The effects of orientation and attention during surround suppression of small image features: A 7 Tesla fMRI study

Abstract

Although V1 responses are driven primarily by elements within a neuron's receptive field, which subtends about 1° visual angle in parafoveal regions, previous work has shown that localized fMRI responses to visual elements reflect not only local feature encoding but also long-range pattern attributes. However, separating the response to an image feature from the response to the surrounding stimulus and studying the interactions between these two responses demands both spatial precision and signal independence, which may be challenging to attain with fMRI. The present study used 7 Tesla fMRI with 1.2-mm resolution to measure the interactions between small sinusoidal grating patches (targets) at 3° eccentricity and surrounds of various sizes and orientations to test the conditions under which localized, context-dependent fMRI responses could be predicted from either psychophysical or electrophysiological data. Targets were presented at 8%, 16%, and 32% contrast while manipulating (a) spatial extent of parallel (strongly suppressive) or orthogonal (weakly suppressive) surrounds, (b) locus of attention, (c) stimulus onset asynchrony between target and surround, and (d) blocked versus event-related design. In all experiments, the V1 fMRI signal was lower when target stimuli were flanked by parallel versus orthogonal context. Attention amplified fMRI responses to all stimuli but did not show a selective effect on central target responses or a measurable effect on orientation-dependent surround suppression. Suppression of the V1 fMRI response by parallel surrounds was stronger than predicted from psychophysics but showed a better match to previous electrophysiological reports.

Suppression and facilitation of human neural responses

“…Feed-forward and feedback connections between EVC and MT are thought to play an important role in spatial context processing 20 ; our data alone are not sufficient to determine the extent to which MT inherits suppression from EVC and / or drives this suppression via feedback. However, surround suppression in early visual areas (e.g., V1, V2) is well established in animal models 20 , and suppressed fMRI responses in these areas generally correspond with perceptual suppression during contrast judgments [48][49][50] . This raises the possibility that some amount of spatial suppression during motion discrimination may be attributed to neural suppression in EVC.…”

Characterizing the effects of feature salience and top-down attention in the early visual system

“…Other studies have tested for effects of orientation-defined salience using more complex displays of multiple spatially separated gratings or lines, akin to the displays commonly used in behavioral investigations of attention and visual search (e.g., Nothdurft 1993). Results from these neuroimaging studies have been mixed: some studies find that a salient, uniquely oriented item evokes stronger responses in V1 (Schallmo et al 2016;Zhang et al 2012), whereas others find no reliable differences in early visual areas (Beck and Kastner 2005;Bogler et al 2013) or more complex interactions that depend on top-down spatial attention (Flevaris and Murray 2015;Hopf et al 2004). Our understanding of visual salience and its neural bases relies critically on testing with multi-item displays, which inform much of our knowledge of the mechanisms of attention and visual search.…”

Weaker neural suppression in autism

“…Weaker neural suppression in ASD does not seem to extend to earlier regions of visual cortex; we found weaker suppression among participants with ASD in the fMRI response within foveal hMT+ ( Figure 2D-F), but not in the foveal region of early visual cortex (EVC) near the occipital pole (at the confluence of V1, V2, and V3; Figure 2G-I) that provides input to area MT. At first, this finding may appear at-odds with the notion of narrower top-down modulation in ASD, since top-down effects such as spatial attention are known to modulate responses in V1 [63][64][65][66][67][68] . However, the magnitude of these modulatory effects varies greatly across different regions of visual cortex.…”

“…Spatial smoothing and normalization to a canonical template were not performed; all analyses were based on within-subject ROIs. ROIs were defined in each hemisphere in the space of the functional data using a standard correlational analysis 1,68 , taking the top 20 most-significant voxels with an initial threshold of p < 0.05 (Bonferroni corrected for multiple comparisons). In a few cases, there were not 20 voxels that satisfied this threshold, thus the threshold was relaxed to include 20 contiguous voxels.…”