Biomedical subjects
Publications and source records attributed to M W Van Hof.
Rabbits trained to discriminate vertical vs. oblique striations are unable to discriminate angular differences of 5 degrees. In the present study the instabilities around the roll axis of the eyes were measured during visual discrimination. The results indicate that these instabilities are one of the causes of the rabbit's inability to discriminate angular differences of 5 degrees.
Rabbits were trained to discriminate between vertical and horizontal striations. After the 90% correct level had been reached transfer to vertical and horizontal rows of dots was studied. It was found that performance depended on the distance between adjacent dots. With angular dot separations (measured from the centers) larger than 4.9 degrees, performance was below the 75% correct level. This result is discussed in relation to the size of the receptive fields of orientation-selective neurons in the binocular visual cortex.
In a group of 14 rabbits aged between 14 and 21 days a hemidecortication was performed in the left hemisphere. A normal monopedal lateral hopping response developed in both forelegs. At adult age, 13 weeks after the first lesion, the remaining cortex on the right side was ablated. Three days afterwards the hopping response was tested again. In all animals, the hopping response remained positive in the right foreleg, contralateral to the lesioning at early age. However, in 10 out of 14 animals the response was absent on the left side, contralateral to the recent lesion. These results show that the hopping response is not mediated by corticofugal connections of the remaining hemisphere of the foreleg contralateral to the early lesion.
Twenty eight rabbits were trained to discriminate striated patterns of different orientation, first binocularly, than with each eye separately. There was no evidence of systematic dominance of either the left or the right hemisphere.
The role of the visual cortex in brightness and pattern vision was re-examined in the rabbit. Animals were trained on both a brightness and a horizontal-vertical striation discrimination. Bilateral removal of the dura mater overlying the entire visual cortex produced no impairments in either brightness or pattern vision. When the visual cortex was devascularized by removal of the pia mater bilaterally, total loss of pattern vision was found with no impairment of brightness discrimination ability. The significance of the finding that the effects of slowly developing degenerative lesions are the same as those of surgical ablation was discussed.
Rabbits were trained to discriminate vertical vs horizontal striations. After reaching criterion one of the patterns was randomly replaced by striations oriented at 135 degrees. Other rabbits were trained to discriminate 45 degrees vs 135 degrees striations. After reaching criterion, one of the patterns was randomly replaced by vertical or horizontal striations. It was found that performance remained at the 90% correct level when the unrewarded pattern was present. When this was not the case performance dropped to 60-70% correct. It was concluded that in the discrimination apparatus used in this study, the rabbit's choice is mainly controlled by the unrewarded stimulus.
Brightness and tilt discrimination were studied in rabbits in which one eye had been enucleated either 1 day or 3 months after birth. When tested at the age of 5 months no differences in performance were found between both groups of animals.
Interocular transfer of mirror image striation discrimination was studied in rabbits in such a way that uninterrupted series of monocular trials were avoided. It was found that the results were not different from those obtained in a previous study in which one eye was occluded by a textile mask. It was concluded that the low level of interocular transfer was not due to position habit developed during prolonged monocular training.
Both normal and commissure-sectioned rabbits were examined over a variety of visual discrimination tasks. The visual displays were projected at various distances within the frontal visual field of the rabbit. The learning of either a black-white or a vertical-horizontal problem was found to be normal in commissure-cut animals. Furthermore, no deficit in angular acuity was observed. However, when they were tested with the patterns displayed at 25 cm distance, large impairments were found in the commissure-sectioned animals.
Rabbits were trained on a brightness and a vertical vs horizontal discrimination. After bilateral removal of the visual cortex the threshold of brightness discrimination was significantly higher. In addition, there was a severe impairment of striated pattern discrimination.
In a two-choice discrimination box, rabbits were trained to discriminate vertical vs horizontal striations. After criterion was reached, the same striations were presented again, but now the patterns moved sinusoidally. All animals immediately transferred to moving patterns. After that, it was attempted to train the animals to discriminate between moving vs non-moving vertical striations. Even after prolonged training, none of the animals learned this discrimination task.
Binocular brightness and striated pattern discrimination were studied in Dutch-belted rabbits. A comparison was made between normal animals and animals in which a unilateral lesion of the occipital lobe had been made three months after birth. No difference between the two groups was found in brightness discrimination learning. However, normal animals were found to do better both in acquisition and accuracy of striated pattern discrimination.
Previous studies in rabbits have shown that a unilateral ablation of the occipital lobe leads to a severe impairment of the ability to discriminate striated patterns. Considerable retraining is required in order to reach the 90% correct level with the eye contralateral to the lesion. In the present study the effects of a unilateral and bilateral ablation are compared. It is concluded that the rudimentary ability to discriminate patterns with the eye contralateral to a unilateral ablation of the occipital lobe is mediated by the remaining ipsilateral cortical projection and not by subcortical structures.
The sharpness of a trans-scleral image was examined in the enucleated albino rabbit eye. Profound changes in the focus of the image were produced by changes in the relative position of the object in the visual field. These findings are discussed in relation to the view that the rabbit is frontally myopic and laterally hypermetropic.
Pattern discrimination was studied in rabbits in which a unilateral ablation of the occipital lobe had been made either at adult age or a few days after birth. It was found that a postoperative stay in an enriched environment did not improve striated pattern discrimination with the eye contralateral to the lesion.
Adult rabbits make flexion and extension movements with fore- and hindlimbs while swimming. This type of behavior develops between 8 and 15 days after birth. It is concluded that by observing swimming movements the maturation of subcortical structures related to locomotion can be studied in unrestrained rabbits.
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