Naoum P. Issa

Sleep enhances plasticity in the developing visual cortex.

During a critical period of brain development, occluding the vision of one eye causes a rapid remodeling of the visual cortex and its inputs. Sleep has been linked to other processes thought to depend on synaptic remodeling, but a role for sleep in this form of cortical plasticity has not been demonstrated. We found that sleep enhanced the effects of a preceding period of monocular deprivation on visual cortical responses, but wakefulness in complete darkness did not do so. The enhancement of plasticity by sleep was at least as great as that produced by an equal amount of additional deprivation. These findings demonstrate that sleep and sleep loss modify experience-dependent cortical plasticity in vivo. They suggest that sleep in early life may play a crucial role in brain development.

Functional Imaging of Primary Visual Cortex Using Flavoprotein Autofluorescence

Neuronal autofluorescence, which results from the oxidation of flavoproteins in the electron transport chain, has recently been used to map cortical responses to sensory stimuli. This approach could represent a substantial improvement over other optical imaging methods because it is a direct (i.e., nonhemodynamic) measure of neuronal metabolism. However, its application to functional imaging has been limited because strong responses have been reported only in rodents. In this study, we demonstrate that autofluorescence imaging (AFI) can be used to map the functional organization of primary visual cortex in both mouse and cat. In cat area 17, orientation preference maps generated by AFI had the classic pinwheel structure and matched those generated by intrinsic signal imaging in the same imaged field. The spatiotemporal profile of the autofluorescence signal had several advantages over intrinsic signal imaging, including spatially restricted fluorescence throughout its response duration, reduced susceptibility to vascular artifacts, an improved spatial response profile, and a faster time course. These results indicate that AFI is a robust and useful measure of large-scale cortical activity patterns in visual mammals.

Hair-bundle stiffness dominates the elastic reactance to otolithic-membrane shear

Efficient transduction by acousticolateralis organs requires that a stimulus force principally deflect hair bundles, rather than flex other structural elements. Hair bundles might therefore be expected to provide a large fraction of the impedence to shear motions of otolithic membranes and other accessory structures. We measured the stiffness for shear motions of the bullfrogs saccular otolithic membrane, and determined the stiffness due to a single hair bundle and its associated extracellular filaments; this component is termed the elemental stiffness. Stiffness measurements were made by displacing the base of a flexible probe whose tip was coupled to the otolithic membrane, and simultaneously measuring the flexion of the probe and the displacement of the membrane. The average elemental stiffness, about 1350 microN.m-1, only modestly exceeded the stiffness of individual hair bundles. The hair bundles therefore provide the dominant component of stiffness in the bullfrogs sacculus, and thus account for a significant component of impedance to otolithic-membrane shear. As a corollary, stiffness changes or active movements in hair bundles should influence the mechanical responses of this and other receptor organs.

Rapid and sensitive mapping of long-range connections in vitro using flavoprotein autofluorescence imaging combined with laser photostimulation.

We investigated the use of flavoprotein autofluorescence (FA) as a tool to map long-range neural connections and combined FA with laser-uncaging of glutamate to facilitate rapid long-range mapping in vitro. Using the somatosensory thalamocortical slice, we determined that the spatial resolution of FA is >or=100-200 microm and that the sensitivity for detecting thalamocortical synaptic activity approximates that of whole cell recording. Blockade of ionotropic glutamate receptors with DNQX and AP5 abolished cortical responses to electrical thalamic stimulation. The combination of FA with photostimulation using caged glutamate revealed robust long-distance connectivity patterns that could be readily assessed in slices from the somatosensory, auditory, and visual systems that contained thalamocortical, corticothalamic, or corticocortical connections. We mapped the projection from the ventral posterior nucleus of thalamus (VPM) to the primary somatosensory cortex-barrel field and confirmed topography that had been previously described using more laborious methods. We also produced a novel map of the projections from the VPM to the thalamic reticular nucleus, showing precise topography along the dorsoventral axis. Importantly, only about 30 s were needed to generate the connectivity map (six stimulus locations). These data suggest that FA is a sensitive tool for exploring and measuring connectivity and, when coupled with glutamate photostimulation, can rapidly map long-range projections in a single animal.

Models and Measurements of Functional Maps in V1

The organization of primary visual cortex has been heavily studied for nearly 50 years, and in the last 20 years functional imaging has provided high-resolution maps of its tangential organization. Recently, however, the usefulness of maps like those of orientation and spatial frequency (SF) preference has been called into question because they do not, by themselves, predict how moving images are represented in V1. In this review, we discuss a model for cortical responses (the spatiotemporal filtering model) that specifies the types of cortical maps needed to predict distributed activity within V1. We then review the structure and interrelationships of several of these maps, including those of orientation, SF, and temporal frequency preference. Finally, we discuss tests of the model and the sufficiency of the requisite maps in predicting distributed cortical responses. Although the spatiotemporal filtering model does not account for all responses within V1, it does, with reasonable accuracy, predict population responses to a variety of complex stimuli.

Glomerular activation patterns and the perception of odor mixtures

Odor mixtures can produce several qualitatively different percepts; it is not known at which stage of processing these are determined. We asked if activity within the first stage of olfactory processing, the glomerular layer of the olfactory bulb, predicts odor mixture perception. We characterized how mice respond to components after training to five different mixture ratios of pentanal and hexanal, and found two types of responses: elemental perception and overshadowing. We then used intrinsic signal imaging to observe glomerular activity in response to the same mixtures and their components. As has been previously described, glomerular activity patterns produced by mixtures resemble the linear combination of responses to components. Mice trained to identify mixtures with more hexanal than pentanal recognized hexanal but not pentanal when the odorants were presented alone (overshadowing). Consistent with these behavioral responses, the imaged activity pattern in response to mixtures was similar to that produced to hexanal alone. Moreover, there was no significant effect of glomerular inhibition in the imaged response. In contrast, the glomerular activity patterns did not predict elemental perception: when trained to identify mixtures with more pentanal than hexanal, mice recognized both components equally well, even with highly overlapping activation patterns. This suggests that spatial activity patterns within the olfactory bulb are not always sufficient to specify component recognition in mixtures.

Fibroblast Growth Factor 8 Organizes the Neocortical Area Map and Regulates Sensory Map Topography

The concept of an “organizer” is basic to embryology. An organizer is a portion of the embryo producing signals that lead to the creation of a patterned mature structure from an embryonic primordium. Fibroblast growth factor 8 (FGF8) is a morphogen that disperses from a rostromedial source in the neocortical primordium (NP), forms a rostral-to-caudal (R/C) gradient, and regulates embryonic and neonatal R/C patterns of gene expression in neocortex. Whether FGF8 also has organizer activity that generates the postnatal neocortical area map is uncertain. To test this possibility, new sources of FGF8 were introduced into the mouse NP with in utero microelectroporation at embryonic day 10.5, close to the estimated peak of area patterning. Results differed depending on the position of ectopic FGF8. Ectopic FGF8 in the caudalmost NP could duplicate somatosensory cortex (S1) and primary visual cortex (V1). FGF8 delivered to the midlateral NP generated a sulcus separating rostral and caudal portions of the NP, in effect creating duplicate NPs. In the caudal NP, ectopic FGF8 induced a second, inclusive area map, containing frontal cortex, S1, V1, and primary auditory areas. Moreover, duplicate S1 showed plasticity to sensory deprivation, and duplicate V1 responded to visual stimuli. Our findings implicate FGF8 as an organizer signal, and its source in the rostromedial telencephalon as an organizer of the neocortical area map.

Subcortical Representation of Non-Fourier Image Features

A fundamental goal of visual neuroscience is to identify the neural pathways representing different image features. It is widely argued that the early stages of these pathways represent linear features of the visual scene and that the nonlinearities necessary to represent complex visual patterns are introduced later in cortex. We tested this by comparing the responses of subcortical and cortical neurons to interference patterns constructed by summing sinusoidal gratings. Although a linear mechanism can detect the component gratings, a nonlinear mechanism is required to detect an interference pattern resulting from their sum. Consistent with in vitro retinal ganglion cell recordings, we found that interference patterns are represented subcortically by cat LGN Y-cells, but not X-cells. Linear and nonlinear tuning properties of LGN Y-cells were then characterized and compared quantitatively with those of cortical area 18 neurons responsive to interference patterns. This comparison revealed a high degree of similarity between the two neural populations, including the following: (1) the representation of similar spatial frequencies in both their linear and nonlinear responses, (2) comparable orientation selectivity for the high spatial frequency carrier of interference patterns, and (3) the same difference in their temporal frequency selectivity for drifting gratings versus the envelope of interference patterns. The present findings demonstrate that the nonlinear subcortical Y-cell pathway represents complex visual patterns and likely underlies cortical responses to interference patterns. We suggest that linear and nonlinear mechanisms important for encoding visual scenes emerge in parallel through distinct pathways originating at the retina.

Infusion of nerve growth factor (NGF) into kitten visual cortex increases immunoreactivity for NGF, NGF receptors, and choline acetyltransferase in basal forebrain without affecting ocular dominance plasticity or column development.

Intracerebroventricular or intracortical administration of nerve growth factor (NGF) has been shown to block or attenuate visual cortical plasticity in the rat. In cats and ferrets, the effects of exogenous NGF on development and plasticity of visual cortex have been reported to be small or nonexistent. To determine whether locally delivered NGF affects ocular dominance column formation or the plasticity produced by monocular deprivation in cats at the height of the critical period, we infused recombinant human NGF into the primary visual cortex of kittens using an implanted cannula minipump. NGF had no effect on the normal developmental segregation of geniculocortical afferents into ocular dominance columns as determined both physiologically and anatomically. The plasticity of binocular visual cortical responses induced by monocular deprivation was also normal in regions of immunohistochemically detectable NGF infusion, as measured using intrinsic signal optical imaging and single-unit electrophysiology. Immunohistochemical analysis of the basal forebrain regions of the same animals demonstrated that the NGF infused into cortex was biologically active, producing an increase in the number of NGF-, TrkA-, p75(NTR)-, and choline acetyltransferase-positive neurons in basal forebrain nuclei in the hemisphere ipsilateral to the NGF minipump compared to the contralateral basal forebrain neurons. We conclude that NGF delivered locally to axon terminals of cholinergic basal forebrain neurons resulted in increases in protein expression at the cell body through retrograde signaling.

Characterization of fluo-3 labelling of dense bodies at the hair cell's presynaptic active zone

SummaryThe presynaptic active zone is the critical region of a chemical synapse at which Ca2+ entry provokes neurotransmitter release by exocytotic fusion of synaptic vesicles. To facilitate investigations of synaptic function, we have identified a group of fluorescent substances that label individual active zones in living hair cells. The Ca2+ indicator fluo-3, the compound studied in most detail, binds to the presynaptic dense bodies that are characteristic of active zones in hair cells and other cells that tonically release transmitter. The indicators binding is reversible, with a dissociation constant of approximately 350 μm. Because fluo-3 that is bound to a presynaptic dense body continues to detect Ca2+ with an unaltered dissociation constant, the binding of this substance provides a valuable tool for exploration of the Ca2+ concentration at the site of vesicle fusion.