Browsing by Subject "Center-surround interaction"

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Open Access
Behavioral and neural investigation on the effect of spatial attention on surround suppression
(2023-09) Kınıklıoğlu, Merve
When a visual stimulus is presented together with other stimuli surrounding it, behavioral sensitivity and neural responses may change, often reduce, compared to when the same stimulus is presented alone. This is commonly referred to as center-surround interaction or surround suppression, and it is one of the most fundamental mechanisms in biological vision. It is well documented that in motion perception, center-surround interaction is affected by the size and contrast of the stimulus. As the size of a drifting grating increases, motion direction discrimination performance, as well as neural activity in one of the main cortical motion processing areas, medial temporal complex (MT+), decreases if the grating has high contrast (surround suppression). Whereas, when the size increases within certain limits, both the discrimination performance and the neural activity in MT+ may increase if the grating has low contrast (surround facilitation). On the other hand, spatial attention is known to modulate surround suppression both in humans and non-human animals with static stimuli. No previous study, how-ever, has directly and systematically investigated the effect of the spatial extent of attention on surround suppression in human motion perception. The studies presented in this dissertation aim to investigate the effect of the extent of spatial attention on center-surround interaction in visual motion processing. In our experiments, we used two attention conditions and a novel stimulus design, where a ‘center’ and a ‘surround’ drifting grating were presented to the participants. Under one of the attention conditions, which we call the ‘narrow-attention’ condition, participants performed a task that limited their attention to the central part of the stimulus. Under the other attention condition, which we call the ‘wide-attention’ condition, participants performed tasks that required them to extend their attention to both the center and surround gratings. Using this experimental paradigm, we measured motion direction discrimination thresholds behaviorally and cortical activity with fMRI. Behaviorally, we found increased thresholds, that is, stronger surround suppression, under the wide attention condition. In the hu-man homolog of MT+ (hMT+), we found that increasing the spatial extent of attention leads to reduced cortical responses, that is, to stronger neural suppression. This was not the case for the activity in the primary visual cortex (V1). Finally, we show that a parsimonious computational model that incorporates spatial attention and response normalization can successfully predict the response patterns in hMT+ and V1. Furthermore, the model could provide a link between cortical responses and behavioral thresholds. Overall, our findings and analyses showed that the behavioral effect can be successfully predicted by hMT+ activity. These results reveal the critical role of spatial attention on surround suppression, namely that surround suppression in motion perception becomes stronger with a wider attention field, and reveal possible cortical mechanisms underpinning the effect.
Open Access
Characterizing surround suppression in motion direction perception
(2021-06) Kurt, Aslı Gül
Visual perception is often achieved by surround modulation mechanisms, which help us pool individual information in our visual ﬁeld. This mechanism is also prominent in motion perception, namely motion discrimination. A center-surround antagonistic organization aids motion perception using these modula-tion mechanisms, which are facilitation or suppression. Decisions to which modu-lation type will take place are usually manipulated by changing size and contrast of drifting Gabor disc stimuli. Because a systematic investigation of these mech-anisms is not conducted in prior research, we have used three diﬀerent types of stimulus (small disc, annular, and large disc grating) to see whether there is a fa-cilitative or suppressive pattern between duration thresholds of these stimuli. To achieve this, we performed a behavioral study, that would assess motion discrim-ination thresholds of human participants for these stimuli via adaptive staircase procedures. Then, we looked for three possible pooling regimes by comparing duration thresholds of small disc-annular-large disc grating stimulus sets: eﬃ-cient or ineﬃcient (weak or strong suppression) pooling. We found that duration thresholds of large disc gratings were almost greater than small disc or annular gratings almost all the time in both contrast levels, which indicated a strong sup-pression. We have also conducted simulations of divisive normalization models and observed that modulation mechanisms were more prominent in simulated duration thresholds using MT parameters from literature than V1. Although previous literature suggests that MT region produces facilitative or suppressive eﬀects within itself, we are not able to postulate a speciﬁc brain region creating strong suppressive pooling we observed in our behavioral data. This is why this study is a critical next step for future neuroimaging studies.
Open Access
Contrast affects fMRI activity in middle temporal cortex related to center-surround interaction in motion perception
(Frontiers Research Foundation, 2016) Türkozer, Halide B.; Pamir, Zahide; Boyacı, Hüseyin
As the size of a high contrast drifting Gabor patch increases, perceiving its direction of motion becomes harder. However, the same behavioral effect is not observed for a low contrast Gabor patch. Neuronal mechanisms underlying this size-contrast interaction are not well understood. Here using psychophysical methods and functional magnetic resonance imaging (fMRI), we investigated the neural correlates of this behavioral effect. In the behavioral experiments, motion direction discrimination thresholds were assessed for drifting Gabor patches with different sizes and contrasts. Thresholds increased significantly as the size of the stimulus increased for high contrast (65%) but did not change for low contrast (2%) stimuli. In the fMRI experiment, cortical activity was recorded while observers viewed drifting Gabor patches with different contrasts and sizes. We found that the activity in middle temporal (MT) area increased with size at low contrast, but did not change at high contrast. Taken together, our results show that MT activity reflects the size-contrast interaction in motion perception. © 2016 Turkozer, Pamir and Boyaci.