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.
Highlights Cerebral visual impairment (CVI) is associated with impaired global motion processing. Mean motion coherence thresholds was higher in individuals with CVI. fMRI responses in area hMT+ showed an aberrant response profile in CVI. White matter tract reconstruction revealed cortico-cortical dysmyelination in CVI.
Spatial patterns presented on the tongue using electro-tactile sensory substitution devices (SSDs) have been suggested to be recognized better by tracing the pattern with the tip of the tongue. We examined if the functional benefit of tracing is overcoming the poor sensitivity or low spatial resolution at the back of the tongue or alternatively compensating for limited information processing capacity by fixating on a segment of the spatial pattern at a time. Using a commercially available SSD, the Brainport, we compared letter recognition performance in three presentation modes; tracing, static, and drawing. Stimulation intensity was either constant or increased from the tip to the back of the tongue to partially compensate for the decreasing sensitivity. Recognition was significantly better for tracing, compared to static and drawing conditions. confusion analyses showed that letters were confused based on their characteristics presented near the tip in static and drawing conditions. the results suggest that recognition performance is limited by the poor spatial resolution at the back of the tongue, and tracing seems to be an effective strategy to overcome this. Compensating for limited information processing capacity or poor sensitivity by drawing or increasing intensity at the back, respectively, does not improve the performance. Visual-to-tactile sensory substitution devices (SSDs) convey visual information to a blind person through touch. Since the seminal work of Bach-y-Rita in the 1960s 1 , tactile representation of visual information has been studied on various body parts including the back 1,2 , abdomen 3-5 , fingers 6-8 , and tongue 9-11. The tongue has been suggested to be a good platform for SSDs because it has higher sensitivity and spatial resolution compared to other body parts 9,12. Users can recognize simple shapes, differentiate a small set of objects, detect motion, point to and avoid high-contrast-staged obstacles when the stimulus is presented sufficiently on the tongue (roughly 30 seconds) 9,13-17. However, navigation and object recognition tasks in real-life situations, where the scene is usually dynamic, may require these tasks to be achieved at shorter times. Vincent, et al. 18 showed that when a stimulus is presented for a short time (0.5 seconds), performance in discriminating basic shapes was not significantly above chance and performance in discriminating different line orientations deteriorated substantially compared to those reported by previous studies. They attributed the better performance in the earlier studies to the longer stimulation durations. This allowed participants to explore the stimulus actively and serially using the tip of the tongue ("tip" from here on), which may be required for spatial recognition through tactile input 19. Our experience with the BrainPort, an FDA-cleared electro-tactile tongue SSD (Wicab Inc., Madison, WI), is consistent with Vincent, et al. 18. The BrainPort delivers the visual information acquired through a head-mounted camera to a 20 × 20 grid...
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