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PubMed1987

Surface distribution of flash-evoked and pattern reversal-evoked potentials in hooded rats.

Simultaneous recording from 21 electrode sites in a 4 X 4 mm area over the posterior cortex was used to determine the surface distribution of all major peaks which constitute flash-evoked potentials (FEPs) and pattern reversal evoked-potentials (PREPs) in hooded rats. Topographical maps were constructed with respect to Bregma and midline reference points. The data indicate that not all of the peaks which constitute either evoked potential have their greatest amplitude within the classically defined primary visual cortex. Further, since the FEPs were produced by uniform stimulation, the data suggest that surface regions of the rat visual cortex differ in ways other than simply the portion of the visual field from which information is received.

Animals↗
PubMed1980

Preoperative overtraining protects against form learning deficits after lateral occipital lesions in Galago senegalensis.

Bushbabies (Galago senegalensis) trained on a form discrimination task in a two-choice apparatus prior to partial disruption of the central field representation of vision in striate cortex were found to be protected from postoperative deficit. These same animals were deficient in learning novel form discriminations. Pre- and postoperative tests of these subjects on discrimination of fine stripe patterns and of small food objects gave no evidence of reduced epicritic visual capacities. Results are discussed in terms of an interpretive role in form learning for areas of central field representation in primary visual cortex.

Animals↗
PubMed1990

Heterogeneity of chandelier neurons in monkey neocortex: corticotropin-releasing factor- and parvalbumin-immunoreactive populations.

Chandelier neurons are a unique subclass of cortical nonpyramidal neurons. The axons of these neurons terminate in distinctive vertically arrayed cartridges that synapse on the axon initial segment of pyramidal neurons. In this study, the rapid Golgi method and immunohistochemical techniques were used to characterize the morphology, regional distribution, laminar location, and biochemical content of chandelier neurons in the prefrontal and occipital cortices of three monkey species. As in our previous studies of visual areas V1 and V2 (Lund: Journal of Comparative Neurology 257:60-92, 1987; Lund et al.: Journal of Comparative Neurology 202:19-45, 1981, 276:1-29, 1988), Golgi impregnations of areas 46 and 9 of macaque prefrontal cortex show chandelier neurons to be present in layers 2 through superficial 5. The vertical arrays of terminal boutons (axon cartridges) typical of this neuron class are also present in layers 2-6 of the prefrontal cortex, but are not found in layer 1 or the subcortical white matter. In immunohistochemical studies, a calcium-binding protein, parvalbumin, and a neuropeptide, corticotropin-releasing factor (CRF), identify rod-like structures that are morphologically similar to the axon cartridges of chandelier neurons seen in the Golgi material. In addition, both parvalbumin- and CRF-immunoreactive cartridges are located below the somata of unlabeled pyramidal neurons and appear to outline the axon initial segment of these neurons. However, we find that parvalbumin and CRF are present in only subpopulations of chandelier axon cartridges. For example, in adult primary visual cortex, parvalbumin-labeled cartridges are present in very low numbers only in layers 2-3, whereas in prefrontal and occipital association cortices these cartridges are a very prominent component of layers 2-superficial 3 and are present in much lower density in the deeper cortical layers. In contrast to these findings in adult macaque monkeys, prefrontal and occipital association cortices of infant macaque monkeys contain a very high density of parvalbumin-labeled cartridges in layer 4 and relatively few in the superficial cortical layers. Furthermore, in adult squirrel monkey prefrontal cortex, CRF-labeled cartridges are predominately present in layer 4, but these CRF-immunoreactive structures have not been observed in the homologous regions of infant or adult macaque monkeys. These findings indicate that even for neurons of such distinctive morphology and presumably constant functional role as chandelier neurons, factors such as regional and laminar location, age, and primate species are associated with differences in the biochemical content of subpopulations of these neurons.

Animals↗
PubMed2006

Spatially specific FMRI repetition effects in human visual cortex.

The functional MRI (fMRI) response to a pair of identical, successively presented stimuli can result in a smaller signal than the presentation of two nonidentical stimuli. This "repetition effect" has become a frequently used tool to make inferences about neural selectivity in specific cortical areas. However, little is known about the mechanism(s) underlying the effect. In particular, despite many successful applications of the technique in higher visual areas, repetition effects in lower visual areas [e.g., primary visual cortex (V1)] have been more difficult to characterize. One property that is well understood in early visual areas is the mapping of visual field locations to specific areas of the cortex (i.e., retinotopy). We used the retinotopic organization of V1 to activate progressively different populations of neurons in a rapid fMRI experimental design. We observed a repetition effect (reduced signal) when localized stimulus elements were repeated in identical locations. We show that this effect is spatially tuned and largely independent of both interstimulus interval (100-800 ms) and the focus of attention. Using the same timing parameters for which we observed a large effect of spatial position, we also examined the response to orientation changes and observed no effect of an orientation change on the response to repeated stimuli in V1 but significant effects in other retinotopic areas. Given these results, we discuss the possible causes of these repetition effects as well as the implications for interpreting other experiments that use this potentially powerful imaging technique.

Female↗
PubMed2006

Studies of human visual pathophysiology with visual evoked potentials.

Visual evoked potentials (VEPs) offer reproducible and quantitative data on the function of the visual pathways and the visual cortex. Pattern reversal VEPs to full-field stimulation are best suited to evaluate anterior visual pathways while hemi-field stimulation is most effective in the assessment of post-chiasmal function. However, visual information is processed simultaneously via multiple parallel channels and each channel constitutes a set of sequential processes. We outline the major parallel pathways of the visual system from the retina to the primary visual cortex and higher visual areas via lateral geniculate nucleus that receive visual input. There is no best method of stimulus selection, rather visual stimuli and VEPs' recording should be tailored to answer specific clinical and/or research questions. Newly developed techniques that can assess the functions of extrastriate as well as striate cortices are discussed. Finally, an algorithm of sequential steps to evaluate the various levels of visual processing is proposed and its clinical use revisited.

Brain Diseases↗
PubMedDate not supplied

Long-term dendritic spine stability in the adult cortex.

The structural dynamics of synapses probably has a crucial role in the development and plasticity of the nervous system. In the mammalian brain, the vast majority of excitatory axo-dendritic synapses occur on dendritic specializations called 'spines'. However, little is known about their long-term changes in the intact developing or adult animal. To address this question we developed a transcranial two-photon imaging technique to follow identified spines of layer-5 pyramidal neurons in the primary visual cortex of living transgenic mice expressing yellow fluorescent protein. Here we show that filopodia-like dendritic protrusions, extending and retracting over hours, are abundant in young animals but virtually absent from the adult. In young mice, within the 'critical period' for visual cortex development, approximately 73% of spines remain stable over a one-month interval; most changes are associated with spine elimination. In contrast, in adult mice, the overwhelming majority of spines (approximately 96%) remain stable over the same interval with a half-life greater than 13 months. These results indicate that spines, initially plastic during development, become remarkably stable in the adult, providing a potential structural basis for long-term information storage.

Absorptiometry, Photon↗
PubMed2002

Corticothalamic interactions in the transfer of visual information.

Thalamic function does not stand apart, as a discrete processing step, from the cortical circuitry. The thalamus receives extensive feedback from the cortex and this influences the firing pattern, synchronization and sensory response mode of relay cells. A crucial question concerns the extent to which the feedback simply controls the state and transmission mode of relay cells and the extent to which the feedback participates in the specific processing of sensory information. Using examples from experiments examining the influence of feedback from the visual cortex to the lateral geniculate nucleus (LGN), we argue that thalamic mechanisms are selectively focused by visually driven feedback to optimize the thalamic contribution to segmentation and global integration. This involves effects on both the temporal and spatial parameters characterizing the responses of LGN cells and includes, for example, motion-driven feedback effects from MT (middle temporal visual area) relayed via layer 6 of V1 (primary visual cortex).

Animals↗
PubMed1998

Visual responses of neurones in the second visual area of flying foxes (Pteropus poliocephalus) after lesions of striate cortex.

1. The first (V1) and second (V2) cortical visual areas exist in all mammals. However, the functional relationship between these areas varies between species. While in monkeys the responses of V2 cells depend on inputs from V1, in all non-primates studied so far V2 cells largely retain responsiveness to photic stimuli after destruction of V1. 2. We studied the visual responsiveness of neurones in V2 of flying foxes after total or partial lesions of the primary visual cortex (V1). The main finding was that visual responses can be evoked in the region of V2 corresponding, in visuotopic co-ordinates, to the lesioned portion of V1 ('lesion projection zone'; LPZ). 3. The visuotopic organization of V2 was not altered by V1 lesions. 4. The proportion of neurones with strong visual responses was significantly lower within the LPZs (31.5 %) than outside these zones, or in non-lesioned control hemispheres ( > 70 %). LPZ cells showed weak direction and orientation bias, and responded consistently only at low spatial and temporal frequencies. 5. The data demonstrate that the functional relationship between V1 and V2 of flying foxes resembles that observed in non-primate mammals. This observation contrasts with the 'primate-like' characteristics of the flying fox visual system reported by previous studies.

Animals↗
PubMedDate not supplied

Flash evoked cortical and subcortical potentials in neuro-ophthalmology.

Visual evoked cortical and sub-cortical potentials are complementary to the electroretinogram in neuro-ophthalmology. By combining monocular stimulation with recordings from over the visual cortex of each cerebral hemisphere it is possible to accurately locate lesions which are both pre and post chiasmal. Chiasmal lesions can also be correctly identified using these techniques. The visual evoked sub-cortical potential, probably arising from a lateral geniculate or tectal site allows further post chiasmal variation. Flash and pattern reversal stimulation provide different information about the functioning of the visual system including the primary visual cortex and the association areas.

Brain Diseases↗
PubMed1992

Magnetic resonance imaging mapping of brain function. Human visual cortex.

Magnetic resonance imaging (MRI) studies of human brain activity are described. Task-induced changes in brain cognitive state were measured using high-speed MRI techniques sensitive to changes in cerebral blood volume (CBV), blood flow (CBF), and blood oxygenation. These techniques were used to generate the first functional MRI maps of human task activation, by using a visual stimulus paradigm. The methodology of MRI brain mapping and results from the investigation of the functional organization and frequency response of human primary visual cortex (V1) are presented.

Brain Mapping↗
PubMed2000

Dynamic regulation of BDNF and NT-3 expression during visual system development.

Recent studies have proposed roles for neurotrophins in the formation and plasticity of ocular dominance columns as well as in the regulation of dendritic arborization in visual cortex of higher mammals. To assess potential roles for neurotrophins in these processes, we have examined the developmental expression of BDNF and NT-3 mRNA in the cat's visual system using in situ hybridization. BDNF and NT-3 mRNAs are dynamically regulated in many CNS structures during embryonic and postnatal development, and both mRNAs undergo striking developmental changes in laminar specificity and levels of expression within primary visual cortex during the critical period for ocular dominance column formation. Within visual cortex, BDNF mRNA is found in neurons in deep cortical layers (5 and 6) prior to eye opening, and in both deep and superficial layers (2 and 3) shortly afterwards. Within layer 4, the target of thalamocortical axons, BDNF mRNA is low initially and rises to high levels by the end of the critical period for ocular dominance column formation. NT-3 mRNA is first detectable in small stellate neurons at the base of layer 4 (4c) after eye opening, and levels decrease near the end of the critical period. BDNF and NT-3 mRNAs can be detected in the lateral geniculate nucleus at birth, and levels peak during the critical period. In both structures, BDNF mRNA expression is maintained into adulthood, while NT-3 is undetectable in the adult. The presence and dynamic regulation of these neurotrophins in visual structures is consistent with suggested roles for both of these neurotrophins in axonal and dendritic remodeling known to accompany the formation of ocular dominance columns.

Age Factors↗
PubMed2003

Electrical activity in visual cortex associated with combined auditory and visual stimulation in temporal sequences known to be associated with a visual illusion.

When a subject views a visual stimulus paired with a brief click, a second click occurring approximately 80 ms later produces the hallucination of a second visual stimulus. We have used combinations of visual and sound stimuli to evoke cortical activity and have recorded the associated event-related potentials. We have recorded EPs in a conventional manner, and have calculated from multichannel recordings the Laplacian derivations to determine if the currents were generated in primary visual cortex. Clicks alone do not cause significant activity in V1, but if paired with pattern stimulation, modify the evoked potential. The timing of this extra activity almost certainly excludes "feed back" activation from higher centres, and can most simply be explained if sound-activated thalamo-cortical input can rapidly produce extra activity in 'primed' visual cortex. This finding has general implications for cortical function, for the generation of the hallucination and for 'blindsight'.

Auditory Perception↗
PubMed2004

Retinol-binding protein gene is highly expressed in higher-order association areas of the primate neocortex.

The neocortex consists of histochemically, connectionally, and functionally distinguishable areas. Recently, molecular biological techniques have enabled us to find rare types of genes expressed in specific neocortical areas. We previously reported occ1 gene as preferentially expressed in the primary visual cortex (V1), using the differential display method. Here, by differential display, we found selective and strong expression of the serum retinol-binding protein (RBP) gene, in higher-order association areas. In V1, RBP mRNA was expressed only in the superficial part of layer II, but its expression increased, involving deeper layers, along the visual pathway. In visual association areas such as TE, RBP mRNA was strongly expressed in both supra- and infragranular layers. In primary auditory and somatosensory areas, as in V1, RBP expression was low, and restricted to the upper part of the supragranular layers. The laminar pattern of RBP expression is in marked contrast with that of occ1; and in early visual areas where both genes are expressed, these occur in distinct sublayers within the supragranular layers. In neonatal monkeys, the area-specific expression pattern of RBP was less distinct, suggesting that the characteristic expression of RBP in higher-order association areas is mainly established postnatally.

Animals↗
PubMed2003

Expression of neuroserpin in the visual cortex of the mouse during the developmental critical period.

The neuronal serine protease inhibitor neuroserpin is widely expressed in the developing and adult brain. In the neocortex, neuroserpin is displayed particularly during the period of synaptic specification and refinement, indicating a role as modulator of extracellular proteolytic processes. The synaptic connections of the visual system of the mouse are shaped during early postnatal life by an activity-dependent process. We have studied the expression of the neuronal serine protease inhibitor neuroserpin in the primary visual cortex of mice from birth until the end of the critical period by means of reverse transcription polymerase chain reaction and in situ hybridization. The localization and the level of expression were constant throughout this period. Monocular deprivation with an eyelid sutured induced a decrease in neuroserpin expression in neurons of area 17 after 1 week of deprivation, the decrease being more pronounced on the side contralateral to the closed eye. The expression of neuroserpin in the visual cortex during the critical period and its decrease in parallel to the refinement of synaptic contacts after visual deprivation suggests a regulative role of neuroserpin on these processes.

Age Factors↗
PubMedDate not supplied

Segregation of pathways leading from area V2 to areas V4 and V5 of macaque monkey visual cortex.

V5 and V4 are areas of macaque monkey prestriate visual cortex that are specialized for involvement in different aspects of visual perception, namely motion for V5 (refs 1-4) and colour vision, with other possible functions, for V4 (refs 2, 5-9). Thus, it is unlikely that they should be fed the same information for further processing, yet both receive a strong input from patches of the upper layers of V2 (refs 10, 11), the area immediately adjoining the primary visual cortex, V1. V2, however, seems to comprise functionally distinct subregions, which can be revealed by staining the tissue for the mitochondrial enzyme cytochrome oxidase. Here we report that V4 and V5 are connected with separate cytochrome oxidase-defined subregions of V2, suggesting that cortical pathways dealing with motion and colour perception are segregated in their passage through V2, and reinforcing evidence for functional specialization in the visual cortex.

Animals↗
PubMed2003

Is slowness a learning principle of the visual cortex?

Slow feature analysis is an algorithm for extracting slowly varying features from a quickly varying signal. It has been shown in network simulations on one-dimensional stimuli that visual invariances to shift and other transformations can be learned in an unsupervised fashion based on slow feature analysis. More recently, we have shown that slow feature analysis applied to image sequences generated from natural images using a range of spatial transformations results in units that share many properties with complex and hypercomplex cells of the primary visual cortex. We find cells responsive to Gabor stimuli with phase invariance, sharpened or widened orientation or frequency tuning, secondary response lobes, end-stopping, and cells selective for direction of motion. These results indicate that slowness may be an important principle of self-organization in the visual cortex.

Journal Article↗
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↗
PubMed1994

Simple cells may lie at the basis of mach bands: evidence from physiological studies in the cat's visual cortex.

Mach bands are a visual illusion evoked by a luminance ramp dividing two luminance plateaux (blurred edges), but not by sharp edges. Recently, two physiology-based models have tried to cope with the psychophysical data concerning this phenomenon. The basic components of both models are neurons with even- or odd-symmetric receptive fields (RFs). Both models predict that odd-symmetric cells respond better to sharp edges, while even-symmetric cells respond better to blurred ones. We have measured the responses of 34 primary visual cortex simple cells of the cat to blurred edges of various degrees. Twenty-one cells had RFs of even symmetry, responding best to blurred edges than to sharp ones. The rest were odd-symmetric cells, of which 12 responded best to sharp edges, and only one exceptional cell responded best to a 0.85 degrees-wide edge. Thus, the different cell types responded as predicted by the two different Mach band models. Simple cells may thus serve as the physiological basis of the psychophysical phenomenon of Mach bands. Furthermore, our evidence suggests the existence of inhibition between odd- and even-symmetric cells, as predicted by one of the models.

Animals↗