Distinct patterns of surround modulation in V1 and hMT+

Modulation of a neuron’s responses by the stimuli presented outside of its classical receptive field is ubiquitous in the visual system. This “surround modulation” mechanism is believed to be critical for efficient processing and leads to many well-known perceptual effects. The details of surround modulation, how-ever, are still not fully understood. One of the open questions is related to the differences in surround modulation mechanisms in different cortical areas, and their interactions. 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 a low contrast, and deteriorates if it has a high contrast. We first replicated the findings in the literature with a behavioral experiment using small and large (1.06 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, but they increased more at high contrast. Whereas in hMT+ with increasing size cortical responses remained unchanged at high contrast, and increased at low contrast. These findings show that surround modulation in V1 and hMT+ are distinct. Furthermore these findings provide evidence that the size-contrast interaction in motion perception is likely to originate in hMT+.


Results
5.56; p < 0.001; M =0.003, SEM = 0.0006). Also, two-tailed paired-samples Student's 141 t-tests showed that SI was significantly higher for low-contrast stimuli compared to 142 that for high-contrast stimuli (t(10) = 6.97; p < 0.001). These results clearly replicate 143 the size-contrast interaction in motion perception when stimuli is presented at the 144 periphery.  Siemens AG, Erlangen, Germany) with a 32-channel array head coil. MR sessions 158 started with a structural run followed by two region of interest (ROI) localizer and 159 four experimental functional runs, totaling approximately 1 hour in duration. One 160 localizer run was used to identify the hMT+ region, the other one was for localizing 161 the sub-regions of hMT+ and V1 that process the input from the visual field that cor-162 respond to the position and size of the small Gabors (see below 3.1.3 "Visual Stimuli Figure 1: Left plot shows group mean (N = 11) of duration thresholds. For lowcontrast stimuli, discrimination threshold decreases as size gets bigger. On the contrary, for high-contrast stimuli, discrimination threshold increases as size gets bigger. Right plot shows mean size indices (SIs) for 2% and 99% contrast levels. SI is defined as the difference in sensitivity (1 / threshold) between large and small Gabor patches. For low-contrast stimuli, SI is positive which indicates that sensitivity increases as size gets bigger, i.e. spatial facilitation. On the contrary, for high-contrast stimuli, sensitivity decreases as size gets bigger, i.e. spatial suppression. These results replicate the size-contrast interaction in motion perception when stimuli is presented at the periphery. Error bars represent ±SEM. (*p < 0.001).
anatomical sequence (TR: 2600 ms, spatial resolution: 1 mm 3 isotropic, number of 165 slices: 176). Functional images were acquired with a T2*-weighted gradient-recalled  This cycle is repeated for 6 times within a run. Large (8.05 degree) and small (1.67 degree) drifting Gabor patches were presented in alternating active blocks. The contrast was kept constant in a run (2% or 99%), and two runs were conducted for each contrast. Participants were required to keep fixation at the central mark, and perform a demanding fixation task. task as described before. All subsequent analyses were performed on the experimental 225 data extracted from these ROIs.

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ROI within hMT+. We first created masks using the hMT+ regions functionally 227 identified as explained before. We then localized the hMT+ ROIs within these masked 228 regions. Specifically we identified the voxels that are selectively more responsive to 229 the drifting Gabor patch by contrasting the responses between active and control 230 blocks using GLM.

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ROI within V1. We identified V1 ROI using the data from the localizer run with the 232 aid of anatomical landmarks. Specifically, using GLM, cortical regions responding 233 preferentially more in the active blocks when contrasted with control blocks and 234 located anteriorly in the calcarine sulcus were identified as the V1 ROI.

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In this experiment, we recorded and analyzed BOLD responses in hMT+ and V1 while   (Turkozer et al., 2016;Schallmo et al., 2018), we expected a larger 286 SI for low contrast compared to high contrast. Indeed, paired sample t-test results 287 revealed that the SIs were statistically significantly different, the SI for low-contrast 288 being greater than that for the high-contrast (t(5) = 3.20, p = 0.024).

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showed that BOLD responses increased significantly with size both at high and low 293 contrast conditions. Critically, this increase was greater when the stimuli had high 294 contrast compared to low contrast. This pattern was inconsistent with the perceptual 295 effect, and surprisingly it was different than the pattern observed in hMT+. We contrast (F(1,5) = 17.96, p = 0.008).

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As we did for the hMT+ data, here too we performed further analyses on SIs. Figure   302 4 shows SIs plotted for individual participants, as well as the group mean. At low 303 contrast, group mean of SI was significantly greater than zero (M SI = 0.425, SEM 304 = 0.143; one sample t-test, t(5)= 2.98, p = 0.031). Average SI value was positive at 305 high contrast, as well (M SI = 1.596, SEM = 0.178; one sample t-test, t(5) = 8.98, 306 p < 0.001). Furthermore, we performed paired-sample t-test, and found that the SI