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PubMed1994

Effects of luminance and external temporal noise on flicker sensitivity as a function of stimulus size at various eccentricities.

We studied how the dependence of flicker sensitivity on stimulus size was affected by the eccentricity of the stimulus at high luminance, at low luminance (with quantal noise), and at high luminance with the addition of pure white temporal noise. Flicker sensitivity was measured as a function of stimulus size for temporal frequencies of 1-30 Hz with uniform sinusoidally flickering spots. Sensitivity first increased with increasing stimulus size but then the increase saturated. At high luminance the saturation took place at larger stimulus sizes with increasing eccentricity. Without externally added temporal noise the maximum sensitivity was higher at the fovea than in the periphery at temporal frequencies of 1-10 Hz, but at 30 Hz this situation reversed. Therefore only the ascending parts of the spatial integration curves from various eccentricities could be superimposed by size scaling. E2, the eccentricity at which the spatial scale doubles, was found to be 2.2-2.7 deg for 1-10 Hz but 4.4 deg for 30 Hz. When enough temporal noise was added, performance at all stimulus sizes studied could be made independent of eccentricity by spatial scaling, since noise reduced maximum sensitivities to a constant level at all eccentricities. E2 was found to be 4.1 deg for 3 Hz and 7.2 deg for 30 Hz. When light level was reduced by 3 log10 units, foveal and peripheral flicker sensitivity functions almost superimposed at all stimulus sizes studied. Hence, at 1 and 3 Hz E2 was very large, about 70 and 22 deg, respectively. At 10 and 30 Hz no size scaling was needed and E2 was therefore infinite.

Adult↗
PubMed1996

Brightness contrast from inhomogeneous surrounds.

The luminance of a test within an inhomogeneous ("checkerboard") surround was adjusted to match the brightness of a comparison patch within a uniform surround. All stimuli were achromatic. Both surrounds had the same space-averaged luminance. With an incremental comparison patch, a test-within-checkerboard at a luminance between the luminances of the brighter and dimmer checks appears dimmer than if viewed within the uniform surround. A decremental comparison patch, however, is matched by a test luminance that is little affected by the inhomogeneity of the surround. In general, the brightness of the test is mediated neither by the space-averaged luminance of an inhomogeneous surround, nor by any equivalent uniform surround, regardless of luminance. We consider alternative models for the brightness of a region that is neither strictly an increment nor decrement with respect to contiguous surrounding surfaces.

Adaptation, Ocular↗
PubMed1996

A comparison of word recognition and reading performance in foveal and peripheral vision.

Word recognition thresholds and rapid serial visual presentation (RSVP) reading rates for both unrelated words and meaningful sentences were examined across the visual field. Both word recognition thresholds and RSVP reading rates for unrelated words can be equated across the visual field by an increase in size scale. RSVP reading rates for meaningful sentences cannot be equated across the visual field, with the fovea demonstrating a qualitative superiority over the periphery, irrespective of scale. The results suggest that the early visual machinery which underlies word recognition is the same across the visual field apart from a change in scale, whilst the periphery is qualitatively inferior to the fovea at interpreting sentences with meaning.

Adult↗
PubMed1996

Learning pop-out detection: specificities to stimulus characteristics.

Training induces dramatic improvement in the performance of pop-out detection. In this study, we examined the specificities of this improvement to stimulus characteristics. We found that learning is specific within basic visual dimensions: orientation, size and position. Accordingly, following training with one set of orientations, rotating target and distractors by 30 deg or more substantially hampers performance. Furthermore, rotation of either target or distractors alone greatly increases threshold. Learning is not transferred to reduced-size stimuli. Position specificity near fixation may be finer than 0.7 deg. On the other hand, learning transfers to the untrained eye, to expanded images, to mirror image transformations and to homologous positions across the midline (near fixation). Thus, learning must occur at a processing level which is early enough to maintain fine separability along basic stimulus dimensions, yet sufficiently high to manifest the described generalizations. We suggest that the site of early perceptual learning is one of the cortical areas which receive input from primary visual cortex, V1, and where top-down attentional control is present.

Adolescent↗
PubMed1989

Visual field asymmetries in numerical size comparisons of digits, words, and signs.

Visual field asymmetries were examined in American Sign Language-English bilinguals for speeded numerical size judgments of pairs of digits, number words, and number signs. Physical size of the number pairs was either congruent or incongruent with their numerical size. The results revealed a greater left visual field (LVF) interference for numbers represented as digits and a greater right visual field (RVF) interference for numbers represented as words or signs. Subjects' performance on number words and signs was also influenced by their skill in English and ASL: interference was greater in the RVF in the subjects' better language but was greater in the LVF for the less skilled language. These findings suggest that lateralization of numerical size judgments is moderated by the mode of number presentation and by prior language experience.

Adult↗
PubMed1994

Lilliputian hallucinations and medical illness.

Lilliputian hallucinations have been described in patients with delirium, schizophrenia, seizure disorders, visual disturbances, and brain tumors. The authors report two cases of patients with lilliputian hallucinations, one with AIDS-dementia complex and the other with dementia following head trauma. This is the first time that lilliputian hallucinations have been described in association with such medical conditions.

AIDS Dementia Complex↗
PubMed1996

Perceptual and attentional asymmetries in schizophrenia: further evidence for a left hemisphere deficit.

Increasing evidence suggests the presence of a lateralizing attentional deficit in schizophrenia. In the present study, 23 unmedicated patients with schizophrenia (mean age = 32.1 years) and 14 age- and sex-matched normal subjects were studied with the Global/Local Task to provide converging evidence for the presence of a left-hemisphere-associated attentional deficit in schizophrenia. This task is sensitive to the integrity of mechanisms involved in discriminating local and global elements of stimuli. Previous research has linked the discrimination of local targets to left hemispheric processes and the discrimination of global targets to right hemispheric processes. As predicted, patients were impaired in the detection of local level targets, consistent with a left hemispheric deficit. The degree of impairment correlated with the patient's level of auditory hallucinations. These results are consistent with previous studies showing an asymmetrical attentional deficit in schizophrenia and left hemispheric dysfunction. The correlation between this deficit and auditory hallucinations is consistent with a hypothesized relationship between this symptom in schizophrenia and left temporal pathology.

Adult↗
PubMed1981

Acuity, luminance, and monocular deprivation in the cat.

Visual acuity was determined for both eyes of long-term monocular-deprived cats over a wide luminance range. The influence of luminance on the rate of pattern vision recovery was also examined. Unlike strabismic humans and cats, the acuity deficit of monocularly deprived cats is not luminance dependent. This acuity is much worse at all luminance levels in the deprived eye. Likewise, initial testing of the deprived eye at low luminance levels does not facilitate recovery of pattern vision. Therefore, monocular lid fusion results in a visual deficit that differs from the effects of strabismus but resembles the effects of anisometropia in man.

Animals↗
PubMed1986

Size-threshold changes after lesions of the visual telencephalon in pigeons.

Fifteen pigeons were tested in a psychophysical procedure that determined the limits of their ability to detect differences in the sizes of stimuli. The results indicated that intact pigeons can reliably discriminate an annulus 3.0 mm in diameter from one that is 3.9 mm in diameter. In the first experiment, pigeons with lesions of the ectostriatum that spared the medial 15% were unimpaired in their size-discrimination ability. Those cases in which the lesions involved both the medial and lateral regions of the ectostriatum were greatly impaired. In a second experiment, these findings were replicated. In some cases, the electrode trajectory was varied to rule out possible effects from non-ectostriatal structures. In addition, the second study indicated that destruction of the medial ectostriatum with the lateral regions intact had no measurable effect on size-difference thresholds. The medial region of ectostriatum is part of the termination field of a second tectofugal pathway to the telencephalon. This pathway passes from the optic tectum to nucleus dorsolateralis posterior thalami and then to the neostriatum intermedium, including the medial ectostriatum. An examination of the data of the present experiment and those of other behavioral studies of the ectostriatum suggest that the medial ectostriatum may be involved in the processing of visual information with low spatial-frequency components and the lateral ectostriatum may be processing information about the high-frequency composition of stimuli.

Animals↗
PubMed1993

The bilateral reach to grasp movement.

This study investigated the kinematic organization of bilateral reach to grasp movements. In Experiment 1 non-homologous bilateral movements were performed. One limb reached to grasp an object using whole hand prehension; the contralateral limb simultaneously reached to grasp an object using precision grip. Corresponding unilateral movements were assessed. Movement duration for each limb in the bilateral condition was similar. However, with earlier temporal settings for peak wrist acceleration and velocity, the limb performing precision grip showed a longer approach (deceleration) phase to the object. Unilateral precision grip movements showed a longer movement duration and deceleration phase than unilateral whole hand prehension movements. In Experiment 2 homologous bilateral movements were assessed. Both limbs performed either a reach and whole hand prehension or a reach and precision grip. Again the precision grip movements showed longer movement and deceleration times. Experiment 3 consisted of bilateral non-homologous pointing movements and a pointing movement with one limb while reaching to grasp with the contralateral limb. It was found that the earlier temporal settings of peak acceleration and velocity with the precision grip limb of the non-homologous bilateral task (Expt. 1) were largely due to the performance of distal grasping actions. It is concluded that a kinematic parameterization which is independent to each limb is evident for bilateral tasks which require functionally independent actions.

Adolescent↗
PubMed1994

Arpeggio and fractionated digit movements used in prehension by rats.

Although it is well known that rats can use one limb to reach for and retrieve food, the way that the paw and digits are used to grasp the food has not been described. Using results obtained from a high speed videorecording procedure, the present study describes how small and large pieces of food are located by the paw and grasped with the digits in a reaching task. As the rat pronates a paw over food, the tips of digits 5 through 2 are successively placed and spread over the target area in an arpeggio movement. The target area is then palpated by downward movements of the palm. If food is not present, the paw is withdrawn without grasping. If food is contacted, the food is manipulated and grasped by the digits with grasp patterns that depend upon food size. Large food pellets are contacted with the pad of digit 3 and then grasped between digits 3 and 4. Small food pellets are first contacted with pad of digit 4 and are grasped between digits 4 and 5. Digit 5 can be partially flexed medially so that the food is held in a modified power grip between digits 4 and 5. The results show that rats use a whole paw movement to position the digits in an arpeggio fashion and they use fractionated digit movements to grasp food. The results are discussed in relation to the possible comparative and anatomical significance of the movements.

Animals↗
PubMed1984

An investigation of hemispheric specialization and visual event related potentials in discriminations of line length.

The purpose of the present study was to examine possible hemispheric differences in discriminations of line length and to determine whether the visual event related potential (ERP) recorded from over left and right hemispheres was related to performance. Twelve right-handed subjects participated on each of 2 separate days. The visual ERPs were derived from left occipital (O1) and right occipital (O2) placements. Eye blinks and vertical and lateral eye movements were also monitored. A single vertical 1.0 cm line (Standard) was displayed for 40 ms and was followed 2 s later by either a 0.9 cm, 1.0 cm or 1.1 cm (Comparison) line (also 40 ms). Subjects were asked to judge whether the second of the lines (Comparison) was "shorter" or "longer" than the first. Stimuli appeared at the center of a screen or at 1 degree 24' to the left and right of central fixation. Accuracy of line length judgements were similar in all 3 visual fields indicating a lack of hemispheric performance asymmetry for this type of discrimination task. In addition, there was no evidence of hemispheric asymmetry in the ERPs. The ambiguous discrimination involving the 1.0 cm standard and comparison lines seems to have influenced P3 latencies at both hemispheres. Namely, P3 latency was significantly longer in response to the 1.0 cm line than to the 0.9 cm or 1.1 cm lines. We propose that the ambiguous discrimination required a greater time for simulus evaluation and that this was reflected in the delayed P3 latencies.

Adult↗
PubMed1993

The effects of attention and context on the spatial and magnitude components of the early responses of the event-related potential elicited by a rare stimulus.

Sokolov's (1963) model-comparator theory of orienting and attention theorizes that different events underlie passive and active processing of sensory information presented in different contexts. The following study investigates changes in event-related potentials (ERPs) related to this theory by presenting stimuli inter- and intra-modally during passive and active processing tasks. Model-comparator theory proposes that novel events are detected by a mismatch discrimination process made between incoming and previously presented stimuli during preattentive processing. Central processing is engaged when this mismatch is relevant to the organism. To explore how engaging central processing, induced by instructional priming, affects preattentive processing, a 'truly' passive task was compared with a standard active task. This 'truly' passive task is different from the distracted one normally used to control direction of attention in ERP experiments in that it did not instruct subjects attention towards any task. ERP data were modeled as a dipole whose trajectory moved through voltage space. Our results suggest that both the spatial components and the magnitude of the dipole trajectory changed as functions of both passive and active processing and the context in which stimuli were presented. Our results also suggest that the trajectory and magnitudes of certain ERP components reflect processes proposed by model-comparator theory.

Adult↗
PubMedDate not supplied

Age-related changes in pattern visual evoked potentials: differential effects of luminance, contrast and check size.

We recorded visual evoked potentials (VEPs) to checkerboard pattern-reversal stimulation in 109 normal subjects (51 males and 59 females; aged 19-84 years) in order to study the aging effect on the multiple channels of the visual system in humans. Transient VEPs to 3 check sizes (15', 30' and 50') were obtained by monocular stimulation. Two test conditions were employed: (1) a high luminance (180 cd/m2) and a low luminance (11 cd/m2) both with a fixed contrast (90%), and (2) a high contrast (85%) and a low contrast (10%) both at a fixed luminance (57 cd/m2). The major features of our results included: (1) the presence of a curvilinear relationship between P100 latency and age for all conditions, while the P100 amplitude did not show any such aging effect, (2) the age-latency function was similar between the two luminance conditions, while it was different between the two contrast conditions, and (3) the differential age effect on the P100 latency caused by changes in contrast depended on the check size. These results suggest that age-related changes in the human visual system are not uniform, but rather are different in the specific functional subdivisions. It is thus hypothesized that aging may differentially influence the separate channels of the human visual system.

Adult↗
PubMed1986

Role of input factors in visual-field asymmetries.

This paper examines the implications of lateral tachistoscopic presentation of information for the processing efficiency of the intact cerebral hemispheres. Considering that the understanding of the processes underlying the particular competences of each hemisphere may require, as a preliminary step, the specification of the characteristics of the input on which the brain operates, anatomical, physiological, and psychophysical consequences of briefly stimulating the retinal periphery for the representation of information in the brain are outlined, with special reference to the spatial-frequency spectral composition of the stimuli. Retinal eccentricity and brief exposure duration converge to making the representation of information qualitatively different from the information that the brain normally operates on, which constrains the interpretation of findings with respect to the normal functions of the cerebral hemispheres. A review and discussion of empirical findings relevant to these issues suggest that manipulation of procedural variables may differentially affect the processing efficiency of the cerebral hemispheres and indicate that a given pattern of visual-field asymmetry may be overdetermined by a multitude of variables interacting in complex ways.

Discrimination Learning↗
PubMed1991

Inter-versus intrahemispheric concordance of judgments in a nonexplicit memory task.

Performance for explicit probed recollection of an item's presentation is usually superior when initial presentation and probe are directed to the same hemisphere than when they are directed to opposite hemispheres. The present study explored whether a same-hemisphere advantage could also be observed for a nonexplicit memory task by examining concordance for initial and subsequent ratings of line length. Results indicated that mean ratings of line length differed less between initial and subsequent presentation when the lines were viewed in the same rather than opposite visual fields. This finding held even though mean ratings for initial presentation of items in the right and left visual fields did not differ. These findings suggest that the same-hemisphere advantage is a relatively basic characteristic of memorial processing in the hemispheres.

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↗