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Results for “Pattern Recognition, Visual”

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PubMed1989

An electrophysiological investigation of the spatial distribution of attention to colored stimuli in focused and divided attention conditions.

In the present experiment ERPs were recorded to colored stimulus bars (red or blue) which were randomly presented at one out of eight different spatial locations on a visual display. The locations were situated on a hemi-circle around the fixation point, with four locations lying in each visual half-field. The subjects were instructed to attend to one stimulus location (the most peripheral left or right location: the relevant location) in a focused attention condition and to all four locations within the same visual half-field in a divided attention condition, and were instructed to respond to stimulus bars in one color presented at the relevant location(s). In the focused attention condition, spatial attention resulted in early positivity in the P1 latency range (ca. 100-175 ms), followed by a prolonged negativity in the N1, P2, and N2 latency range (ca. 175-350 ms). These effects generalized to locations in the same visual half-field as the relevant location. The effect of attending and responding to the target color consisted of a number of different effects. An early anterior positivity, occipital negativity was observed for the relevant location and for locations in the same visual half-field as the relevant location, but not for the locations in the opposite visual field. A later central negativity (N2b) appeared to be confined to the relevant location and one location adjacent to it. Finally, a late parietal positivity (P3b) was exclusively evoked by target stimuli at the relevant location. In the divided attention condition, the ERPs evoked by stimuli presented at each of the four locations within the relevant visual half-field showed increased early positivity (enhancing P1 amplitude) as compared to the ERPs to stimuli in the opposite (irrelevant) visual half-field. The early color selection effect was also found to all stimuli within the relevant half-field, but the N2b component was evoked both by stimuli within the relevant half-field and by stimuli at the location in the irrelevant half-field closest to the midline. The P3b was present to target stimuli at the three most lateral positions within the relevant half-field but it was absent to the relevant location closest to the midline. The data suggested that the attentional spotlight encompassed at least approximately 2 degrees in the focused attention condition, and an efficient selection by visual half-field in the divided attention situation.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1994

Task decision difficulty: effects on ERPs in a same-different letter classification task.

A "same-different" letter pair choice reaction time task was used to compare reaction time, performance accuracy, and P3 amplitude and latency at three levels of task difficulty. Stimulus set was held constant across tasks, and task difficulty was manipulated by instructions to the subject. Subjects delivered a button-press response to designate whether members of each letter pair matched or mismatched on the basis of their physical identity (low difficulty), name identity (medium difficulty), or category identity (vowels/consonants, high difficulty). Task decision difficulty was confirmed by slower reaction times and reduced response accuracy. P3 amplitude was inversely related to task decision difficulty; relatively larger P3s were associated with matched (vs. mismatched) letter pairs. These findings evidence direct effects of task difficulty on P3 amplitude, possibly as the result of "equivocation" related to more difficult task judgments.

Adult↗

PubMed1995

Processing of multi-dimensional stimuli: P300 component of the event-related brain potential during mental comparison of compound digits.

This study explored the processing of multi-dimensional stimuli during mental comparisons by means of behavioral and event-related potential (ERP) measures. As multidimensional stimuli served compound digits with separable physical and semantic dimensions, each graduated in size. Fourteen adult volunteers were presented with the digits zero through nine, which were constructed by suitably arranged small digits of the same number set. The subject's task was to decide whether the physically larger digit was numerically larger or smaller than the physically smaller one. Large and small numbers as well as the numerical differences between them appeared, equiprobably, in randomized order. Reaction time (RT), frequency of errors, and ERPs registered from Fz, Cz and Pz (time constant 5.00 s; 30 Hz frequency cut off) were selectively averaged according to three dimensions (i) the numerical differences between simultaneously presented large and small digits, (ii) the 10 physically large digits, and (iii) the 10 physically small digits. As a reflection of the symbolic distance effect, the mean RTs and errors decreased and the P300 amplitude increased with increasing differences between the numbers compared. The P300 amplitude changed in a U-shaped function of the numerical value of the physically large digits. This result is in accordance with the adaptation-level hypothesis and indicates that prior to numerical comparisons the physically large numbers were primarily encoded and arranged into the internal number scale. The fact that the U-shaped amplitude trend was found for the physically large but not for the physically small digits is interpreted as an expression of the advantage of global information. The approach of separation task-relevant stimulus dimensions on the basis of P300 changes is proposed as an informative method for the study of multi-dimensional-stimulus processing in multi-task environments.

Adult↗

PubMed1996

Aging, stimulus identification and the effect of probability: an event-related potential study.

Event-related potentials (ERPs) of younger and older subjects were compared in a simple reaction time (SRT) and in two GoNogo (20% and 80% target probability) tasks. At the T5 location, the NA component (the difference between the ERPs elicited by the frequent stimuli in the GoNogo tasks on the one hand, and the ERPs in the SRT task on the other hand) emerged earlier in the younger group. The N2b was larger in the younger group, and in this group the rare stimuli of the 80% GoNogo task elicited an enlarged N2. When compared to the older group, the stimulus probability in the younger group had a larger effect on the amplitude of the late positivity. The results show age-related changes at an early stage of the information processing activity, and larger sensitivity of the orienting system in the younger subjects.

Adolescent↗

PubMed1983

Visual evoked potentials in spinocerebellar degenerations.

Pattern reversal visual evoked potentials were studied in 21 patients with spinocerebellar ataxias among whom 6 had Friedreich's ataxia, 10 had hereditary spastic ataxia and 5 had spinocerebellar degeneration with slow eye movements (olivopontocerebellar degeneration). The VEP abnormalities found in 4 cases of Friedreich's ataxia and one with spinocerebellar degeneration with slow eye movements, consisted of, bilaterally absent VEP in 3 patients and bilaterally abnormal responses with asymmetry in two. All the patients with spastic ataxia had normal VEP latencies. The N 70 - P 100 amplitudes, in patients with hereditary ataxias were significantly reduced compared to controls (P less than 0.001). The VEP abnormalities correlated best with neuroophthalmic findings, but had no relation to age, sex, inheritance or duration of illness. The VEP findings are probably suggestive of progressive nerve fibre loss in the visual pathways with associated slowing of conduction. The higher incidence of visual pathway involvement in Friedreich's ataxia compared to other hereditary ataxias as reported in recent studies is confirmed.

Adult↗

PubMedDate not supplied

The objective assessment of visual contrast sensitivity by pattern reversal visual evoked potentials in diabetes.

Recent studies have described abnormalities of visual evoked potentials and pattern electroretinography in diabetics without retinopathy. The visual contrast sensitivity, determined by psychophysical tests, has proved to be abnormal in diabetic patients with and without clinical retinopathy. In this study we evaluated contrast sensitivity function using both electrophysiologic and psychophysical methods. The objective assessment of functional visual contrast sensitivity was superior to psychophysical evaluation in the detection of contrast sensitivity alterations. No relationships were found between contrast sensitivity dysfunction and abnormalities of pattern electroretinography or fluorescein angiography. Our data suggest that functional visual deficits might precede background retinopathy and that the involvement of foveal function is early and very frequent in diabetic patients, even if they have normal visual acuity.

Adolescent↗

PubMed2005

Simulated nystagmus reduces pattern-reversal more strongly than pattern-onset multifocal visual evoked potentials.

OBJECTIVE: In patients with nystagmus conventional pattern-reversal visual evoked potentials are severely degraded, while sizeable pattern-onset VEPs can often still be obtained. We tested whether this differential effect of retinal image motion on pattern-reversal and pattern-onset responses also applies to multifocal VEPs (mfVEPs). METHODS: In eight subjects with normal oculomotor behaviour and vision we recorded pattern-reversal and pattern-onset mfVEPs from an occipital electrode pair to 60 locations of a scaled dartboard-pattern, and to 64 locations of a uniform checkerboard-pattern. Subjects viewed the stimulus monocularly via a mirror, which was placed close to the eye and driven by a scanner with a 4 Hz sawtooth waveform at an amplitude of 2 degrees to simulate horizontal jerk nystagmus and of 0 degrees for the reference condition. RESULTS: For the scaled dartboard-stimulus we observed an eccentricity-dependent effect of induced retinal image motion on mfVEP responses: in the central visual field (0-0.5 degrees), pattern-reversal and pattern-onset responses were reduced by 73 and 42%, respectively. In the periphery (10-16 degrees), only pattern-reversal responses were reduced (by 27%), while pattern-onset responses were enhanced by 39%. Pattern-onset responses to the uniform checkerboard stimulus were enhanced by 17%, while pattern-reversal responses were reduced by 27%. CONCLUSIONS: Pattern-onset mfVEPs are more efficient than pattern-reversal mfVEPs, if retinal image motion is superimposed onto the stimulus. This is in close correspondence to previous reports on conventional VEPs. SIGNIFICANCE: This study demonstrates a differential effect of retinal image motion on pattern-reversal and pattern-onset mfVEPs and thus suggests stimulus conditions to enhance the efficiency of mfVEP recordings in patients with moderate nystagmus.

Adult↗

PubMed2004

Flexibility in spatial and non-spatial feature grouping: an event-related potentials study.

Early perceptual grouping was studied using 256-channels event-related potentials in a choice response task. The task involved the detection of a triangle configuration of Gabor patches among patches of different spatial frequencies. The influence of two task-irrelevant factors was compared. One was the spatial proximity relation between the target patches and the other their relative orientation, a non-spatial relationship. Non-spatial effects were predominant in early peaks N64 and P100 in the occipital areas, and were reduced in size for later peaks. Spatial effects started from N180 in the occipital areas and continued in P250 and P430 in the central areas, increasing in size with time. These findings constitute a case of reversal of the usual order of spatial and non-spatial feature processing, illustrating that the flexibility in the early visual system may be greater than previously assumed.

Adult↗

PubMed2005

Electrical brain responses evoked by human faces in acute psychosis.

Patients with schizophrenia are known to have behavioral deficits in recognizing faces and facial expressions. However, the ability to process simple visual stimuli appears to be intact in first-episode psychosis. The aim of this study was to examine complex visual processing, especially the event-related potentials (ERPs) elicited by human faces, in early psychosis. Never-medicated patients in acute psychosis (n=18) were compared with healthy controls (n=19). Photographs of human faces were presented in a classic oddball paradigm requiring a motor response to a smiling face. Cerebral sources of ERPs were analyzed of the averaged responses, using minimum norm estimates, and dipole models. Face-sensitive response at 145 ms after the face stimuli was of significantly higher amplitude in our never-medicated patients, and the activity distribution between the groups was clearly different. At the early phase of a psychotic illness, these alterations in face-related neuronal network processes represent perceptual disturbance in psychosis, possibly including state and trait, as well as potential physiological compensatory features.

Acute Disease↗

PubMed2005

Correspondence of cue activity to reward activity in the macaque orbitofrontal cortex.

It has been reported that neurons in the orbitofrontal cortex (OFC) respond to emotionally significant events such as reward-predicting cues and/or the reward itself. The responses to reward-predicting cues are considered to carry the information of the predicted reward. However, few studies have focused on the relationship of the neuronal activity during a cue period with that during a reward period. We can infer that the cue responses of OFC neurons are correlated to the reward responses if they carry the information of the predicted reward. In this study, we focused on neurons that showed responses during both the cue and reward periods, and compared the response characteristics between these periods. We found 94 of 369 OFC neurons showed significant responses during both the cue and reward periods, and 43 of which preserved their selectivity between these periods. Furthermore, population analysis showed that stronger cue responses corresponded to stronger reward responses, and stronger reward responses corresponded to stronger cue responses. These results suggest that individual neurons in the OFC associate visual information with reward information, and contribute to the prediction of future rewards by forming reward representations.

Animals↗

PubMed2006

Memory activation enhances EEG abnormality in mild cognitive impairment.

This exploratory study investigated EEG power changes during memory activation in patients with amnestic mild cognitive impairment (MCI). Twelve MCI patients and 16 age-matched controls underwent EEG registration during two conventional EEG conditions ('eyes closed' and 'eyes open') and three memory conditions ('word memory', 'picture memory' and 'animal fluency'). For all conditions, EEG power in the theta (4-8 Hz), lower alpha (8-10.5 Hz) and upper alpha (10.5-13 Hz) bands were expressed as percentile changes compared to 'eyes closed'. MCI patients showed significantly less decrease in the lower alpha band than controls (p=0.04) during picture memory activation. The word memory task showed a trend towards a similar effect (p=0.09). This study suggests that memory activation reveals EEG differences between MCI patients and controls while conventional EEG conditions do not.

Aged↗

PubMed2006

The VESPA: a method for the rapid estimation of a visual evoked potential.

Faster and less obtrusive means for measuring a Visual Evoked Potential would be valuable in clinical testing and basic neuroscience research. This study presents a method for accomplishing this by smoothly modulating the luminance of a visual stimulus using a stochastic process. Despite its visually unobtrusive nature, the rich statistical structure of the stimulus enables rapid estimation of the visual system's impulse response. The profile of these responses, which we call VESPAs, correlates with standard VEPs, with r=0.91, p<10(-28) for the group average. The time taken to obtain a VESPA with a given signal-to-noise ratio compares favorably to that required to obtain a VEP with a similar level of certainty. Additionally, we show that VESPA responses to two independent stimuli can be obtained simultaneously, which could drastically reduce the time required to collect responses to multiple stimuli. The new method appears to provide a useful alternative to standard VEP methods, and to have potential application both in clinical practice and to the study of sensory and perceptual functions.

Adult↗

PubMed2005

Progressive N170 habituation to unattended repeated faces.

This study utilized a location-matching task to investigate whether the face-sensitive N170 event-related potential component would habituate in its response to the repeated presentation of same face stimuli when face identity was irrelevant to the experimental task. N170 amplitude decreased progressively with repeated presentation of the same face vs. sequential presentation of novel faces. This N170 habituation to face identity repetition occurred only for faces at unattended spatial locations, likely representing a relatively pure observation of automatic early face processing.

Adolescent↗

PubMed2006

Automatic face identity encoding at the N170.

The N170 event-related potential component is currently under investigation for its role in face identity processing. Using a location-matching paradigm, in which face identity is task irrelevant, we observed a progressive decrease in N170 amplitude to multiple repetitions of upright faces presented at unattended locations. In contrast, we did not observe N170 habituation for repeat presentations of inverted faces. The findings suggest that the N170 repetition effects reflect early face identity processes that are part of familiarity acquisition of new faces.

Adult↗

PubMed2002

Electrophysiological indexes of illusory contours perception in humans.

The present study investigated brain mechanisms underlying the perception of illusory contours, using recordings of event-related potentials of the brain (ERPs) in right-handed individuals. Forty different stimuli were presented randomly 1600 times in foveal vision; twenty of them produced the perception of illusory contours of a Kanizsa square, the remaining were obtained rotating outwards the inducers and they did not produce any illusory percept. Half of them had white inducers on a black background and vice versa; half of them were symmetrical and the other half asymmetrical. In lateral occipital areas illusory percepts produced larger evoked responses starting as early as 145 ms post-stimulus with the N1 peak. ERP data did not provide evidence of right-sided lateralisation of the processes underlying illusory contours formation at sensory level, as suggested by some neuroimaging and neuropsychological studies. The two cerebral hemispheres were differently activated while the subjective patterns formation progressed through neural processing stages. Indeed, brain response to illusory contours was more pronounced in the left occipital area at N2 component level (about 250 ms post-stimulus) and at right parietal sites at the latency of P300 component. Both background luminance and stimulus symmetry interacted with illusory boundaries formation. Present results confirm the hypothesis that the integration of contours arises at early stages of visual processing and highlight the primary role of edges continuity and boundary alignment in illusory contours perception.

Adult↗

PubMed2003

Visual evoked potentials to line- and luminance-defined triangles.

Processing of line- and luminance-defined triangles was investigated by studying visual potentials (VEPs) evoked by triangles. Twenty-six subjects were randomly presented with line, grey, and illusory triangles. Relative to VEPs elicited by grey and illusory triangles, VEPs to line triangles included P220s that were smaller at frontal sites but larger at occipital sites, and N260s that were smaller over both temporal and occipital sites. It is proposed that, in contrast to triangle defined by a line, illusory and luminance-defined triangles include information involved in the processing of surface.

Adult↗

PubMed1997

Responses of complex cells in cat area 17 to apparent motion of random pixel arrays.

The characteristics of directionally selective cells in area 17 of the cat are studied using moving random pixel arrays (RPAs) with 50% white and 50% black pixels. The apparent motion stimulus is similar to that used in human psychophysics [Fredericksen et al. (1993). Vision Research, 33, pp. 1193-1205]. We compare motion sensitivity measured with single-step pixel lifetimes and unlimited pixel lifetimes. A motion stimulus with a single-step pixel lifetime contains directional motion energy primarily at one combination of spatial displacement and temporal delay. We recorded the responses of complex cells to different combinations of displacement and delay to describe their spatio-temporal correlation characteristics. The response to motion of RPAs with unlimited lifetime is strongest along the preferred speed line in a delay vs displacement size diagram. When using an RPA with a single-step pixel lifetime, the cells are responsive to a much smaller range of spatial displacements and temporal delays of the stimulus. The maximum displacement that still gives a directionally selective response is larger when the preferred speed of the cell is higher. It is on average about three times smaller than the receptive field size.

Animals↗

PubMed1997

Responses to chromatic and luminance contrast in glaucoma: a psychophysical and electrophysiological study.

Increasing anatomical evidence indicates that large retinal ganglion cells (M-cells) are preferentially damaged in primary open angle glaucoma (OAG), while the smaller ganglion cells (P-cells) are relatively spared. In 13 patients with defined OAG and modest visual field defects, we evaluated the responses to stimuli that are expected to involve primarily the function of the P-pathway and compared them with those of control subjects. The psychophysical contrast sensitivity (CS), the PERG and the VEPs were measured for red-green gratings of pure chromatic contrast, as well as yellow-black gratings of pure luminance contrast. As compared with controls, OAG patients had reduced CS for both luminance and chromatic contrast stimuli by about 6 dB. PERGs and VEPs to luminance stimuli were little affected, whereas those to chromatic stimuli were both reduced in amplitude and delayed. These results indicate that visual dysfunction in glaucoma is not selective for the M-pathway, and that responses to equiluminant colour-contrast stimuli may be of diagnostic value.

Aged↗