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PubMed2002

Plastic reaction of the rat visual corticocollicular connection after contralateral retinal deafferentiation at the neonatal or adult stage: axonal growth versus reactive synaptogenesis.

The effects of neonatal or adult enucleation on the final adult pattern of the rat visual corticocollicular (C-Co) connection were studied using the anterograde tracer biotinylated dextranamine 10,000 (BDA) iontophoretically injected in the primary visual cortex. In control animals, column-shaped terminal fields limited to a small portion of the collicular surface were observed. Synaptic boutons were present in all superficial strata of the superior colliculus (SC), with the highest density in the ventral part of the stratum griseum superficiale (SGS). Neonatal enucleation caused a considerable expansion of the contralateral visual C-Co terminal fields, which occupied almost the entire collicular surface, suggesting that axonal sprouting had occurred. In addition, terminal boutons tended to localize more dorsally in these cases compared with controls. Following enucleation in adult animals, no changes were observed with respect to the extension of the terminal fields, although a plastic reaction leading to an increase in the bouton density in the stratum zonale (SZ) and upper SGS was found, reflecting a process of reactive synaptogenesis at these levels. These results show that both neonatal and adult visual C-Co fibers react in response to retinal ablation, although this reaction shows distinct characteristics. Molecular factors, such as growth-associated cytoskeletal proteins operating in the cortical origin, and extracellular matrix components and myelin-associated axonal growth inhibitors acting on the collicular target very likely account for these differences.

Age Factors↗
PubMed2002

Evidence from V1 connections for both dorsal and ventral subdivisions of V3 in three species of New World monkeys.

We used patterns of connections of primary visual cortex (V1) to reevaluate differing proposals on the organization of extrastriate cortex in three species of New World monkeys. Several fluorescent tracers and the bidirectional tracer cholera toxin B subunit (CTB) were injected into dorsal V1 (representing the lower visual quadrant) and ventral V1 (representing the upper visual quadrant) of titi, squirrel, and owl monkeys. Labeled cells and terminals were plotted on brain sections cut parallel to the surface of flattened cortex and were related to architectonic boundaries. The results provided compelling evidence for both dorsal V3 with dorsal V1 connections and ventral V3 with ventral V1 connections. The connection pattern indicated that V3 represents the visual hemifield as a mirror image of V2. In addition, V3 could be recognized by a weak banding pattern in brain sections processed for cytochrome oxidase. V1 has connections with at least 12 subdivisions of visual cortex, with half of the connections involving V2 and 20% V3. Comparable results were obtained from all three species, suggesting that visual cortex is similarly organized.

Animals↗
PubMed2003

Retinotopic pathways providing motion-selective information to the lobula from peripheral elementary motion-detecting circuits.

Recordings from afferent channels from the medulla supplying deep neuropils of the fly's optic lobes reveal different filter properties among the three classes of afferent neurons: transmedullary cells, T2 neurons, and Y cells. Whereas transmedullary cells respond to local flicker stimuli without discriminating these from directional or oriented motion, the T2 afferent neurons show clear motion orientation selectivity, which corresponds closely with a morphological bias in the orientation of their dendrites and could also be influenced by systems of local recurrent neurons in the medulla. A Y cell having a clearly defined terminal in the lobula, but having dendrite-like processes in the medulla and, possibly, the lobula plate, discriminates the direction of motion and its orientation. These results demonstrate unambiguously that the lobula receives information about motion and that the channels carrying it are distinct from those supplying wide-field motion-selective neurons in the lobula plate. Furthermore, recordings from a newly identified recurrent neuron linking the lobula back to the inner medulla demonstrate that the lobula discriminates nondirectional edge motion from flicker, thereby reflecting a property of this neuropil that is comparable with that of primary visual cortex in cats. The present findings support the proposal that elementary motion detecting circuits supply several parallel channels through the medulla, which segregate to, but are not shared by, the lobula and the lobula plate. The results are discussed in the context of other intracellular recordings from retinotopic neurons and with analogous findings from mammalian visual systems.

Afferent Pathways↗
PubMed2006

Synaptic organization of projections from the amygdala to visual cortical areas TE and V1 in the macaque monkey.

The primate amygdaloid complex projects to a number of visual cortices, including area V1, primary visual cortex, and area TE, a higher-order unimodal visual area involved in object recognition. We investigated the synaptic organization of these projections by injecting anterograde tracers into the amygdaloid complex of Macaca fascicularis monkeys and examining labeled boutons in areas TE and V1 using the electron microscope. The 256 boutons examined in area TE formed 263 synapses. Two hundred twenty-three (84%) of these were asymmetric synapses onto dendritic spines and 40 (15%) were asymmetric synapses onto dendritic shafts. Nine boutons (3.5%) formed double asymmetric synapses, generally on dendritic spines, and 2 (1%) of the boutons did not form a synapse. The 200 boutons examined in area V1 formed 211 synapses. One hundred eighty-nine (90%) were asymmetric synapses onto dendritic spines and 22 (10%) were asymmetric synapses onto dendritic shafts. Eleven boutons (5.5%) formed double synapses, usually with dendritic spines. We conclude from these observations that the amygdaloid complex provides an excitatory input to areas TE and V1 that primarily influences spiny, probably pyramidal, neurons in these cortices.

Amygdala↗
PubMed2006

Contribution of feedforward thalamic afferents and corticogeniculate feedback to the spatial summation area of macaque V1 and LGN.

Neurons in the primary visual cortex (V1) respond best to oriented gratings of optimal size within their receptive field (RF) and are suppressed by larger gratings involving the nonclassical RF surround. A V1 neuron's optimal stimulus size is larger at lower stimulus contrast. A central question in visual neuroscience is what circuits generate the size tuning of V1 cells. We recently demonstrated that V1 horizontal connections integrate signals within a region of the RF center corresponding to the V1 neuron's optimal stimulus size at low contrast; extrastriate feedback connections to V1, instead, are longer range and can integrate signals from the most distant regions of the V1 cell's RF surround. Here, we have determined the contribution of geniculocortical feedforward and corticogeniculate feedback connections to the size-tuning of macaque V1 and lateral geniculate (LGN) neurons, respectively. Specifically, we have quantitatively compared the visuotopic extent of geniculate feedforward afferents to V1 with the size of the RF center and surround of neurons in the V1 input layers and the visuotopic extent of V1 feedback connections to the LGN with the RF size of cells in V1 layer 6, where these connections originate. We find geniculate feedforward connections to provide visuotopic information to V1 that is spatially coextensive with the V1 neuron's optimal stimulus size measured with high-contrast gratings. V1 feedback connections restrict their influence to an LGN region visuotopically coextensive with the size of the minimum response field (or classical RF) of V1 layer 6 cells and commensurate with the LGN region from which they receive feedforward connections.

Animals↗
PubMed1976

On the striate area of the human isocortex. A Golgi- and pigmentarchitectonic study.

The distribution pattern of the pigment in various layers of the primary visual cortex of man is described by means of a method for the stereomicroscopical demonstration of neurolipofuscins in sections up to 1,000 mum thick. This paper presents the striate area as being composed of 12 cellular layers, which can be clearly delineated by a successive examination of both the Nissland the pigment preparations. Layers I, II, and III resemble more or less the corresponding laminae of other isocortical fields. The pyramids within the deep portion of layer III are unusually small and additionally are intermingled with thorny stellate cells which are a characteristic component of layer IV. Accordingly, this band of cells is called IIIc-IVa. The subjacent layers IVb and IVcalpha are poorly pigmented. In Nissl-preparations layer IVb, which contains only a few cells, is followed by a broad band of cells tightly packed together, which includes layers IVcalpha, IVcbeta, IVd, and Va. These layers cannot be reliably delineated in Nissl-preparations but appear in pigment preparations as distinct and sharply limited laminae. Particularly, layer IVcbeta, which is built up by minute thorny stellate cells, is marked by coarse and intensely stained lipofuscin granules, whereas IVd is mainly populated by minute pyramids which are almost devoid of pigment deposits. A remarkably narrowed layer Va, the pyramids of which store a medium number of lipofuscin grains, completes this series (IVcalpha, IVcbeta, IVd, Va). The cell-sparse layer Vb is almost devoid of pigment granules. Layer VIa has specialized pyramidal cells containing finely grained pigment. The relatively broad layer VIb shows loosely arranged cells poorly endowed with lipofuscin granules.

Brain Mapping↗
PubMed1978

Functional properties of the corticotectal projection in the golden hamster.

Approximately 31% of the cells recorded in the hamster's superior colliculus could be activated by stimulation of the ipsilateral primary visual cortex. While cortically activated cells were encountered in all laminae of the colliculus where visual cells were isolated, the highest probability of driving visual cells was observed in the deeper laminae, that is, those ventral to the stratum opticum. Response latency, jitter (latency variability), latency shifts as a function of shock intensity, thresholds, and spike numbers did not vary as a function of depth in the colliculus. There was a clear correspondence between the visual fields of the best cortical stimulus points and the receptive fields of cortically activated cells recorded in the superficial laminae of the colliculus. However, there was considerably less retinotopic fidelity for the cortical areas from which cells isolated in the deeper laminae could be driven. This suggests a greater degree of convergence from relatively widespread cortical regions upon visual cells of the deeper laminae. The visal organization) of the cortically activated cells did not differ appreciably from the overall sample of visual cells recorded in the colliculus. Only 3 of the 159 cells tested were driven by stimulation of the contralateral visual cortex and two of these were responsive only at very long latencies.

Animals↗
PubMed1985

Somatostatin (SRIF)-like immunoreactivity in subcortical and cortical visual centers of the rat.

The distribution of neuronal elements containing immunoreactive somatostatin (I-SRIF) in the rat central visual pathway was examined by light-microscopic immunocytochemistry. These studies were concerned with the location and morphology of neurons and innervated cells and the distribution of fiber and terminal plexuses in the primary visual cortex (area 17), visual association areas 18 and 18a, the superior colliculus, the lateral geniculate nucleus, and the pretectum. In the superior colliculus, I-SRIF-containing fibers and perikarya were distributed predominantly in the superficial, or visual, layers; these elements were moderately dense and occupied the entire mediolateral extent of these layers. In the intermediate and deep layers, immunoreactive neurons were widely scattered, and fibers were located mainly in the medial third. Immunoreactive cell populations in the superior colliculus included small bipolar neurons with fusiform perikarya and multipolar neurons with round to ovoid perikarya. In the pretectum, the peptide was demonstrable in large and small multipolar neurons of the nucleus of the optic tract and in the posterior and olivary pretectal nuclei. I-SRIF-containing neurons were also present in the nucleus of the posterior commissure, the nucleus of Edinger-Westphal, and the ventral division of the lateral geniculate nucleus. In the visual cortex, the peptide was present in all layers and in a variety of morphologically defined cell populations, including some which are presumed excitatory (pyramidal and bipolar cells) and others which are presumed inhibitory (bitufted and stellate cells). Our data suggest that somatostatin is involved in visual and visuomotor reflex pathways and in the horizontal optokinetic nystagmus reflex pathway. These results provide a foundation for further studies to evaluate the role of this peptide in visual processes.

Animals↗
PubMed1986

Laminar distribution of receptors in monkey (Macaca fascicularis) geniculostriate system.

We have examined the laminar distributions of eight types of receptor in the primary visual cortex (area 17) and the lateral geniculate nucleus (LGN) of the macaque monkey. The receptor populations and subpopulations examined included those selective for gamma-aminobutyric acid (GABA) (using [3H]-muscimol as ligand), L-glutamate-related receptors (using [3H]-L-glutamate and [3H]-AMPA), muscarinic acetylcholine (using [3H]-quinuclidinyl benzilate--QNB and [3H]-N-methyl scopolamine--NMS), cholecystokinin (CCK) (using [3H] pentagastrin), benzodiazepine (using [3H]-flunitrazepam), and adenosine (using [3H]-cyclohexyladenosine--CHA). Each of the receptors examined exhibited characteristic and differing laminar patterns of binding in the striate cortex. Perhaps reflecting the high density of cell bodies and synapses in layer 4C, most receptors, except those labelled by [3H]-L-glutamate or [3H]-AMPA, showed dense concentrations in this layer. Layers 4B and 5, which contain relatively few cell bodies and heavy myelin concentrations, were in general lightly labelled. Layer 6 showed relatively heavy labelling when [3H]-AMPA (quisqualate) or [3H]-pentagastrin (CCK) were used as ligands. The superficial layers of the cortex were zones of relative concentration of GABA, benzodiazepine, acetylcholine, glutamate-related, and adenosine receptors. In general, the binding patterns resembled those previously described for cat visual cortex, but there were also some clear differences. The distributions of all of these receptors likely reflect the differential input substances to different laminae of the visual cortex. Of the receptors examined, only those for GABA, benzodiazepine, and acetylcholine were found in substantial concentration in the LGN. Of these, GABA and benzodiazepine receptors showed especially dense binding in the magnocellular layers of the LGN compared to the parvicellular layers.

Animals↗
PubMed1987

Anatomical demonstration of ocular segregation in the retinogeniculocortical pathway of the New World capuchin monkey (Cebus apella).

We describe the architecture of the dorsal lateral geniculate nucleus and primary visual cortex (striate cortex; area 17) of the New World capuchin monkey (Cebus apella) on the basis of the distribution of cell bodies and cytochrome oxidase histochemistry. Changes in staining for cytochrome oxidase following unilateral enucleation served to indicate the organization of the representation of the two eyes in the retinogeniculocortical pathway. The number and disposition of eye-specific layers within the lateral geniculate nucleus of Cebus are consistent with the common plan of geniculate organization in anthropoid primates, and the radial organization of area 17 fits the pattern common to New World squirrel and Old World macaque monkeys, including the presence of cytochrome-oxidase-rich zones in supragranular and deeper cortical layers (Horton: Philos. Trans. R. Soc. Lond. [Biol.] 304:199-253, '84). Our principal finding is that cytochrome oxidase histochemistry following unilateral eye removal unequivocally reveals ocular dominance columns in the striate cortex of Cebus. As in the macaque (Hubel: Nature 292:762-764, '82), ocular dominance columns extend through the thickness of cortex and blobs are centered on columns, but the array of columns viewed tangentially is less orderly or more mosaic than in the macaque, and there is apparently significant overlap between columns. The presence of well-defined ocular dominance columns in Cebus, as in Ateles (Florence, Conley, and Casagrande: J. Comp. Neurol. 243:234-248, '86) but not in other New World monkeys examined previously, emphasizes the phylogenetic lability of binocular segregation in the primate visual cortex. In addition, the present results indicate significant differences with respect to the tangential organization of the ocular dominance domain between primate species in which ocular dominance columns are present.

Animals↗
PubMed1988

Sequential development of connections between striate and extrastriate visual cortical areas in the rat.

In these experiments we have asked whether the projection from the rat's primary visual cortex, area 17, to the extrastriate visual cortical area 18a is formed in a sequence and whether that sequence resembles the pattern of inside-out cortical neurogenesis. For this purpose fluorescent retrograde tracers were injected into area 18a at different postnatal ages (P1, P5, adult). Animals survived until 3-4 weeks of age, after migration is complete and neurons have arrived at their final laminar location. In the ipsilateral cortex, P1 injections retrogradely labeled cells in layers 5 and 6 of area 17. Labeling after P5 injections extended into more superficial layers and included the bottom of layer 2/3 and layers 4-6. After P5, more labeled cells were found at the top of layer 2/3, producing the adult laminar pattern, where the projection originates predominantly from layer 2/3. A similar sequence of laminar labeling was observed in the transcallosal connection of area 18a. This sequence of labeling, deep layers before superficial, resembles the pattern in which cortical neurons are born and indicates that axons arrive at their cortical targets in the order the cells were generated.

Animals↗
PubMed1992

Postnatal development of area 17 callosal connections in Tupaia.

The goal of the present study was to investigate the pattern of maturation of callosal projecting neurons in a well-studied mammalian visual system with unique structural and functional properties. Studies of the distribution pattern of interhemispheric connections in the adult tree shrew primary visual cortex reveal not only a high concentration of labeled neurons along the area 17/18 border, as in standard experimental animals such as the cat and monkey, but also numerous callosal projecting neurons in the adjacent dorsal part of area 17, which largely corresponds to the binocular visual field (Kretz and Rager, Exp. Brain Res. 82:271, '90). Callosal projections were anatomically traced in 11 tree shrews (Tupaia belangeri) at various ages between postnatal day 7 (7, 9, 10, 13, 15, 17, 19, and 26 days old) and adulthood (107 days old). In each animal, four injections of wheat germ agglutinin conjugated to horseradish peroxidase were made in a standard configuration into the striate cortex of one hemisphere. In young tree shrews only 7 and 9 days old, heavily labeled terminal axon structures could be seen in the white matter and in layer VI of the opposite hemisphere. Only a few labeled neurons, however, were detected in layer III. The small number of labeled neurons indicated that early in postnatal development, only a few callosal axons had invaded the upper cortical layers. By 10 days of age, the number of supragranular neurons was increasing and the maximal value was counted in a 13-day-old tree shrew. A sharp decline in the number of labeled supragranular neurons was noticed--about 94% in our case--between days 13 and 15. In animals more than 15 days old, the distribution pattern and the density of the neurons looked like the pattern seen in the adult Tupaia brain. The labeled cells were mostly concentrated in layers II and III. The majority of neurons resembled typical pyramidal cells. However, some of the neurons in sublayer IIIc had elongated cell bodies oriented parallel to the laminar boundaries. In contrast to the supragranular cells found in all stages investigated, small populations of labeled cells in layer VI were observed in 9- to 17-day-old tree shrews only. In young postnatal animals 7 to 13 days old, a peculiar cell type was labeled on the ipsilateral side. In coronal sections these cell bodies formed a continuous band that extended from the ventricular wall to the subcortical white matter. These cells might belong to a population of cells still in migration.

Animals↗
PubMed1993

Distribution of [3H]QNB and [125I]alpha-bungarotoxin binding and acetylcholinesterase activity in visual system and hippocampal structures of eleven mammalian species.

This study assessed interspecies differences in regional brain distribution of [3H]QNB binding, [125I]alpha-bungarotoxin binding and acetylcholinesterase activity, by autoradiographic and histochemical methods. Eleven mammalian species were examined, including carnivores (cat, dog), a lagomorph (rabbit), and rodents (squirrel, guinea pig, gerbil, hamster, vole, lemming, rat, mouse). Comparisons were based on primary visual system structures (superior colliculus, lateral geniculate nucleus, primary visual cortex) and the hippocampal formation. The two radioligands differed greatly in the degree of interspecies variation: while the pattern of [3H]QNB binding was quite similar across species, [125I]alpha-bungarotoxin showed striking interspecies diversity. This contrast was most obvious in laminar patterns of the visual cortex and hippocampal formation. Regional distributions of acetylcholinesterase staining were fairly diverse, and were unlike the patterns of either [3H]QNB or [125I]alpha-bungarotoxin. The two ligands showed more consistency in overall levels across species than did acetylcholinesterase. Possible correlates of the differences in interspecies diversity are discussed.

Acetylcholinesterase↗
PubMed1995

Morphology and connections of neurons in area 17 projecting to the extrastriate areas MT and 19DM and to the superior colliculus in the monkey Callithrix jacchus.

Neurons of area 17, the primary visual cortex, project to various anatomically and physiologically different extrastriate areas and subcortical regions. In the present investigation, we addressed the question of whether the efferent neurons in area 17 can contribute to functional diversity between these regions. We approached this question by analyzing the dendritic morphology of neurons in area 17 projecting to area MT, area 19DM, and the superior colliculus in the new world simian primate Callithrix jacchus, because dendritic morphology is an important factor in determining physiological properties of nerve cells. Retrograde transport of fluorochromes injected into the target regions, and intracellular injections of Lucifer yellow in the prelabelled neurons, revealed the following. 1) Morphologically identical large pyramidal cells in layer VI of area 17 project to all three targets. Some of them possess axon collaterals to two or all three targets, suggesting that they provide common information to all three areas. 2) Pyramidal cells in layer IIIc projecting to area MT form a morphologically homogeneous population. 3) Three small to medium-sized pyramidal cell types in layers IIIa-c, spiny stellate cells in layer IIIc, and another large pyramidal cell type in layer VI project to area 19DM. 4) Pyramidal cells in the lower two-thirds of layer V in area 17 project to the superior colliculus. In conclusion, we have shown that in Callithrix one efferent pathway may originate from several cell types. However, with the exception of the large cells in layer VI, efferent cells projecting to area MT, area 19DM, and the superior colliculus were morphologically distinct. This suggests that functional differences between brain regions could arise in part from morphological heterogeneity between and within the efferent cell populations.

Animals↗
PubMed2001

Frequency dependence and gender effects in visual cortical regions involved in temporal frequency dependent pattern processing.

Neural response to flickering stimuli has been shown to be frequency dependent in the primary visual cortex. Controversial gender differences in blood oxygen level dependent (BOLD) amplitude upon 6 and 8 Hz visual stimulation have been reported. In order to analyze frequency and gender effects in early visual processing we employed a passive graded task paradigm with a dartboard stimulus combining eight temporal frequencies from 0 to 22 Hz in one run. Activation maps were calculated within Statistical Parametric Mapping, and BOLD amplitudes were estimated for each frequency within the striate and extrastriate visual cortex. The BOLD amplitude was found to steadily rise up to 8 Hz in BA 17 and 18 with an activation plateau at higher frequencies. In addition, we observed a laterality effect in the striate cortex with higher BOLD contrasts in the right hemisphere in men and in women. BOLD response rises similarly in men and women up to 8 Hz but with lower amplitudes in women at 4, 8, and 12 Hz (30% lower). No frequency effect above 1 Hz was found in the extrastriate visual cortex. There was also a regional specific gender difference. Men activated more in the right lingual gyrus (BA 18) and the right cerebellum compared with women, whereas women showed more activation in the right inferior temporal gyrus (BA 17). The study indicates that frequency dependent processing at the cortical level is limited to the striate cortex and may be associated with a more global information processing (right hemisphere dominance), particularly in men. The finding of significantly lower BOLD amplitudes in women despite previously shown larger VEP (visual evoked potential) amplitudes might suggest gender differences in cerebral hemodynamics (baseline rCBV, rCBF, or neurovascular coupling). The regional distinction points at additional differences in psychological processing even when using a simple visual stimulus.

Adult↗
PubMed2007

Spatiotemporal analysis of the cortical sources of the steady-state visual evoked potential.

This study aimed to characterize the neural generators of the steady-state visual evoked potential (SSVEP) to repetitive, 6 Hz pattern-reversal stimulation. Multichannel scalp recordings of SSVEPs and dipole modeling techniques were combined with functional magnetic resonance imaging (fMRI) and retinotopic mapping in order to estimate the locations of the cortical sources giving rise to the SSVEP elicited by pattern reversal. The time-varying SSVEP scalp topography indicated contributions from two major cortical sources, which were localized in the medial occipital and mid-temporal regions of the contralateral hemisphere. Colocalization of dipole locations with fMRI activation sites indicated that these two major sources of the SSVEP were located in primary visual cortex (V1) and in the motion sensitive (MT/V5) areas, respectively. Minor contributions from mid-occipital (V3A) and ventral occipital (V4/V8) areas were also considered. Comparison of SSVEP phase information with timing information collected in a previous transient VEP study (Di Russo et al. [2005] Neuroimage 24:874-886) suggested that the sequence of cortical activation is similar for steady-state and transient stimulation. These results provide a detailed spatiotemporal profile of the cortical origins of the SSVEP, which should enhance its use as an efficient clinical tool for evaluating visual-cortical dysfunction as well as an investigative probe of the cortical mechanisms of visual-perceptual processing.

Adolescent↗
PubMedDate not supplied

Fast spin-echo characteristics of visual stimulation-induced signal changes in the human brain.

A fast spin-echo (FSE) technique used in a conventional MR imaging scanner has been successfully developed for obtaining functional MR images with high spatial resolution and multiple slices. Our preliminary visual stimulation studies using the FSE technique show that the nuclear MR signal increases by 2.6% during activation in the primary visual cortex. These results provide evidence that the diffusion of tissue water molecules plays a key role in determining functional MR signal amplitude. Because the FSE functional MR imaging signal is extremely sensitive to microvascular (brain capillaries) hemodynamics, the FSE technique can be a powerful tool for studying the neuronal activity of the human brain.

Adult↗
PubMed2005

Repeated visual hallucinations in Parkinson's disease as disturbed external/internal perceptions: focused review and a new integrative model.

Visual hallucinations (VH) in Parkinson's disease (PD) are a chronic complication in 30 to 60% of treated patients and have a multifaceted phenomenology. Flickering, faultive impressions, and illusionary misperceptions precede the core syndrome of stereotyped, colorful images. The patient variably recognizes these images as hallucinations, being rarely irritated or frightened and more often amused as a bystander. Although studies on VH in PD focus on several research domains, no comprehensive, unified theory has been developed to study their pathophysiology. We have adapted Hobson's work on the states of consciousness and propose a model integrating seemingly disparate data on VH. We suggest that VH should be considered as a dysregulation of the gating and filtering of external perception and internal image production. Contributive elements and anatomical links for the model include poor primary vision, reduced activation of primary visual cortex, aberrant activation of associative visual and frontal cortex, lack of suppression or spontaneous emergence of internally generated imagery through the ponto-geniculo-occipital system, intrusion of rapid eye movement dreaming imagery into wakefulness, errative changes of the brainstem filtering capacities through fluctuating vigilance, and medication-related overactivation of mesolimbic systems. Different etiologies likely produce different phenomenologies and the prognosis may not be uniform. This new conceptual framework permits an anatomical view of VH and suggests new, testable hypotheses regarding their pathophysiology and therapy.

Brain Chemistry↗