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Biomedical subjects

S Magnussen

Publications and source records attributed to S Magnussen.

At least 19 recordsLinked to original sources
PubMed2001

Filling-in of the foveal blue scotoma.

The blue-blindness (tritanopia) of the human foveola normally goes unnoticed but can be directly visualized by having observers view a flickering, monochromatic, short-wavelength field. The blue scotoma appears as a tiny dark spot in central vision, the visibility of which depends upon the wavelength of the field and the temporal frequency of modulation. Comparisons of fading times as a function of flicker frequency for the blue scotoma, foveal afterimages and optically stabilized images indicate a common time course, consistent with the hypothesis that perceptual filling-in of the foveal blue scotoma reflects the operation of neural processes similar to those involved in fading and regeneration of stabilized images.

Afterimage↗
PubMed2000

Brain regions involved in spatial frequency discrimination: evidence from fMRI.

The cortical areas underlying successive spatial-frequency discrimination were explored using functional magnetic resonance imaging (fMRI). In a steady-state, block-design paradigm, 12 subjects viewed a single fixation cross during a rest period, followed by an activation period consisting of the presentation of horizontal (distractors) and vertical (targets) sinewave gratings. Two tasks were performed: in the control task, subjects pressed a button after the second vertical grating was presented within each trial; in the discrimination task, subjects decided which target grating had the higher spatial frequency. Post-processing consisted of off-line image registration to correct for head motion, spatial and temporal smoothing, and cross-correlation between each voxel time course and a phase-shifted stimulus time profile. The results indicate that striate, extrastriate, parietal, and prefrontal areas show significant BOLD (blood oxygen level dependent) effects during both discrimination and control tasks, with consistently higher activity levels in the discrimination task.

Discrimination, Psychological↗
PubMed2000

What limits simultaneous discrimination accuracy?

Discrimination accuracy decreases when viewers simultaneously monitor two perceptually distinct stimulus components for changes in a common property, e.g. contrast [Magnussen & Greenlee (1997). Journal of Experimental Psychology: Human Perception and Performance, 23, 1603-1616; Olzak & Wickens (1997). Perception, 26, 1101-1120]. We ask whether the limitation is in monitoring two components or in making dual decisions about a single property. Using the same uncertainty paradigm as Magnussen and Greenlee, we find no evidence of a processing limitation when viewers simultaneously monitor one component (1.25 c/d) for a possible change in contrast and a second component (5 c/d) for a possible change in spatial frequency, regardless of whether the components are spatially separated or superimposed. The limitation is in making dual decisions about a single property.

Attention↗
PubMed2000

Low-level memory processes in vision.

Psychophysical studies of the short-term memory for attributes or dimensions of the visual stimulus that are known to be important in early visual processing (spatial frequency, orientation, contrast, motion and color) identify a low-level perceptual memory mechanism. This proposed mechanism is located early in the visual processing stream, prior to the structural description system responsible for shape priming but beyond primary visual cortex (V1); it is composed of a series of parallel, special-purpose perceptual mechanisms with independent but limited processing resources. Each mechanism is devoted to the analysis of a single dimension and is coupled to a memory store.

Humans↗
PubMed1999

The psychophysics of perceptual memory.

Psychophysical studies of short-term memory for attributes or dimensions of the visual stimulus known to be important in early visual processing--spatial frequency, orientation, contrast, motion--identify an early perceptual memory system. The proposed system, which may be part of the Schacter-Tulving perceptual representation system (PRS), is located early in the visual processing stream, prior to the structural description system responsible for shape priming but beyond primary visual cortex (V1), and consists of a series of parallel special-purpose perceptual mechanisms with independent but limited processing resources, where each mechanism is devoted to the analysis of a single stimulus dimension and is coupled to a memory store. The experimental evidence for this hypothesis is reviewed.

Evoked Potentials↗
PubMed1999

Effects of moderate doses of alcohol on immediate recall of numbers: some implications for information technology.

OBJECTIVE: Moderate doses of alcohol will have detrimental effects on memory functions used in various aspects of human interaction with information technology. The need to deal with multidigit numbers while under alcohol intoxication (e.g., making telephone calls) is a reality of life. To determine these relations, the effects of alcohol on immediate ordered recall were studied in a practical number-dialing task. METHOD: Immediate ordered recall of eight-digit numbers, presented visually or auditorily, was tested in read-and-dial or listen-and-dial forced ordered recall tasks with male subjects (N = 11) under two levels of blood alcohol concentration (BrAC = 0.05% and BrAC = 0.1%) and a no-alcohol placebo control condition (BrAC = 0.0%). RESULTS: With increasing alcohol level, immediate ordered recall of whole numbers (all eight digits correct and in the right order) fell by 9% (visual presentation) and 15% (auditory presentation). No significant interaction effect between alcohol level and digit position in the number was found for visually presented numbers, while a weak, but significant, effect was found for auditorily presented numbers (most prominent at digit positions 5, 6 and 7). This is probably due to the difference in rehearsal opportunities in the two presentation modes. No significant interaction effects between alcohol level and presentation modality were found. CONCLUSIONS: The results, tailored to a practical everyday task of dialing unfamiliar eight-digit telephone numbers, show that even moderate doses of alcohol will affect the performance of an already fragile short-term memory system engaged whenever reproduction of digit strings is required.

Acoustic Stimulation↗
PubMed1999

Asymmetry of left versus right unilateral cleft impairments: an experimental study of face perception.

OBJECTIVE: Previous psychosocial studies of adults born with cleft lip and palate have provided circumstantial evidence that surgically repaired right-sided unilateral clefts may be more disfiguring than left-sided clefts. The present study asked if such asymmetries are physiognomic asymmetries or arise "in the eye of the beholder," representing perceptual processes in face recognition. DESIGN: Color slides of 160 children (6 years of age) and young teenagers (16 years of age) were rated by subjects for perceived disfigurement. Sixty of the subjects had unilateral complete cleft lip and palate (30 had a right-sided cleft and 30 had a left-sided cleft), 60 had unilateral cleft lip/alveolus (30 right-sided and 30 left-sided clefts), 32 children had bilateral cleft lip and palate, and 8 children had cleft palate only. Faces were shown in normal and in mirror-reversed versions; the order in which faces were shown was randomized, as were other stimulus factors such as cleft type, age, and gender. SETTING: The study was conducted as a classroom-type experiment at the Vision Laboratory, Department of Psychology, Oslo, Norway. PARTICIPANTS: Thirty-seven students of psychology at the University of Oslo, who were ignorant of the purpose of the study, acted as subjects. MAIN OUTCOME MEASURE: Subjects rated perceived disfigurement using a visual analog scale. RESULTS: Modest but highly consistent hemifacial asymmetries in judged disfigurement were found, with left-sided unilateral clefts rated as less disfiguring than right-sided unilateral clefts. Unilateral clefts were judged as being less disfiguring than the bilateral clefts, and cleft lip/alveolus was judged as being less disfiguring than cleft lip and palate. The patterns of facial judgments were almost identical in the normal and reversed-slides conditions. CONCLUSIONS: Asymmetries between left- and right-sided clefts reside in physiognomic factors rather than in hemispheric asymmetries controlling the perceptual process of face judgment.

Adolescent↗
PubMed1998

Electrophysiological localization of brain regions involved in perceptual memory.

Event-related potentials (ERP) were recorded during perceptual discrimination and short-term memory, varying the interstimulus interval (1-10 s) in delayed spatial frequency discrimination. Accuracy of discrimination remained unimpaired across this time interval, but choice reaction times increased. A brain source localization (BESA) model showed that the activity of the parietal and right temporal sources increased with long retention intervals in a sequential activation pattern where a long-latency component of the parietal source specific to the memory condition was observed, the latency of which matched a memory-related increase in choice reaction times in the cognitive task. It is suggested that the temporal sources are involved in encoding and storage of visual information, and the parietal source is involved in memory retrieval.

Electroencephalography↗
PubMed1998

Limited-capacity mechanisms of visual discrimination.

Discrimination thresholds of spatial frequency and choice reaction times (RT) were measured in three subjects who performed a dual-judgment delayed discrimination task. Two reference gratings were presented side-by-side with a 0-800 msec stimulus onset asynchrony (SOA), which were followed after a 5-sec retention interval by two test gratings. Subjects judged which component changed and which interval had the higher spatial frequency (SF). Thresholds in the dual-judgment task were four to six times higher than thresholds in single-judgment tasks. The SOA had only a moderate effect on discrimination thresholds, whereas the difference between the spatial frequencies of the two components had a highly significant effect. The discrimination thresholds increase with increasing spatial frequency difference for the lower SF component, while they decrease for the higher SF component. An analysis of the distribution of possible error types indicated that all subjects tended to respond more frequently that the higher SF component changed. This tendency led to more errors on trials where the low SF component changed. A posthoc analysis revealed, in two of the three subjects, a significant correlation between delta f/f and RT such that higher delta f/f values were associated with lower RTs and vice versa.

Analysis of Variance↗
PubMed1998

Representation of orientation and spatial frequency in perception and memory: a choice reaction-time analysis.

Discrimination and short-term memory for the orientation of sinusoidal gratings that differed in spatial frequency, and for the spatial frequency of gratings that differed in orientation, were measured in a same-different task with 0-10 s interstimulus intervals (ISI) between test and reference stimuli. Introducing a difference between test and reference stimuli on a second dimension, or increasing ISI, did not impair spatial discrimination in terms of accuracy, but choice reaction times for correct decisions were prolonged by both manipulations. Results suggest that perceptual discrimination is based on representations in which orientation and spatial frequency are conjointly coded and that decisions are reached by a serial process scanning multiple-tuned, labeled channels; short-term memory may involve reactivation of these channels.

Analysis of Variance↗
PubMed1998

Spatial-frequency discrimination, brain lateralisation, and acute intake of alcohol.

The effect of alcohol (breath-alcohol level of 0.1%) on perceptual discrimination of low (1.5 cycles deg-1) and high (8 cycles deg-1) spatial frequencies in the left and right visual field was measured in eighteen right-handed males, in a double-blind, balanced placebo design. Discrimination thresholds for briefly (180 ms) presented sinusoidal gratings were determined by two-alternative forced-choice judgments with four interleaving psychophysical staircases providing random trial-to-trial variation of reference spatial frequency and visual field, in addition to a random (+/- 10%) jitter of reference spatial frequency. Alcohol produced overall higher discrimination thresholds but did not alter the visual-field balance: no main effect of visual field was observed, but in both placebo and alcohol conditions spatial frequency interacted with visual field in the direction predicted by the spatial-frequency hypothesis of hemispheric asymmetry in visual-information processing, with left-visual-field/right-hemisphere superiority in discrimination of low spatial frequencies and right-visual-field/left-hemisphere superiority in discrimination of high spatial frequencies.

Adult↗
PubMed1997

Hidden visual capabilities in mentally retarded subjects diagnosed as deaf-blind.

The visual acuity of twelve multi-handicapped, mentally retarded subjects, diagnosed as deaf-blind, was measured on two occasions with the Teller Acuity Cards (TAC). Eight subjects scored above the criterion for legally blind and the results of six of these indicated various degrees of poor to approaching-normal eyesight. To evaluate high-level vision four subjects were tested with the Fagan Test, assessing visual recognition memory for faces subsequent to familiarization with the preferential looking technique. The results for three subjects showed evidence for perceptual recognition. It is concluded that TAC combined with the Fagan Test may detect perceptual capacities unnoticed by clinical observation.

Adolescent↗
PubMed1997

Competition and sharing of processing resources in visual discrimination.

Discrimination thresholds for spatial frequency and contrast tested individually were compared with dual discrimination of contrast and spatial frequency, and dual discrimination of 2 contrast or spatial frequency components. The components were presented overlapping, forming a compound grating or as side-by-side simple gratings. When observers had to judge contrast and spatial frequency simultaneously, discrimination thresholds increased by an amount predicted by a model of stimulus uncertainty for orthogonal dimensions (1.7); when they had to judge 2 frequency or contrast components, discrimination thresholds increased by a factor of 3-6 compared with the single-judgment task. The relative spatial location of the components did not interact with task complexity. The results are consistent with a model assuming a set of parallel special-purpose attentional mechanisms.

Analysis of Variance↗
PubMed1996

Age comparisons of serial position effects in short-term memory.

Memory for serial position in four-item lists of words or abstract designs was tested at retention intervals of 5-25 seconds in two subject groups, aged 25-35 and 60-75 years; accuracy and choice reaction times (RT) were recorded. Increasing the retention interval transformed the serial position curve from recency to primacy but produced no overall reduction in memory performance in terms of accuracy. RTs varied as a function of both serial position and retention interval. Under all conditions the memory of older subjects was less accurate and they exhibited longer RTs compared to young subjects, but the age differences did not interact with list position or retention interval for either response indicator. The results suggest that age differences in memory can be explained by a single factor of mental speed limiting encoding efficiency and affecting decision times.

Adult↗
PubMed1996

Parallel processing in visual short-term memory.

Visual short-term memory for the contrast and spatial frequency of sinusoidal gratings was measured in a delayed discrimination task in which the 2 stimuli to be compared were separated in time by 1-10 s interstimulus intervals (ISIs). Delayed discrimination thresholds for spatial frequency and contrast were compared, both when the 2 types of thresholds were measured in separate blocks of trials and when the 2 types of measures were randomly intermixed in an uncertainty paradigm, which required participants to process information about both dimensions on each trial. In both cases, accuracy of memory for spatial frequency was independent of ISI, but memory for contrast decreased as ISI increased. Performance was lower in the uncertainty case, but only by an amount predicted by statistical decision theory for independent sources. The results are consistent with a model assuming a set of parallel special-purpose visual discrimination and short-term memory mechanisms.

Differential Threshold↗
PubMed1996

Serial position effects in visual short-term memory for words and abstract spatial patterns.

Two experiments tested the effects of list position, and retention-interval in recognition for two distinct stimulus categories in young adults. Stimulus categories were spatial abstract patterns and words presented on a computer screen. At short delay intervals recency effects predominates and at longer delay intervals a primacy effect predominates in both experiments, indicating similar basic memory processes producing the serial position functions for the two different categories of visual stimuli, but as length of retention-interval increases, memory for first list items improves for words and remains constant for abstract patterns. Recency functions are similar for both stimulus categories tested.

Adult↗
PubMed1995

Preferential looking: a sudden shift in visual fixation pattern at 32 months of age.

Observations of eye movements of young children in a modified preferential looking task suggest a change in the visual looking pattern taking place during a very brief time between 32 and 33 months of age. In the younger children, a grating stimulus elicited eye movements towards the target followed by a few seconds of focused attention; in the older children a visual avoidance behavior was observed where subjects looked consistently in the opposite direction of the target. An analysis of video recordings suggests that this avoidance pattern was a composite of a very brief initial target-directed eye movement which was quickly arrested, and followed by eye movements in the opposite direction, initiating a search of the visual scene.

Attention↗