Biomedical subjects

S Johnstone

Publications and source records attributed to S Johnstone.

34 records · Page 2Linked to original sources

PubMed1990

Delay, discriminatory, and modality specific neurons in striatum and pallidum during short-term memory tasks.

The function of the striatum and its integrative capacities is addressed. The activity of single neurons in behaving Macaca mulatta is studied. Two new classes of neurons were found. In the striatum, but not in the pallidum, these neurons were visual modality specific. These neurons may represent a conjunction of sensory, mnemonic and motor information, and may be concerned with decisions about the emission or withholding of movements. The frontal dysfunction hypothesis of cognitive deficits in Parkinson's disease is considered.

Animals↗

PubMed1989

Spatial and orientation specific integration in the tilt illusion.

Lateral inhibition across a population of cells in visual cortex which are tuned to local orientation has been proposed and widely accepted as a basic process in the analysis of contour in the visual field. The tilt illusion is usually explained in terms of this inhibition. Experiments are reported which cast new light on the analysis of visual orientation. It is shown that tilt illusions may be obtained with very thin inducing annuli which are spatially remote from the test figure. In experiments in which remote crossed-grating plaids were used, an illusion which was pattern (global) rather than component (local) selective was seen. It is difficult to account for these observations in terms of local inhibitory mechanisms. Rather, the results support the existence of a secondary mechanism which is involved in basic orientation analytic processes. The relevance of these observations to models of visual contour analysis is discussed.

Attention↗

PubMed1989

Two-dimensional tilt illusions induced by orthogonal plaid patterns: effects of plaid motion, orientation, spatial separation, and spatial frequency.

Tilt illusions occur when a drifting vertical test grating is surrounded by a drifting plaid pattern composed of orthogonal moving gratings. The angular function of this illusion was measured as the plaid orientation (and therefore its drift direction) varied over a 180 degrees range. This was done when the test and inducing stimuli abutted and had the same spatial frequency, and when the test and inducing stimuli either differed in frequency by an octave, or were spatially separated by a 2 deg blank annulus, or both differed in frequency and were also separated by the annulus (experiments 1-4). The obtained angular function was virtually identical to that obtained previously with the rod and frame effect and other cases involving orthogonal inducing components, with evidence for illusions induced both by real-line components and by virtual axes of symmetry. Although the magnitude of the illusion was very similar in all four experiments, there was evidence to suggest that largest real-line effects occurred in the abutting same-frequency condition, with a pattern of results similar to that obtained previously with the simple one-dimensional tilt illusion. On the other hand, virtual-axis effects were more prominent with gaps between test and inducing stimuli. A fifth, repeated-measures, experiment confirmed this pattern of results. It is suggested that this pattern-induced tilt effect reflects both striate and extrastriate mechanisms and that the apparent influence of spatially distal virtual axes of symmetry upon perceived orientation implies the existence of AND-gate mechanisms, or conjunction detectors, in the orientation domain.

Attention↗

PubMed1989

Orientation illusions induced by briefly flashed plaids.

Orientation illusions which occur when a vertical grating is surrounded by a plaid can be induced either by one of the plaid's orthogonal component gratings or by a virtual axis of symmetry of the plaid, whichever is nearest vertical. In six experiments in which such illusion displays were flashed for durations between 15 and 405 ms, it was found that when these two-dimensional illusions are induced by a component grating (direct effects) the illusions increase monotonically as duration decreases, from 1 degree-2 degrees to about 6 degrees-7 degrees, over this range. Effects induced by axes of symmetry (indirect effects), in contrast, begin to occur only at longer durations: for short exposures, illusions are directionally opposite and large, similar to direct effects. These results suggest that only plaid-component-selective mechanisms operate at the shortest exposure durations and that additional time is required to extract more global higher-order pattern structure. The data are discussed in relation to sustained and transient mechanisms and also with respect to recent reports of more global processing mechanisms in extrastriate cortex and related data on one-dimensional tilt illusions.

Adult↗

PubMed1989

Pattern orientation, not motion, determines the two-dimensional tilt illusion.

In the usual tilt illusion (TI) configuration, an inducing stimulus which has a single orientation is used psychophysically to explore orientation analysis in the human visual system. Recently, this approach has been extended to the use of inducing stimuli which have two orientations. Such a two-dimensional (2-D) stimulus permits investigation of the low-level analysis of visual patterns. Prior experimentation has left it unclear whether it is the spatial or the motion properties of a moving crossed-grating plaid which determine two-dimensional tilt illusions (2-D TIs) because these two parameters previously were perfectly correlated. In the present experiments pattern orientation and motion were decoupled. It is shown that 2-D TIs are determined by the spatial properties of an inducing annulus and not by its motion properties. The results also support the existence of a mechanism which extracts axes of symmetry, and which is difficult to account for in terms of local cross-orientation domain inhibition.

Attention↗

PubMed1988

The different mechanisms of the direct and indirect tilt illusions.

Both the tilt illusion and aftereffect exhibit indirect effects under certain conditions: these are negative (assimilation) effects which occur with large (70-90 deg) angular separations between test and inducing gratings. They are opposite in direction to the positive, and much larger, contrast effects which occur at smaller (10-15 deg) separations. Evidence from six experiments shows that stimulus manipulations which reduce direct effects have little or no effect on indirect effects and vice versa, suggesting that the two effects have different determinants. It is proposed that direct effects arise from lateral inhibitory interactions between populations of neurones in striate cortex and that indirect effects occur at a higher level, possibly in areas concerned with stimulus-specific interactions beyond the classic receptive field. The implications of the data for theories of the tilt illusion are considered.

Figural Aftereffect↗

PubMed1988

Psychophysical evidence for an extrastriate contribution to a pattern-selective motion aftereffect.

A stationary vertical test grating appears to drift to the left after adaptation to an inducing grating drifting to the right, this being known as the motion aftereffect (MAE). Pattern-specific motion aftereffects (PSMAEs) induced by superimposed pairs of gratings in which the component gratings drift up and down but the observer sees a single coherent plaid drifting to the right have been investigated. Two experiments are reported in which it is demonstrated that the PSMAE is tuned more to the motion of the pattern than to the orientation and direction of motion of the component gratings. However, when subjects adapt to the component gratings in alternation, aftereffect magnitude is dependent upon the individual grating orientations and motion directions. These results can be interpreted in terms of extrastriate contributions to the PSMAE, possibly arising from the middle temporal area, where some cells, unlike those in striate cortex (V1), are tuned to pattern motion rather than to component motion.

Figural Aftereffect↗

PubMed1988

The differential effects of brief exposures and surrounding contours on direct and indirect tilt illusions.

Four experiments in which logarithmic intervals between 25 and 1600 ms were used for stimulus duration in tests for the tilt illusion are reported. It is demonstrated that the direct and the indirect tilt illusions both increase in magnitude inversely with length of stimulus presentation. The data suggest that whereas the direct effect peaks with a value of about +7 degrees at the shortest flash duration used (25 ms), peak indirect effects (of about +2 degrees) do not occur at this duration. In addition, whereas direct effects level out after 100 ms stimulus exposure times, to the usual magnitude obtained with long presentations (about +2 degrees), indirect effects reach their standard magnitude (-0.5 degrees to -1.0 degrees) later, at exposures of about 400 ms. Even at very short flash durations, a luminance square frame surrounding the illusion display reduces the indirect effect by two thirds of its magnitude but has no effect at all on the direct effect. It is suggested that direct effects arise early in visual processing, in area V1, where there are transient mechanisms and where corruption of orientation analysis by the inducing grating would occur prior to later, extrastriate, global analysis of the surrounding peripheral frame. Indirect effects, on the other hand, may arise later, along the sustained parvocellular colour-form pathway, where more global processing occurs and susceptibility to surrounding fields might be expected.

Adult↗

PubMed1987

The influence of auditory stimulation on a visual oblique effect.

It has previously been reported by Smets that there is an increase in the magnitude of the monocular oblique effect when a 70 dB(A), 1 kHz acoustic stimulus is presented contralaterally, but not ipsilaterally, to the viewing eye. This finding was interpreted as one which provided difficulties both for data-driven models of information processing and for the cortical simple cell explanation of the oblique effect. There are several logical and methodological difficulties in Smets's paper, and in the two experiments reported here the effect found by Smets was not replicated. The failure to observe the effect was robust under conditions that maximised the possibility of neural interaction. It is concluded that acoustic stimulation does not affect the magnitude of the oblique effect.

Attention↗

PubMed1987

Possible neural substrates for orientation analysis and perception.

Recent research into the response properties of extrastriate visual cortical mechanisms has revealed single-cell functional organisation which closely parallels certain global and apparently emergent properties of psychophysical observation. An attempt is made to relate previous data on orientation illusions and aftereffects to these extrastriate mechanisms and new data which cannot be explained adequately by V1 (striate) orientation channels are discussed. Conversely, properties of cells in areas such as V3, V4, MT, and others seem to provide an obvious neural substrate for global interactions. It is suggested that psychological 'explanations' couched in terms of 'hypotheses' or 'cognitive problem solving' lack heuristic value, and that, in contrast, the properties of extrastriate cells can suggest novel experimental psychophysical paradigms which are designed to probe these higher-order global mechanisms more or less selectively.

Animals↗

PubMed1986

The role of a functionally distinct IgM anti-type III pneumococcal polysaccharide (SIII) in low-dose paralysis to SIII in mice.

Prior treatment (priming) with a weakly immunogenic dose of Type III pneumococcal polysaccharide (SIII) results in the development of an antigen-specific state of unresponsiveness termed low-dose paralysis which is believed to be mediated by suppressor T cells. The present findings show that the passive administration of functionally distinct non-complement-fixing (NCF) IgM anti-SIII antibodies either in monoclonal form or from protein-A absorbed immune serum could significantly suppress the direct plaque-forming cell (PFC) response to an immunogenic dose of SIII administered concurrently. The degree of suppression was comparable with that induced by low-dose paralysis. Low-dose paralysis was consistently induced in athymic (nude) mice 4 days, but not 3 days, after priming with a low dose of SIII, and was associated with the delayed appearance of NCF-IgM anti-SIII in the serum of athymic mice. In contrast, low-dose paralysis was readily induced in normal BALB/c mice 3 days after priming when NCF-IgM anti-SIII antibodies were present. Comparable inhibiton of the direct anti-SIII PFC response was observed when Concanavalin A (Con A) or NCF-IgM anti-SIII serum was administered with SIII antigen. That Con A and NCF-IgM anti-SIII together did not produce additive suppression was attributed to the adsorption of NCF-IgM anti-SIII antibodies to Con A. Complement-dependent single radial haemolysis mediated by CF hybrid IgM/A or CF-IgM anti-SIII serum was blocked by monoclonal NCF-IgM or IgA anti-SIII antibodies and indicated that each of the antibodies was specific for the same SIII-determinant. Evidence is presented to show that low-dose paralysis in the CF-IgM response to SIII is not mediated by suppressor T cells but can be attributed to highly avid NCF-IgM anti-SIII antibodies, formed preferentially to low doses of SIII, being able to reduce the immunogenicity of SIII administered subsequently. We propose that low-dose paralysis to SIII is the result of an immunobiological function of highly avid NCF-IgM anti-SIII antibodies which not only confer resistance against capsulated pneumococci but preferentially bind soluble SIII-antigen to reduce its immunogenicity and thereby protect specific CF-IgM positive B cells from being rendered tolerant by direct contact with higher doses of SIII antigen.

Animals↗

PubMed1985

Regulation of immunity and tolerance to type III pneumococcal polysaccharide (SIII) by functionally distinct IgM anti-SIII antibodies.

When BALB/c mice and athymic (nude) mice are injected intraperitoneally (i.p.) with pneumococcal type III polysaccharide (SIII), their antibodies as measured by passive haemagglutination (HA) are inhibited more easily by high doses of SIII than antibody measured by passive haemolysis (HL). The HA activity, due mainly to a highly avid non-complement-fixing (NCF) type of IgM, was further distinguished from the HL activity (CF-IgM, or CF-IgM plus CF hybrid IgM/A anti-SIII antibodies) by the failure of the NCF-IgM anti-SIII to bind to protein-A of Staphylococcus aureus (Sa). High-dose tolerance in the HL anti-SIII antibody response of BALB/c and athymic mice was induced only in the absence of circulating NCF-IgM anti-SIII antibodies. The presence of NCF-IgM anti-SIII antibodies formed to multiple daily increasing amounts of SIII, commencing with 0.01 micrograms SIII, decreased the magnitude of the HL anti-SIII response to subsequent daily increments of SIII antigen injected into BALB/c and athymic (nude) mice. Thus, the effect on the HL anti-SIII response was independent of T-cells. The concomitant administration of NCF-IgM anti-SIII rendered SIII less tolerogenic in primed mice. In contrast to the HL activity, the NCF-IgM anti-SIII antibodies were induced to low doses of SIII, conferred protection against viable pneumococci, but did not precipitate the soluble antigen in agar. It is proposed that immune paralysis (as defined by the failure of SIII-injected mice to resist pneumococcal challenge) is not necessarily a condition of total unresponsiveness but is due to an absence of protective NCF-IgM anti-SIII antibodies. Thus, immune paralysis can co-exist with either the presence or absence of non-protective CF-IgM or CF-IgM/A anti-SIII antibodies.

Animals↗

PubMed1984

Serum IgM/A, IgA and functionally distinct IgM anti-type III pneumococcal polysaccharide (SIII) antibodies in BALB/c and athymic (nude) mice.

The influence of hereditary absence of thymus upon the synthesis of IgA, complement-fixing (CF) hybrid IgM/A, CF-IgM and non-CF-IgM antibodies to pneumococcal type III polysaccharide (SIII) injected into BALB/c and athymic nude mice was studied. Techniques involved the differential absorption of the serum antibodies by protein-A of Staphylococcus aureus (Sa), coprecipitation in gels with 125I-SIII and autoradiography. IgM/A anti-SIII activity was not demonstrable in nude mice but was produced in significant amounts, by day 5, in BALB/c mice injected with SIII. By day 8, nude mice produced more IgA anti-SIII antibodies than BALB/c mice injected with the same antigen. IgA anti-SIII antibodies were not detected in either strain 5 days after SIII administration. The absence of hybrid IgM/A anti-SIII antibodies in athymic mice, prior to the appearance of monotypic IgA anti-SIII antibodies at day 8, suggests that IgM/A and not IgA synthesis is largely T cell-dependent. The evidence also implies that hybrid IgM/A antibody production, maximal on day 5 in BALB/c mice, and absent from nude mice, is not an essential product in the switching from IgM to IgA synthesis. Both strains of mice produced comparable amounts of complement-fixing (CF)-IgM and NCF-IgM anti-SIII antibodies, with the production of non-complement-fixing (NCF)-IgM anti-SIII in athymic mice being delayed. Results indicate that attempts to quantitate the levels of IgA by assays incorporating anti-IgA anti-sera may be complicated by the presence of IgM/A hybrid antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1983

Enrichment of differentiated, stellate astrocytes in cerebellar interneuron cultures as studied by GFAP immunofluorescence and autoradiographic uptake patterns with [3H]D-aspartate and [3H]GABA.

A study was undertaken to correlate the morphology expressed by astroglial cells in post-natal cerebellar, interneuron-enriched primary cultures, and the ability of these cells to accumulate putative neurotransmitter amino acids. Astroglial cell morphology, as studied by GFAP immunofluorescence staining showed considerable changes during the culture period considered (up to 12 days in vitro). While the total number of GFAP-positive cells decreased with time (cell multiplication was prevented by cytosine arabinoside), a progressive enrichment of stellate astrocytes (cells bearing multiple radially arranged processes) and a striking increase in size of these cells was noted. In 12 DIV cultures stellate astrocytes accounted for 70-80% of the astrocytes present, and could reach a diameter of over 300 micron. The L-glutamate analogue, [3H]D-aspartate, was avidly taken up by all the astrocytes, independently of their shape and stage of differentiation. Astroglial cell morphology as delineated by [3H]D-aspartate autoradiography was identical to that evidenced by GFAP staining. On the other hand, [3H]GABA was accumulated in substantial amounts only by the stellate astrocytes, that is by the cells showing greater morphological differentiation. Astrocytes of other shapes were only lightly labelled by [3H]GABA in 2 DIV and 5 DIV cultures, and even less at later stages. Even within the stellate astrocyte population, the extent of [3H]GABA labelling was very variable, from one cell to another. Autoradiographic examinations and the determination of the IC50s for GABA uptake inhibitors consistently indicated that the GABA transport system present in stellate astrocytes did not have the features generally attributed to a glial transport system. In fact, beta-alanine was a very weak inhibitor, while nipecotic acid and ACHC were strongly inhibitory; DABA inhibitory potency fell somewhere in between. [3H]GABA uptake into the inhibitory interneurons present in the cultures showed similar sensitivity to GABA transport inhibitors.

Animals↗

PubMed1977

Cutaneous cross-modal interactions: variations in punctate pressure sensitivity with skin surface temperature.

We determined the effects of adapting the skin surface to temperatures 3 degreees C, 8 degrees C, or 12 degrees C above and below physiological zero for 0, 2, 5, 10, 15, or 30 min on punctate pressure sensitivity. Temperatures above physiological zero reduced punctate pressure sensitivity and temperatures below physiological zero enhanced punctate pressure sensitivity. As the skin adapted to warmth, punctate pressure sensitivity increased suggesting a cross modal adaptation effect, however there was no indication of a decrease in punctate pressure sensitivity as the skin adapted to cooling. The results are considered to be consonant with a system of mechanoreceptors which also respond to temperature changes and a system in which there are points of convergence in the somesthetic pathways at which pressure and temperature information interact and are integrated.

Adaptation, Physiological↗

PubMed1974

The recovery of anaerobic bacteria from swabs.

When a standard sample of simulated exudate containing known numbers of anaerobic bacteria was taken up on a swab and plated on solid medium, the number of colonies subsequently cultured represented a very small proportion of the original sample. Evidence is produced that the apparent loss is not primarily attributable to inactivation on the swab but rather to retention of organisms on the swab. This was demonstrable with Clostridium welchii and with Bacteroides species that have hitherto been regarded as relatively oxygen-sensitive.When stock strains of Bacteroides species were held for some hours on swabs, some progressive loss of viability was demonstrable. A measure of protection was afforded when these organisms were held aerobically on blood agar medium, but a very exacting anaerobe and some wild strains of faecal anaerobes showed gradual inactivation under these conditions.These findings may have important implications in relation to currently employed bacteriological sampling procedures with swabs in clinical practice.

Aerobiosis↗