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At least 649 records · Page 36Linked to original sources
PubMed2005

Time-frequency analysis of target detection reveals an early interface between bottom-up and top-down processes in the gamma-band.

The early visual gamma-band response is an oscillatory signal evoked approximately 100 ms after stimulation. While some studies have found effects of various cognitive processes on this signal, such effects could not be replicated in other studies. Accordingly, some authors have claimed that evoked gamma-band activity reflects merely sensory functions. To resolve these conflicting positions, we conducted a target detection experiment in which the feature that defined the target could be distributed over a large or a small part of the entire stimulus. Only targets covering a larger area of the entire stimulus evoked stronger gamma-band activity than standards although the over-all stimulus size was identical for all stimuli. This increase in evoked activity resulted from stronger oscillatory power and not exclusively from stronger phase-locking. In contrast, N1 and P3 amplitudes were larger for target stimuli irrespective of the distribution of the relevant stimulus feature. These results are consistent with the notion that early gamma-band activity is generated by feature-selective neural assemblies the activity of which can in fact be modulated by top-down processes. This interaction, however, may be only detectable in scalp-recorded EEG if it affects a sufficient number of neural assemblies.

Adult↗
PubMed1997

Similar electrophysiological correlates of texture segregation induced by luminance, orientation, motion and stereo.

Certain local features induce preattentive texture segregation. Recently, components in the visual evoked potential (VEP) associated with preattentive texture segregation (tsVEPs) have been demonstrated. To assess the similarity and dissimilarity of visual processing across visual dimensions, we compared VEPs and tsVEPs in texture segregation by luminance, orientation, motion and stereo disparity. We found tsVEPs across these four visual dimensions to be remarkably similar when compared to the "low-level" VEPs. The tsVEPs were always negative; their implicit time, peak latency and amplitude were (in msec/msec/microV): 91/234/-5.7, luminance; 84/257/-3.9, orientation; 80/295/-8.3, motion; and 95/310/-5.0 for stereo. The cross-correlation function, as a quantitative measure for similarity, on average was higher for the tsVEPs by a factor of 4.2 as compared to the low-level VEPs (P < 0.0001). The results suggest (1) that the tsVEPs represent activity of neural mechanisms that have generalised to some degree across visual dimensions; and (2) that these hypothetical generalisation mechanisms might exist already in the primary visual cortex.

Evoked Potentials, Visual↗
PubMed1997

The development of chromatic and achromatic contrast sensitivity in infancy as tested with the sweep VEP.

Swept-contrast visual evoked potential (VEP) techniques were used to measure the development of contrast sensitivity functions (CSFs) for achromatic and red/green isoluminant chromatic gratings. Subjects were infants of 8, 14, 20 and 32 weeks of age, and adults. Stimuli were 20 deg, 0.3-4 cyc/deg sinusoidal gratings, counterphased at 6 Hz and modulated through white. Achromatic and chromatic CSFs for all ages could be fit simultaneously with a double exponential equation of a common, lowpass shape. Both achromatic and chromatic CSFs exhibited developmental shifts in both sensitivity and spatial scale. From 8 weeks to adulthood, sensitivity increased by 0.64 log units for achromatic gratings and by 0.91 log units for chromatic gratings, yielding an 0.27 log unit larger sensitivity change for chromatic than for achromatic stimuli. Spatial scale shifts were closely similar across achromatic and chromatic CSFs, and were consistent with the factor of about four predicted on the basis of changes in foveal receptor packing density and eye size. The question of uniform vs differential loss of sensitivity for chromatic vs achromatic stimuli at fixed spatial frequencies is discussed.

Adult↗
PubMed1998

Development of directional motion symmetry in the monocular visually evoked potential of infant monkeys.

Motion processing in humans and monkeys exhibit a directional asymmetry during infancy which is not present in adults except following abnormal visual rearing conditions. To characterize the time course for maturation of a symmetric response, we measured the monocular visually evoked potential (MVEP) response to 0.26 c/deg gratings oscillating horizontally at 6 Hz in 13 infant rhesus monkeys between 1 and 52 weeks of age. An asymmetric (F1) and a symmetric (F2) frequency component were extracted from the MVEP using Fourier analysis. At early ages the asymmetric F1 component measured from the two eyes exhibited a 180 deg interocular phase shift, demonstrating that there was a directional bias in opposite directions between the left and right eyes. Although our methods could not determine whether the bias was in the nasal or temporal direction, our results would be consistent with a nasal bias, as has been observed in previous motion studies. Magnitude of the asymmetry was quantified in the form of an asymmetry index, F1/(F1 + F2). Based on developmental changes in the asymmetry index, and phase and amplitudes of F1 and F2, we conclude that the MVEP loses its directional asymmetry at 6 weeks of age. The development of directional motion symmetry observed in monkeys over the first 6 weeks is similar to that observed in humans over the first 5 months.

Aging↗
PubMed1998

Temporal analysis of the chromatic flash VEP--separate colour and luminance contrast components.

Temporal analysis of the chromatic flash visual evoked potential (VEP) was studied in human subjects with normal and anomalous colour vision using a deterministic pseudo-random binary stimulus (VERIS). Five experiments were carried out on four normal subjects investigating heterochromatic red-green exchange and single colour/achromatic (either red/grey or green/grey) exchange over a wide range of luminance ratios for the two stimuli, the effects of lowered mean luminance on the chromatic VEP and the effects of colour desaturation at constant mean luminance and constant luminance contrast. Finally, the performance of three dichromats, a protanope and two deuteranopes, on heterochromatic exchange VEP and on colour desaturation were investigated. In contrast to the chromatic electroretinogram, which shows great symmetry with respect to luminance ratio on opposite sides of the isoluminant point, the chromatic VEP demonstrated a distinct asymmetry when the colours exchanged included red. On the red side of isoluminance (red more luminant than green), a wave with longer latency and altered waveform became dominant. The effects of green stimulation were indistinguishable from those of achromatic stimulation at the same luminance contrast over the whole range of chromatic contrast and for all levels of desaturation studied. Desaturation of red with constant luminance contrast (desaturated red/grey stimulation) resulted in a systematic alteration in the evoked waveform. Subtraction of the achromatic first- and second-order responses from responses recorded in the red desaturation series resulted in remarkably uniform waveforms, with peak amplitudes growing linearly with saturation. The absence of interaction between achromatic and coloured components for all (including the most intense colour) stimulus parameters used suggests that the generators of these components are separate. Recordings from the dichromats showed that the contrast response minimum shifted from the point of photopic isoluminance to the point of zero cone contrast (at the silent substitution point) for the remaining cone type. The waveforms recorded with a series of luminance ratios were much simpler than those recorded from trichromats and symmetrical with respect to their isoluminant points. Despite the indication of the presence of L cones of apparently normal spectral sensitivity in the deuteranopes (on the basis of flicker photometry), there was no evidence for a red-sensitive component in the desaturation or heterochromatic stimulation series. The results are discussed in terms of the possibility of separate generation of chromatic and achromatic contributions to the VEP.

Color Perception↗
PubMed2001

Different processes are involved in human brain for shape and face comparisons.

Fifteen subjects participated in a matching task of visual stimuli. Two sequentially presented stimuli in a pair were the same shape (shape match), different shapes (shape mismatch), same human face (face match) or different faces (face mismatch). All four kinds of stimulus pairs were of equal probability. The shape mismatch pairs elicited a negative event-related potential component N270 (Peak latency: 262.1+/-16.5 ms, P4) after the second stimulus onset, while the face mismatch evoked N270 with longer peak latency (301.2+/-19.8 ms, P4) and N450. There exists a specific system for mismatch processing of a complex stimulus (face) in the human brain.

Cerebral Cortex↗
PubMed2002

Magnetic responses of human visual cortex to illusory contours.

To examine the neural mechanism underlying illusory-contour perception, we measured the magnetic responses of the human visual cortex to an abutting-line grating inducing illusory contours (test stimulus) and a non-abutting-line grating (control stimulus) using the technique of magnetoencephalography (MEG). In the initial latency period of 60-80 ms, the MEG response to the test stimulus was nearly identical with that to the control stimulus, but in the subsequent period of 80-150 ms, the former was larger than the latter. The origin of the peak MEG response to the test stimulus was estimated to be in the vicinity of striate cortex/extrastriate visual cortex for two of the four subjects. These results suggest that, in accord with those of the previous electrophysiological and functional magnetic resonance imaging studies, illusory-contour signals are generated in the very early stage(s) of processing in the primate visual cortex.

Adult↗
PubMed1996

Mental representations of morphologically complex words: an event-related potential study with adult humans.

Event related brain potentials (ERPs) were recorded from 13 subjects while performing a lexical decision task. Regular and irregular German principles were presented twice (identical repetition) or were primed by their related infinitives (morphological repetition). Regular participles primed by their infinitives displayed ERP waveforms similar to those evoked by identical repetitions, namely a large positivity with an onset latency of about 200 ms. Irregular participles, by contrast, showed no reliable morphological repetition effects. The results are discussed in terms of the controversy between approaches proposing a single representational mechanism for regular and irregular inflection and the "dual mechanism' account that suggests an associative memory for the storage of irregulars and a rule-based symbolic processor for generating regular forms.

Adolescent↗
PubMed1997

Visual evoked cortical magnetic fields to pattern reversal stimulation.

We studied visual evoked magnetic fields to pattern reversal stimulation in six healthy subjects. Similar to the N75-P100-N145 components in visual evoked potentials, triphasic deflections, N75m-P100m-N145m, were clearly observed around the midoccipital position. A very small component, P50m, was occasionally observed preceding the N75m. Equivalent current dipoles (ECDs) of the main deflection, P100m, to quadrant-field stimulation were estimated near or around the calcarine fissure contralateral to the stimulation. The vertical ECD location of the P100m to the upper quadrant-field stimulation was estimated significantly lower (0.81 +/- 0.45 cm) than those to lower stimulation. These results were compatible with the retinotopic organization of the visual cortex (cruciform model) and suggested that the P100m originated in the striate cortex. The small P50m, although only a small number of ECDs could be estimated reliably, was located in the contralateral visual cortex. ECDs of the N75m were estimated mainly near or around the contralateral calcarine fissure. ECDs of the N145m were estimated also retinotopically, but with a greater vertical distance (2.90 +/- 1.09 cm) between upper and lower quadrant-field stimulation. MR-overlaid ECDs of the N145m suggested that these originated in the extrastriate cortex. No ECD was estimated when a probe was placed at the midfrontal position.

Adult↗
PubMed2003

Face versus non-face object perception and the 'other-race' effect: a spatio-temporal event-related potential study.

OBJECTIVE: To investigate a modulation of the N170 face-sensitive component related to the perception of other-race (OR) and same-race (SR) faces, as well as differences in face and non-face object processing, by combining different methods of event-related potential (ERP) signal analysis. METHODS: Sixty-two channel ERPs were recorded in 12 Caucasian subjects presented with Caucasian and Asian faces along with non-face objects. Surface data were submitted to classical waveforms and ERP map topography analysis. Underlying brain sources were estimated with two inverse solutions (BESA and LORETA). RESULTS: The N170 face component was identical for both race faces. This component and its topography revealed a face specific pattern regardless of race. However, in this time period OR faces evoked significantly stronger medial occipital activity than SR faces. Moreover, in terms of maps, at around 170 ms face-specific activity significantly preceded non-face object activity by 25 ms. These ERP maps were followed by similar activation patterns across conditions around 190-300 ms, most likely reflecting the activation of visually derived semantic information. CONCLUSIONS: The N170 was not sensitive to the race of the faces. However, a possible pre-attentive process associated to the relatively stronger unfamiliarity for OR faces was found in medial occipital area. Moreover, our data provide further information on the time-course of face and non-face object processing.

Adult↗
PubMed2003

From immature to mature pattern ERG and VEP.

Electrophysiological parameters in children reflect not only pathological processes, but also maturation of the visual system. This review investigates PERG and PVEP changes in infants and schoolchildren in order to establish the time scale for electrophysiological maturation of the visual system. Studies using pattern-reversal and pattern-onset stimulation are presented. Maturational changes have been found to be rapid in infants and gradual in schoolchildren. PERG age-related changes in infants were seen as a decrease in latency and in schoolchildren as a decrease in amplitude. PVEP age-related changes in infants were seen as a decrease in latency, an increase in amplitude and the development of the waveform. In schoolchildren, PVEP changes were more gradual, with a decrease in latency, a decrease in amplitude and transformation of the waveform. In conclusion, electrophysiological maturation proceeds until adulthood. Therefore, in paediatric clinical work, normative values in infants and schoolchildren are an important factor in differentiating maturation of the visual system from pathological processes.

Adolescent↗
PubMed1991

Modulations of sensory-evoked brain potentials indicate changes in perceptual processing during visual-spatial priming.

Reaction time (RT) differences to visual stimuli as a function of expectancy have been attributed to changes in perceptual processing or entirely to shifts in decision and response criteria. To help distinguish between these competing interpretations, event-related brain potentials (ERPs) were recorded to lateralized flashes delivered to visual field locations precued by a central arrow (valid stimuli) or not precued (invalid stimuli). Validly cued stimuli in both simple and choice RT tasks elicited consistent amplitude enhancements of the early, sensory-evoked PI component of the ERP recorded at scalp sites overlying lateral prestriate visual cortex (90-130 ms poststimulus). In contrast, the subsequent N1 component (150-200 ms) was enhanced by validly cued stimuli in the choice RT task condition only. These electrophysiological findings support models proposing that the behavioral effects of precuing expected target locations are due, at least in part, to changes in sensory-perceptual processing. Furthermore, these data provide specific information regarding the neural mechanisms underlying such effects.

Adolescent↗
PubMed1981

[Visually evoked cortical potentials (VECP) in patients with doubtful impairment of central vision (author's transl)].

Visual evoked cortical potential (VECP) to non-structured, brightness (luminance) stimuli do not provide an appropriate means for assessing the degree of impaired central vision. In comparison, visually evoked cortical potentials to structured (patterned) stimuli presented by pattern reversal of equal mean luminance are highly susceptible to visual changes caused by reduced central vision. Employing different check sizes, the use of checkerboard pattern reversal VECPs permits, independently of the patient, to assess the impairment of central vision.

Accommodation, Ocular↗
PubMed1998

Anatomy of word and sentence meaning.

Reading and listening involve complex psychological processes that recruit many brain areas. The anatomy of processing English words has been studied by a variety of imaging methods. Although there is widespread agreement on the general anatomical areas involved in comprehending words, there are still disputes about the computations that go on in these areas. Examination of the time relations (circuitry) among these anatomical areas can aid in understanding their computations. In this paper, we concentrate on tasks that involve obtaining the meaning of a word in isolation or in relation to a sentence. Our current data support a finding in the literature that frontal semantic areas are active well before posterior areas. We use the subject's attention to amplify relevant brain areas involved either in semantic classification or in judging the relation of the word to a sentence to test the hypothesis that frontal areas are concerned with lexical semantics and posterior areas are more involved in comprehension of propositions that involve several words.

Adult↗
PubMed1998

Figure-ground activity in primary visual cortex is suppressed by anesthesia.

By means of their small receptive fields (RFs), neurons in primary visual cortex perform highly localized analyses of the visual scene, far removed from our normal unified experience of vision. Local image elements coded by the RF are put into more global context, however, by means of modulation of the responses of the V1 neurons. Contextual modulation has been shown to follow closely the perceptual interpretation of the scene as a whole. This would suggest that some aspects of contextual modulation can be recorded only in awake and perceiving animals. In this study, multi-unit activity was recorded with implanted electrodes from primary visual cortex of awake, fixating monkeys viewing textured displays in which figure and ground regions were segregated by differences in either orientation or motion. Contextual modulation was isolated from local RF processing, by keeping RF stimulation identical across trials while sampling responses for various positions of the RF relative to figure and ground. Contextual modulation was observed to unfold spatially and temporally in a way that closely resembles the figure-ground percept. When recording was repeated, but with the animals anesthetized, the figure-ground related modulatory activity was selectively suppressed. RF tuning properties, however, remained unaffected. The results show that the modulatory activity is functionally distinct from the RF properties. V1 thus hosts distinct regimes of activity that are mediated by separate mechanisms and that depend differentially on the animal being awake or anesthetized.

Anesthesia, Inhalation↗
PubMed1995

Visual evoked potentials in early Alzheimer's dementia: an exploratory study.

Topographical maps of visual evoked potentials (VEPs) were recorded from 10 possible Alzheimer's disease (AD) patients and 10 control subjects. The purpose of the study was to determine if the two types of VEPs could function as a diagnostic screening for AD. Results of the statistical analysis did not reveal any latency differences between VEPs for the components elicited by either the pattern shift visual evoked potential (PSVEP)--or flash visual evoked potential (FVEP)--elicited components for AD patients compared with the control subjects; however, the information provided insight into results that are frequently lost with conventional evoked potential data. Statistically significant differences in amplitude were found between the P1 and the N2 of the PSVEP at 124, 126, and 130 ms, and at 116 ms for the FVEP.

Aged↗
PubMed2006

Electrophysiological evidence for temporal dissociation between spatial attention and sensory competition during human face processing.

Scalp event-related potential (ERP) studies in humans indicate that face processes taking place between 130 and 170 ms after stimulus onset at posterior sites (N170) are strongly reduced when another face stimulus is processed concurrently or has been presented shortly before for a prolonged period. These observations suggest that neural representations of individual faces compete in the occipitotemporal cortex as early as 130 ms. Here, we tested the respective role of spatial attention and sensory competition in accounting for the amplitude reduction of the N170 during concurrent face stimulation. ERPs time locked to a lateralized face stimulus were recorded while subjects were fixating either a face or a controlled scrambled-face stimulus (context factor) and were engaged in either a high- or a low-attentional load task at fixation (task factor). The N170 amplitude to the lateralized face stimulus was reduced both when the central stimulus was a face compared with a scrambled face and when the attentional load at fixation was high. However, these effects of context and task factors were largely additive. Most importantly, spatial attention modulated visual processes as early as 80 ms after stimulus onset, whereas sensory competition effects started at about 130 ms. These results provide strong evidence that the N170 in response to faces is modulated by spatial attention, and also that spatial attention and sensory competition do not reflect the same mechanisms of early selection of visual information in the extrastriate cortex.

Adult↗