Noise as a Trigger for Headaches: Relationship Between Exposure and Sensitivity

OBJECTIVE

This study investigated how triggers acquire the capacity to precipitate headaches.

BACKGROUND

Traditional clinical advice is that the best way to prevent headache/migraine is to avoid the triggers. Avoidance of anxiety-eliciting stimuli, however, results in sensitization to the stimuli, so is there a danger that avoidance of migraine/headache triggers results in decreased tolerance for the triggers?

DESIGN

One hundred and fifty subjects, 60 of whom suffered from regular headaches, were randomly assigned to 5 experimental conditions, defined by length of exposure to the headache trigger of noise.

METHODS

Subjects attended a laboratory session divided into 3 phases: preintervention test, intervention (1 of 5 levels of exposure to the trigger), and postintervention test. Response to the intervention was measured in terms of noise tolerance, sensitivity to noise, and nociceptive response to noise.

RESULTS

A curvilinear relationship was found between length of exposure to the trigger and pain response for individuals who do not suffer from regular headaches, that is, short exposure was associated with sensitization and prolonged exposure with desensitization. The relationship for headache patients was less clear.

CONCLUSIONS

The findings are consistent with the proposition that 1 etiological pathway to suffering from frequent headaches is via trying to avoid, or escape from, potential trigger factors. These results suggest that the traditional clinical advice to headache patients, that the best way to prevent migraine/headache is to avoid the triggers, runs the risk of establishing an insidious sensitization process thereby increasing headache frequency.

Random wiring in the midget pathway of primate retina

The present study addresses the questions of how topographically organized neuronal populations are connected, and whether there is anatomical evidence for color-selective wiring in retinal pathways for red-green color vision. The connectivity of OFF midget bipolar and OFF midget ganglion cells was studied in the peripheral retina of dichromatic ("red-green color blind") and trichromatic ("color normal") marmosets (Callithrix jacchus). Midget bipolar cells were identified immunohistochemically. Midget ganglion cells were retrogradely labeled from the lateral geniculate nucleus and photofilled. Comparable results were obtained from all retinas studied. Between 3 and 16 bipolar terminals converge onto each ganglion cell. Nearly all bipolar terminals investigated show regions of colocalization (areas of presumed synaptic contacts) with ganglion cell dendrites. This contact area makes up approximately 14% of the axon surface area for a typical midget bipolar cell. The output from individual midget bipolar axons is often shared between midget ganglion cells so that, on average, <70% of the axon terminal area of a midget bipolar cell shows overlap with the dendritic field of a given midget ganglion cell. We conclude that there is no morphological evidence of red-green color selectivity in the connections between midget bipolar and midget ganglion cell mosaics. Furthermore, the results suggest that convergence is based on local interactions between axons and dendrites rather than cell-by-cell recognition between members of each mosaic.

Chromatic organization of ganglion cell receptive fields in the peripheral retina

This study addresses the chromatic properties of receptive fields in the subcortical visual pathway of primates. There is agreement that, in the central visual field, many cells belonging to the parvocellular (PC) division of the subcortical pathway show red-green opponent responses, that a subgroup of cells belonging to the koniocellular (KC) pathway shows blue-yellow opponent responses, and that magnocellular (MC) pathway cells show only weak signs of chromatic input. However, the chromatic properties of ganglion cells in the peripheral retina are poorly understood. Here, we measured the temporal-chromatic properties of ganglion cells in extracellular in vivo recordings from peripheral macaque retina. We show that the chromatic responsivity of peripheral KC ("blue-on") and MC cells is very similar to that of their counterparts in the foveal retina. Cone-opponent responses are expressed only at low temporal frequencies (<10 Hz) in the majority of peripheral PC cells, and some peripheral PC cells show non-opponent response properties. With these exceptions, the chromatic properties of ganglion cells are essentially preserved throughout the first 50 degrees of visual eccentricity. The main change seen in passing from foveal to peripheral retina is that all ganglion cell classes become more responsive to high temporal-frequency modulation.

Contribution of chromatic aberrations to color signals in the primate visual system

We measured responses to red-green color variation in parvocellular (PC) neurons in the lateral geniculate nucleus of dichromatic ("red-green color blind") marmoset monkeys. Although these animals lack distinct visual pigments to distinguish between wavelengths in this range, many of the colored stimuli nevertheless produced robust responses in PC cells. We show that these responses, which are restricted to high stimulus spatial frequencies (fine image details), arise from chromatic aberrations in the eye. The neural signals produced by chromatic aberrations are of comparable magnitude to signals produced by high-frequency luminance (LUM) modulation and thus could influence cortical pathways for processing of color and object recognition. The fact that genetically "color-blind" primates are not necessarily blind to wavelength-dependent contours in the visual world may have enabled red-green color vision to become linked with high-acuity spatial vision during primate evolution.

A randomized controlled trial of psychological interventions for postnatal depression

OBJECTIVES

First, to establish the efficacy of psychological interventions versus routine primary care for the management of postnatal depression (PND). Secondly, to provide a direct comparison of cognitive-behavioural therapy (CBT) versus counselling and, finally, to compare the relative value of group and individual delivery formats.

DESIGN

The study involved 192 depressed women drawn from a large community screening programme in Melbourne, Australia and allocated to cognitive behaviour therapy, counselling, or routine primary care. Baseline and post-intervention measures of depression and anxiety were collected in the form of validated self-report inventories.

METHOD

Women were screened in the community and diagnosis of depression confirmed with a standardized psychiatric interview. Interventions were of 12 weeks duration, including three partner sessions, and adhered to a structured manual.

RESULTS

Psychological intervention per se was superior to routine care in terms of reductions in both depression and anxiety following intervention.

CONCLUSIONS

For those women with PND, psychological intervention is a better option than routine care, leading to clinically significant reduction of symptoms. Counselling may be as effective as group cognitive behaviour therapy. The benefits of psychological intervention may be maximized by being delivered on a one-to-one basis.

Spatial coding and response redundancy in parallel visual pathways of the marmoset Callithrix jacchus

Many neurons in the primary visual cortex (area V1) show pronounced selectivity for the orientation and spatial frequency of visual stimuli, whereas most neurons in subcortical afferent streams show little selectivity for these stimulus attributes. It has been suggested that this transformation is a functional sign of increased coding efficiency, whereby the redundancy (or overlap in response properties) is reduced at consecutive levels of visual processing. Here we compared experimentally the response redundancy in area V1 with that in the three main dorsal thalamic afferent streams, the parvocellular (PC), koniocellular (KC), and magnocellular (MC) divisions of the dorsal lateral geniculate nucleus (LGN) in marmosets. The spatial frequency and orientation tuning of single cells in the LGN and area V1 were measured, using luminance contrast sine-wave gratings. A joint spatial frequency-orientation response selectivity profile was calculated for each cell. Response redundancy for each population was estimated by cross-multiplication of the joint selectivity profiles for pairs of cells. We show that when estimated in this way, redundancy in LGN neurons is approximately double that of neurons in cortical area V1. However, there are differences between LGN subdivisions, such that the KC pathway has a spatial representation that lies between the redundant code of the PC and MC pathways and the more efficient sparse spatial code of area V1.

Effects of Noise and a Stressor on Head Pain

OBJECTIVE

Can causal relationships be established between negative affect (NA) and headaches, and noise (N) and headaches? Do NA and N interact to cause headaches? Do NA and N cause headaches by means of the same or different physiological mechanisms? Are the answers to these questions a function of diagnostic status?

BACKGROUND

A functional model of chronic headaches has been proposed that seeks to understand the variance in headaches by focusing on the controlling variables, that is, the antecedents and consequences. This study is one in a series investigating the immediate antecedents of headaches, namely the trigger factors.

DESIGN

Twenty-four subjects with migraine and 44 subjects with tension-type headache were randomly assigned to one of four experimental conditions defined by the presence or absence of two antecedent challenges, a stressor (S) designed to induce NA and N.

METHODS

The S challenge consisted of difficult-to-solve anagrams accompanied by failure feedback. The N challenge consisted of 50 dB of white N. Laboratory sessions were divided into adaptation, baseline, challenge, and recovery phases. Responses were measured in terms of headache intensity ratings, forehead electromyographic activity, heart rate, blood pressure, and temporal pulse amplitude (TPA).

RESULTS AND CONCLUSIONS

Both NA and N precipitated headaches. These two factors did not interact in triggering headaches. Headaches induced by N were associated with elevated TPA but headaches induced by NA were not associated with significant physiological changes. Diagnosis was not related to any of the outcomes.

Mosaic properties of midget and parasol ganglion cells in the marmoset retina

We measured mosaic properties of midget and parasol ganglion cells in the retina of a New World monkey, the common marmosetCallithrix jacchus. We addressed the functional specialization of these populations for color and spatial vision, by comparing the mosaic of ganglion cells in dichromatic (“red–green color blind”) and trichromatic marmosets. Ganglion cells were labelled by photolytic amplification of retrograde marker (“photofilling”) following injections into the lateral geniculate nucleus, or by intracellular injection in anin vitroretinal preparation. The dendritic-field size, shape, and overlap of neighboring cells were measured. We show that in marmosets, both midget and parasol cells exhibit a radial bias, so that the long axis of the dendritic field points towards the fovea. The radial bias is similar for parasol cells and midget cells, despite the fact that midget cell dendritic fields are more elongated than are those of parasol cells. The dendritic fields of midget ganglion cells from the same (ON or OFF) response-type array show very little overlap, consistent with the low coverage of the midget mosaic in humans. No large differences in radial bias, or overlap, were seen on comparing retinae from dichromatic and trichromatic animals. These data suggest that radial bias in ganglion cell populations is a consistent feature of the primate retina. Furthermore, they suggest that the mosaic properties of the midget cell population are associated with high spatial resolution rather than being specifically associated with trichromatic color vision.

Synaptic connectivity in the midget-parvocellular pathway of primate central retina

The synaptic connectivity of OFF midget bipolar cells was investigated in the central retina of two primate species, the New World common marmoset monkey, Callithrix jacchus, and the Old World macaque monkey, Macaca fascicularis. In marmosets, dichromatic and trichromatic animals were compared. Bipolar output synapses were identified with antibodies against ribbon proteins (kinesin, C-terminal binding protein 2) or with an antiserum that recognizes postsynaptic glutamate receptor clusters (GluR4). The midget bipolar cells were identified immunocytochemically with antibodies to CD15 (marmoset) or an antiserum to recoverin (macaque). In marmosets, midget ganglion cells were retrogradely labeled from the parvocellular layers of the dorsal lateral geniculate nucleus. Consistent with previous studies of Old World primates, in marmoset, midget bipolar cells contacted midget ganglion cells at a ratio of 1:1. The number of output synapses made by OFF midget bipolar cells was quantified for 104 cells in two dichromatic marmosets, 108 cells in one trichromatic marmoset, and 118 cells in one macaque. The number of output synapses was comparable for all animals, ranging from 10-71 in the dichromatic marmoset (average 29.7 +/- 12.4 SD), 12-86 in the trichromatic marmoset (average 28.6 +/- 11.7 SD) and 9-48 in the macaque (average 26.5 +/- 9.3 SD) per axon terminal. In all animals the number of output synapses per axon terminal showed a unimodal distribution. Our results suggest that the midget circuitry is comparable in dichromatic and trichromatic animals.

Colour through the thalamus

Visual perception in humans and other primates depends on the retino-thalamo-cortical pathway. This pathway begins with retinal ganglion cells, which have axonal terminations in the lateral geniculate nucleus (LGN) of the thalamus. Each ganglion cell axon provides input to one or more LGN relay neurones and, in turn, nearly all the LGN relay neurones project to the primary visual cortex. Thus, this pathway forms the dominant functional input to cortical mechanisms for colour vision, as well as for other aspects of conscious visual perception. In this review, recent progress in understanding the transmission of signals for colour vision through the LGN is summarised, with emphasis on studies which provide links between function and structure.

Latitudinal variation in genetic divergence of populations and the potential for future speciation

The increase in biological diversity with decreasing latitude is widely appreciated but the cause of the pattern is unknown. This pattern reflects latitudinal variation in both the origin of new species (cladogenesis) and the number of species that coexist. Here we address latitudinal variation in species origination, by examining population genetic processes that influence speciation. Previous data suggest a greater number of speciation events at lower latitudes. If speciation events occur more frequently at lower latitudes, we predicted that genetic divergence among populations within species, an important component of cladogenesis, should be greater among lower latitude populations. We tested this prediction using within-species patterns of mtDNA variation across 60 vertebrate species that collectively spanned six continents, two oceans, and 119 degrees latitude. We found greater genetic divergence of populations, controlling for geographic distance, at lower latitudes within species. This pattern remained statistically significant after removing populations that occur in localities previously covered by continental glaciers during the last glaciation. Results suggest that lower latitude populations within species exhibit greater evolutionary independence, increasing the likelihood that mutation, recombination, selection, and/or drift will lead to divergence of traits important for reproductive isolation and speciation. Results are consistent with a greater influence of seasonality, reduced energy, and/or glacial (Milankovitch) cycles acting on higher latitude populations, and represent one of the few tests of predictions of latitudinal variation in speciation rates using population genetic data.

Chromatic and spatial properties of parvocellular cells in the lateral geniculate nucleus of the marmoset (Callithrix jacchus)

The parvocellular (PC) division of the afferent visual pathway is considered to carry neuronal signals which underlie the red-green dimension of colour vision as well as high-resolution spatial vision. In order to understand the origin of these signals, and the way in which they are combined, the responses of PC cells in dichromatic ('red-green colour-blind') and trichromatic marmosets were compared. Visual stimuli included coloured and achromatic gratings, and spatially uniform red and green lights presented at varying temporal phases and frequencies.The sensitivity of PC cells to red-green chromatic modulation was found to depend primarily on the spectral separation between the medium- and long-wavelength-sensitive cone pigments (20 or 7 nm) in the two trichromatic marmoset phenotypes studied. The temporal frequency dependence of chromatic sensitivity was consistent with centre-surround interactions. Some evidence for chromatic selectivity was seen in peripheral PC cells. The receptive field dimensions of parvocellular cells were similar in dichromatic and trichromatic animals, but the achromatic contrast sensitivity of cells was slightly higher (by about 30%) in dichromats than in trichromats. These data support the hypothesis that the primary role of the PC is to transmit high-acuity spatial signals, with red-green opponent signals appearing as an additional response dimension in trichromatic animals.

Characterization of a complex chemosensory signal transduction system which controls twitching motility in Pseudomonas aeruginosa

Virulence of the opportunistic pathogen Pseudomonas aeruginosa involves the coordinate expression of a wide range of virulence factors including type IV pili which are required for colonization of host tissues and are associated with a form of surface translocation termed twitching motility. Twitching motility in P. aeruginosa is controlled by a complex signal transduction pathway which shares many modules in common with chemosensory systems controlling flagella rotation in bacteria and which is composed, in part, of the previously described proteins PilG, PilH, PilI, PilJ and PilK. Here we describe another three components of this pathway: ChpA, ChpB and ChpC, as well as two downstream genes, ChpD and ChpE, which may also be involved. The central component of the pathway, ChpA, possesses nine potential sites of phosphorylation: six histidine-containing phosphotransfer (HPt) domains, two novel serine- and threonine-containing phosphotransfer (SPt, TPt) domains and a CheY-like receiver domain at its C-terminus, and as such represents one of the most complex signalling proteins yet described in nature. We show that the Chp chemosensory system controls twitching motility and type IV pili biogenesis through control of pili assembly and/or retraction as well as expression of the pilin subunit gene pilA. The Chp system is also required for full virulence in a mouse model of acute pneumonia.

Glutamate receptors at bipolar synapses in the inner plexiform layer of primate retina: light microscopic analysis

At least 10 different types of bipolar cells have been distinguished in the primate retina. The axon terminals of these cells stratify in distinct strata in the inner plexiform layer and are involved in parallel pathways to distinct types of ganglion cells. Ionotropic glutamate receptor (GluR) subunits also show a stratified distribution in the inner plexiform layer. Here, we investigated whether different types of bipolar cells are associated with different types of ionotropic glutamate receptors in the inner retina of a New World primate, the common marmoset Callithrix jacchus. Vertical cryostat sections through central retina were double labeled with immunohistochemical markers for bipolar cell types and with antibodies to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits GluR1 to 4, kainate receptor subunits GluR6/7, and the NR1C2' subunit of the N-methyl-D-aspartate (NMDA) receptor. The axon terminals of bipolar cell types were reconstructed from confocal sections, and the colocalized immunoreactive puncta were quantified. For all bipolar cell types, immunoreactive puncta for the AMPA receptor subunits GluR2, 2/3, and 4 were colocalized at highest densities, whereas GluR1-immunoreactive puncta were expressed at very low densities. The kainate receptor subunits GluR6/7 were predominantly associated with diffuse bipolar (DB6) and rod bipolar cells. The NMDA receptor subunit NR1C2' was specifically colocalized with flat midget and DB3 axons. These findings suggest that rod and cone bipolar cell types contribute to multiple but distinct glutamate receptor pathways in primate retina.

Interactions between color and luminance in the perception of orientation

At the early stages of visual processing in humans and other primates, chromatic signals are carried to primary visual cortex (V1) via two chromatic channels and a third achromatic (luminance) channel. The sensitivities of the channels define the three cardinal axes of color space. A long-standing though controversial hypothesis is that the cortical pathways for color and form perception maintain this early segregation with the luminance channel dominating form perception and the chromatic channels driving color perception. Here we show that a simple interaction between orientation channels (the tilt illusion) is influenced by both chromatic and luminance mechanisms. We measured the effect of oriented surround gratings upon the perceived orientation of a test grating as a function of the axes of color space along which the gratings were modulated. We found that the effect of a surround stimulus on the perceived orientation of the test is largest when both are modulated along the same axis of color space, regardless of whether that is a cardinal axis. These results show that color and orientation are intimately coupled in visual processing. Further, they suggest that the cardinal chromatic axes have no special status at the level(s) of visual cortex at which the tilt illusion is mediated.

Coping and caregivers of people with dementia

PURPOSE

To critically review the research based on Lazarus and Folkman's (1984) stress and coping model, in respect to the coping of those caring for persons with dementia in the community, in an attempt to establish its implications for interventions aimed at improving caregiver adjustment.

METHOD

Published material on the coping of caregivers of persons with dementia was identified through computerized literature searches (Med-line, Psych-Info) to December 1999, employing search terms including Alzheimer's disease, dementia, caregiving, caregiver burden, adaptation, psychological, coping, and stress. Studies were chosen to be considered in detail, based on the reviewer's opinion that they would contribute to an understanding of the current state of the research and its clinical implications. This material was then critically reviewed against the tenets of Lazarus and Folkman's (1984) model.

RESULTS

Sixteen studies were selected to be included in the review, 12 cross-sectional and 4 longitudinal. Seven of the studies did not incorporate coping measures specific to caregiving and/or assess coping in respect of specific caregiver problems. Nine of the studies did do this. The research suggests that a general tendency towards problemsolving and acceptance styles of coping is likely to be advantageous to caregivers of people with dementia.

CONCLUSIONS

Despite this finding, it is concluded that the ability of the research to inform the clinician is severely limited. It is proposed that while longitudinal studies considered specific caregiver problems which incorporate coping measures specific to the caregiving task may improve understanding, a substantial revision of methodology and perspective may be required to produce findings that are likely to influence practice.

Modulation sensitivity of ganglion cells in peripheral retina of macaque

There is ample psychophysical evidence that flicker is more salient in the peripheral than the central visual field, but the physiological basis of this eccentricity-dependant change is unclear. Here, we compared responsivity to temporal modulation of ganglion cells in central and peripheral primate retina. Above 30 Hz modulation frequency, both magnocellular (MC) and parvocellular (PC) pathway cells are more responsive in peripheral retina. This suggests that an increase in high-frequency temporal responsiveness arises in outer retina before the MC and PC pathways diverge. In both central and peripheral retina, the critical fusion frequency of MC cells is higher than that of PC cells. This result is consistent with other evidence that psychophysical flicker sensitivity is mediated by the MC pathway.

Expression and distribution of ionotropic glutamate receptor subunits on parasol ganglion cells in the primate retina

The response properties of postreceptoral sensory neurones are determined by the properties of their input neurones, by intrinsic membrane properties, and by the properties of neurotransmitter receptors on the soma and dendritic tree. We previously showed that inhibitory neurotransmitter (GABA(A) and glycine) receptors on a well-characterised sensory neurone, the parasol ganglion cell in the primate retina, are segregated towards the distal part of the dendritic tree. Here we studied the distribution of excitatory ionotropic glutamate receptor subunits on the dendrites of parasol cells in the retina of a New World monkey, the marmoset, Callithrix jacchus. Individual ganglion cells were intracellularly injected in an in vitro retinal wholemount preparation. Ionotropic glutamate receptor subunits. including AMPA (GluR1-4), kainate (GluR6/7), NMDA (NR1C2') subunits, and the orphan receptors delta1 and delta2 were visualized with immunocytochemical methods. Immunoreactive puncta that colocalized with the dendrites of ganglion cells were analyzed using standard and/or confocal light microscopy. Colocalized puncta were present on parasol dendrites for all subunits studied, but their density was much lower (approximately 1/5) than previously reported for inhibitory (GABA and glycine) receptors. Segregation of the glutamate receptor clusters (GluR1, GluR6/7 subunits) to the peripheral dendrites was less marked than that shown for GABA and glycine receptor clusters. No sign of segregation of colocalized puncta to the peripheral part of the dendritic field was seen with antibodies to the GluR2, GluR2/3, GluR4, delta1/2, or NR1C2' subunits. The results suggest that although there is diverse expression of glutamate receptor subtypes, the glutamatergic synapses form only a small proportion of the total synaptic input to primate ganglion cells. They further suggest that the processes which control distribution of excitatory and inhibitory synapses on the dendritic field of ganglion cells are, at least to some extent, independent.

Extraclassical receptive field properties of parvocellular, magnocellular, and koniocellular cells in the primate lateral geniculate nucleus

Descriptions of receptive fields at subcortical levels of the visual system have mostly considered only the classical receptive field (CRF). A suppressive extraclassical receptive field (ECRF) has been demonstrated in relay cells within the primate lateral geniculate nucleus (LGN), but the quantitative properties and specific influence of the ECRF on the distinct magnocellular (MC), koniocellular (KC), and parvocellular (PC) pathways are not known. Here we quantified the effect of ECRF stimulation on visually responsive cells in the LGN of a diurnal New World primate, the marmoset. We show that for all cells, visually evoked responses are reduced by stimulation of the ECRF. The magnitude of the suppression is greatest for MC cells and smallest for PC cells. The effect of ECRF stimulation on KC cells is variable but always suppressive. We refer to these effects as extraclassical inhibition (ECI). The contrast-response relationship of the ECI parallels that of CRF-induced excitation for each cell class: for MC cells, ECI contrast threshold is close to 10% and the ECI saturates at 50% contrast, but the contrast dependence of ECI on PC cells is more linear. The ECI also contributes to contrast-dependent changes in spatial summation: on average for all LGN cells the radius of the excitatory spatial summation field (measured from aperture-tuning curves) at low contrast is 1.31 times that at high contrast. No consistent effects of orientation on ECI were seen. The data suggest that the suppressive component of the ECRF seen in cortical neurons could primarily be inherited from subcortical input streams.

Bipolar cell diversity in the primate retina: morphologic and immunocytochemical analysis of a new world monkey, the marmoset Callithrix jacchus

The aim of this study was to identify the bipolar cell types in the retina of a New World monkey, the common marmoset, and compare them with those found in the Old World macaque monkey. Retinal whole-mounts, sections, or both, were stained by using DiI labeling and immunohistochemical methods. Semithin sections were analyzed by using quantitative methods. We show that the same morphologic types of bipolar cell as described for the Old World macaque monkey by Boycott and Wässle (Boycott and Wässle [1991] Eur. J. Neurosci. 3:1069-1088) are present in marmoset retina: two types of midget bipolar cells, six type of diffuse bipolar cells, a blue cone bipolar cell, and one type of rod bipolar cell. The pattern of staining with different immunohistochemical markers ("fingerprint") of each bipolar cell type in marmoset was also the same as described for macaque, with one exception: the flat midget bipolar cell (FMB) class is labeled by antibodies to recoverin in macaque but is labeled by antibodies to CD15 in marmoset. The labeled FMB cells in marmoset make contact with multiple cone photoreceptors throughout most of the extrafoveal retina. The spatial density of bipolar cells in marmoset is shown to be sufficient to support one-to-one connectivity of midget bipolar and ganglion cells in the fovea and to allow for parallel pathways to ganglion cells throughout the retina. Quantitative differences in the morphology and receptor connectivity between marmoset and macaque can be related to differences in cone and rod photoreceptor density between the species. We conclude that bipolar cell diversity is a preserved feature of the primate retina.

Spatial properties of koniocellular cells in the lateral geniculate nucleus of the marmoset Callithrix jacchus

1. The receptive field dimensions, contrast sensitivity and linearity of spatial summation of koniocellular (KC), parvocellular (PC) and magnocellular (MC) cells in the lateral geniculate nucleus (LGN) of 11 adult marmosets were measured using achromatic sinusoidal gratings. 2. The receptive field centre diameter of cells in each (PC, KC and MC) class increases with distance from the fovea. There is substantial overlap in centre size between the three cell classes at any eccentricity, but the PC cells have, on average, the smallest centres and the KC cells have the largest. Some PC and KC cells did not respond at all to the grating stimulus. 3. The contrast sensitivity of the receptive field centre mechanism in KC cells decreases in proportion to the centre area. A similar trend was seen for the surround mechanism. These characteristics are common to PC and MC cells, suggesting that they originate at an early stage of visual processing in the retina. 4. The KC cells showed, in general, lower peak evoked discharge rates than PC or MC cells. The spontaneous discharge rate of KC cells was lower than that of PC cells and similar to that of MC cells. 5. The majority of cells in all divisions of the LGN show linear spatial summation. A few cells did show non-linear spatial summation; these cells were predominantly located in the MC and ventral KC layers. 6. The ventral KC layers below and between the MC layers contain cells with larger and more transiently responding receptive fields than cells in the more dorsal KC layers. 7. We conclude that many of the contrast-dependent spatial properties of cells in the marmoset LGN are common to PC, MC and KC cells. The main difference between KC cells and the other two classes is that there is more variability in their response properties, and they are less responsive to high spatial frequencies.

How do trigger factors acquire the capacity to precipitate headaches?

This study tested two contrasting theories of how trigger factors acquire the capacity to precipitate headaches. The sample consisted of 110 participants, of whom 48 suffered from regular headaches. Participants were exposed to a validated headache trigger factor for one of five exposure durations. The trigger used was "visual disturbance" (flicker, glare and eyestrain) induced by a very bright, stroboscopic light. Response to the stimulus was measured by participant ratings of the degree of visual disturbance and head pain caused by the stimulus. As expected, the headache sufferers experienced more visual disturbance and head pain in response to the stimulus than the non-headache individuals. Longer exposure to the stimulus was associated with a subsequent reduction in pain ratings in response to the stimulus. This desensitization effect supported an avoidance model of how trigger factors acquire the capacity to precipitate headaches. The findings of this study have implications for the etiology of headache disorders. Also, the findings imply that the traditional clinical advice that the best way to prevent migraine and headache is to avoid the factors that trigger them, may be counterproductive, as any short-term gains may be more than wiped out by decreased tolerance for the trigger factors.

Molecular mechanisms of thiamine utilization

Thiamine is required for all tissues and is found in high concentrations in skeletal muscle, heart, liver, kidneys and brain. A state of severe depletion is seen in patients on a strict thiamine-deficient diet in 18 days, but the most common cause of thiamine deficiency in affluent countries is alcoholism. Thiamine diphosphate is the active form of thiamine, and it serves as a cofactor for several enzymes involved primarily in carbohydrate catabolism. The enzymes are important in the biosynthesis of a number of cell constituents, including neurotransmitters, and for the production of reducing equivalents used in oxidant stress defenses and in biosyntheses and for synthesis of pentoses used as nucleic acid precursors. Because of the latter fact, thiamine utilization is increased in tumor cells. Thiamine uptake by the small intestines and by cells within various organs is mediated by a saturable, high affinity transport system. Alcohol affects thiamine uptake and other aspects of thiamine utilization, and these effects may contribute to the prevalence of thiamine deficiency in alcoholics. The major manifestations of thiamine deficiency in humans involve the cardiovascular (wet beriberi) and nervous (dry beriberi, or neuropathy and/or Wernicke-Korsakoff syndrome) systems. A number of inborn errors of metabolism have been described in which clinical improvements can be documented following administration of pharmacological doses of thiamine, such as thiamine-responsive megaloblastic anemia. Substantial efforts are being made to understand the genetic and biochemical determinants of inter-individual differences in susceptibility to development of thiamine deficiency-related disorders and of the differential vulnerabilities of tissues and cell types to thiamine deficiency.