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PubMed1983

Retinal and cortical pattern responses: a comparison of infants and adults.

Retinal and cortical responses to pattern reversal stimulation were recorded simultaneously in three human infants of average age 3.5 months and two adults of average age 28.5 years. The relative power of retinal signals as a function of spatial frequency for both adults and infants was very similar and extrapolated to a threshold of about 30 c/deg [6/6 (20/20)] as did the relative power of the adult cortical signal. The infant cortical signal was relatively more attenuated at higher spatial frequencies and extrapolated to a threshold of 8.5 c/deg [6/22.2 (20/74)]. A greater relative maturity of retinal as compared with cortical neural function in 3.5-month-olds is inferred.

Adult↗
PubMed1997

Development of pattern visual evoked potentials: longitudinal measurements in human infants.

PURPOSE: This experiment used longitudinal testing to trace the emergence of the major components of pattern visual evoked potentials (VEPs) in infants, using two paradigms: large-checkerboard pattern reversal and low spatial frequency pattern onset. METHODS: Testing with both pattern-reversal and pattern-onset stimuli was performed on the same infants. Testing was conducted at weekly intervals during the first three postnatal months, and at intervals of 2 weeks to 1 month thereafter. RESULTS: The pattern-reversal and early pattern-onset responses recorded within individual subjects showed remarkably systematic developmental sequences. The broad, positive component seen at 200 to 250 ms in infants could be traced readily through the developmental sequence, to become the more sharply tuned positive component seen at about 100 ms in adults. Responses to low spatial frequency pattern onsets in infants were larger and more reliable than those in adults. The late components of the pattern-onset response, generally attributed to pattern offset, emerged later and with more complex changes. In all cases, response amplitude was much more variable than response latency, both within and between subjects. CONCLUSIONS: Frequent VEP recording in a longitudinal design can reveal systematic and detailed transitions of wave-form during development.

Adolescent↗
PubMed2005

Position-specificity of facial adaptation.

We investigated the representation of objects' position at the higher, shape-selective stages of visual processing by testing the position-specificity of the behavioural and neural effects of facial adaptation. Here, we show that facial after-effects evoked by adaptation to both upright and upside-down faces are significantly larger when the adaptor and test faces are presented on the same retinal position than when they are displayed in different hemifields. Our event-related potential recordings revealed that adaptation effects measured on the amplitude of the N170 event-related potential component over the hemisphere that was contralateral to the test face stimulus also show strong position-specificity. These findings suggest that face adaptation effects are only partially translation invariant and facial after-effects measured with peripheral test stimuli primarily reflect the adaptation processes in the contralateral hemisphere.

Adaptation, Physiological↗
PubMed2006

Causal influences in the human brain during face discrimination: a short-window directed transfer function approach.

In this letter, we considered the application of parametric spectral analysis, namely a short-window directed transfer function (DTF) approach, to multichannel electroencephalography (EEG) data during a face discrimination task. We identified causal influences between occipitoparietal and centrofrontal electrode sites, the timing of which corresponded to previously reported EEG face-selective components. More importantly we present evidence that there are both feedforward and feedback influences, a finding that is in direct contrast to current computational models of perceptual discrimination and decision making which tend to favor a purely feedforward processing scheme.

Algorithms↗
PubMed2003

Semantic categorization in the human brain: spatiotemporal dynamics revealed by magnetoencephalography.

We examined the cortical representation of semantic categorization using magnetic source imaging in a task that revealed both dissociations among superordinate categories and associations among different base-level concepts within these categories. Around 200 ms after stimulus onset, the spatiotemporal correlation of brain activity elicited by base-level concepts was greater within than across superordinate categories in the right temporal lobe. Unsupervised clustering of data showed similar categorization between 210 and 450 ms mainly in the left hemisphere. This pattern suggests that well-defined semantic categories are represented in spatially distinct, macroscopically separable neural networks, independent of physical stimulus properties. In contrast, a broader, task-required categorization (natural/man-made) was not evident in our data. The perceptual dynamics of the categorization process is initially evident in the extrastriate areas of the right hemisphere; this activation is followed by higher-level activity along the ventral processing stream, implicating primarily the left temporal lobe.

Adult↗
PubMed2006

Repetition suppression of induced gamma band responses is eliminated by task switching.

The formation of cortical object representations requires the activation of cell assemblies, correlated by induced oscillatory bursts of activity > 20 Hz (induced gamma band responses; iGBRs). One marker of the functional dynamics within such cell assemblies is the suppression of iGBRs elicited by repeated stimuli. This effect is commonly interpreted as a signature of 'sharpening' processes within cell-assemblies, which are behaviourally mirrored in repetition priming effects. The present study investigates whether the sharpening of primed objects is an automatic consequence of repeated stimulus processing, or whether it depends on task demands. Participants performed either a 'living/non-living' or a 'bigger/smaller than a shoebox' classification on repeated pictures of everyday objects. We contrasted repetition-related iGBR effects after the same task was used for initial and repeated presentations (no-switch condition) with repetitions after a task-switch occurred (switch condition). Furthermore, we complemented iGBR analysis by examining other brain responses known to be modulated by repetition-related memory processes (evoked gamma oscillations and event-related potentials; ERPs). The results obtained for the 'no-switch' condition replicated previous findings of repetition suppression of iGBRs at 200-300 ms after stimulus onset. Source modelling showed that this effect was distributed over widespread cortical areas. By contrast, after a task-switch no iGBR suppression was found. We concluded that iGBRs reflect the sharpening of a cell assembly only within the same task. After a task switch the complete object representation is reactivated. The ERP (220-380 ms) revealed suppression effects independent of task demands in bilateral posterior areas and might indicate correlates of repetition priming in perceptual structures.

Adult↗
PubMed1990

Event-related potentials in a visual discrimination task: negative waves related to detection and attention.

Event-related potential effects of deviant stimuli were investigated in a visual discrimination task. The stimuli (two angles within a frame) were either frequent (Standard) or one of two types of infrequent deviant (Deviant 1 or Deviant 2) stimuli. In comparison to the Standard stimuli, for Deviant 1 the two angles differed in their orientation, whereas for Deviant 2 the angles were identical but the frame was thicker. In Condition 1 the subjects counted the number of Deviant 1 stimuli. Of the 13 subjects, 12 did not detect the fact that some of the frames were thicker in this condition (i.e., for the Deviant 2 stimuli in Condition 2). The task in Condition 2 was the same (i.e., the target was Deviant 1), but the subjects were instructed about the thicker frame of Deviant 2 stimuli. In Condition 3, Deviant 2 stimuli became the targets. In comparison to the Standard, Deviant 1 elicited two posterior negative waves in the 120-180 and 240-300 ms latency ranges respectively. In addition, when Deviant 1 was the target stimulus (Conditions 1 and 2), this stimulus elicited the N2 and the P300 as well. In contrast, the irrelevant Deviant 2 elicited no such waves. In the target position (Condition 3), Deviant 2 elicited the second posterior negativity, the N2 and the late positivity. The earlier negativity is considered to be a correlate of processes connected to the automatic detection of the deviant features, whereas the later negativity is considered to be related to attentive processes, i.e., this wave is considered to be a member of the family of processing negativities.

Adult↗
PubMed1994

Electrophysiological correlates of feature analysis during visual search.

Event-related brain potentials (ERPs) were recorded from normal young adults during visual search tasks in which the stimulus arrays contained either eight identical items (homogeneous arrays) or seven identical items and one deviant item (pop-out arrays). Four experiments were conducted in which different classes of stimulus arrays were designated targets and the remaining stimulus arrays were designated nontargets. In Experiments 1 and 2, both target and nontarget pop-out stimuli elicited an enhanced anterior N2 wave and a contralaterally larger posterior P1 wave, but Experiments 3 and 4 demonstrated that these components do not reflect fully automatic pop-out detection processes. In all four experiments, target pop-outs elicited enlarged anterior P2, posterior N2, occipital P3, and parietal P3 waves. The target-elicited posterior N2 wave contained a contralateral subcomponent (N2pc) that exhibited a focus over occipital cortex in maps of current source density. The overall pattern of results was consistent with guided search models in which preattentive stimulus information is used to guide attention to task-relevant stimuli.

Adolescent↗
PubMed1995

Selective processing of two-dimensional visual stimuli in young and old subjects: electrophysiological analysis.

In this study, we compared selection of conjunctions of spatial frequency and orientation in young and old subjects, using performance and event-related brain potential (ERP) measures. Responses to target conjunctions were slightly but significantly slower in older subjects; lateralized readiness potential (LRP) onsets did not differ between groups. Older subjects made more false-positive responses to stimuli sharing only spatial frequency with the target. Difference waves associated with selection of separate dimensions included frontal positivity and occipital and central negativity and were 20-50 ms slower in older subjects. Later parts of these difference waves had smaller amplitudes in older subjects. Sensory discrimination, assessed through the effects of physical stimulus parameters on ERPs, was not clearly affected by age. Data are interpreted in terms of a relationship in older subjects between longer latencies of difference waves but equal LRP onsets and a higher false-alarm rate.

Adult↗
PubMed1997

An ERP study of visual spatial attention and letter target detection for isoluminant and nonisoluminant stimuli.

The event-related potential (ERP) effects of visual spatial attention and letter target detection for stimuli presented against a (nonisoluminant) dark background or against an isoluminant grey background were investigated. The goal was to study how the perceptual variable of luminance would influence early ERP reflections of selective attention. Such effects could further substantiate the claim that selective attention operates at the level of early perceptual processing and could provide evidence regarding the role of different visual routes in selective attention. Isoluminance increased the peak latency of the early ERP deflections (NP80, P1, and N1) by 40-50 ms. The ERP effects of spatial attention, consisting of P1 and N1 amplitude enhancements, were similarly delayed by isoluminance, supporting the idea that early selective processing is strongly dependent on bottom-up perceptual processing. P300 latency and reaction time were delayed by 70-75 ms, the additional delay probably reflecting that isoluminance affected decision processes in addition to perceptual processes. Isoluminance left the scalp topographies of the early ERP deflections largely unaffected, although a slight shift of the N1 topography in the isoluminant condition toward more inferior lateral posterior regions of the scalp could have reflected an increased contribution from ventral (occipitotemporal) brain areas. Relative to nontarget letters, targets presented at both attended and unattended spatial positions elicited an early contralaterally dominant lateral occipitotemporal negativity (N2pc). This ERP component is proposed to reflect an early, partly automatic process of template matching, consistent with indications from spatiotemporal dipole modelling that the N2pc was generated in inferior occipitotemporal brain regions.

Adult↗
PubMed1988

Pattern reversal visual evoked potential among men at risk for alcoholism.

The biological sons of male alcoholics, deemed to be at high risk (HR) for the development of alcoholism, were compared to control males, aged 18 to 21, using measures of the visual evoked potential elicited by checkerboard pattern reversal. Overall, the HR and control groups were not distinguished on the basis of visual evoked potential measures acquired from the occipital scalp region; however, when comparisons were restricted to right-handed subjects, the HR subjects showed more symmetry in a positive component with approximate latency of 242 ms compared with control subjects. The results are discussed in relation to hemispheric differences and alcoholism.

Adolescent↗
PubMed1982

The effects of visual field changes and ocular hypertension on the visual evoked potential.

The effects of visual field changes and ocular hypertension on visual evoked potentials were investigated by photopic ERG and by luminance and pattern-reversal EPs on 116 glaucomatous and on 7 normal eyes. The problem was approached by way of four investigations: Firstly, which nerve structures are affected by glaucoma and how do visual field defects caused by glaucoma influence the EP? The results show a functional diminution of all intraocular nerve structures in which the prelaminary part of the optic nerve is most affected. The EPs, especially the pattern-reversal EPs, are markedly diminished if the visual field defects extend inside the 10 degree boundary. Differences in the visual field defects of both eyes and the course of the sickness can be well observed by the EPs. Secondly, which preoperative prognosis for visual acuity produced by the EP can be given to patients who have a dense cataract in addition to glaucoma? A postoperative improvement of the visual acuity can be expected if the L-EPs are within the standard deviation. If the EP is distinctly diminished and does not increase with increasing stimulus intensity, then there is no hope for an improvement of the visual acuity after the operation. Thirdly, does a decrease of intraocular pressure in chronic and acute glaucoma influence the EP? In acute glaucoma with pressure levels of 50 mmHg or more, and sometimes in chronic glaucoma with pressure levels of about 30 mmHg, an increase of the amplitude of the EP and an improvement of the visual field could be noticed after pressure regulation. Fourthly, what is the behaviour of the EP in normal and glaucomatous eyes at experimentally elevated intraocular pressure? The amplitudes of the ERG components show a gradual decrease in normal as well as in glaucomatous eyes when intraocular pressure is increased and are maintained when intraocular pressure reaches systolic ophthalmic blood pressure. On the other hand, the EPs show a strong decrease in amplitude when intraocular pressure exceeds the mean ophthalmic blood pressure, particularly in the case of glaucomatous eyes.

Evoked Potentials, Visual↗