Biomedical subjects

A M Norcia

Publications and source records attributed to A M Norcia.

At least 19 recordsLinked to original sources

PubMed1997

Abnormal long-range spatial interactions in amblyopia.

Neural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia.

Amblyopia↗

PubMed1996

Development of grating acuity and contrast sensitivity in the central and peripheral visual field of the human infant.

Central and peripheral visual functions were measured simultaneously in 39 infants from 10 to 39 weeks old using a dual-frequency VEP technique. Central acuity and contrast sensitivity over a 4 deg circular field were measured at 6 or 8 Hz. Peripheral acuity and contrast sensitivity were measured simultaneously at the other rate with a semi-circular stimulus extending from 8 to 16 deg. The EEG was analyzed at 12 and 16 Hz to determine the separate responses for the central and peripheral fields. Both central and peripheral VEP acuity developed over the age range tested. Central acuity improved by about a factor of 2.6 over the age range tested, while peripheral acuity improved by about a factor of 2.2. Central acuity was always higher by an average factor of about 2.3. Contrast sensitivity showed similar development for the central and peripheral fields with an early rapid rise in sensitivity.

Adult↗

PubMed1996

Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex.

Long-range spatial interactions in human visual cortex were explored using a lateral masking paradigm. Visual evoked potentials (VEPs) elicited by a Gabor signal presented in isolation or in the presence of two flanking high-contrast Gabor signals (masks) were measured. Response amplitude and phase were recorded for a vertically oriented test, for horizontal and vertical masks and for combinations of vertical tests and vertical or horizontal masks. The amplitudes and phases of the test alone and mask alone responses were added coherently to predict the amplitude for collinear and orthogonal lateral masking conditions. Additivity failures were taken as evidence for neural interactions. At a target-to-mask distance of 2 deg, VEP amplitude exceeded the linear prediction for test contrasts in the range of 8-16% for the collinear, co-axial target/mask combination. Measured response phase also led predicted response phase over the same range of contrast. The VEP amplitudes were less than the linear prediction in the orthogonal target/mask combination and measured response phase lagged the predicted phase. Significant facilitation occurred with collinear test/mask combinations up to at least 3 deg of separation (nine wavelengths). Co-oriented, but non-collinear test/mask combinations (oblique test and mask, horizontal test and mask) did not produce facilitation. Contrast gain thus appears to be set over considerable distances in a configuration-specific fashion.

Contrast Sensitivity↗

PubMed1996

Abnormal motion processing and binocularity: infantile esotropia as a model system for effects of early interruptions of binocularity.

Infantile esotropia, a common form of strabismus with onset prior to 6 months of age, occurs at a time of rapid visual development. While monocular visual acuity is relatively unaffected in these patients, the majority of them fail to achieve fully normal stereopsis. In addition, these patients show a spectrum of abnormalities in their ocular following responses, visual perception and visual evoked potentials (VEPs) that suggest a failure to develop a normal complement of motion processing mechanisms. While abnormalities of of stereopsis have been studied for many years, motion processing in strabismus is a rapidly evolving area of current research. Motion mechanisms are normally binocular and may form a distinct binocular sub-system. This review summarises which is known about sensory and motor abnormalities in infantile esotropia, with special emphasis on recent motion VEP recordings. The monocular motion VEP shows directional biases early in infancy that are consistent with a nasalward/temporalward response bias. Patients with infantile esotropia maintain their neonatal biases beyond the age at which they normally disappear. The motion VEP biases persist into visual maturity in patients whose strabismus is treated after about 2 years of age. Treatment prior to age 2 can lessen the magnitude of the motion VEP asymmetry and these improvements can be maintained into visual maturity. A recording from the striate cortex of a visually deprived macaque monkey indicates that the motion VEP asymmetry arises early in the visual pathway.

Esotropia↗

PubMed1996

[Measuring vernier acuity and motion perception with the parameter sweep VEP].

BACKGROUND: The visual abilities of infants and small children, who are unable to communicate verbally, can be determined with "objective" visual acuity tests as, e.g., preferential looking (PL) or visual evoked potentials (VEP). Both methods provide an estimate of grating visual acuity, but are unable to determine optotype visual acuity. Grating acuity, however, is not an optimum indicator for visual performance, as it is less affected than optotype acuity by pathological changes. The diagnostic relevance of objective methods may be improved by testing visual functions that are more strongly degraded by a malfunction of the fovea than is grating acuity. Visual functions such as vernier acuity and relative motion sensitivity are potential candidates. METHODS: Characteristic properties of vernier- and motion-VEP have been determined on adult subjects with a rapid-sweep-technique at various eccentricities. RESULTS: Both vernier and motion stimuli elicited VEP-responses at the 2nd harmonic (F2) of the stimulus frequency. A VEP-response at the fundamental frequency (F1) could be recorded with the vernier stimulus only. Foveal VEP-thresholds were very similar to psychophysical thresholds lying in the range from 11 to 25 arcsecs. The eccentricity dependence of the VEP-threshold recorded with vernier- and motion stimuli shows different cortical magnifications and supports the notion that the F1 response is related to the detection of the vernier offset, whereas the F2 response is generated by motion components of the stimuli. CONCLUSIONS: Vernier and relative motion-related VEP thresholds can be recorded with the steady-state VEP. They may provide more sensitive tests of foveal vision loss than grating visual acuity tests.

Attention↗

PubMed1995

Spatially distributed responses induced by contrast reversal in cat visual cortex.

Field potentials were recorded from cat striate cortex, either between an epidural screw electrode and a cannula-electrode inserted deep in the gray matter (transcortical recording) or with a pair of metal microelectrodes. Electrodes were placed bilaterally near the cortical projection of the area centralis. The horizontal separation of the recording tips was approximately 2 mm and approximately 300 microns, respectively. The area of the visual field providing input to the recording site (receptive field) was determined by measuring the field potentials generated by contrast reversal of high-contrast, achromatic bar gratings. Five-degree-diameter grating patches were presented individually over a large area of the visual field. The gratings were contrast-reversed at 4, 6 or 10 Hz, while also being swept in spatial frequency between 0.56 and 5.24 c/deg. The receptive fields were approximately 20 deg across or more, substantially larger than expected on the basis of cortical retinotopy. Responses were also elicited by stimulation of the hemi-field contralateral to that contributing to the classical receptive field, implicating the presence of a callosal projection. The large, spatially distributed receptive fields consisted of patches of high and low sensitivity. Continuous cortical infusion of either 100 microM tetrodotoxin or 10 mM muscimol at the recording site totally suppressed the transcortically recorded field potentials, proving that the local field potentials were generated postsynaptically. The present findings suggest that a cluster of cortical cells near the projection site of the area centralis receives input from remote cortical regions to an extent that is comparable with that of anatomically demonstrated long-range lateral connections.

Animals↗

PubMed1995

Application of adaptive filtering to steady-state evoked response.

A new method to detect steady-state evoked potentials (EPs) is presented. The technique is based on a two-weight recursive least squares (RLS) adaptive filter and the Tcirc2 statistic. Simulations with known sinusoids buried in Gaussian noise and in EEG noise indicate that the adaptive filter can detect signals at 3 or 4 times lower signal-to-noise ratios that the discrete Fourier transform (DFT). Qualitatively similar results were obtained with human visual evoked potential recordings.

Computer Simulation↗

PubMed1995

Plasticity of human motion processing mechanisms following surgery for infantile esotropia.

Monocular oscillatory-motion visual evoked potentials (VEPs) were measured in prospective and retrospective groups of infantile esotropia patients who had been aligned surgically at different ages. A nasalward-temporal response bias that is present prior to surgery was reduced below pre-surgery levels in the prospective group. Patients in the retrospective group who had been aligned before 2 yr of age showed lower levels of response asymmetry than those who were aligned after age 2. The data imply that binocular motion processing mechanisms in infantile esotropia patients are capable of some degree of recovery, and that this plasticity is restricted to a critical period of visual development.

Adolescent↗

PubMed1995

An adaptive filter for steady-state evoked responses.

A 2-weight adaptive filter that determines the amplitude and phase of steady-state evoked potentials is presented. Reference signals are derived from the visual stimulator that are related to corresponding harmonics of the response and the filter weights are adjusted so as to minimize the squared estimation error between the reference and the recorded signal using the recursive least squares (RLS) method. The filter, which acts as an adaptive bandpass filter, is followed by a detector based on the T2circ statistic. The performance of the RLS adaptive filter was compared to that of the conventional Discrete Fourier Transform (DFT) and the filtered DFT of Tang and Norcia in a series of simulations with known sinusoids buried in Gaussian noise and in EEG noise. In the simulations, the RLS adaptive filter detected signals at about 3-4 times lower signal to noise ratios than did the DFT. The RLS filter also outperformed the filtered DFT. Qualitatively similar results were obtained with human visual evoked potential recordings. The adaptive RLS filter significantly outperforms both the DFT and filtered DFT and is much simpler to implement than the filtered DFT method of Tang and Norcia.

Algorithms↗

PubMed1994

The development of motion sensitivity during the first year of life.

Using the sweep visual evoked potential (VEP), we have measured oscillatory displacement thresholds (OMTs) in 49 infants ranging in age from 7 to 54 weeks of age. The stimuli were high-contrast (80%), sine-wave gratings (1 c/deg) undergoing oscillatory displacements at 6 Hz. In addition to the motion thresholds, contrast thresholds for phase-reversing (6 Hz), 1 c/deg gratings were measured in the same session for 26 infants. In the main experiment, responses were recorded at the second harmonic (F2) of the stimulus frequency (12 Hz) under binocular viewing conditions. Our main finding is that, over the age range during which infants' peak contrast sensitivity (CS) first develops to within a factor of 2 of adult CS (9-12 weeks), infants' sensitivity to grating displacement is a factor of approximately 10 less than adults'. Moreover, infants' sensitivity to oscillatory motion undergoes relatively little development over the period between 2 and 15 months postnatal, gradually achieving a factor of 4.5 below adult values by 1 yr of age. Averaged over the entire age range tested, infants' OMTs were 167 sec arc, a factor of 6.4 times higher than the average OMT (26 sec arc) for 13 adults tested under identical conditions. In contrast, the infants' average CS for reversing gratings averaged only a factor of 2.5 less than the adults' average CS. In a second experiment, we took advantage of a developmental asymmetry in the monocular oscillatory motion VEP which allows for unambiguous identification of direction selective responses from very young infants. Monocular motion VEPs were measured in five infants (8-14 weeks) and their data analyzed at the fundamental frequency (F1). Responses at F1 were present in the monocular motion VEP from each infant and were 180 deg out of phase between the two eyes, identifying them as directional cortical responses with a nasalward/temporalward bias. These directional thresholds were equal to or lower than the symmetric (F2) thresholds. The presence of directional asymmetry in the motion VEP and the similarity of the monocular F1 and F2 OMTs support the notion that the OMTs measured in the main experiment were, in fact, derived from the responses of directionally selective cells in visual cortex. These data also imply that the OMTs are not derived from local contrast-reversal responses. Other models to explain infants' relative insensitivity to oscillatory motion are discussed.

Adult↗

PubMed1993

Does chromatic sensitivity develop more slowly than luminance sensitivity?

Chromatic sensitivity is very low in humans during the first few months of life. We examined whether low chromatic sensitivity reflects a deficiency among chromatic mechanisms or whether it is simply a manifestation of poor visual sensitivity in general. The sweep VEP was used to measure contrast sensitivity to gratings varying in the mixture of red and green components. For infants from 2 to 8 weeks of age, sensitivity to all mixtures was lower than color-normal adults' sensitivity, but infant and adult ratios of luminance/chromatic sensitivity were similar. This finding is consistent with the hypothesis that infants have functional MWS and LWS cones and the requisite post-receptor chromatic mechanisms to compare their signals.

Age Factors↗

PubMed1992

Contrast dependence of the oscillatory motion threshold across the visual field.

Observer sensitivity to oscillatory step displacements of sine-wave gratings was investigated at various loci in the visual field (0-30 degrees) as a function of contrast. Detection thresholds at 10 Hz and high grating contrasts were approximately 11-15 arcsec in the fovea and 37-47 arcsec at 30 degrees eccentricity. At any given contrast, threshold displacement increases linearly with eccentricity. The data provide evidence against an interpretation based on cortical magnification, because the slope and the scale-free x intercept of the eccentricity function vary strongly with contrast. While foveal thresholds for high-contrast gratings are in the range of the hyperacuities, the oscillatory motion threshold falls off an order of magnitude more slowly than the traditional hyperacuities. Rather than conceiving of the oscillatory motion threshold as a spatial acuity limited by cortical magnification, we suggest an alternative approach that is based on a form of contrast discrimination. Oscillatory motion can be decomposed into the sum of a modulating counterphase grating and a static masking grating, both of which are in spatial quadrature (i.e., 90 degrees out of phase). At low grating contrast, oscillatory motion can be detected when the counterphase component exceeds a constant contrast value. Above a critical contrast value of the static component Cscrit, threshold rises as a power function of contrast with a slope near 1.0. The critical contrast value Cscrit increases linearly with eccentricity, indicating that oscillating gratings observed with the peripheral visual field are less easily masked compared with foveally fixated gratings.

Contrast Sensitivity↗

PubMed1991

Cortical recovery from effects of monocular deprivation caused by diffusion and occlusion.

Two forms of visual deficit were induced in 12 pairs of kittens (4-6 weeks of age) by monocular lid suture combined with either an opaque soft contact lens ('occlusion' amblyopia) or a clear lens ('diffusion' amblyopia) which had been kept behind the sutured eyelids for 5-9 weeks. The kittens were then reverse-sutured at the age of 9-15 weeks, and the previously open eye was occluded with an opaque lens for the next 5-7 weeks. We compared across the two groups the proportion of binocularly driven cells (group 2-6) and cells predominantly activated by stimulation of the initially deprived eye (groups 1-3). Both values were significantly higher in the 'occlusion' kitten, though the difference was small, compared to the 'diffusion' kitten. In addition, we measured visual acuity and peak contrast sensitivity for the two eyes in 4 of the 12 pairs, using the sweep VEP method applied under anesthesia and paralysis. The 'occlusion' kitten showed consistently better acuity and higher peak contrast sensitivity than the 'diffusion' kitten, when the initially deprived eye was tested. Taken together, the present results suggest that there is a difference in the depth of amblyopia caused by monocular 'occlusion' and monocular 'diffusion'. The cortical effects of the latter are more difficult to reverse than those of the former.

Amblyopia↗

PubMed1991

Theory of eccentric photorefraction (photoretinoscopy): astigmatic eyes.

An optical analysis of eccentric photorefraction (photoretinoscopy) of astigmatic eyes is presented. The size and the angular tilt of the dark crescent appearing in the subject's pupil are derived as a function of five variables: the ametropia of the eye (Dsph, Dcyl, axis), the eccentricity of the flash, e, and the distance of the camera from the subject's eye, dc. A simplified solution and a solution of the inverse problem, which enable one to calculate the degree of ametropia from the size and the tilt of the crescent, are also presented. If the crescent is smaller than the pupil, both the size and the tilt of the dark crescent are independent of the pupil size. The angular tilt of the crescent is also independent of the eccentricity. Characteristic changes of the crescent as a function of the cylinder axis are illustrated for compound and mixed astigmatisms. The validity of the theoretical predictions was experimentally verified on a model eye.

Astigmatism↗

PubMed1991

Anomalous motion VEPs in infants and in infantile esotropia.

Visual evoked potentials (VEPs) were recorded monocularly in response to vertical gratings that underwent oscillatory apparent motion at a temporal frequency of 10 Hz. In normal infants 6 months or younger and in patients with a history of constant strabismus onset before 6 months of age, the oscillatory motion VEP contains a prominent first harmonic component that is temporally 180 degrees out of phase in the two eyes. This pattern is not seen in normal adults and is consistent with the presence of a nasalward/temporalward asymmetry of cortical responsiveness in infants and in patients with early onset strabismus.

Adolescent↗

PubMed1990

Development of contrast sensitivity in the human infant.

Contrast sensitivity and grating acuity were measured using the sweep VEP method in a group of 48 infants from 2 to 40 weeks of age and in a group of 10 adults. Sinusoidal gratings were reversed in contrast at 12 alternations per sec at a space-average luminance of 220 cd/m2. During 10 sec trials, either the contrast or the spatial frequency was increased in a series of 19 steps. Thresholds were estimated by extrapolation of the VEP response functions to zero amplitude. The contrast threshold at low spatial frequencies developed rapidly from 7% contrast at 2-3 weeks to an asymptote of 0.5% at 9 weeks. For adults, maximum sensitivity at low spatial frequencies was 0.32-0.22%. The sweep VEP estimate of grating acuity showed a gradual increase in spatial frequency with age, starting at 5 c/deg during the first month and reaching 16.3 c/deg at 8 months. The mean adult acuity was 31.9 c/deg. There appeared to be two phases in the development of contrast sensitivity and acuity. Between 4 and 9 weeks overall contrast sensitivity increased by a factor of 4-5 at all spatial frequencies. Beyond 9 weeks, contrast sensitivity at low spatial frequencies remained constant, while sensitivity increased systematically at higher spatial frequencies.

Adult↗

PubMed1989

The development of monocular and binocular VEP acuity.

The development of monocular and binocular grating acuity was measured in 87 infants, 2-52 weeks of age, using the sweep VEP technique. Average monocular and binocular acuity growth functions were nearly identical, with a small (less than 0.2 octaves) binocular acuity superiority occurring only under 6 months. Interocular acuity differences were small (averaging less than 1/4 octave, unsigned, with a 95% confidence interval of less than +/- 0.6 octaves) and were not significant at any age. These characteristics make the sweep VEP technique a potentially sensitive tool for the detection of monocular visual losses in the early stages of amblyopia.

Child Development↗

PubMed1989

Measurement of spatial contrast sensitivity with the swept contrast VEP.

Contrast response functions (CRFs) for the VEP were obtained with a Discrete Fourier Transform (DFT) technique employing swept contrast gratings. VEP CRFs in infants were found to have a form similar to those observed in adults, being linear functions of log contrast over a range of near-threshold contrasts. CRFs with low and high contrast lobes were present in infants, as they are in adults. Contrast thresholds were estimated by extrapolation of the CRF to zero microvolts. The effects of additive EEG noise and of the DFT data window on the shape of the measured CRF are considered. For large signals, the measured CRF is nearly independent of the additive noise, but at small signal values additive noise introduces a small bias towards larger amplitudes. The VEP signal-plus-noise distribution was modeled as a family of Rice distributions in order to evaluate the effects of bias on the estimates of threshold. The amount of bias depends inversely upon the slope of the CRF. The amount of bias introduced by a smoothing window also depends upon slope of the CRF as well as the sweep rate. The combined effects of additive noise and window bias were such that the total bias was nearly independent of CRF slope. Sweep VEP contrast thresholds were shown empirically to be unaffected by changes in the range of contrast swept.

Adult↗