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PubMed1975

Central projection of optic tract from translocated eyes in the leopard frog (Rana pipiens).

In Rana pipiens embryos, eye anlagen were moved to the evacuated ear position, where they continued to differentiate and sent their optic nerve fibers into the hindbrain. Upon entering the medulla, the optic fibers turned caudally, penetrated the spinal cord, and traversed the dorsolateral white matter to the caudal end. We found this pattern of growth in every animal; the optic fibers did not enter the tecta. These results suggest the existence within the neural tube of a three-dimensional gradient system to which embryonic optic fibers are responsive and which may guide the normal development of the visual pathway.

Animals↗
PubMed2005

Polarization-sensitive and light-sensitive neurons in two parallel pathways passing through the anterior optic tubercle in the locust brain.

Many migrating animals use a sun compass for long-range navigation. One of the guiding cues used by insects is the polarization pattern of the blue sky. In the desert locust Schistocerca gregaria, neurons of the central complex, a neuropil in the center of the brain, are sensitive to polarized light and might serve a key role in compass navigation. Visual pathways to the central complex include signal processing in the upper and lower units of the anterior optic tubercle. To determine whether these pathways carry polarization-vision signals, we have recorded the responses of interneurons of the optic tubercle of the locust to visual stimuli including polarized light. All neurons of the lower unit but only one of five recorded neurons of the upper unit of the tubercle were sensitive to linearly polarized light presented in the dorsal visual field. These neurons showed polarization opponency, or a sinusoidal modulation of activity, during stimulation through a rotating polarizer. Two types of bilateral interneurons preferred particular e-vector orientations, reflecting the presence of bilateral pairs of these neurons in the brain. We show here for the first time neurons with projections to the lateral accessory lobe that are suited to provide polarization input to the central complex. All neurons of the tubercle, furthermore, responded to unpolarized light, mostly with tonic activity changes. These responses strongly depended on stimulus position and might reflect navigation-relevant signals such as direct sunlight or visual landmarks that are integrated with polarization responses in neurons of the lower unit.

Action Potentials↗
PubMed1993

Spatial selectivity in the anterior ectosylvian visual area of the cat.

Pattern reversal visually evoked potentials (PR-VEPs) were recorded in the anterior ectosylvian visual area (AEV) of the cat, and the dependence of the PR-VEPs on the spatial frequency (SF) and the mean luminance of the stimulation was studied. Six SFs between 0.025 and 1 cycles per degree (c/d) and four light density levels between 2 x 10(-2) and 117 cd/m2 (candela per square meter) were tested. The latency of the first positive component (P1) and the amplitude measured between P1 and the first negative (N1) component were analyzed. The dependence of the PR-VEP latencies on the SF displayed a significant relationship, with a minimum at around 0.05 c/d. This relationship was consistent at all luminance levels tested. The dependence of the amplitudes on the SF gave a "low-pass" tuning curve at every luminance level. The higher the luminance level, the steeper was the curve. The cut-off SF of these curves was at around 1 c/d under photopic conditions and around 0.6 c/d under scotopic conditions. The contrast dependence of the PR-VEP amplitudes exhibited saturation at 0.5, which was much higher than that of either the striate cortex or other extrastriate visual cortical areas, but similar to that of the superior colliculus. These results provide further evidence in support of the notion of a tecto-extrageniculo-extrastriate visual pathway functioning separately from the geniculo-striate mechanisms.

Animals↗
PubMed1987

Control of recurrent inhibition of the lateral geniculate nucleus by afferents from the superior colliculus of the rabbit: a possible mechanism of saccadic suppression.

Stimulation of the ipsilateral superior colliculus elicited a short burst of discharges of the recurrent inhibitory interneurones in the geniculocortical pathway of the rabbit. The most effective stimulating sites for this excitation were located in the deep layers rather than the superficial layers of the superior colliculus. The short latency of the response (2.3 +/- 0.6 ms) implied an oligo-synaptic excitatory pathway from the deep layers of the superior colliculus to the recurrent interneurones located in the caudal reticular nucleus of the thalamus. Following the excitation of the interneurone, there was a prolonged inhibition which started 10-30 ms and ended 150 ms after the collicular stimulation. The maximal inhibition occurred 50-70 ms after the stimulation. The effects of collicular stimulation on the recurrent inhibitory interneurones may be concerned with the inhibition of the visual pathway during saccades and with the disinhibition of "facilitation" during fixation of a new visual target.

Animals↗
PubMed2000

Development of connections in the human visual system during fetal mid-gestation: a DiI-tracing study.

Animal studies have shown that connections between the retina, lateral geniculate nucleus (LGN), and visual cortex begin to develop prenatally. To study the development of these connections in humans, regions of fixed brain from fetuses of 20-22 gestational weeks (GW) were injected with the fluorescent tracer DiI. Placement of DiI in the optic nerve or tract labeled retinogeniculate projections. In the LGN, these projections were already segregated into eye-specific layers by 20 GW. Retinogeniculate segregation thus preceded cellular lamination of the LGN, which did not commence until 22 GW. Thalamocortical axons, labeled from DiI injections into the optic radiations, densely innervated the subplate, but did not significantly innervate the cortical plate. This pattern was consistent with observations of a "waiting period" in animals, when thalamocortical axons synapse in the subplate for days or weeks before entering the cortical plate. Cortical efferent neurons (labeled retrogradely from the optic radiations) were located in the subplate and deep layers of the cortical plate. In summary, human visual connections are partially formed by mid-gestation, and undergo further refinement during and after this period. The program for prenatal development of visual pathways appears remarkably similar between humans and other primates.

Carbocyanines↗
PubMed2003

Branching thalamic afferents link action and perception.

Recent observations of single axons and review of older literature show that axons afferent to the thalamus commonly branch, sending one branch to the thalamus and another to a motor or premotor center of the brain stem. That is, the messages that the thalamus relays to the cerebral cortex can be regarded as copies of motor instructions. This pattern of axonal branching is reviewed, particularly for the somatosensory and the visual pathways. The extent to which this anatomical evidence relates to views that link action to perception is explored. Most pathways going through the thalamus to the cortex are already involved in motor mechanisms. These motor links occur before and during activity in the parallel and hierarchical corticocortical circuitry that currently forms the focus of many studies of perceptual processing.

Afferent Pathways↗
PubMed1995

Receptive field expansion in adult visual cortex is linked to dynamic changes in strength of cortical connections.

1. Receptive field (RF) sizes of neurons in adult primary visual cortex are dynamic, expanding and contracting in response to alternate stimulation outside and within the RF over periods ranging from seconds to minutes. The substrate for this dynamic expansion was shown to lie in cortex, as opposed to subcortical parts of the visual pathway. The present study was designed to examine changes in cortical connection strengths that could underlie this observed plasticity by measuring the changes in cross-correlation histograms between pairs of primary visual cortex neurons that are induced to dynamically change their RF sizes. 2. Visually driven neural activity was recorded from single units in the superficial layers of primary visual cortex in adult cats, with two independent electrodes separated by 0.1-5 mm at their tips, and cross-correlated on-line. The neurons were then conditioned by stimulation with an "artificial scotoma," a field of flashing random dots filling the region of visual space around a blank rectangle enclosing the RFs of the recorded neurons. The neuronal RFs were tested for expansion and their visually driven output again cross-correlated. After this, the neurons were stimulated vigorously through their RF centers to induce the field to collapse, and the visually driven output from the collapsed RFs was again cross-correlated. Cross-correlograms obtained before and after conditioning, and after RF collapse, were normalized by their flanks to control for changes in peak size due solely to fluctuations in spike rate. 3. A total of 37 pairs of neurons that showed distinct cross-correlogram peaks, and whose RF borders were clearly discernible both before and after conditioning, were used in the final analysis. Of these neuron pairs, conditioning led to a clear expansion of RF boundaries in 28 pairs, whereas in 9 pairs the RFs did not expand. RFs that did expand showed no significant shifts in their orientation preference, orientation selectivity, or ocularity. 4. When the RFs of a pair of neurons expanded with conditioning, the area of the associated flank-normalized cross-correlogram peaks also increased (by a factor ranging from 0.84 up to 3.5). Correlograms returned to their preconditioning values when RFs collapsed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2003

Visual selective behavior can be triggered by a feed-forward process.

The ventral visual pathway implements object recognition and categorization in a hierarchy of processing areas with neuronal selectivities of increasing complexity. The presence of massive feedback connections within this hierarchy raises the possibility that normal visual processing relies on the use of computational loops. It is not known, however, whether object recognition can be performed at all without such loops (i.e., in a purely feed-forward mode). By analyzing the time course of reaction times in a masked natural scene categorization paradigm, we show that the human visual system can generate selective motor responses based on a single feed-forward pass. We confirm these results using a more constrained letter discrimination task, in which the rapid succession of a target and mask is actually perceived as a distractor. We show that a masked stimulus presented for only 26 msec - and often not consciously perceived - can fully determine the earliest selective motor responses: The neural representations of the stimulus and mask are thus kept separated during a short period corresponding to the feed-forward "sweep." Therefore, feedback loops do not appear to be "mandatory" for visual processing. Rather, we found that such loops allow the masked stimulus to reverberate in the visual system and affect behavior for nearly 150 msec after the feed-forward sweep.

Attention↗
PubMed1981

[Morphological differentiation in connection with the functional specialization of the mammalian visual system (author's transl)].

In the mammals there are two visual pathways: 1. retino-geniculo-cortical (as a rule area 17) and 2. retino-colliculo-thalamo-(pulvino-) cortical (mainly areas 18 and 19). In connection with the visual specialization the first pathway only shows on the subcortical level (dorsal lateral geniculate nucleus) and on the cortical level (area 17) morphological differentiations. In contrast, the differentiation of parts of the second pathway (pulvinar complex and associative cortex) is the expression of a higher evolution of this system. In the dLGN the input-zones of the ipsi- and contralateral eye are separated more and more. This becomes visible by a cytoarchitectonic lamination of the dLGN. In area 17 layer IV is--in connection with the specialization--increasingly divided in sublayers where the fibers of the X- and Y-system terminate separately.

Animals↗
PubMed1991

Detection of optic pathway misrouting in the human albino neonate.

The diagnosis of albinism is indicated by the presence of visual pathway misrouting in which temporal retinal fibers erroneously decussate at the optic chiasm disrupting the normal topographical distribution of retinal geniculate-cortical projections. Detection of misrouted fibers is effected by non-invasive electrophysiological assessment of the topographical representation of the visual evoked potential (VEP) following full field monocular stimulation. By combining appropriate state defined neonatal recording procedures with the albino VEP test paradigm, the presence of aberrant optic pathway projections was detected in a five-day-old full-term infant. The electrophysiological signature pathognomonic to albinism was observed within a long (300 ms) latency window of an otherwise normal neonatal luminance flash response. The results of this study indicate that the VEP misrouting test can be extended to reliable albino diagnosis within the neonatal period.

Albinism↗
PubMed1986

The effects of pretectal and superior collicular lesions on binocular vision.

Monkeys with mid-brain lesions involving the pre-tectum and superior colliculi often have an odd stare, as if the visual axes were parallel and the animal was looking into the distance. Such a visuomotor abnormality could lead to double-vision for objects close to the animal. The experiments reported here were designed to test this hypothesis of diplopia in monkeys with combined bilateral ablations of the colliculi and frontal eye-fields (area 8). These animals performed better on a task of visually-guided reaching, and in a series of visual pattern discriminations, when they viewed stimuli monocularly rather than binocularly. Monkeys with other cortical lesions showed no such monocular superiority. We propose that an abnormality of vergence eye movements provides a simple explanation for some of the so-called perceptual impairments that follow damage to the mid-brain visual pathways in monkeys.

Animals↗
PubMed1994

Changes in metabolic activity in the hyperstriatum of the chick before and after hatching.

Changes in metabolic activity in the hyperstriatal regions of the chick forebrain have been assessed just prior to and after hatching using [14C] 2-deoxyglucose (2-DG) autoradiography. Embryos were injected on day E19, followed by either exposure to light for 30 min or being held in darkness. Other embryos were injected on day E20, after pipping of the egg shell had occurred, and chicks were injected on day 1 (D1) after hatching, followed by light exposure. In the E19 groups metabolic activity in visual regions of the hyperstriatum accessorium (HA) was significantly higher than that in the hyperstriatum dorsale (HD), the region which receives the thalamofugal visual projections. The result was the same in both the light and dark exposed embryos, indicating that the high level of activity in HA on day E19 is not visually driven and that HA may be processing inputs from other sensory modalities. At stage E20 the activities of HA and HD did not differ and by day 1 post-hatching HD activity exceeded that of HA. Activity in HA fell between E19 and E20, while in HD activity rose between E20 and D1. The developmental sequence of metabolic activity levels in the intermediate medial hyperstriatum ventrale (IMHV), a region involved with imprinting memory formation, was higher on E19 and D1 than on E20. E20 is thus a quiescent period of neural activity in the hyperstriatum prior to hatching. Although a small number of the embryos showed distinct hemispheric asymmetries in metabolic activity, overall there was no significant asymmetry in the embryo groups. The implications of these results for imprinting and early perceptual processing are discussed: it appears that HA activity may be inhibited or limited during the sensitive period for visual imprinting, thereby temporarily diminishing the importance of the thalamofugal visual pathway relative to the tectofugal pathway in the imprinting process.

Animals↗
PubMed2004

Transient homonymous hemianopia and positive visual phenomena in nonketotic hyperglycemic patients.

PURPOSE: To report a case of a transient homonymous hemianopia in a patient with nonketotic hyperglycemia with negative magnetic resonance imaging (MRI) scans. DESIGN: Case report. METHOD: A 72-year-old woman with diabetes mellitus was referred to the neuro-ophthalmology service with hyperglycemia and multiple visual complaints. RESULTS: The patient was found to have a dense left homonymous hemianopia on examination. An MRI scan of the brain was negative for a lesion affecting the visual pathways. Resolution of the homonymous hemianopia and the patient's symptoms came with normalization of her blood glucose levels. CONCLUSION: Nonketotic hyperglycemia is an important diagnostic consideration in patients who present with homonymous visual field defects but have negative neuroimaging studies.

Aged↗
PubMed2006

Evaluating shape after-effects with radial frequency patterns.

Mechanisms selective for complex shape are vulnerable to adaptation techniques historically used to probe those underlying performance in lower-level visual tasks. We explored the nature of these shape after-effects using radial frequency patterns. Adapting to a radial frequency pattern resulted in a strong and systematic after-effect of a pattern that was 180 degrees out of phase with the adapting pattern. This after-effect was characterized as both a shift in the point of subjective equality and an increase in response uncertainty. The after-effect transferred across adapting pattern contrast and adaptor amplitude, suggesting an involvement from shape-specific mechanisms located at higher processing stages along the visual pathway. Moreover, our results suggested that the shift in the point of subjective equality was guided by global processing mechanisms, whereas the increase in uncertainty reflected activity from local processing mechanisms. Together, these results suggest that shape-specific after-effects reflect gain control processes at various stages of processing along the ventral pathway.

Adaptation, Physiological↗
PubMed1990

Representation of the ipsilateral visual field in the transition zone between areas 17 and 18 of the cat's cerebral cortex.

The representation of the visual field in the architectonically defined transition zone between areas 17 and 18 of cat cerebral cortex was assessed by recording the activities and plotting the receptive fields of neurons at 2327 sites along 148 electrode penetrations made in 19 cats. The results show that the transition zone contains a significant representation of the ipsilateral visual hemifield although not all elevations in the visual field are represented to the same extent. The shape of the field region represented resembles an hour glass, for the region represented is narrowest on the 0-deg horizontal meridian and increasingly wider at progressively more positive and negative elevations. When receptive-field centers are considered, the extent of the representation reaches to -2.5 deg on the 0-deg horizontal meridian and to 10 or more degrees towards the field periphery. When receptive-field areas are considered, the representation at the 0-deg horizontal meridian extends to -3.6 deg and to beyond 20 deg at other elevations. In contrast, the visual-field representations in flanking areas 17 and 18 are essentially limited to the contralateral hemifield. The presence of a distinct representation of part of the ipsilateral hemifield in the transition zone suggests that the zone may have connections distinctly different from those of the adjacent areas. The observations bear on the problems of understanding the visual pathways in hypopigmented cats and binocular disparity mechanisms about the midline.

Animals↗
PubMed2005

Object onset and parvocellular guidance of attentional allocation.

The parvocellular visual pathway in the primate brain is known to be involved with the processing of color. However, a subject of debate is whether an abrupt change in color, conveyed via this pathway, is capable of automatically attracting attention. It has been shown that the appearance of new objects defined solely by color is indeed capable of modulating attention. However, given evidence suggesting that the visual system is particularly sensitive to new onsets, it is unclear to what extent such results reflect effects of color change per se, rather than effects of object onset. We assessed attentional capture by color change that occurred as a result of either new objects appearing or already-present "old" objects changing color. Results showed that although new object onsets accrued attention, changing the color of old objects did not. We conclude that abrupt color change per se is not sufficient to capture attention.

Adult↗
PubMed1977

The dynamics of the pi 1 colour mechanism: further evidence for two sites of adaptation.

1. The visual pathway that determines Stiles's Pi(1) colour mechanism was isolated by the auxiliary field technique and studied under dynamic conditions of light adaptation and recovery by threshold measurements.2. The time courses of adaptation to Pi(1)-equated short wave-length (mu </= 500 nm) and long wave-length (mu >/= 550 nm) fields are very distinct: a large and relatively long-enduring transient threshold elevation occurs at the onset of the long wave-length, but not of the short wave-length fields.3. Similarly, the time courses of recovery from Pi(1)-equated long and short wave-length fields are quite distinctive: a large and relatively long enduring transient (;transient tritanopia') occurs at the offset of the long wave-length, but not of the short wave-length fields.4. The wave-lengths of the fields which cause the adaptation transients coincide with those shown previously (Pugh, 1976) to combine non-additively with mu = 430 nm fields in effecting Pi(1) adaptation. The failure of the time course of Pi(1) adaptation to be spectrally ;univariant' combines with the failures of field-additivity to demonstrate that signals from the long and/or middle wave-length sensitive cones affect the adaptation state of the Pi(1) pathway.5. The adaptation transients are not observed in the pathways that determine Pi(4) and Pi(5). Thus, instantaneous signals from the middle and/or long wave-length sensitive cones are not the cause of the transients. Rather the cause must lie in the path by which those cones transmit their signals to the Pi(1) pathway or in the Pi(1) pathway itself.6. The off-transient can be diminished by adding an adequately intense short wave-length field to a long wave-length field that would normally cause it. The Pi(1) pathway must receive chromatically opponent signals.

Adaptation, Ocular↗
PubMed2004

Retinal axon guidance: novel mechanisms for steering.

Axons from the retina traverse different molecular territories as they navigate to the tectum. A single territory might span only a few cell diameters and harbour multiple guidance cues, many of which are beginning to be characterized. Also present in the pathway are 'modulators' that influence a growth cone's response to a coincident signal but do not guide growth directly. An emerging principle is that the growth cone, itself, changes molecularly as it journeys through the visual pathway. Growing retinal axons contain mRNAs, ubiquitinating and apoptotic enzymes, translation and degradation machinery. Guidance cues can trigger rapid and local synthesis, degradation and endocytosis of proteins, providing a fast and flexible way for growth cones to respond to cues in their microenvironment and to alter their responsiveness. The data raise the idea that the localized synthesis and downregulation of proteins might help to steer retinal axon growth and, further, might contribute to the changing character of a growth cone as it ages.

Animals↗