Contrast integration across space

AbstractContrast integration across space was studied in respect to stimulus extent and the spatial layout, using high-contrast stimuli. Contrast discrimination thresholds were measured (2AFC) by either increasing the size of a peripheral (2.4°) Gabor signal (GS: λ=0.08°) or by increasing the number of GS elements in a circular arrangement. The supra-threshold mask (pedestal) was either increased with the target or fixed at maximal size and had 30% contrast. For stimuli with an increasing size of both the pedestal and the increment target, we find approximately constant discrimination thresholds. Contrast discrimination improved linearly on a log–log scale with slopes average of −1/4 (fourth-root summation) when the size of the Gabor target was increased but the mask was kept at maximal size, indicating contrast integration across space. Taken together, these results indicate balanced spatial integration of both contrast increment and pedestal, resulting object-size invariant contrast discrimination. Contrast discrimination was found to improve as well when the number of aligned Gabor elements was increased (both pedestal and increment), pointing to independent contrast normalization for disconnected (sparsely positioned) stimuli. The results indicate a complex pattern of spatial integration involved in contrast discrimination, possibly depending on image segmentation

The relationship between the subjective and objective aspects of visual filling-in

AbstractWe explored the relationship between filling-in processes and the known increase in detection sensitivity observed for targets presented between collinear flankers. Filling-in was probed using a Yes/No detection task by measuring the false-positive reports (false-alarm, FA) and hit rate (Hit) for a low-contrast Gabor target with different target-flankers distances. Observers increased the number of reports on the presence of a target (FA and Hit) when the flankers’ distance was within the known range of facilitatory lateral interactions. This bias in reporting was reduced with blocked stimulation, when the target-flanker distance was kept fixed across trials. When different distances were mixed by trials the bias followed the pattern of lateral interactions across distance. The effect was maximal when flankers and targets were aligned. These false perceptions are most likely the result of a filling-in process by lateral excitation that produces illusory contours

Abnormal Long-range Spatial Interactions in Amblyopia

AbstractNeural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia. © 1997 Elsevier Science Ltd. All rights reserved

Long-Lasting, Long-Range Detection Facilitation

We examined the time course of threshold reduction in the Gabor lateral masking paradigm (Polat and Sagi 1993). Contrast detection thresholds were measured (2AFC) for a briefly presented (36 msec) foveal Gabor signal (GS), preceded by a presentation (90 msec) of two high-contrast GS flanked masks, with Stimulus Onset Asynchrony (SOA) varying from 0 to 16,290 msec. Using target-to-mask separations of 3 and 12 (=0.15 ffi , GS wavelength), the 3 separated GS masks enhanced target threshold by 0.25 log units at SOA=0 and by 0.17 log units at 2,700 msec. At 12 separation, threshold was enhanced by 0.11 log units at SOA=0 and by 0.14 log units at 2,700 msec. Long-range (12) and short-range (3) enhancements persisted for over 16 seconds. Delayed and simultaneous enhancement depended on the stimulus configuration (maximal for collinear target and masks), local parameters (orientation, spatial frequency, and phase), and the presented eye (dichoptic vs. monoptic). The results suggest that spat..

Long-Lasting, Long-Range Detection Facilitation

We examined the time course of threshold reduction in the Gabor lateral masking paradigm (Polat and Sagi 1993). Contrast detection thresholds were measured (2AFC) for a briefly presented (36 msec) foveal Gabor signal (GS), preceded by a presentation (90 msec) of two high-contrast GS flanked masks, with Stimulus Onset Asynchrony (SOA) varying from 0 to 16,290 msec. Using target-to-mask separations of 3 and 12 (=0.15 ffi , GS wavelength), the 3 separated GS masks enhanced target threshold by 0.25 log units at SOA=0 and by 0.17 log units at 2,700 msec. At 12 separation, threshold was enhanced by 0.11 log units at SOA=0 and by 0.14 log units at 2,700 msec. Long-range (12) and short-range (3) enhancements persisted for over 16 seconds. Delayed and simultaneous enhancement depended on the stimulus configuration (maximal for collinear target and masks), local parameters (orientation, spatial frequency, and phase), and the presented eye (dichoptic vs. monoptic). The results suggest that spat..