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PubMed2003

Pooling speed information in complex tasks: estimation of average speed and detection of nonplanarity.

To gain insight into how speeds are combined in structure-from-motion, we compared performance for estimating the mean speed and performance for detecting deviations from planarity. The stimuli showed a center dot surrounded by an annulus of dots. In one (plane) condition, the stimuli simulated a rotating plane. In a two alternative forced choice (2AFC) task, the subject had to choose in which of two stimuli the center dot moved in the plane. In another (cloud) condition, the same dot locations and speeds were used but now assigned to different dots. Such a stimulus resembles a translating and rotating cloud of dots. In this case, the subject had to choose the stimulus in which the center dot moved with the mean speed of the surrounding dots. Performance was measured as a function of deformation/slant. Although location and speeds were the same in both conditions, performance was much poorer in the cloud condition. Subsequent experiments and an ideal observer model point to a plausible explanation: in detecting deviations from planarity, the visual system can focus on the most reliable pieces of information (the slower dots, closest to the test dot). Although performance could benefit by taking more dots into account, performance barely improved with an increase in the number of dots. This may reflect a limited processing capacity of the visual system.

Humans↗
PubMed1999

Visual sensitivity in the presence of a patterned background.

The sensitivity of the visual system depends on ambient illumination: Sensitivity is reduced in the presence of a bright, uniform background. We asked how sensitivity is adjusted when the background is spatially detailed and therefore contains both luminance peaks and troughs in the neighborhood of a foreground object. A test flash was superimposed on a static sinusoidal grating. As the grating's spatial frequency increased, sensitivity for flash detection declined, regardless of whether the flash was superimposed on a peak or a trough of the grating. We studied the mechanisms underlying this loss of sensitivity by delivering the test stimulus through one eye and the background through the other. The conclusion is that three mechanisms are involved. Luminance adaptation and a masking process adjust sensitivity at low- and mid-range spatial frequencies, respectively. The third mechanism, a contrast gain control, is localized (it occurs at spatial frequencies approaching the limit for resolution) and fast (complete in half a second), and it results from early processing in the visual pathway (it is absent during dichoptic viewing). This local adjustment of sensitivity may help to protect the clarity of even the smallest details in the visual scene.

Adaptation, Ocular↗
PubMed1976

Matching forms as a function of complexity, type of match, and retention interval.

The present experiment tested the hypothesis that nominal processing increases as stimulus complexity increases. Subjects indicated whether two 4- or 12-sided forms, separated by an interval of .5 or 4.0 sec., were the same or different. "Same" responses corresponded to matches for physical or nominal identity. Longer RTs for high complexity than low complexity forms suggest that complexity affects the efficiency of visual processing rather than the occurrence of nominal processing. An interaction between type of match and interval, due to the longer RTs for matches of nominally identical forms at only the .5-sec. interval, indicates that at this interval, matches for physical and nominal identity are made with visual and nominal representations respectively.

Form Perception↗
PubMed2000

Is visual thinking "imageless thought"?

Two measures of visual problem solving ability and two measures of visual-imaging ability were administered to 99 subjects. Scores on the Emergent Patterns Test, a problem-solving test which requires subjects to think about specific combinations of visual icons and to identify the emergent patterns, did not correlate significantly with scores on either the Vividness of Visual Imagery Questionnaire or the Brightness of Visual Imagery Measure. Scores on the Block Visualization Test, which requires subjects to think about blocks being painted and diced and to answer questions about the resulting pieces of block, did not correlate significantly with the scores on the Vividness of Visual Imagery Questionnaire and correlated negatively with the scores on the Brightness of Visual Imagery Measure. The three nonsignificant correlations support nineteenth century arguments that vivid imagers tend to construct sensory representations of unconscious visual thoughts, whereas "imageless thinkers" tend not to. The one negative correlation further may suggest that the conscious construction of visual imagery can sometimes interfere with the underlying processes of unconscious visual thinking.

Humans↗
PubMed1990

Letter identification and lateral masking in dyslexic and average readers.

Lateral masking and letter identification in dyslexic and average readers were investigated with a methodology that corrected for some of the weaknesses of the Geiger and Lettvin studies (1986, 1987). Target letters were presented alone or embedded within three-letter arrays at retinal locations from 0 degrees to 15 degrees to the right or left of a fixation point. A foveal letter, which was the same as or different from a target letter, appeared as a distractor, and comparisons were made between scaled and unscaled stimuli. Dyslexic readers were found to be better than average readers at detecting scaled letters embedded in an array in some of the peripheral locations tested. Unlike the results of Geiger and Lettvin, however, this finding of better letter detection in the periphery by dyslexics was limited to selected conditions of the study. Reading groups were also found to differ in responding to the foveal distractor in the letter task. Detection of letters by average readers was affected by the type of distractor, but this variable was not found to influence the dyslexic readers. These findings suggest some differences between dyslexic and average readers in attention to stimuli presented at multiple locations in the visual field.

Adult↗
PubMed1987

Pupil response as an objective measure of visual acuity.

Pupillary responses to brief presentations of sinusoidal grating patterns were measured. The results show that the amplitude of the pupillary responses varies systematically with grating spatial frequency. Gratings of spatial frequencies in the range 1-5 c deg-1 elicited the maximum percentage change in pupil area, with higher and lower spatial frequency gratings producing smaller responses. The bandpass nature of the pupil response function was compared to contrast sensitivity functions obtained under the same viewing conditions. The effects of defocus and eccentric fixation on both contrast sensitivity and pupil response functions were qualitatively similar. Grating acuities calculated from contrast sensitivity data correlated well with similar data extracted from pupil response functions. The results suggest that automated pupillometry can provide an attractive means for objective measurement of visual acuity.

Fixation, Ocular↗
PubMed1998

The contralateral organization of visual memory: a theoretical concept and a research tool.

Contralateral-control methods can be applied to psychophysiology and in particular to the study of visual memory. Visual memory possesses some degree of hemispheric organization, so that visual memory traces for laterally presented stimuli are stronger or more durable in the hemisphere contralateral to the hemifield where the stimuli were first presented. I first introduce the concept of hemispheric organization of function. Then I discuss how hemispheric organization can be exploited for obtaining information about the time course and brain localization of psychological processes, using a contralateral-control method. Behavioral and event-related brain potential data support the hemispheric organization view of visual memory, and the contralateral-control method, in conjunction with the recording of the event-related optical signal, can be used to reveal the existence of memory-driven processes in early stations of the visual system.

Animals↗
PubMed1994

The role of surface information in object recognition: studies of a visual form agnosic and normal subjects.

Three experiments were conducted to explore the role of colour and other surface properties in object recognition. The effects of manipulating the availability of surface-based information on object naming in a patient with visual form agnosia and in two age-matched control subjects were examined in experiment 1. The objects were presented under seven different viewing conditions ranging from a full view of the actual objects to line drawings of those same objects. The presence of colour and other surface properties aided the recognition of natural objects such as fruits and vegetables in both the patient and the control subjects. Experiment 2 was focused on four of the critical viewing conditions used in experiment 1 but with a large sample of normal subjects. As in experiment 1, it was found that surface properties, particularly colour, aided the naming of natural objects. The presence of colour did not facilitate the naming of manufactured objects. Experiment 3 was focused on possible ways by which colour could assist in the recognition of natural objects and it was found that object naming was facilitated only if the objects were presented in their usual colour. The results of the experiments show that colour does improve recognition for some types of objects and that the improvement occurs at a high level of visual analysis.

Adult↗
PubMed2005

The influence of training length on generalization of visual feature assemblies in honeybees.

Generalization is a fundamental cognitive ability that allows treating similar stimuli as equivalents, and thus responding to them in the same manner. Here, we show that after training free-flying bees with a single, constant pair of patterns made of four quadrants, each displaying different oriented gratings, bees extract the orientation information corresponding to each quadrant and integrate it in a generic layout that preserved the spatial relationship between oriented edges. Our results show that the amount of experience with the training patterns is critical to determine or not generalization to novel stimuli sharing the layout of the rewarded stimulus. Increasing experience results in higher generalization levels reflected in significant responding to novel stimuli. With ongoing training, redundant information seems to be eliminated and reduced to the minimum that is necessary and sufficient to solve the task. Controlling precisely the level of experience of individuals is therefore crucial in experiments on visual recognition.

Analysis of Variance↗
PubMed2004

Wider recognition in peripheral vision common to different subtypes of dyslexia.

Italian children (n = 125) were classified into dyslexics, poor readers and ordinary readers. The dyslexics were further classified into the Boder and Bakker subtypes. The children were tested with the form-resolving field (FRF), which measures central and peripheral visual recognition. Dyslexics show higher correct identification of letters in the periphery, supporting the notion of a different distribution of lateral masking. A numerical characterization of individual FRFs--C2R--reliably distinguishes between dyslexics and ordinary readers. The wider distribution of recognition, similar across the various subtypes of dyslexia, suggests a general characteristic of visual perception, and possibly a different visual-attentional mode.

Adolescent↗
PubMed1985

Optic nerve dysfunction during gravity inversion. Pattern reversal visual evoked potentials.

The intraocular pressure (IOP) in humans approximately doubles when a head-down, or inverted, position is assumed. Simultaneously, the ophthalmic artery pressure increases by an even greater absolute magnitude. We recorded pattern reversal visual evoked potentials (PRVEPs) in 16 subjects in both the inverted and upright positions. In ten of the 16 subjects, we performed an additional experiment. On a different day, we obtained control PRVEPs, and then, using a scleral suction cup, we artificially increased the IOPs to the same values reached when the subjects were previously inverted. We found that the PRVEP amplitudes were significantly reduced in both experiments compared with corresponding control recordings, without changes in latency or wave form. Furthermore, the degree of amplitude reduction was even greater during gravity inversion than with suction cup elevation of IOP. The effects of IOP on optic nerve function as measured by the PRVEP appear to be dependent more on the IOP than on vascular perfusion. We also concluded that gravity inversion activities pose potential risks to the eyes.

Adult↗
PubMed2005

Enhanced extrastriate visual response to bandpass spatial frequency filtered fearful faces: time course and topographic evoked-potentials mapping.

We compared electrical brain responses to fearful vs. neutral facial expressions in healthy volunteers while they performed an orthogonal gender decision task. Face stimuli either had a broadband spatial-frequency content, or were filtered to create either low spatial-frequency (LSF) or high spatial-frequency (HSF) faces, always overlapped with their complementary SF content in upside-down orientation to preserve the total stimulus energy. We tested the hypothesis that the coarse LSF content of faces might be responsible for an early modulation of event-related potentials (ERPs) to fearful expressions. Consistent with previous findings, we show that broadband images of fearful faces, relative to neutral faces, elicit a higher global field power of approximately 130 ms poststimulus onset, corresponding to an increased P1 component over lateral occipital electrodes, with neural sources located within the extrastriate visual cortex. Bandpass filtering of faces strongly affected the latency and amplitude of ERPs, with a suppression of the normal N170 response for both LSF and HSF faces, irrespective of expression. Critically, we found that LSF information from fearful faces, unlike HSF information, produced a right-lateralized enhancement of the lateral occipital P1, without any change in the scalp topography, relative to unfiltered (broadband) fearful faces. These results demonstrate that an early P1 response to fear expression depends on a visual pathway preferentially tuned to coarse-magnocellular inputs, and can persist unchanged even when the N170 generators are disrupted by SF filtering.

Adult↗
PubMed1990

Electroretinograms and pattern visually evoked cortical potentials in central areolar choroidal dystrophy.

We describe three patients with central areolar choroidal dystrophy whose electroretinograms (ERGs) and pattern visually evoked cortical potentials (VECPs) confirmed their macular dysfunction. Visual fields measured by Goldmann perimetry showed central relative scotomata corresponding to a dystrophic lesion. Dark adaptation curves were slightly abnormal either in the first curve or in the second curve, depending on the visual acuity. Color vision was disturbed irregularly. Fluorescein angiography revealed a loss of the choriocapillaries, and a hyperfluorescent border outlined the dystrophic lesion. Single-flash dark-adapted ERGs and scotopic ERGs were almost normal, while photopic and flicker ERGs showed slightly attenuated amplitudes. Conversely, VECPs to pattern reversal stimulation were greatly affected for both transient and steady-state stimuli. We conclude that, in central areolar choroidal dystrophy, pattern VECPs can provide more information on macular dysfunction than does the ERG.

Adult↗
PubMed1989

Visual evoked potential abnormalities in chiasmal lesions.

Pattern reversal visual evoked potentials (VEPs) were recorded from 38 patients with lesions affecting the chiasmal area. Lesions were confirmed by computer tomography and all patients had ophthalmologic examination. VEPs to full-field stimulation (0-16 degrees r) were compared with those obtained with half-field stimulation. Changes in VEPs were seen as a nonrecordable or attenuated P 100 (abnormal amplitude ratio) or as a prolonged P 100. Analysis of the records showed that temporal half-field stimulation (crossed fibers) yielded a higher rate of abnormal responses (80%) than full-field stimulation (66%). The most frequent abnormality in the former stimulation was a nonrecordable P 100 (42%) and in the latter an abnormal amplitude ratio of P 100 (41%). When the uncrossed fibers were stimulated with the nasal half-field, abnormalities were detected in 32% of responses. Lesions in the region of the sella turcica were also associated with a high incidence of delayed responses (39% of patients when crossed fibers were stimulated). However, the magnitude of the delays was smaller (1-32 ms) compared with delays in patients with demyelinating disease. Findings of this study show that half-field stimulation assists in the interpretation of responses to full-field stimulation. In addition, half-field stimulation can reveal abnormalities that are not detected with full-field stimulation.

Adolescent↗
PubMed1992

Clinical application of motion-onset visual evoked potentials.

The results of motion-onset visual evoked potentials and pattern-reversal visual evoked potentials were compared in 5 adults with amblyopia, in 13 patients with unilateral retrobulbar neuritis and in 62 patients with multiple sclerosis. While the pattern-reversal visual evoked potentials had reduced amplitudes and prolonged latencies in all amblyopic eyes, the motion-onset visual evoked potentials were normal. Thus, motion-onset visual evoked potentials cannot be used for diagnosis of amblyopia. In patients with retrobulbar neuritis, both types of visual evoked potentials were delayed on stimulation of the affected eye. The latency increase was, however, greater for pattern-reversal visual evoked potentials than for motion-onset visual evoked potentials. Examination of the patients with multiple sclerosis showed that the additional use of motion-onset visual evoked potentials increased the sensitivity of the investigation. In some patients, only the motion-onset visual evoked potentials had pathologic latency increases, whereas the pattern-reversal visual evoked potentials stayed within normal limits.

Adult↗
PubMed1982

Foveal inhibition and facilitation caused by remote grating jerks: interaction between long-range and short-range effects.

Periodic oscillation of a luminance grating imaged upon the peripheral retina reduces the threshold visibility of a foveally presented test spot. This new effect has been named the "jerk effect". The present investigation is concerned with the effect of a single jerk of the remote grating on threshold sensitivity. Foveal sensitivity changes were measured for different delays between grating jerk and test spot presentation. For 0.38 degrees, 100 ms test spot, long-range transient inhibition was found for all delays, with a maximal effect between 0 and 30 ms delay. By combining the jerk effect with the Westheimer paradigm, both facilitatory and inhibitory long-range effects could be demonstrated. For facilitation to occur, it was necessary that the steady background extended into the sensitization zone of the Westheimer area. Inhibition was the only result for smaller backgrounds. Reduced visibility is consistent with the hypothesis that peripheral transient mechanisms inhibit foveal sustained mechanisms. Enhanced visibility indicates that thresholds depend on an interaction between foveal-sustained and foveal-transient units. Transient peripheral stimulation and steady backgrounds of increasing diameter change the balance of inhibitory and facilitatory processes between these units.

Evoked Potentials, Visual↗
PubMed1997

Prognostic value of pattern reversal visual-evoked potentials in idiopathic epiretinal membrane.

BACKGROUND: Prognostically favorable factors for epiretinal membrane removal have been described in the literature by several authors. Little information, however, is available about the objective assessment of the preoperative macular function. This study reports the results of idiopathic epiretinal membrane removal and the prognostic value of preoperative, pattern reversal visual-evoked potentials (PRVEPS) in recovery of visual acuity (VA). METHODS: In 60 patients (60 eyes) with idiopathic epiretinal membrane we performed PRVEP examination preoperatively. All eyes were operated on by standard three-port vitrectomy with membrane removal. Two eyes were excluded because of postoperative complications. Follow-up VA was compared with preoperative VA for the 58 study eyes and correlated with preoperative PRVEP parameters. RESULTS: The mean preoperative VA was 0.2, the mean postoperative VA, 0.4. The PRVEP was recordable in 74%, 67% and 36% of cases for check sizes of 17, 10 and 7 arcmin respectively. Twenty patients (50%) had an increase in VA of two lines or more, in 25 patients (43%) VA remained within one line of the preoperative value, and in 4 patients (7%) VA decreased by two lines or more. The mean preoperative VA was not significantly different between the group with an improved VA and the group that did not benefit from membrane removal. Of the PRVEP parameters, only the N80 latency for the 17' check size was significantly associated with postoperative visual outcome. CONCLUSION: The PRVEP is applicable as a predictor for visual outcome in cases of epiretinal membrane removal. For the 17' pattern size we found a significant association of the combination of recordability and delayed N80 latency with visual outcome.

Adolescent↗