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Biomedical subjects

J Saarinen

Publications and source records attributed to J Saarinen.

69 records · Page 4Linked to original sources
PubMed1992

Representation of abstract attributes of auditory stimuli in the human brain.

Representations of abstract attributes of auditory stimuli in the human brain were demonstrated using the mismatch negativity (MMN), an event-related potential component elicited by a change in a repetitive sound. Stimuli were pairs of sinusoidal tones. There were two types of tone pairs in each block, standard (p = 85%) and deviant pairs (p = 15%), delivered in a random order. Standard and deviant tone pairs differed only in the direction of within-pair frequency change. In addition, the frequency levels of both the standard and deviant pairs varied randomly within a wide range in a block; thus the standard pairs shared the direction of the within-pair frequency change but not the absolute frequency level. Correspondingly, the deviant pairs only shared the opposite direction of the within-pair change. Nevertheless, the deviant tone pairs elicited MMN, implying that even the direction of the within-pair frequency change of the standard stimuli, and not just their absolute frequencies, developed a neural representation.

Acoustic Stimulation↗
PubMed1991

The speed of attentional shifts in the visual field.

The scanning speed of focal visual attention was measured directly by flashing a sequence of two, three, or four numerals one by one at random retinal positions and at a distance from each other to avoid interference between the numerals. Each numeral was followed by a mask pattern so the observers had to move their focal attention in the visual field in synchrony and at the same speed as the presentation rate of the numerals in order to recognize every numeral in the stimulus sequence. Observers could recognize the numerals orders of magnitude above the theoretical chance level of performance even when the presentation rate was as fast as 33 ms per numeral. However, the temporal order of the numerals was perceived rather poorly at the fast presentation rates and for the sequences of four numerals.

Attention↗
PubMed1989

Analysis of spatial structure in eccentric vision.

The analysis of spatial structure, ie, the encoding of relative positions between pattern elements, was studied in central and eccentric vision. In a two-alternative forced-choice task the observer had to discriminate between two patterns consisting of short line segments. At each trial the two patterns were flashed for 140 msec and the observer indicated whether the patterns were identical or mirror symmetric. Psychometric functions were measured by changing pattern size at each eccentricity in order to find the threshold size allowing 75% of correct responses. The scaling factor, required for discriminating between mirror symmetric and identical patterns independent of eccentricity, was found to be similar to the size-scaling proposed by Levi et al (Vision Res 25:963, 1985) for vernier acuity tasks.

Differential Threshold↗
PubMed1988

Detection of mirror symmetry in random dot patterns at different eccentricities.

Differences between central and eccentric vision in the detection of mirror symmetry were investigated using patterns consisting of small dots. In each trial, a dot pattern was flashed for 140 msec and the subject had to decide whether the upper and lower parts of the dot pattern contained mirror symmetric pairs or whether the dots were distributed totally randomly. Detectability of mirror symmetry declined with increasing eccentricity when the retinal size of the dot patterns was constant at different eccentricities and when the size of the patterns in eccentric vision was size-scaled by the cortical magnification factor derived from the frequency of ganglion cells (M-scaling). The drop in detectability was more rapid for constant-size patterns than for M-scaled patterns. The results seem to support the notion that the encoding of positional information in eccentric vision is inferior to that of central vision even when the test patterns are M-scaled.

Form Perception↗
PubMed1988

The effect of exposure duration on the analysis of spatial structure in eccentric vision.

There is some evidence from grating experiments that the transient presentation of a stimulus pattern interferes with the encoding of positional relationships between pattern elements (i.e. the analysis of spatial structure) more in eccentric vision than in central vision. The present study investigated the effect of exposure duration on the analysis of spatial structure in eccentric vision using a task in which the observer discriminated between two mirror symmetric patterns consisting of short line segments. In each trial, the two patterns were flashed for 140 or 500 ms, and the observer had to decide whether the patterns were identical or mirror symmetric. Both constant-size and size-scaled patterns were used in eccentric vision. The longer exposure duration slightly increased the proportion of correct responses in eccentric vision but performance remained distinctly inferior to that in central vision.

Humans↗
PubMed1988

Rapid discrimination of visual patterns in children and adults.

Rapid visual discrimination in children (9 to 11 yr.) and adults was investigated using two tasks in which the subject had to search for a target pattern embedded in 35 background patterns. The time available for inspecting the search arrays was altered by varying the stimulus duration. In one task, there was a large difference in the feature or 'texton' content of patterns between the target and the background; in the other, this difference was small. In the first task, the children could detect the target pattern in a briefly flashed search array with high accuracy, because the target 'popped out' perceptually from the array, but in the second task the same detection rate was not reached until the stimulus duration was much longer, since a serial searching strategy was required. Results achieved by children and adults were similar. It seems that serial visual search is as efficient in children as in adults.

Adult↗
PubMed1988

Rapid discrimination of Julesz's texton patterns in patients with cerebral lesions.

Rapid visual discrimination in patients with unilateral cerebral lesions was investigated using a search task. Both the exposure duration of search arrays and the difference in the texton content between the target pattern and the background patterns were varied. Patients could detect the targets with a large texton difference more rapidly than the targets with a small difference.

Brain Diseases↗
PubMed1987

Perception of positional relationships between line segments in eccentric vision.

The encoding of positional relationships between pattern elements in eccentric vision was studied with different patterns consisting of the same short line segments in different positions. In each trial two stimulus patterns were flashed one above the other and the subject had to decide whether the patterns were identical or mirror symmetric (experiment 1) or whether the patterns were the same or different (experiments 2 and 3). The ability to discriminate between identical and mirror symmetric patterns was reduced in eccentric vision, even when the patterns were size-scaled according to the cortical magnification factor and thus the patterns were similarly visible at the different eccentricities. The results agree with the notion that eccentric vision is inferior to central vision in tasks which require proper encoding of information about the relative positions of pattern elements.

Attention↗
PubMed1987

Texture discrimination at different eccentricities.

Differences in preattentive texture discrimination between central vision and peripheral vision were studied with textures composed of random dots. The subject had to discriminate between two textures whose first-order statistics were kept identical but whose second-order statistics were different. For textures of constant retinal size the discrimination was easy in central vision, but the decrease of visual acuity with increasing eccentricity made the textures unresolvable in peripheral vision. When the textures were scaled by the cortical magnification factor derived from the frequency of retinal ganglion cells so that the calculated neural representations of the textures became similar at different eccentricities, texture discrimination became independent of visual field location. This indicates that preattentive texture discrimination based on differences in second-order statistics of random dots operates similarly in central vision and peripheral vision.

Discrimination, Psychological↗
PubMed1985

Self-organized formation of colour maps in a model cortex.

A report is presented of computer simulations which demonstrate the applicability of self-organization principles to the formation of a cortical colour map. A two-dimensional array of cortical units can be shown to become selectively sensitive to different colour stimuli in an orderly fashion. The precortical part of the simulation model contains three wavelength-sensitive receptors with overlapping sensitivity distributions, and a simple opponent processing stage. Each cortical unit receives the same activity from the precortical stage by adaptive connections. Initially the connections are arbitrary; during self-organization they are changed so that different way. Self-organization of the connections can be shown to take place even when the stimulation of the receptors is totally random, ie the wavelength and purity (and in some simulations the distribution) of each stimulus are selected randomly.

Color Perception↗
PubMed1983

Contrast sensitivity in monocular glaucoma.

The results of this selected group of glaucoma patients demonstrate that the effect of glaucoma on the contrast sensitivity can be assessed only after careful exclusion of all other causes for decrease in contrast sensitivity. Further development of methods is necessary.

Aged↗
PubMedDate not supplied

Carotid artery surgery--critical analysis of indications in a regional Finnish vascular center.

BACKGROUND: The purpose of this study was to analyze retrospectively the treatment of patients referred for carotid artery stenosis to a vascular surgical unit in the 1990's. Main attention was paid to the appropriateness of the indications for CEA. MATERIAL AND METHODS: In the Pirkanmaa region (population of 440 000), all carotid surgery is performed in the regional University Hospital. All new referrals for vascular surgery because of carotid stenosis or bruit in 1990, 1992, 1994, 1996 and 1998 were included and case records reviewed. RESULTS: Four hundred patients were referred. Indication for referral was a neurologic event in 46.2%, indefinite symptom in 27.9% and asymptomatic stenosis or carotid bruit in 25.9%. Most patients underwent carotid ultrasound as first imaging (93.7%). Almost half of the patients were operated (n=176). The 30-day combined stroke and death rate was 6.5%. Appropriate indication for CEA was found in 31.6%. Over half (57.0%) of the indications were considered uncertain and 11.4% inappropriate, most of these patients having asymptomatic stenosis. CONCLUSION: Patients with asymptomatic stenosis underwent CEA relatively often and the proportion of inappropriate indications was too high. Evaluation of indications and perioperative complications is highly important in carotid surgery.

Carotid Stenosis↗
PubMedDate not supplied

Preoperative physical examination for primary vascular access--reliability in determining vessel quality.

BACKGROUND AND AIMS: The purpose of this prospective clinical study was to evaluate the reliability and adequacy of preoperative physical examination in determining the quality of vessels prior to primary vascular access procedure by performing peroperative completion fistulography. MATERIAL AND METHODS: 26 consecutive patients who were scheduled for primary vascular access surgery, between July 2001 and June 2002, were included. Findings between the preoperative physical examination and peroperative completion fistulography were compared. RESULTS: Of the 26 patients that were initially enrolled in the study, 4 patients were excluded because physical examination showed poor superficial arm veins and 2 patients had not undergone access procedure by the end of the study. The remaining twenty patients constituted the actual study group. The arteriovenous fistula could be performed at the chosen level and way in all 20 patients. The findings between preoperative physical examination and peroperative fistulography were compatible and the specificity of physical examination to detect patent inflow and outflow vessels was 100%. Due to the fact that 4 patients in whom a poor vein was suspected were excluded, the sensitivity could not be assessed. CONCLUSIONS: Preoperative physical examination seems to be reliable and adequate method in determining vessel quality prior primary vascular access surgery. According to our study, its specificity is high in determining patent inflow and out-flow vessels. However, because of exclusion of patients with suspected problem, sensitivity cannot be determined.

Adolescent↗