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P Apkarian

Publications and source records attributed to P Apkarian.

34 records · Page 2Linked to original sources
PubMed1991

Detection of optic pathway misrouting in the human albino neonate.

The diagnosis of albinism is indicated by the presence of visual pathway misrouting in which temporal retinal fibers erroneously decussate at the optic chiasm disrupting the normal topographical distribution of retinal geniculate-cortical projections. Detection of misrouted fibers is effected by non-invasive electrophysiological assessment of the topographical representation of the visual evoked potential (VEP) following full field monocular stimulation. By combining appropriate state defined neonatal recording procedures with the albino VEP test paradigm, the presence of aberrant optic pathway projections was detected in a five-day-old full-term infant. The electrophysiological signature pathognomonic to albinism was observed within a long (300 ms) latency window of an otherwise normal neonatal luminance flash response. The results of this study indicate that the VEP misrouting test can be extended to reliable albino diagnosis within the neonatal period.

Albinism↗
PubMed1991

VEP projections in congenital nystagmus; VEP asymmetry in albinism: a comparison study.

Visual evoked potential (VEP) asymmetry in which a preponderance of nasal and temporal retina afferents project to the contralateral hemisphere after full-field monocular stimulation is considered specific to albinism. Some reports, however, suggest that patients with congenital nystagmus (CN) share the albino-like visual pathway anomaly. To examine the clinical specificity of albino misrouting, VEP topography was assessed in ten patients with congenital nystagmus and in ten age-matched albino patients. As an additional control, the VEP response profiles from eight albino patients with no nystagmus were also evaluated. The results are definitive; VEP contralateral asymmetry reflecting temporal retinal misprojection is evinced only in albino patients. Furthermore, ocular-motor instabilities in CN cannot be readily attributed to albino-type misrouted retinal-cortical projections.

Albinism↗
PubMed1989

Visual evoked potentials in Prader-Willi syndrome.

Oculocutaneous, electrophysiological, and cytogenetic factors were evaluated in 14 patients with Prader-Willi syndrome and in three controls, two albinos and a normal observer. In a substantial number of PW patients chromosomal anomalies, particularly deletions of the long arm of chromosome 15 as well as hypopigmentation of hair, skin, and eye have been identified. In the genetic condition of albinism, hypopigmentation related to neural ectoderm derivatives is associated with reduced visual acuity, foveal hypoplasia, and aberrant retinogeniculocortical projections. The latter can be observed by visual evoked potential (VEP) assessment of hemispheric response symmetry. To determine the possible neural ectodermal origin of hypopigmentation and its involvement in ocular development and optic pathway integrity, the potential distributions of the pattern onset/offset VEP were examined. Our results show hypopigmentation in 13 of our 14 PW patients and a chromosome abnormality in 6; no correlation between these two features was found. None of the PW patients showed the characteristic contralateral hemispheric asymmetry seen in albinism. On the other hand their VEP profiles were found to be atypical, rendering waveform and cortical topography difficult to interpret. Analysis suggests that in the absence of VEP evidence for optic pathway misprojection, PW hypopigmentation is probably of neural crest origin.

Adolescent↗
PubMed1989

Global stereopsis in human albinos.

The presence of global stereopsis was examined in 18 clinically diagnosed albinos; four non-albino controls were also tested including two observers with congenital nystagmus. Stereopsis was evaluated with standard clinical stereo tests and with TV generated random dot stereograms. The latter test involved electrophysiological measures of vertical eye movement tracking in response to a stimulus target. For either test procedure, global stereopsis could be demonstrated in a significant number of albinos across varying phenotypes. These results are of interest in view of electrophysiological investigations in albino animal models which indicate a paucity of binocularly driven cortical neurons in visual areas 17, 18 and 19. While stereopsis may be mediated in our albinos via residual appropriately projecting retino-geniculo-cortical fibres, we suggest that inter and intra cortical communication via corpus callosal connections may play a primary role in providing the adequate neural substrate for albino binocularity.

Adolescent↗
PubMed1987

Origin of notches in CSF: optical or neural?

Grating contrast sensitivity was measured across a range of 1 to 32 cycles per degree (c/deg) in normal observers with a computer-automated method of ascending limits. Monocular contrast sensitivity functions (CSF) were obtained for vertical, oblique and horizontal orientations, with or without full refractive correction. Small amounts of astigmatic error resulted in loss of sensitivity at selective spatial frequencies. Coincident with these CSF "notches" was the presence of monocular diplopia induced, in this study, by the condition of astigmatic error. Experimental manipulation of the selective spatial frequency losses was possible by the introduction of slight cylindrical defocus and by changes in grating orientation. Determination of the angular displacement and orientation of the monocular double images allowed prediction of the spatial frequencies which would show reduced sensitivity due to partial cancellation of contrast. The close fit between the predicted and measured sensitivity loss supports the suggestion that refractive error can affect narrowly-tuned notches. These results indicate that before the presence of a notch in the CSF can be attributed to neural abnormality, an optical cause must be eliminated.

Adult↗
PubMed1986

The use of a system analysis approach to electrodiagnostic (ERG and VEP) assessment.

To evaluate the integrity of a biological system and its constituent functional units, a systematic study of input-output relations adopted from engineering has proven appropriate. With such an approach, sequential analysis can be implemented to probe the various parameter extractions along, for example, the visual system. The a priori assumption in this approach is that the visual world is processed along functionally separate pathways yielding distinct percepts such as contrast and motion. This so-called channel approach has proven useful not only to basic vision research but also for clinical application. The present overview shows that on the basis of the ERG or VEP, a type of functional anatomy can be performed with the biological system of interest remaining intact. Finally, it will be demonstrated that electrophysiological output parameters of the visual system can also serve as a non-invasive entry to investigate general systemic disorders.

Adult↗
PubMed1985

The oculomotor behaviour of human albinos.

Horizontal and vertical eye movements were recorded in 16 human albinos with a scleral search coil technique. Spontaneous nystagmus, responses to target steps, voluntary pursuit and optokinetic nystagmus (OKN) were assessed, including the effects of selective stimulation of the nasal or temporal halves of the retina. The results suggest a subdivision of albinos into three classes of oculomotor behaviour. Class I (n = 11) is characterized by vigorous spontaneous nystagmus (of the pendular unidirectional jerk or bidirectional jerk type), the absence of true horizontal OKN but the presence of the ability to control the direction of gaze in an imprecise way. In Class II (n = 2) there is a vigorous unidirectional jerk nystagmus which reverses in direction spontaneously or as a result of visual stimulation. Moving stimuli typically elicited inverted pursuit, the smooth eye movements having a direction opposite to that of the stimulus movement. Class III (n = 3) is characterized by very little or no spontaneous nystagmus and virtually normal oculomotor responses. Only pursuit of motion in the temporal direction, projected onto the temporal half retina, was defective. In all three classes, vertical eye movements were disturbed much less than horizontal. Anomalous visual projections (confirmed in all subjects by asymmetrical monocular visual evoked cortical potentials) are a likely basic cause underlying the oculomotor instability, but the large intersubject differences show that the eventual consequences of misrouting and secondary adaptations can vary widely among subjects.

Adolescent↗
PubMed1984

Component specificity in albino VEP asymmetry: maturation of the visual pathway anomaly.

The existence of an isolated albino asymmetry component was investigated in the visual evoked potential (pattern appearance/disappearance response) of 96 albinos ranging in age from 3 months to 65 years. Misrouted optic pathway projections in albinism produce VEP asymmetry across the left and right occiput upon monocular whole field stimulation of a checkerboard pattern (element size, 55'). Our results indicate that the asymmetry which reflects the albino feature is contralateral and is restricted to an early period of the appearance response corresponding to the latency of the CI component. Further support for CI component specificity in albino asymmetry is provided by half-field stimulation in non-albino controls and by the results of a component maturation study in children under 6 years of age.

Age Factors↗
PubMed1983

Albinism: phenotype or genotype?

As part of a combined ophthalmological, genetic, clinical, biochemical, ultrastructural and electro-physiological study of albinism we have examined over one hundred albinos, together with their heterozygote family members. Given this substantial number, we have developed a diagnostic protocol to facilitate albino classification and heterozygote detection. The major difficulty in the detection and differential diagnosis of albinism is that for a given albino, not all symptoms, either ophthalmological or cutaneous, may be manifest, whereas several may be in a non-albino. To compensate for the wide diversity in albino expression, diagnosis is typically based on the results of combinations of tests.

Adolescent↗
PubMed1981

Binocular facilitation in the visual-evoked potential of strabismic amblyopes.

Electrophysiological correlates of abnormal binocular function were investigated by studying the monocular and binocular visual-evoked potentials (VEPs) of strabismic amblyopes. Steady-state evoked potentials were elicited by sinusoidal gratings which were temporally modulated in counterphase and logarithmically swept in spatial frequency from 0.2 to 20 c/deg in a 20 sec epoch. The VEP spatial frequency tuning functions of amblyopic eyes are abnormal in that they tend to show a reduced cutoff spatial frequency in relation to the nonamblyopic fellow eye. In contrast, the binocular interactions in strabismic amblyopes are similar in many respects to those of normal observers. These binocular interactions depend upon the spatial frequency and contrast of the stimulus, and may vary from inhibition to marked facilitation. The results suggest that some binocular function may be preserved in the cortex of patients with strabismic amblyopia, and that caution should be exercised in attempting to use the steady-state VEP to differentiate normal from abnormal binocular vision.

Adolescent↗
PubMed1979

Rapid assessment of visual function: an electronic sweep technique for the pattern visual evoked potential.

We have developed an electronic spatial frequency sweep technique for electrophysiological assessment of visual acuity and pattern vision. The technique allows an accurate and reliable measurement of VEPs to a full range of spatial frequencies in just 10 sec. Because the measurements are so rapid, the technique suggests several new improvements in the assessment of visual function. Sweeping spatial frequency linearly and extrapolating the high-frequency region of the VEP spatial-tuning function to zero voltage allows an estimate of acuity which correlates highly with psychophysical estimates of acuity. Variants of the procedure are appropriate for the assessment of refracture error, determination of equality of visual function for the two eyes and of binocular interactions, and for sequential assessment of therapeutic conditions.

Amblyopia↗
PubMed1978

Multiple spatial-frequency tuning of electrical responses from human visual cortex.

Human occipital potentials evoked by stimulation with a counterphase flickering grating were recorded by a digital narrowband filter technique. The data showed a surprising degree of narrow tuning to particular spatial frequencies in addition to the expected narrow temporal frequency tuning. At each temporal frequency, there could be two or more peaks of different spatial frequencies, each distinct from the whole field flicker response. Variations in this multiple spatial frequency tuning were investigated as a function of luminance, electrode location, and temporal frequency for several observers. The results are interpreted in terms of many stimulus-specific resonant neural circuits within the brain, and suggest that it is possible to make a highly detailed exploration of the responses of neural circuits to visual stimulation.

Electroencephalography↗
PubMedDate not supplied

Contrast sensitivity and the diagnosis dysthyroid optic neuropathy.

Contrast sensitivity function (CSF) was investigated in 19 patients (34 eyes) with clinical signs and symptoms of dysthyroid optic neuropathy (DON). CSF was disturbed in 33 eyes and was shown to improve after orbital decompression. These results indicate that the CSF may be a useful supplementary test of visual function in patients with DON.

Adult↗
PubMedDate not supplied

Temporal frequency responsivity shows multiple maturational phases: state-dependent visual evoked potential luminance flicker fusion from birth to 9 months.

Maturation of temporal resolution was investigated in a visual evoked potential study in 77 infants from birth to 9 months of age. Luminance evoked potential measures in response to homogeneous sinusoidal flickering light (1-64 Hz) were recorded under behavioral state-defined conditions. Behavioral state was determined by direct observation and by polygraphic recording of the electroencephalogram (EEG), eye movements (EOG), muscle activity (EMG), heart rate (ECG), and respiration. Temporal-frequency functions of the amplitude of the fundamental response across the temporal-frequency range were recorded during sleep and wakefulness. The highest temporal-frequency response recorded during wakefulness was accepted as a measure for inclusion in a growth function of temporal-frequency responsiveness. The resulting temporal resolution frequency vs. age function showed three separate maturational phases. Maturational phases were defined as (1) an initial slow phase from 1-32 days postnatal during which maturation of temporal vision is unremarkable; (2) an intermediate rapid phase of improvement from age 26 to 170 days; and (3) an overlapping but final slow phase from 151 to at least 270 days during which adult-like flicker resolution is approximated. This study suggests that the multiple maturational phases of the infant's responses to flickering light are due to maturational differences, which correspond with maturation of structural factors of brain function. Finally, across the age span tested, high-frequency responsivity was influenced significantly by the degree of infant arousal.

Adult↗