Partitioning two components of BOLD activation suppression in flanker effects.

Chien-Chung Chen

doi:10.3389/fnins.2014.00149

Abstract

The presence of a visual stimulus not only increases the blood oxygenation level dependent (BOLD) activation in its retinotopic regions in the visual cortex but also suppresses the activation of the nearby regions. Here we investigated whether there are multiple components for such lateral effects by using the m-sequence paradigm to measure the stimulus spatial configuration specific BOLD activation. The central target (2 cyc/deg grating) was centered on a fixation point while the flanking stimulus was placed 2° away and was located on axes that were either collinear or orthogonal to the target's orientation. Three types of flankers were used: gratings whose orientation was the same as the central stimulus, gratings which were orthogonal to the stimulus, and random dots. The onset and offset of each stimulus were determined by shifted copies of an 8-bit long m-sequence. The duration of each state of the sequence was 2 s or 1TR. The first order activation, computed as the waveform recorded following on-states minus that recorded after off-states, determined the retinotopic regions for each stimulus. We then computed BOLD activation waveforms for the target under various flanker conditions. All flankers reduced the activation to the target. The suppressive effect was largest following the presence of the iso-orientation collinear flankers. Our result suggests two types of BOLD signal suppression: general suppression, which occurs whenever a flanker is presented and is insensitive to the spatial configuration of the stimuli, and spatial configuration dependent suppression, which may be related to the collinear flanker effect.

Highlights

The visual response to a stimulus can be modulated by another stimulus
Whereas a visual cortical neuron only responds to visual stimuli projected onto its receptive field (Hubel and Wiesel, 1962; DeAngelis et al, 1993), this response can be modulated by the presence of other visual stimuli presented outside its classical receptive field (Blakemore and Tobin, 1972; Nelson and Frost, 1985; Knierim and Van Essen, 1992; Sillito et al, 1995; Polat et al, 1998; Sengpiel et al, 1998; Kapadia et al, 1999, 2000; Chen et al, 2001; Freeman et al, 2001; Angelucci et al, 2002; Cavanaugh et al, 2002)
It is known that the presence of a visual stimulus produces an increment of blood oxygenation level dependent (BOLD) activation in the corresponding retinotopic regions for that stimulus, there is a sustained reduction in BOLD activation in the neighboring brain regions (Logothetis, 2002; Shmuel et al, 2002, 2006; Smith et al, 2004; Chen et al, 2005)

Summary

INTRODUCTION

The visual response to a stimulus can be modulated by another stimulus. For instance, in the Ebbinghaus effect, a target circle surrounded by large circles appears to be smaller than the same target surrounded by small circles; in simultaneous contrast (Wallach, 1948; Gilchrist, 2006), a patch of gray on a dark background appears brighter than the same patch on a bright background; and, in particular, in the flanker effect (Polat and Sagi, 1993, 1994; Chen and Tyler, 2001, 2008), the visibility of a low contrast periodic pattern (target) increases when it is flanked by collinear and iso-oriented patterns (flankers). In psychophysical or electrophysiological experiments, the lateral effect can be measured by comparing the visual performance or cell response to a visual target in the presence of a spatial context with those without a context. This approach, may not be directly applicable to an fMRI study. The experimental result may tag a neural mechanism that is unrelated to the context effect in perception To avoid such risk, the better strategy is to compare activation to the stimuli that are known to cause a difference in perception. We were able to have multiple flankers in one fast event-related run and keep our experiment to a reasonable length

METHODS

RESULT

Findings

DISCUSSION