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PubMed2006

Increases in alpha oscillatory power reflect an active retinotopic mechanism for distracter suppression during sustained visuospatial attention.

Human electrophysiological (EEG) studies have demonstrated the involvement of alpha band (8- to 14-Hz) oscillations in the anticipatory biasing of attention. In the context of visual spatial attention within bilateral stimulus arrays, alpha has exhibited greater amplitude over parietooccipital cortex contralateral to the hemifield required to be ignored, relative to that measured when the same hemifield is to be attended. Whether this differential effect arises solely from alpha desynchronization (decreases) over the "attending" hemisphere, from synchronization (increases) over the "ignoring" hemisphere, or both, has not been fully resolved. This is because of the confounding effect of externally evoked desynchronization that occurs involuntarily in response to visual cues. Here, bilateral flickering stimuli were presented simultaneously and continuously over entire trial blocks, such that externally evoked alpha desynchronization is equated in precue baseline and postcue intervals. Equivalent random letter sequences were superimposed on the left and right flicker stimuli. Subjects were required to count the presentations of the target letter "X" at the cued hemifield over an 8-s period and ignore the sequence in the opposite hemifield. The data showed significant increases in alpha power over the ignoring hemisphere relative to the precue baseline, observable for both cue directions. A strong attentional bias necessitated by the subjective difficulty in gating the distracting letter sequence is reflected in a large effect size of 2.1 (eta2 = 0.82), measured from the attention x hemisphere interaction. This strongly suggests that alpha synchronization reflects an active attentional suppression mechanism, rather than a passive one reflecting "idling" circuits.

Adult↗

PubMed1993

Responses of the starburst amacrine cells to moving stimuli.

1. Rabbit retinas were isolated from the eye and incubated in the presence of 3H-choline. Samples of retina taken from a defined midperipheral eccentricity were spread over the domed end of a fiberoptic bundle that formed the floor of a superfusion chamber. The rate of release of labeled acetylcholine by the starburst amacrine cells was studied. 2. When the retina was stimulated by moving gratings, the cells vigorously increased their secretion of acetylcholine. Responses were observed when the bars were as small as 60 microns in width. Systematically varying the spatial and temporal frequency of stimulation revealed that temporal frequency was the dominant variable: the cells responded best to stimuli of 1-4 Hz, whether those stimuli were flashing lights, fine gratings moving slowly, or coarse gratings moving rapidly. 3. With temporal frequency constant, the cells' responses decreased as the spatial frequency of the grating increased. The decreased response to fine gratings is most likely due, at least in part, to lateral interactions that become stronger as the light and dark bars become more closely spaced. These could occur in either the outer or inner retina. 4. The velocity tuning curve for the starburst cells' release of acetylcholine matched fairly well the velocity tuning of ON-OFF directionally selective cells in the rabbit. It did not correspond at all well with the tuning curve for the ON directionally selective cells. If the ON cells receive input from the starburst cells, that input appears to be quite indirect.(ABSTRACT TRUNCATED AT 400 WORDS)

Acceleration↗

PubMed1993

Information encoding and the responses of single neurons in the primate temporal visual cortex.

1. The possibility of temporal encoding in the spike trains of single neurons recorded in the temporal lobe visual cortical areas of rhesus macaques was analyzed with the use of principal component and information theory analyses of smoothed spike trains. The neurons analyzed had responses selective for faces. 2. Provided that a correction was applied to earlier methods of principal component analysis used for neuronal spike trains, it was shown that the first principal component provides by a great extent the most information, with the second and third adding only small proportions (on average 18.8 and 8.4%, respectively). 3. It was shown that the magnitude of the second and higher principal components is even smaller if the spike train analysis is started after the onset of the neuronal response, instead of before the neuronal response has started. This suggests that variations in response latency are at least a part of what is reflected by the second and higher principal components. 4. The first principal component was correlated with the mean firing rate of the neurons. The second and higher principal components reflected at least partly the onset properties of the neuronal responses, such as response latency differences between the stimuli. 5. A considerable proportion of the information available from principal components 1-3 is available in the firing rate of the neuron. 6. Periods of the firing rate of as little as 50 or even 20 ms are sufficient to give a reasonable estimate of the firing rate of the neuron. 7. Information theory analysis showed that in short epochs (e.g., 50 ms) the information available from the firing rate can be as high, on average, as 84.4% of that available from the firing rate calculated over 400 ms, and 52.0% of that available from principal components 1-3 in the 400-ms period. It was also found that 44.0% of the information calculated from the first three principal components is available in the firing rates calculated over epochs as short as 20 ms. 8. More information was available near the start of the neuronal response, and the information available from short epochs became less later in the neuronal response. 9. Taken together, these analyses provide evidence that a short period of firing taken close to the start of the neuronal response provides a reasonable proportion of the total information that would be available if a long period of neuronal firing (e.g., 400 ms) were utilized to extract it, even if temporal encoding were used.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Focal attention produces spatially selective processing in visual cortical areas V1, V2, and V4 in the presence of competing stimuli.

1. The activity of single neurons was recorded in Macaca mulatta monkeys while they performed tasks requiring them to select a cued stimulus from an array of three to eight stimuli and report the orientation of that stimulus. Stimuli were presented in a circular array centered on the fixation target and scaled to place a single stimulus element within the receptive field of the neuron under study. The timing of the cuing event permitted the directing of visual attention to the spatial location of the correct stimulus before its presentation. 2. The effects of focal attention were examined in cortical visual areas V1, V2, and V4, where a total of 672 neurons were isolated with complete studies obtained for 94 V1, 74 V2, and 74 V4 neurons with receptive-field center eccentricities in the range 1.8-8 degrees. Under certain conditions, directed focal attention results in changes in the response of V1, V2, and V4 neurons to otherwise identical stimuli at spatially specific locations. 3. More than one-third of the neurons in each area displayed differential sensitivity when attention was directed toward versus away from the spatial location of the receptive field just before and during stimulus presentation. Both relative increases and decreases in neural activity were observed in association with attention directed at receptive-field stimuli. 4. The presence of multiple competing stimuli in the visual field was a major factor determining the presence or absence of differential sensitivity. About two-thirds of the neurons that were differentially sensitive to the attending condition in the presence of competing stimuli were not differentially sensitive when single stimuli were presented in control studies. For V1 and V2 neurons the presence of only a few (3-4) competing stimuli was sufficient for a majority of the neurons studied; a majority of the V4 neurons required six to eight stimuli in the array before significant differences between attending conditions occurred. 5. For V1 and V2 neurons the neuronal sensitivity differences between attending conditions were observed primarily at or near the peak of the orientation tuning sensitivity for each neuron; the differences were evident over a broader range of orientations in V4 neurons. 6. In conclusion, neural correlates of focal attentive processes can be observed in visual cortical processing in areas V1 and V2 as well as area V4 under conditions that require stimulus feature analysis and selective spatial processing within a field of competing stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Effect of stimulus contrast on simultaneous steady-state pattern reversal electroretinogram and visual evoked response.

We studied the effect of stimulus pattern contrast on the simultaneously recorded pattern reversal electroretinogram (PERG) and pattern reversal visual evoked response (PVER) at 5 check sizes. The stimulus contrast ranged from 24 to 95% in 6 steps of 0.1- or 0.15-log unit increments. PERG amplitude showed a linear increase with an increase in pattern contrast at all check sizes. PVER amplitude showed an increase up to a certain pattern contrast, but was saturated upon further increase in stimulus contrast at all check sizes. The PERG amplitude-check size function curve showed a flattened, inverted-U shape only with the highest contrast and low-pass filter shape or flat shape with lower contrasts. The PVER amplitude-check size function curve showed an inverted-U shape (bandpass filter shape) at all contrasts. These characteristics to the changes in contrast and the spatial frequency tuning suggest that PVER is processed in a different location of visual pathway than PERG.

Adult↗

PubMed1993

Effect of hemifield stimulation on simultaneous steady-state pattern reversal electroretinogram and visual evoked response.

Steady-state pattern reversal electroretinograms (PERG) and pattern reversal visual evoked responses (PVER) were recorded simultaneously in 4 normal subjects using hemifield stimulation of the upper/lower and nasal/temporal conditions with 95 and 60% stimulus contrasts. A square-wave checker-board pattern (check size 40 min of arc) was used. The temporal frequency (reversal rate) was 6 Hz (12 reversals/s). With nasal/temporal hemifield stimulation, neither the PERG nor the PVER amplitudes differed significantly with either stimulus contrast. With the upper/lower hemifield stimulation, PERG amplitudes were not significantly different; PVER showed a significantly larger amplitude for lower than for upper hemifield stimulation with both contrasts (ANOVA test: p = 0.0064, 95% contrast; p = 0.0018, 60% contrast). PVER amplitudes recorded with lower hemifield stimulation were 2.05 and 2.63 times larger than those elicited with upper hemifield stimulation, for the 95 and 60% contrasts, respectively. The difference in response to the upper/lower hemifield stimulation, observed only in PVER, suggests that the lower stimulus field dominancy may be processed in a visual pathway proximal to the retinal level.

Adult↗

PubMed1990

Influence of different half-field stimulations on the pattern reversal visually evoked cortical potential.

In 30 healthy subjects the visually evoked cortical potential following full-field and upper, lower, temporal and nasal half-field pattern-reversal stimulation was registered. No significantly differing latencies for peak N90 were to be seen. The responses of peak P120 for upper half-field stimulation showed a significantly longer latency than for full-field, lower, temporal and nasal half-field stimulation. Responses to upper and lower half-field stimulation showed no potential reversal. Nasal and temporal half-field stimulation evoked responses with no significantly differing P120 latencies.

Adolescent↗

PubMed2002

Masking interrupts figure-ground signals in V1.

In a backward masking paradigm, a target stimulus is rapidly (<100 msec) followed by a second stimulus. This typically results in a dramatic decrease in the visibility of the target stimulus. It has been shown that masking reduces responses in V1. It is not known, however, which process in V1 is affected by the mask. In the past, we have shown that in V1, modulations of neural activity that are specifically related to figure-ground segregation can be recorded. Here, we recorded from awake macaque monkeys, engaged in a task where they had to detect figures from background in a pattern backward masking paradigm. We show that the V1 figure-ground signals are selectively and fully suppressed at target-mask intervals that psychophysically result in the target being invisible. Initial response transients, signalling the features that make up the scene, are not affected. As figure-ground modulations depend on feedback from extrastriate areas, these results suggest that masking selectively interrupts the recurrent interactions between V1 and higher visual areas.

Animals↗

PubMed1996

Topography of auditory and visual P300 in normal children.

Auditory and visual P300 recordings were performed on 39 normal, right-handed individuals from age 6 through 15, using 31 evenly spaced scalp electrodes. Amplitude at the P300 peak and latency to this peak at each electrode site were measured. Age was significantly correlated with the 31-electrode mean for auditory and visual P300 latencies, but not for amplitudes. The younger age group (6-10) had longer auditory and visual P300 latencies than the older age group. Visual P300 amplitudes were of an overall larger magnitude than auditory amplitudes. There were no other differences including significant topographical differences in P300 amplitudes or latencies by gender, age group, modality, or side of scalp. Radial current density maps on group-averaged auditory and visual P300 waveforms at the group mean P300 latency at Cz, showed a right centroparietal sink surrounded by sources. This suggests a major right centroparietal P300 generator. Description of the normal topography of the P300, and demonstration of the lack of topographic differences by gender, age group, modality, or side of scalp, may facilitate the meaningful examination of P300 topography in cognitive disorders. Such an examination might lead to better diagnostic tools and more appropriate treatment of cognitive disorders in children.

Adolescent↗

PubMed2000

Normal development of bilateral field advantage and evoked potential interhemispheric transmission time.

Implications of the developmental progression of myelination of the corpus callosum were studied using evoked potential interhemispheric transmission time (EP-IHTT) and the bilateral field advantage (BFA) in letter matching. Forty-two normal children aged 7 to 17 years were asked to respond regarding whether 2 letters matched when presented either unilaterally (both in the same visual field) or bilaterally (1 letter in each field). Evoked potentials were recorded with bilateral midparietal electrodes during unilateral presentations of the letter-matching task. Age-related changes were found for both EP-IHTT and BFA. BFA in reaction time in the visual letter-matching task increased significantly with age. Decreasing EP-IHTT with age was also evident (although only in a statistical trend). These findings lend support to the hypothesis that increased callosal myelination during late childhood has functional significance. Callosal maturation appears to result in faster interhemispheric transfer and increasing ability to integrate information across the midline.

Adolescent↗

PubMed2006

Dissociation between local field potentials and spiking activity in macaque inferior temporal cortex reveals diagnosticity-based encoding of complex objects.

Neurons in the inferior temporal (IT) cortex respond selectively to complex objects, and maintain their selectivity despite partial occlusion. However, relatively little is known about how the occlusion of different shape parts influences responses in the IT cortex. Here, we determine experimentally which parts of complex objects monkeys are relying on in a discrimination task. We then study the effect of occlusion of parts with different behavioral relevance on neural responses in the IT cortex at the level of spiking activity and local field potentials (LFPs). For both spiking activity and LFPs, we found that the diagnostic object parts, which were important for behavioral judgments, were preferentially represented in the IT cortex. Our data show that the effects of diagnosticity grew systematically stronger along a posterior-anterior axis for LFPs, but were evenly distributed for single units, suggesting that diagnosticity is first encoded in the posterior IT cortex. Our findings highlight the power of combined analysis of field potentials and spiking activity for mapping structure to computational function in the brain.

Action Potentials↗

PubMed1993

Assessment of prepubertal and postpubertal boys and girls at risk for developing alcoholism with P300 from a visual discrimination task.

A total of 86 children (45 male) between the ages of 8-18 who were from families that were either at high- or low-risk for developing alcoholism were evaluated in a visual event-related potential paradigm. The children responded to two target conditions (hard and easy) and a blank condition by pressing an appropriate button. P300 amplitude was reduced in prepubescent boys who were at high-risk for developing alcoholism. Reaction time data for type of target indicated that, while latency was longer for the hard condition compared to the easy one, no significant differences by risk status were found. Performance accuracy did not differ by group status. These behavioral findings suggest that P300 amplitude is an electrophysiological marker of alcoholism risk in young boys.

Adolescent↗

PubMed1993

Brainstem frequency-following responses and cortical event-related potentials during attention.

Human brainstem frequency-following responses (FFRs) and cortical event-related potentials (ERPs) were evoked by a low-frequency (230 Hz) tone during directed attention. ERPs showed significant amplitude differences consistent with expected attention effects, viz., largest to attended stimuli and smallest to ignored stimuli. The ERP data thereby confirm that attention effectively modulated cortical responses. The FFR, however, did not differ between conditions. The present results agree with one earlier FFR study and a majority of studies using click stimuli to elicit the brainstem auditory evoked response (BAER). However, several BAER studies and two recent FFR studies have shown that attention can influence human brainstem responses. The present results are therefore interpreted in the context of specific task requirements that optimize early selective attention effects.

Adolescent↗

PubMed1996

Emotional information processing and visual evoked brain potentials.

Visual evoked potentials to emotional slides presented for 2 sec. were investigated in 13 subjects. 73 emotional slides (pleasant, unpleasant, and neutral) were selected from a standardized set of photographic slides, the 1988 International Affective Picture System of Lang, Ohman, and Vaitl Visual evoked potentials were recorded from three head locations, frontal, central and parietal (Fz, Cz, and Pz). Analyses were performed in the two latency ranges: 300-400 msec. and 400-500 msec. Analyses showed an arousal effect, as indicated by a quadratic trend, indicating that emotional slides (both pleasant and unpleasant) gave higher cortical positivity than neutral ones, for all components. In addition, in the two latency epochs, larger positivities were found at Pz, compared to Fz and Cz, whereas Fz and Cz did not differ from each other.

Adult↗

PubMed1983

Pattern component ratio in pattern-reversal VEP: normative data and clinical applications.

It has been well established that there is a reliable implicit time to the VEP positive component (ca 100 msec) in response to pattern reversal. This has become a valuable test of the status of the visual system. Recent data suggest that activity around 200 msec is another useful measure of the cortical response to pattern alternation. We have carried out a normative study of the relative amplitudes of the two components. Among adults with normal visual acuity activity around 200 msec is directly related to the spatial frequency of the stimulus, peaking markedly in amplitude at a narrow range of spatial frequencies. This is in sharp contrast to the earlier component, which does not exhibit such sensitive size differentiation. Recently we have used the ratio between the late and early component amplitudes as a quantitative indicator of the efficiency of higher-level processing of pattern information. We present our normative data and illustrate the effects of decreased visual responses not related to known peripheral optical problems. The results of this work are also interpreted as showing basic characteristics of information processing in the visual system, in that higher spatial frequencies are processed by a continuum of rather sharply tuned cortical processes, with the higher spatial frequencies being processed later in time.

Evoked Potentials, Visual↗

PubMed1997

Local luminance and pattern reversal stimuli yield different visual evoked potential topography.

We studied how the stimulation of quadrants of the visual field affect brain potential topography, and we compared activity elicited by conventional pattern reversal or by local luminance stimuli. The method of quasi-simultaneous stimulation of many small visual field elements by binary m-sequences allowed us to reconstruct the potentials evoked at each of 54 visual field locations independently. Data from all field elements within each quadrant and in the whole stimulation field were summed and compared to those elicited by checkerboard reversal stimuli presented in the four quadrants or as full field stimuli. In twenty-two healthy adults evoked brain activity was recorded in 30 channels with an electrode array densely spaced over the occipital brain areas. With local flash stimuli as well as with checkerboard reversal the topographical distributions of cortical activation changed significantly with retinal stimulus location. Analysis of three components occurring between 50 and 240 ms revealed significant differences between pattern reversal and local luminance evoked brain activity. Reversal stimuli yielded not only larger amplitudes but also a completely different component structure and topography. Our results illustrate that different neuronal generators are activated by pattern reversal and local luminance stimuli although visual field location of the stimuli was identical indicating that the same retinal and cortical structures respond in a different way depending on stimulation mode.

Adult↗

PubMed1988

Steady-state visual evoked potentials to asymmetrical contrast.

The temporal frequency components in the steady-state visual evoked response (VEP) depend on the method of stimulus presentation; a first harmonic is generated to "on-off" patterns while a second harmonic occurs to both "on-off" and counterphase patterns. This study examined the VEP response in humans to patterns between these two extremes. In the main experiment, a 1 c/d sinusoidal grating was phase reversed sinusoidally at 8 Hz. The DC offset, however, was adjusted such that two different levels of peak contrast occurred during a temporal cycle. Within this context, a counterphase pattern would represent complete contrast symmetry and an "on-off" pattern would represent maximum contrast asymmetry during a temporal cycle. With this manipulation, the a) integrated luminance change; b) local luminance change; and c) total contrast remained constant. Only mean contrast varied. The amplitude of the first harmonic strongly depended on mean contrast. The amplitude and phase of the second harmonic, however, changed little across experimental conditions suggesting a dependence on one or more of the above three listed stimulus attributes which remained constant. In a supplementary experiment, this phase constancy was confirmed using other temporal frequencies.

Adult↗

PubMed1995

Processing relative clauses varying on syntactic and semantic dimensions: an analysis with event-related potentials.

Event-related potentials were used to study how parsing of German relative clauses is influenced by semantic information. Subjects read well-formed sentences containing either a subject or an object relative clause and answered questions concerning the thematic roles expressed in those sentences. Half of the sentences contained past participles that on grounds of semantic plausibility biased either a subject or an object relative reading; the other half contained past participles that provided no semantic information favoring either reading. The past participle elicited an N400 component, larger in amplitude for neutral than for semantically biased verbs, but this occurred only in the case of subject relative clauses. More specific effects were obtained only for a subgroup of subjects, when these were grouped into fast and slow comprehenders on the basis of their question-answering reaction times. Fast comprehenders showed larger N400 amplitudes for neutral than for semantically biased past participles in general and larger N400s for the latter when there was a bias for an object relative reading as opposed to a subject relative reading. Syntactic ambiguity resolution, indicated by an auxiliary in sentence final position, was associated in this subgroup with a positive component (P345), larger in amplitude for auxiliaries indicating an object relative reading than for those indicating a subject relative reading. The latter component was independent of semantically biasing information given by a preceding past participle. Implications of these findings for models of language comprehension are considered.

Adult↗