1,721,097 research outputs found
Statistical validity of the results obtained in small sample size experiments. A parametric test simulation [Validità statistica dei risultati in esperimenti a bassa numerosità campionaria. Simulazione di un test parametrico]
No full textMany authors have criticized small sample size experiments because of the lack of statistical reliability, on the basis of the statistical power and considering a subjective evaluation of prior probability of the null Hypothesis H0 (Cohen, 1994; Chertow, Palevsky e Green, 2006). The aim of the present study is to test the reliability of significant results obtained from the small sample size experiments in comparison with larger ones. The different samples (10, 20, 40, 80, 160) are obtained by monte Carlo simulation, representing two conditions: H0 true and H0 false. As parametric procedure, the linear regression analysis has been used. Thus, the frequency of the type i error and the false positive rate probability (FPRP) have been evaluated. The frequency of the i type error is around 5%, independently on the sample size. Moreover, the FPRP values obtained in the small size samples are comparable to the values obtained in the larger samples. In conclusion the significant results obtained in small size samples are reliable and have statistical validity as well as those obtained in larger samples. This is true even from a Bayesian point of view when a non-informative a priori probability of H0 is taken
Visual search during motion perception
No full textEye movements during motion tracking or smooth pursuit have been widely investigated. However, it is still unclear how eye movements and motion perception system are related. We aim to analyze the pattern of gaze during coherent motion perception in absence of tracking at the very early stage of the motion perception process. Moreover, we would measure if good performances in motion perception are guided by specific gaze pattern. We measured the gaze in twenty-five healthy subjects using eye tracker system (SMI500) during motion perception tasks (coherent motion test), and during visual search of static elements (i.e. searching coloured dots within a group of white dots). The subjects are divided in two groups: good motion perception performers (scoring above the 50% of the total trials) and bad motion performers. The gaze is analyzed by means of specific parameters obtained from the eye movements recordings: the length of the eye movement track plotted on a bidimensional plane, the area of the confidence ellipse of the eye position, the standardized length (obtained by the ratio between the length and the area of the confidence ellipse), the flattening and the slope of the confidence ellipse. Significant differences in gaze patterns are found between motion perception and static visual search tasks. The gaze during motion perception is characterized by stronger flattening of confidence ellipse and longer length of eye movement track. The good performers have significantly longer length and standardized length of the eye movements track as compared to bad performers. These results show that the motion detection requires longer gaze path with respect to static visual search and that the path length is longer in good than in bad performers. In conclusion, the efficiency in motion detection is related to longer length of the gaze path (i.e. mean velocity) more than to the area scanned by the different gaze positions
Coherent motion and coherent form perception in developmental dyslexia
No full textThe role of visual perception in Developmental Dyslexia (DD) is still controversial. Although the Magnocellular theory of DD has been widely criticized, it is still unclear if the deficit in motion perception is specific for the Magnocellular system or if it is connected to a general difficulty in discriminating signal to noise.
The aim of this study is to investigate dorsal and ventral pathways in different conditions of noise in a population of DD. Seventeen DD and forty-three typically developed (TD) children (age 7-12 years) participated in the study. All the subjects performed the Motion coherence test and the Form coherence test. In the motion coherence test the ability in discriminating the direction of luminance coherent moving dots in different levels of noise is reported. The Form test measures the ability in form recognition in different condition of noise.
The MANOVA showed that DD differed from TD both in the Motion and in the Form coherence tests. Moreover, these differences are significant only in intermediate noise conditions.
These results are in accordance with the noise exclusion theory of DD and added important information about the DD deficit in discriminating signal to noise
L'effetto Simon: il suo decorso temporale con stimoli visivi e risposte oculo-motorie
No full textL’effetto Simon consiste in una prestazione più veloce ed accurata quando
la posizione della risposta corrisponde alla posizione dello stimolo anche se questa
non è rilevante per il compito. Il presente lavoro ha studiato l’emergere dell’effetto Simon
e il suo decorso temporale con stimoli visivi e risposte oculo-motorie utilizzando
un paradigma di tipo standard: stimoli e risposte condividevano infatti una relazione
anatomico-spaziale. I risultati hanno mostrato un effetto (1) ampio e significativo sia
nei tempi di reazione saccadici che negli errori e (2) con un decorso temporale descrescente,
supportando l’ipotesi che esso sia dovuto ad un meccanismo visuomotorio
The Effect of Luminance Condition on Form, Form-from-Motion and Motion Perception
Full text linkThis study investigated to what extent rod-dominated vision affects motion and form perception accuracy. Twenty-nine healthy subjects took part in the experiment. Form coherence (FC), form-from-motion (FFM) and motion coherence (MC) tests were assessed in low-light (rod-dominated vision) and high-light (cone-dominated vision) conditions. For each test we determined the accuracy by evaluating the correct detection obtained in five levels of coherence (corresponding to different signal-to-noise ratio). The results evidenced that motion, form and form-from-motion accuracy decreased in low-light condition. Furthermore, light condition effect was differently mediated by noise according to the type of task. The motion perception is affected only at high noise levels, while form discrimination was globally affected at all the levels, also in absence of noise, both for static (FC) and dynamic stimuli (FFM). We conclude that in rod-dominated vision form-from-motion perception is more defected than form and motion perception. We hypothesized that our results are due to the integration between M and P cells in FFM test increases the form perception accuracy in high-light condition but this advantage is completely lost during low-light condition, when the rods need to integrate information both from M and P cells
Motion perception, form discrimination and visual motor integration abilities in mTBI patients
No full textMotion-from-Form perception in Scotopic and in Photopic light conditions
No full textHuman visual perception is globally worsened under dim light condition.
The aim of this study was to investigate how rod-dominated vision
affects motion perception in a motion-from-form (MFF) and motion coherence
(MC) task. Thirty healthy subjects (11 males, mean age 21.3 years)
participated in the experiment. MFF and MC were tested both in scotopic
and photopic light conditions. Both the MFF and MC stimuli consisted of
nine empty circles moving coherently at a constant velocity of 10.7°/sec
in one of eight directions (cardinal and oblique). Five noise-levels were
used: starting from zero, the noise increased by 6 units in each following
level. In the MFF test 169 empty white squares acted as stationary background.
Nine target-stimuli were drawn in place of the same number
of squares. To make the shape appear to move, in the subsequent frame
each target-stimulus reverted to square while the next square changed
to circle, following a specific direction. The noise consisted of circle positioned
randomly in the space, irrespectively to their previous position. In
the MC test the target-stimuli moved coherently on a black background
and the noise was obtained by stimuli moving in a Brownian manner. The
subjects had to identify the directions of the coherent motion in both tests.
The accuracy was evaluated for each condition. ANOVA showed a significant
global effect of light condition, evidencing disrupted performances
in rod-dominated vision, significant differences between the types of stimuli
(lower scores in MFF as compared to MC) and a significant interaction
stimuli type by light condition. In conclusion, the results confirm that in
scotopic light levels motion perception is not significantly altered using
MC, while it is degraded in higher order motion stimuli, such as MFF
A test for the evaluation of form perception ability: Form Coherence Test
No full textThe aim of this study is the assessment of the applicability of the Form Coherence Test in a population of infants and adolescents. The Form Coherence Test is a computerized test for the evaluation of form perception ability. The subject is asked to discriminate a shape-stimulus among eight possible forms. The form perception is obtained by spatial coherent luminance dots visualized on a black background. Stimuli are presented in five levels of coherence which determines the difficulty of the task: starting from the 100% of coherence level, the number 1 of the coherently aligned dots exponentially decreases while the number of non-coherent dots (noise) increases. A sample of two hundred and fifty children (range 5-12 years) took part in the study. Accuracy is measured as the number of correct answers for each coherence level and for each stimulus. The Generalized Estimating Equation Model shows that the form perception accuracy decreased significantly in the subsequent coherence levels, starting from the 100% to 45%. The number of correct answers raised significantly with age. Age also affects the identification of the eight forms characterized by different difficulty. The results showed that Form Coherence Test is an instrument appropriate for the considered ages and allowed to evaluate the subject’s performance at different coherence level taking into account the age
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