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PubMed1990

Organization of geniculocortical projections in turtles: isoazimuth lamellae in the visual cortex.

The projection from the dorsal lateral geniculate complex to the visual cortex in Pseudemys and Chrysemys turtles was examined by using the anterograde transport of horseradish peroxidase (HRP) in vitro and the retrograde transport of HRP in vivo. In vitro HRP injections into the lateral forebrain bundle were used to fill geniculocortical axons anterogradely, which were then analyzed in cortical wholemount preparations. Geniculocortical axons gain access to the visual cortex along its entire rostral-caudal extent. They course in slightly curved trajectories for up to 2 mm from the lateral edge of the cortex through both the lateral (or pallial thickening) and medial parts of Desan's cortical area D2. Single axons are of fine caliber. They tend to cross each other and sometimes branch in the pallial thickening, but are generally unbranched in the medial part of D2. They bear small, fusiform varicosities at irregular intervals along their lengths. Although axons show small variations in the number of varicosities per 100 microns segment, no consistent variation in varicosity number as a function of distance could be detected. These results indicate that geniculocortical axons project to the visual cortex in an orderly pattern. The retrograde transport experiments provide some clue as to the significance of this pattern. Small, ionotophoretic injections of HRP in the visual cortex retrogradely labeled neurons in the dorsal lateral geniculate complex. Injections in the rostral visual cortex retrogradely labeled neurons in the caudal pole of the geniculate complex. Injections at progressively more caudal loci within the visual cortex labeled neurons at progressively more rostral loci within the geniculate complex. Thus, there is a representation of the rostral-caudal axis of the geniculate complex along the caudal-rostral axis of the visual cortex. Consistent with the anterograde transport experiments that showed individual geniculocortical axons coursing through both lateral and medial parts of the visual cortex, HRP injections restricted to the medial edge of the visual cortex retrogradely labeled neurons along the entire dorsal-ventral axis of the geniculate complex at the appropriate rostral-caudal position. The neurophysiological studies of Mazurskaya ('72: J. Evol. Biochem. Physiol. 8:550-555; respond to a small, moving stimulus anywhere in visual space, implying a convergence of inputs from all points in visual space somewhere along the retinogeniculocortical pathway. The experiments reported here suggest a convergence in the geniculocortical projections of information along the vertical meridians, or azimuth lines, of visual space onto neurons lying along lateral to medial transects through the visual cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Synaptic organization of cortico-cortical connections from the primary visual cortex to the posteromedial lateral suprasylvian visual area in the cat.

The synaptic organization of the projection from the cat striate visual cortex to the posteromedial lateral suprasylvian cortical area (PMLS) was examined. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was iontophorectically delivered into area 17, and anterogradely labeled fibers were revealed in PMLS by means of an immunocytochemical detection method. Most axons and presumptive terminal swellings were found in layers III and IV. The neuronal elements (n = 190) that were postsynaptic to anterogradely labeled boutons were quantitatively analyzed. All anterogradely labeled cortico-cortical boutons (n = 182) established type 1 synapses. The results show that 83% of the postsynaptic targets were dendritic spines, probably belonging to pyramidal cells. Dendritic shafts constituted 17% of the targets. The dendritic shafts postsynaptic to cortico-cortical boutons were studied for the presence of gamma-aminobutyric acid (GABA) with a postembedding immunogold method. Most dendritic shafts (85%) that were tested were found to be GABA-positive, demonstrating that they originate from local inhibitory neurons. Taking into account that most postsynaptic targets were spines and extending the results of the immunocytochemical testing to the total population of postsynaptic dendrites, it was calculated that at least 14% of targets originated from GABA-positive cells. Thus cortico-cortical axons establish direct monosynpatic connections mainly with pyramidal and to a lesser extent with GABAergic nonpyramidal neurons in area PMLS, providing both feedforward excitation and feedforward inhibition to a visual associational area known to be involved in the processing of motion information. The results are consistent with previously demonstrated deficits in physiological properties of neurons in PMLS following removal of cortico-cortical afferents.

Animals↗

PubMed1992

Activity-dependent decrease in NMDA receptor responses during development of the visual cortex.

Plasticity of the developing visual system has been regarded as the best model for changes of neuronal connections under the influence of the environment. N-methyl-D-aspartate (NMDA) receptors are crucial for experience-dependent synaptic modifications that occur in the developing visual cortex. NMDA-mediated excitatory postsynaptic currents (EPSCs) in layer IV neurons of the visual cortex lasted longer in young rats than in adult rats, and the duration of the EPSCs became progressively shorter, in parallel with the developmental reduction in synaptic plasticity. This decrease in NMDA receptor-mediated EPSC duration is delayed when the animals are reared in the dark, a condition that prolongs developmental plasticity, and is prevented by treatment with tetrodotoxin, a procedure that inhibits neural activity. Application of L-glutamate to outside-out patches excised from layer IV neurons of young, but not of adult, rats activated prolonged bursts of NMDA channel openings. A modification of the NMDA receptor gating properties may therefore account for the age-dependent decline of visual cortical plasticity.

Action Potentials↗

PubMed1979

Neurophysiological mechanisms of recovery from visual cortex damage in cats: properties of lateral suprasylvian visual area neurons following behavioral recovery.

Damage to visual cortical areas 17, 18, and 19 in the cat produces severe and long-lasting deficits in performance of form and pattern discriminations. However, with extensive retraining the animals are able to recover their ability to discriminate form and pattern stimuli. Recent behavioral experiments from this laboratory have shown that a nearby region of cortex, the lateral suprasylvian visual area (LS area), plays an important role in this recovery (Wood et al., 1974; Baumann and Spear, 1977b). The present experiment investigated the underlying neurophysiological mechanisms of the recovery by recording from single neurons in the LS area of cats which had recovered from long-term visual cortex damage. Five adult cats received bilateral removal of areas 17, 18, and 19. They were then trained to criterion on two-choice brightness, form, and pattern discriminations. Recording from LS area neurons was carried out after the behavioral training, from 3 to 7 months after the visual cortex lesions. The properties of these neurons were compared to those of LS area neurons in normal cats (Spear and Baumann, 1975) and in cats with acute or short-term visual cortex damage and no behavioral recovery (Spear and Baumann, 1979). The results showed that all of the changes from normal which were produced by acute visual cortex damage were also present after the behavioral recovery. Moreover, all of the response properties of LS area neurons which remain after acute visual cortex damage were present in similar form after the behavioral recovery. There was no evidence for any functional reorganization in the LS area concomitant with its role in the behavioral recovery. These results suggest that functional reorganization plays little or no role in recovery from visual cortex damage in adult cats. Rather, the recovery of form and pattern discrimination ability appears to be based upon the functioning of residual neural processes in the LS area which remain after the visual cortex damage.

Animals↗

PubMed1992

Sex differences in cortical thickness and the dendritic tree in the monocular and binocular subfields of the rat visual cortex at weaning age.

The visual cortex of adult rats is sexually dimorphic at both the gross size and dendritic levels [Brain Res., 295 (1984) 27-34; J. Comp. Neurol., in press]. In addition, sex differences in the dendritic tree are dynamic and can be altered by environmental conditions imposed at weaning [Brain Res., 295 (1984) 27-34]. The present study examines sex differences in cortical thickness and in the dendritic tree of the monocular (Oc1M) and binocular (Oc1B) subfields in littermate male and female pairs of Long-Evans rats at weaning age (25 days). From Nissl-stained tissue, it was found that the whole cortex and layer II-IV of Oc1B was thicker in males than females. No sex differences were found in the thickness of Oc1M. Golgi-Cox-stained pyramidal neurons in layer III from the Oc1M and Oc1B regions were quantified in 7 littermate pairs of weaning-age rats. There were no sex differences in the basilar tree, while the apical oblique branches were sexually dimorphic, especially in the monocular region. Females had greater total dendritic length and longer terminal branches in Oc1M compared to males. Females also had longer bifurcating branches in both Oc1M and Oc1B than males. The present study found that sex differences at weaning age do not completely mirror the dimorphisms found in the visual cortex of the adult rat. This study also indicates that related subfields can differ in their morphology and should be examined separately.

Analysis of Variance↗

PubMed1990

Functional compensation in the lateral suprasylvian visual area following bilateral visual cortex damage in kittens.

Previous studies have shown that functional compensation is present in the cat's posteromedial lateral suprasylvian (PMLS) area of cortex after damage to areas 17, 18, and 19 (visual cortex) early in life but not after damage in adults. These studies all have investigated animals with a unilateral visual cortex lesion, whereas all behavioral studies of compensation for early visual cortex damage have investigated animals with a bilateral lesion. In the present experiment, we investigated whether functional compensation also is present in PMLS cortex after a bilateral visual cortex lesion early in life. We recorded from single neurons in the PMLS cortex of adult cats that had received a bilateral lesion of areas 17, 18, and 19 on the day of birth or at 8 weeks of age. We found that PMLS cells in both groups of cats had functional compensation (normal direction selectivity and ocular dominance) similar to that seen after a unilateral lesion at the same ages. These results are consistent with the hypothesis that PMLS cortex is involved in the behavioral compensation seen after early visual cortex damage. In addition, the results indicate that inputs from contralateral visual cortex are not necessary for the development of functional compensation seen in PMLS cortex.

Age Factors↗

PubMed1991

Relation of cortical cell orientation selectivity to alignment of receptive fields of the geniculocortical afferents that arborize within a single orientation column in ferret visual cortex.

Neurons in the primary visual cortex of higher mammals are arranged in columns, and the neurons in each column respond best to light-dark borders of particular orientations. The basis of cortical cell orientation selectivity is not known. One possible mechanism would be for cortical cells to receive input from several lateral geniculate nucleus (LGN) neurons with receptive fields that are aligned in the visual field (Hubel and Wiesel, 1962). We have investigated the relationship between the arrangement of the receptive fields of geniculocortical afferents and the orientation preferences of cortical cells in the orientation columns to which the afferents provide visual input. Radial microelectrode penetrations were made into primary visual cortex of anesthetized adult sable ferrets. Cortical cells were recorded throughout the depth of the cortex, and their orientation preferences were determined. Cortical cell responses were then eliminated by superfusion of the cortex with either kainic acid (Zahs and Stryker, 1988) or muscimol. After the drug treatment, responses from many single units with distinct receptive fields were recorded. These responses were presumed to be those of geniculocortical afferents, because they had the response properties characteristic of LGN neurons, and because they could be recorded only in cortical layers that receive geniculate input. In 16 of 18 cases, the afferent receptive fields recorded in a single penetration covered an elongated region of visual space. In these penetrations, the best-fit line through the centers of the afferent receptive fields generally paralleled the preferred orientation of cortical cells recorded at the same site in cortex. These results are consistent with the Hubel and Wiesel (1962) model for the construction of oriented visual cortical receptive fields from geniculate inputs with aligned receptive fields.

Afferent Pathways↗

PubMed1978

Synaptic development in the rabbit superior colliculus and visual cortex.

The development of synapses in the visual cortex (VC) and superior colliculus (SC) of the rabbit has been examined with the electron microscope. In both areas, the number of synapses reaches adult levels by 20--25 days of postnatal age, but the development in the visual cortex is delayed in comparison to that in the superior colliculus. When S synapses (spheroidal vesicles, asymmetric thickening) are compared with F synapses (flattened vesicles, symmetric thickening), even greater differences are seen. In both the VC and SC, S synapses develop earlier than F synapses, though there is considerable overlap. Of interest is that fact that synapses in the visual cortex seem to overshoot their adult levels late in development, suggesting that an excess of synapses may be formed in this system. Multiple synapses, probably of retinal origin, increase in the first 3 weeks of synaptic development in the SC, but never are present in significant proportions in the VC. Synapse formation most often is characterized by formation of a junction and a postsynaptic thickening, followed by acquisition of synaptic vesicles. After 15 days, there is only a small number of such "non-vesicle synapses" in either the SC or VC.

Age Factors↗

PubMed1990

'Real-motion' cells in area V3A of macaque visual cortex.

The stability of visual perception despite eye movements suggests the existence, in the visual system, of neural elements able to recognize whether a movement of an image occurring in a particular part of the retina is the consequence of an actual movement that occurred in the visual field, or self-induced by an ocular movement while the object was still in the field of view. Recordings from single neurons in area V3A of awake macaque monkeys were made to check the existence of such a type of neurons (called 'real-motion' cells; see Galletti et al. 1984, 1988) in this prestriate area of the visual cortex. A total of 119 neurons were recorded from area V3A. They were highly sensitive to the orientation of the visual stimuli, being on average more sensitive than V1 and V2 neurons. Almost all of them were sensitive to a large range of velocities of stimulus movement and about one half to the direction of it. In order to assess whether they gave different responses to the movement of a stimulus and to that of its retinal image alone (self-induced by an eye movement while the stimulus was still), a comparison was made between neuronal responses obtained when a moving stimulus swept a stationary receptive field (during steady fixation) and when a moving receptive field swept a stationary stimulus (during tracking eye movement). The receptive field stimulation at retinal level was physically the same in both cases, but only in the first was there actual movement of the visual stimulus. Control trials, where the monkeys performed tracking eye movements without any intentional receptive field stimulation, were also carried out. For a number of neurons, the test was repeated in darkness and against a textured visual background. Eighty-seven neurons were fully studied to assess whether they were real-motion cells. About 48% of them (42/87) showed significant differences between responses to stimulus versus eye movement. The great majority of these cells (36/42) were real-motion cells, in that they showed a weaker response to visual stimulation during tracking than to the actual stimulus movement during steady fixation. On average, the reduction in visual response during eye movement was 64.0 +/- 15.7% (SD).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1977

[Types of receptive fields of neurons in different laminae of the rabbit visual cortex].

Neurons of the primary rabbit visual cortex were classified into 7 large groups according to features of their receptive fields. The neurons with receptive fields were mostly revealed in the layers IV and VI, those with uniform directional receptive fields--in layer V, those with simple I-in layer VI, those with simple II--in layer II + III, and those with hypercomplex--in layer IV. The neurons with concentric receptive fields and those without responses to visual stimuli were equally distributed over the layers. The data obtained in rabbits, cats, and monkeys suggest that the tendency towards function stratification of primary visual cortex is successive in an evolutionary order of mammals.

Animals↗

PubMed1990

Ultrastructure of PkC(II/III)-immunopositive structures in rat primary visual cortex.

In the primary visual cortex of adult rats the cellular and subcellular distribution of protein kinase C isozymes II and III (PkCII/III) was examined by immunohistochemical methods with a monoclonal antibody against PkCII/III. Strong PkC(II/III)-immunoreactivity was found in neurons and astrocytes. Immunopositive neurons exhibited morphological features characteristic for both pyramidal and non-pyramidal cells. They were distributed in layers II through VI but were concentrated in layers II/III. At the electron microscopic level immunoprecipitate was found predominantly in distinct regions of the somata, except the nuclei, and only a few labeled dendrites and axons were seen. Two different patterns of cytoplasmic immunoreactivity could be distinguished. In most neurons, PkC(II/III)-staining was confined to cytoplasmic spots associated with the Golgi complex, while a few neurons exhibited additional labeling in the vicinity of the cell membrane. Moreover, PkC(II/III)-immunoreactivity was present in numerous astroglial processes and in the perikaryal cytoplasm of a subpopulation of astrocytes. The present data provide morphological indications for specific functions of PkC isozymes II and III in neurons as well as in astrocytes.

Animals↗

PubMed1990

Effects of NMDA antagonists on developmental plasticity in kitten visual cortex.

The existence of Hebb synapses in the visual cortex of young kittens has long been postulated. A mechanism for the correlation of activity in simultaneously active pre- and postsynaptic neurons could be provided by the properties of the N-methyl-D-aspartate (NMDA) receptor and its associated Ca2+ channel, which opens in a transmitter- and voltage-dependent manner. We have studied the effects on cortical plasticity of blocking NMDA receptors in different ways with competitive and non-competitive NMDA antagonists. In our first approach, the non-competitive NMDA antagonist ketamine, a short-acting dissociative anaesthetic, was injected systemically after each of a series of brief monocular exposures. This procedure prevented the development of an ocular dominance shift towards the experienced eye in the visual cortex. Other short-acting anaesthetics, such as xylazine or methohexital, while providing the same depth of anaesthesia, did not have the same effect on ocular dominance plasticity. We conclude, therefore, that ketamine quite specifically interferes with synaptic consolidation in the visual cortex. In order to establish a role of NMDA receptors for cortical plasticity directly in the visual cortex, we performed another series of experiments: 2-amino-5-phosphono-valerate (APV), a competitive NMDA antagonist, was infused intracortically by means of implanted osmotic minipumps in kittens, which were monocularly deprived for 1-2 weeks. Within a radius of 4-5 mm, the expected ocular dominance shift was prevented or reduced. In addition, however, physiologically determined cell density and responsiveness to visual stimuli were grossly abnormal around the infusion site, and histological cell density was also reduced. Similar effects were found when MK801 (a non-competitive NMDA antagonist) was used in the same type of experiment. The outcome of both experimental approaches makes it very likely that NMDA antagonists somehow interfere with cortical plasticity. Their mode of action, however, remains ambiguous. Although it is quite possible that blocking of the NMDA channel prevents the Hebbian correlative process necessary for synaptic consolidation, more complex effects, such as an interference with a neurotrophic action normally exerted via the NMDA receptor, may have to be taken into account as well.

2-Amino-5-phosphonovalerate↗

PubMed1992

The effects of selective glutamate receptor antagonists on synchronized firing bursts in layer III of rat visual cortex.

In the rat visual cortex in vitro, single-shock stimulations applied to the border between layer VI and the white matter evoke synchronized burst-firing by units in layer III. We have examined the effects of glutamate receptor antagonists on this activity, with antagonists applied via the bath to allow correlation of effects with concentrations. All synaptically driven components (recorded extracellularly as field potential 'S2' spikes, dipoles 'W1' and 'W2', and coinciding single-unit spikes) were inhibited by greater than 90% in 1.0 mM kynurenic acid and in 3 or 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, which selectively blocks AMPA/kainate receptors). S2 spike amplitudes were reduced by half in 0.7 microM CNQX. 2-Amino-5-phosphonovalerate (APV), a specific blocker of NMDA receptors, did not prevent S2 spike burst or horizontal spread of bursting within layer III. However, APV reduced the duration of synchronized bursts and the slower potentials which followed. In Mg(2+)-free medium, new components appeared which were APV-sensitive: (1) low amplitude spikes, distributed spatially like S2 spike, but recurring more slowly, and (2) slow potentials, distributed spatially like W1 and W2 potentials, but lasting for hundreds of milliseconds. The amplitudes of these spikes were reduced by half in 3 microM D-APV. Our data imply that: (1) glutamate receptors play a major role in mediating local, excitatory neurotransmission in the supragranular layers of neocortex, with NMDA and AMPA/kainate subtypes each subserving somewhat different functions; (2) AMPA/kainate receptors mediate rapid excitatory transmission between layer III neurons, responsible for driving the first 15 ms of synchronized bursts; (3) currents gated by NMDA receptors determine the duration of coherent firing bursts, and drive asynchronous neuronal firing following bursts; and (4) under conditions which circumvent block by extracellular Mg2+, activation of NMDA receptors greatly enhances and prolongs the response to single-shock stimulations. In vivo, activation of layer III neurons is likely to depend significantly upon currents gated by NMDA receptors whenever repetitively firing excitatory inputs summed over several tens of milliseconds provide enough depolarization to lift block by extracellular Mg2+.

2-Amino-5-phosphonovalerate↗

PubMed1992

Periodic-pattern-selective cells in monkey visual cortex.

To study the visual processing of periodic and aperiodic patterns, we have analyzed neuronal responses in areas V1 and V2 of the visual cortex of alert monkeys during behaviorally induced fixation of gaze. Receptive field eccentricities ranged between 0.5 degrees and 4 degrees. We found cells that responded vigorously to gratings, but weakly or not all to bars and edges. In some cells the aperiodic stimuli even reduced the activity below the spontaneous level. The distribution of a bar-grating response index indicated a discrete population of "grating cells" characterized by more than 10-fold superiority of gratings. We estimated that these cells have a frequency of 4% in V1 and 1.6% in V2, and that about 4 million grafting cells of V1 subserve the central 4 degrees of vision. The converse, cells that responded to isolated bars but not to gratings of any periodicity, was also observed. The grating cells of V1 were mostly (23 of 26) found in layers 2, 3, and 4B. They preferred spatial frequencies between 2.6 and 19 cycles/degree (median, 9.3), with tuning widths at half-amplitude between 0.4 and 1.4 octaves (median, 1.0). Their tunings were narrower, and their preferred frequencies higher, than those of other cells on average. Grating cells were also narrowly tuned for orientation. Those of V2 were similarly selective. The responses of grating cells depended critically on the number of cycles of the gratings. With square waves of optimum periodicity responses required a minimum of 2-6 grating cycles and leveled off at 4-14 (median, 7.5). The corresponding receptive field widths were 0.34-2.4 degrees (median, 0.78 degrees) for V1 and 0.72-2.4 degrees (median, 1.4 degrees) for V2. Grating cells typically gave unmodulated responses to drifting gratings, were unselective for direction of motion, and were strongly activated also by stationary gratings. Half of those of V1 were monocular, the others binocular, some showing strong binocular facilitation and disparity sensitivity. Length summation was usually monotonic, but strong end-inhibition was also observed. In contrast to other cells, grating cells were not activated by harmonic components. Spatial-frequency response curves for sine-wave, square-wave, and line gratings were similar. Square-wave gratings of one-third the preferred frequency failed to excite the cells, while the isolated 3f component (f = the fundamental of the square wave) of these gratings evoked strong responses. In spite of the nonlinear features, grating cells had low contrast thresholds.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1976

Alterations in receptive field properties of superior colliculus cells produced by visual cortex ablation in infant and adult cats.

To determine if functional alterations in the superior colliculus might account for recovery of visual behaviors following visual cortex removal in infant cats, the receptive field characteristics of single units in the superior colliculus of cats whose visual cortex was removed within the first week of life were compared with those of cats which sustained visual cortex lesions in adulthood and with those of normal cats. In the normal superior colliculus, 90% of all cells responded to moving stimuli irrespective of shape or orientation. Sixty-four percent of these units were directionally selective, responding well to movement in one direction but poorly or not at all to movement in the opposite direction. Ninety percent of units were binocular, the vast majority of these responding equally to stimulation of either eye or showing only slight preference for stimulation of the contralateral eye. Responses to stationary flashes of light were observed in only 33% of all visually activated cells in the normal superior colliculus. After visual cortex ablation in adult cats, only six percent of movement sensitive cells were directionally selective. Binocular preference was shifted following adult visual cortex lesions such that sixty percent of all cells responded exclusively or predominantly to stimulation of the contralateral eye. Seventy-one percent of all visually responsive units responded to stationary lights flashed on or off within their receptive field boundaries. Lesions limited primarily to area 17 had the same effect as larger lesions of visual cortex. Infant visual cortex lesions resulted in receptive field alterations similar to those observed after adult ablation. Only fifteen percent of motion sensitive units were directionally selective. Seventy-one percent responded exclusively or predominantly to stimulation of the contralateral eye. Seventy-six percent of visually responsive cells were activated by stationary light. Lesions largely confined to area 17 produced the same alterations as more extensive lesions of visual cortex. Thus, no evidence was found that the superior colliculus is involved in the functional reorganization presumed to occur following visual cortex ablation in infant cats. Recovery of visual behaviors following neonatal injury may therefore not involve alterations in the receptive fields of single cells.

Animals↗

PubMed1991

Specialized vascularization of the primate visual cortex.

We have analyzed blood vessel distribution in the primary and secondary visual cortices of the squirrel monkey in relation to cortical modules, laminae, and cytoarchitectonic areas. Measurements of microvessel length in tangential sections through the primary visual cortex showed that blobs are more richly vascularized than intervening cortical regions. Thus, the mean total length of microvessel profiles per unit was 42% greater within these cortical modules than within adjacent (interblob) areas. Total microvessel length per unit area in another class of module, the stripes in the secondary visual cortex, was 27% greater than in interstripe regions. Microvessel distribution also varied systematically from layer to layer in the primary visual cortex, being greatest in lamina IVc. Finally, the overall microvessel length per unit area in sections of the primary visual cortex was 26% greater than that in the secondary visual cortex. These observations indicate that the modular, laminar, and regional organization of the primate visual cortex is reflected in the underlying distribution of cortical microvessels. These vascular patterns should be discernable in living animals with vascular contrast agents and appropriate imaging techniques.

Animals↗

PubMed1990

Development of connections to and from the visual cortex in the wallaby (Macropus eugenii).

The time course of the development of connections between the visual cortex and the main subcortical visual structures, as well as intrahemispheric and interhemispheric connections, has been studied in the marsupial wallaby (Macropus eugenii) to compare its development with that of placental mammals. Pouch young are born prior to retinal innervation of the primary visual centers and spend a protracted period of development in the pouch, making them ideal for visual, developmental studies. Horseradish peroxidase conjugated to wheatgerm agglutinin was injected into either the presumptive visual cortex or the superior colliculus in young of varying ages. Thalamocortical projections from the dorsal lateral geniculate and lateral posterior nuclei reach the presumptive visual cortex between 12 and 15 days after birth. Descending cortical connections form later. Corticogeniculate axons are first detected in the geniculate and lateral posterior nucleus at 48 days after birth, while corticocollicular axons first reach the superior colliculus at 71 days and, by 81 days, have innervated the superficial layers. Intrahemispheric and interhemispheric connections form even later. By 99 days intrahemispheric axons from area 17 have accumulated in visual association areas but are yet to invade layers III and IV, their major termination zones in adult, while axons projecting back to area 17 have also reached their target area. At this time interhemispheric axons from area 17 have begun to accumulate in the opposite visual cortex, although they have not invaded the cortical layers. By 111 days cortical cells projecting to the opposite visual cortex are first labelled. These have a more widespread distribution in area 17 at 111 and 122 days compared to the adult, where they are confined to the 17/18 border. The results show that the marsupial wallaby has a timetable of similar sequence, but different relative timing, in the formation of cortical connections compared to that of placental mammals. In the first half of the period between conception and eye opening, the timing in the wallaby precedes considerably that in placental mammals. Ascending connections from the thalamus develop relatively earlier in the wallaby but descending collicular connections are delayed until the same relative time that they appear in placental mammals.

Afferent Pathways↗

PubMed1991

Flashed pattern-induced activity in the visual system: I. The short latency evoked response recorded from the cat visual cortex.

Previous methods for estimating visual acuity have used the visual evoked response conventionally the late visual evoked response components or the steady-state potential. The present experiments were undertaken to evaluate the possible use of short latency flashed pattern evoked responses in estimating pattern dependent activity in the cat visual system. Recordings were made from the skull bone and the dura above the primary visual cortex and intracortically. The visual evoked responses to patterned (checks) and non-patterned light flashes of high intensity and short duration were recorded. The visual evoked response activity recorded from the cortical surface had an onset latency of 14-15 ms. The initial positive-negative potential sequences of the responses were similar for patterned and non-patterned stimuli, however a difference was recorded from 35-40 ms after stimulus. The smallest check size which separated a pattern from a non-pattern VER was in the order of 10 min of arc. The results indicate that the short-latency cortical VER may be used to estimate visual resolution.

Animals↗