Distinct patterns of surround modulation in V1 and hMT+
Description
Here we study patterns of surround modulation in primary visual cortex (V1) and middle temporal complex (hMT+) utilizing a well-studied effect in motion perception, where human observers' ability to discriminate the drift direction of a grating improves as its size gets bigger if the grating has low contrast, and deteriorates if it has high contrast. We first replicated the findings in the literature with a behavioral experiment using small and large (1.67 and 8.05 degrees of visual angle) drifting gratings with either low (2%) or high (99%) contrast presented at the periphery. Next, using functional MRI, we found that in V1 with increasing size cortical responses increased at both contrast levels. Whereas in hMT+ with increasing size cortical responses remained unchanged or decreased at high contrast, and increased at low contrast, reflecting the perceptual effect. 3.1.3. Visual Stimuli & Experimental Design Visual stimuli were drifting gratings. Two size (small: 1.67°, large: 8.05°) and two contrast levels (2% and 99%) were tested. Size (diameter) was defined as six times the standard deviation of the Gaussian envelope in the fMRI experiment. FMRI experimental code was written in Python using the built-in methods of the Psychopy package, whereas the behavioral experiment was written using custom Java modules developed by us. Therefore the formulation and implementation were slightly different in the behavioral experiment. Nevertheless, the stimuli were ensured to have the same sizes in both experiments. Due to the limits of the visual display system, gratings were presented at ± 8.02 degrees of horizontal eccentricity (was 9.06° in the behavioral experiment), and drifted with a rate of 6°/s (was 4°/s in the behavioral experiment) either upward or downward for the duration of 12 s. Both gratings drifted in the same direction simultaneously and alternated direction every 2 s to avoid motion adaptation. A functional run was composed of “active” and “control” blocks, each lasting for 12 s. In the active blocks, drifting gratings and a central fixation mark were presented, whilst in the control blocks, only the fixation mark remained visible. In alternating active blocks, small and large drifting gratings were shown, each repeated 6 times in a run. The contrast level was kept constant within a run. Two experimental runs were conducted for each contrast level in a session. The runs started with an initial blank period of 24 s to allow hemodynamic response to reach a steady state. The total duration of a functional run was around 5 min.