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PubMed1999

Integration and saturation within the circadian photic entrainment pathway of hamsters.

The sensitivity of the visual pathway that subserves circadian entrainment was measured in hamsters after prior stimulation and using trains of multiple pulses. Immediately after subsaturating stimulation in the late subjective night, there was a significant decrease in responsiveness that persisted for at least 1 h. The reduced responsiveness was not due to light adaptation (shifting of the stimulus-response curve) but rather to response saturation, which appeared to reduce the sensitivity to subsequent stimulation and limit the maximum response of the pacemaker. The system, therefore, integrates the total number of photons delivered in two light stimuli separated in time by up to 1 h. The responsiveness was also measured using stimulus trains containing 10-1,000 individual pulses of equal irradiance and equal total photons. Results suggest that this pathway is responsive to the total photons delivered in all of the stimuli and is not responsive to light onsets or offsets associated with individual stimuli. These data outline several fundamental characteristics of phase shifting for the circadian photic entrainment pathway in hamsters. Knowledge of these characteristics is important for designing and interpreting results of future studies to dissect the cellular and molecular nature of the mammalian circadian clock and for understanding how visual information affects the cellular clock during entrainment.

Adaptation, Ocular↗
PubMed1983

The effects of lesions of telencephalic visual structures on visual discriminative performance in turtles (Chrysemys picta picta).

Ascending thalamotelencephalic visual pathways that terminate in specific telencephalic regions have been described in all reptiles studied. Although the anatomical data suggests that such telencephalic regions may play a role in visual processing in reptiles, few behavioral data are available. In the present study, the effects of destruction of either the core nucleus (CN) of the dorsal ventricular ridge (telencephalic terminus of the tectothalamofugal pathway) or the dorsal cortex (telencephalic terminus of the retinothalamofugal pathway) on visual discriminative performance in the turtle were examined. Following extensive bilateral destruction of the CN, turtles were severely impaired in their performance of both a simultaneous pattern discrimination and a simultaneous visual intensity discrimination. The extent of the discriminative impairment was found to be specifically correlated with the amount of CN damage. In contrast to the effects of CN lesions, lesions of the dorsal cortex had no evident effect on the performance of either a simultaneous pattern discrimination or a simultaneous visual intensity discrimination. The present results suggest that, as in birds and mammals, telencephalic visual areas play an important role in visual functions in reptiles. As in at least some birds (such as pigeons), the telencephalic terminus of the tectothalamofugal visual pathway appears to play a larger, or at least more readily measurable, role in visual discrimination than does the telencephalic terminus of the retinothalamofugal pathway.

Animals↗
PubMed2005

Haphazard neural connections underlie the visual deficits of cats with strabismic or deprivation amblyopia.

Identification of the neural basis of the visual deficits experienced by humans with amblyopia, particularly when associated with strabismus (strabismic amblyopia), has proved to be difficult in part because of the inability to observe directly the neural changes at various levels of the human visual pathway. Much of our knowledge has necessarily been obtained on the basis of sophisticated psychophysical studies as well as from electrophysiological explorations on the visual pathways in animal models of amblyopia. This study combines these two approaches to the problem by employing similar psychophysical probes of performance on animal models of two forms of amblyopia (deprivation and strabismic) to those employed earlier on human amblyopes (Hess & Field, 1994, Vis. Res., 34, 13397-13406). The tests explore two competing explanations for the visual deficits, namely an evenly distributed loss of neural connections (undersampling) with the amblyopic eye as opposed to disordered connections with this eye (neural disarray). Unexpectedly, the results in animal models of deprivation amblyopia were not in accord with expectations based upon an even distribution of lost connections with the amblyopic eye. However, the results were similar to those observed in a strabismic amblyopic animal and to strabismic amblyopic humans. We suggest that deprivation amblyopia may be accompanied by an uneven loss of connections that results in effective neural disarray. By contrast, amblyopia associated with strabismus might arise from neural disarray of a different origin such as an alteration of intrinsic cortical connections.

Amblyopia↗
PubMed1983

Visual evoked potential abnormalities in Charcot-Marie-Tooth disease and comparison with Friedreich's ataxia.

Pattern-reversal visual evoked potentials (VEPs), recorded in 15 visually asymptomatic patients fulfilling the clinical and electrophysiological criteria of Charcot-Marie-Tooth disease (CMTD), were abnormal in 5 and possibly abnormal in another 3. Five patients showed a prolongation of P100 latency, one a reduction of amplitude and one a possibly abnormal "scotomatous" waveform. In 9 cases abnormalities were detected on neuro-ophthalmological examination. These were poorly correlated with VEP abnormalities, except for patients with 2 or more clinical eye signs. Relative central scotomata were found in the patient with an abnormal waveform. VEP abnormalities, where present, were usually fairly comparable in the 2 eyes. In comparison with a group of Friedrich's ataxia cases there was a lower overall incidence of VEP abnormalities in CMTD, but little to suggest a qualitative difference in the nature of the visual pathway pathology. All 4 patients with unequivocally abnormal VEPs had experienced atypical symptoms suggestive of CNS involvement. In none of these was it possible to sustain an alternative diagnosis. It is concluded that a minor degree of visual pathway involvement may be present in many CMTD cases, in spite of the fact that optic atrophy is only rarely reported, and that the VEP latency may reflect the degree to which other parts of the CNS are involved.

Adolescent↗
PubMed1986

Reduction of axonal transport in the rat optic system after direct application of methylmercury.

Fast axonal transport of proteins in the optic nerve and tract was quantified by scintillation counts of protein-bound radioactivity along the visual pathway after an intraocular injection of [3H]proline. In control rats the label traveled at a rate of about 60 mm/day, reaching the optic chiasm at 4 h and the lateral geniculate body at 8 h postinjection. When methylmercury was injected simultaneously with [3H]proline, the label traveled at a rate of about 30 mm/day. At 8 h postinjection, the labeled protein had reached the optic chiasm, but the more distal pathway was unlabeled. The same pattern was observed histologically by emulsion autoradiography of the pathway. Some label was detected in the lateral geniculate of methylmercury-treated animals at 8 h, but this may have resulted from local incorporation, as judged by a similar level of labeling in the contralateral visual pathway. Alternatively, it may be the case that a small fraction of the axons in the treated pathway continued to transport proteins in a normal fashion. The very heavy label observed throughout the pathway in controls was present only in the proximal half of the pathway in methylmercury-treated rats. Methylmercury significantly reduced incorporation of [3H]proline in the rat retina, but this reduction was not as great as the effect in the optic nerve. In contrast, cycloheximide, a potent protein synthesis inhibitor, reduced labeled protein in the optic nerve only to the same extent as it reduced incorporation. These results suggest that methylmercury's effect on transport is not dependent solely on its effects on protein synthesis, but represents a separate mechanism of neurotoxicity.

Animals↗
PubMed1999

Blindsight in subjects with homonymous visual field defects.

Brain damage in the visual system can lead to apparently blind visual areas. However, more elaborate testing indicates that some visual ability may still exist for specific stimuli in the otherwise blind regions. This phenomenon is called 'blindsight' if subjects report no conscious awareness of visual stimuli but when forced to guess, nevertheless perform better than chance. It has mainly been suggested that secondary visual pathways are responsible for this phenomenon. However, no published study has clearly shown the neural mechanism responsible for blindsight. Furthermore, experimental artifacts may have been responsible for the appearance of the phenomenon in some subjects. In the present study, the visual fields of nine subjects were mapped and residual visual performance was examined in many areas using three different experimental procedures. Artifacts such as stray light or eye movements were well controlled. In addition, confidence ratings were required after each trial in the forced-choice tests. The results show that only one subject with a lesion in the optic radiation had blindsight in two discrete areas of the affected visual field. Spared optic radiation fibers of the main (primary) geniculo-striate visual pathway were most likely to account for this finding.

Adult↗
PubMed1975

Considerations of vision and cerebral function during hypotension.

The currently held theory of preserved consciousness with impaired vision during positive acceleration is based on a maintained intraocular pressure and selective impairment of conduction in the peripheral visual pathway, though the experimental evidence does not wholly support this theory. During hypotension, intraocular pressure follows closely the arterial pressure, and conduction along the optic nerve is preserved as long as the electroretinogram persists. It is proposed that, though the mechanism of visual impairment during hypotension is related to events in the peripheral visual pathway, there are likely to be active forebrain events which preserve consciousness.

Acceleration↗
PubMed2006

Homonymous hemianopia in stroke.

BACKGROUND: Previous reports have suggested that most cases of homonymous hemianopia (HH) are caused by occipital stroke. However, these reports have not always been supported by brain imaging. METHODS: We reviewed the medical records of all patients seen in our unit between 1989 and 2004 who had HH documented by formal perimetry or confrontation visual fields and had undergone brain imaging. HHs were divided into those caused by stroke and by non-stroke conditions. The clinical and visual field characteristics were compared in the two groups. RESULTS: Among 850 patients with 902 HHs, 629 (69.7%) resulted from stroke, of which 531 (84.4%) were from infarction and 98 (15.6%) from primary intraparenchymal hemorrhage. Non-stroke causes included head trauma (123), brain tumor (102), neurosurgical procedures (22), multiple sclerosis (13), and miscellaneous conditions (13). Occipital lesions most commonly resulted from stroke. The configuration of the HH did not predict where in the retrochiasmal visual pathway the responsible lesion lay. CONCLUSIONS: Ischemic stroke causes most HHs from lesions in the occipital lobe that generally do not produce other neurologic manifestations. The configuration of the HH does not predict the location of the lesion within the retrochiasmal visual pathway.

Adolescent↗
PubMedDate not supplied

Contribution to the theory of prosthetic vision.

By way of extracellular, electrical stimulation of the visual pathway, the various approaches to vision prosthesis aim to provide crude, patterned vision to individuals with profound blindness. Common to all approaches is the implantable electrode array and the rendering of phosphenes-the actuated percepts occupying the visual field of the implantee. Thus prosthetic vision may be simulated, and underlying theories as to how to render it efficacious developed. We review the field of simulated prosthetic vision. Furthermore, with retinal prosthesis in mind, we suggest a revised approach-an approach with regard to sampling theory, the vertebrate central visual pathway, and eye movements. The parallel development of prosthetic vision theory, explored via simulation and bioengineering issues surrounding neurostimulator design and implantation has bearing on the success of clinical trials by numerous groups in coming years.

Biomedical Engineering↗
PubMed1998

Visually evoked cortical potentials in awake cats during saccadic eye movements.

Visually evoked potentials (VEPs) measured under conditions of retinal image stabilization that minimized the influences of visual masking and smearing were averaged from electroencephalographic records measured from striate cortex of three cats. The amplitudes of the VEPs increased around saccade initiation. The grating-evoked potentials obtained at different times relative to the saccade exhibited changes in waveform shape that could be attributed to a saccade-evoked potential. The changes in the shape of the waveform were reasonably accounted for by the summation of the grating-evoked potential (produced when the cat did not make a saccade) and an appropriately timed saccade-evoked potential. The fundamental amplitudes of the residual potentials were computed and found to vary across the time course of the saccade. These observations suggest that there are other influences besides visual masking that are exerted early in the visual pathway to modulate visual processing during saccadic eye movements. A corollary discharge process is the most likely candidate to exert these influences.

Animals↗
PubMed1989

Cortical integration of parallel pathways in the visual system of primates.

Injections of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP) were placed in the middle temporal visual area (MT) of squirrel monkeys to reveal the distributions of interconnections with functionally distinct modules in areas 17 and 18. In agreement with previous reports, brain sections cut parallel to the surface of manually flattened cortex and reacted for cytochrome oxidase (CO) revealed CO dense blobs in area 17 and alternating CO dense thick and thin bands separated by CO light interbands in area 18. Alternate sections stained for myelin indicated that the CO light interblobs and interbands are more densely myelinated than the CO dense blobs and bands. Our major finding is that projections from MT to areas 17 and 18 are both to modules projecting to MT and modules projecting to other targets. In area 17, the cells in the middle layers projecting to injection sites in MT typically were distributed in several short merging and diverging rows, suggesting the convergence of projections from several matched orientation columns in area 17 to the restricted injection site in MT. Backward projections from MT to more superficial layers in area 17 were distributed more evenly across cortex and over a wider area of cortex. These terminations were dense throughout the interblob cortex which includes all orientation columns and neurons projecting to area 18, but were light over the blobs. As previously reported, neurons in area 18 projecting to MT were located in one set of the CO dense bands. However, these bands appeared to be thin rather than thick.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2006

Correctable and non-correctable visual impairment in a population-based sample of 12-year-old Australian children.

PURPOSE: To document the prevalence of correctable and non-correctable visual impairment in a representative sample of Australian children, predominantly age 12 years. DESIGN: Population-based cross-sectional study. METHODS: Logarithm of the minimum angle of resolution (logMAR) visual acuity was measured in both eyes unaided, with spectacles if worn, and after subjective refraction if required, in 2353 children, examined during 2004 to 2005. Cycloplegic autorefraction (using cyclopentolate) and dilated fundus examination were performed. Using a cut-off of 0.3 logMAR units (<20/40), presenting visual impairment was defined using unaided visual acuity if spectacles were not worn or with usual correction if spectacles were worn. Impairment not eliminated by refraction was considered non-correctable; any difference between this and presenting impairment was defined as correctable impairment. Myopia was defined as spherical equivalent refraction (SER) < or =-0.50 diopters (D), hyperopia as SER > or =+2.0 diopters, anisometropia as SER difference > or =1.00 diopters, and astigmatism as cylinder > or =1.0 diopters. Amblyopia was defined as corrected visual acuity <0.3 logMAR not attributable to an underlying structural eye or visual pathway abnormality. RESULTS: Visual impairment was found in the worse eye of 117 children (5.0%) and comprised correctable (82%) and non-correctable impairment (18%). Correctable impairment was due to myopia in 67 (69.8%), hyperopia in 11 (11.5%) and astigmatism in 32 subjects (33.3%). Causes of non-correctable impairment were: amblyopia 66.7%, congenital glaucoma 9.5%, optic nerve hypoplasia 9.5%, congenital nystagmus 4.8%, and cortical blindness 4.8%. CONCLUSIONS: Visual impairment had a relatively low prevalence in this older childhood population, a large proportion of which was correctable by refraction alone.

Adolescent↗
PubMed2004

Fos-tau-LacZ mice expose light-activated pathways in the visual system.

We have employed fos-tau-LacZ (FTL) transgenic mice to examine functional activation in the visual areas of the nervous system. The FTL mice express the marker gene lacZ in neurons and their processes following many different stimuli, and allow the imaging of activation from the level of the entire brain surface through individual neurons and their projections. Analysis of FTL expression in the retinas of mice following diurnal exposure to light shows that bipolar cells, specific classes of amacrine cells, ganglion cells, and a dense network of processes in the inner plexiform layer are functionally activated. In animals deprived of light, there is almost no activity in the retina. In the lateral geniculate nucleus (LGN), light exposure appears responsible for FTL expression in dorsal nuclei, but not for expression in the ventral nuclei or the intergeniculate leaflet. In the superficial layers of the superior colliculus, FTL expression is highly dependent on light exposure. Similarly, light exposure is required for FTL expression in primary visual cortex (area 17), but some expression remains in area 18 of dark-adapted animals. Finally, using mice with one or both eyes missing, we have determined which parts of the visual system are dependent on the presence of a functional connectivity from the eye. These data demonstrate the usefulness of the FTL mice to map functional activation within the entire visual system. Furthermore, we can capture visual activation in a conscious animal. Our findings give an insight into the architecture of activity within the retina and throughout the visual system.

Animals↗
PubMed2004

Action-specific extrapolation of target motion in human visual system.

Neuropsychological studies have indicated two distinct visual pathways in our brain, one dedicated to conscious perception and one to visuomotor control. Some psychophysical results support this idea with normal subjects, but they are still controversial. This study provides new psychophysical evidence for the dissociation by showing action-specific extrapolation of the visual target trajectory. When a moving target disappears, the perceived final position is liable to be shifted forward (representational momentum). In experiment 1, larger and more robust forward shifts were found when the position was directly touched without seeing the screen (open-loop pointing) than when the position was judged perceptually. The most striking dissociation was that fixation did not affect the forward shift in open-loop pointing while it almost abolished the shifts in perceptual judgements. In experiment 2, this action-specific result was found to disappear after a response delay of 4000 ms. Experiments 3 and 4 confirmed that the results were not affected by the external reference frames. The specific forward shifts found in open-loop pointing suggest that the visuomotor system compensates for the neural delays by extrapolating the target motion. The results, together with earlier findings, lead to a psychophysical double dissociation of the two visual pathways.

Adult↗
PubMed2005

The psychophysics of glaucoma: improving the structure/function relationship.

Perimetry of some kind remains an important tool in the detection, diagnosis and monitoring of glaucomatous damage to the visual pathway. However, recent studies have served to reinforce the suspicion that conventional perimetry does not possess the sensitivity to detect the earliest signs of functional loss resulting from glaucoma. The relationship between differential light threshold and ganglion cell loss is extremely weak and, in the early stages of glaucoma, non-existent. Alternative, more novel perimetric techniques seem to offer promise of better detectability for early loss by claiming to tap in to one or other of the separate parallel pathways of the visual system. While some of these tests show potential for better detection and monitoring of glaucoma, the reasons why this might be so are not always clearly formulated or represented. This leads to misunderstanding of what the test actually measures and of the glaucomatous disease process itself. This paper seeks to revisit and review the theory underlying psychophysical testing of visual function related to glaucoma and stresses the importance of developing tests that are based on a firm theoretical understanding of visual function and processing in order to both detect glaucoma at an earlier stage and better understand the mechanisms of loss from the disease process.

Glaucoma↗
PubMed2003

Neural networks active during tactile form perception: common and differential activity during macrospatial and microspatial tasks.

Prior studies have shown that tactile perception recruits activity not only in somatosensory but also in visual cortical areas. The present study used functional magnetic resonance imaging to investigate the distribution of neural activity during tactile perception of 2D form. In a macrospatial form task, raised letters (uppercase T and V) were presented upside-down. In a microspatial form task, a bar, either with or without a gap, was presented. Stimuli were applied to the immobilized right index fingerpad. Six neurologically normal volunteers were studied in a block design paradigm, with alternating blocks of rest and covert discrimination between the two alternatives for a task. Each task was studied in a separate run. Contrasting macrospatial form discrimination against rest revealed activity in an extensive, bilateral network of cortical and subcortical regions, including areas of somatosensory cortex and the intraparietal sulcus (IPS), occipito-temporal cortex, dorsal and ventral premotor cortex, medial superior frontal cortex, lateral inferior frontal cortex, thalamus and cerebellar hemispheres. Contrasting (microspatial) gap detection against rest showed activity in a similar network, with the notable exception of the occipito-temporal cortical regions. A direct contrast between the two tasks yielded greater activity for the macrospatial than microspatial task in these occipito-temporal regions bilaterally, and also in foci near the right IPS and in the right cerebellar hemisphere. The occipito-temporal cortical activations were in the lateral occipital complex, a part of the ventral visual pathway active during visual form perception. Thus, macrospatial form perception preferentially recruits this region of extrastriate visual cortex, compared to microspatial form perception.

Adult↗
PubMed2005

White matter development during childhood and adolescence: a cross-sectional diffusion tensor imaging study.

Maturation of brain white matter pathways is an important factor in cognitive, behavioral, emotional and motor development during childhood and adolescence. In this study, we investigate white matter maturation as reflected by changes in anisotropy and white matter density with age. Thirty-four children and adolescents aged 6-19 years received diffusion-weighted magnetic resonance imaging scans. Among these, 30 children and adolescents also received high-resolution T1-weighed anatomical scans. A linear regression model was used to correlate fractional anisotropy (FA) values with age on a voxel-by-voxel basis. Within the regions that showed significant FA changes with age, a post hoc analysis was performed to investigate white matter density changes. With increasing age, FA values increased in prefrontal regions, in the internal capsule as well as in basal ganglia and thalamic pathways, the ventral visual pathways, and the corpus callosum. The posterior limb of the internal capsule, intrathalamic connections, and the corpus callosum showed the most significant overlaps between white matter density and FA changes with age. This study demonstrates that during childhood and adolescence, white matter anisotropy changes in brain regions that are important for attention, motor skills, cognitive ability, and memory. This typical developmental trajectory may be altered in individuals with disorders of development, cognition and behavior.

Adolescent↗
PubMed1994

Evidence for excitatory amino acid neurotransmitters in forward and feedback corticocortical pathways within rat visual cortex.

It is a commonly accepted notion that cells which make projections between the multiple cortical areas found in the mammalian visual system are excitatory, but there is little direct evidence that this is the case. Here we demonstrate using retrograde tracing with D-[3H]aspartate that connections in the rat which project from lower to higher visual areas (i.e. forward) and those which project from higher to lower areas (i.e. feedback) may use excitatory amino acid neurotransmitters. Following injection into the primary visual cortex, clusters of retrogradely labelled cells were found in several extrastriate areas within the cytoarchitectonic subdivisions 18a ('areas' LM, AL, PX, FLX, RL, AX) and 18b ('area' MX), and in the retrosplenial cortex. In all of these areas D-[3H]aspartate-labelled cells were surrounded by diffuse label which may represent anterograde labelling of axon terminals. This suggests that both legs of reciprocal intracortical circuits have similar chemospecificity. To directly demonstrate excitatory amino acid localization in forward projections, D-[3H]aspartate was injected into extrastriate area LM. As expected, the results revealed retrogradely labelled neurons within area 17. Outside area 17, LM injections labelled neurons in AL, PX, FLX, RL, AX and MX. Taken in the context of the hierarchy of areas in rat cerebral cortex (Coogan and Burkhalter, J. Neurosci., 13, 3749-3772, 1993), these results show that D-[3H]aspartate labels: (1) forward connections from area 17 to LM, AL, PX, RL, AX and MX, (2) feedback connections from LM, AL, FLX, PX, RL, AX and MX to area 17, (3) feedback connections from AL, PX, RL, AX and MX to LM, and (4) lateral connections between FLX and LM. These findings strongly indicate that both forward and feedback connections as well as lateral connections at several different levels of the cortical hierarchy use excitatory amino acid neurotransmitters.

Animals↗