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PubMed2006

Assessment of stimulus-induced changes in human V1 visual field maps.

Visual cortex contains a set of field maps in which nearby scene points are represented in the responses of nearby neurons. We tested a recent hypothesis that the visual field map in primary visual cortex (V1) is dynamic, changing in response to stimulus motion direction. The original experimental report replicates, but further experimental and analytical investigations do not support, the interpretation of the results. The V1 map remains invariant when measured using stimuli moving in different directions. The measurements can be explained by small and systematic response amplitude differences that arise when probing with stimuli moving in different directions.

Algorithms↗

PubMedDate not supplied

Seeing visual hallucinations with functional magnetic resonance imaging.

We have used blood oxygenation level dependent imaging with functional magnetic resonance imaging (fMRI) to investigate the visual cortex response to photic stimulation during and in the absence of continuous visual hallucinations. A patient with cortical Lewy body dementia who experienced persistent and vivid complex hallucinations underwent fMRI on and off treatment with risperidone. When he was not hallucinating, photic stimulation produced a normal bilateral activation in striate cortex. During hallucinations, very limited activation in striate cortex could be induced. We interpret this result as indicating that at least part of the activity in the brain responsible for the experience of visual hallucinations is located in the primary visual cortex.

Hallucinations↗

PubMedDate not supplied

From neural oscillations to variational problems in the visual cortex.

Aim of this study is to provide a formal link between connectionist neural models and variational psycophysical ones. We show that the solution of phase difference equation of weakly connected neural oscillators gamma-converges as the dimension of the grid tends to 0, to the gradient flow relative to the Mumford-Shah functional in a Riemannian space. The Riemannian metric is directly induced by the pattern of neural connections. Next, we embed the energy functional in the specific geometry of the functional space of the primary visual cortex, that is described in terms of a subRiemannian Heisenberg space. Namely, we introduce the Mumford-Shah functional with the Heisenberg metric and discuss the applicability of our main gamma-convergence result to subRiemannian spaces.

Animals↗

PubMed1989

Synaptogenesis in visual cortex of normal and preterm monkeys: evidence for intrinsic regulation of synaptic overproduction.

We used quantitative electron microscopy to determine the effect of precocious visual experience on the time course, magnitude, and pattern of perinatal synaptic overproduction in the primary visual cortex of the rhesus monkey. Fetuses were delivered by caesarean section 3 weeks before term, exposed to normal light intensity and day/night cycles, and killed within the first postnatal month, together with age-matched controls that were delivered at term. We found that premature visual stimulation does not affect the rate of synaptic accretion and overproduction. Both of these processes proceed in relation to the time of conception rather than to the time of delivery. In contrast, the size, type, and laminar distribution of synapses were significantly different between preterm and control infants. The changes and differences in these parameters correlate with the duration of visual stimulation and become less pronounced with age. If visual experience in infancy influences the maturation of the visual cortex, it must do so predominantly by strengthening, modifying, and/or eliminating synapses that have already been formed, rather than by regulating the rate of synapse production.

Aging↗

PubMed2002

Vascularization in the primate visual cortex during development.

We studied the relationship between vascularization and neuronal activity in the visual cortex during postnatal development in the primate. Analyses were focused on layer IVC that displays a sequential pattern of maturation for the magno- and parvocellular systems in separate sublayers, respectively IVC alpha and IVC beta. Cytochrome oxidase and endogenous alkaline phosphatase histochemistry was used to analyse, on the same sections, the laminar patterns of cortical activity and vessel density in the primary visual cortex of the marmoset (Callithrix jacchus). Experiments were carried out in five young and two adult animals. We showed that the temporal pattern of angiogenesis differs in layer IVC alpha and IVC beta. During the first postnatal month, vessel density is higher in IVC alpha than in IVC beta and runs parallel to cytochrome oxidase intensity. In 2-month-old animals, both vessel densities and cytochrome oxidase activity are similar in IVC alpha and IVC beta. In adults, the vessel densities in IVC alpha and IVC beta are the reverse of those observed during the first postnatal month. Vessel diameter does not account for this evolution in vascular patterns. In the discussion, we suggest that such a developmental time-course of angiogenesis might be linked to the synaptogenesis requirements that proceed differently for the magno- and parvocellular systems in the primate striate cortex.

Alkaline Phosphatase↗

PubMed1999

Enhanced NR2A subunit expression and decreased NMDA receptor decay time at the onset of ocular dominance plasticity in the ferret.

Enhanced NR2A subunit expression and decreased NMDA receptor decay time at the onset of ocular dominance plasticity in the ferret. The NMDA subtype of glutamate receptor is known to exhibit marked changes in subunit composition and functional properties during neural development. The prevailing idea is that NMDA receptor-mediated synaptic responses decrease in duration after the peak of cortical plasticity in rodents. Accordingly, it is believed that shortening of the NMDA receptor-mediated current underlies the developmental reduction of ocular dominance plasticity. However, some previous evidence actually suggests that the duration of NMDA receptor currents decreases before the peak of plasticity. In the present study, we have examined the time course of NMDA receptor changes and how they correlate with the critical period of ocular dominance plasticity in the visual cortex of a highly binocular animal, the ferret. The expression of NMDA receptor subunits NR1, NR2A, and NR2B was examined in animals ranging in age from postnatal day 16 to adult using Western blotting. Functional properties of NMDA receptors in layer IV cortical neurons were studied using whole cell patch-clamp techniques in an in vitro slice preparation of ferret primary visual cortex. We observed a remarkable increase in NR1 and NR2A, but not NR2B, expression after eye opening. The NMDA receptor-mediated synaptic currents showed an abrupt decrease in decay time concurrent with the increase in NR2A subunit expression. Importantly, these changes occurred in parallel with increased ocular dominance plasticity reported in the ferret. In conclusion, molecular changes leading to decreased duration of the NMDA receptor excitatory postsynaptic current may be a requirement for the onset, rather than the end, of the critical period of ocular dominance plasticity.

Aging↗

PubMed2000

Task-related modulation of visual cortex.

We performed a series of experiments to quantify the effects of task performance on cortical activity in early visual areas. Functional magnetic resonance imaging (fMRI) was used to measure cortical activity in several cortical visual areas including primary visual cortex (V1) and the MT complex (MT+) as subjects performed a variety of threshold-level visual psychophysical tasks. Performing speed, direction, and contrast discrimination tasks produced strong modulations of cortical activity. For example, one experiment tested for selective modulations of MT+ activity as subjects alternated between performing contrast and speed discrimination tasks. MT+ responses modulated in phase with the periods of time during which subjects performed the speed discrimination task; that is, MT+ activity was higher during speed discrimination than during contrast discrimination. Task-related modulations were consistent across repeated measurements in each subject; however, significant individual differences were observed between subjects. Together, the results suggest 1) that specific changes in the cognitive/behavioral state of a subject can exert selective and reliable modulations of cortical activity in early visual cortex, even in V1; 2) that there are significant individual differences in these modulations; and 3) that visual areas and pathways that are highly sensitive to small changes in a given stimulus feature (such as contrast or speed) are selectively modulated during discrimination judgments on that feature. Increasing the gain of the relevant neuronal signals in this way may improve their signal-to-noise to help optimize task performance.

Attention↗

PubMed1982

Visual development: early experience with torsionally disparate images.

Environmental influences on the developing primary visual cortex of kittens were studied by exposing dark reared kittens to prism-induced interocular rotational disparities of 32 degrees, the visual input rotated equally and oppositely in the two eyes. The present report describes preliminary results obtained from two kittens that received this altered visual exposure during 1-6 hours each day from 4 until 12 weeks of age. Subsequent single-unit recordings from the striate cortex revealed three major changes in functional cortical visual physiology. First, there was a disruption in binocularity, with many more cells being monocularly driven in the rotated conditions compared to control conditions. Second, there was an increased variance in the distribution of cells' interocular differences in preferred stimulus orientation (interocular orientation disparity, or IOD) as compared to control conditions. Third, changes were noted in orientation tuning and in the distribution of orientation preferences: cells most selective for orientation tended to be in the extreme ocular dominance groups, and monocular cells were often the most highly selective; also, both binocular and monocular cells showed a tendency for preferred orientations for both eyes to fall near the horizontal or vertical (+/- 22.5 degrees). Thus, a large optically-induced orientation disparity between the two eyes' visual fields during the critical period can modify the characteristics of striate cortical neurons, particularly binocularity and IOD. In addition, these results indicate that an inherent cortical mechanism may ensure the encoding of horizontal and vertical orientation specificities for a subclass of primary visual cortical neurons.

Aging↗

PubMed2001

Apical tuft input efficacy in layer 5 pyramidal cells from rat visual cortex.

1. The integration of synaptic inputs to the apical dendrite of layer 5 neocortical pyramidal cells was studied using compartment model simulations. The goal was to characterize the generation of regenerative responses to synaptic inputs under two conditions: (a) where there was an absence of background synaptic input, and (b) when the entire cell surface was subjected to a uniform blanket of synaptic background conductance such that somatic input resistance was reduced 5-fold. 2. Dendritic morphology corresponded to a layer 5 thick-trunked pyramidal cell from rat primary visual cortex at postnatal day 28 (P28), with distribution of dendritic active currents guided by the electrophysiological characteristics of the apical trunk reported in this cell type. Response characteristics for two dendritic channel distributions were compared, one of which supported Ca(2+) spikes in the apical dendrite. 3. In the absence of background, synaptic input to the apical tuft was surprisingly effective in eliciting somatic firing when compared with input to apical oblique branches. This result obtained even when the tuft membrane was the least excitable in the dendritic tree. 4. The special efficacy of tuft input arose because its electrotonic characteristics favour development of a sustained depolarization which charged the apex of the apical trunk to its firing threshold; once initiated in the distal trunk, firing propagated inward to the soma. This mechanism did not depend upon the presence of depolarizing channels in tuft membrane, but did require an excitable apical trunk. 5. Rather than disconnect the tuft, background synaptic conductance enhanced the efficacy advantage enjoyed by input arriving there. This counterintuitive result arose because background reduced the subthreshold spread of voltage, and so diminished the ability of the excitation of various individual oblique branches to combine to charge the relatively thick adjacent trunk. In contrast, drive from the depolarized tuft is exerted at a single critical point, the apex of the distal trunk, and so was relatively undiminished by the background. Further, once initiation at the apex occurred, background had little effect on inward propagation along the trunk. 6. We conclude that synaptic input to the apical tuft of layer 5 cells may be unexpectedly effective in triggering cell firing in vivo. The advantage in efficacy was not dependent upon the characteristics of tuft membrane excitability, but rather stemmed from the geometry of the tuft and its junction with the distal apical trunk. The efficacy of tuft input was, however, critically dependent upon inward propagation, suggesting that modulation of membrane currents which affect propagation in the apical trunk might sensitively control the efficacy of tuft input.

Action Potentials↗

PubMed1988

Callosal projections between areas 17 in the adult tree shrew (Tupaia belangeri).

In the primary visual cortex (area 17) of the tree shrew (Tupaia belangeri) neurons projecting to the contralateral area 17 via the corpus callosum were identified by horseradish peroxidase histochemistry (HRP, WGA-HRP). The distribution of homotopic and heterotopic connections was studied. We found that a narrow stripe of area 17 close to the dorsal area 17/18 border - which corresponds to the visual field along the vertical meridian - is connected via homotopic callosal projections. The adjacent dorsal part of area 17, which largely corresponds to the binocular visual field, is connected via homotopic as well as heterotopic projections. Heterotopic projections originate in the cortical stripe along the area 17/18 border and their contralateral targets are displaced medially. Callosal neurons are located mostly in supragranular but also occur in infragranular layers. The supragranular neurons in general are pyramidal cells. In addition to these findings, we confirmed earlier reports on ipsilateral projections of the primary visual area to the dLGN, the claustrum, area 18 and other visual areas.

Animals↗

PubMed2002

Effects of hyperoxia on human sensorimotor cortex activity produced by electrical stimulation of the median nerve: a functional magnetic resonance imaging study.

This study investigated the effect of hyperoxia on sensorimotorcortical activity resulting from electrical stimulation of the median nerve, using functional magnetic resonance imaging (fMRI). Nine volunteers underwent stimulation at 5 and 100 Hz while breathing 21% FIO(2) (fraction of inspired oxygen) or 100% FIO(2). fMRI data were correlated with a stimulus predictor curve, transformed into Talairach space and averaged by group. Normoxic (21% FIO(2)) and hyperoxic (100% FIO(2)) sensorimotor activation volumes were compared using Student's t-test. There were no significant differences between the primary somatosensory/primary motor/Brodmann area 40 (SI/MI/Ba40) and secondary somatosensory cortex (SII) activation volumes for normoxia and hyperoxia. (P>0.05). There was no difference between SI/MI/Ba40 and SII activations at 5 and 100 Hz. In contrast to results previously reported for primary visual cortex (V1), hyperoxia did not enhance sensorimotor cortical activation in area SI/MI/Ba40 or SII. These results indicate that there is regional heterogeneity of the fMRI response to hyperoxia in the cerebral cortex.

Adult↗

PubMed2002

Functional brain imaging using fMRI and optical topography in infancy.

We performed functional magnetic resonance imaging and optical topography over the visual cortex of subjects during sedation with pentobarbital, and 8-Hz flickering light was intermittently projected onto their eyelids. Two age groups were analyzed: infants <60 days old and those >60 days old (corrected for gestational age at birth). The stimulus-related signal change was positive in the lateral geniculate nucleus regardless of the infant's age, but it reversed in the primary visual cortex from positive in the infants less than 60 days old to negative in the infants more than 60 days old (Experiment 1). We also investigated spontaneous changes in the cerebral oxygenation state of neonates and infants aged 1 month during quiet sleeping by using a form of multi-channel near-infrared spectroscopy: non-invasive optical topography. Spatially synchronized oscillations of changes in the concentration of oxy-hemoglobin (oxy-Hb) and deoxy-Hb were observed throughout the occipital cortex in neonates but not in the infants aged 1 month. Time series analysis based on the theory of non-linear oscillations showed that the mean periods of the oscillation for each infant ranged from 11 to 18s. The phase lag of oxy-Hb relative to deoxy-Hb was stable at about 3 pi/4 in neonates but in the infant aged 1 month, time lag was unstable. These findings may be due to rapid synaptogenesis in early life.

Journal Article↗

PubMed1997

Regional cerebral blood flow and oxygen metabolism during migraine with and without aura.

Eleven cases of migraine with and without aura were investigated with positron emission tomography (PET). Regional cerebral blood flow (rCBF), oxygen metabolism (rCMRO2) and oxygen extraction (rOER) were measured during baseline (n = 11), aura (n = 6), headache (n = 10) and after treatment with sumatriptan (n = 4). Data were analysed using an ROI-based approach from 26 different anatomically defined regions, and also an exploratory approach whereby all subjects were normalized to a stereotactic brain atlas; t-maps were constructed by depicting significant changes between states. The exploratory approach revealed a region corresponding to the primary visual cortex with significant reductions in rCBF (23.1%) and rCMRO2 (22.5%), but no change in rOER during the headache phase compared to baseline. These data suggest that cerebral ischemia was not the primary cause of the attacks in these cases.

Adult↗

PubMed2001

Enhancing fMRI contrast in awake-behaving primates using intravascular magnetite dextran nanopartieles.

Functional MRI in awake-behaving primates is an emerging tool for bridging the gap between human fMRI and neurophysiology information from nonhuman primates. We report the use of magnetite dextran nanoparticles (Feridex) as a blood-pool agent to enhance fMRI contrast-to-noise (CNR) in primate FMRI. The intravascular half-life of the magnetite dextran was long compared to lanthanide chelates (T(1/2) = 198 min) with shortened T(2) relaxation observed in blood and cerebral cortex. Greater than 3-fold enhancement in the percentage MR signal change was observed using nanoparticles (13%) compared with conventional BOLD fMRI (4%). The calculated regional cerebral blood volume in macaque primary visual cortex increased 32% with photic stimulation. The increased CNR allows greater flexibility in the design of awake-behaving primate fMRI studies with the potential for improvements in resolution and significantly shortened imaging times.

Animals↗

PubMed1991

Postnatal development of thalamic recipient neurons in the monkey striate cortex: III. Somatic inhibitory synapse acquisition by spiny stellate neurons of layer 4C.

The development of type 2 (Colonnier, '81) synapses on the cell bodies of thalamic recipient spiny stellate neurons in layers 4C alpha and 4C beta of primary visual cortex neurons was examined over the first 36 postnatal weeks and in the adult monkey. The type 2 synapses, known to be GABAergic (Ribak, '78) and therefore presumed to be inhibitory, developed faster on the alpha neurons than the beta neurons. Both neuron groups show a marked increase and then decline in the percentage of the somatic membrane covered by type 2 synaptic appositions during this 36-week time period. The time course of the type 2 synapses development is compared to that of the spine synapse development described in previous studies (Lund and Holbach, '91; Lund et al., '91), and it is clear that on both neuron groups this inhibitory synapse population is put in place and refined later than the spine synapses. These findings suggest that each cortical neural circuit has a unique time course for its early development within an overall time window (Rakic et al., '86), or sensitive period (Hubel and Wiesel, '70). Visual deprivation, although causing the alpha and beta neurons to adopt a more similar temporal and numerical developmental pattern than normal, did not prevent acquisition and loss phases of type 2 synapses or the assumption of a normal numerical loading by 36 weeks of age.

Aging↗

PubMed2004

GABAA receptor maturation in relation to eye opening in the rat visual cortex.

Changes in subunit composition of N-methyl-D-aspartate (NMDA) receptors have been reported to be affected by visual experience and may therefore form a major aspect of neuronal plasticity in the CNS during development. In contrast, putative alterations in the expression and functioning of the inhibitory GABAA receptor around eye opening have not been well defined yet. Here we describe the timing of changes in GABAA receptor subunit expression and the related synaptic functioning in the neonatal rat visual cortex and the influence of visual experience on this process. Quantitative analysis of all GABAA receptor subunit transcripts revealed a marked alpha3 to alpha1 subunit switch, in addition to a change in alpha4 and alpha5 expression. The changes were correlated with an acceleration of the decay of spontaneous inhibitory postsynaptic currents (sIPSCs). Both changes in receptor expression and synaptic functioning were initiated well before eye opening. Moreover, dark rearing could not prevent the robust upregulation of alpha1 or the change in sIPSC kinetics, indicating that this is not dependent of sensory (visual) input. Upon eye opening a positive correlation was observed between a faster decay of the sIPSCs and an increase in sIPSC frequency, which was absent in dark-reared animals. Thus, lack of extrinsic input to the cortex does not affect overall developmental regulation of synaptic functioning of GABAA receptors. However, we cannot exclude the possibility that visual experience is involved in proper shaping of the inhibitory network of the primary visual cortex.

Animals↗

PubMed2001

Effects of experimental strabismus on the architecture of macaque monkey striate cortex.

Strabismus, a misalignment of the eyes, results in a loss of binocular visual function in humans. The effects are similar in monkeys, where a loss of binocular convergence onto single cortical neurons is always found. Changes in the anatomical organization of primary visual cortex (V1) may be associated with these physiological deficits, yet few have been reported. We examined the distributions of several anatomical markers in V1 of two experimentally strabismic Macaca nemestrina monkeys. Staining patterns in tangential sections were related to the ocular dominance (OD) column structure as deduced from cytochrome oxidase (CO) staining. CO staining appears roughly normal in the superficial layers, but in layer 4C, one eye's columns were pale. Thin, dark stripes falling near OD column borders are evident in Nissl-stained sections in all layers and in immunoreactivity for calbindin, especially in layers 3 and 4B. The monoclonal antibody SMI32, which labels a neurofilament protein found in pyramidal cells, is reduced in one eye's columns and absent at OD column borders. The pale SMI32 columns are those that are dark with CO in layer 4. Gallyas staining for myelin reveals thin stripes through layers 2-5; the dark stripes fall at OD column centers. All these changes appear to be related to the loss of binocularity in cortical neurons, which has its most profound effects near OD column borders.

Animals↗

PubMed2005

Paired-pulse transcranial magnetic stimulation protocol applied to visual cortex of anaesthetized cat: effects on visually evoked single-unit activity.

In this study, we tested the paired-pulse transcranial magnetic stimulation (ppTMS) protocol - a conditioning stimulus (CS) given at variable intervals prior to a test stimulus (TS) - for visually evoked single-unit activity in cat primary visual cortex. We defined the TS as being supra-threshold when it caused a significant increase or decrease in the visually evoked activity. By systematically varying the interstimulus interval (ISI) between 2 and 30 ms and the strength of CS within the range 15-130% of TS, we found a clear dependence of the ppTMS effect on CS strength but little relation to ISI. The CS effect was strongest with an ISI of 3 ms and steadily declined for longer ISIs. A switch from enhancement of intracortical inhibition at short ISIs (2-5 ms, SICI) to intracortical facilitation (ICF) at longer ISIs (7-30 ms), as demonstrated for human motor cortex, was not evident. Whether the CS caused facilitation or suppression of the TS effect mainly depended on the strength of CS and the polarity of the TS effect: within a range of 60-130% a positive correlation between ppTMS and TS effect was evident, resulting in a stronger facilitation if the TS caused facilitation of visual activity, and more suppression if the TS was suppressive by itself. The correlation inverted when CS was reduced to 15-30%. The ppTMS effect was not simply the sum of the CS and TS effect, it was much smaller at weak CS strength (15-50%) but stronger than the sum of CS and TS effects at CS strength 60-100%. Differences in the physiological state between sensory and motor cortices and the interactions of paired synaptic inputs are discussed as possible reasons for the partly different effects of ppTMS in cat visual cortex and human motor cortex.

Action Potentials↗