Gradient Of Inhibition Research Articles

Distractor-target interactions during directed visual attention

The spatial extent of directed visual attention (DVA) was examined in a series of experiments using precuing in a suprathreshold luminance detection (reaction time) paradigm. Previous findings (Hughes, H. C. and Zimba, L. D. J. Exp. Psychol.; Human Percept Perf., 1985, 11, 409-430) indicated that, in an empty visual field, the effects of DVA were primarily manifest as a uniform elevation of response times to all probe targets in the hemifield contralateral to the observer's expectancy. The present experiments were designed to determine whether increased spatial selectivity could be found when luminous markers indicated the exact location of the expected visual target. To maintain equivalent states of adaptation in both hemifields, luminous markers were also present at the same location in the contralateral hemifield. In general, hemifield effects were again obtained, but with two notable exceptions. First, marking locations in the unattended hemifield produced a local increase (enhanced interference) in RTs above the level characteristic of other locations within that hemifield. Second, when multiple locations were indicated with identical luminous markers, graded costs were obtained in both hemifields. However, scaling the markers according to estimates of cortical magnification factor (M) substantially reduced the slope of these inhibitory gradients, and the results once again approached those characteristic of an unstructured visual field. The findings suggest that when attention is directed to a marked location along the horizontal meridian, a transition in performance typically occurs at the vertical meridian. In addition, irrelevant stimuli some distance from the attentional focus interfere with detection times to unexpected targets that appear in the same vicinity. This interference may relate to an enhanced susceptibility to spatial interactions between the distractors and target away from the attentional focus. The interference appears to extend over a constant area of visual cortex, since it is reduced when the markers are M-scaled.

Quantitative analysis of an inhibitory gradient field in the hydrozoan stolon.

An inhibitory field which emanates from the mobile tips of elongating stolons of colonial hydroids has been identified and analyzed. It extends proximally with decreasing intensity for about 400-700 μm and ensures that branching sites occur at appropriate distances along the stolon. The local strengths of inhibition within the field have been measured with a new method which permits high temporal and spatial resolution. Kinetic studies reveal three characteristics. First, inhibition decays rather rapidly after removal of its source. The half life is about 30 min. Second, loss of inhibition immediately triggers initiation of future tip formation. Third, restoration and spreading of inhibition are slow processes which take 8-24 h to recover 90% of the original inhibitory levels.

Two determinants of the peak shift in human voluntary stimulus generalization

Two hundred and forty college students were divided into two groups, with training stimuli (from the brightness dimension) selected to produce small and large adaptation level shifts between discrimination training (to respond “same” to S+ and “different” to S—) and gen-eralization testing. These were further divided into three groups with discriminations expected to yield positive (toward brighter values), negative (toward dimmer values), or zero post-discrimination peak shifts. Half the subjects received brief discrimination training while half received extended training. A further group of 60 subjects were given exposure to the stimuli comparable to that of the extended training subjects, but were asked to rate the stimuli instead of being given discrimination training. The results indicated that two independent, additive sources of shift were active. One source, occurring in all groups, was interpreted as being due to the change in adaptation level from training to test. The other source of shift, occurring only in the groups with extended discrimination training, was interpreted as due to the establishment of asymmetrical decision criteria; the more traditional interpretation in terms of the interaction between excitatory and inhibitory gradients of response strength was also considered.

Signalled Reinforcement and Dimensional Stimulus Control

Summary For four pigeons reinforcement was signalled in one component of a multiple variable-interval 1-min variable-interval 1-min schedule, and two pigeons were trained on a multiple variable-interval 1-min extinction schedule. Following training each bird was given a resistance-to-extinction generalization test, followed by an additional training session, and a final resistance-to-reinforcement generalization test. Test stimuli were combined with presence or absence of the signal. Generalization tests uniformly showed differential dimensional stimulus control in the presence and absence of the signal for three of the four signalled-reinforcement subjects. However, excitatory dimension stimulus control was obtained for the one signalled-reinforcement bird that did not respond differentially during training. Similarly, nondifferential, flat, or inhibitory gradients were obtained for the two extinction subjects. These data support an analysis of signalled reinforcement as a conditional discrimination.

INHIBITORY STIMULUS CONTROL FOLLOWING ERRORLESS DISCRIMINATION LEARNING1

Three generalization procedures were used to investigate inhibitory stimulus control following discrimination learning with few errors. Three groups of pigeons acquired a discrimination between a green stimulus (the positive stimulus) and a vertical or horizontal line (the negative stimulus) through differential autoshaping followed by multiple schedule presentation of the two stimuli with gradually increasing stimulus durations. Genereralization testing was along a line-tilt continuum. For one group, the test involved a resistance-to-reinforcement procedure in which responses to all line tilts were reinforced on a variable-interval schedule. For a second group, also tested with the resistance-to-reinforcement procedure, the lines were superimposed on the green field that formerly served as the positive stimulus. A third group was tested in extinction with the combined stimuli. Control groups had no discrimination training but responding to green was nondifferentially reinforced. The control subjects responded more to all line tilts during testing than did the comparable experimental subjects, indicating that the negative stimulus had become an inhibitory stimulus. Both resistance-to-reinforcement groups revealed inhibitory gradients around the negative stimulus, but the gradient for the extinction group was relatively flat. These data are consistent with others that modify Terrace's early conclusion concerning the failure of inhibition to develop during errorless training.

GENERALIZATION OF EXCITATION AND INHIBITION AFTER DIFFERENT AMOUNTS OF TRAINING OF AN AVOIDANCE BASELINE1

After acquisition of a treadle-pressing response maintained by an avoidance contingency, four groups of pigeons received interdimensional discrimination training. For two groups, the positive stimulus was a 1000-Hertz tone correlated with the avoidance schedule and the negative stimulus was noise correlated with extinction. The discriminative stimuli were reversed for the other two groups. For two groups, the test stimuli were presented during extinction without any shocks during the test stimuli, for the other two groups, an unavoidable shock was presented during each test stimulus. Generalization was measured daily during discrimination training by randomly substituting each of six test frequencies for the 1000-Hertz tone. When the 1000-Hertz tone was correlated with the avoidance schedule, the number of responses to it increased and the excitatory gradient became steeper as a function of the amount of training. When the 1000-Hertz tone was associated with extinction, the number of responses to it decreased as a function of days of training and the inhibitory gradient became slightly steeper, provided that responding to the test stimulus was elevated by unavoidable shock. The training effects parallel those obtained with positive reinforcement.

The budding region as source of diffusible inhibitors of head and foot regeneration inHydra viridis.

An implication of Crick's (1970, 1971) "source-sink" model of diffusion gradients is that the addition of sources along the length of a gradient would cause it to bulge away from linearity in the direction of the source. The activity of the budding region ofHydra viridis as a source of the diffusible inhibitors of head and of foot regeneration is investigated in this light, by placing multiple series of budding regions along the length of the inhibitory gradients of head and of foot regeneration previously described (Shostak, 1972, 1973).Samples of 30-50 animals were used to determine the frequencies of head and of foot regeneration at each graft border formed by grafting 2 to 5 gastric-plus budding regions in tandem, the distal one having a terminal apical head, and the proximal one part of a host animal having terminal basal peduncle and foot. These frequencies were compared to the corresponding frequencies at appropriate distances along the lengths of the inhibitory gradients, as computed from the linear equations for these gradients based on earlier work. The curves for the regeneration of heads and feet for animals with three or fewer additional budding regions deviate in the direction of greater inhibition, but do not differ significantly from the gradients on animals having only grafted gastric regions. The curves for animals with four additional budding regions, however, bulge out toward greater inhibition, and differ with statistical significance from the linear inhibitory gradients at the fourth graft border.The results show, therefore, that the budding region is a source of both the inhibitors of head and of foot regeneration also produced by the head and foot, respectively. The suggestion arises that the diffusible inhibitors produced by the morphogenetically active regions ofHydra have no effect on the normal homeostatic processes and budding occurring in these regions, but normally prevent regeneration that might otherwise occur during cellular turnover.

Postdiscrimination generalization as a function of testing procedure: Steep inhibitory gradients

Three pigeons received interdimensional discrimination training to criterion. With responses to other test stimuli always reinforced during testing, gradients around S+ were inverted v-shaped (exictatory) when S+ responding was reinforced, steep and v-shaped (inhibitory) when S+ responding was nonreinforced, and very steep and v-shaped when S+ responding was both nonreinforced, and punished with a 7-sec S- presentation.

Bipolar Inhibitory Gradients' Influence on the Budding Region ofHydra viridis

A head inhibitory gradient arising in the vicinity of a hydra's head, and a foot inhibitory gradient arising from the animal's foot are thought to intersect in the budding region. To investigate the possible influences of these gradients on budding, samples were prepared of 25 or more animals having as many as five gastric regions or gastric-plus-budding regions grafted in tandem. The segments of grafted animals were considered equivalent except for distance from the head and foot. The average frequencies of regeneration of budding regions on grafted gastric regions, and the average number of buds on grafted budding regions were determined for each sample. Both of these variables changed as functions of distance from the head in U-shaped curves as the hypothesis predicts. The arm of the U closest to the head indicates a stronger degree of inhibition than the arm closest to the foot since there are less regeneration and fewer buds close to the head. Evidence for segments on which ectopic heads regenerated also shows inhibition of budding due to a head. The slopes of the arms of the U-shaped curves do not resemble the slopes of the curves for head and for foot inhibition, however, particularly since the slopes for budding inhibition do not decrease as a function of the number of grafted segments. It is doubtful, therefore, that the observed bipolar inhibition of the budding region is due to the inhibitors of head and foot formation.

Inhibitory gradients and behavioural contrast in rats with lesions of the fornix

Rats with lesions of the fornix showed behavioural contrast during the acquisition of a successive discrimination between white noise (S+) and a 670 Hz tone (S−). Inhibitory generalisation around S− was then tested with the resistance-to-reinforcement technique. The fornix-lesioned rats showed sharper inhibitory gradients than the controls. During the test their response rate rose from their old S− rate to their old S+ rate more slowly than did the controls', and was at the beginning of the test significantly lower than the controls'. The implication of these results for theories of hippocampal function was discussed.

Evidence of morphogenetically significant diffusion gradients in Hydra viridis lengthened by grafting

ABSTRACT Morphogenetic inhibitory gradients were produced in Hydra viridis having 2–5 gastric regions or 2–5 peduncles grafted in tandem. The frequency of head regeneration at graft borders changed as a function of distance from the terminal head, and the frequency of foot regeneration changed as a function of distance from the terminal foot. The object of the study was to characterize and compare these morphogenetic inhibitory gradients in light of Crick’s (1970) model of diffusion gradients. Heads regenerate more easily on gastric regions and feet on peduncles. This is shown in several ways: (1) on gastric regions more heads and fewer isolated tentacles and ‘spikes’ (isolated hypostomes) form, while the opposite is true on peduncles; (2) more heads are regenerated at graft borders between gastric regions than at those between peduncles at comparable distances from the terminal head, while more feet are regenerated at graft borders between peduncles than at those between gastric regions at comparable distances from the terminal foot; (3) the Y intercepts for gradients of the frequencies of head and of foot regeneration as functions of distance (which are the theoretical frequencies of regeneration at the ends of the animal) are higher for head regeneration on gastric regions than on peduncles, and for foot regeneration on peduncles than on gastric regions. This is explained in terms of relative differences in either competence to regenerate or sensitivity to inhibitors. The slopes of the frequencies of head and of foot regeneration as functions of distance from the respective ends change inversely with the numbers of gastric regions and peduncles grafted. This is explained in terms of changes in the distance between the sources of diffusible inhibitors and the sinks which use them up. The results are taken to support Crick’s model in which a linear gradient of a diffusible substance is produced between its source and sink.

Inhibitory gradients of head and foot regeneration in Hydra viridis

The existence and characteristics of antiparallel inhibitory gradients of head and foot morphogenesis were examined in grafted Hydra viridis. The frequencies of regeneration at the borders between grafted pieces was employed as an assay for morphogenesis and, conversely, for inhibition. Four types of experiments were performed: (1) graft borders were formed either with or without lengthening animals by the addition of regions; (2) the sources of the hypothetical inhibitors were removed; (3) additional sources were supplied; (4) additional time was allowed in making three part composite animals between grafting the first two pieces together and adding the third piece. Inhibitory gradients affecting the ability of graft borders to regenerate heads, budding regions, and feet appeared when animals were lengthened to a total of three gastric regions or three peduncles. The lengths of the animal's body performing the function of the “sinks” in Crick's (1970) formulation of gradients exceed Crick's prediction. The possibility that the animal's terminal head is the sole source of head inhibition and that the animal's terminal foot is the sole source of foot inhibition appears to be overruled by the evidence from the ablation and addition experiments. The experiments involving delaying the addition of a piece showed that the gradients are stable, and that the time for the inhibition of regeneration to become irreversible is only a fraction of the time for regeneration to become determined.

CONTRAST AND STIMULUS GENERALIZATION FOLLOWING PROLONGED DISCRIMINATION TRAINING1

Different groups of pigeons received discrimination training in which the reinforcement-associated and extinction-associated stimuli were respectively either (a) a line tilt vs a blank key, (b) a blank key vs a line tilt, or (c) two different line tilts. The high response rates that developed to the positive stimulus in all groups during discrimination learning were maintained over 64 sessions of training. After these sessions, all subjects were tested for stimulus generalization along the line-tilt dimension. Gradients of relative (per cent) generalization around the stimulus associated with reinforcement (so-called excitatory gradients) and around the stimulus associated with extinction (so-called inhibitory gradients) were as steep as they typically are after much briefer training periods. These results do not support several of Terrace's predictions on the basis of the hypothesis that emotional responses develop to the stimulus associated with extinction during discrimination learning with errors, but eventually dissipate after extended training.

Generalization and discrimination of shape orientation in the pigeon.

Pigeons learned to peck a green key on which parallelogram-shapes were projected; they then received generalization tests in which the orientation of the parallelogram was varied. Nondifferential training produced very little eventual stimulus control along the orientation dimension, but when training included S- trials (absence of the parallelogram) subjects responded consistently more to certain orientations than to others. Unlike typical results for visual generalization (e.g., line-tilt), the tilt gradients obtained for this complex stimulus were bimodal, supporting predictions on the basis of human perceptual data. However, unimodal gradients could be produced by specific discrimination training along the orientation dimension. Other forms of intradimensional training also produced relatively steep gradients, often characterized by unexpected but consistent secondary peaks. An attempt to obtain inhibitory gradients (S+: green key; S-: parallelogram on a green background) resulted in virtually zero responding all along the shape-orientation dimension; therefore, specific inhibitory control could not be evaluated. All these experiments suggest that definition of this complex stimulus dimension in terms of mere "angular orientation" is inappropriate, and alternative interpretations are discussed.

Generalization gradients of inhibition following auditory discrimination learning.

A more direct method than the usual ones for obtaining inhibitory gradients requires that the dimension of the nonreinforced stimulus selected for testing be orthogonal to the dimensions of the reinforced stimulus. In that case, the test points along the inhibitory gradient are equally distant from the reinforced stimulus. An attempt was made to realize this condition by obtaining inhibitory gradients along the frequency dimension of a pure tone after discrimination training in which the nonreinforced stimulus was a pure tone (or tones), and the reinforced stimulus was either white noise or the absence of a tone. The results showed that some degree of specific inhibitory control was exerted by the frequency of the tone, although the gradients were broad and shallow in slope.A further experiment was conducted to see whether the modification of an excitatory gradient resulting from training to discriminate neighboring tones could arise from a simple interaction of inhibitory and excitatory gradients. The results indicated that it could not, since discrimination training produced a concentration of responding in the vicinity of the reinforced stimulus which cannot be derived from any plausible gradient of inhibition.

The Shadow Reaction of Diadema Antillarum Philippi

ABSTRACT Light is shown to suppress the shadow reactions of Diadema spines to a degree which varies with its intensity. Inhibition can occur when there is spatial separation between the areas shaded and illuminated in the radial nerve and skin. The degree of inhibition is affected by the position and size of the area lighted. In both skin and radial nerves, uniform meridional gradients of inhibition are found, inhibition being maximal when the areas lighted and shaded are near or coincide, decreasing as these areas are moved apart. The effect of light may be reversed when it is projected at more than a critical distance from the shadow. Gradients in the skin which run parallel to the ambitus, show maximal inhibition at the ambulacral margins, so that the inhibitory gradient corresponds with that of sensitivity to shadows. Interaction between excitation and inhibition may occur in the radial nerve or at the periphery and there are several pathways for excitation and inhibition. The findings are discussed in relation to ‘off’ effects and receptor fields in retinae.