Spatial attention improves reliability of fMRI retinotopic mapping signals in occipital and parietal cortex

Abstract

Spatial attention improves visual perception and increases the amplitude of neural responses in visual cortex. In addition, spatial attention tasks and fMRI have been used to discover topographic visual field representations in regions outside visual cortex. We therefore hypothesized that requiring subjects to attend to a retinotopic mapping stimulus would facilitate the characterization of visual field representations in a number of cortical areas. In our study, subjects attended either a central fixation point or a wedge-shaped stimulus that rotated about the fixation point. Response reliability was assessed by computing coherence between the fMRI time series and a sinusoid with the same frequency as the rotating wedge stimulus. When subjects attended to the rotating wedge instead of ignoring it, the reliability of retinotopic mapping signals increased by approximately 50% in early visual cortical areas (V1, V2, V3, V3A/B, V4) and ventral occipital cortex (VO1) and by approximately 75% in lateral occipital (LO1, LO2) and posterior parietal (IPS0, IPS1 and IPS2) cortical areas. Additionally, one 5-minute run of retinotopic mapping in the attention-to-wedge condition produced responses as reliable as the average of three to five (early visual cortex) or more than five (lateral occipital, ventral occipital, and posterior parietal cortex) attention-to-fixation runs. These results demonstrate that allocating attention to the retinotopic mapping stimulus substantially reduces the amount of scanning time needed to determine the visual field representations in occipital and parietal topographic cortical areas. Attention significantly increased response reliability in every cortical area we examined and may therefore be a general mechanism for improving the fidelity of neural representations of sensory stimuli at multiple levels of the cortical processing hierarchy.

Low attention impairs optimal incorporation of prior knowledge in perceptual decisions

“…The assumption of variance reduction is supported by recent research (Bressler & Silver, 2010;Pestilli et al, 2011;Wyart et al, 2012). From a computational point of view, attention has recently been interpreted as facilitating inferring the level of uncertainty or precision during hierarchical perception, thus minimizing the error prediction signal (Feldman & Friston, 2010).…”

“…According to our Common Criterion Model, in addition to increasing the strength of the perceptual signal (possibly by increasing activity in visual cortical areas that represent the attended location and suppressing cortical activity that represents unattended locations (Desimone & Duncan, 1995)), attention also reduces the trial-by-trial variability of the internal perceptual response (Bressler & Silver, 2010;Pestilli, Carrasco, Heeger, & Gardner, 2011;Wyart et al, 2012). Importantly, motivated by previous empirical evidence within a signal detection theoretic framework (Gorea & Sagi, 2000;Zak, Katkov, Gorea, & Sagi, 2012), the model also presupposes that subjects use a single common decision criterion for discriminating both attended and unattended stimuli.…”

Functional Heterogeneity in Posterior Parietal Cortex Across Attention and Episodic Memory Retrieval

“…The stimulus and task in the attention mapping procedure were modeled after those used previously (Bressler and Silver 2010). During fMRI, a black and white, contrast-reversing (7.5 Hz) checkerboard wedge stimulus was continuously presented on a gray background, systematically rotating about a central fixation point (Engel et al 1994;Sereno et al 1995;Engel et al 1997).…”

“…During fMRI, a black and white, contrast-reversing (7.5 Hz) checkerboard wedge stimulus was continuously presented on a gray background, systematically rotating about a central fixation point (Engel et al 1994;Sereno et al 1995;Engel et al 1997). The check size within the stimulus was scaled according to the cortical magnification factor in human V1 (Slotnick et al 2001;Bressler and Silver 2010), and the stimulus contrast was 100%. The wedge subtended 45°and extended from 0.5°(inner radius) to 16°(outer radius) of visual angle, except when the wedge was at the top and bottom of the field of view (the dimensions of the mirror used for stimulus viewing extended 13°f rom fixation along the vertical meridian).…”

“…The boundaries of visual field representations in posterior parietal areas IPS0-5 and SPL1 were defined using established phase-encoded retinotopic mapping methods (Engel et al 1994;Tootell et al 1998;Silver et al 2005;Swisher et al 2007;Konen and Kastner 2008a;Bressler and Silver 2010;Szczepanski et al 2010). Maps were defined with a coherence threshold of 0.15 for each subject, a value that is consistent with prior work (e.g., Brewer et al 2005).…”