Biomedical subjects

A M Skoczenski

Publications and source records attributed to A M Skoczenski.

6 recordsLinked to original sources

PubMed2001

Normalization models applied to orientation masking in the human infant.

Human infants can discriminate the orientation of lines within the first week after birth (Atkinson et al., 1988; Slater et al., 1988) but have immature orientation-selective pattern masking until after 6 months of age (Morrone and Burr, 1986). Here the development of orientation processing is further examined using a visual-evoked potential paradigm and normalization models of pattern masking. Contrast response functions were measured for 1 cycle per degree (cpd) gratings, counterphase-reversed in contrast at either 3.3 or 5.5 Hz. A second 1 cpd, 20% contrast, 8.3 Hz grating of either the same or orthogonal orientation was added as a mask. Evoked responses associated with the test grating, the mask, and intermodulation between the two were individually extracted using spectral analysis of the scalp-recorded EEG. Adults exhibited orientation selectivity in the masking of their test component responses and in nonlinear intermodulation between the test and mask stimuli. Infants <5 months old, however, demonstrated nonselective masking or a reversed selectivity in their responses to the test component, with adult-like orientation selectivity in their intermodulation responses. Within the context of a normalization model of pattern masking, the results are consistent with the existence of oriented filters early in life the responses of which are normalized immaturely until approximately 5 months of age.

Adult↗

PubMed1999

Development of VEP Vernier acuity and grating acuity in human infants.

PURPOSE: To compare the developmental sequences of two basic measures of pattern vision, Vernier acuity and grating acuity, using steady state visual-evoked potentials (VEPs) and an analysis designed to isolate pattern-specific responses from those due to motion in the Vernier stimulus. METHODS: The authors recorded VEPs from 57 healthy full-term infants and 4 adults. The grating acuity stimulus was a sinusoidal grating, temporally modulated (appearance-disappearance) at a rate of 3 Hz, with spatial frequency decreasing in linear steps during each 10-second trial. The Vernier acuity stimulus was a vertical square-wave grating with portions of each bar temporally modulated to make offsets appear and disappear at a rate of 3 Hz. Vernier offset size changed in log steps from small to large offsets. The authors recorded each observer's electroencephalogram (EEG) during multiple presentations of each stimulus type, and the EEG was digitized and filtered to obtain the amplitude and phase of the response at the first two harmonics of the stimulus temporal frequency. Thresholds were estimated with an extrapolation technique that took into account the signal-to-noise ratio and phase of the response. RESULTS: VEP Vernier acuity and grating acuity develop at different rates, with grating acuity approaching adult levels earlier than Vernier acuity. The within-subject relationship between VEP Vernier acuity and grating acuity follows the same developmental trajectory established by previous psychophysical studies of humans and monkeys. CONCLUSIONS: This VEP technique provides a rapid estimate of Vernier acuity in infants. VEP Vernier acuity remains strikingly immature throughout the first year of life, similar to behavioral Vernier acuity. Because Vernier acuity is a sensitive measure of amblyopia, this VEP test may be useful in the future to identify amblyopia and to follow its treatment progress in pediatric patients.

Adult↗

PubMed1998

Neural noise limitations on infant visual sensitivity.

Visual contrast sensitivity is poor in newborn human infants, but improves rapidly to approach adult levels by 8 months of age. During this period, infant sensitivity can be limited by physical factors affecting photon capture, such as eye size and photoreceptor density. Here we show that infant visual sensitivity is also limited by high levels of noise in the neural transduction process. Using a non-invasive electrophysiological measurement and a visual noise titration technique, we have found that intrinsic neural noise in neonates is approximately nine times higher than in adults. As intrinsic neural noise decreases during infancy, contrast sensitivity improves proportionally, suggesting that neural noise places critical limits on contrast sensitivity throughout development. Moreover, contrast gain control, an inhibitory process that adjusts visual responses to changing stimulation, is in place and operating in infants as young as 6 weeks of age, in spite of high levels of neural noise and significant immaturities in contrast sensitivity. The contrast gain control that we observed in human neonates may serve as a building block for more complex forms of visual inhibition, which develop later in infancy.

Adult↗

PubMed1996

Retinal illuminance and contrast sensitivity in human infants.

Several investigators have related infants' low contrast sensitivity to immaturities in the optics and receptor lattice of the immature eye. A critical element in the modeling is how much the lower photon catch of the immature retina reduces sensitivity; the assumptions vary from square-root to Weber's law and lead to very different modeling outcomes. We measured the relationship between retinal illuminance and contrast sensitivity at different spatial frequencies. The sweep visual-evoked potential was used to measure thresholds in 2- and 3-month olds and adults over a 2.5-log-unit range of illuminances. The contrast threshold vs illuminance functions were fit by power functions. The best-fitting exponents for adults were about -0.5 at higher spatial frequencies (consistent with square-root law) and lower at lower frequencies. The best-fitting exponents for 2- and 3-month olds were -0.2 to -0.35 which indicates that threshold is less affected by changes in illuminance than is the case in adults. These results suggest that none of the models relating optical and receptoral immaturities to infants' spatial vision has assumed an appropriate relationship between lower photon catch and contrast sensitivity. Once the models are modified to incorporate the relationship obtained in the present experiment, the predictions fall well short of explaining 2-month olds' low contrast sensitivity.

Adult↗

PubMed1995

Assessment of vernier acuity development using the "equivalent intrinsic blur" paradigm.

Vernier acuity in human infants is more than two orders of magnitude poorer than in adults and does not appear to reach adult levels until well beyond the first postnatal year. One source of these developmental differences in vernier acuity may be the presence of levels of intrinsic blur that are higher in infants than in adults. We investigated this hypothesis by measuring vernier acuity in 3-month-olds, 5-month-olds, and adults using stimuli blurred by two-dimensional Gaussian filters. Experiment 1 showed that more stimulus blur is required to degrade vernier acuity in infants than in adults. From these data we estimated that the level of equivalent intrinsic blur for this vernier acuity task decreased by approx. 1.5 log units between 3 months of age and adulthood. These results also suggested that this reduction of equivalent intrinsic blur can account entirely for the improvement in vernier acuity between 3 and 5 months postnatal. However, the large further improvement which occurs between 5 months of age and adulthood cannot be explained solely by equivalent intrinsic blur. In Expt 2, we measured vernier acuity in 3-month-olds and adults using a non-blurred stimulus with the same luminance contrast as the most-blurred stimulus used in Expt 1. Infants' and adults' thresholds were degraded slightly, relative to the non-blurred stimulus from Expt 1, but still were significantly better than the most blurred condition of Expt 1. This suggests that the results from the first experiment were not due simply to the reduction in overall luminance contrast which occurs when stimuli are blurred.

Adult↗

PubMed1992

Spatiotemporal factors in infant position sensitivity: single bar stimuli.

The ability of 3-month-old human infants to detect the presence of spatial misalignments in single-bar stimuli was investigated in a series of spatiotemporal stimulus manipulations. Both discrete and sinusoidal positional offsets, either stationary or temporally modulated, were presented using the forced-choice preferential looking technique. When discrete offsets were presented in alteration with the absence of offsets, thresholds were a factor of two worse than previous estimates of infant vernier acuity using grating stimuli. However, when offsets (discrete or sinusoidal) were presented in continuous motion, mean threshold was 22-24 arcmin, comparable to previous estimates using gratings with moving discrete offsets. For stimuli containing continuous motion, lowering velocity reduced infants' positional sensitivity. However, when either velocity or temporal frequency was held constant, the most important determinant of positional sensitivity was the sharpness of the offsets. The results from the temporally-modulated stimuli suggest that a simple local flicker mechanism cannot account for sensitivity when continuous motion is used in the stimulus; rather, a local motion mechanism may govern sensitivity in these conditions. We characterize this mechanism as one of "local motion" because it does have a position-sensitive component. The two stationary stimulus conditions in the present study indicate that infants can use a position-sensitive mechanism when no temporal modulation is present.

Humans↗