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Biomedical subjects

Martin Heil

Publications and source records attributed to Martin Heil.

At least 19 recordsLinked to original sources
PubMed2007

Maintaining readiness for mental rotation interferes with perceptual processes in children but with response selection in adults.

Previous work revealed that mental rotation is not purely inserted into a same-different discrimination task. Instead, response time (RT) is slowed to upright stimuli in blocks containing rotated stimuli compared to RT to the same upright stimuli in pure upright blocks. This interference effect is a result of maintaining readiness for mental rotation. In two experiments we investigated previous evidence that these costs depend upon distinct sub-processes for children and for adults. In Experiment 1, the maintaining costs turned out to be independent of the visual quality of the stimulus for adults but not so for children. Experiment 2 revealed that the maintaining costs were greatly reduced for adults when they performed mental rotation as a go-no-go task, but not so for children. Taken together, both experiments provide evidence that whereas perceptual processes seem to be important for school-age children to maintain readiness for mental rotation, response selection is relevant for adults.

Adult↗
PubMed2006

Induced resistance enzymes in wild plants-do 'early birds' escape from pathogen attack?

Systemic acquired resistance (SAR) of plants to pathogens is a well-defined phenomenon. The underlying signalling pathways and its application in crop protection are intensively studied. However, most studies are conducted on crop plants or on Arabidopsis as a model plant. The taxonomic distribution of this phenomenon and its dependence on life history are thus largely unknown. We quantified activities of three classes of resistance-related enzymes in 18 plant species to investigate whether plants with varying life histories differ in their investment in disease resistance. Enzyme activities were quantified in untreated plants, and in plants induced with BION, a chemical resistance elicitor. All species showed constitutive activities of chitinase, peroxidase, or glucanase. However, constitutive chitinase activities varied by 30 times, and peroxidase by 50 times, among species. Several species did not respond to the induction treatment, while enzyme activities in other species increased more than threefold after BION application. Plant species differ dramatically in the presence and inducibility of resistance enzymes. This variation could be related to life history: While all resistance enzymes were significantly induced in larger perennial plants that flower during summer, spring geophytes hardly showed inducible resistance. These plants grow in an environment that is characterised by a low-pathogen pressure, and thus may simply 'escape' from infection. Our study presents the first comparative data set on resistance-related enzymes in noncultivated plants. The current view on SAR-narrowed by the concentration on cultivated crops-is not sufficient to understand the ecological and evolutionary relevance of this widespread plant trait.

Animal Feed↗
PubMed2006

Conceptual priming for environmental sounds and words: an ERP study.

In this study we examined conceptual priming using environmental sounds and visually displayed words. Priming for sounds and words was observed in response latency as well as in event-related potentials. Reactions were faster when a related word followed an environmental sound and vice versa. Moreover both stimulus types produced an N400-effect for unrelated compared to related trials. The N400-effect had an earlier onset for environmental sounds than for words. The results support the theoretical notion that conceptual processing may be similar for verbal and non-verbal stimuli.

Acoustic Stimulation↗
PubMed2006

'Winner-take-all' competition among real and illusory words.

'Winner-take-all' networks are an important theoretical construct in visual neuroscience. An implication of winner-take-all networks is that a stimulus that wins the competition for selection receives maximal activation and will be consciously perceived, even when selection was incorrect. In this study, competition was induced between physically presented ('real') and self-constructed ('illusory') words. Semantic activation was probed by recording event-related potential responses to a downstream target word. The results showed that only words reported by participants triggered a spread of activation in the semantic system, whereas non-reported words failed to prime the target. Both effects were independent of whether the potential primes were 'real' or 'illusory'. Our findings indicate that neural 'winner-take-all' networks extend to the processing of lexical units.

Adult↗
PubMed2006

Phenotypic plasticity of cyanogenesis in lima bean Phaseolus lunatus-activity and activation of beta-glucosidase.

Cyanogenesis, the release of toxic HCN from damaged plant tissues, is generally considered as a constitutive plant defense. We found phenotypic plasticity of cyanogenesis in young leaves of lima bean Phaseolus lunatus based on increased activity of the beta-glucosidase in response to herbivore attack. Two aspects of plant cyanogenesis have to be considered in ecological analyses: (1) the cyanogenic potential (HCNp), which indicates the total amount of cyanide-containing compounds present in a given tissue, and (2) the cyanogenic capacity (HCNc), representing the release of HCN per unit time. This release is catalyzed by specific beta-glucosidases, whose activity is a crucial parameter determining overall toxicity. Enzymatic activity of beta-glucosidase-and, in consequence, the rate of HCN release-was increased significantly after 72 hr of incubation with spider mites as compared to non-infested leaves. Feeding by L1 larvae of Mexican bean beetles also led to enhanced enzymatic activity, whereas mechanical damage of leaf tissue had no effect on beta-glucosidase activity and the release of HCN. The results place plant cyanogenesis in the group of induced resistance traits, whose degree of activity depends on the feeding by a particular herbivore.

Animals↗
PubMed2006

One-armed artificial receptors for the binding of polar tetrapeptides in water: probing the substrate selectivity of a combinatorial receptor library.

We have recently developed a new class of one-armed artificial receptors 1 for the binding of the polar tetrapeptide N-Ac-D-Glu-L-Lys-D-Ala-D-Ala-OH (EKAA) 2 in water using a combined combinatorial and statistical approach. We have now further probed the substrate selectivity of this receptor library 1 by screening a second tetrapeptide substrate (3) with the inverse sequence N-Ac-D-Ala-D-Ala-L-Lys-D-Glu-OH (AAKE). This "inverse" substrate is also efficiently bound by our receptors, with K(ass) approximately 6000 M(-1) for the best receptors, as determined both by a quantitative on-bead binding assay and by UV and fluorescence titration studies in free solution. Hence, the inverse tetrapeptide 3 is in general bound two to three times less efficiently than the "normal" peptide 2 (K(ass) approximately 17,000 M(-1)), even though the complexation mainly involves long-range electrostatic interactions and both the receptor and substrate are rather flexible. Molecular modeling and ab initio calculations have been used to rationalize the observed substrate selectivity and to analyze the various binding interactions within the complex.

Combinatorial Chemistry Techniques↗
PubMed2006

Semantic activation, letter search and N400: A reply to Mari-Beffa, Valdes, Cullen, Catena and Houghton (2005).

From a behavioural point of view, the automaticity of semantic activation was denied because letter search eliminates semantic priming effects on RT. From a neuroscience point of view, two recent studies used the N400 to test this claim with antithetic results. Here, we replicate N400 priming effects in the letter search task. The negative results might be traced back to the use of an inappropriate reference electrode. Thus, the theory of semantic activation being an automatic process should not be abandoned prematurely.

Brain Mapping↗
PubMed2006

Violation of pure insertion during mental rotation is independent of stimulus type, task, and subjects' age.

The experiment validated and extended the finding of Ilan and Miller (1994) [A violation of pure insertion: mental rotation and choice reaction time. Journal of Experimental Psychology: Human Perception and Psychophysics 20, 520-536] that the mental rotation process is not purely inserted into a mirror-normal discrimination task. In contrast to their work we used other experimental stimuli (drawings of animals instead of characters), a different task (same/different comparison) and investigated this effect under a developmental perspective. Adults and children between the age of 8 and 10 took significantly longer to respond to upright drawings of animals in conditions containing rotated stimuli than in conditions containing only upright stimuli, indicating a violation of pure insertion. In general, we found evidence that the violation of pure insertion during a mental rotation task itself can be generalised across stimulus type, task, and subject populations. However, for children this effect was independent of the format of the stimuli, while for adults the effect was larger for mirror-imaged than for identical objects. This suggests that the violation of pure insertion might occur at different processing stages as a function of age.

Adult↗
PubMed2006

Priming of indirect defences.

Leaf damage induces in many plant species the secretion of extrafloral nectar (EFN) and/or the release of specific odours (volatile organic compounds, VOCs). Both traits attract carnivorous arthropods and function as indirect plant defences by increasing the predation pressure on attacking herbivores. We have conducted field experiments in Mexico and found that plants that had been exposed to VOCs thereafter responded to subsequent leaf damage with an increased EFN secretion. VOCs 'primed' the plants to respond more efficiently once they were attacked themselves. Such priming effects are well known for induced resistance to pathogens, yet this is the first description of VOCs priming EFN secretion, a taxonomically widespread anti-herbivore defence. Plants can use chemical signals in their environment to assess the risk of herbivory and make use of this information to adjust their own defensive strategy accordingly.

Adaptation, Physiological↗
PubMed2005

Material-specific long-term memory representations of faces and spatial positions: evidence from slow event-related brain potentials.

Motivated by models that propose material-specific cortical long-term memory representations we expected different topographies of event-related slow waves of the EEG during cued retrieval of two distinct types of information (faces and spatial positions), which are assumed to be processed and stored in topographically distinct cortical areas, i.e., in either the ventral or the dorsal visual pathway. Seventeen participants learned associations either between words and spatial positions or between words and faces. Each word was associated with either one or two positions or faces. In a cued recall test, one day later, participants saw two words and had to decide whether these were linked to each other via an associated spatial position or a face. Slow event-related potentials (ERPs) of the EEG were recorded from 61 scalp electrodes during both acquisition and recall. Response times increased monotonically with the number of faces and positions to be reactivated. Negative slow ERPs showed a comparable topography during anticipation learning and cued recall, but dissociated topographically for positions and faces. The maximum of the negativity increased when items were presented repetitively (compared to the first presentation) during learning, and also with the number of the to-be-reactivated associations during retrieval. These results are consistent with an information-processing model that assumes material-specific cortical representations of episodic memory contents, which are established as localized cortical cell assemblies during encoding, and which are being reactivated during recall.

Adult↗
PubMed2005

Quantification of invertase activity in ants under field conditions.

Invertases (EC 3.2.1.26) are hydrolases that cleave sucrose into the monosacccharides, glucose, and fructose. They play a central role in carbohydrate metabolism of plants and animals. Methods presented so far to quantify invertase activity in ants or other animals have been hampered by the variability in both substrates and products of the enzymatic reaction in animals whose carbohydrate metabolism is highly active. Our method is based on a spectrophotometric quantification of the kinetics of glucose release. We first obtained an equilibrium state summarizing reactions of any carbohydrates and enzymes that are present in the extract. Sucrose was then added to quantify invertase activity as newly released glucose. Invertase activities differed significantly among species of ants. Variances were lowest among individuals from the same colony and highest among different species. When preparations were made from ants of the same species, invertase activity was linearly related to the number of ants used for extraction. Our method does not require ants to be kept on specific substrates prior to the experiment, or expensive or large equipment. It, thus, appears suitable for dealing with a broad range of physiological, ecological, and evolutionary questions.

Animals↗
PubMed2004

Evolutionary change from induced to constitutive expression of an indirect plant resistance.

Induced plant resistance traits are expressed in response to attack and occur throughout the plant kingdom. Despite their general occurrence, the evolution of such resistances has rarely been investigated. Here we report that extrafloral nectar, a usually inducible trait, is constitutively secreted by Central American Acacia species that are obligately inhabited by ants. Extrafloral nectar is secreted as an indirect resistance, attracting ants that defend plants against herbivores. Leaf damage induces extrafloral nectar secretion in several plant species; among these are various Acacia species and other Fabaceae investigated here. In contrast, Acacia species obligately inhabited by symbiotic ants nourish these ants by secreting extrafloral nectar constitutively at high rates that are not affected by leaf damage. The phylogeny of the genus Acacia and closely related genera indicate that the inducibility of extrafloral nectar is the plesiomorphic or 'original' state, whereas the constitutive extrafloral nectar flow is derived within Acacia. A constitutive resistance trait has evolved from an inducible one, obviously in response to particular functional demands.

Acacia↗
PubMed2004

Priming illusory words: an ERP approach.

Repetition blindness (RB) was used to investigate whether illusory words emerge at a lexical-perceptual or a semantic-reconstructional level. Illusory words were evoked by the rapid serial visual presentation (RSVP) of two real words and a word fragment. The initial words share the same string of letters ("CREEP"-"SHEEP"), producing a free-floating word fragment ("SH"). This fragment is likely to be linked to a subsequently presented fragment ("IFT") if both combine to a meaningful word ("SHIFT"). The processing level of the illusions was probed by prime words preceding the RSVP sequence which were semantically related or unrelated to the second real word or to the illusion. Behavioural and electrophysiological correlates of the semantic priming effect were recorded in 14 subjects. Real words related to the prime were perceived more frequently, and evoked widespread N400-like effect in the event-related brain potentials (ERPs). An ERP effect of the same polarity was obtained for illusory words, however, its latency was delayed and the topographical distribution was restricted to left posterior electrode positions. These differences suggest that priming might affect real and illusory words at different levels of word processing: access to real words is facilitated at a semantic level, whereas lexical activation apparently accompanies the generation of illusory words.

Adult↗
PubMed2004

Direct defense or ecological costs: responses of herbivorous beetles to volatiles released by wild Lima bean (Phaseolus lunatus).

In response to feeding damage, Lima bean releases herbivore-induced plant volatiles (HIPV), which are generally assumed to attract carnivorous arthropods as an indirect defense. While many studies have focused on such tritrophic interactions, few have investigated effects of HIPV on herbivores. I used natural herbivores of wild Lima bean and studied their responses to jasmonic acid-induced plants in an olfactometer and in feeding trials. Both Cerotoma ruficornis and Gynandrobrotica guerreroensis (Chrysomelidae) significantly preferred control plants to induced ones in the olfactometer, and they avoided feeding on induced plants. In contrast, Curculionidae significantly preferred HIPV of the induced plant to those of the control in one plant pair and did not choose in the case of a second pair. In feeding trials, no choice occurred in the first plant pair, while control leaves were preferred in the second. Release of HIPV deterred Chrysomelid herbivores and, thus, acted as a direct defense. This may be an important addition to indirect defensive effects. Whether or not HIPV released by induced plants attracted herbivorous Curculionidae, thus incurring ecological costs, varied among plants. Such differences could be related to various HIPV blends released by individual plants.

Animals↗
PubMed2004

Automatic semantic activation is no myth: semantic context effects on the N400 in the letter-search task in the absence of response time effects.

The claim that semantic activation is an automatic process was recently called a myth, on the basis of the finding that if letter search is performed on a prime word, semantic priming effects on response time are eliminated, whereas repetition effects are preserved. The absence of semantic activation, however, cannot be validly inferred from the lack of response time effects, and converging evidence is needed. To this end, we examined the event-related potential correlate of priming, the N400 amplitude modulation, in a letter-search priming paradigm. Our experiment replicated the response time effects and demonstrated that the N400 amplitude successfully differentiates cross-case repetition priming, semantic priming, and neutral conditions. The results clearly indicate that the meaning of the prime word was processed and that semantic activation indeed was present in the letter-search task. The notion that semantic activation is an automatic process should not be abandoned prematurely.

Adult↗