Psychological distance to reward in monkeys

A concurrent-chains schedule was employed to examine crab-eating macaques' choice between segmented and unsegmented fixed-interval 30-s schedules. A stimulus change occurred during one of the fixed-interval schedules (segmented schedule), while no stimulus change occurred during the other (unsegmented schedule). The subjects were exposed to three consecutive stimulus-change conditions, in an attempt to explore whether correlation of the stimulus changes with a fixed interval of nonreinforcement affects the preference between the alternatives. When the stimulus change always occurred at a half point of the segmented schedule (the first and third conditions), all subjects preferred the fixed-interval schedule without stimulus changes. This replicated previous findings with pigeons and humans. On the other hand, when the intervals before and after the stimulus change varied on every entry to the segmented schedule with the sum of them being constant (the second condition), the subjects were indifferent to the alternatives. These results are consistent with the view that the preference for unsegmented schedules over segmented ones might be due to aversiveness which the initial component in the segmented schedule possesses as a result of its correlation with a fixed interval of nonreinforcement.     

Chemosensory behavior: the path from stimulus to response

In the past year, candidates have been identified for two long-sought classes of molecules, insect odorant receptors and mammalian taste receptors. In addition, genes directing receptor gene expression and the development of specific chemosensory neurons have been described in Drosophila melanogaster and Caenorhabditis elegans. Finally, recent physiological experiments have provided new insights into the mechanisms by which chemosensory information is processed.     

Zebrafish larvae use stimulus intensity and contrast to estimate distance to prey

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Dynamic integration of reward and stimulus information in perceptual decision-making

In perceptual decision-making, ideal decision-makers should bias their choices toward alternatives associated with larger rewards, and the extent of the bias should decrease as stimulus sensitivity increases. When responses must be made at different times after stimulus onset, stimulus sensitivity grows with time from zero to a final asymptotic level. Are decision makers able to produce responses that are more biased if they are made soon after stimulus onset, but less biased if they are made after more evidence has been accumulated? If so, how close to optimal can they come in doing this, and how might their performance be achieved mechanistically? We report an experiment in which the payoff for each alternative is indicated before stimulus onset. Processing time is controlled by a “go” cue occurring at different times post stimulus onset, requiring a response within msec. Reward bias does start high when processing time is short and decreases as sensitivity increases, leveling off at a non-zero value. However, the degree of bias is sub-optimal for shorter processing times. We present a mechanistic account of participants'' performance within the framework of the leaky competing accumulator model [1], in which accumulators for each alternative accumulate noisy information subject to leakage and mutual inhibition. The leveling off of accuracy is attributed to mutual inhibition between the accumulators, allowing the accumulator that gathers the most evidence early in a trial to suppress the alternative. Three ways reward might affect decision making in this framework are considered. One of the three, in which reward affects the starting point of the evidence accumulation process, is consistent with the qualitative pattern of the observed reward bias effect, while the other two are not. Incorporating this assumption into the leaky competing accumulator model, we are able to provide close quantitative fits to individual participant data.     

The orienting response: stimulus factors and response measures

This paper outlines some of the basic ideas of the orienting response (OR) that have developed from the classical writings of E.N. Sokolov, in particular the effects of stimulus novelty, intensity, and significance upon the OR, and predictions about these effects on a range of physiological measures traditionally associated with the OR. Such measures include the GSR, respiration, heart rate, vascular responses, EEG, and pupil diameter. Unfortunately, many of the predictions of classical OR theory do not hold up when such a fine-grain analysis is undertaken. Possible conceptualizations of the discrepancies between Sokolovian predictions and empirical data are considered--should we accept such discrepancies as merely reflecting the imperfect nature of many OR indices, or seek other regularizing principles? Preliminary Process Theory has been proposed as one alternative account of the existing data base, and similarities and differences between it and Sokolovian theory are explored. The need for further investigation of such problems, in the very foundations of OR theory, appears to be of fundamental importance to the future status of the OR. An appendix provides a discussion between Barry and Sokolov on some of these issues.     

From response to stimulus: adaptive sampling in sensory physiology

Sensory systems extract behaviorally relevant information from a continuous stream of complex high-dimensional input signals. Understanding the detailed dynamics and precise neural code, even of a single neuron, is therefore a non-trivial task. Automated closed-loop approaches that integrate data analysis in the experimental design ease the investigation of sensory systems in three directions: First, adaptive sampling speeds up the data acquisition and thus increases the yield of an experiment. Second, model-driven stimulus exploration improves the quality of experimental data needed to discriminate between alternative hypotheses. Third, information-theoretic data analyses open up novel ways to search for those stimuli that are most efficient in driving a given neuron in terms of its firing rate or coding quality. Examples from different sensory systems show that, in all three directions, substantial progress can be achieved once rapid online data analysis, adaptive sampling, and computational modeling are tightly integrated into experiments.     

Glycogen metabolism: the integrated cellular response to a bi-directional metabolic stimulus.

The control exerted by phosphorylase a over the I-conversion of glycogen synthase (1) does not apply to glycogen synthesis, as judged from experiments with intact human leucocytes. 1. In incubated leucocytes conversion of glycogen synthase (GS) (E.C.2.4.1.11) to GS-I is preceeded by inactivation of glycogen phosphorylase a (GPh-a). 2. By the addition of latex particles a flash-activation of GPh-a can be elicited within 10–30 sec. in the intact cells. 3. GPh is inactivated when a glucose load is given. 4. When, in glycogen depleted cells, glucose is given immediately after latex the I-conversion of GS is completely abolished, and GS-I remains low for 20 min although GPh is rapidly inactivated after the glucose is given. 5. However, glycogen synthesis as compared with a control in which only glucose is given, and in which a large I-conversion occurs, is not the least depressed. 6. This apparent contradiction is completely resolved taking into account a newly discovered, intermediate form of GS (2) (GS-R). 7. An important physiological role can be proposed for GS-R.     

Hippocampal representation of touch-guided behavior in rats: persistent and independent traces of stimulus and reward location

Understanding the mechanisms by which sensory experiences are stored remains a compelling challenge for neuroscience. Previous work has described how the activity of neurons in the sensory cortex allows rats to discriminate the physical features of an object contacted with their whiskers. But to date there is no evidence about how neurons represent the behavioural significance of tactile stimuli, or how they are encoded in memory. To investigate these issues, we recorded single-unit firing and local field potentials from the CA1 region of hippocampus while rats performed a task in which tactile stimuli specified reward location. On each trial the rat touched a textured plate with its whiskers, and then turned towards the Left or Right water spout. Two textures were associated with each reward location. To determine the influence of the rat's position on sensory coding, we placed it on a second platform in the same room where it performed the identical texture discrimination task. Over 25 percent of the sampled neurons encoded texture identity--their firing differed for two stimuli associated with the same reward location--and over 50 percent of neurons encoded the reward location with which the stimuli were associated. The neuronal population carried texture and reward location signals continuously, from the moment of stimulus contact until the end of reward collection. The set of neurons discriminating between one texture pair was found to be independent of, and partially overlapping, the set of neurons encoding the discrimination between a different texture pair. In a given neuron, the presence of a tactile signal was uncorrelated with the presence, magnitude, or timing of reward location signals. These experiments indicate that neurons in CA1 form a texture representation independently of the action the stimulus is associated with and retain the stimulus representation through reward collection.     

Determination of the number of conserved chromosomal segments between species

Genomic divergence between species can be quantified in terms of the number of chromosomal rearrangements that have occurred in the respective genomes following their divergence from a common ancestor. These rearrangements disrupt the structural similarity between genomes, with each rearrangement producing additional, albeit shorter, conserved segments. Here we propose a simple statistical approach on the basis of the distribution of the number of markers in contiguous sets of autosomal markers (CSAMs) to estimate the number of conserved segments. CSAM identification requires information on the relative locations of orthologous markers in one genome and only the chromosome number on which each marker resides in the other genome. We propose a simple mathematical model that can account for the effect of the nonuniformity of the breakpoints and markers on the observed distribution of the number of markers in different conserved segments. Computer simulations show that the number of CSAMs increases linearly with the number of chromosomal rearrangements under a variety of conditions. Using the CSAM approach, the estimate of the number of conserved segments between human and mouse genomes is 529 +/- 84, with a mean conserved segment length of 2.8 cM. This length is <40% of that currently accepted for human and mouse genomes. This means that the mouse and human genomes have diverged at a rate of approximately 1.15 rearrangements per million years. By contrast, mouse and rat are diverging at a rate of only approximately 0.74 rearrangements per million years.     

On the effects of a constant sub-threshold conditioning stimulus upon the response to a constant current test stimulus

The strength of a test stimulus, which is just adequate to produce a response after a sub-threshold conditioning stimulus, is calculated on the basis of Rashevsky's two-factor theory as a function of the strength and duration of the conditioning stimulus. The results are compared with available data for one of a family of curves and found to be in satisfactory agreement.     

Developmental and genotypic differences in the response of pea stem segments to auxin

The objective of this investigation was to examine the response to exogenous auxin (indole-3-acetic acid; IAA)of stem segments at two developmental stages. The standard auxin response of excised stem segments and intact plants consists of an initial growth response and a prolonged growth response. We found that this biphasic response does not occur in internodes at very early stages. Stem segments of light grown pea of various genotypes were cut when the fourth internode was at 6–13% of full expansion (early-expansion) or at 18–25% of full expansion (mid-expansion). Length measurements of excised segments were made after 48 hours of incubation on buffer with or without auxin. An angular position transducer linked to a computerized data collection system provided high-resolution measurement of growth of stacks of segments incubated in buffer over 20 hours. Early-expansion segments of all genotypes deviated from the standard auxin response, while mid-expansion segments responded in a manner consistent with previous reports. Early-expansion segments of tall, light-grown plants were unique in showing an auxin-induced inhibition of growth. The auxin-induced inhibition correlated with high endogenous auxin content, as determined by HPLC and GC/MS, across genotypes and between early-expansion and mid-expansion segments of tall plants. Measurement of ethylene evolved from stem segments in response to auxin, and treatment of segments with the ethylene action inhibitor, norbornadiene, showed the inhibition to be mediated in part by heightened ethylene sensitivity. Growth of early-expansion segments of dwarf and severe dwarf plants was stimulated by exogenous auxin, but the growth rate increase was delayed compared to that in mid-expansion segments. This is the first time that such a growth response, termed the delayed growth response has been emonstrated. It is concluded that developmental stage and endogenous hormone content affect tissue response to exogenous auxin.     

In vitro response of rat gastrointestinal segments to cholecystokinin and bombesin

In previous studies of the rat gastrointestinal (GI) tract, we have demonstrated specific binding of cholecystokinin (CCK) to the pylorus and of bombesin (BN) to the gastric fundus, gastric antrum, duodenum, and ileum. We now present the results of an investigation of the in vitro response of the same regions of the rat GI tract to CCK-8 (the active octapeptide of CCK) and BN. Sections of rat fundus, antrum, pylorus, duodenum, and ileum were suspended in a Tyrode buffer and attached to an isometric pressure transducer in a longitudinal orientation. Dose-response curves to CCK-8 and BN were generated for each tissue. CCK-8 consistently induced a change only in pylorus, while BN induced a response from fundus, antrum and duodenum. With the exception of the lack of ileal response to BN, the regions of the rat GI tract which biologically respond (i.e., contract or relax) to CCK-8 or BN were the same regions in which we have located BN and CCK-8 binding sites. This correlation supports the hypothesis that GI function is modified by specified hormone-receptor interactions with these peptides.     

Cardiodynamic response to psychological and cold pressor stress: Further evidence for stimulus response specificity and directional fractionation

In the present study 36 police officers were exposed to a psychological stressor (IQ quiz) and to cold pressor stress while several cardiovascular variables were monitored. Impedance cardiography was used to provide measures of heart rate, stroke volume, cardiac output, myocardial contractility, and total peripheral resistance. In addition, measures of systolic and diastolic blood pressure and peripheral skin temperature were obtained. A multivariate analysis of variance (MANOVA) indicated that significant increases in diastolic and systolic blood pressure during the cold pressor test were mediated by large increases in total peripheral resistance, whereas blood pressure elevation during the IQ quiz were accompanied by significant increases in heart rate and, to a lesser extent, cardiac output. Peripheral skin temperature decreased in response to each stressor. Additional analysis indicated a degree of stimulus specificity for several variables. For example, diastolic blood pressure showed greater increases to cold pressor than quiz, whereas systolic blood pressure increased more with the psychological than the physical stressor. Directional fractionation occurred for both myocardial contractility and cardiac output.     

Cardiodynamic response to psychological and cold pressor stress: further evidence for stimulus response specificity and directional fractionation

In the present study 36 police officers were exposed to a psychological stressor (IQ quiz) and to cold pressor stress while several cardiovascular variables were monitored. Impedance cardiography was used to provide measures of heart rate, stroke volume, cardiac output, myocardial contractility, and total peripheral resistance. In addition, measures of systolic and diastolic blood pressure and peripheral skin temperature were obtained. A multivariate analysis of variance (MANOVA) indicated that significant increases in diastolic and systolic blood pressure during the cold pressor test were mediated by large increases in total peripheral resistance, whereas blood pressure elevation during the IQ quiz were accompanied by significant increases in heart rate and, to a lesser extent, cardiac output. Peripheral skin temperature decreased in response to each stressor. Additional analysis indicated a degree of stimulus specificity for several variables. For example, diastolic blood pressure showed greater increases to cold pressor than quiz, whereas systolic blood pressure increased more with the psychological than the physical stressor. Directional fractionation occurred for both myocardial contractility and cardiac output.     

Involvement of neuromedin S in the oxytocin release response to suckling stimulus

We recently identified neuromedin S (NMS) from the rat hypothalamus as an endogenous ligand for the FM-4/TGR-1 receptor distinct from neuromedin U. In the present study, we examined the role of NMS in the oxytocin release response to suckling stimulation by rat pups. Intracerebroventricular (icv) injection of NMS induced cFos expression in the paraventricular nucleus and supraoptic nucleus. Double immunohistochemical analysis revealed induction of cFos expression in a proportion of oxytocinergic neurons in both nuclei. In addition, icv injection of NMS stimulated oxytocin release dose-dependently in intact rats, and increased milk secretion in lactating rats. On the other hand, icv injection of anti-NMS antiserum into lactating rats significantly suppressed suckling-induced milk ejection. These results suggest that, in the rat, endogenous NMS plays an important role in the oxytocin release response to the suckling stimulus.