Suzannah Bliss Tieman

The nature of perceptual deficits in visually deprived cats.

Summary Different types of visual deprivation lead to different patterns of deficits determined neurophysiologically. This study attempted to determined whether these different types of visual deprivation would lead to different patterns of behavioral deficits. Kittens were reared with one or both eyes suturned shut or with contact lenses alternately occluding either or both eyes. Following 13–25 weeks of deprivation these kittens were tested for the ability to learn and to intercularly transfer discimination of luminous flux, grid oreintation and pattern (upright versus inverted triangles). Generalization test were administered to determine the basis on which these discriminations were performed. The amount of intercoular transfer obtained was not proportiona to the number of binuclear cells that would be expected to result from the different kinds of deprivation. Specifically, the alternating mononuclear deprivation, which should have led to a 10-fold reduction in the proportion of such units, had no effect on intercular transfer. Interocular appears to be more difficult if an animals uses different cues to master the discrimination with the two eyes as may be the case for the monucularly deprive animals. The learning data suggest that normal visual perception of patterned stimuli is dependent upon the presence of an adequate number of cells with highly selective response characteristics. A decrease in the proportion of such cells retards the learning visual discriminations. The fewer the remaining cells, the greater the retardation. With reduced set of cells with selective receptive fields, the organism can still extract some form cues — such as grid orientation — from the environment, but finds it very difficult to extract higher order configurational properties. Overall, the results of this series of experiments provide evidence that highly selective cells in cat visual cortex are essential for normal pattern perception.

N-acetylaspartylglutamate immunoreactivity in neurons of the cat's visual system.

The acidic dipeptide, N-acetylaspartylglutamate (NAAG) was identified immunohistochemically within neurons of the cats visual system. In the retina, NAAG-like immunoreactivity was observed in some horizontal and amacrine cells at the inner and outer margins of the bipolar cell layer. NAAG-like immunoreactivity was also observed in many retinal ganglion cell bodies, their neurites, and the neuropil of their target areas, the lateral geniculate nucleus (LGN) and the superior colliculus. Additionally, peptide immunoreactivity was also seen in the projection neurons of the LGN, in cells of the pulvinar nucleus, and in the pyramidal cells of layers III and V in areas 17, 18 and 19 of the cerebral cortex. These data suggest that NAAG or a structurally related molecule may have a prominent role in the communication of visual signals at retinal, thalamic and cortical levels.

The blood supply of the cat's visual cortex and its postnatal development

We examined the blood supply of the cats visual cortex using alkaline phosphatase histochemistry to demonstrate the capillary endothelial cells. In the adult, layer 4 is marked by a band that is of obviously greater density, extends throughout areas 17 and 18, and ends abruptly at the 18/19 border. We quantified blood vessel density in area 17, observing a 23% greater density in layer 4 than in supragranular and infragranular layers. This difference reflects a laminar difference in metabolic rate. In three animals studied using the metabolic marker 2-deoxyglucose, layer 4 was 25% denser than the other layers. The band of greater density in layer 4 is not present in newborn kittens, but becomes apparent at about 5 weeks of age. Early in development, the endothelial cells form filopodia as the capillaries grow and branch. The density of blood vessels decreases slightly during the first week of postnatal life, but increases between 1 and 6 weeks of age, so that by 6 weeks, the blood supply of the visual cortex resembles that seen in the adult. This pattern resembles that of cortical metabolism seen with 2-deoxyglucose [J. Cereb. Blood Flow Metab. 11 (1991) 35], but the increase in vascular density precedes that in glucose metabolism.

Alternating monocular exposure increases the spacing of ocularity domains in area 17 of cats

Goodhill (1993) has recently suggested that the spacing of ocularity domains in visual cortex is not solely an intrinsic property of cortex, but is determined, at least in part, by the degree of correlation in the activity of the two eyes. In support of this model, Löwel (1994) has shown that strabismus, which decorrelates the activity of the two eyes, increases the spacing of ocular dominance columns in area 17, but not area 18, of the cat. As a further test of Goodhills model, in this paper we examine the effects of another rearing procedure that decorrelates the activity of the two eyes, namely alternating monocular exposure (AME). Cats were reared either normally (9 cats) or with AME (21 cats). We labeled their ocularity domains by one of three methods: ocular dominance columns by 2-deoxyglucose (14 cats), and ocular dominance patches by transneuronal transport (14 cats), or by injections of tracer into single layers of the lateral geniculate nucleus (LGN; 2 cats). The spacing of ocular dominance was 11% greater in the AME cats than in the normal cats (0.976 vs. 0.877 mm). These results are similar to those previously reported for strabismic cats, although the effect is less striking. We thus confirm that decorrelating the activity of the two eyes increases the spacing of cortical ocularity domains. Our results further suggest that the degree of decorrelation affects the extent of that increase.

Interocular transfer of mirror image discriminations by chiasm-sectioned monkeys.

Abstract Chiasm-sectioned monkeys, trained monocularly, learned to discriminate left-right (L-R) mirror images faster than up-down (U-D) in contrast to normal monkeys Unlike those of an earlier report, the chiasm-sectioned monkeys in our experiment consistently failed to choose the mirror image stimulus when tested for interocular transfer of discriminations of L-R mirror images under a variety of conditions. Compared to normal monkeys or to their own performance on U-D mirror images, however, they did show a deficit in interocular transfer of the L-R discriminations, performing near chance when first tested with the untrained eye. We attribute the chance performance to a conflict between two response tendencies: verdical performance based on unreversed sensory information transferred by the callosum, and reversed performance based on masking by the bitemporal hemianopia. Reversal occurs because one half of an asymmetric stimulus, seen by the testing eye, resembles the opposite half of its mirror image, seen by the training eye. Related asymmetries in attention may play a significant role in the difficulty that normal animals often show in discriminating L-R mirror images.

Effect of eye removal on N-acetylaspartylglutamate immunoreactivity in retinal targets of the cat

The endogenous brain dipeptide N-acetylaspartylglutamate (NAAG) has previously been demonstrated in the somata of retinal ganglion cells and the neuropil of retinal targets. In this paper we report that the NAAG immunoreactivity of the neuropil in the retinal targets is dependent on an intact optic pathway. Removal of one eye produced a marked decrease in the staining of the neuropil in layer A of the contralateral geniculate nucleus (LGN) and layer A1 of the ipsilateral LGN. There was also decreased staining in the superficial layers of the superior colliculus contralateral to the removal. These results suggest that NAAG is present in the terminals of retinal ganglion cells and is consistent with a role for NAAG in visual synaptic transmission.

Morphological Changes in the Geniculocortical Pathway Associated with Monocular Deprivationa

To summarize (Fig. 10), the structural consequences of monocular deprivation include the following changes: the relay cells in the binocular segments of the deprived geniculate layers shrink and contain less of the possible neurotransmitter NAAG. These changes appear to be secondary to a loss of terminal synaptic arbor. Certainly, deprived geniculocortical cells project to smaller ocular dominance patches in layer IV of visual cortex, where they make fewer and abnormal synapses. As a result, they activate ocular activation columns that, in addition to being small, are faint and usually fail to extend into extragranular layers. This failure to extend to other layers probably results from a failure of the poorly activated deprived-eye cells in layer IV to compete successfully with neighboring experienced-eye cells in layer IV, resulting in a loss of connections from layer IV to other layers (Fig. 11). Thus, the primary effect of monocular deprivation is probably the disruption of the geniculocortical synapse, with the other changes, such as cell size, and possibly the change in neurotransmitter content, being secondary. The disrupted synapse would result in poorly driven cortical cells and faint ocular activation columns, which in turn would bias a secondary competition for access to cells in extragranular layers. There are certain general principles that unite the findings presented in this chapter with the others in this session. First, there are similarities in the types of morphological changes observed, for example, changes in the number and size of synaptic terminals, as well as mitochondrial changes. This implies that there are similar changes during development and adult plasticity and also similar changes in vertebrates and invertebrates. Second, it is not so much the amount of activity that determines these changes, but the pattern of activity. In my results, the relative imbalance in activity is important, but not the absolute amount (for example, the columns activated by the 8-hr eye of an AME 8/1 are different from those activated by the 8-hr eye of an AME 8/8). Similarly, the binocular segment, where there was an imbalance and competition could occur, was affected, whereas the monocular segment, where there was no imbalance and competition could not occur, was not. Finally, the recent results of Reiter and Stryker suggest that monocular deprivation produces changes only when the activity of the presynaptic cell and the postsynaptic cell are correlated.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular injection in fixed slices: obtaining complete dendritic arbors of large cells.

Intracellular injection of Lucifer yellow into fixed brain slices is widely used to demonstrate dendritic morphology. A major limitation of this technique is that large dendritic arbors are usually truncated at the cut surfaces. Here we describe modifications that allowed us to obtain complete dendritic arbors of large spiny stellate cells. Lucifer Yellow cadaverine biotin-X (LY-X) was injected into individual neurons within 300-1000 microm thick aldehyde-fixed slices of kitten visual cortex. Subsequently, the LY-X was histochemically reacted using standard ABC methods to obtain a permanent record of the injected cells. Dendrites, studded with a variety of dendritic spines, were darkly labeled and well defined against virtually no background. Somatic spines, dendritic varicosities and growth cones were common in the younger animals. Computer-assisted reconstructions demonstrated that, in older animals, the dendritic arbors of cells injected in 300 microm slices were truncated, whereas the arbors of cells injected deep within thick slices were complete. The modifications described here remove the most critical limitation of intracellular injection in slices, allowing quantitative analysis of even large dendritic arbors.

Interhemispheric comparison of mirror image stimuli in chiasm-sectioned monkeys.

Abstract Normal and chiasm-sectioned monkeys compared a sample stimulus presented to one eye with two possible matching stimuli presented to the other eye. The possible matches were either up-down (U-D) or left-right (L-R) mirror images of each other. The normal monkeys matched correctly on all tests; the chiasm-sectioned monkeys matched the U-D patterns correctly but were inconsistent with the L-R patterns, matching veridically on one problem while matching to the mirror image on another problem. This variability in performance cannot be explained by anatomical models such as the one proposed by Noble, which assumes that the commissural fibers reverse the cortical representation of a stimulus left for right. Behavioral mechanisms based on the bitemporal hemianopia caused by section of the optic chiasm, however, can interpret both the present data and the pre-existing data on interocular reversal of mirror image discriminations.

A computer-assisted video technique for preparing high resolution pictures and stereograms from thick specimens.

A computer-assisted video technique is presented for rapidly and accurately gathering, storing and depicting three-dimensional anatomical structures in thick specimens. Several optical sections through the specimen are combined to produce high-resolution photographs with essentially infinite depth-of-field. Further, the depth information implicit in the series of optical sections makes the creation of stereoscopic pairs relatively simple. The technique employs a real-time digitizing frame store and a computer. A video camera is attached to a microscope and successive optical sections are stored digitally as the plane of focus is systematically changed. After storage, the image of each optical section is enhanced to emphasize elements that are sharply focussed. The final two-dimensional image is generated by selecting for each point in the final picture the darkest grey value occurring at the corresponding point in any of the pictures in the through-focus series. A picture with essentially infinite depth-of-field is produced when points of correspondence in the series are determined by a ray passing normal to the plane of optical section. Right and left pictures for a stereoscopic pair are produced when points of correspondence are determined by a ray slanting either left or right of normal. This technique is illustrated with cobalt chloride-filled neurons from whole-mounted cricket ganglia, with HRP-filled axons from whole-mounted goldfish tectum, with Golgi-Kopsch-impregnated neurons from cat visual cortex, and with sections of cobalt chloride-filled antennal afferents in cricket.