SEARCH · PubMed Health

Results for “Pattern Recognition, Visual”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,639 records · Page 91Linked to original sources

PubMed2000

Deconvolution of event-related fMRI responses in fast-rate experimental designs: tracking amplitude variations.

Recent developments towards event-related functional magnetic resonance imaging has greatly extended the range of experimental designs. If the events occur in rapid succession, the corresponding time-locked responses overlap significantly and need to be deconvolved in order to separate the contributions of different events. Here we present a deconvolution approach, which is especially aimed at the analysis of fMRI data where sequence- or context-related responses are expected. For this purpose, we make the assumption of a hemodynamic response function (HDR) with constant yet not predefined shape but with possibly variable amplitudes. This approach reduces the number of variables to be estimated but still keeps the solutions flexible with respect to the shape. Consequently, statistical efficiency is improved. Temporal variations of the HDR strength are directly indicated by the amplitudes derived by the algorithm. Both the estimation efficiency and statistical inference are further supported by an improved estimation of the noise covariance. Using synthesized data sets, both differently shaped HDRs and varying amplitude factors were correctly identified. The gain in statistical sensitivity led to improved ratios of false- and true-positive detection rates for synthetic activations in these data. In an event-related fMRI experiment with a human subject, different HDR amplitudes could be derived corresponding to stimulation at different visual stimulus contrasts. Finally, in a visual spatial attention experiment we obtained different fMRI response amplitudes depending on the sequences of attention conditions.

Algorithms↗

PubMed2004

Visual activation in prefrontal cortex is stronger in monkeys than in humans.

The prefrontal cortex supports many cognitive abilities, which humans share to some degree with monkeys. The specialized functions of the prefrontal cortex depend both on the nature of its inputs from other brain regions and on distinctive aspects of local processing. We used functional MRI to compare prefrontal activity between monkey and human subjects when they viewed identical images of objects, either intact or scrambled. Visual object-related activation of the lateral prefrontal cortex was observed in both species, but was stronger in monkeys than in humans, both in magnitude (factors 2-3) and in spatial extent (fivefold or more as a percentage of prefrontal volume). This difference was observed for two different stimulus sets, at two field strengths, and over a range of tasks. These results suggest that there may be more volitional control over visual processing in humans than in monkeys.

Animals↗

PubMed2005

Emerging neurophysiological specialization for letter strings.

In adult readers, printed words and other letter strings activate specialized visual functions within 200 msec, as evident from neurophysiological recordings of brain activity. These fast, specialized responses to letter strings are thought to develop through plastic changes in the visual system. However, it is unknown whether this specialization emerges only with the onset of word reading, or represents a precursor of literacy. We compared 6-year-old kindergarten children who could not yet read words to adult readers. Both age groups detected immediate repetitions of visually presented words, pseudowords, symbol strings, and pictures during event-related potential (ERP) mapping. Maps from seven corresponding ERP segments in children and adults were analyzed regarding fast (<250 msec) and slow (>300 msec) specialization for letter strings. Adults reliably differentiated words through increased fast (<150 msec) occipito-temporal N1 activity from symbols. Children showed a later, more mid-occipital N1 with marginal word-symbol differences, which were absent in those children with low letter knowledge. Children with high letter knowledge showed some fast sensitivity to letter strings, which was confined to right occipito-temporal sites, unlike the stronger adult N1 specialization. This suggests that a critical degree of early literacy induces some immature, but fast, specialization for letter strings before word reading becomes possible. Children also differentiated words from symbols in later segments through increased right occipito-temporal negativity for words. This slow specialization for letter strings was not modulated by letter knowledge and was absent in adults, possibly reflecting a visual precursor of literacy due to visual familiarity with letter strings.

Adult↗

PubMed2007

Distinction between perceptual and attentional processing in working memory tasks: a study of phase-locked and induced oscillatory brain dynamics.

Working memory involves the short-term storage and manipulation of information necessary for cognitive performance, including comprehension, learning, reasoning and planning. Although electroencephalogram (EEG) rhythms are modulated during working memory, the temporal relationship of EEG oscillations with the eliciting event has not been well studied. In particular, the dynamics of the neural network supporting memory processes may be best captured in induced oscillations, characterized by a loose temporal link with the stimulus. In order to differentiate induced from evoked functional processes, the present study proposes a time-frequency analysis of the 3 to 30 Hz EEG oscillatory activity in a verbal n-back working memory paradigm. Control tasks were designed to identify oscillatory activity related to stimulus presentation (passive task) and focused attention to the stimulus (detection task). Evoked theta activity (4-8 Hz) phase-locked to the visual stimulus was evidenced in the parieto-occipital region for all tasks. In parallel, induced theta activity was recorded in the frontal region for detection and n-back memory tasks, but not for the passive task, suggesting its dependency on focused attention to the stimulus. Sustained induced oscillatory activity was identified in relation to working memory in the theta and beta (15-25 Hz) frequency bands, larger for the highest memory load. Its late occurrence limited to nonmatched items suggests that it could be related to item retention and active maintenance for further task requirements. Induced theta and beta activities displayed respectively a frontal and parietal topographical distribution, providing further functional information on the fronto-posterior network supporting working memory.

Adult↗

PubMed2005

A computational analysis of separating motion signals in transparent random dot kinematograms.

When multiple motion directions are presented simultaneously within the same region of the visual field human observers see motion transparency. This perceptual phenomenon requires from the visual system to separate different motion signal distributions, which are characterised by distinct means that correspond to the different dot directions and variances that are determined by the signal and processing noise. Averaging of local motion signals can be employed to reduce noise components, but such pooling could at the same time lead to the averaging of different directional signal components, arising from spatially adjacent dots moving in different directions, which would reduce the visibility of transparent directions. To study the theoretical limitations of encoding transparent motion by a biologically plausible motion detector network, the distributions of motion directions signalled by a motion detector model (2DMD) were analysed here for Random Dot Kinematograms (RDKs). In sparse dot RDKs with two randomly interleaved motion directions, the angular separation that still allows us to separate two directions is limited by the internal noise in the system. Under the present conditions direction differences down to 30 deg could be separated. Correspondingly, in a transparent motion stimulus containing multiple motion directions, more than eight directions could be separated. When this computational analysis is compared to some published psychophysical data, it appears that the experimental results do not reach the predicted limits. Whereas the computer simulations demonstrate that even an unsophisticated motion detector network would be appropriate to represent a considerable number of motion directions simultaneously within the same region, human observers usually are restricted to seeing not more than two or three directions under comparable conditions. This raises the question why human observers do not make full use of information that could be easily extracted from the representation of motion signals at the early stages of the visual system.

Computer Simulation↗

PubMed2004

Selective attention in schizophrenia: sparing and loss of executive control.

OBJECTIVE: Auditory selective attention involves top-down modulation of sensory processing to selectively filter relevant from irrelevant information. Patients with schizophrenia have deficits in auditory selective attention, but whether these attention deficits are evident at the earliest stages of auditory processing or emerge later in the processing stream is unknown. METHOD: Event-related potentials were used to assess the integrity and time course of auditory attention in schizophrenia during a cross-modal selective attention task. A random sequence of equiprobable loud and soft speech sounds and bright and dim checkerboard patterns occurred every 800 to 1200 msec. A button press was required in response to soft speech sounds in the auditory attention task and to dim checkerboards in the visual attention task. Event-related potentials were recorded from 15 patients with DSM-IV schizophrenia and 16 age- and gender-matched healthy comparison subjects. RESULTS: As early as 50 msec, both patients and comparison subjects showed evidence of cross-modal selective attention. These attention effects were sustained until 300 msec in comparison subjects but were no longer evident in patients by 100 msec. CONCLUSIONS: Schizophrenia patients implemented an early attentional filter but failed to sustain selective attention later in the processing stream. This suggests that initially intact executive control of auditory attention cannot be sustained in schizophrenia, perhaps implicating dysfunction in frontotemporal pathways.

Acoustic Stimulation↗

PubMed2006

Gender differences in hemispheric asymmetry for face processing.

BACKGROUND: Current cognitive neuroscience models predict a right-hemispheric dominance for face processing in humans. However, neuroimaging and electromagnetic data in the literature provide conflicting evidence of a right-sided brain asymmetry for decoding the structural properties of faces. The purpose of this study was to investigate whether this inconsistency might be due to gender differences in hemispheric asymmetry. RESULTS: In this study, event-related brain potentials (ERPs) were recorded in 40 healthy, strictly right-handed individuals (20 women and 20 men) while they observed infants' faces expressing a variety of emotions. Early face-sensitive P1 and N1 responses to neutral vs. affective expressions were measured over the occipital/temporal cortices, and the responses were analyzed according to viewer gender. Along with a strong right hemispheric dominance for men, the results showed a lack of asymmetry for face processing in the amplitude of the occipito-temporal N1 response in women to both neutral and affective faces. CONCLUSION: Men showed an asymmetric functioning of visual cortex while decoding faces and expressions, whereas women showed a more bilateral functioning. These results indicate the importance of gender effects in the lateralization of the occipito-temporal response during the processing of face identity, structure, familiarity, or affective content.

Adult↗

PubMed1985

Visual evoked potentials by different check sizes in patients with multiple sclerosis.

Visual evoked potentials (VEPs) were obtained using three checks of different size (25', 50', and 100' of arc) in 20 normal controls and 1,006 patients. The test was abnormal in 270 patients regardless of the check size, whereas in 53 patients, a prolongation of P100 latency occurred only with checks of a certain size. The procedure was most sensitive in detecting abnormalities with combined application of 25' and 100' checks and least sensitive with the 50' checks alone. In occasional subjects showing W-shaped responses, the use of another check size often elicited the usual V-shaped pattern, alleviating the problem of selecting the peak for latency determination.

Adolescent↗

PubMed1985

Spatial pattern as a stimulus to the pupillary system.

Pupillary responses to the contrast reversal of a checkerboard pattern of variable check size and contrast were measured. The results show that the pupillary system is responsive to the contrast reversal. It is argued that this response is the light reflex caused by the local luminance change. The summation characteristics of the pupillary afferent are considered. It has also been found that the pupil size, without any influence from contrast reversal, systematically changes with the changes in the check size. Whether this pupillary change is caused by the variation in accommodative performance with changing spatial factors of visual stimulus is discussed.

Adult↗

PubMed1993

Direction selective climbing fiber responses to horizontal and vertical optokinetic stimuli in the cat cerebellar flocculus.

In ketamine anesthetized cats, complex spike (CS) responses to movement of a large-field visual surround (optokinetic stimulus) were recorded in the cerebellar flocculus. Two types of Purkinje cells were found. One type showed increased CS activity to optokinetic stimuli directed contralaterally to the recording site and decreased activity to stimuli directed ipsilaterally. The modulation occurred in both monocular (responses to the stimuli for only the ipsilateral eye) and binocular forms (dominant eye, ipsilateral). The other type of cell showed increased CS activity during upward optokinetic stimuli and decreased activity during downward stimuli. The responses occurred only in the binocular form. The dominant eye was either ipsilateral or contralateral with respect to the recording site. Thus, there are two types of Purkinje cells in the cat flocculus: horizontal type cell showing CS modulation during horizontal optokinetic stimuli, and vertical type cell showing CS modulation during vertical optokinetic stimuli.

Animals↗

PubMed1982

Hemispheric sex differences in response to apparently moving stimuli as indicated by visual evoked potentials.

This research was designed to determine whether visual event related potentials (ERPs) recorded from over the two occipital hemispheres would indicate differential processing of apparently moving and stationary stimuli. Previous findings in the literature led us to consider a male-female comparison as part of our experimental design. Nine male and nine female subjects were screened for right-handedness and visual abilities (acuity, depth and phoria). Each participated in three experimental sessions over a three day period in which ERPs were recorded from O1 and O2 in response to stimuli presented in left, central and right visual fields. The main finding was that, for female subjects, right hemispheric derived ERPs were larger in amplitude than left hemisphere recordings for apparently moving stimuli presented centrally. Males showed no hemispheric amplitude differences. However, for male subjects, the left hemisphere ERP latencies to motion were longer than those recorded from over O2 with central visual field presentations. It was speculated that different attentional strategies in the two sexes led to the observed result.

Adolescent↗

PubMed1984

Power spectral analysis of visual evoked potentials in multiple sclerosis.

Power spectral analysis (PSA) was performed on the visual evoked potentials (VEPs) to counterphased checkerboard stimuli from 98 eyes in 49 patients with multiple sclerosis (MS) and 54 eyes of 27 normal volunteers. Attenuation of high frequency components of the transient visual evoked potential was found in 53% of MS patients and 4% of controls. Loss of high frequency components was poorly correlated with prolonged latency (r = 0.346). The consideration of both PSA and latency of the VEP increased the percentage of MS patients exhibiting visual pathway conduction abnormalities from 61% to 86%. The use of PSA in the diagnosis of MS is useful in increasing detection especially in cases where deformed waveforms preclude a reliable estimation of latency.

Adult↗

PubMed1991

Mechanisms of attentional selection: temporally modulated priority tags.

When a single abrupt onset appears in a multielement display, it captures attention. When multiple onset elements occur, they have conditional priority over no-onset elements such that a limited number of onsets can be serviced with high priority in visual search (Yantis & Johnson, 1990). We report three experiments in which we assess two possible mechanisms for attentional prioritization: a priority queue into which a fixed number of high-priority elements are placed for early servicing during search, and a mechanism that temporarily tags all high-priority elements for early servicing or more frequent sampling. We manipulated the visual quality or inter-letter confusability of the stimuli to prolong encoding and/or comparison operations; this manipulation led to a decrease in the estimated number of elements serviced with high priority. We conclude that a mechanism incorporating temporally decaying priority tags is implicated in servicing multiple abrupt onsets in visual search.

Adult↗

PubMed2000

Progression of visual evoked potential abnormalities in multiple sclerosis and optic neuritis.

To investigate visual signal processing in patients with optic neuritis, suspected multiple sclerosis, confirmed multiple sclerosis (MS), and optic neuritis combined with MS, pattern reversal visual evoked potentials (VEPs) were recorded in patients and normal subjects. The amplitude and latency of the first positive peak, P100 were determined to assess electrical conduction in patients as compared to normal subjects. Suspected MS patients did not differ from normal subjects in peak latencies or amplitudes. The P100 amplitude was reduced in optic neuritis, confirmed MS and optic neuritis combined with confirmed MS. The P100 and N145 latencies were prolonged in optic neuritis patients and confirmed MS patients as compared to normal subjects. The main characteristic of optic neuritis was P100 amplitude reduction, and of confirmed MS was P100 latency delay. There was a progression of the P100 latency delay and of the P100 amplitude decrement in optic neuritis, confirmed MS, and optic neuritis combined with confirmed MS. These results indicate a progression of demyelination in optic neuritis, confirmed MS, and optic neuritis combined with confirmed MS.

Adolescent↗

PubMedDate not supplied

Use of a stenopeic semiocclusor enhances vep diagnosis.

PURPOSE: We conducted a study to assess the capability of a 2-mm stenopeic semiocclusor in improving the sensitivity of the standard full-field pattern-visual evoked potential technique to disclose abnormalities of the pre-chiasmatic visual pathway. METHODS: Nineteen control subjects and fifty-five patients with a diagnosis of definite or probable Multiple Sclerosis (MS) were evaluated. All subjects were seated in front of a 19" TV monitor depicting a black and white square pattern, standard condition (R). Two trials of 100 stimulus repetitions at 1.8 Hz, recording at Oz- Fz were performed for each eye. After two minutes the procedure was replicated using an stenopeic semiocclusor, central field condition (S). The main parameters quantified were the P100 latency in each eye (a) and P100 inter-ocular difference (b). RESULTS: Under condition R abnormalities were detected in 42 patients for parameter "a", and in 39 patients for parameter "b". If both parameters, a + b, were considered 48 patients (87%) showed abnormalities. Under condition S, abnormalities were detected in 49 patients for parameter "a" and 41 patients for parameter "b". When both parameters, a + b, were considered 53 patients (96%) showed abnormalities. DISCUSSION: The main point of interest of this study is the fact that when a pattern reversal stimulus is restricted to the central area the evoked response shows a P100 peak with prolonged latency in comparison to the full-field stimulus. Additionally, used in combination with the standard full-field technique it improves the abnormality detection rate whilst adding little extra time to the procedure.

Adolescent↗

PubMed2006

[Electrophysiological tests in early and differential diagnosis of some hereditary retinal and optic nerve diseases].

PURPOSE: The goal of our study was to determine the value of electrophysiological tests in early and differential diagnosis of some hereditary retinal and optic nerve diseases. MATERIAL AND METHODS: Review article on the basis of own experiences and results of another authors concerning to EOG, ERG, PERG, MfERG and VEP tests in evaluation of hereditary, stationary and progressive retinal diseases, as well as optic nerve diseases. RESULTS: Electrophysiological tests can be abnormal even in patients without fundus changes seen in routine ophthalmological examination. CONCLUSIONS: Electrophysiological tests are useful in early and differential diagnosis of some hereditary retinal and optic nerve diseases. General ophthalmologists should remember about its application, especially in difficult diagnostic cases.

Diagnosis, Differential↗

PubMed1989

The P300 amplitude of the event-related brain potential indicates changes within a frame-of-reference.

The starting point of this study was the fact that investigations in the frame-of-reference research were mostly performed on the behavioural level. The present study aimed at studying behaviour of P300 amplitude of the event-related brain potential in a frame-of-reference investigation. Ten healthy volunteers participated in the investigation. Test material consisted of horizontal bars of different lengths presented successively on a screen. Each bar (serial stimulus) was preceded by a prestimulus (anchor) of constant length. Each serial stimulus was to be evaluated according to a 9 step rating scale. There were no significant differences in the overt length judgements of the serial stimuli depending on the 3 different anchor lengths. However, the mean P300 amplitude increased with growing dissimilarity between the length of the anchor and serial stimulus length. Thus, it is assumed that the adaptation level on the internal dimension of stimulus length, which was assumed to be influenced by the anchor stimuli, led to the different behaviour of P300 amplitude.

Adult↗

PubMed1989

The pattern electroretinogram: technical aspects and clinical significance.

The pattern electroretinogram (PERG), thought to be generated in retinal ganglion cells, is a recent addition to the neurophysiologic assessment of retinal function. Factors that affect the PERG include type of pattern-reversal stimulus, spatial frequency of the pattern, luminance and contrast of the pattern, method of waveform processing, type of electrode employed, electrode montage, background illumination, position of stimulus on the retina, temporal frequency of pattern reversal, refractive error, and patient age. Clinical entities reported to affect the PERG include glaucoma and ocular hypertension, optic neuritis, other optic neuropathies, maculopathies, and amblyopia. Technical aspects and clinical significance of the PERG are reviewed.

Electroretinography↗