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At least 793 records · Page 44Linked to original sources

PubMed1994

Visual repetition priming for words relies on access to the visual input lexicon: evidence from a dyslexic patient.

In this study we tested the hypothesis that visual repetition priming for words depends upon the accessibility of lexical units in the visual input lexicon. For this purpose, we investigated a dyslexic patient, A.M., whose neuropsychological performances suggested an impaired access to the lexical route of reading. According to the predictions, Experiments 1 and 2 demonstrated deficient priming in tests involving the visual presentation of words (Word Identification and Stem Completion). In Experiment 3, we demonstrated that A.M.'s deficient priming was specific for visually presented words, in that the auditory presentation elicited a normal priming effect (auditory Stem Completion). These data are discussed in the light of a theoretical framework suggesting a fractionation of the modalities by which repetition priming can be elicited, each mediated by a particular memory subsystem.

Attention↗

PubMed1995

Visual processing of looming and time to contact throughout the visual field.

We measured discrimination threshold for time to contact with a simulated approaching object at 20 locations between 0 and 32 deg eccentricity in the left, right, upper, and lower visual fields. We also measured discrimination threshold for rate of expansion at the same 20 locations. At 0 deg eccentricity, discrimination of trial-to-trial variations in time to contact was virtually unaffected by simultaneous trial-to-trial variations of both rate of expansion and starting size, discrimination of trial-to-trial variations in rate of expansion was virtually unaffected by simultaneous trial-to-trial variations of both time to contact and starting size, and discrimination of trial-to-trial variations in starting size was virtually unaffected by simultaneous trial-to-trial variations of both time to contact and rate of expansion. We conclude that, in foveal vision, time to contact, rate of expansion and size can be processed simultaneously, independently and in parallel. Our main finding was that this independence progressively decreased as eccentricity increased. For example, in peripheral, but not in foveal vision, variations in rate of expansion produced illusory variations in time to contact. A secondary finding was that the effect of eccentricity on discrimination threshold for the task-relevant variable (whether time to contact or rate of expansion) was considerably less than the effect of eccentricity on visual acuity and on several other aspects of visual performance. We suggest that visual processing of time to contact is developed by exposure to optic flow patterns created by self-locomotion.

Adult↗

PubMedDate not supplied

Use of visually evoked responses in evaluation of visual blurring in brain-damaged patients.

Pattern visual evoked potentials were recorded in brain-damaged patients who complained of fluctuation of vision causing visual blurring. Continuous prolonged pattern stimulation revealed marked variability of P100 amplitudes. In contrast, normal subjects and brain-damaged patients who did not complain of visual blurring showed stable P100 amplitudes. Fluctuation of vision thus seems to have an electrophysiological correlate in terms of P100 amplitude lability, which can be objectively assessed by prolonged continuous recording of pattern visual evoked potentials.

Adult↗

PubMed2004

Reliance on visual information after stroke. Part II: Effectiveness of a balance rehabilitation program with visual cue deprivation after stroke: a randomized controlled trial.

OBJECTIVE: To test the hypothesis that balance rehabilitation with visual cue deprivation improves balance more effectively than rehabilitation with free vision. DESIGN: Single-blind, randomized controlled trial. SETTING: Public rehabilitation center in France. PARTICIPANTS: Twenty patients with hemiplegia after a single-hemisphere stroke that occurred at least 12 months before the study. INTERVENTION: Patients were randomly assigned to 1 of 2 balance rehabilitation programs-with and without visual cue deprivation. In all other respects, the programs were identical. Each lasted for 1 hour and was implemented 5 days a week for 4 weeks. All patients completed the program. Mean outcome measures Balance under 6 sensory conditions was assessed by computerized dynamic posturography (EquiTest), gait velocity, timed stair climbing, and self-assessment of ease of gait before and after program completion. RESULTS: After completing the program, balance, gait velocity, and self-assessment of gait improved significantly in all patients. The improvements in gait velocity (P= .03) and timed stair climbing (P= .01) correlated significantly with improved balance. Balance improved more in the vision-deprived group than in the free-vision group. CONCLUSIONS: Balance improved more after rehabilitation with visual deprivation than with free vision. Visual overuse may be a compensatory strategy for coping with initial imbalance exacerbated by traditional rehabilitation.

Cues↗

PubMed2004

Prediction of visual perceptions with artificial neural networks in a visual prosthesis for the blind.

Within the framework of the OPTIVIP project, an optic nerve based visual prosthesis is developed in order to restore partial vision to the blind. One of the main challenges is to understand, decode and model the physiological process linking the stimulating parameters to the visual sensations produced in the visual field of a blind volunteer. We propose to use adaptive neural techniques. Two prediction models are investigated. The first one is a grey-box model exploiting the neurophysiological knowledge available up to now. It combines a neurophysiological model with artificial neural networks, such as multi-layer perceptrons and radial basis function networks, in order to predict the features of the visual perceptions. The second model is entirely of the black-box type. We show that both models provide satisfactory prediction tools and achieve similar prediction accuracies. Moreover, we demonstrate that significant improvement (25%) was gained with respect to linear statistical methods, suggesting that the biological process is strongly non-linear.

Blindness↗

PubMed2006

Age and novelty: event-related potentials to visual stimuli within an auditory oddball--visual detection task.

Age-related change of event-related potentials to novel visual stimuli was investigated while participants attended to both auditory and visual stimulation. Meaningful but irrelevant pictures (novel stimuli) were presented to younger (mean=21.8, range=18-26 years) and older (mean=70.0, range=60-78) participants (10 in each group). The participants were performing an auditory oddball task and counting silently the changes of a visually presented letter. In the younger group novel stimuli elicited a posterior positivity in the 220-255 ms range. This component habituated to the repetition of the same picture. In the older group this component had longer latency, and did not habituate. A later positivity had shorter latency and larger amplitude in the younger group, but this positivity was preceded by a negative component (N2b) only in the elderly. Results show decreased sensitivity to the content of the visual stimuli in an earlier stage of novelty processing in the elderly, and the age-related slowing of both orientation-related and task-related processes.

Acoustic Stimulation↗

PubMed2005

A psychophysical study of visual extinction: ipsilesional distractor interference with contralesional orientation thresholds in visual hemineglect patients.

Visual extinction was investigated in left (n=15) and right (n=25) brain-damaged patients with or without visual neglect, and in normal control subjects (n=14), using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. The location of the target grating was always cued, making the distractor grating task irrelevant. Even after equalizing the visibility of left and right hemifield stimuli, neglect patients still displayed an increased interference effect from an ipsilesional distractor (and no interference from a contralesional distractor). Left or right brain-damaged controls did not show this asymmetric interference of irrelevant distractors, even the patients who demonstrated extinction on standard extinction testing. This suggests that visual extinction is a critical component of the visual neglect syndrome and that it involves an attentional deficit.

Adult↗

PubMed2006

Visual recalibration and selective adaptation in auditory-visual speech perception: Contrasting build-up courses.

Exposure to incongruent auditory and visual speech produces both visual recalibration and selective adaptation of auditory speech identification. In an earlier study, exposure to an ambiguous auditory utterance (intermediate between /aba/ and /ada/) dubbed onto the video of a face articulating either /aba/ or /ada/, recalibrated the perceived identity of auditory targets in the direction of the visual component, while exposure to congruent non-ambiguous /aba/ or /ada/ pairs created selective adaptation, i.e. a shift of perceived identity in the opposite direction [Bertelson, P., Vroomen, J., & de Gelder, B. (2003). Visual recalibration of auditory speech identification: a McGurk aftereffect. Psychological Science, 14, 592-597]. Here, we examined the build-up course of the after-effects produced by the same two types of bimodal adapters, over a 1-256 range of presentations. The (negative) after-effects of non-ambiguous congruent adapters increased monotonically across that range, while those of ambiguous incongruent adapters followed a curvilinear course, going up and then down with increasing exposure. This pattern is discussed in terms of an asynchronous interaction between recalibration and selective adaptation processes.

Adaptation, Physiological↗

PubMed2005

Visual processing in schizophrenia: Structural equation modeling of visual masking performance.

Schizophrenic patients consistently demonstrate performance deficits on visual masking procedures. In visual masking, the subject's ability to process a target stimulus is reduced by another stimulus (mask) presented either before (forward masking) or after (backward masking) the target. Masking procedures employed in schizophrenia research have used several experimental paradigms. Most early studies have used high-energy masks (i.e., the mask is stronger than the target) and spatially overlapping target and mask. More recently, studies have begun to employ relatively weak (i.e., low-energy) masks, as well as masks that surround, but do not spatially overlap, the target. Data for forward and backward masking components of four masking conditions (target location and identification with a high-energy mask, target identification with a low-energy mask, and target identification with equal energy paracontrast/metacontrast) were collected from 75 patients with schizophrenia. Based on theoretical distinctions among masking procedures, we compared four models of visual masking using structural equation modeling. Although high zero-order correlations were found among the masking parameters, a four-factor model, in which factors were separated on the type of response (target location and identification), the shape of the function (monotonic and non-monotonic), and the overlap of the stimuli (overlapping and non-overlapping), provided the best fit for the data. These findings suggest that the four masking procedures used in this study may tap unique aspects of visual processing and are not redundant. The results also support theories of the different mechanisms underlying performance on these measures.

Adult↗

PubMed1999

Visual paralexias in a Spanish-speaking patient with acquired dyslexia: a consequence of visual and semantic impairments?

We report the case of a Spanish patient SC who misread 55 per cent of the single words shown to her. SC's reading accuracy was affected by word imageability and frequency. Nonword reading was very poor. The majority of SC's errors to real-word targets bore a close visual similarity to the items that elicited them, but there was no indication of an effect of serial position on the probability that a letter from a target word would be incorporated into the error made to that word. SC made some visual errors in object naming and also showed evidence of a general semantic impairment. We consider the similarity between SC and patient AB reported by Lambon Ralph and Ellis (1997), and suggest that the very high levels of visual errors shown by these two patients may reflect a combination of visual and semantic impairments.

Aged↗

PubMed1985

A case of visual agnosia showing a disorder of pre-semantic visual classification.

A case (J.R.) of associative visual agnosia (loss of meaning for visually presented objects without any serious sensory disorder) is reported. When asked to match a given name to sample the patient made both visual and semantic errors. Whereas semantic errors were reduced by priming, visual errors were intractable. It is argued that there is a disorder at the stage of pre-semantic abstraction, similar to that postulated elsewhere for some cases of deep dyslexia, contributing to the agnosia.

Adult↗

PubMed2003

Hemispatial neglect: its effects on visual perception and visually guided grasping.

Hemispatial neglect is a neurological disorder characterized by a failure to represent information appearing in the hemispace contralateral to a brain lesion. In addition to the perceptual consequences of hemispatial neglect, several authors have reported that hemispatial neglect impairs visually guided movements. Others have reported that the extent of the impairment depends on the type of visually guided task. Finally, in some cases, neglect has been shown to impair visual perception without affecting visuomotor control in relation to the very same stimuli. While neglect patients may be able to successfully pick up an object they have difficulty perceiving in its entirety, it does not mean that they are picking up the object in the same way that a neurologically intact individual would. In the current study, patients with hemispatial neglect were presented with irregularly shaped objects, directly in front of them, that lacked clear symmetry and required an analysis of their entire contour in order to calculate stable grasp points. In a perceptual discrimination task, the neglect patients had difficulty distinguishing one object from another on the basis of their shape. In a grasping task, the neglect patients showed more variance in the position of their grasp on the target objects than their control subjects, with an overall shift to the relative right side of the presented objects. The perceptual and visuomotor deficits seen in patients with hemispatial neglect deficits may be the result of an inability to form good structural representations of the entire object for use in visual perception and visuomotor control.

Adult↗

PubMed2000

Visual search and visual working memory in patients with chronic focal cortical lesions.

Visually guided behavior is known to involve temporo-parietal, inferotemporal, and prefrontal cortex and each of these areas appears to contribute to visual working memory. We explored the extent to which chronic lesions in one of these cortical areas affect visually guided oculomotor performance. We also explore whether possible impairments become more pronounced with increasing memory load. With this aim we recorded saccadic eye movements in 19 patients with a chronic focal postsurgical lesion in either temporo-parietal, inferior temporal or prefrontal cortex. Their results are compared to those of 19 age-matched volunteers. The subjects performed three different visual search tasks with increasing memory load: Instructed search, cue-guided search and memory-guided search. In addition, the latter task was performed with a short (1 s) and a long (6 s) delay. All tasks required the subjects to make a saccade to a single target presented together with one or three distractors. The results indicate that patients with inferotemporal lesions make the most task-related errors. Saccadic reaction times (SRTs) were significantly prolonged in patients with temporo-parietal and prefrontal lesions, but were unaffected in the patients with lesions in the inferotemporal cortex. The spatial accuracy of saccades was lowest in patients with temporo-parietal lesions. An increase in memory load led to more errors, to longer reaction times and to lower saccadic precision. However, the effect was similar across the three patient groups and the controls. An error analysis indicated that both patients and controls tended to weight global (luminance contrast and form) features higher than local features (line-segment orientation) when making difficult perceptual decisions.

Adult↗

PubMed1999

Long term depression is expressed during postnatal development in rat visual cortex: a role for visual experience.

Long term forms of synaptic plasticity and in particular LTD/LTP are both present in the mammalian visual cortex. However, while LTP is not inducible in adulthood LTD can be elicited in the mature brain, but its developmental pattern is unknown. Aim of this work was to investigate whether LTD is expressed during postnatal development and if it is modulated by visual experience. To investigate these points we have used rat primary visual cortex slices taken at different stages of functional maturation process, i.e., postnatal day 17 (P17), P23 and P30-35. LTD was assessed by measuring the amplitude of extracellular field potentials recorded in cortical layers 2/3 and elicited by low frequency stimulation to the white matter. LTD was expressed at all ages investigated without significant differences between age groups. These data indicate that LTD developmental expression is not temporally related with the period of functional maturation of rat visual cortex. Dark rearing from birth to P23 resulted in a reduction of LTD amplitude while light deprivation from P17 to P30 did not affect LTD expression in comparison to age matched control values. We suggest that light imprinting is essential for a normal LTD expression during postnatal development.

Animals↗

PubMed1999

Visual deprivation increases capability of layer II/III for epileptiform activity in the rat visual cortical slices.

Effects of visual deprivation on the induction of epileptiform activity were studied in layer II/III of 29-39-day-old rat primary visual cortex. Field potentials were evoked by stimulation of layer IV in slices from control (CON) and dark-reared (DR) rats. Picrotoxin (PTX)-induced epileptiform activity was characterized by spontaneous and evoked epileptic field potentials (EFPs). The results showed that DR slices demonstrate greater susceptibility for induction of spontaneous EFP. PTX-induced changes in the characteristics of evoked field potentials also showed higher tendency of DR animals to generate epileptiform activity. In both groups, field potentials consisted of pEPSP(1) (population excitatory postsynaptic potential 1, i.e., first negativity) and pEPSP(2) (second negativity), respectively. There was no significant difference between the characteristics of field potentials in CON and DR slices. PTX significantly increased amplitude and duration of pEPSP(2), but it had no significant effect on pEPSP(1). Effects of PTX on pEPSP(2) were significantly higher in DR slices. It is concluded that visual deprivation results in a heightened potential in layer II/III of the rat visual cortex to generate PTX-induced epileptiform activity.

Action Potentials↗

PubMed2000

Visually guided locomotion and computation of time-to-collision in the mongolian gerbil (Meriones unguiculatus): the effects of frontal and visual cortical lesions.

Past research has indicated that many species use the time-to-collision variable but little is known about its neural underpinnings in rodents. In a set of three experiments we set out to replicate and extend the findings of Sun et al. (Sun H-J, Carey DP, Goodale MA. Exp Brain Res 1992;91:171-175) in a visually guided task in Mongolian gerbils, and then investigated the effects of lesions to different cortical areas. We trained Mongolian gerbils to run in the dark toward a target on a computer screen. In some trials the target changed in size as the animal ran toward it in such a way as to produce 'virtual targets' if the animals were using time-to-collision or contact information. In experiment 1 we confirmed that gerbils use time-to-contact information to modulate their speed of running toward a target. In experiment 2 we established that visual cortex lesions attenuate the ability of lesioned animals to use information from the visual target to guide their run, while frontal cortex lesioned animals are not as severely affected. In experiment 3 we found that small radio-frequency lesions, of either area VI or of the lateral extrastriate regions of the visual cortex also affected the use of information from the target to modulate locomotion.

Animals↗

PubMed1999

Visual contribution to self-induced body sway frequencies and visual perception of male professional dancers.

We studied the degree of dependence on vision, for postural control and for perception, among male adult dancers and untrained subjects. First, body sways were analyzed on a free seesaw platform. Fast Fourier transform processing allowed spectral frequency analysis of the platform sways recorded by an accelerometer. Secondly, a visual dependence test, the rod and frame test (RFT) was used. Professional dancers were significantly more stable and less dependent on vision for postural control and for perception than untrained subjects. Presumably, professional dance training strengthens the accuracy of proprioceptive inputs and shifts sensorimotor dominance from vision to proprioception. For the dancers, there was interaction between the RFT visual dependence and the visual control of posture: the less visual-dependent they were for the RFT, the more stable they were in dynamic balance conditions.

Adolescent↗

PubMed2001

Bidirectional modulation of basal forebrain N-methyl-D-aspartate receptor function differentially affects visual attention but not visual discrimination performance.

Basal forebrain neuronal circuits, specifically the corticopetal cholinergic system, mediate attentional abilities. The effects of infusions of N-methyl-D-aspartate (NMDA) and the competitive NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) into the basal forebrain were assessed in rats trained in an operant task designed to generate measures of sustained attention performance. Control animals were trained in a cued visual discrimination task devoid of explicit demands on attentional performance, but involving similar basic operant components as the sustained attention task. The effects of intrabasalis infusions of NMDA (1, 3 and 6nmol) and APV (3, 10 and 20nmol) were tested in separate groups of animals. Infusion of neither drug affected the animals' response accuracy in the cued visual discrimination task, indicating that performance in this task remains insufficient to activate basal forebrain NMDA receptors. Infusions of APV in sustained attention task-performing animals selectively decreased the animals' ability to detect visual signals, but spared their ability to reject non-signal events. Conversely, infusions of NMDA into the basal forebrain did not affect the animals' hit rate but increased their number of false alarms, i.e. "claims" for signals in non-signal trials. The concentrations of NMDA infused into the basal forebrain did not result in neurotoxic effects as demonstrated by a separate experiment, which indicated neurodegeneration following the infusion of 30 nmol NMDA as visualized by the Fluoro-Jade method.The effects of APV correspond with the attentional consequences of other manipulations known to impair the functions of cortical cholinergic input. Conversely, the effects of NMDA infusions agree with the hypothesis that overactivity of cortical cholinergic inputs mediates an abnormal overprocessing of the stimulus situation. Basal forebrain NMDA receptor manipulations assist in determining the role of this neuronal system in cognitive processes.

2-Amino-5-phosphonovalerate↗