Imaging orientation selectivity: decoding conscious perception in V1.
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Action Potentials↗
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Results for “Pattern Recognition, Visual”
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Early neural activity in Necker-cube reversal: evidence for low-level processing of a gestalt phenomenon.
Normally we experience the visual world as stable. Ambiguous figures provide a fascinating exception: On prolonged inspection, the "Necker cube" undergoes a sudden, unavoidable reversal of its perceived front-back orientation. What happens in the brain when spontaneously switching between these equally likely interpretations? Does neural processing differ between an endogenously perceived reversal of a physically unchanged ambiguous stimulus and an exogenously caused reversal of an unambiguous stimulus? A refined EEG paradigm to measure such endogenous events uncovered an early electrophysiological correlate of this spontaneous reversal, a negativity beginning at 160 ms. Comparing across nine electrode locations suggests that this component originates in early visual areas. An EEG component of similar shape and scalp distribution, but 50 ms earlier, was evoked by an external reversal of unambiguous figures. Perceptual disambiguation seems to be accomplished by the same structures that represent objects per se, and to occur early in the visual stream. This suggests that low-level mechanisms play a crucial role in resolving perceptual ambiguity.
Adult↗
[Clinical significance of objective vision assessment using visually evoked cortical potentials induced by rapid pattern sequences of different spatial frequency].
BACKGROUND: In patients where reliable subjective assessment of visual acuity is impossible, further diagnostics should be enhanced by an objective method. PATIENTS AND METHODS: A group of 34 patients was examined by objective assessment of visual acuity using visual evoked potentials (VEP) as described by Hajek and Zrenner in 1988. The presentation of five checkerboards with different spatial frequency in repetitive sequences on a TV-monitor elicits a series of transient visual evoked potentials. Shape and amplitude of each wavelet depends on check size and directly reflect a spatial tuning function with a low- and high-frequency cut-off. This amplitude is described by a polynomial fit (2nd order). The function's intersection with the x-axis at higher spatial frequencies leads to an estimation of the visual acuity. RESULT: This result is compared to the subjectively determined visual acuity. In the majority of the presented cases the suspected malingering was confirmed. CONCLUSION: Patients with suspected malingering represent the primary indication of the described method.
Adult↗
[Fast frequency electroretinogram differentiates ocular hypertension].
On the basis of the 30 Hz responses it was possible to divide the patients (total 38 eyes) into two groups. The PERGs of one group (abnormal OHT) resembled responses of typical glaucomatous eyes, with greatly diminished or abolished 30 Hz amplitudes. The PERGs of the other group (normal OHT) were indistinguishable from those of normal eyes.
Diagnosis, Differential↗
[Use of the scanning laser ophthalmoscope for recording pattern electroretinography and visual evoked cortical potentials].
With a scanning laser ophthalmoscope (SLO) variable patterned stimuli can be projected onto the retina. During alternation of these patterns visual evoked cortical potentials and pattern ERGs can be recorded. The configurations of the SLO-elicited potentials and peak latencies correspond to those evoked during conventional stimulation. During pattern stimulation the fundus and alternating pattern stimuli are observed simultaneously on a video monitor. Thus the examiner always knows the exact location of the stimulus on the retina. Scanning laser ophthalmoscopy could be a clinically interesting method of recording evoked potentials, because it enables the patient's retina to be viewed continuously at low light levels and makes electrophysiological examination of a defined region of the retina possible.
Electroretinography↗
Short-term visual recognition memory in pigeons.
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Animals↗
My body or yours? The effect of visual perspective on cortical body representations.
A human body part, such as a foot, may be observed from an egocentric perspective (consistent with looking at one's own body, e.g. top of the foot, toes pointing up) or from an allocentric perspective (only consistent with looking at someone else, e.g. top of the foot, toes pointing downwards). We found that the right extrastriate body area (EBA) response to images of body parts was enhanced for body parts presented from an allocentric perspective. Other areas of extrastriate cortex which responded robustly to images of bodies, including the right lateral occipital complex, right MT and left EBA, nevertheless did not distinguish between the two perspectives. A region of primary somatosensory cortex showed the reverse selectivity: the blood oxygen level-dependent response to body parts presented from an allocentric perspective was suppressed. These results help to illuminate the integration of visual and tactile information by which the brain identifies seen body parts as belonging to the self or to another person.
Adult↗
Electrophysiological markers of visuocortical development.
The development of noninvasive techniques for the assessment of functional brain maturation is critical. The present study analyzed 63 babies' and children's (27 days to 5.5 years) cerebral responses to a pattern-reversal visual stimulation using high-density (128 electrodes) electrophysiological recordings. Developmental data were further compared with those of young adults (n = 16). Tremendous changes in pattern visual evoked potentials (pVEPs) morphology were observed between 7 and 24 months characterized by the emergence of negative components labeled "N70" and "N145" and the reduction of the P100 amplitude. The adult pattern of response appears from 24 months onward. Spectral density values show an increase of higher frequencies with age. Coherence values show a reduction between 3 and 23 months of age as well as a further increase toward adulthood between areas implicated in visual processing. These results are discussed in light of developmental features such as synaptogenesis, myelination, and neuronal networks refinement.
Adult↗
The sightless view: neural correlates of occluded objects.
The relationship between neural activity and object perception has received considerable attention using stimulus manipulations such as masking or dichoptic presentation. Here we investigate the same problem by occluding objects with an opaque screen that acts to dissociate the direct perception of the object from the awareness of its presence. We used functional magnetic resonance imaging to measure brain activity when subjects viewed objects (faces and houses) that underwent occlusion and found that the response of the majority of the fusiform face area (FFA) and lateral occipital cortex is the same whether the object is visible or occluded. This suggests that when objects are directly viewed, activity within object-selective regions may reflect the awareness of presence, not the direct perception, of the object. Additionally, we identify a region of premotor cortex that is selectively activated by occlusion of either object type, suggesting its generic involvement with processing occluded objects.
Adult↗
Automatic grapheme processing in the left occipitotemporal cortex.
In a previous event-related fMRI study, we showed that the left occipitotemporal cortex was specifically involved in the abstract categorization of visually presented alphabetic symbols. Here, we duplicate the fMRI study by measuring visual event-related potentials to verify whether the left posterior cortex supports the processing of graphemic representations at a perceptual, prelexical level. N170 amplitudes elicited by the categorically ambiguous stimulus 'O' were about twice as small in the left as in the right occipitotemporal region, and comparable to that of other letters on the left, and to that of other geometric figures on the right. The side asymmetry suggests that a graphemic module is unilaterally implemented in the left hemisphere, where it automatically processes heteromorphic representations that do not depend on the physical characteristics of the signal, whereas the difference in amplitude suggests that this left graphemic module is in series with, precedes and does not transmit information to, or inhibits a general shape-analysis module.
Adult↗
Stepwise emergence of the face-sensitive N170 event-related potential component.
The present study used a parametric design to characterize early event-related potentials (ERP) to face stimuli embedded in gradually decreasing random noise levels. For both N170 and the vertex positive potential (VPP) there was a linear increase in amplitude and decrease in latency with decreasing levels of noise. In contrast, the earlier visual P1 component was stable across noise levels. The P1/N170 dissociation suggests not only a functional dissociation between low and high-level visual processing of faces but also that the N170 reflects the integration of sensorial information into a unitary representation. In addition, the N170/VPP association supports the view that they reflect the same processes operating when viewing faces.
Adult↗
Spatiotemporal analyses of the N170 for human faces, animal faces and objects in natural scenes.
We assessed the specificity to human faces of the N170 ERP component in the context of natural scenes. Subjects categorized photographs containing human faces, animal faces and various objects. Spatiotemporal topography analyses were performed on the individual ERP data. ERPs elicited by animal faces were similar to human faces ERPs but with a delayed face activity. In the N170 time window, ERPs to human and animal faces had a different topography compared with object ERPs. Such data suggest that N170 generators might process various stimuli with a coarse facial organization and show the care that must be taken in comparing scalp signal to faces and other objects as they are probably generated, at least partially, by different cortical sources.
Adult↗
Transient inability to distinguish between faces: electrophysiologic studies.
It is not known with certainty at which level of face processing by the cortex the distinction between a familiar and an unfamiliar face is made. Subdural electrodes were implanted under the fusiform gyrus of the right temporal lobe in a patient who developed an unusual inability to distinguish differences between faces as part of the epileptic aura ("all faces looked the same"). A cortical region located posterior to the epileptic focus was identified that exhibited a maximum evoked response to the presentation of facial images (N165), but not to objects, scenes, or character strings. Evoked potentials elicited by a variety of visual images indicated that any perturbation away from novel whole-face stimuli produced submaximal responses from this region of the right temporal lobe. Electrical stimulation of this region resulted in an impairment of face discrimination. It was found that presentation of familiar faces (grandmother, treating physician) produced a different response from that observed for novel faces. These observations demonstrate that within 165 msec of face presentation, and before the conscious precept of face familiarity has formed, this cortical region has already begun to distinguish between a familiar and an unfamiliar face.
Adult↗
Aging and pattern visual evoked potentials.
Visual abilities decline during senescence. Age-related declines cannot be attributed entirely to changes in optical factors (senile miosis and opacification of the ocular media) and must therefore be caused by changes in the visual pathways. Because pattern visual evoked potentials (VEPs) provide information regarding the sensory function and integrity of the visual system, recent research on age-related changes in VEPs is reviewed. The results of this review show that stimulus parameters of VEPs (luminance, contrast, spatial and temporal frequencies) modify the age-related changes in VEPs. The visual system consists of multiple, parallel channels which process different information. Therefore, it is hypothesized that aging may affect the specific functional subdivisions in the visual pathways differentially.
Adolescent↗
Psychological defense mechanisms and electroencephalographic arousal.
The aim of this study was to examine the relationship between psychological defense as measured by the Kragh tachistoscopic Defense Mechanisms Test (DMT), and general arousal properties of the individual as measured with electroencephalogram (EEG). The DMT assesses defense by presenting neutral and threatening pictures with very short exposure times. EEG characteristics were measured in the 8-12 Hz EEG frequency band during DMT testing. Twenty-one male subjects participated in the study. All were US Air Force personnel on active duty. Personnel with high defense mechanisms, defined in this connection as slow perception of the threat, tended to have higher cortical arousal after Stimulus exposure than the subjects with low defense mechanisms. The differences occurred mainly with new stimulus material without any threatening content. The differences in arousal response to novel stimuli may contribute to an understanding of the relationships between the Kragh DMT test and performance in life-threatening situations.
Adult↗
Early and late selection in young and old adults: an event-related potential study.
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Adolescent↗
An ERP study on visual spatial priming with peripheral onsets.
Visual event-related potentials were measured for peripheral target stimuli that were preceded by a peripheral square. Targets appeared either at the same location as the square or in the opposite visual hemifield. In Experiment 1, 75% of the trials were same-location trials, and in Experiment 2, same- and opposite-location trials were equiprobable. The subject's overt response was dependent either on the identity or on the location of the target. In both experiments, opposite-location targets elicited an enhanced P1 at posterior electrodes ipsilateral to the position of the letter. This enhancement may be due to a sensory inhibition of same-location targets. Same-location targets elicited an enhanced negativity between 130 and 300 ms, with a first peak located parietally and a second peak broadly distributed over midline electrodes. This effect was larger in Experiment 1 than in Experiment 2 and is interpreted as enhanced processing of same-location targets due to an attentional orienting process elicited by the peripheral square.
Adult↗
Pattern-shift visual evoked responses in cluster headache.
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Adult↗