Human Visual Pathway Research Articles

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.

The implications of foveal splitting for saccade planning in reading

The human fovea and visual pathways are precisely split: information in one hemifield is initially projected to the contralateral visual cortex. This fundamental anatomical constraint on word recognition in reading has been largely ignored in eye movement research. We explore the consequences of this constraint through analyses of a large corpus of eye movement data, and demonstrate that aspects of saccade planning (target selection, initial landing position) are sensitive to both hemispheres, estimated uncertainty about the identity of the currently fixated word. We interpret these findings in terms of a hemispheric division of labour. We suggest that anatomical, visual and lexical factors all contribute to the decision of where to send the eyes next in reading.

Visual stimulation, 1H MR spectroscopy and fMRI of the human visual pathways.

The purpose was to assess changes in lactate content and other brain metabolites under visual stimulation in optical chiasm, optic radiations and occipital cortex using multiple voxel MR spectroscopy (MRS). 1H chemical shift imaging (CSI) examinations of transverse planes centered to include the above structures were performed in four subjects at an echo time of 135 ms. Functional MRI (fMRI) was used to confirm the presence of activity in the visual cortex during the visual stimulation. Spectral maps of optical chiasm were of poor quality due to field disturbances caused by nearby large blood vessels and/or eye movements. The optic radiations and the occipital lobe did not show any significant MR spectral change upon visual stimulation, i.e., the peak areas of inositol, choline, creatine, glutamate and N-acetylaspartate were not affected. Reproducible lactate signals were not observed. fMRI confirmed the presence of strong activations in stimulated visual cortex. Prolonged visual stimulation did not cause significant changes in MR spectra. Any signal observed near the 1.33 ppm resonance frequency of the lactate methyl-group was artifactual, originating from lipid signals from outside the volume of interest (VOI). Previous claims about changes in lactate levels in the visual cortex upon visual stimulation may have been based on such erroneous observations.

'stuff' versus 'things': Neural processing of the material properties and geometric form of objects in human visual pathways

'stuff' versus 'things': Neural processing of the material properties and geometric form of objects in human visual pathways

Human lateral geniculate nucleus and visual cortex respond to screen flicker.

The first electrophysiological study of the human lateral geniculate nucleus (LGN), optic radiation, striate, and extrastriate visual areas is presented in the context of presurgical evaluation of three epileptic patients (Patients 1, 2, and 3). Visual-evoked potentials to pattern reversal and face presentation were recorded with depth intracranial electrodes implanted stereotactically. For Patient 1, electrode anatomical registration, structural magnetic resonance imaging, and electrophysiological responses confirmed the location of two contacts in the geniculate body and one in the optic radiation. The first responses peaked approximately 40 milliseconds in the LGN in Patient 1 and 60 milliseconds in the V1/V2 complex in Patients 2 and 3. Moreover, steady state visual-evoked potentials evoked by the unperceived but commonly experienced video-screen flicker were recorded in the LGN, optic radiation, and V1/V2 visual areas. This study provides topographic and temporal propagation characteristics of steady state visual-evoked potentials along human visual pathways. We discuss the possible relationship between the oscillating signal recorded in subcortical and cortical areas and the electroencephalogram abnormalities observed in patients suffering from photosensitive epilepsy, particularly video-game epilepsy. The consequences of high temporal frequency visual stimuli delivered by ubiquitous video screens on epilepsy, headaches, and eyestrain must be considered.

Multifocal ERGs and VEPs: Noninvasive studies of the electrical activity of the human visual pathway

Multifocal ERGs and VEPs: Noninvasive studies of the electrical activity of the human visual pathway

Reorganization of human cortical maps caused by inherited photoreceptor abnormalities.

We describe a compelling demonstration of large-scale developmental reorganization in the human visual pathways. The developmental reorganization was observed in rod monochromats, a rare group of congenitally colorblind individuals who virtually lack cone photoreceptor function. Normal controls had a cortical region, spanning several square centimeters, that responded to signals initiated in the all-cone foveola but was inactive under rod viewing conditions; in rod monochromats this cortical region responded powerfully to rod-initiated signals. The measurements trace a causal pathway that begins with a genetic anomaly that directly influences sensory cells and ultimately results in a substantial central reorganization.

The Primary Visual Pathway in Humans Is Regulated According to Long-Term Light Exposure through the Action of a Nonclassical Photopigment

The mammalian eye shows marked adaptations to time of day. Some of these modifications are not acute responses to short-term light exposure but rely upon assessments of the photic environment made over several hours. In the past, all attempts at a mechanistic understanding have assumed that these adaptations originate with light detection by one or other of the classical photoreceptor cells (rods or cones). However, previous work has demonstrated that the mammalian eye contains non-rod, non-cone photoreceptors. This study aimed to determine whether such photoreceptors contribute to retinal adaptation. In the human retina, second-order processing of signals originating in cones takes significantly longer at night than during the day. Long-term light exposure at night is capable of reversing this effect. Here, we employed the cone ERG as a tool to examine the properties of the irradiance measurement pathway driving this reversal. Our findings indicate that this pathway (1) integrates irradiance measures over time periods ranging from at least 15 to 120 min; (2) responds to relatively bright light, having a dynamic range almost entirely outside the sensitivity of rods; (3) acts on the cone pathway primarily through a local retinal mechanism; and (4) detects light via an opsin:vitamin A photopigment (lambda(max) approximately 483 nm). A photopigment with a spectral sensitivity profile quite different from those of the classical rod and cone opsins but matching the standard profile of an opsin:vitamin A-based pigment drives adaptations of the human primary cone visual pathway according to time of day.

Object-form topology in the ventral temporal lobe: Response to

In her thoughtful comment on the functional organization of ventral temporal cortex, Isabel Gauthier clearly outlines the hypotheses that have been proposed and are currently being investigated with functional brain imaging [Gauthier, I. (2000) What constrains the organization of the ventral temporal cortex? Trends Cognit. Sci. 4, 1–2] 1 Gauthier I. What constrains the organization of the ventral temporal cortex?. Trends Cognit. Sci. 2000; 4: 1-2 Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar . One of those models, which we call the object form topology hypothesis, was proposed by us in a recent paper 2 Ishai A. et al. Distributed representation of objects in the human ventral visual pathway. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 9379-9384 Crossref PubMed Scopus (705) Google Scholar that motivated Gauthier’s comment. In this reply, we would like to explain why we have proposed the object-form topology hypothesis and clarify what we believe are its most salient aspects.

Distributed representation of objects in the human ventral visual pathway.

Brain imaging and electrophysiological recording studies in humans have reported discrete cortical regions in posterior ventral temporal cortex that respond preferentially to faces, buildings, and letters. These findings suggest a category-specific anatomically segregated modular organization of the object vision pathway. Here we present data from a functional MRI study in which we found three distinct regions of ventral temporal cortex that responded preferentially to faces and two categories of other objects, namely houses and chairs, and had a highly consistent topological arrangement. Although the data could be interpreted as evidence for separate modules, we found that each category also evoked significant responses in the regions that responded maximally to other stimuli. Moreover, each category was associated with its own differential pattern of response across ventral temporal cortex. These results indicate that the representation of an object is not restricted to a region that responds maximally to that object, but rather is distributed across a broader expanse of cortex. We propose that the functional architecture of the ventral visual pathway is not a mosaic of category-specific modules but instead is a continuous representation of information about object form that has a highly consistent and orderly topological arrangement.

High spatial resolution functional magnetic resonance imaging at very-high-magnetic field.

Although neuroimaging methods have been used successfully to map large-scale neurocognitive networks distributed across the human cortex, functional mapping and differentiation of localized brain organization within a small structure has been limited by inadequate sensitivity for high spatial resolution imaging. Functional magnetic resonance imaging (fMRI) technique based on blood oxygenation level-dependent (BOLD) contrast has become one of the most useful neuroimaging techniques. It has been used extensively to study human brain function from sensory perception to cognitive performance. However, the majority of these studies used a relatively low spatial resolution (typically with a voxel size of 3.1 x 3.1 x 5.0 mm3), which is incapable of mapping on the millimeter and submillimeter spatial scale. In this article, we review the technical aspects of the high-resolution fMRI technique and the sensitivity and spatial specificity of BOLD-based fMRI. We demonstrate applications of high-resolution fMRI in studying the human visual pathway from the lateral geniculate nucleus in the thalamus to the ocular dominance columns in the primary visual cortex. Most results were obtained at very-high-magnetic fields (3.0 and 4.0 Tesla). They reveal that high-resolution fMRI at very-high-magnetic field is promising for functional mapping of brain organization from large cortical networks, small nuclei, and even to cellular layer structures.

A sequence of object-processing stages revealed by fMRI in the human occipital lobe

Functional magnetic resonance imaging was used in combined functional selectivity and retinotopic mapping tests to reveal object-related visual areas in the human occipital lobe. Subjects were tested with right, left, up, or down hemivisual field stimuli which were composed of images of natural objects (faces, animals, man-made objects) or highly scrambled (1,024 elements) versions of the same images. In a similar fashion, the horizontal and vertical meridians were mapped to define the borders of these areas. Concurrently, the same cortical sites were tested for their sensitivity to image-scrambling by varying the number of scrambled picture fragments (from 16-1,024) while controlling for the Fourier power spectrum of the pictures and their order of presentation. Our results reveal a stagewise decrease in retinotopy and an increase in sensitivity to image-scrambling. Three main distinct foci were found in the human visual object recognition pathway (Ungerleider and Haxby [1994]: Curr Opin Neurobiol 4:157-165): 1) Retinotopic primary areas V1-3 did not exhibit significant reduction in activation to scrambled images. 2) Areas V4v (Sereno et al., [1995]: Science 268:889-893) and V3A (De Yoe et al., [1996]: Proc Natl Acad Sci USA 93:2382-2386; Tootell et al., [1997]: J Neurosci 71:7060-7078) manifested both retinotopy and decreased activation to highly scrambled images. 3) The essentially nonretinotopic lateral occipital complex (LO) (Malach et al., [1995]: Proc Natl Acad Sci USA 92:8135-8139; Tootell et al., [1996]: Trends Neurosci 19:481-489) exhibited the highest sensitivity to image scrambling, and appears to be homologous to macaque the infero-temporal (IT) cortex (Tanaka [1996]: Curr Opin Neurobiol 523-529). Breaking the images into 64, 256, or 1,024 randomly scrambled blocks reduced activation in LO voxels. However, many LO voxels remained significantly activated by mildly scrambled images (16 blocks). These results suggest the existence of object-fragment representation in LO.

A sequence of object‐processing stages revealed by fMRI in the human occipital lobe

Functional magnetic resonance imaging was used in combined functional selectivity and retinotopic mapping tests to reveal object-related visual areas in the human occpital lobe. Subjects were tested with right, left, up, or down hemivisual field stimuli which were composed of images of natural objects (faces, animals, man-made objects) or highly scrambled (1,024 elements) versions of the same images. In a similar fashion, the horizontal and vertical meridians were mapped to define the borders of these areas. Concurrently, the same cortical sites were tested for their sensitivity to image-scrambling by varying the number of scrambled picture fragments (from 16–1,024) while controlling for the Fourier power spectrum of the pictures and their order of presentation. Our results reveal a stagewise decrease in retinotopy and an increase in sensitivity to image-scrambling. Three main distinct foci were found in the human visual object recognition pathway (Ungerleider and Haxby [1994]: Curr Opin Neurobiol 4:157–165): 1) Retinotopic primary areas V1–3 did not exhibit significant reduction in activation to scrambled images. 2) Areas V4v (Sereno et al., [1995]: Science 268:889–893) and V3A (DeYoe et al., [1996]: Proc Natl Acad Sci USA 93:2382–2386; Tootell et al., [1997]: J Neurosci 71:7060–7078) manifested both retinotopy and decreased activation to highly scrambled images. 3) The essentially nonretinotopic lateral occipital complex (LO) (Malach et al., [1995]: Proc Natl Acad Sci USA 92:8135–8139; Tootell et al., [1996]: Trends Neurosci 19:481–489) exhibited the highest sensitivity to image scrambling, and appears to be homologous to macaque the infero-temporal (IT) cortex (Tanaka [1996]: Curr Opin Neurobiol 523–529). Breaking the images into 64, 256, or 1,024 randomly scrambled blocks reduced activation in LO voxels. However, many LO voxels remained significantly activated by mildly scrambled images (16 blocks). These results suggest the existence of object-fragment representation in LO. Hum. Brain Mapping 6:316–328, 1998. © 1998 Wiley-Liss, Inc.

Stepwise decrease in VEP latencies and the process of myelination in the human visual pathway

To assess the progress in myelination in the developing human brain, a prospective longitudinal study of flash visual evoked potentials (VEPs) was performed in 22 healthy preterm infants with the same gestational age at birth (between 30 weeks 0 day and 31 weeks 0 day). The individual curves of the changes in the N1a peak latency (the early peak of the N1 wave) decrease not linearly but in a stepwise pattern in the preterm period. Twenty-one infants out of the 22 have one or more `acceleration week(s)' in which the latency decreases at a rate of more than 6 ms per week. These stepwise decreases in the latency may reflect a synchronized progress in myelination in several parts of the visual pathway. A detailed analysis of the `acceleration weeks' in relation to postmenstrual age (PMA) indicates that they most prominently occur at 37 weeks PMA. At 37 weeks an initiation of myelination in the optic radiation has been demonstrated in post-mortem studies. We propose that a longitudinal follow-up study of VEPs can be accepted as a functional in vivo evaluation of myelination in the developing human brain.

Cyclopean Discrimination Thresholds for the Direction and Speed of Motion in Depth

We measured just-noticeable differences in the direction and speed of motion in depth of cyclopean and monocularly visible targets. Our stimulus set comprised different combinations of (dφ/dt)/(dδ/dt), dδ/dt, dφ/dt and Δδ, where dφ/dt was the angular frontal plane speed of the binocularly-fused target, dδ/dt was its rate of change of disparity and Δδ was its disparity displacement. Our three subjects based their direction discriminations entirely on the task-relevant variable (dφ/dt)/(dδ/dt), and based their speed discriminations entirely on the task-relevant variable dδ/dt. They ignored all task-irrelevant variables in both tasks. Performance on both tasks was the same for motion within the horizontal and vertical meridians. Direction discrimination threshold rose significantly as the reference direction grew more oblique with respect to a line passing midway between the eyes and perpendicular to the frontal plane. Performance on the direction discrimination task was significantly better for the noncyclopean than for the cyclopean target, but the difference was not great. For the cyclopean target, the lowest value of the direction discrimination threshold was 0.70 deg (mean of three observers and two meridians). The Weber fraction for discriminating speed was not significantly different for the cyclopean and monocularly visible targets, and did not depend on the direction of motion in depth. The lowest values (mean of three observers and two meridians) were 0.12 (cyclopean) and 0.10 (noncyclopean). Findings did not scale for viewing distance. We propose that the human visual pathway contains: (a) a cyclopean mechanism sensitive to variations in the ratio (dφ/dt)/(dδ/dt) that is comparatively insensitive to both dφ/dt and dδ/dt; and (b) a speed-sensitive cyclopean mechanism that responds to variations in the value of dδ/dt, but is comparatively insensitive to dφ/dt. We also propose that a single speed-sensitive mechanism determines speed discrimination thresholds for both cyclopean and monocularly visible targets. Copyright © 1996 Elsevier Science Ltd.

Evidence for a neural mechanism that encodes angles

We measured the discrimination threshold (Δθ) Th for angle θ, where θ was either the angle of a Vee composed of two straight lines contained within the frontoparallel plane or the angle of intersection of two straight lines contained within the frontoparallel plane. The two-line pattern was rotated bodily through a random angle between trials with the aim of eliminating the absolute orientation of one or the other line as a reliable cue to the task. We report evidence that this aim was achieved. Our main conclusion is that the ability to discriminate a change in angle θ cannot entirely be explained in terms of the ability to discriminate changes in the orientations of the individual lines that comprise the Vee. We propose that the human visual pathway contains a neural mechanism that encodes the difference in the orientations of two simultaneously-presented straight lines. Discrimination threshold for angle (Δθ) Th is roughly twice orientation discrimination threshold for an isolated line. When subjects cannot use the orientation of one or another line as a cue to the task, the plot of (Δθ) Th vs θ is approximately flat between θ = 20 and 160 deg.

Pattern reversal visual evoked potentials in minor head injury.

Pattern reversal visual evoked potentials (PR-VEPs) have been recorded in 50 patients with minor head injury (MHI) on days 1 and 30 after trauma and the data compared to 20 normals. None of the patients had visual complaints. The aim was to investigate a possible visual pathway affection in MHI and test the usefulness of PR-VEPs as an objective noninvasive tool in the detection of a possible subclinical affection of the visual system in MHI. P100 latency and amplitude had no significant difference compared to normals. Comparison of patient data on days 1 and 30 after trauma showed a significant latency decrease and amplitude increase on day 30, compared to day 1. These alterations were not age dependent. Our data suggest affection of the human visual pathway in MHI. PR-VEP recording seems to be a useful, objective, noninvasive tool, helping to identify possible subclinical affections of the visual pathway in MHI.

Preattentive discrimination of relative phase modelled by interacting gabor or by difference-of-gaussian filters

A computer simulation of the human preattentive visual pathway used Gabor and difference-of-Gaussian (DOG) filters to model two-dimensional relative phase discrimination. There is a hierarchy of hexagonally packed levels: (i) an image layer; (ii) an intermediate level of on- and off-centre DOG filters; (iii) a partial set of broadband oriented Gabor-like filters with high-level DOG filters in parallel. Connections between layers use half-wave rectification and a compressive nonlinearity. Local feedback interactions between oriented Gabor filters, together with spatial averaging, allow the model to discriminate both two-dimensional relative phase and orientation differences. There were two separate simulations for die Gabor filters, one with even-symmetric filters and another with odd-symmetric. Textures were formed by superposing three high contrast sine-wave gratings with successive rotations of 60°. Relative phase and global orientation were the varied parameters. Psychophysical rating data for peripheral viewing of texture pairs resemble the results from the even-symmetric simulation, showing discrimination of relative phase and orientation. In contrast, the DOG filters in the model simulate only the relative phase aspects of the data.

Magnetic resonance imaging of the visual pathways in human albinos.

Neuroanatomical and electrophysiological studies of albino visual pathways have demonstrated that retinogeniculate axons arising from the temporal retina decussate abnormally in the optic chiasm to synapse in the contralateral lateral geniculate nucleus (LGN). Anomalies in the LGN secondarily disrupt normal geniculo-cortical and interhemispheric cortico-cortical (callosal) visual connections. It is not known whether retinogeniculate misrouting affects the size or configuration of the afferent visual pathways in human albinos. We used T1-weighted coronal and sagittal magnetic resonance imaging (MRI) to examine the prechiasmatic intracranial optic nerves, optic chiasm, and corpus callosum in 10 human albinos. In all subjects, these structures were normal in size and configuration. Despite the complex cascade of aberrant central neuronal connections, the human albino visual pathways and their interhemispheric connections appear normal in size and configuration when viewed with MRI.

Dissociation of discrimination thresholds for time to contact and for rate of angular expansion

It is well known that, if a rigid sphere is moving at constant speed towards the eye along the line of sight then, for small values of θ, T = θ/gq, where T is the time to contact, θ is the instantaneous angular size and θ is the rate of increase of angular size. We describe a rationale and an experimental procedure for demonstrating empirically when subjects base discrimination of time to contact on trial-to-trial variations of (θ/θ) rather than on variations of θ or on variations of Δθ (the change of angular size during a presentation). Discrimination threshold for the ratio (θ/θ) was 0.070–0.13, and was independent of mean time to contact over a range of at least T = 1.0–4.0 sec. We conclude that the human visual pathway contains a mechanism that is sensitive to the ratio (θ/θ) rather independently of the values of θ and θ. Using a different procedure we demonstrated empirically that subjects based discriminations of rate of expansion on trial-to-trial variations of θ rather than variations of time to contact or on variations of Δθ. Discrimination threshold for rate of expansion was 0.85-0.14, and was independent of mean time to contact over a range of at least T = 1.0−4.0 sec. We conclude that the human visual pathway contains a mechanism that is sensitive to rate of expansion rather independently of time to contact or absolute change in size. When rate of expansion and time to contact were both available as cues, discrimination threshold was on average lower than when only one of the two cues was present. We conclude that there is some summation of the two cues. Our data can be explained by a small modification to a model previously put forward to account for data on threshold elevations and aftereffects caused by adapting to changing-size stimulation. This model incorporates a filter that is strongly activated by isotropic, homogeneous two-dimensional expansion of the retinal image and whose output is inversely proportional to time to contact (i.e. the more urgent the demand for evasive action, the stronger the output of the filter).