Biomedical subjects

Jonathan B Levitt

Publications and source records attributed to Jonathan B Levitt.

7 recordsLinked to original sources

PubMed2006

The use of a novel and simple method of revealing neural fibers to show the regression of the lateral geniculate nucleus in the naked mole-rat (Heterocephalus glaber).

The lateral geniculate nucleus (LG) is an important subcortical nucleus in the visual system. It receives primary projections from the retina and relays these to central visual structures. Although there are studies on the retina and visual cortex of animals with regressed vision, little is known about the LG in such animals. The strictly subterranean naked mole-rat (Heterocephalus glaber) has markedly reduced visual acuity with concomitant pronounced changes in the visual cortex. We used a novel method to reveal myelinated neural fibers in a histological study assessing if the LG shows regressive changes commensurate with the level of reliance on vision by this rodent. Myelin detection here relies on significant differences in visible light reflection between neural fibers and the gray matter. Moreover, this simple method does not interfere with further staining for additional analyses. This method reveals that the contribution of the LG to brain volume in the naked mole-rat is less than a third of that of the rat. This shows that the retinogeniculocortical system in the naked mole-rat is considerably smaller than that of rodents that rely heavily on their visual system, but is nevertheless less regressed than that of the extensively studied blind mole-rat; this may facilitate limited responses to visual stimuli.

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PubMed2005

Feedback connections to ferret striate cortex: direct evidence for visuotopic convergence of feedback inputs.

Interareal feedback connections are a fundamental aspect of cortical architecture, yet many aspects of their organization and functional relevance remain poorly understood. Previous studies have investigated the topography of feedback projections from extrastriate cortex to macaque area 17. We have extended this analysis to the ferret. We made restricted injections of cholera toxin B (CTb) into ferret area 17 and mapped the distribution of retrogradely labeled cells in extrastriate cortex. In addition to extensive label spreading within area 17, we found dense cell label in areas 18, 19, and 21 and the suprasylvian cortex and sparser connections from the lateral temporal and posterior parietal cortex. We made extensive physiological assessments of magnification factors in the extrastriate visual cortex and used these measures to convert the spread of labeled cortex in millimeters into a span in degrees of visual field. We also directly measured the visuotopic extents of receptive fields in the regions containing labeled cells in cases in which we made both CTb injections and physiological recordings in the same animals; we then compared the aggregate receptive field (ARF) of the labeled region in each extrastriate area with that of the injection site. In areas 18, 19, and 21, receptive fields of cells in regions containing labeled neurons overlapped those at the injection site but spanned a greater distance in visual space than the ARF of the injection site. The broad visuotopic extent of feedback connections is consistent with the suggestion that they contribute to response modulation by stimuli beyond the classical receptive field.

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PubMed2005

A new chamber method for mounting tissue sections.

We describe a method for mounting tissue sections onto slides that is faster and especially useful for those sections too thin or too large to be mounted easily by conventional methods. We have devised a chamber system for mounting tissue sections onto slides under buffer solution. Because of the buoyancy of sections in the buffer solution and the elimination of fluid surface tension, it is easy to move, turn, unfold, and spread even quite large or thin tissue sections. In-solution-mounting in this chamber also greatly reduces the force of the brush tip used to maneuver sections onto the slide, thus resulting in less damage to the tissue sections. This chamber greatly facilitates mounting multiple tissue sections onto a single slide. The new method is applicable to protocols that stain tissue sections either before (e.g. cytochrome oxidase) or after (e.g. cresyl violet) section mounting.

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PubMed2002

Circuits for local and global signal integration in primary visual cortex.

Contrast-dependent changes in spatial summation and contextual modulation of primary visual cortex (V1) neuron responses to stimulation of their receptive field reveal long-distance integration of visual signals within V1, well beyond the classical receptive field (cRF) of single neurons. To identify the cortical circuits mediating these long-distance computations, we have used a combination of anatomical and physiological recording methods to determine the spatial scale and retinotopic logic of intra-areal V1 horizontal connections and inter-areal feedback connections to V1. We have then compared the spatial scales of these connectional systems to the spatial dimensions of the cRF, spatial summation field (SF), and modulatory surround field of macaque V1 neurons. We find that monosynaptic horizontal connections within area V1 are of an appropriate spatial scale to mediate interactions within the SF of V1 neurons and to underlie contrast-dependent changes in SF size. Contrary to common beliefs, these connections cannot fully account for the dimensions of the surround field. The spatial scale of feedback circuits from extrastriate cortex to V1 is, instead, commensurate with the full spatial range of center-surround interactions. Thus these connections could represent an anatomical substrate for contextual modulation and global-to-local integration of visual signals. Feedback projections connect corresponding and equal-sized regions of the visual field in striate and extrastriate cortices and cover anisotropic parts of visual space, unlike V1 horizontal connections that are isotropic in the macaque. V1 isotropic connectivity demonstrates that anisotropic horizontal connections are not necessary to generate orientation selectivity. Anisotropic feedback connections may play a role in contour completion.

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PubMed2002

Anatomical origins of the classical receptive field and modulatory surround field of single neurons in macaque visual cortical area V1.

From the analyses of our own and others' anatomical and physiological data for the macaque visual system, we arrive at a conclusion that three pathways can provide the V1 neuron with access to information from the visual field and affect its response. First, direct thalamic input can determine the size of the initial activating RF at high contrast. Second, lateral connections can enlarge the RF at low contrast by pooling information from larger regions of cortex that are otherwise ineffective when high contrast thalamic input is driving the cortical neuron. Thirdly, feedback from extrastriate cortex (possibly together with overlap or interdigitation of coactive lateral connectional fields within V1) can provide a large and stimulus specific surround modulatory field. The stimulus specificity of the interactions between the center and surround fields, may be due to the orderly, matching structure and different scales of intra-areal and feedback projection excitatory pathways. The observed activity changes of single recorded excitatory neurons could be a result of the relative weight of excitation on the excitatory neurons themselves and on local inhibitory interneurons that synapse on them. Inhibitory basket neurons, driven by the local excitatory neurons, could govern local interactions between cortical patches of different tuning properties, resulting in more distant changes in excitatory input in the laterally connected intra-areal neuronal pools.

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PubMedDate not supplied

The spatial extent over which neurons in macaque striate cortex pool visual signals.

We recorded activity of single units in macaque monkey primary visual cortex (V1) to define the retinotopic extent of the visual inputs that drive or modulate V1 neuron responses in parafoveal and peripheral (calcarine) cortex. We used high-contrast drifting grating stimuli to define the extent of the area over which responses summate and the extent of the receptive-field surround. We found responses of most V1 cells to summate over 1 deg, with a suppressive surround typically twice that in diameter, though for some cells (even in parafoveal V1) surrounds exceeded 13 deg in diameter. Surprisingly, we found no significant laminar differences in these dimensions or in the strength of surround suppression. We found that surround suppression in most cells arises from both the ends and sides of the receptive field. Our measures indicate that the strongest modulatory input arises from regions immediately adjacent to the excitatory summation area. These physiological measures suggest that the high-contrast summation field of V1 neurons can be accounted for by the sum of lateral geniculate nucleus (LGN) inputs offered to the local cortical column, with monosynaptic lateral connections within area V1 adding the larger dimensions of the low-contrast summation field and the near surround. Neither of these inputs suffice to explain the largest surrounds, which most likely derive from feedback from extrastriate visual areas.

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PubMedDate not supplied

Retinotopic organization of ferret suprasylvian cortex.

The retinotopic organization of striate and several extrastriate areas of ferret cortex has been established. Here we describe the representation of the visual field on the Suprasylvian visual area (Ssy). This cortical region runs mediolaterally along the posterior bank of the suprasylvian sulcus, and is distinct from adjoining areas in anatomical architecture. The Ssy lies immediately rostral to visual area 21, medial to lateral temporal areas, and lateral to posterior parietal areas. In electrophysiological experiments we made extracellular recordings in adult ferrets. We find that single and multiunit receptive fields range in size from 2 deg x 4 deg to 21 deg x 52 deg. The total visual field representation in Ssy spans over 70 deg in azimuth in the contralateral hemifield (with a small incursion into the ipsilateral hemifield), and from +36 deg to -30 deg in elevation. There are often two representations of the horizontal meridian. Furthermore, the location of the transition from upper to lower fields varies among animals. General features of topography are confirmed in anatomical experiments in which we made tracer injections into different locations in Ssy, and determined the location of retrograde label in area 17. Both isoelevation and isoazimuth lines can span substantial rostrocaudal and mediolateral distances in cortex, sometimes forming closed contours. This topography results in cortical magnifications averaging 0.07 mm/deg in elevation and 0.06 mm/deg in azimuth; however, some contours can run in such a way that it is possible to move a large distance on cortex without moving in the visual field. Because of these irregularities, Ssy contains a coarse representation of the contralateral visual field.

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