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Biomedical subjects

R D Freeman

Publications and source records attributed to R D Freeman.

At least 109 records · Page 6Linked to original sources
PubMed1982

Contrast sensitivity in children.

We have used a spatial two-alternative, forced-choice staircase technique to measure contrast sensitivities for sinusoidally modulated gratings. Subjects, all of whom were untrained observers, consisted of children of ages 2-16 yr and adults. Our testing method was completely successful with children who were over 3.5 yr but failed with those below 2.5 yr. Mean contrast sensitivities of the youngest group from which data were obtained (2.5-4.5 yr) were 0.35 log units lower than those of adults. However, there was very little difference between these two groups in the middle range of spatial frequencies tested. A gradual increase with age of contrast sensitivities was found up to about 8 yr. This change is probably due to a combination of neural development and non-visual factors.

Adolescent↗
PubMed1982

Brief periods of monocular deprivation in kittens: effects of delay prior to physiological study.

1. Brief periods of selective visual exposure in 4-wk postnatal kittens can cause pronounced alterations in striate cortex. The processes underlying these changes may be similar to those of other short-term neural events such as memory or learning, and attempts have been made to link these areas. In particular, it has been reported that the effects of selective visual exposures are consolidated if a delay is interposed between rearing and physiological study. 2. We have tested this notion directly. Normally reared kittens were monocularly occluded for 8 or 24 h on postnatal day 29. Half of the 8-h and of the 24-h groups were tested physiologically immediately following monocular exposure, while the other halves were recorded after an intervening session of 8 or 48 h, respectively, during which the kittens were kept in darkness. 3. Extracellular responses of cortical neurons were analyzed, and primary attention was paid to absolute and relative interocular response strengths of each cell. Ocular dominance distributions from all kittens were significantly abnormal, but invariably, effects were less extensive for those who had experienced a period of dark rearing prior to physiological study. 4. These results are clearly at odds with the idea that the effects of monocular deprivation were consolidated by allowing a period of delay between exposure and physiological recording. On the contrary, during the time the kittens spent in darkness, it appears that some functional recovery of binocular pathways occurred. 5. To explore the minimal period of monocular occlusion that could cause cortical disruption, we studied an additional group of normally reared kittens that were occluded for only 4 h on postnatal day 29. Effects for this group were small but ocular dominance patterns were significantly abnormal. 6. An alternative procedure was used for the delay period between 24-h monocular occlusion and physiological study. Prior to recording, these kittens spent 48 h in a normally illuminated colony. Ocular dominance histograms were normal, indicating that complete recovery had occurred from the initial monocular deprivation. 7. Histological reconstructions were made to estimate effects in different cortical laminae, but conclusions must be tentative because of small samples. In the group that had been monocularly occluded for 4 h and then recorded, inactivation of binocular pathways was most apparent in layers IV and V. This results suggest that the process deprivation disconnection during monocular deprivation occurs frist in these layers. Findings from the groups in which a delay was imposed between monocular exposure and recording indicate that functional binocular recovery occurs frist in layer IV and above and last in layer VI.

Animals↗
PubMed1981

Effects of strabismus on development of cortico-geniculate projections in the kitten.

In six kittens reared with surgically induced strabismus, three each of convergent and divergent types, we studied 208 striate cortex cells. Of these, 22 were identified as projecting to the dorsal lateral geniculate nucleus (LGN) on the basis of antidromic activation from LGN and of histological localization within cortical layer VI. We classified these cortico-geniculate cells according to their axonal conduction velocities which in normal cats, are generally grouped into slow, intermediate, or fast categories. Proportions and mean conduction velocities of slow and fast groups were approximately the same as in the normal cat, but the intermediate group was almost entirely missing with only one cell classified as such. Our results suggest that development of the intermediate group, which has previously been implicated in functional binocular vision, may be impaired selectively by strabismus.

Aging↗
PubMed1981

Ocular dominance in kitten cortex: induced changes of single cells while they are recorded.

We have monitored extracellularly individual neurons in the striate cortex of 4-week-old unparalyzed kittens with the aim of changing the ocular dominance of these cells during recording. To do this, we elicited conjugate eye movements using a bipolar stimulating electrode positioned in the internal medullary lamina (IML) of the thalamus. During electrical stimulation of this region, one eye was occluded and the other was visually activated with optimal stimuli. Receptive fields were studied subjectively and objectively and relative response strengths were assessed. Of 42 cells studied in detail, 62% underwent changes on ocular dominance following conditioning periods of, generally, 15-20 min. Control experiments suggest that this plasticity is: age-related; requires both visual stimulation and activation of pathways associated with eye movement; and does not appear to be caused solely by increased arousal levels.

Animals↗
PubMed1981

Contrast sensitivity in anisometropic amblyopia.

Contrast sensitivity functions were measured for sinusoidal gratings from a sample of 10 anisometropic amblyopes. A high spatial frequency deficit was found from tests of the amblyopic eyes of all subjects. This defect decreased with spatial frequency and was correlated with the magnitude of anisometropia. Controls were instituted to rule out psychophysical method and residual defocus as possible causes of these effects. At low spatial frequencies, there were small differences between the two eyes. For some subjects, sensitivities of the amblyopic eyes appeared actually higher than normal whereas the reverse was found for most of the others. Additional tests demonstrated that the low-frequency differences could be accounted for by magnification differences (aniseikonia) between the two eyes. These findings are consistent with the idea that monocular contrast deprivation is the causal agent in anisometropic amblyopia.

Adolescent↗
PubMed1980

Rescaling of the retinal map of visual space during growth of the kitten's eye.

We have measured the angle between the visual axis and the axis projected from the center of the optic disk in 35 cats ranging in age from two weeks to adulthood. Our results show that this angle, a, declines from around 27 degrees in very young kittens to about 16 degrees in adult cats, with most of the change occurring during the first 6 weeks after birth. We interpret this change as reflecting a progressive contraction of the area of object space projected onto the retina. For this to occur, the posterior nodal distance of the eye's optical system must increase by a larger factor than the transverse extent of the retina. This process undoubtedly contributes to maturation of the kitten's visual function, causing a reduction of the size of neuronal receptive fields and an enhancement of spatial resolution.

Age Factors↗
PubMed1980

Cortical effects of daily sequential stimulation of right and left eyes in the kitten.

Beginning near the peak of the sensitive period to monocular deprivation, kittens were reared in darkness except for daily sessions during which the left eye was exposed first followed immediately by an equal amount of right eye exposure. The notion was that the sequence of stimulation may be an important determinant in cortical representation of each eye. Although study of single neurons in area 17 showed that nearly all cells were monocular, no systematic imbalance was found in the numbers of units controlled by each eye.

Animals↗
PubMed1980

Corneal radius of curvature of the kitten and the cat.

Keratometric measurements were made from 38 kittens and cats in a closed breeding colony. Data were obtained on changes of radius of curvature of the cornea as a function of age and weight. The measurements also provided data on estimated changes with growth of corneal astigmatism.

Age Factors↗
PubMed1979

Cortical plasticity in monocularly deprived immobilized kittens depends on eye movement.

A marked reduction of binocular cells in striate cortex is found if 4-week-old kittens are visually stimulated monocularly while anesthetized and held in a stereotaxic apparatus. If the kittens are paralyzed and artificially respirated, changes are not found unless an eye is moved mechanically. It appears that eye movement and visual stimulation are necessary conditions for deactivation of binocular connections, but neither is sufficient to induce such changes alone.

Animals↗
PubMed1979

Effects of brief uniocular 'patching' on kitten visual cortex.

Studies of the striate cortex of the kitten are reviewed with reference to the effects of very brief periods of uniocular occlusion. It is shown that changes occur in binocular connections within 4 hours after the vision of an eye is blocked. Effects are also found if the occlusion is instituted while an animal is anaesthetized but not if it is paralysed in addition. If 4-hour unilateral 'patching' sessions are given daily, a cumulative uniocular deprivation effect is found if the animals are kept in darkness between the patching sessions. However, the visual cortex is nearly normal if the kittens are allowed binocular vision between patching periods.

Anesthesia↗