Biomedical subjects

Behzad Mansouri

Publications and source records attributed to Behzad Mansouri.

6 recordsLinked to original sources

PubMed2006

The global processing deficit in amblyopia involves noise segregation.

Some studies have reported deficits in amblyopia for global form and motion integration, whereas other studies have shown global integration of form and motion information to be normal in amblyopia. Here, we attempt to resolve this discrepancy by showing that amblyopes only exhibit selective performance deficits on global tasks that contain noise as well as signal. We hypothesized that signal integration is normal, but noise segregation is not. We used comparable global orientation and motion direction discrimination tasks to measure integration performance in the presence of controlled amounts of pedestal noise (i.e., elements whose orientations or directions were randomly selected). We modelled the performance using an equivalent noise model, which has the parameters of internal noise and number of samples. Our results show that amblyopic eyes can integrate form (i.e., orientation) and motion information (i.e., motion direction) similarly to normals when all the information is signal (i.e., no pedestal noise). However, introducing pedestal noise perturbs the performance of the amblyopic eyes significantly more than that of the normal eyes.

Amblyopia↗

PubMed2006

Integration of local motion is normal in amblyopia.

We investigate the global integration of local motion direction signals in amblyopia, in a task where performance is equated between normal and amblyopic eyes at the single element level. We use an equivalent noise model to derive the parameters of internal noise and number of samples, both of which we show are normal in amblyopia for this task. This result is in apparent conflict with a previous study in amblyopes showing that global motion processing is defective in global coherence tasks [Vision Res. 43, 729 (2003)]. A similar discrepancy between the normalcy of signal integration [Vision Res. 44, 2955 (2004)] and anomalous global coherence form processing has also been reported [Vision Res. 45, 449 (2005)]. We suggest that these discrepancies for form and motion processing in amblyopia point to a selective problem in separating signal from noise in the typical global coherence task.

Adult↗

PubMed2005

Detection, discrimination and integration of second-order orientation information in strabismic and anisometropic amblyopia.

To better understand the nature of the cortical deficit in amblyopia we undertook a systematic investigation of second-order processing in 8 amblyopic and 8 normal observers. We investigated local detection, discrimination and global integration. Our local stimulus consisted of a Gaussian patch of fractal noise multiplied by a 1-d sinusoidal modulator. Our global stimulus consisted of an array of such elements. We revealed second-order detection deficits for stimuli with equi-visible carriers. Orientation discrimination for an isolated second-order patch was comparable in normal and amblyopic eyes. We showed that pure integration of second-order patterns can be normal in amblyopia.

Adult↗

PubMed2005

Integration, segregation, and binocular combination.

The human visual system can accurately judge the mean of a distribution of different orientation samples. We ask whether the site of this integration is before or after the sites of binocular combination and disparity processing. Furthermore, we are interested in whether the efficiency with which local orientation information is integrated depends on the eye of origin. Our results suggest that orientation integration occurs after binocular integration but before disparity coding. We show that the effectiveness of added orientation noise is not only less than expected on signal or noise grounds but also that it depends on the dominance of the eye to which it is presented, suggesting an interocular opponent interaction in which the dominant eye input has higher gain.

Dominance, Ocular↗

PubMed2004

Integration of orientation information in amblyopia.

A recent report suggests that amblyopes are deficient in processing local orientation at supra-threshold contrasts. To determine whether amblyopes are also poor at integrating local orientation signals, we assessed performance for an orientation integration task in which the orientations of static signals are integrated across space. Our results show that amblyopic visual systems can integrate local static oriented signals with the same level of efficiency as normal visual systems. Although internal noise was slightly elevated, there was no indication that fewer samples were used to achieve optimal performance. This finding suggests normal integration of local orientation signals in amblyopia.

Adult↗

PubMed2003

Integration of first- and second-order orientation.

The problem of how visual information such as orientation is combined across space bears on key visual abiities, such as texture perception. Orientation signals can be derived from both luminance and contrast, but it is not well understood how such information is pooled or how these different orientation signals interact in the integration process. We measured orientation discrimination thresholds for arrays of equivisible first-order and second-order Gabors. Thresholds were measured as the orientation variability in the arrays increased, and we estimated the number of samples (or efficiency) and internal noise of the mechanism being used. Observers were able to judge the mean orientation of arrays of either first- or second-order Gabors. For arrays of first-order and arrays of second-order Gabors, estimates of the number of samples used increased as the number of Gabors increased. When judging the orientation of arrays of either order, observers were able to ignore randomly oriented Gabors of the opposite order. If observers did not know which Gabor type carried the more useful orientation information, they tended to use the information from first-order Gabors (even when this was poorer information). Observers were unable to combine information from first- and second-order Gabors, though this would have improved their performance. The visual system appears to have separate integrators for combining local orientation across space for luminance- and contrast-defined features.

Discrimination, Psychological↗