Separate Neural Mechanisms Research Articles

Multiple mechanisms of visual-spatial attention: recent evidence from human electrophysiology

Natural visual scenes contain vast quantities of information—far more than the visual system can process in a short period of time-and spatial attention is therefore used to focus the visual system's processing resources onto a subset of the incoming visual information. Most psychological theories of attention posit a single mechanism for this focusing of attention, but recent electrophysiological studies have provided evidence that the visual system employs several separable neural mechanisms of spatial attention. This paper describes the evidence for multiple attentional mechanisms and suggests links between these neurophysiologically defined mechanisms and specific functional processes that have been proposed in psychological theories of attention.

Error accumulation and error correction in sequential pointing movements.

Human subjects pointed, without seeing their arm, at visual targets presented in repeated sequences in a frontal plane. Required movement direction could change within the sequence by 0, 45, 90, 135 or 180 degrees. Hand position was recorded contact-free in three dimensions (3D). From the recordings, the pointing errors towards each target were transformed into a Cartesian coordinate system with the x-axis representing the mean direction of all movements towards that target. We then investigated the relationship between successive errors by applying linear regression analysis separately to the three Cartesian error components. For the x-component, we found that successive errors were positively correlated throughout the experiment, which confirms our previous finding that errors in sequential pointing tend to accumulate (Bock and Eckmiller 1986; Bock et al. 1990). Correlation dropped by nearly 50% following a direction change of 90 degrees or more, suggesting that accumulation is reduced but not abolished by large changes in movement direction. The slope of the regression line averaged 0.6, which indicates the existence of a complementary trend towards error correction, contributing about 40% towards motor performance. Changes of movement direction affected slope and correlation in a closely similar way, suggesting that reduced accumulation is paralleled by increased correction. For the y- and z-components, we found that successive errors were positively correlated as well, but were not reduced following even large direction changes. This apparent discrepancy can be resolved by assuming separate neural mechanisms for amplitude and for direction control, differing in their sensitivity to direction changes.(ABSTRACT TRUNCATED AT 250 WORDS)

Two separate neural mechanisms of brightness induction

A particular rate of quantal absorption by photoreceptors may result in a dim or an intense percept, depending on light stimulating other parts of the retina. The brightness of an object in a natural scene, therefore, depends on the amount of light reflected from the object in comparison to light from other parts of the scene. We show this phenomenon is mediated by two separate neural mechanisms at distinct levels of the visual system. The first mechanism depends on retinal image contrast between adjacent regions. The second mechanism depends on the binocularly fused “cyclopian” representation and is influenced by more remote, noncontiguous areas of the visual field.

Kinesthetic acuity in humans with an emphasis on the fingers

A signal detection model of static-position and movement mechanisms involved in detecting angular displacements of joints will be presented along with experimental data. The model predicts that separate neural mechanisms exist for position and movement and the data indicate that the static-position mechanism is absent at the interphalangeal joints of the fingers. The issue of what consequences result from a lack of a static-position sense will then be explored with tests of position matching accuracy comparing joints with and without this sense. A metric for kinesthetic acuity using information transfer analysis was used from which an estimate of channel capacity could be made. These data indicate surprisingly poor kinesthetic acuity for the fingers with channel capacities from ca. 1.4 bits or 2.5 discriminable levels over a 90-deg range for the proximal interphalangeal joint of the index finger (no static-position sense) to 1.8 bits of 3.5 levels for the metacarpophalangeal joint (static-position sense present). A possible anatomical basis for this lack of a static-position sense at interphalangeal joints will be discussed along with speculations on why the fingers, where one might expect a very high kinesthetic acuity, have such a low acuity.

A single brain stem substrate mediates the motivational effects of both opiates and food in nondeprived rats but not in deprived rats.

Drug-naive and morphine-dependent rats both preferred places paired with morphine over unfamiliar neutral places. Morphine-dependent, but not naive, rats avoided places paired with the lack of morphine (i.e., withdrawal). Food-sated and food-deprived rats both preferred places paired with food over unfamiliar neutral places. Food-deprived, but not sated, rats avoided places paired with the lack of food (i.e., hunger). Lesions of the tegmental pedunculopontine nucleus (TPP) blocked the morphine- and food-conditioned place preferences in drug-naive and food-sated rats, respectively. TPP lesions failed to block morphine- and food-conditioned place preferences as well as morphine withdrawal-conditioned and hunger-conditioned place aversions in morphine-dependent and food-deprived rats, respectively. These results suggest that separate neural mechanisms subserve deprivation- and non-deprivation-induced motivation.

Capsaicin-sensitive vagal afferents and CCK in inhibition of gastric motor function induced by intestinal nutrients

The role of vagal afferent pathways and cholecystokinin (CCK) in mediating changes in gastric motor function after a meal was investigated in urethane-anesthetized rats. Proximal gastric motor function was measured manometrically, and nutrients were infused into an isolated segment of duodenum. Inhibition of gastric motility in response to duodenal infusion of protein (peptone or casein), but not carbohydrate (glucose), was significantly attenuated by administration of the CCK antagonist, L364,718. Selective ablation of vagal afferents by perineural treatment with the sensory neurotoxin, capsaicin, significantly reduced responses to both duodenal protein and glucose. These results suggest that protein in the duodenum decreases proximal gastric motor function via release of CCK and a vagal capsaicin-sensitive afferent pathway. In contrast, glucose acts via a capsaicin-sensitive vagal pathway not involving CCK. Thus separate neural and hormonal mechanisms mediate the effects of different nutrients in the duodenal feedback regulation of gastric motor function.

Sexual motivation: A neural and behavioural analysis of the mechanisms underlying appetitive and copulatory responses of male rats

Experiments investigating the neural mechanisms underlying the expression of masculine sexual behaviour are discussed in the context of the hypothesis set out by Frank Beach that suggested the existence of separate sexual arousal and performance mechanisms. The results indicate that the medial preoptic area is crucially involved in consummatory aspects of sexual behaviour: lesions and chemical manipulations of the area profoundly affect mounts, intromissions and ejaculation, but tend not to alter appetitive sexual responses. By contrast, ventral striatal dopamine-dependent mechanisms primarily affect appetitive sexual responses, measured in a variety of paradigms, but tend not to alter copulatory behaviour itself. Finally, associative mechanisms, for example those by which arbitrary environmental stimuli come to control appetitive sexual responses through their predictive association with sexual reinforcement, are shown to depend at least in part on interactions between the basolateral amygdala and dopamine-dependent events in the ventral striatum. Thus, diverse neural and behavioural procedures have revealed that separable neural mechanisms appear to be involved more or less selectively with different components of the male rat's sexual response system. It may still be useful to conceptualize separate sexual arousal and intromission/ejaculatory mechanisms when studying the neuroendocrine basis of sexual behaviour. However, a major challenge is to understand the way in which elements of the telencephalic limbic system, the striatum and preoptic area, some of which are targets for the action of sex steroids, interact to produce an integrated pattern of sexual behaviour.

Effects of progressive hypoxia on parasternal, costal, and crural diaphragm activation.

The distribution of motor drive to the costal and crural diaphragm and parasternal intercostal muscles was evaluated during progressive isocapnic hypoxia in anesthetized dogs. Bipolar stainless steel wire electrodes were placed unilaterally into the costal and crural portions of the diaphragm and into the parasternal intercostal muscle in the second or third intercostal space. Both peak and rate of rise of electromyographic activity of each chest wall muscle increased in curvilinear fashion in response to progressive hypoxia. Both crural and parasternal intercostal responses, however, were greater than those of the costal diaphragm. The onset of crural activation preceded that of the costal portion of the diaphragm and parasternal intercostal muscle activation. Despite differences in the degree of activation among the various chest wall muscles, the rate of increase in activation for any given muscle was linearly related to the rate of increases for the other two. This suggests that respiratory drive during progressive hypoxia increases in fixed proportion to the different chest wall inspiratory muscles. Our findings lend further support to the concept that the costal and crural diaphragm are governed by separate neural control mechanisms and, therefore, may be considered separate muscles.

Lower esophageal sphincter function in the cat: role of central innervation assessed by transient vagal blockade.

Studies were performed on four cats to assess the role of extrinsic vagal innervation in the control of lower esophageal sphincter (LES) function. Both cervical vagal nerves were blocked transiently by cooling. LES pressure was measured using a multilumen manometry tube. LES relaxation was assessed during intraesophageal balloon distension in both the striated and smooth muscle portions of the esophagus. Bilateral vagal nerve blockade lowered the mean LES pressure from 58 +/- 17 to 29 +/- 9 mmHg (P less than 0.01). During vagal blockade, balloon distension in the striated muscle esophagus further reduced sphincter pressure to 16 +/- 4 mmHg (P less than 0.01) and that in the smooth muscle esophagus to 15 +/- 3 mmHg (P less than 0.01). Swallow-induced LES relaxation was abolished during bilateral vagal nerve blockade. During vagal blockade, atropine reduced LES pressure to 10 +/- 1 mmHg, phentolamine to 13 +/- 6 mmHg, and hexamethonium to 10 +/- 4 mmHg (all P less than 0.01). We conclude that 1) normal LES tone in the cat is mediated primarily by two separate neural mechanisms: a vagal cholinergic mechanism and a nonvagal mechanism that utilizes both alpha-adrenergic and cholinergic receptors; 2) local, intramural mechanisms of high threshold are present in the striated and smooth muscle cat esophagus to allow distension-induced reflex inhibition of the LES; and 3) swallow-induced LES relaxation is dependent on vagally mediated central nervous system connections.

An early and a late developing system for learning and retention in infant monkeys.

On the evidence that memory formation and habit formation represent two qualitatively different learning processes based on separate neural mechanisms, the functional development of these two processes was followed ontogenetically. Separate groups of rhesus monkeys of different ages were tested in delayed nonmatching-to-sample and 24-hr concurrent discrimination learning, considered to be measures of recognition memory and discrimination habit formation, respectively. The youngest group of infant monkeys failed to learn the nonmatching task until they were approximately 4 months old. With further maturation, learning ability on this task gradually improved, yet it did not reach adult levels of proficiency even at 1 year of age. Postlearning evaluation with long delays and lists confirmed this slow ontogenetic development of recognition memory to adult levels of function. By contrast, infant monkeys 3-4 months old learned to discriminate long lists of object-pairs about as quickly as adult monkeys despite the use of 24-hr intertrial intervals. This striking dissociation in the ability of infants on the two tasks closely resembles the dissociation first found in adult monkeys rendered amnesic by limbic lesions. The results suggest that whereas the nonlimbic habit system matures early in infancy, the limbic-dependent memory system develops only slowly.

Temporal properties of binocular mechanisms in the human visual system.

A dichoptic stimulation paradigm was used to determine the degree to which the two monocular images must match in terms of the temporal properties to yield facilitation in binocular grating detection. Several converging lines of evidence point to the existence of two separate neural mechanisms in binocular detection. One of these mechanisms is selective for temporal frequency and limited in its capacity to integrate information from the two eyes over time. The other mechanism is much less selective for temporal frequency and integrates over a longer period of time. At threshold these two separate mechanisms behave independently and exhibit similar degrees of binocular summation.

Comparison of ovarian and hypothalamic obesity syndromes in the female rat: effects of diet palatability on food intake and body weight.

Both ovariectomy and hypothalamic knife cuts produced hyperphagia and obesity in adult female rats. The ovarian obesity, however, unlike hypothalamic obesity, was virtually independent of diet palatability. Ovariectomized rats became obese on quinine-adulterated diets, which completely blocked hypothalamic obesity, and they displayed little further weight gain when given a high-fat diet, which greatly potentiated hypothalamic obesity. Ovarian and hypothalamic obesity were also found to be additive irrespective of diet condition when both surgical treatments were combined in the same animal; that is, ovariectomy increased the food intake and body weight of knife-cut animals given the quinine or high-fat diet. In contrast to their dissimilar feeding effects, ovariectomy, hypothalamic cuts, and the combined surgeries, did not differentially alter the aversion to a .01% quinine solution. The results indicate that ovarian obesity and hypothalamic obesity represent two different feeding disorders and are mediated by separate neural mechanisms. The functional nature of these disorders is discussed in light of recent body weight set point interpretations.

Quantitative studies of single-cell properties in monkey striate cortex. V. Multivariate statistical analyses and models

1. Several statistical analyses were performed on 205 S-type and CX-type cells which had been completely analyzed on 12 response variables: orientation tuning, end stopping, spontaneous activity, response variability, direction selectivity, contrast selectivity for flashed or moving stimuli, selectivity for interaction of contrast and direction of stimulus movement, spatial-frequency selectivity, spatial separation of subfields responding to light increment of light decrement, sustained/transient response to flash, receptive-field size, and ocular dominance. 2. Correlation of these variables showed that within any cell group, these response variables vary independently. 3. A multivariate discriminant analysis showed that orientation specificity, receptive-field size, interaction of direction and contrast specificity ocular dominance, and spontaneous activity, taken together can adequately assign cells into the S-type or CX-type subgroups. 4. Various models of visual cortex are examined in view of the findings reported here and in the previous papers of this series, which suggest that a) orientation and direction selectivities are produced by separate neural mechanisms, b) there may be hierarchy among simple (S type) cells, and c) complex (CX-type) cells appear to receive a prominent S-type cell input.

Quantitative studies of single-cell properties in monkey striate cortex. II. Orientation specificity and ocular dominance.

1. Quantitative analyses of orientation specificity and ocular dominance were carried out in striate cortex of the rhesus monkey. 2. Sharpness of orientation selectivity was greater for simple (S type) than for complex (CX type) cells. CX-type cells became more broadly tuned in the deeper cortical layers: S-type cells were equally well tuned throughout the cortex. 3. Sharpness of orientation selectivity for S-type cells was similar at all retinal eccentricities studied (0 degrees - 20 degrees from the fovea):in CX-type cells orientation selectivity decreased slightly with increasing eccentricity. 4. The orientation tuning of binocular cells was similar when mapped separately through each eye. 5. Orientation selectivity and direction selectivity are independent of each other, suggesting that separate neural mechanisms give rise to them. 6. More CX-type cells can be binocularly activated than S-type cells (88% versus 49%). The ocular dominance of S-type cells is similar in all cortical layers: for CX-type cells there is an increase in the number of cells in ocular-dominance category 4 in layers 5 and 6.

Hormonal modification of sexually dimorphic behavior

1. Female animals exposed to stimulation by testosterone in utero or very soon after birth may exhibit changes in adult mating behavior which are indicative of two independent types of modification. One is defeminization, and another quite different one is masculinization. The behavioral modifications are believed to represent changes in separate neural mechanisms, either of which can be altered without changing the other. The implication is, of course, that the CNS is originally bisexual with respect to its behavioral potential. 2. Independent changes which can be produced in males by eliminating the effects of endogenous testosterone during early development are demasculinization and/or defeminization. Here again, separate neural mechanisms are postulated. 3. In rodents and carnivores there are at least two types of behavior which are normally dimorphic in adulthood and are subject to modification by manipulation of androgenic stimulation during very early development. One type, represented by mating behavior, depends heavily upon concurrent hormonal stimulation in adulthood during the time it is being manifested. The other, represented by urination behavior in female dogs, is hormonally modifiable in utero but independent of gonadal hormones in adulthood. Female dogs genitally masculinized by testosterone pre- and neonatally exhibit male mounting responses as adults but only if they are injected with testosterone propionate before and during the time of testing. The same females display the male urination pattern as adults, but this behavior occurs without any concurrent androgenic stimulation and is in fact unaffected by testosterone injections. It would appear that certain sex differences in behavior which normally appear at the time of puberty are dependent upon both prenatal and pubertal hormonal action, whereas other differences may depend only upon hormonal action during development and emerge at the normal age of puberty without any additional endocrine facilitation.