Behavioural and electrophysiological analysis of strabismus in cats: modern context.

We studied the effects of paralytic strabismus on visual behaviour and binocularity of cortical visual mechanisms by immobilization of one eye in adult cats. Visual discrimination abilities of the immobilized eye were significantly diminished despite extensive training with one or both eyes. The deficits are not caused by the immobilization itself but appear to reflect an adaptive mechanism to deal with double vision. The deficits with the immobilized eye persisted even after section of the optic chiasm, which effectively removes the direct cortical competition of the two eyes. Single-cell electrophysiological recordings showed that cortical cellular responses are modified by the immobilization, with loss of binocularity in some cells and shifting of receptive fields of other cells that continued to respond to both eyes.

Why do birds sleep with one eye open? Light exposure of the chick embryo as a determinant of monocular sleep.

Together with some aquatic mammals, birds exhibit a unique behavioral and electrophysiological state called "unihemispheric sleep," in which one cerebral hemisphere is awake and the other is sleeping. Slow-wave sleep in one hemisphere is associated with closure of the contralateral eye, while the eye contralateral to the awake hemisphere is open; closure of both eyes, in contrast, is associated with bihemispheric slow-wave sleep or with REM sleep. During the last few days of incubation, the chick's embryo is turned in the egg so that it occludes its left eye, whereas light entering through the shell can stimulate the right eye. Here we show that in the first two days after hatching, chicks coming from eggs incubated in the light prevalently slept with their right eye open, whereas those coming from eggs incubated in the dark prevalently slept with their left eye open. Thus, asymmetric light stimulation in the embryo can modulate the left-right direction of eye opening during post-hatching monocular sleep.

Four paradigms to study visual--spatial attention of myopic subjects.

We describe four protocols for evaluating the attentional skills of myopic and control subjects in target stimulus detection tasks; simple reaction times (RT) are recorded. Two protocols are designed to study both automatic and voluntary orienting of attention. Modified implicit orienting paradigms [Q. J. Exp. Psychol. 32A (1980) 2; J. Exp. Psychol. Gen. 109 (1980) 160] are used in which cues elicit a shift of attention but gaze is maintained on a central fixation point. A third paradigm is designed to study the time-course of focusing; two circles (focusing cues) of different diameters are randomly presented on a point of the visual space where attention has been previously shifted. Seven SOAs (stimulus onset asynchrony) are used. The fourth paradigm was designed to evaluate visual search; three circular arrays of four, eight and twelve stimuli are randomly presented around a fixation point and subjects have to detect the target stimulus inside one of the circles (the other circles are distracters). Since some attentional deficits are associated with myopia [Invest. Ophthalmol. Vis. Sci. 38 (1997) 1725; Cogn. Brain Res. 8 (1999) 369], these protocols could be useful both in the detection of deficits in subjects on the verge of becoming myopics and in the development of cognitive training programs to reduce attentional deficits.

Orienting of visual attention in dyslexia: evidence for asymmetric hemispheric control of attention.

The control of attentional orienting was studied in children with specific reading disorder (SRD) or dyslexia, and it was compared with that of normal readers. We used the covert orienting paradigm to measure subjects' reaction times for target detection both in valid and invalid cue conditions, either in the left or in the right visual fields. In experiment 1, we investigated exogenous orienting. The cue consisted of a peripheral abrupt onset and the cue-target delay was 350 ms. As compared with normal readers, in dyslexics the cue effect was absent in the right visual field, whereas in the left visual field a greater cue effect was observed. No visual field asymmetry was found in normal readers. In experiment 2, we investigated endogenous orienting. The cue was shown centrally and the cue-target delay was 750 ms. In dyslexics and normal readers, orienting of attention was present in both visual fields. However, in the invalid condition, dyslexic children showed significantly slower reaction times in the left visual field than in the right visual field. These results were interpreted as being due to an asymmetric control of visual spatial attention, possibly related with a posterior attention mechanism deficit in the right parietal cortex and/or an interhemispheric dysfunction and/or an impairment of cerebellar functions.

Can expectancy influence hemispheric asymmetries?

We carried out three experiments with the aim of verifying a critical assumption of Kinsbourne's (Acta Psychol., 33 (1970), 193-201; Attention and Performance V, London: Academic press, (1975), pp. 81-96) 'dynamic' attentional hypothesis of hemispheric asymmetries, namely, that asymmetries arise only when subjects know in advance what type of stimulus and/or cognitive mode they are about to be engaged with. We used a paradigm modified from Posner (J. Exp. Psychol., 109 (1980), 160-174) to study the effects of non-spatial 'cognitive' cueing on hemispheric asymmetries using a lexical decision and a visuo-spatial discrimination task (acute vs. obtuse angles). While we did not find significant overall hemispheric asymmetries with the spatial material, we found a consistent advantage of the left hemisphere in the lexical decision task. In Experiment 2 where the cue was presented in central vision and only the stimuli were lateralised and in Experiment 3 where both cue and stimuli were lateralised to the same hemisphere, the left hemisphere advantage did not interact with the effect of cueing. In contrast, in Experiment 4, where only the cue was lateralised and the stimuli were centrally presented, the left hemisphere advantage in the lexical decision task emerged only following invalid cueing. While the results of Experiments 2 and 3 are not in keeping with Kinsbourne's hypothesis, the result of Experiment 4 shows that some pre-exposural mechanisms may indeed affect the emergence of hemispheric asymmetries. A differential susceptibility in 'disengaging' from the processing mode induced by an invalid cue might represent another interesting example of hemispheric difference.

Neuroendocrine responses to experimentally-induced psychological stress in healthy humans.

Previous studies of hormonal and neurophysiological changes in response to psychological stress in humans have produced contrasting findings due to differing experimental procedures and consistent individual variability. Habituation effects, which influence physiological coping in response to exposure to repeated stress, need to be investigated more extensively. In the present study, twenty healthy male subjects were each exposed twice to the same psychosocial stressor (Stroop Color Word Interference task, public speaking and mental arithmetic in front of an audience) during a first session (day 1) and a second session (day 8). Plasma concentrations of norepinephrine (NE), epinephrine (EPI), adrenocorticotropic hormone (ACTH), cortisol (CORT) and prolactin (PRL) were measured immediately before the beginning of the tests and at their end, 30 min later, on both experimental days. For the total group, NE, EPI, ACTH, and CORT levels were significantly elevated, and PRL levels were significantly decreased, after stress exposure on day 1. ACTH and CORT levels showed less significant increases after stress on day 8. In contrast, NE and EPI responses to stress were not significantly blunted, and PRL response was unchanged on day 8. Cluster analysis revealed two groups of subjects who showed different habituation patterns for ACTH and CORT. The first group (n=12) of subjects showed a reduction of ACTH and CORT responses to stress on day 8. The subjects of the second group (n=8) displayed a significant increase of ACTH and cortisol in response to stress on day 8, without any habituation effect. These results increase the evidence concerning the involvement of the HPA axis and catecholamines in response to psychological stress, and suggest that possible individual differences in the neuroendocrine coping mechanisms may affect mood regulation and the state of health.

Automatic and voluntary focusing of attention.

In this study, we investigate whether attentional focusing, like attentional orienting, comprises two independent mechanisms. We provide direct empirical evidence in favor of the existence of two mechanisms--one exogenous, or automatic, and one endogenous, or voluntary--that play a role in adjusting the size of the focus of attention. When a new object suddenly occurs in the visual field, the focus is first automatically fitted to it, and then an endogenous effort has to be exerted to maintain attention in the focused mode. Also, we provide evidence that voluntary focusing needs a perceptual object in order to operate.

Visual-spatial attention in developmental dyslexia.

Orienting and focusing of visual attention are two processes strictly involved in reading. They were studied in a group of dyslexic children and normal readers. Shifting of attention by both peripheral and central visual cues was studied by means of the covert orienting paradigm. Focusing, consisting in the ability to control the size of the attentional focus, was investigated using simple reaction times in central vision. Results showed that dyslexics had a specific disability in the shifting of attention caused by a peripheral cue at short SOAs, and were also able to maintain attention focused for short periods of time only, presumably not long enough for efficient visual processing. Our results support the suggestion that visual selective attention deficits in disabled readers may be due to a specific difficulty in orienting and focusing.

Visuospatial attention in myopia.

Four experiments were conducted to ascertain whether myopia is associated with deficits of visuospatial attention. In myopic and emmetropic control subjects, we studied: (1) automatic and voluntary orienting of attention, (2) focusing of attention and (3) performance on a visual search task. The results indicated that automatic orienting was defective in myopics and their performance in visual search was less efficient than that of controls. By contrast, myopics showed no deficits in voluntary orienting and in focusing.

Visual lateralization and monocular sleep in the domestic chick.

Behavioural sleep during the first 2 weeks of life was investigated in female chicks reared with an imprinting object or in social (visual) isolation. Binocular sleep tended to decrease and monocular sleep to increase with age in both rearing conditions. In chicks reared with an imprinted object. during the first week, monocular sleep with either right or left eye closure occurred with approximately the same frequency, except that on day 5 in which right eye closure dominated; during the second week, however, there was a clear bias towards more monocular sleep with left eye closure. During the second week, the pattern of monocular sleep was similar in both rearing conditions, but during the first week chicks reared with the imprinting object showed relatively more right eye closure compared to chicks reared without the imprinting object, an effect that might tentatively be associated with consolidation of imprinting memories in the left hemisphere. Binocular sleep occurred in all four body postures adopted by chicks during sleep: standing sleep, sleep with bill forward, sleep with bill on the ground, and sleep with head on the ground. Monocular sleep, in contrast, only occurred when chicks adopted the bill forward posture. When the colour of the imprinting object was suddenly changed on day 8, a striking shift towards predominant right eye closure during monocular sleep was observed. The same occurred when the imprinting object was suddenly removed from the home-cage on day 8, but not with other types of changes (i.e., when a novel different object was inserted into the home-cage or when a novel-coloured imprinting object was inserted into the home-cage together with the original one). It is argued that this phenomenon could be associated with right hemisphere involvement in response to novelty.

Binocular behavior of split-brain cats which have previously learned monocularly opposite visual discriminations.

Twelve adult split-brain cats were tested binocularly in visual tasks which had been previously learned monocularly in a two-choice paradigm. Eight experimental cats learned two opposite tasks with two eyes because contingencies of reinforcement changed with the open eye. Four control cats learned the same tasks but contingencies of reinforcement did not change with the open eye and therefore they learned the same problems with the two eyes. Thereafter, cats were submitted binocularly to the same tasks but in a free-choice paradigm. Experimental cats showed extinction of the discriminative response in 12 out of 16 binocular testings; in four the extinction criterion was not reached. In control cats no extinction behavior was observed in seven out of eight testings. It is suggested that extinction of the discriminative response in experimental cats could be caused by an inhibitory effect build-up because the two hemispheres attempted to control binocular behavior in opposite ways. Alternatively, these cats may develop a response alternative to discrimination in which one hemisphere takes the control of subcortical motor and/or attentional centers. In four testings no extinction was recorded for experimental cats and it is likely that control of those centers shifted from one hemisphere to the other every few trials.

Interaction between the hemispheres in split-brain cats.

Functional interactions between the two hemispheres were studied in adult split-brain cats. The aims were to assess whether monocular learning developed independently or that there were clues for interactions between the two sides of the brain during acquisition of opposite learning tasks. Experimental cats learned two visual pattern discriminations in which one pattern was positive for the right eye, whereas the other pattern was positive for the left eye. Control cats learned the same problems, but the same pattern was positive for both eyes. The open eye was changed from one session to the next in both groups of cats. In general, monocular performances of experimental cats were asymmetrical because they learned better and faster with one eye than with the other eye. Instead, no differences between the eyes were found in control cats. Statistical analysis of the data indicated that learning in experimental cats was significantly slower than learning in control cats, and that the difference between monocular performances was significantly greater for the experimental group than for the control group. The slower and asymmetrical monocular learning of experimental cats may reflect a conflict and a competition between the hemispheres for the control of learning behaviour, resulting in the dominance of one of them. Thus, some information about the stimuli must have been transmitted via the remaining interhemispheric connections. Symmetrical monocular learning of control group indicated that the competition for the control of behaviour was not present because there was no conflict between the hemispheres.

The internal horizontal cell of the frog: spatial summation.

The receptive field properties of internal horizontal cells (IHCs) were studied in the frog Rana pipiens. The space constant of each cell was determined by the use of stimulus spots of various sizes or by moving a light slit across the retina, varied from 100-500 microns by the former and 100-720 microns by the latter method. A similar range of values has been reported for Xenopus although its IHCs are much larger than the IHCs of Rana. Apparently, coupling among the IHCs is more efficient in the latter than in the former retina. The large range in values suggests substantial variation in coupling efficiency among cells of the same retina.

The internal horizontal cell of the frog: spatial summation.

The receptive field properties of internal horizontal cells (IHCs) were studied in the frog Rana pipiens. The space constant of each cell was determined by the use of stimulus spots of various sizes or by moving a light slit across the retina, varied from 100-500 microns by the former and 100-720 microns by the latter method. A similar range of values has been reported for Xenopus although its IHCs are much larger than the IHCs of Rana. Apparently, coupling among the IHCs is more efficient in the latter than in the former retina. The large range in values suggests substantial variation in coupling efficiency among cells of the same retina.

Interocular transfer of extinction of visual pattern discriminations in split-chiasm and split-brain cats.

The interocular transfer of the extinction of visual pattern discrimination was studied in cats with either section of optic chiasm (split-chiasm cats) or combined sections of chiasm and forebrain commissures (split-brain cats). Visual pattern discriminations were monocularly learned and their interocular transfer was assessed through the opposite eye. Then, learning was unilaterally extinguished and interocular transfer of extinction was tested on the other side. In split-chiasm cats, the total number of trials to extinction criterion (EC) was significantly lower with the second eye than with the first eye, indicating a successful interocular transfer. In split-brain cats, EC with the second eye was attained faster than with the first in some performances; it was similar with both eyes in other tasks, and with the second eye was higher than with the first in still other tasks. Statistical analysis applied to this group of split-brain cats pointed out that extinction performances with the two eyes were not significantly different. These findings suggest that interocular transfer of extinction was abolished in split-brain cats and that memory for extinction was unilaterally established in the absence of forebrain commissures.

Visual cells in the inferior colliculus of the cat.

Extracellular recordings from 91 cells of the pericentral nucleus (ICP) of the inferior colliculus of the cat revealed that 83 of them were responsive to auditory stimuli and the other 8 to visual stimuli. All visually driven cells were binocular and showed large receptive fields located in the contralateral hemifield. The best stimulus was either a spot or a bar moved in any direction across the receptive field. No directional selectivity was found. It is suggested that the visual input to the ICP participates in an integrated reflex-orienting behavior, in which the visual information is important for the localization of the sound source.

The outer horizontal cell of the frog retina: morphology, receptor input, and function.

The outer horizontal cell (OHC) of the frog Rana pipiens was studied by light and electron microscopy of Golgi-stained and horseradish peroxidase-injected cells. Responses of OHCs were recorded with intracellular electrodes. The OHCs are probably axonless cells. Their dendritic terminals are lateral processes at synaptic ribbons and are involved frequently in reciprocal invaginating contacts with receptors, ie, contacts characterized by invagination of receptor membrane deeply into the horizontal cell process. Two classes of OHC were distinguished on the basis of size and receptor contacts. Small OHCs (dendritic area about 5,000 micron2) contact all classes of receptor. These cells were not successfully penetrated with microelectrodes. Giant OHCs (dendritic area about 30,000 micron2) apparently contact only blue-sensitive rods and red-sensitive cones. They generate chromaticity or C-type responses, hyperpolarizing to short and depolarizing to long wavelength light.

Perfect interocular transfer of visual pattern discriminations in split-chiasm cats trained with fading.

It has been reported that the interocular transfer of visual pattern discrimination is imperfect in split-chiasm cats. Perhaps the efficiency of callosal transfer is to some extent determined by the training procedure employed. To test that possibility, we trained 3 split-chiasm cats with a Classical Procedure used in interocular transfer studies (CP cats) and 4 split-chiasm cats with Fading (Fading cats). The Fading procedure consisted of the presentation of two stimuli which differed maximally at the beginning of training; through a series of gradual steps their difference was reduced to a minimal value required to control the desired discriminative behavior. Confirming previous studies, CP cats showed a drop in the discriminative performance when the viewing eye was changed from trained to untrained and they needed additional trials to reattain criterion with the untrained eye. Fading cats, however, showed no difference when the viewing eye was changed; criterion was immediately reached with the untrained eye. It is suggested that the perfect interocular transfer shown by the Fading paradigm represents efficient callosal transmission and stabilized, well organized memory traces.

The internal horizontal cell of the frog. Analysis of receptor input.

The inner horizontal cell (IHC) of the retina of Rana pipiens was studied by light and electron microscopy of Golgi stained or horseradish peroxidase injected cells. Responses of IHCs were recorded with intracellular electrodes. Both axon and dendritic terminals of the IHC make synaptic contact with all classes of receptor. The terminals occur as lateral or medial processes at synaptic ribbons. Reciprocal invaginations of receptor into horizontal cell process are common. Different classes of receptor are contacted in about the proportions with which they occur in the retina. No tendency for contacts of one type of receptor to occur on a particular part of the cell was found. The IHC generates L-type S-potentials. Both rod and cone input is evident in the waveform of the response, its spectral sensitivity, and the effect on it of adaptation.