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PubMed2002

Spatial dynamics of receptive fields in cat primary visual cortex related to the temporal structure of thalamocortical feedforward activity. Experiments and models.

We investigated how changes in the temporal firing rate of thalamocortical activity affect the spatiotemporal structure of receptive field (RF) subunits in cat primary visual cortex. Spike activity of 67 neurons (48 simple, 19 complex cells) was extracellulary recorded from area 17/18 of anesthetized and paralyzed cats. A total of 107 subfields (on/off) were mapped by applying a reverse correlation technique to the activity elicited by bright and dark rectangles flashed for 300 ms in a 20x10 grid. We found that the width of the (suprathreshold) discharge fields shrank on average by 22% during this 300-ms-long stimulus presentation time. Fifty-eight subfields (54%) shrank by more than 20% of peak width and only ten (less than 10%) showed a slight increase over time. The main size reduction took place 40-60 ms after response onset, which corresponded to the transition from transient peak firing to tonic visual activity in thalamocortical relay cells (TC). The experimentally obtained RFs were then fitted with the aid of a neural field model of the primary visual pathway. Assuming a Gaussian-shaped spatial sensitivity profile across the RF subfield width, the model allowed us to estimate the subthreshold RF (depolarization field, D-field) from the minimal discharge field (MDF). The model allowed us to test to what degree the temporal dynamics of thalamocortical activity contributes to the spatiotemporal changes of cortical RFs. To this end, we performed the fitting procedure either with a pure feedforward model or with a field model that also included intracortical feedback. Spatial and temporal parameters obtained from fits of the experimental RFs matched closely to those achieved by simulating a pure feedforward system with the field model but were not compatible with additional intracortical feedback. Thus, our results show that dot stimulation, which optimally excites thalamocortical cells, leads to a shrinkage with respect to the size of the RF subfield at the first transient response of visual cortical RFs which seems mainly due to a change in the thalamic firing pattern. In these experiments little or no influence from intracortical sources was observed, which, however, may play a role when using more complex visual stimuli.

Action Potentials↗
PubMed1992

First-order analysis of optical flow in monkey brain.

Optical flow is a rich source of information about the three-dimensional motion and structure of the visual environment. Little is known of how the brain derives this information. One possibility is that it analyzes first-order elementary components of optical flow, such as expansion, rotation, and shear. Using a combination of physiological recordings and modeling techniques, we investigated the contribution of the middle superior temporal area (MST), a third-order cortical area in the dorsal visual pathway that receives inputs from the medial temporal area (MT). The results show (i) that MST cells, but not MT cells, are selective for elementary flow components (EFCs) alone or their combination with translation, (ii) that MST cells selective for an EFC do not extract this component from a more complex motion pattern, and (iii) that position invariance as observed in MST is compatible with an input arrangement from MT cells matching the selectivity of MST neurons.

Animals↗
PubMed2003

Single-unit recording in the lateral geniculate nucleus of the awake behaving monkey.

In recent years, recording neuronal activity in the awake, behaving primate brain has become established as one of the major tools available to study the neuronal specificity of the initiation and control of various behaviors. Primates have traditionally been used in these studies because of their ability to perform more complex behaviors closely akin to those of humans, a desirable prerequisite since our ultimate aim is to elucidate the neuronal correlates of human behaviors. A wealth of knowledge has accumulated on the sensory and motor systems such as vision, audition, and eye movements. For more demanding behaviors where the main focus has been on attention, recordings in awake primates have begun to yield valuable data on the centers of the brain that are reactive to different attributes of this behavior. As a result, various hypotheses of the origin and distribution of attentional effects have evolved. For instance, visual attentional effects have been described not only in the higher cortical area (V4) but also in areas earlier in the visual pathway which presumably involve a feedback mechanism in the latter region. Here we outline the ways in which we have successfully used these methods to make single-cell recordings in awake macaques to show how certain behavioral paradigms affect neurons of the thalamus (with emphasis on the lateral geniculate nucleus). As we have done with established techniques these methods can be readily adapted to incorporate most behaviors needed to be tested and allow recordings to be made in virtually any part of the brain.

Action Potentials↗
PubMed1992

Development of the visual system of the chick--a review.

This article reviews recent and earlier findings that yield the present knowledge about the embryonic development of retina, tectum, and the retinotectal projection in the chick. Data and concepts dealing with cell proliferation, migration, and differentiation, the processes underlying the generation of cytoarchitecture in the nervous system are discussed for the avian visual system. Emphasis is also laid on the presentation of hypotheses and experiments about directed axonal growth along the visual pathway and concerning the mechanisms responsible for the establishment of specific connections between retinal ganglion cells and their central targets. Among the results, the following topics deserve special attention: 1) Investigations of morphogenetic factors in vitro, and the application of recombinant retroviruses in vivo to study cell lineages rendered new insights into the processes of cell determination and differentiation. The evolving picture in this progressing field is discussed. At present, however, the research of retinal and tectal histogenesis is still largely in the state of morphological description. 2) Both systems, retina and optic tectum, develop independently from each other but in corresponding spatio-temporal patterns, which provide that ingrowing retinal axons encounter receptive target tissue at appropriate locations at the time when connections are due to be formed. 3) Possible mechanisms of directed fibre growth are being elucidated by increasing efforts in research devoted to cell surface molecules, neurotrophic, and inhibitory substances, and their receptors. The axons of the primary visual pathway seem to be guided by local cues on glial endfeet and perhaps in the extracellular matrix, but so far, instructive molecules to which functional significance can be assigned have eluded discovery. 4) The question, how the retinotopic projection upon the tectum is created during development, remains still unsolved, although most results point to modified forms of the chemoaffinity hypothesis for its explanation. Sequential maturation and growth, selective fasciculation of orderly entering axons, recognition of positional tectal markers, and functionally controlled refinement may together contribute to the correct retinotectal projection.

Animals↗
PubMed1998

Retinal axon regeneration in the lizard Gallotia galloti in the presence of CNS myelin and oligodendrocytes.

Retinal ganglion cell (RGC) axons in lizards (reptiles) were found to regenerate after optic nerve injury. To determine whether regeneration occurs because the visual pathway has growth-supporting glia cells or whether RGC axons regrow despite the presence of neurite growth-inhibitory components, the substrate properties of lizard optic nerve myelin and of oligodendrocytes were analyzed in vitro, using rat dorsal root ganglion (DRG) neurons. In addition, the response of lizard RGC axons upon contact with rat and reptilian oligodendrocytes or with myelin proteins from the mammalian central nervous system (CNS) was monitored. Lizard optic nerve myelin inhibited extension of rat DRG neurites, and lizard oligodendrocytes elicited DRG growth cone collapse. Both effects were partially reversed by antibody IN-1 against mammalian 35/250 kD neurite growth inhibitors, and IN-1 stained myelinated fiber tracts in the lizard CNS. However, lizard RGC growth cones grew freely across oligodendrocytes from the rat and the reptilian CNS. Mammalian CNS myelin proteins reconstituted into liposomes and added to elongating lizard RGC axons caused at most a transient collapse reaction. Growth cones always recovered within an hour and regrew. Thus, lizard CNS myelin and oligodendrocytes possess nonpermissive substrate properties for DRG neurons--like corresponding structures and cells in the mammalian CNS, including mammalian-like neurite growth inhibitors. Lizard RGC axons, however, appear to be far less sensitive to these inhibitory substrate components and therefore may be able to regenerate through the visual pathway despite the presence of myelin and oligodendrocytes that block growth of DRG neurites.

Animals↗
PubMed2004

Underlying principles of visual shape selectivity in posterior inferotemporal cortex.

Object perception depends on shape processing in the ventral visual pathway, which in monkeys culminates in inferotemporal cortex (IT). Here we provide a description of fundamental quantitative principles governing neural selectivity for complex shape in IT. By measuring responses to large, parametric sets of two-dimensional (2D) silhouette shapes, we found that neurons in posterior IT (Brodmann's areas TEO and posterior TE) integrate information about multiple contour elements (straight and curved edge fragments of the type represented in lower-level areas) using both linear and nonlinear mechanisms. This results in complex, distributed response patterns that cannot be characterized solely in terms of example stimuli. We explained these response patterns with tuning functions in multidimensional shape space and accurately predicted neural responses to the widely varying shapes in our stimulus set. Integration of contour element information in earlier stages of IT represents an important step in the transformation from low-level shape signals to complex object representation.

Action Potentials↗
PubMed2001

Electrophysiological and anatomical evidence for a direct projection from the nucleus of the basal optic root to the nucleus rotundus in pigeons.

A direct projection of the nucleus of the basal optic root (nBOR) onto the nucleus rotundus (Rt) in the pigeon would link the accessory optic system to the ascending tectofugal pathway and could thus combine self- and object-motion processes. In this study, injections of retrograde tracers into the Rt revealed some cells in central nBOR to project onto the ipsilateral Rt. Contrary, injections into the diencephalic component of the ascending thalamofugal pathway resulted in massive labeling of neurons in dorsal nBOR. Single unit recordings showed that visual nBOR units could be activated by antidromic stimulation through the Rt. Successful collision tests applied to nBOR cells revealed that the connection between nBOR and Rt is direct. These data provide strong evidence for a direct and differential projection of nBOR subcomponents onto the thalamic relays of the two ascending visual pathways.

Animals↗
PubMed1986

Visual evoked cortical potentials and pattern electroretinograms in Parkinson's disease and control subjects.

Parkinson's disease patients have been shown to have abnormal visually evoked cortical potentials (VEPs) to pattern stimulation. Whereas dopamine is not an important neurotransmitter in the central visual pathways, the retina is rich in dopamine and, together with previous animal and human studies, this suggests that the abnormal VEPs in Parkinson's disease patients may be due to a biochemical and electrophysiological disorder in the retina. This hypothesis has been examined by studying the VEPs and pattern electroretinogram (PERG) of Parkinson's disease patients and matched control subjects. The amplitudes of the cortical and retinal evoked potentials were significantly reduced in Parkinson's disease patients compared with the control subjects and this could not be attributed to any particular feature of the disease or its treatment. There was a significant relationship between the VEP P100 latency and the PERG amplitude. Moreover for those subjects in whom there was an interocular difference in both cortical and retinal evoked potentials, the abnormality was more commonly found in the potentials from the same eye. These findings suggest that the abnormality of the VEP in Parkinson's disease patients is, at least in part, secondary to an abnormality of the retina itself.

Aged↗
PubMed1998

Development of a magnocellular function in good and poor primary school-age readers.

BACKGROUND: Abnormal functioning of the transient visual pathway (the M-pathway) has been implicated in specific reading disability (SRD). The aim of this study is to examine the contrast thresholds for flicker-defined form discrimination in primary school children, and to compare its development with reading and mentation development as a means of identifying children at risk of SRD. METHODS: One hundred eighty-seven children (aged 4 to 13 years) and 22 adults (aged 18 to 45 years) were assessed for contrast sensitivity to an illusory, flicker contrast-defined form (the letter 'E')--a task which was designed to rely to a large extent on magnocellular pathway function. Reading age (Neale Analysis of Reading) and mental age (Raven's Coloured Progressive Matrices) were assessed in the children, who had been previously screened for clinically normal binocular vision and refractive anomalies. RESULTS: The ability of primary school-age children to discriminate the orientation of the low contrast flickered letters (from a choice of 4) showed a significant improvement from kindergarten (ages 4 to 6 years) to grade 3 (ages 8 to 10 years) and older age groups. No significant difference was found between good and disabled readers (at least 1-year lag in reading readiness for the kindergarten group and 2-year lag in reading performance for 8 to 10 and 10 to 12-year-olds). CONCLUSIONS: It appears that there is a developmental improvement in perceptual capacity for tasks attributed to magnocellular function, which plateaus at the age of about 8 to 10 years. However, despite the reported reduction of magnocellular function in specific reading disabled children, no significant difference in contrast threshold for flicker-defined letter discrimination was found between good and poor readers.

Adolescent↗
PubMed1994

Scotopic threshold response in complete and incomplete types of congenital stationary night blindness.

PURPOSE: To study the function of the rod visual pathway in the complete and incomplete types of congenital stationary night blindness (CSNB), with special reference to the scotopic threshold response (STR) of electroretinograms (ERGs) METHODS: Using full-field stimuli with light intensities ranging from near absolute threshold to bright, ERG intensity series from two patients with complete CSNB, four patients with incomplete CSNB, and four normal subjects were recorded. RESULTS: Neither the rod b-wave nor the STR was recordable from the patients with complete CSNB. In the patients with incomplete CSNB, the STR was clearly recorded, although the absolute threshold was elevated in accordance with elevation of the psychophysical absolute threshold. The b-wave stimulus threshold was not elevated, and the b-wave amplitude near the threshold was normal. The peak time of the STR was delayed by approximately 80 msec, whereas that of the b-wave was normal. CONCLUSIONS: These STR results indicate that the rod system abnormality in complete CSNB differs from that in incomplete CSNB. Furthermore, the greatly delayed peak time of STR in the patients with incomplete CSNB made the interaction between b-wave and STR different from that in normal subjects.

Adolescent↗
PubMed2006

Dynamics of spatial resolution of single units in the lateral geniculate nucleus of cat during brief visual stimulation.

Sharpness of vision depends on the resolution of details conveyed by individual neurons in the visual pathway. In the dorsal lateral geniculate nucleus (LGN), the neurons have receptive fields with center-surround organization, and spatial resolution may be measured as the inverse of center size. We studied dynamics of receptive field center size of single LGN neurons during the response to briefly (400-500 ms) presented static light or dark spots. Center size was estimated from a series of spatial summation curves made for successive 5-ms intervals during the stimulation period. The center was wide at the start of the response, but shrank rapidly over 50-100 ms after stimulus onset, whereupon it widened slightly. Thereby, the spatial resolution changed from coarse-to-fine with average peak resolution occurring approximately 70 ms after stimulus onset. The changes in spatial resolution did not follow changes of firing rate; peak firing appeared earlier than the maximal spatial resolution. We suggest that the response initially conveys a strong but spatially coarse message that might have a detection and tune-in function, followed by transient transmission of spatially precise information about the stimulus. Experiments with spots presented inside the maximum but outside the minimum center width suggested a dynamic reduction in number of responding neurons during the stimulation; from many responding neurons initially when the field centers are large to fewer responding neurons as the centers shrink. Thereby, there is a change from coarse-to-fine also in the recruitment of responding neurons during brief static stimulation.

Action Potentials↗
PubMed1985

Dissociated foveal and parafoveal visual evoked responses in subacute combined degeneration.

In four patients with subacute combined degeneration, clinically silent involvement of the visual pathways was demonstrated by visual evoked responses following selective foveal and parafoveal stimulation. The characteristic distribution of the lesion is different from that generally found in patients with multiple sclerosis, and affects slow- and fast-conducting fibers in an opposite manner.

Aged↗
PubMed1984

Human sensory stimulation and deprivation: positron emission tomographic results and strategies.

Fluorine-18-labeled fluorodeoxyglucose was used to measure local cerebral glucose metabolism by means of positron emission tomography (PET) in patients and in normal subjects. Various states of audiovisual stimulation and deprivation were explored. Our experience in performing neurobehavioral PET studies in over 145 normal right-handed individuals is described. In normal subjects metabolic left-right symmetry was found in states of partial sensory deprivation (eyes patched or ears plugged). Metabolic asymmetries (right less than left) were observed in subjects with more complete sensory deprivation (eyes patched and ears plugged). Auditory stimulation studies in normal subjects demonstrated metabolic evidence of cerebral lateralization. No correlation between site of metabolic response and side of stimulation was observed. Both the site and the side (left versus right) of maximal metabolic response correlated with the type (verbal versus nonverbal) and content of the stimulus as well as with the strategy used by the subject to solve the listening task. Visual stimuli of increasing complexity produced symmetrical increases in metabolic rate for the primary and secondary visual cortices. Focal stimulation of the central portion of the retina produced focal responses limited to the occipital poles, while full-field visual stimulation produced increased metabolic activity throughout the entire extent of the primary visual cortex. Patients with lesions of the visual pathway that spared the visual cortex itself demonstrated abnormalities in visual cortical metabolic rate that correlated with clinical symptoms. The refinement of neurobehavioral PET studies is discussed in terms of the limitations presently induced by spatial resolution, temporal resolution, anatomical localization accuracy, experimental neuropsychological paradigm design, and data analysis techniques. These limitations, as well as future prospects for using PET to study human brain function in both normal and pathological states, are discussed.

Acoustic Stimulation↗
PubMed1983

Optically induced strabismus results in visual field losses in cats.

Kittens were reared using goggles containing a prism that simulated a convergent strabismus. A bilateral loss of 30 degrees of the contralateral monocular visual field resulted, indicating that the components of the visual pathways representing the nasal visual field are more sensitive to spatially conflicting visual inputs than the components representing the temporal field. Differences in the extent of binocular visual fields occurred, with alternating fixators demonstrating a full field and unilateral fixators demonstrating a binocular field reduction ipsilateral to the deviating eye.

Animals↗
PubMed1982

Hemiretinal differences in the effect of a rotating visual background on the subjective visual vertical.

Sixteen normal subjects were tested for their accuracy in judging the verticality of a visual edge before or during exposure to a moving visual background. All subjects showed a counter-rotation of the subjective visual vertical as a consequence of movement of the visual background. The effect was stronger for binocular than monocular viewing and for nasal than temporal hemiretina stimulation. No hemispheric asymmetries were observed. These results show a predominance of the crossed visual pathways originating from the nasal hemiretinae in a visual effect presumably involving a visual-vestibular interaction.

Adolescent↗
PubMed2004

Functional MRI of visual-spatial processing in neurofibromatosis, type I.

Visual-spatial impairment and neuroanatomical abnormalities are considered hallmark features of neurofibromatosis, type I (NF-I). Numerous studies have demonstrated visual-spatial deficits in children with NF-I, but few relations between these deficits and neuroanatomical abnormalities have been identified. We compared the functional neuroanatomy of cerebral regions involved in the spatial transformation of alphanumeric stimuli in individuals with NF-I and healthy control participants using functional magnetic resonance imaging (fMRI). Given the prevalence of visual pathway abnormalities and visual-spatial deficits in NF-I, we hypothesized that less neuronal hemodynamic activity would be found in occipital and parietal cortices in this group compared with controls. However, NF-I participants relied to a greater degree than controls on posterior cortex (including occipital, parietal, and middle temporal cortices) relative to lateral and inferior frontal regions during visual-spatial analysis. This pattern was significantly related to their behavioral performance on the fMRI task, which in turn was also positively correlated with reading scores. These findings support evidence of frontal cortical anomalies in NF-I and may provide a pathophysiological basis for cognitive deficits in NF-I.

Adolescent↗
PubMed2003

Can the magnocellular pathway read? Evidence from studies of color.

A review of the neurophysiological literature suggests that the magnocellular pathway has adequate spatial-frequency and contrast sensitivity to perceive text under normal contrast conditions (>10%) and also is suppressed by red light. Results from three experiments involving color and reading show that red light impairs reading performance under normal luminance contrast conditions. However in a fourth experiment, isoluminant color text, designed to selectively activate the parvocellular pathway, is easier to read under red light. These discrepant results suggest that the magnocellular pathway is the dominant visual pathway for text perception. Implications for reading models and developmental dyslexia are discussed.

Color Perception↗
PubMed2001

Abnormal activation in the visual cortex after corneal refractive surgery for myopia: demonstration by functional magnetic resonance imaging.

OBJECTIVE: To try to correlate subjective photophobic symptoms with visual pathway modifications (from the retinal image to the visual cortex) after refractive surgery by exploring brain activation on photic stimulation. DESIGN: Noncomparative case series. PARTICIPANTS: Four subjects reporting discomfort produced by luminance (glare, halos, starbursts, or a combination thereof) in one eye after laser in situ keratomileusis (LASIK) were enrolled. The contralateral myopic eye (control) had no visual impairment and had undergone LASIK without complications or had not had previous surgery. METHODS: Functional magnetic resonance imaging was performed during photic stimulation, delivered by an optical fiber, of the affected and unaffected eyes. RESULTS: Functional magnetic resonance imaging provided evidence that most subjective visual symptoms correlated with anatomic flap abnormalities are associated with a higher signal increase in the visual association cortices compared with a nonsymptomatic eye. CONCLUSIONS: Functional magnetic resonance imaging of the visual cortex may help in exploring the mechanisms involved in glare effects after refractive surgery.

Adult↗