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PubMed1994

Positional disparity sensitivity of neurons in the cat accessory optic system.

There is considerable evidence supporting the view that the accessory optic system (AOS) and the closely associated nucleus of the optic tract (NOT) provide visual signals used in the control of optokinetic nystagmus (OKN). In frontal-eyed animals such as the cat and primate, the high degree of overlap in the visual fields of each eye, along with a substantial projection from the visual cortex, gives rise to an increased incidence of binocularly responsive neurons in the AOS. In previous studies, my collaborators and I have shown that visual cortical input to the AOS mediates ipsilateral eye responses and high speed tuning, and can function independently of the contralateral eye. However, beyond fairly gross assessments such as these, the binocular interactions of AOS cells have not been subject to detailed examination. The present study set out to determine whether the responses of binocular cells in the dorsal terminal nucleus (DTN) of the AOS are sensitive to horizontal retinal disparity. Single units were recorded from the DTN of anaesthetized, paralysed cats. A large random-dot pattern was moved under computer control at a constant velocity in the preferred and non-preferred direction. Convergent and divergent disparities were generated by deviating the visual axis of the contralateral (dominant) eye using wedge prisms. The responses of DTN units fell into three categories: (1) cells showing tuned excitatory responses (29% or 7 cells) consisting of a marked facilitation for a single or a limited range of disparities; (2) cells broadly tuned for inhibition (25% or 6 cells); and (3) cells relatively insensitive to disparity (46% or 11 cells), showing a relatively flat response profile across the entire range of disparity conditions, or cells without clear tuning. In summary, this study demonstrates that some AOS cells are sensitive to positional disparity and, therefore, this system may provide signals which specify the plane of motion for ocular stabilization. Some of these results have been presented in brief form [Grasse (1991a) Society of Neuroscience Abstracts, 17, 1380].

Animals↗

PubMed1996

Is the motion system relatively spared in amblyopia? Evidence from cortical evoked responses.

Visual evoked potentials (VEPs) produced by pattern reversal were compared with those elicited by onset of motion in 37 amblyopic children (20 with anisometropic amblyopia, seven with strabismic amblyopia and 10 with both anisometropia and strabismus). The amplitudes and peak latencies of the main P1 peak in the pattern-reversal VEP and of the motion-specific N2 peak in the motion-onset VEP through the amblyopic eye were compared with those through the normal fellow eye. Regardless of the type of amblyopia, the amplitude of the pattern-reversal VEP for full-field stimulation was significantly smaller and its latency significantly longer through the amblyopic eye (P < 0.001). In contrast, neither the amplitudes nor the latencies of the N2 motion-onset VEPs differed significantly between amblyopic and non-amblyopic eyes. For pattern-reversal VEPs through the amblyopic eyes, the extent to which amplitude was reduced and latency prolonged correlated well with the reduction of visual acuity, whereas the amplitudes and latencies of motion-onset VEPs did not vary with visual acuity. Even for stimuli restricted to the central visual field (5 or 2 deg diameter) or to the peripheral field (excluding the central 5 deg), motion-onset responses were indistinguishable through the two eyes, while pattern-reversal responses always differed significantly in amplitude. These results suggest that the source of motion-onset VEPs (probably an extrastriate motion-sensitive area) is less affected in amblyopia than that of pattern-reversal VEPs (probably the striate cortex). The motion pathway, presumably deriving mainly from the magnocellular layers of the lateral geniculate nucleus, may be relatively spared in amblyopia.

Adolescent↗

PubMed1991

Age-related features of the resting pattern-reversal visual evoked response using the dipole localization method and cortical imaging technique.

Two mathematical techniques, the dipole localization method (DLM) and the cortical imaging technique (CIT), are used to analyze the resting visual response to pattern-reversal stimulation. These methods identify certain age-related features of this evoked response that are not found by using standard topographic maps. These features include the symmetry of the N1 and P1 responses. The amplitudes of the N1 and P2 responses and the latency of N2 are also significantly different between old and young groups of test subjects, findings consistent with differences seen in conventional topographical analyses. Theoretical dipole sources and simulated cortical surface maps are also constructed for the "average" normal older subject and one patient with documented progressive frontal lobe degenerative disease. Standard topographical imaging studies of this patient were unremarkable, except for the P300 auditory response. DLM and CIT analyses of the VER components were exceptional and consistent with the clinical diagnosis. These mathematical methods appear to enhance the discriminating power of traditional electrophysiological measures.

Adult↗

PubMed1986

Amplitudes of visually evoked potentials to patterned stimuli: age and sex comparisons.

Electrophysiological age and sex differences in visual pattern responsivity were investigated. Pattern reversal evoked potentials (PREPs) and visual evoked potentials (VEPs) to patterned and unpatterned flashes were recorded from 20 normal subjects in each of 4 groups: young females and males aged 25-35 years and older females and males aged 55-70 years. PREP waves N70-P100 and P100-N150 from the older women were significantly larger than those from subjects in the other groups; mean amplitudes for the young females, young males and older males were not different. A similar effect, unusually large potentials for the older women, was obtained for VEPs, but only for VEPs elicited by patterned flashes and recorded from occipital scalp, i.e., an area overlying visual cortex which is sensitive to lines and edges. Our findings suggest that the visual system of older females is unusually responsive to patterned stimuli.

Adult↗

PubMedDate not supplied

N70 and P100 can be independently affected in multiple sclerosis.

We have studied the relationship between N70 and P100 of the pattern visual evoked potential in 98 patients with multiple sclerosis and in 59 controls. In patients with multiple sclerosis, P100 was either absent or had prolonged latency in 121 eyes (61.7%), while N70 was absent or prolonged in 97 eyes (49.5%). The total number of eyes with either N70 and/or P100 abnormalities was 137 (69.9%). Eighty eyes (40.8%) had abnormal latency of both P100 and N70, while 41 eyes showed P100 delays without corresponding N70 changes. Seventeen eyes had abnormal N70, but normal P100 latency. N70 and P100 appear to be more often absent in the definite rather than in the possible multiple sclerosis group. These data show that N70 and P100 can be independently affected in patients with MS.

Adolescent↗

PubMed1990

Event-related potentials and the semantic matching of pictures.

Event-related potentials (ERPs) were recorded from one midline and three pairs of lateral electrodes while subjects determined whether pairs of sequentially presented pictures were semantically associated. The ERPs evoked by the second picture of each pair differed as a consequence of whether it was associated with its predecessor, such that ERPs to nonassociated pictures were more negative-going than those to associated items. These differences resulted from the modulation of two ERP components, one frontally distributed and centered on an N300 deflection, the other distributed more widely over the scalp and encompassing an N450 deflection. The modulation of N450 is interpreted as further evidence that the "N400" ERP component is sensitive to semantic relationships between nonverbal stimuli. The earlier N300 effects, which do not appear to occur when ERPs are evoked by semantically primed and unprimed words, could suggest that the semantic processing of pictorial stimuli involves neural systems different from those associated with the semantic processing of words.

Adolescent↗

PubMed1992

Event related potentials, reaction time, and cognitive performance in idiopathic Parkinson's disease.

Sixteen non-demented patients with idiopathic Parkinson's disease (PD) with varying degrees of cognitive impairment and sixteen age-, sex- and education-matched normal controls were examined with (1) an auditory oddball paradigm requiring counting or a motor response in separate determinations, (2) a reaction time task with movement time component and (3) a detailed clinical and neuropsychological test battery. Patients were impaired on a number of neuropsychological tests. They also showed an increased P2 and N2 latency, but no significant increase in P3 latency. Their response initiation times and reaction times during the oddball experiment were not different from controls, whereas movement time was significantly increased. Increased peak latencies, particularly for N2, were moderately associated with Parkinsonian motor impairment in patients and with the Benton Multiple Choice Visual Retention Test in patients and controls. Movement time was associated with P3 latency only in controls and in both groups with the Benton Multiple Choice Visual Retention Test. The observed pattern of results suggests that in non-demented PD patients ERP peak latencies, visuo-spatial task performance and Parkinsonian motor impairment share a significant degree of variance. While impairments in neuropsychological tests and delay in the earlier peaks P2 and N2 do not appear to be sensitive to medication with L-DOPA, normal P3 latencies might indicate good pharmacological symptom control in the absence of dementia.

Aged↗

PubMed1994

P300 is unaffected by glucose increase.

The effects of glucose ingestion on the P3(00) event-related brain potential (ERP) were investigated by using a visual stimulus oddball paradigm in which subjects discriminated between checkerboard (target) and horizontal line (standard) stimuli. Subjects were assessed for six consecutive trial blocks that were spaced 20 min apart on two different occasions. For the glucose condition, an initial baseline trial block was recorded followed by ingestion of 100 mg of glucose and the remaining five trial blocks recorded. For the water condition, the same procedure was employed with water ingested instead of glucose. Blood glucose levels, heart rate, and body temperature measures also were obtained before each trial block. P3 amplitude and latency did not change across trial blocks for the glucose/water conditions. No glucose/water effects were observed for the N1, P2, or N2 components as well. Blood glucose levels and heart rate increased for the glucose but not the water condition; body temperature decreased with the ingestion of both glucose and water and then returned to baseline levels. These findings suggest that ERPs are not influenced by increases of blood glucose level and are discussed in the context of previous ERP studies employing glucose manipulations.

Adult↗

PubMed2005

Changes in neural complexity of the EEG during a visual oddball task.

OBJECTIVE: Neural complexity (C(N)) was introduced by Tononi et al. in an information-theoretic framework to capture the balance between functional specialisation and integration in the brain. We hypothesised that C(N) should vary during cognitive processing, specifically during an oddball task. METHODS: In 11 normal human subjects, we recorded from groups of EEG electrodes in the frontal (F), central-parietal (CP) and occipito-temporal (OT) regions during a visual oddball reward conditioning task and calculated C(N) in each region. Three types of visual stimulus (abstract shapes, called neutral, reward and penalty) were presented randomly in three blocks of trials. During the first block, subjects did not know the significance of the stimulus shapes. For the subsequent (conditioning) blocks, subjects were informed that whenever they saw reward or penalty patterns, they would win or lose money, respectively. RESULTS: In regions CP and OT, C(N) was significantly larger in reward and penalty trials than in neutral during all blocks. During a trial, significant changes in C(N) occurred around the ERP peaks N1 and P300 and the effects of reward conditioning on C(N) could be distinguished from penalty. CONCLUSIONS: Our findings support the above hypothesis, indicating that C(N) correlates with both the sensory and cognitive components of stimulus processing. SIGNIFICANCE: This study extends the scope of C(N) in the analysis of cognitive processing.

Adult↗

PubMed2005

EEG phase synchrony differences across visual perception conditions may depend on recording and analysis methods.

OBJECTIVE: (1) To investigate the neural synchrony hypothesis by examining if there was more synchrony for upright than inverted Mooney faces, replicating a previous study; (2) to investigate whether inverted stimuli evoke neural synchrony by comparing them to a new scrambled control condition, less likely to produce face perception. METHODS: Multichannel EEG was recorded via nose reference while participants viewed upright, inverted, and scrambled Mooney face stimuli. Gamma-range spectral power and inter-electrode phase synchrony were calculated via a wavelet-based method for upright stimuli perceived as faces and inverted/scrambled stimuli perceived as non-faces. RESULTS: When the frequency of interest was selected from the upright condition exhibiting maximal spectral power responses (as in the previous study) greater phase synchrony was found in the upright than inverted/scrambled conditions. However, substantial synchrony was present in all conditions, suggesting that choosing the frequency of interest from the upright condition only may have been biased. In addition, artifacts related to nose reference contamination by micro-saccades were found to be differentially present across experimental conditions in the raw EEG. When frequency of interest was selected instead from each experimental condition and the data were transformed to a laplacian 'reference free' derivation, the between-condition phase synchrony differences disappeared. Spectral power differences were robust to the change in reference, but not the combined changes in reference and frequency selection criteria. CONCLUSIONS: Synchrony differences between face/non-face perceptions depend upon frequency selection and recording reference. Optimal selection of these parameters abolishes differential synchrony between conditions. SIGNIFICANCE: Neural synchrony is present not just for face percepts for upright stimuli, but also for non-face percepts achieved for inverted/scrambled Mooney stimuli.

Adult↗

PubMed2004

When does the brain register deviances from standard word spellings?--An ERP study.

Event-related potentials (ERPs) in response to the standard form of words (e.g., taxi) were compared with ERPs in response to letter-altered (e.g., taksi) or case-altered forms (e.g., taXi). The altered forms always resulted in the same reading as the standard forms. First divergences between ERPs were found at around 160 ms. At occipital sites, the peak amplitude of the N160 was higher for standard than letter-altered strings. At frontal and central sites, the standard strings diverged from the altered strings persistently by higher positivity from about 160 ms onwards. These early ERP differences between standard and altered visual word forms speak for early contact between the letter input and stored visual-orthographic representations of words.

Adult↗

PubMed2005

Amplitude modulation of gamma band oscillations at alpha frequency produced by photic driving.

Gamma band response to visual stimulation in humans has been observed to have both burst and resonance properties. Amplitude modulation of gamma activity at low frequencies has been seen in rat hippocampus and modeled in a number of forms. Significant amplitude modulation (p=0.05) of 33 Hz gamma frequency activity at the frequency of an 8 1/3 Hz photic driving stimulus, which also produced strong alpha entrainment, was observed in 67% of the channels in 42 human subjects. Similar amplitude modulation was found at a range of frequencies from greater than 50 Hz to about 28 Hz. The peak of the gamma amplitude modulation curve trailed the peak of the alpha signal by 25 to 30 ms, corresponding to a phase difference of 150 degrees to 180 degrees. The phase consistency of the gamma signal, measured across comparable times of the alpha signal, was least at the minimum amplitude modulation, and largest at the maximum. Although there was no consistent overall relation between the gamma amplitude and alpha amplitude, peak gamma amplitude values were consistently higher during post-target-stimulus alpha suppression, which occurs about 300-750 ms subsequent to stimulus presentation, than they were at the time of maximum alpha activity during the immediate post-stimulus period. It is hypothesized that there is an interaction between the alpha and gamma generating systems, in which gamma triggers alpha activity and is subsequently inhibited by it, thus producing the observed amplitude modulation. The transition from dark to light of the photic driving stimulus begins a phase resetting process in the gamma system and a concomitant burst of gamma activity; this produces an activation in the alpha system, similar to that found in the P1-N1 response in evoked potential experiments, and a subsequent inhibition of gamma production.

Adult↗

PubMed2004

Kanizsa subjective figures capture visual spatial attention: evidence from electrophysiological and behavioral data.

Figural binding and attention are two important processes that help to perceive the outside world. Binding is necessary to link together the different features of single objects which are represented in a distributed fashion in the brain. Attention serves to focus onto a small subset of incoming information. It is still not clear how exactly these two mechanisms operate and interact. We performed two experiments employing illusory Kanizsa figures (KFs) to investigate the temporal order of figural binding and spatial attention. In a visual search task, subjects had to detect the presence of a KF among distractor stimuli. We found only a slight increase of reaction times when increasing the number of distractors, indicating that KFs popped out and drew the perceiver's attention. In a further event-related potential (ERP) study, we used displays of the search task as non-informative cue for a subsequent target choice-reaction task. Enhanced contralateral negative amplitudes (starting at about 230 ms) over ventral occipital areas were found for cue displays which included a KF. For target stimuli, faster reaction times and enhanced ipsilateral N1 amplitudes over occipito-parietal areas were observed for validly (target presentation inside a KF) as compared to invalidly cued targets (target presentation outside a KF). Furthermore, enhanced contralateral N1 amplitudes were found for invalidly cued targets. It might be that interactions between perceptual closure processing of the ventral pathway and spatial target processing of the dorsal pathway contributed to the present result. We conclude that KFs automatically capture spatial attention when used as visual cues.

Adult↗

PubMed2004

Effect of adaptation direction on the motion VEP and perceived speed of drifting gratings.

The N200 amplitude of the motion-onset VEP evoked by a parafoveal grating of variable contrast (0.5-64%), constant speed (2 degrees/s), direction (horizontally rightward), and spatial frequency (2 cpd) was studied before and after adaptation to a stationary or drifting grating (1, 2, or 4 degrees/s rightward or leftward). These results are compared to those for the pattern-appearance VEP. Psychophysical measurements were made simultaneously of the perceived speed. While iso-directional (rightward) adaptation leads to a mean amplitude reduction of 39%, the decrease after counter-directional adaptation has a size of 20%. The post-adaptation matches of perceived speed differ in dependence on the iso-directional adapting speed and decrease on average to 98%, 85%, and 69% of the pre-adapt perceived speed after 1, 2, and 4 degrees/s adapting speeds, respectively. The perceived speed is moderately reduced (83% of the pre-adapt value) after counter-directional adaptation nearly independently of the adapting speed. A model of velocity processing is presented, which enables us to predict the trends of the experimental motion VEP and perceived speed data.

Adaptation, Physiological↗

PubMed1998

Image-based object recognition in man, monkey and machine.

Theories of visual object recognition must solve the problem of recognizing 3D objects given that perceivers only receive 2D patterns of light on their retinae. Recent findings from human psychophysics, neurophysiology and machine vision provide converging evidence for 'image-based' models in which objects are represented as collections of viewpoint-specific local features. This approach is contrasted with 'structural-description' models in which objects are represented as configurations of 3D volumes or parts. We then review recent behavioral results that address the biological plausibility of both approaches, a well as some of their computational advantages and limitations. We conclude that, although the image-based approach holds great promise, it has potential pitfalls that may be best overcome by including structural information. Thus, the most viable model of object recognition may be one that incorporates the most appealing aspects of both image-based and structural description theories.

Animals↗

PubMed1998

Macular pigment and the colour-specificity of visual evoked potentials.

A modified Moreland anomaloscope was used to examine two subjects, one with dense macular pigment, the other with relatively light pigmentation. Chromatic visual evoked potentials (VEPs) were elicited from these two subjects using coarse, isoluminant gratings of different sizes. Colour-specificity was verified by comparing chromatic onset VEPs (reflecting sustained activity) with chromatic reversal VEPs (reflecting mainly transient activity) and also by Fourier analysis (colour-specific responses are dominated by the fundamental, transient activity by the second harmonic). Achromatic (transient-type) intrusions, produced by large blue-green gratings could be related to the extent of subject-specific macular pigmentation.

Adult↗

PubMed1999

Amodal completion in texture visual evoked potentials.

Amodal completion refers to the phenomenological finding of perceiving partly occluded objects as continuing uninterrupted behind an occluder. The outlying problem is how the visual system processes such non-local stimuli because the known processes of early vision are spatially restricted operations which segregate local differences in the visual image, and little is known about their interactions in producing the segmentation of the image into functionally coherent, or global, objects. We recorded human visual evoked potentials (VEPs) to texture stimuli and addressed local/non-local relationships in comparing a condition in which local edges were present, due to texture segregation, with a condition in which, in addition to local edges, textures appeared to continue as surfaces behind gray stripes due to non-local amodal completion. Subtraction of offset from onset responses showed: (1) a difference component due to texture segregation characterized by a negativity with onset at about 95 ms and lasting up to about 280 ms; (2) a further negativity, specifically elicited by amodal completion, with onset at about 142 ms, peaking at 175 ms, and lasting up to about 188 ms. Therefore, amodal completion occurs at an early processing stage of image analysis and the difference component in VEPs can be related to figure-ground perception.

Adult↗

PubMed1998

Development of the spatio-chromatic visual evoked potential (VEP): a longitudinal study.

Most prior visual evoked potential (VEP) research on the development of color vision has employed pattern-reversing stimuli that are not optimal for producing chromatic responses. We measured infant VEPs using low spatial frequency, onset-offset stimuli, modulated along the three axes of a cone-based color space (Derrington et al. [J. Physiol 1984;357, 241-265.]). Three color-normal infants were tested in a longitudinal design over the first postnatal year. One red/green color-deficient infant was also tested at 197 days. We found that VEP responses to S-axis (tritan) stimuli have their initial onset later than responses to red/green (L-M) or achromatic stimuli, and that developmental changes in VEP waveforms are more complex and longer lasting for chromatic than for achromatic stimuli. Possible mechanisms underlying these changes are discussed.

Adult↗