PubMed Health⌕ Search

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 865 records · Page 48Linked to original sources
PubMed1991

[Specificity of visual evoked potential alterations in Alzheimer's disease].

We have studied visual evoked potentials (VEP) in four groups: healthy young and aged subjects and aged patients suffering from depression or Alzheimer's disease (AD). In the flash modality, peak IV delay in AD patients as compared to aged controls corresponded to lengthened III-IV interlatency (IL). When AD and depressed patients were compared, peak IV delay in the former did not reach significance. Normal aging resulted in delayed peaks II and III, without IL modification. In the pattern reversal modality, no difference was observed between AD patients and aged, healthy or depressed controls. Aging delayed peaks P50 to P180. The abnormality exhibited by AD patients was specific to this group. However, the lack of significant difference between AD and depressed groups argues against the ability of VEP to differentiate those conditions.

Adult↗
PubMed1999

The effect of spatial frequency on chromatic and achromatic steady-state visual evoked potentials.

OBJECTIVE: Little is known about the physiological properties of the major components of steady-state visual evoked potentials (VEPs). Based on the hypothesis that isoluminant color and high contrast pattern differentially activate the parvo- and magnocellular pathways, we studied difference in spatial frequency function between chromatic and achromatic VEPs to sinusoidal gratings. METHODS: Steady-state VEPs to isoluminant chromatic (red-green) and high contrast (90%) achromatic (black-white) sinusoidal gratings with nine spatial frequencies (0.5 to 8.0 cycles/degrees (cpd)) at 4 Hz (8 reversals/s) were recorded in 13 normal subjects. VEPs were Fourier analyzed to obtain phase and amplitude of the second (2F) and fourth (4F) harmonic responses. RESULTS: The 2F amplitude of chromatic VEPs decreased above 4.0 cpd in a low-pass function while that of achromatic VEPs showed a band-pass function with a peak at 4.0 cpd. The 4F amplitude of chromatic VEPs was not affected significantly by spatial frequency whereas that of achromatic VEPs exhibited a high-pass function. The phases of 2F and 4F showed a non-monotonic function of spatial frequency in both chromatic and achromatic stimuli with peaks at middle spatial frequencies. CONCLUSION: Chromatic and achromatic visual stimuli differently affected 2F and 4F components, which thus suggests that 2F and 4F components are generated from different neuronal subgroups largely in the parvocellular pathway.

Adolescent↗
PubMed2000

Predictive value of novel stimuli modifies visual event-related potentials and behavior.

OBJECTIVE: We examined how behavioral context influences novelty processing by varying the degree that a novel event predicted the occurrence of a subsequent target stimulus. METHODS: Visual event-related potentials (ERPs) and reaction times (RTs) were recorded in 3 detection experiments (23 subjects). The predictive value of a novel stimulus on the occurrence of a subsequent target was varied as was novel-target pairing intervals (200-900 ms). In Experiment 1, novel stimuli always preceded a target, in Experiment 2, 40% of novel stimuli were followed by a target, and in Experiment 3, novel stimuli occurred randomly. RESULTS: In Experiment 1, RTs following 100% predictive novels were shortened for targets at all spatial locations and novel-target pairing intervals. Novel stimuli predicting a target generated a central negativity peaking at 300 ms and reduced P3a and P3b ERPs. In Experiments 2 and 3, target RTs were prolonged only when novel and target stimuli were presented in the same spatial location at short ISIs (200 ms). The central novel N2 was smaller in amplitude in comparison to Experiment 1, and novelty P3a and target extrastriate N2 and posterior scalp P3b ERPs were enhanced. CONCLUSIONS: The enhanced N2 for 100% predictive novel stimuli appears to index an alerting system facilitating behavioral detection. The same novel stimuli with no predictive value distract attention and generate a different ERP pattern characterized by increased novelty P3a and target P3b responses. The results indicate that behavioral context determines how novel stimuli are processed and influence behavior.

Adult↗
PubMed2000

A study of the effects of contrast change on pattern VEPS, and the transition between onset, reversal and offset modes of stimulation.

We studied the relationship between the visual evoked potential (VEP) components by tracing the transition from onset/offset mode of stimulation to the reversal mode by a series of contrast change steps. VEPs on the ipsilateral and contralateral side of the scalp with respect to the left half-field checkerboard stimulus (checksizes: 12', 50' and 80') were recorded in 15 subjects. Eight contrast steps for each checksize were recorded. Each test step consisted of the alternation of a constant high contrast checkerboard (A), with a second checkerboard (B) in which the contrast was changed. Checkerboard B was initially of identical spatial phase to A, but contrast was reduced systematically until B was a uniform grey field (i.e. onset/offset). In subsequent steps checkerboard B was of opposite spatial phase and contrast was increased until the final step when B was of equal high contrast (i.e. full reversal). All ipsilateral and contralateral onset components, ipsilateral offset components, and the reversal P100 component significantly enlarged with increasing contrast. The extent of amplitude change with contrast was greatest for offset, followed by onset and then reversal. A clear association could be discerned between all offset and reversal components. Onset CI and CII appeared to be related to the reversal P100 and N145, respectively. When small 12' checks were used, onset Co appeared to have common features with the reversal N80. The contralateral onset P105 component did not have a comparable component in the reversal mode.

Adult↗
PubMed2003

Effect of stimulus check size on multifocal visual evoked potentials.

In this study we examined the effects of varying stimulus check size on multifocal visual evoked potential (VEP). We also evaluated the currently used cortical scaling of stimulus segments. The ObjectiVision multifocal objective perimeter stimulates the eye with random check patterns at 56 cortically scaled segments within the visual field extending to a radius of 26 degrees. All cortically scaled segments have equal number of checks, which gradually increase in size from the center to the periphery, proportional to the size of the segment. Stimuli with 9, 16, 25, 36 and 49 checks/segment were tested on 10 eyes belonging to 10 normal subjects. The check size varied inversely with number of checks per segment. VEP was recorded using bipolar occipital cross electrodes (7 min/eye), the amplitude and latency of responses obtained were compared with the check size at different eccentricities. Our findings suggest that the existing setting with 16 checks/segment subtending 26' to 140' from center to periphery, is the most effective amongst all the check sizes. Decreasing the check size prolongs the latency in the central field only. Cortical scaling of segments generates responses of the same order of magnitude throughout the field, but could be improved slightly to enhance the signal from the outer two rings.

Adult↗
PubMed2004

The dependency of simultaneously recorded retinal and cortical potentials on temporal frequency.

To optimize the simultaneous recording of retinal and cortical potentials, we tried to identify the most sensitive condition for pattern-electroretinogram (PERG) and pattern visual evoked potential (VEP) concerning the temporal frequency. In the same session PERGs and VEPs were elicited by checkerboard patterns with 5 temporal frequencies ranging from 8 rps to 33 rps. For data analysis the steady-state responses were Fourier analysed. We evaluated whether a statistically significant response was present, estimated the magnitude of the response at the stimulus frequencies tested and estimated it's significance. For PERG less dependence on temporal frequency was evident compared to VEP. The magnitude of the VEP response was larger than that of the PERG. However the rate of statistically significant responses for the PERG compared to that of the VEP was similar for the small checksize and even higher for the large checksize. The results permit a simultaneous recording in the range of temporal frequencies, where high responses are acquired from both levels, retinal and cortical.

Adult↗
PubMed2000

Overt and covert identification of fragmented objects inferred from performance and electrophysiological measures.

The authors investigated visual processing leading to object identification by manipulating the number of fragments and nature of the study. During the study, participants either named or drew objects in Experiment 1 and drew them all in Experiment 2. During the test, participants made an identification judgment at each of 6 different fragmentation levels for studied and new objects. Fewer fragments were needed to identify studied than unstudied objects. Reaction times were faster for studied than unstudied objects both at identification and at the preceding level. Event-related brain potentials (ERPs) to unidentified objects were characterized by a late negativity in contrast to a positivity to identified objects. ERPs to studied but not to new objects contained a smaller and later version of the identification positivity at level just prior to identification, which was not due to differential response confidence. Much covert visual analysis and even object identification may precede overt identification, depending on the nature of prior experience.

Adult↗
PubMed1987

Brain potentials as indices of orthographic and phonological interaction during word matching.

The interaction between orthographic and phonological codes in a same-different judgment task was studied by requiring subjects to decide if two visually presented words either looked alike or rhymed. Word pairs were selected from four different lists. Words rhymed and looked alike, rhymed but did not look alike, looked alike but did not rhyme, or neither looked alike nor rhymed. Reaction time and percent error increased whenever there was a conflict between the orthography and phonology of the words. The N200 component of the event-related brain potential (ERP) indicated that subjects were capable of detecting phonological differences between words within 260 ms from the presentation of a word pair. The amplitude of the N200s also varied with the degree of mismatch between words. N200s were largest when both the orthography and phonology mismatched, of intermediate amplitude when either orthography or phonology mismatched, and smallest when both orthography and phonology matched. P300 latency was consistent with reaction time, increasing whenever there was a conflict between the two codes. Taken together, behavioral measures and the ERP data suggest that the extraction of the orthographic and phonological aspects of words occurs early in the information processing sequence.

Evoked Potentials, Visual↗
PubMed2006

The orthographic neighbourhood frequency effect in French: a letter-case manipulation study.

The neighbourhood frequency effect was investigated by a letter-case manipulation of French words presented in the go/no-go lexical decision task. Accented words were presented both in lower-case letters with written accents and upper-case letters with no written accents, two usual typographies in French. This procedure addressed the stimulus-matching problem by using intra-word comparisons as some words have different orthographic neighbourhoods in the two cases. Neighbourhood frequency was varied across case change for half of the words while it was held constant for the other half. The results showed an interaction between letter-case and neighbourhood constancy, so the inhibitory neighbourhood frequency effect could not be attributed to interstimulus comparisons or to typographical change. Implications are discussed in current models of visual word recognition.

Choice Behavior↗
PubMed1998

Comparison of pattern-onset, -reversal and -offset VEPs in treated amblyopia.

PURPOSE: There are differences in the properties of visual evoked potentials (VEPs) to various forms of pattern stimulation and it is not clear how these differences reflect macular and parmacular function in amblyopic and normal eyes. We assessed pattern-onset, -reversal and -offset VEPs from amblyopic eyes and compared them with the responses from the fellow eyes, and from controls, to gauge the relative effectiveness of these stimulus modes. METHODS: The three modes of pattern stimulation were presented sequentially in a single recording run to enable direct comparisons to be made for identical recording conditions. Half-field stimulation was used, as this elicits components over the ipsilateral and contralateral occipital scalp relative to the stimulated half-field, which reflect stimulation of macular and paramacular areas of the visual field. Eighteen amblyopes treated by occlusion and 20 control children were studied. RESULTS: Multivariate analysis of variance showed significant differences between ambylopic and fellow eyes in amblyopes: pattern-onset components were significantly attenuated and ipsilateral reversal components were significantly prolonged in amblyopic eyes. When fellow eyes of amblyopes were compared with the eyes of controls, the reversal P100 and offset P110 and N165 components showed significant differences. CONCLUSIONS: Ipsilateral reversal components and onset CII and contralateral P105 were the most affected in amblyopic eyes. The subnormal findings for the fellow eyes of amblyopes suggest that occlusion may have a long-standing physiological effect on the patched eye, not normally clinically apparent.

Adolescent↗
PubMed2000

Mechanisms of human motion perception: combining evidence from evoked potentials, behavioural performance and computational modelling.

Based on single cell recordings in monkey, it has been suggested that neural activity can be related directly to psychophysically measured threshold behaviour. Here, we investigated in humans whether evoked potentials correlate with behavioural measurements like discrimination thresholds and reaction time. Subjects were asked to report the perceived direction of object motion stimuli which contained variable amounts of coherent motion. Simultaneously, we recorded evoked potentials with a multielectrode array, or measured the reaction time. We show here that motion coherence had a strong influence on both amplitude and latency of the evoked potential. Stronger motion signals evoked stronger and faster cortical responses. The latency reduction of the motion onset response with increasing coherence correlated very well with the concurrent decrease in reaction time. Taken together, these results suggest that temporal integration is an important step in analysing motion signals to generate a reliable behavioural response. We stimulated a two-dimensional array of correlation-type motion detectors with the same motion sequences, and analysed the distribution of local motion signals according to signal detection theory. Performance resembled that of human subjects when the decision strategy was optimized so as to exclude small signals and, in particular, when the ideal observer had some knowledge about a region of interest in which the object was to be expected.

Cerebral Cortex↗
PubMed1983

Application of pattern evoked potential techniques for evaluating infant perceptual systems.

An evoked potential (EP) test was developed to aid in the diagnostic evaluation of a 16-week-old infant of initially unknown pathology and degree of perceptual deficit. Patterned stimuli were presented at two repetition rates to challenge the infant's visual and auditory information processing capabilities. The data were compared to those obtained from a "normal" infant of the same age and sex and, where possible, to normative population data. The EP data indicated the following: (1) the "suspect" infant's occipital response to visual pattern was abnormal in terms of waveform characteristics and in the relationship of its amplitude to spatial frequency of pattern; (2) in comparing brain response to 1 Hz and 6 Hz stimulation rates, the "suspect" infant's occipital EPs were abnormally attenuated for the faster presentation rate when a patterned visual display was part of the stimulus complex; (3) an auditory speech sound produced equivalent EP effects in both infants. These data indicated that the "suspect" infant's perceptual deficit was restricted to the processing of visual pattern information, possibly due to abnormal functioning of the geniculo-striate system. Subsequent tests and the diagnosis of ocular albinism corroborated the results of the abnormalities suggested by the EP procedure.

Auditory Perceptual Disorders↗
PubMed2006

Effects of long-term object familiarity on event-related potentials in the monkey.

Although some change in the neural representation of an object must occur as it becomes familiar, the nature of this change is not fully understood. In humans, it has been shown that the N170-an evoked visual potential-is enhanced for classes of objects for which people have visual expertise. In this study, we explored whether monkeys show a similar modulation in event-related potential (ERP) amplitude as a result of long-term familiarity by recording ERPs with chronically implanted electrodes over extended training periods spanning many sessions. In each of 3 experiments, we found larger amplitude visual evoked responses to highly familiar images for the time period of 120-250 ms after stimulus onset. This difference was found when the monkeys were trained in an individual-level discrimination task, in a task that required only color discrimination, and even following a viewing-only task. We thus observed this familiarity effect across several tasks and different object categories and further found that the difference between "familiar" and "novel" became smaller as the animals gained experience with the previously unfamiliar objects across multiple test sessions. These data suggest that changes in visual responses associated with familiarity are evident early in the evoked visual response, are robust, and may be automatic, driven at least in part by repeated object exposure.

Animals↗
PubMed2006

Cingulate activation increases dynamically with response speed under stimulus unpredictability.

Functional magnetic resonance imaging studies of cognition require repeated and consistent engagement of the cognitive process under investigation. Activation is generally averaged across trials that are assumed to tax a specific mental operation or state, whereas intraindividual variability in performance between trials is usually considered error variance. A more recent analysis approach postulates that these fluctuations can reflect variation in the very process taxed by the particular trial type. In the present study, participants responded to targets presented randomly in 1 of 4 peripheral locations. By employing a function of reaction time (RT) of individual trials as a linear regressor, brain regions were identified whose activation varied with RT on a trial-by-trial basis. Whole-brain analysis revealed that the anterior cingulate, posterior cingulate, and left angular/superior temporal gyri were more active in trials with faster RT but only when the target location was unpredictable. No such association was seen in trials where the target location was predicted by a central cue. These results suggest a role for the cingulate and angular gyri in the dynamic regulation of attention to unpredictable events. This is in accordance with the function of a default network that is active in the absence of top-down-focused attention and is thought to continuously provide resources for broad and spontaneous information gathering. Exploiting intertrial performance variability may be particularly suitable for capturing such spontaneous and elusive phenomena as stimulus-driven processes of attention.

Adaptation, Physiological↗
PubMed1999

Differential processing of part-to-whole and part-to-part face priming: an ERP study.

We provide electrophysiological evidence supporting the hypothesis that part and whole face processing involve distinct functional mechanisms. We used a congruency judgment task and studied part-to-whole and part-to-part priming effects. Neither part-to-whole nor part-to-part conditions elicited early congruency effects on face-specific ERP components, suggesting that activation of the internal representations should occur later on. However, these components showed differential responsiveness to whole faces and isolated eyes. In addition, although late ERP components were affected when the eye targets were not associated with the prime in both conditions, their temporal and topographical features depended on the latter. These differential effects suggest the existence of distributed neural networks in the inferior temporal cortex where part and whole facial representations may be stored.

Adult↗
PubMed2001

Early visual processing deficits in schizophrenia: impaired P1 generation revealed by high-density electrical mapping.

Integrity of early visual sensory processing in schizophrenia was assessed using the well characterized P1 and N1 components of the visual evoked potential (VEP) as our dependent measures. VEPs were recorded in response to successively less fragmented line drawings of common objects. P1 amplitudes were significantly reduced across all stimulus conditions for patients versus controls. Further, this decrement was relatively greater at parieto-occipital than occipito-temporal electrode sites. No differences in N1 amplitude were found. The finding of P1 deficits in patients, particularly over dorsal scalp, supports the view that schizophrenia is associated with impairment of early dorsal visual stream processing. On the other hand, the finding of normal N1 amplitudes in patients suggests that early stages of ventral stream processing may be relatively more intact. These results imply that the cognitive impairment seen in schizophrenia is not just due to deficits in higher order aspects of cognition but also encompasses significant deficits in early sensory processing.

Adult↗
PubMed2005

Development of visual-evoked potentials to radially modulated concentric patterns.

The visual processing of radially modulated concentric patterns was studied in human participants, aged 3-22 years, by recording event-related potentials. These stimuli are known to activate the fusiform face area as well as area V4 in normal adults. The electrophysiological data showed a P1 latency that reached a maturation asymptote before 3 years of age, whereas that of N1 and P2 became adultlike by 13 years of age. In addition, the distribution of the P2 component over the scalp was focalized in the primary visual cortex before adolescence and became distributed over the entire brain after adolescence. Radially modulated concentric stimuli thus induce brain activation that is not mature until 13 years of age.

Adolescent↗