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PubMed1993

Brain potential differences related to spatial attention in migraineurs with and without aura symptoms support supposed differences in activation.

It is to be expected that differences in electrical activity of the brain between migraine patients with aura and those without aura can only be revealed by stimuli that provoke visual spatial processing, i.e. stimuli that trigger so called endogenous Visual Evoked Potential (VEP) activity. This is not the case for the flashes and checkerboard reversals. Those stimuli elicited exogenous activity only. During and between attacks the blood flow of migraineurs with aura changes in the posterior cerebral part of the brain, which is assumed to be specialized in the processing of spatial aspects of visual stimuli. Reaction times (RTs), early and late Event Related Potential (ERP) differences were compared at 12 scalp positions for two groups of migraineurs (with and without aura) and a control group. They had to perform a passive attention task, checkerboard reversals, and an active attention task, where attention was either divided into or focussed at spatial locations. In agreement with many studies on migraine, checkerboard stimuli did not differ on any early components. However, RTs were faster for migraineurs with aura and their early components were different when stimuli were highly attended. This is probably because these stimuli can relatively easily trigger cortical activity due to an over activated central mechanism and an enhanced level of attention.

Adult↗
PubMed1994

Frontal P300 decrements in antisocial personality disorder.

Event-related potentials (ERPs) were recorded from 83, 21- to 25-year-old nonalcoholic men varying in alcoholism vulnerability due to the presence/absence of an alcoholic family history or a personal history of antisocial personality disorder. ERPs were elicited by a visual oddball task in which the target was presented more frequently than the nontarget, in order to elicit impulsive or perseverative responding. Analyses of N200 and P300 revealed no group differences in the nontarget response. However, analyses of the target response revealed a significantly smaller P300 in the antisocial personality (ASP)+ group compared with the ASP- groups. The P300 decrement was limited to frontal electrode sites and is interpreted as indicating the presence of subtle anterior brain dysfunction among ASP+ subjects.

Adult↗
PubMed1983

The responses of cells in macaque lateral geniculate nucleus to sinusoidal gratings.

Responses of cells in the parvocellular (p.c.l.) and magnocellular (m.c.l.) layers of the macaque lateral geniculate nucleus to sine-wave gratings were studied. Both p.c.l. and m.c.l. cells responded best at a temporal frequency (drift rate) of 10-20 Hz. P.c.l. cells responded at temporal frequencies lower than 1 Hz; m.c.l. cells did not. With coloured- or white-black luminance-modulated gratings, responses of m.c.l. cells were weaker at low than at medium spatial frequencies. With coloured gratings, p.c.l. cell responses were not attenuated at low spatial frequencies. With white gratings a few p.c.l. cells did show such attenuation. Optimal responses from p.c.l. cells were obtained with coloured gratings; white gratings evoked weaker responses. With a grating of a colour causing suppression of a p.c.l. cell's activity, the modulation of firing was much less than with a grating of a colour excitatory for the cell. M.c.l. on- and off-centre cells responded equally well to moving gratings. The ability of p.c.l. cells to resolve fine gratings was dependent on cell type as well as on the colour of grating used. The ability of m.c.l. cells to resolve fine gratings was comparable to that of p.c.l. cells. The contrast sensitivity of m.c.l. cells was much higher than that of p.c.l. cells. This may account for their ability to resolve fine gratings, despite their larger centre size. In comparison with luminance-modulated gratings, chromatically modulated gratings could evoke larger or smaller responses, depending on p.c.l. cell type and the colours in the grating. M.c.l. cells responded poorly or not at all.

Animals↗
PubMed1983

Spatio-temporal interactions in cat retinal ganglion cells showing linear spatial summation.

The spatio-temporal characteristics of cat retinal ganglion cells showing linear summation have been studied by measuring both magnitude and phase of the responses of these cells to drifting or sinusoidally contrast-modulated sinusoidal grating patterns. It has been demonstrated not only that X cells behave approximately linearly when responding with amplitudes of less than about 10 impulses/sec to stimuli of low contrast but also that cells of another type with larger receptive field centres (Q cells) behave approximately linearly under the same conditions. These Q cells appear to form a homogeneous group which is probably a subset of the tonic W cells (Stone & Fukuda, 1974) or sluggish centre-surround cells (Cleland & Levick, 1974). The over-all spatio-temporal frequency characteristics of cells showing linear spatial summation are not separable in space and time. The form of the spatial frequency responsivity function of these cells depends upon the temporal frequency at which it is measured while the temporal phase of their resonse measured at any constant temporal frequency depends upon the spatial frequency of the stimulus. The behaviour of X and Q cells is quite well explained by an extension of the model in which signals from centre and surround mechanisms with radially Gaussian weighting functions are summed to provide the drive to the retinal ganglion cell. While the general form of the temporal frequency response characteristics of these ganglion cells are probably provided by the characteristics of elements common to the centre and surround pathways, the spatio-temporal interactions can be explained by assuming that the surround signal is delayed relative to the centre signal by a few milliseconds.

Animals↗
PubMed1988

Stereoscopic evoked responses to crossed and uncrossed disparity accompanying simulated refractive error.

Evoked potentials were recorded to the occurrence of a disparate stimulus in dynamic random dot stereograms. Seven adult males, all of whom had vision which was normal or corrected to normal, participated in the experiment. Subjects viewed 100 ms duration stimuli which embodied 30 arc min of either crossed or uncrossed disparity under four conditions of spherical overcorrection: -0.25, +1.0, +2.0, +3.0 dioptres. The first condition, essentially normal refraction, yielded reliable behavioural reports of the stimulus and clear evoked potentials to both crossed and uncrossed disparity. With increasing overcorrection the behavioural reports became less reliable, and the evoked potentials were degraded for both conditions of disparity. The responses to the crossed disparity condition, however, showed significantly less degradation in both behavioural and electrophysiological measures. The implications of this finding may be that there are separate cortical subsystems for the processing of crossed and uncrossed disparity and that the former is more robust under non-ideal viewing conditions.

Adult↗
PubMed1989

The visual evoked potential in acute primary angle closure glaucoma.

Visual evoked potentials (VEPs) were elicited from 29 patients who had experienced a previous attack of acute primary angle closure glaucoma. The VEPs were shown to be abnormal in at least one of the measures (latency, amplitude, contrast threshold, or slope) in 72.4% of affected eyes, whereas only 41.4% indicated obvious optic nerve damage. It is notable that 48.1% of fellow eyes with no (known) history of acute pressure rise also showed some form of VEP abnormality. The possible pathophysiological mechanisms operating in both affected and fellow eyes are discussed. It is concluded that, despite the presence of possible artefactual influences, the results probably reflect the presence of primary angle closure glaucoma.

Adult↗
PubMed1995

Detection of glaucomatous damage in patients with osteo-odontokeratoprosthesis.

BACKGROUND: Osteo-odontokeratoprosthesis (OOKP) is an autologous transplantation procedure in which the cornea is replaced by an optical cylinder glued to a biological support. Patients undergoing OOKP surgery may develop a secondary glaucoma whose diagnosis, by means of standard diagnostic procedures, is often doubtful. METHODS: In the present study pattern electroretinograms (PERGs), visual evoked potentials (VEPs), contrast sensitivity, and automated threshold perimetry (Humphrey 30-2) were evaluated in 19 OOKP treated patients with postoperative visual acuities > or = 0.8. Nine patients had had a preoperative secondary glaucoma, while the remaining 10 had no history of glaucoma and normal posterior pole. RESULTS: Results were compared with those obtained from either normal control subjects or from ordinary glaucoma patients. PERG amplitudes and contrast and perimetric sensitivities were reduced in both groups of OOKP patients when compared with normal controls. However, these losses were significantly greater in OOKP patients with glaucoma compared with those with normal posterior pole. VEPs were reduced, compared with controls, only in OOKP patients with glaucoma. These VEP losses were similar to those found in ordinary glaucoma patients. CONCLUSION: Among the tests employed, VEPs showed the best accuracy (79%) in discriminating between glaucomatous and non-glaucomatous OOKP treated eyes. The present results suggest a possible use of the VEP technique for detecting glaucomatous dysfunction after OOKP.

Adult↗
PubMed2005

Processing of kinetic boundaries in macaque V4.

We used gratings and shapes defined by relative motion to study selectivity for static kinetic boundaries in macaque V4 neurons. Kinetic gratings were generated by random pixels moving in opposite directions in the neighboring bars, either parallel to the orientation of the boundary (parallel kinetic grating) or perpendicular to the boundary (orthogonal kinetic grating). Neurons were also tested with static, luminance defined gratings to establish cue invariance. In addition, we used eight shapes defined either by relative motion or by luminance contrast, as used previously to test cue invariance in the infero-temporal (IT) cortex. A sizeable fraction (10-20%) of the V4 neurons responded selectively to kinetic patterns. Most neurons selective for kinetic contours had receptive fields (RFs) within the central 10 degrees of the visual field. Neurons selective for the orientation of kinetic gratings were defined as having similar orientation preferences for the two types of kinetic gratings, and the vast majority of these neurons also retained the same orientation preference for luminance defined gratings. Also, kinetic shape selective neurons had similar shape preferences when the shape was defined by relative motion or by luminance contrast, showing a cue-invariant form processing in V4. Although shape selectivity was weaker in V4 than what has been reported in the IT cortex, cue invariance was similar in the two areas, suggesting that invariance for luminance and motion cues of IT originates in V4. The neurons selective for kinetic patterns tended to be clustered within dorsal V4.

Animals↗
PubMed2006

Effects of spatial attention on contrast response functions in macaque area V4.

Previous single-unit studies of visual cortex have reported that spatial attention modulates responses to different orientations and directions proportionally, such that it does not change the width of tuning functions for these properties. Other studies have suggested that spatial attention causes a leftward shift in contrast response functions, such that its effects on responses to stimuli of different contrasts are not proportional. We have further explored the effects of attention on stimulus-response functions by measuring the responses of 131 individual V4 neurons in two monkeys while they did a task that controlled their spatial attention. Each neuron was tested with a set of stimuli that spanned complete ranges of orientation and contrast during different states of attention. Consistent with earlier reports, attention scaled responses to preferred and nonpreferred orientations proportionally. However, we did not find compelling evidence that the effects were best described by a leftward shift of the contrast response function. The modulation of neuronal responses by attention was well described by either a leftward shift or proportional scaling of the contrast response function. Consideration of differences in experimental design and analysis that may have contributed to this discrepancy suggests that it was premature to exclude a proportional scaling of responses to different contrasts by attention in favor of a leftward shift of contrast response functions. The current results reopen the possibility that the effects of attention on stimulus-response functions are well described by a single proportional increase in a neuron's response to all stimuli.

Animals↗
PubMed1990

Modulation by a moving texture of cat area 18 neuron responses to moving bars.

1. The influence of a moving texture on neuronal responses to a moving bar was tested in 103 area 18 neurons of anesthetized and paralyzed cats. The texture was a two-dimensional noise pattern, the bar moved at optimal speed, and its contrast was adjusted to yield 50% of the maximum response. 2. The moving texture exerted two different but related effects: it suppressed the response of area 18 neurons to the moving bar, and it modulated the direction selectivity of parastriate neurons. These effects were strongest when the texture moved at the same speed or faster than the bar. 3. Genuine suppressive effects of the moving texture were distinguished from lack of summation between bar and texture responses. Suppressive effects of either type were observed in 75% of the area 18 cells and occurred more frequently among C family cells, velocity tuned cells, and in layer 5 than in other groups of cells. 4. The modulation of direction selectivity was distinguished from pseudomodulation because of lack of summation of bar and texture responses. The direction selectivity of 35% of the area 18 cells was modulated by the moving texture. Six different relative direction selectivity (RDS) types were observed in area 18. 5. The neurons of which direction selectivity was modulated by the moving texture occurred predominantly in layers 2-3 and 6, suggesting that they represent a further stage of processing within area 18. 6. Many (75%) area 18 cells responded to the texture moving on its own. Most of these cells respond to isolated features ("grains") in the patterns rather than to the movement of the whole pattern. Cells responding to the movement of the whole pattern were generally C family cells, and their direction selectivity was not modulated by the moving texture. 7. These results are compared with those obtained under identical experimental conditions in area 17. Although suppressive effects are similar in both areas, RDS types are differently distributed in the two areas. 8. The possible origins of the interactions and their functional significance are discussed.

Animals↗
PubMed1993

Speed and direction selectivity of macaque middle temporal neurons.

1. We tested quantitatively the responses of 147 middle temporal (MT) cells to light and dark bars moving at different speeds ranging over a 1,000-fold range (0.5-512 deg/s). 2. We derived the following quantities from the speed-response (SR) curves obtained for opposite directions of motion. Speed selectivity was characterized by the maximum response, optimum speed, upper cutoff speed, response to slow movement, and tuning width. Direction selectivity was characterized by the direction index (DI) averaged over speeds yielding significant responses (MDI) and by the direction index at optimal speed (PDI). 3. There was an excellent correlation between speed characteristics for light and dark bars. These correlations were stronger than the correlations between direction indexes. The strongest correlations were obtained for maximum response and upper cutoff. 4. SR curves were classified into three groups: low pass (25%), tuned (43%), and broadband (28%), leaving 4% unclassified. 5. In the majority (75%) of MT cells, there was an agreement between the typology of speed selectivity for light and dark bars. Cells were classified as tuned (33%), low pass (22%), broadband (19%), and mixed (22%), leaving 4% unclassified. In addition to differences in speed characteristics, these groups also differed in response level, direction selectivity, and distribution of preferred directions. 6. For tuned cells, there was a very tight correlation of most speed characteristics for light and dark bars. 7. Direction selectivity depended on stimulus speed in most neurons, yielding a tuned average speed-DI curve. 8. Speed characteristics, proportions of speed selectivity types, and direction selectivity indexes showed little dependence on laminar position. 9. Speed characteristics and direction selectivity indexes were not dependent on eccentricity. Proportion of speed selectivity types however, changed dramatically with eccentricity: low-pass cells dominated foveally, tuned cells parafoveally, and broadband cells peripherally. 10. There were also small eccentricity effects on the range of optimal speeds shown by tuned cells and on the speed at which direction selectivity decreases in the slow speed range.

Acceleration↗
PubMed1996

Primate striate and prestriate cortical neurons during discrimination. II. separable temporal codes for color and pattern.

1. In the previous paper we reported our analysis of the responses of neurons in cortical areas V1, V2, and V4 to a set of stimuli that consisted of all 36 combinations of six colors and six patterns. Neurons in all three cortical areas simultaneously encoded information about both the color and pattern of the stimulus in the number and temporal distribution of spikes in their responses. To account for this ability, we propose that a neuron's response consists of separable temporal codes representing the color and pattern of the stimulus that are multiplexed together. 2. We used nonlinear regression to fit the model parameters to the data. We used the responses to 30 of the 36 stimuli as a training set to estimate the parameters of the model and the responses to the remaining 6 stimuli as a test set. After training, the model fitted the responses to stimuli in the training sets very well and predicted the responses to stimuli in the test sets. Thus neuronal responses to colored patterns contain separate temporal codes representing color and pattern. 3. After establishing the model parameters, we obtained the waveforms that represented each neuron's temporal codes for the six colors and six patterns of our stimulus set. We then proceeded with a series of analyses to determine whether these waveforms were viable candidates for neuronal codes. Cluster analysis revealed that there were only a few different classes of waveforms representing each color and pattern, and there were many neurons in each class. Further, neurons that used similar waveforms to represent one color or pattern also tended to use similar waveforms to represent other colors or patterns. The waveforms representing five of the six colors and three of the six patterns were similar in the two monkeys used in this study. 4. We compared the shapes of the code waveforms across cortical areas and found no differences among areas in the shapes of the waveforms representing four of the six colors. In contrast, we found that there were differences among areas in the shapes of the waveforms representing all six patterns. These results suggest that messages about color are encoded at an early level and are then propagated upward, but that messages about pattern are altered in each successive cortical area. 5. Our results offer a neurophysiological explanation for the psychophysical evidence that color and form are processed by different channels. We propose that the psychophysical channels for color and pattern arise from the separability of the temporal codes for color and pattern in the responses of single neurons. This hypothesis implies that psychophysical channels correspond to classes of temporal codes rather than to classes of neurons.

Animals↗
PubMed2000

Texture segregation in the human visual cortex: A functional MRI study.

The segregation of visual scenes based on contour information is a fundamental process of early vision. Contours can be defined by simple cues, such as luminance, as well as by more complex cues, such as texture. Single-cell recording studies in monkeys suggest that the neural processing of complex contours starts as early as primary visual cortex. Additionally, lesion studies in monkeys indicate an important contribution of higher order areas to these processes. Using functional MRI, we have investigated the level at which neural correlates of texture segregation can be found in the human visual cortex. Activity evoked by line textures, with and without texture-defined boundaries, was compared in five healthy subjects. Areas V1, V2/VP, V4, TEO, and V3A were activated by both kinds of line textures as compared with blank presentations. Textures with boundaries forming a checkerboard pattern, relative to uniform textures, evoked significantly more activity in areas V4, TEO, less reliably in V3A, but not in V1 or V2/VP. These results provide evidence that higher order areas with large receptive fields play an important role in the segregation of visual scenes based on texture-defined boundaries.

Adult↗
PubMed1984

Influence of diazepam on visual pattern-evoked potentials with due regard to nonstationary effects. Methodological problems.

The effects of diazepam on visual-evoked potentials (VEP) have been studied. The general effects - increase in latencies and decrease in amplitudes of the main peaks of the VEP - were differentiated by segmented averaging (as a measure for the possible lack of stationariness of the VEP) and by psychological testing (to study the influence of personality traits on medication effects).

Adult↗
PubMed1989

Event-related potentials and visual spatial attention: influence of a cholinergic drug.

The effects of two dosages (200 mg, 600 mg, placebo) of a cholinergic nootropic (WEB 1881 FU) were investigated in a visual spatial attention task. Event-related brain potentials (ERPs) were used as a physiological measure of attention as they have previously been shown to be sensitive to allocation of attention to points in space. The typical enhancement of several peaks of the visual ERP due to attention was found in the present experiment. No systematic effect of the medication was revealed, suggesting that the effects of WEB 1881 FU do not extend to early perceptual processes.

Adult↗