abstract
-
This thesis investigated the monocular direction of temporal and nasal stimuli in infants who were newborn, 1 month old and 2 months old. They were shown three stimuli: a blank field, a single line 3°1δ' wide located in the temporal visual field and the same line located in the nasal visual field. Their eye movements were recorded with corneal photography. To assess peripheral detection, I compared the probability that an infant moved his eyes from the centre of the visual field toward a line with the probability that he moved his eyes first in the same direction when the field was blank.
When the lines were located at 10° in the periphery, newborns detected both the temporal line and the nasal line with their left eye (Experiment 1, n = 32). These results showed that newborns can demonstrate both temporal and nasal detection monocularly. When the lines were located further in the periphery, newborns detected the temporal line at 30°. but they showed no evidence of detecting the nasal line at 20°. This was true both when I tested the left eye (Experiment 2, n = 33) and when I tested the right eye (Experiment 3, n = 30).
The results show clearly that in newborns detection in the temporal visual field is better than detection in the nasal visual field. In cats the direct projection from retina through superior colliculus can mediate good detection in the temporal visual field but the Y-pathway through the cortex is necessary for good detection in the nasal visual field. Thus, in the human newborn, the Y-pathway through the cortex might be too immature to mediate good detection in the nasal visual field.
There appear to be major electrophysiological changes in the huaman's geniculo-cortical pathway at about 2 months of age. Moreover, only at that age do human infants begin to show smooth pursuit and symmetrical optokinetic nystagmus, behaviours which in cats depend on the Y-pathway through the cortex. Those data suggest that in human infants, the Y-pathway through the cortex might begin to influence visual behaviour at about 2 months of age. Thus, if poor nasal field detection in human newborns were to reflect an immature Y-pathway through the cortex, infants should show an improvement in nasal field detection at 2 months of age, but not before. Experiments 4 and 5 confirmed that prediction. One-month-olds, like newborns, detected the temporal line at 30° but appeared not to detect the nasal line at 20°(Experiment 4, n = 30). In contrast, 2-month-olds detected both the temporal and the nasal lines (Experiments 5, n = 32).
The data reported in this thesis suggest that, in human infants, good nasal field detection depends at least on the Y-pathway through the cortex. That pathway appears to be too immature to mediate good nasal field detection prior to 2 months of age.