extent that depth perception could not be evaluated, visual acuity and visual fields were also measured. In both of the chiasm-sectioned animals the visual fields were reduced and the visual acuity was substantially lower than in normal cats, with a maximum of about 2 cyc deg-1. Binocular depth thresholds of the chiasm-sectioned cats were worse than those of the normal cat but were better than their own monocular thresholds. These results suggest that the chiasm-sectioned animals were still able to use binocular cues to judge depth and indicate that the indirect pathway through the corpus callosum is sufficient to mediate binocular depth perception.

Depth Perception↗

PubMed1982

A model of the early stages of the human visual system: functional and topological transformations performed in the peripheral visual field.

A model of the early stages of the visual system is presented, with particular reference to the region of the visual field outside the fovea and to the class of retinal and lateral geniculate nucleus cells which are most active in the processing of patter information (X-cells). The main neuroanatomical and neurophysiological properties taken into account are: the linear increase of the receptive fields diameter with eccentricity, the constancy of the overlap factor and the topological transformation operated upon the retinal image by the retino-cortical connection. The type of filtering taking place between the retina and the visual cortex is analyzed and some simulations are presented. It is shown that such a filtering is of a bandpass space variant type, with center frequencies that decrease from the center (i.e. the fovea) toward the periphery of the visual field. This processing is "form invariant" under linear scaling of the input. Moreover, considering the properties of the retino-cortical connection, it is shown that the "cortical images" undergo simple shifts whenever the retinal images are scaled or rotated.

Humans↗

PubMed2001

Application of visual evoked potentials for preoperative estimation of visual function in eyes with dense cataract.

PURPOSE: To determine whether the temporal frequency characteristics of the visual system as determined by visually evoked potentials (VEP) can be used for a preoperative estimation of the visual function in eyes with cataracts. METHODS: Light stimuli driven by a pseudorandom binary sequence (PRBS) of 40950 ms duration were presented and EEG recordings were made from 13 control and 20 patients with cataracts preoperatively and 1 week after cataract operation. The first kernel of the PRBS-VEP was obtained as the first-order cross-correlation function between PRBS and PRBS-VEP. The Fourier transform of this function was used as the temporal frequency characteristic (TFC). RESULTS: The mean +/- standard deviation of the latency and amplitude of the VEP in normal controls were 110.8+/-4.3 ms and 2.01+/-0.67 microV, respectively. A high correlation (r>0.7) between the pre-and postoperative VEP waveform was obtained in 13 eyes (65%), and 14 eyes (70%) in the VEP-TFC curves. The sensitivity of the examination was 73%, 27%, and 91% for the latency, amplitude and TFC of the VEP, respectively. The specificity of the examination was 67%, 100%, and 89% for the same measures. Eleven of 12 eyes with abnormal TFC preoperatively showed retinal or optic nerve lesions postoperatively. False-negative results were seen in cases with delayed corneal edema. CONCLUSION: Postoperative visual function of patients with cataracts can be predicted by preoperative measurement of the TFC obtained by PRBS-VEP.

Adult↗

PubMed1985

Receptive field properties of neurones in visual area 1 and visual area 2 in the baboon.

In order to compare the receptive field properties of cells in the striate area (visual area 1; V1), and the parastriate area (visual area 2; V2), we have recorded from 174 cells in V1 and 112 cells in V2 in five anaesthetized and paralysed baboons (Papio ursinus). The receptive fields were mapped to determine their type, size and position in the visual field, and the binocular interaction, if any. Moving and stationary optimally oriented bars were used to distinguish cells with single "on" or "off" subregions and those with more than one such subregion (S and A types) from those with overlapping "on" or "off" subregion (C and B types). The A types had larger receptive fields than S types and C types had larger receptive fields than B types, but as receptive fields increase in size with eccentricity in V1 and even more rapidly in V2, the distinction between large and small receptive fields has to be defined for the different ranges of eccentricity. In V1 there are more cells with non-oriented receptive fields than in V2. In V1 S cells are found in all cortical layers except layer 5. C cells are absent from layer 4C, but predominate in layer 5. There is a preference for horizontal and vertical orientations in S cells only. The transition in cell properties from V1 to V2 occurs in two stages. There is a strip extending from the V1-V2 border for up to 6 mm containing the representation of the visual field from -2 degrees ipsilateral to +2 degrees (contralateral) azimuth in which the cell type distribution resembles that of V1 more than that of V2. By contrast, in V2 from 2 to 10 degrees there are very few S cells, many more C cells and over three times as many cells driven only by binocular stimulation, as compared to V1.

Animals↗

PubMed2006

Receptive-field modification in rat visual cortex induced by paired visual stimulation and single-cell spiking.

Experience-dependent plasticity of visual cortical receptive fields (RFs) involves synaptic modifications in the underlying neural circuits, but the site and mechanism of these modifications remain to be elucidated. Using in vivo whole-cell recordings, we show that pairing visual stimulation at a given retinal location with spiking of a single neuron in developing rat visual cortex induces rapid RF modifications. The time course of the response to the visual stimulus at the paired RF location is altered, with an enhancement of the response preceding the spike time and a reduction following the spike. Such bidirectional modification is consistent with spike timing-dependent plasticity. Response modification also occurs at nearby locations, the direction and magnitude of which are correlated with the change at the paired location. In addition, changes at unpaired locations show a negative correlation with the initial strength of the response, which may facilitate rapid modification of the spatial RF profile.

Animals↗

PubMed2003

Visually impaired children with posterior ocular malformations: pre- and neonatal data and visual functions.

AIM: To analyse pre- and neonatal data and ocular findings in children with visual impairment caused by posterior ocular malformations. METHODS: Medical records were scrutinized, dried blood spot cards were analysed for virus DNA and ophthalmological assessments were performed in 28 children with optic nerve hypoplasia (ONH) and 10 with optic/chorio-retinal coloboma. RESULTS: Prenatal exposure to possible teratogens was documented in 5/28, herpes simplex virus type 1 DNA was identified in the dried blood spot cards of 1/26 children and neonatal hypoglycaemia in 12/28 children with ONH. The time delay from ocular to endocrinological diagnosis and treatment was 3 years. Children with ONH and severe visual impairment had endocrinopathy more often (11/13) than ONH children with better visual functions (5/15). Prenatal exposure to teratogens or neonatal hypoglycaemia was not identified in any of the children with coloboma. CONCLUSION: Neonatal hypoglycaemia was common in children with ONH. Severe visual impairment predicted endocrinopathy. Analysis of dried blood spot cards could serve as an additional diagnostic tool in children with ocular malformations.

Child↗

PubMed2000

Which ocular and neurologic conditions cause disparate results in visual acuity scores recorded with visually evoked potential and teller acuity cards?

PURPOSE: We investigated whether disparity between visually evoked potential (VEP) acuity scores and Teller Acuity Card (TAC) scores varied according to presence of ocular or neurologic conditions. METHODS: Charts from 175 children (mean age, 34.8 months; range, 3 to 158 months) referred for visual acuity testing were examined. All children had been tested with pattern-alternation VEP and TAC and had undergone a complete eye examination. VEP and TAC acuity scores were relative to age-expected acuity scores for each acuity test. The absence and degree of macular abnormality, retinal abnormality, optic nerve hypoplasia, optic nerve atrophy, cortical visual impairment, developmental delay, cerebral palsy, seizures, and nystagmus were noted. Analysis of variance models were used to determine whether differences between VEP and TAC scores varied according to the presence of specific deficits. Logistic regression analysis determined whether degree of specific deficits was associated with a greater chance of inconsistency between VEP and TAC scores (>0.3 log unit difference). RESULTS: Inconsistent scores were found in 48% of children. Developmental delay was associated with relatively poorer TAC than VEP score, and the chance of inconsistency increased with severity of developmental delay. CONCLUSIONS: Diagnosis-dependent variability exists between TAC and VEP scores. Therefore knowledge of the clinical picture is necessary in interpretation of VEP and TAC scores. It is not clear which test is more useful when a disparity exists, either from this or previous studies. When visual acuity is assessed longitudinally in a given child, then consistency in method for acuity assessment is important.

Aging↗

PubMed2005

Optical imaging of visually evoked responses in the middle temporal area after deactivation of primary visual cortex in adult primates.

The middle temporal area (MT) is a visual area in primates with direct and indirect inputs from the primary visual cortex (V1), a role in visual motion perception, and a suggested role in "blindsight." When V1 is deactivated, some studies report continued activation of MT neurons, which has been attributed to an indirect pathway to MT from the superior colliculus. Here we used muscimol to deactivate V1 while optically imaging visually evoked activity in MT in two primates, owl monkeys and galagos, where MT is exposed on the brain surface. The partial loss of V1 inputs abolished all or nearly all evoked activity in the retinotopically matched part of MT. Low levels of activation that persisted in portions of MT that were unstimulated or retinotopically congruent with the blocked portion of V1 appeared to reflect the spread of activity from stimulated to unstimulated parts of MT. Thus, a significant pathway based on the superior colliculus was not demonstrated.

Aging↗

PubMed2002

The function of bursts of spikes during visual fixation in the awake primate lateral geniculate nucleus and primary visual cortex.

When images are stabilized on the retina, visual perception fades. During voluntary visual fixation, however, constantly occurring small eye movements, including microsaccades, prevent this fading. We previously showed that microsaccades generated bursty firing in the primary visual cortex (area V-1) in the presence of stationary stimuli. Here we examine the neural activity generated by microsaccades in the lateral geniculate nucleus (LGN), and in the area V-1 of the awake monkey, for various functionally relevant stimulus parameters. During visual fixation, microsaccades drove LGN neurons by moving their receptive fields across a stationary stimulus, offering a likely explanation of how microsaccades block fading during normal fixation. Bursts of spikes in the LGN and area V-1 were associated more closely than lone spikes with preceding microsaccades, suggesting that bursts are more reliable than are lone spikes as neural signals for visibility. In area V-1, microsaccade-generated activity, and the number of spikes per burst, was maximal when the bar stimulus centered over a receptive field matched the cell's optimal orientation. This suggested burst size as a neural code for stimuli optimality (and not solely stimuli visibility). As expected, burst size did not vary with stimulus orientation in the LGN. To address the effectiveness of microsaccades in generating neural activity, we compared activity correlated with microsaccades to activity correlated with flashing bars. Onset responses to flashes were about 7 times larger than the responses to the same stimulus moved across the cells' receptive fields by microsaccades, perhaps because of the relative abruptness of flashes.

Animals↗

PubMed2005

Visual mental imagery induces retinotopically organized activation of early visual areas.

There is a long-standing debate as to whether visual mental imagery relies entirely on symbolic (language-like) representations or also relies on depictive (picture-like) representations. We sought to discover whether visual mental imagery could evoke cortical activity with precise visual field topography (retinotopy). Participants received three conditions: the perception condition consisted of a standard retinotopic mapping procedure, where two flickering checkerboard wedges rotated around a central fixation point. The imagery and attention conditions consisted of the same stimulus, but only the outer arcs of the wedges were visible. During imagery, participants mentally reproduced the stimulus wedges, using the stimulus arcs as a guide. The attention condition required either distributed attention or focused attention to where the stimulus wedges would have been. Event-related analysis revealed that the imagery (greater than either form of attention) retinotopic maps were similar to the perception maps. Moreover, blocked analysis revealed similar perception and imagery effects in human motion processing region MT+. These results support the depictive view of visual mental imagery.

Adult↗

PubMed2006

Temporal characteristics of visual receptive fields in primary visual cortex and medial superior temporal cortex areas.

We mapped the receptive fields of 49 cells from primary visual cortex and 19 cells from medial superior temporal cortex in two awake monkeys. The receptive field structures we obtained lasted a mean time of 32.7 ms in primary visual cortex and 38.4 ms in medial superior temporal cortex, showing no statistical difference. This result suggests that both areas have the same time requirements for processing visual information. In primary visual cortex, 100% of cells had conformed the receptive field structure at 65 ms pre-spike, whereas in medial superior temporal cortex it occurred at 150 ms. In both areas, cells with shorter response latencies had receptive field structures with longer durations. This may indicate that cells tend to synchronize their output to other areas.

Action Potentials↗

PubMed2006

Localizing sites of activation in primary visual cortex using visual-evoked potentials and functional magnetic resonance imaging.

This study compared retinotopic map identification in primary visual cortex (V1) using: (i) functional magnetic resonance imaging (fMRI) and (ii) visual evoked potentials (VEPs) coupled with dipole source localization (DSL). A multielectrode array was used to record VEPs while subjects viewed a flickering dartboard pattern modulated by a 16-bit m-sequence. The stimulus preferentially activates V1. Using a common time function DSL algorithm, the primary source of each stimulus patch was found independent of the fMRI. The VEP/DSL and fMRI localization data for each subject were aligned by a rigid translation and rotation. The average distance between VEP and corresponding fMRI sources was 10.8 mm +/- 3.8 mm. To assess the significance of the results, fMRI and DSL solutions were scrambled so the comparisons were no longer for corresponding patches. The average distance between the noncorresponding data sets was 17.2 mm for 50 million scrambles. The probability of the scrambled data yielding a better fit than the real data was p < 10(-7). The combination of multielectrode recording, multiinput visual stimulation and common time function DSL analysis can provide a detailed retinotopic map of visual cortex that has high correspondence with independent fMRI localization analysis on the same subject.

Brain Mapping↗

PubMed1989

Detection and recognition of visual field defects resulting from lesions involving the visual pathways.

A prospective study of visual field defects associated with lesions of the visual pathway was carried out using kinetic and suprathreshold static stimuli with a view to establishing the most effective screening method for these field defects. The 215 abnormal fields so obtained showed that all field defects due to lesions of the visual pathways are detectable within 30 degrees of fixation and that not only is central field testing mandatory in excluding such a field defect, but more peripheral field testing alone is ineffective. This study also revealed that when kinetic fields are charted, it is probably not worthwhile searching for scotomata other than within the most central part of the field. Furthermore, where outer and inner isopter depression is not coextensive, inner isopters are always depressed more than outer isopters when the field defect is due to a lesion of the visual pathway.

Central Nervous System Diseases↗

PubMed1996

Less segregated processing of visual information in V2 than in V1 of the monkey visual cortex.

To test the possibility of cross-talk between parallel pathways dealing with different aspects of visual information such as orientation, direction of motion and colour in cortical area V2, we quantitatively analysed visual responses of 121 V2 cells recorded from anaesthetized and paralysed macaques and compared them with those of 147 V1 cells. A selectivity index of visual responses was calculated for each neuron, which was then classified as selective or not to a particular attribute of visual stimuli. Twenty-one percent of the V2 neurons had dual selectivity to both colour and direction of stimulus motion (C&D cells). In V1, only 5% of the cells were C&D cells. Thus, the proportion of C&D cells significantly increased from V1 to V2. We also carried out cross-correlation analysis of spike trains recorded simultaneously from pairs of V2 neurons or pairs of V1 neurons. In V2, correlated firings could be observed between cells with completely different optimal orientation, such as orthogonal, while it was never observed in V1. The cross-correlation analysis further indicted that functional interactions in V2 were more widespread than those in V1. These results suggest that neurons which have different functional properties become less segregated, and that functional interactions become more widespread in V2 than in V1.

Animals↗

PubMed1998

Delayed visual maturation: pupillary responses implicate subcortical and cortical visual systems.

Vision in very early infancy is probably subserved by subcortical pathways, with many cortical processes only fully emerging by 3 months of age. The improvement of vision in delayed visual maturation (DVM) occurs around this time, and this has given rise to the suggestion that the condition may have a subcortical basis that resolves with the appearance of cortical function. To explore further the role of cortical and subcortical visual systems in DVM we studied the visual development in identical twins, one of whom had type 1b DVM. Two non-invasive methods of investigating visual pathway function were employed: the acuity card procedure (a behavioural response) and luminance and grating pupillometry. While the former reflects both subcortical and cortical function and can be detected at birth, pupil responses to gratings reflect cortical activity alone and normally become measurable at 1 month of age. Development of both behavioural and pupillary responses was delayed in DVM, indicating that although the underlying defect is primarily subcortical, secondarily it delays the emergence of cortically mediated responses. The observed rapidity of improvement--over a very few days and within a narrow age range--suggests a discrete rather than a widespread structural abnormality, the improvement of which is closely linked to postmenstrual age.

Failure to Thrive↗

PubMed1995

The brain's visual world: representation of visual targets in cerebral cortex.

Microelectrode recordings from behaving monkeys have shown that neuronal responses in the visual cerebral cortex can depend greatly on which aspect of the scene is the target of the animal's attention. Accumulating evidence suggests that while the early stages of the visual pathway provide a faithful representation of the retinal image, later stages of processing in the visual cortex hold representations that emphasize the viewer's current interest. By filtering out irrelevant signals and adding information about objects whose presence is remembered or inferred, the cortex creates an edited representation of the visual world that is dynamically modified to suit the immediate goals of the viewer.

Action Potentials↗