A dichoptic projection system for visual psychophysics in fMRI scanners

Abstract

Here we describe a projection system based on the combination of linear polarizing filters, designed to allow dichoptic presentation of visual stimuli during fMRI experiments. Currently available MRI compatible dichoptic presentation systems are either highly expensive, require degradation of the projected stimulus such as the removal of all but one color or are difficult to deploy in a range of scanner environments. The system described here is relatively low in cost, requires no change in stimulus properties and could be used in many types of scanner facilities. We provide a verification of the system demonstrating minimal cross-talk between the images presented to the two eyes.

Differential BOLD Activity Associated with Subjective and Objective Reports during “Blindsight” in Normal Observers

“…Stimuli were video-projected in a dual-projector setup (Thompson et al, 2008) onto a screen behind the head coil at a viewing distance of 100 cm using two identical projectors (LCD EMP74, Epson) with a refresh rate of 60 Hz, and two comparable PCs (2.4 GHz, 3 GB RAM, NVidia GeForce 8500GT) running Windows XP (Microsoft) and Matlab 2008a (The MathWorks). For dichoptic stimulation, linear polarizing filters (TechSpec linear polarizing film, Edmund Optics) were mounted in front of each projector's lens.…”

A very inexpensive MRI-compatible method for dichoptic visual stimulation

“…As Thompson et al (2008) have pointed out, most of the options available for dichoptic stimulus presentation, such as stereoscopes or shutter goggles, are not suitable for use in a high-field environment. Many fMRI experiments of binocular rivalry 1 have resorted to the use of inexpensive red-green (or red-blue) "3-D" glasses (Fang and He, 2005;Meng et al, 2005;Tong et al, 2006) (see Thompson et al, 2008 for a more complete list), but these suffer from difficulties with "cross-talk" between the eyes and also greatly limit the way that stimuli can be viewed -the substrate being a pair of grayscale images, each one tinted either green or red.…”

Dichoptic Viewing Methods for Binocular Rivalry Research: Prospects for Large-Scale Clinical and Genetic Studies

“…In addition, compared to all the other types of dichoptic display, the hardware requirement of a dual-screen and half-silvered mirror makes a PPF system considerably more expensive. For instance, the total cost of the True3Di TM 22-inch PPF monitor is ßUS$4,000, which is the price for a typical linear PPF system (Thompson et al, 2008). However, recent advances in PPF technology mean that cheaper single-screen interleaved PPF monitors are now available (ßUS$300; e.g., AOC e2352Phz).…”

“…While mirrors are typically included in fMRI head coils, modifications to these, while not trivial, are also not particularly onerous. One setup option is the divided bore that involves relatively simple modifications to the bore (e.g., a hanging divider) and the head coil mirror (e.g., a plastic or similar divider; Thompson et al, 2008).…”

“…More generally, eye-tracker calibration should be undertaken for all setups where the eye-tracking camera is not viewing the eye directly (i.e., through mirrors and filters). For PPF setups, eye-gaze detection through the polarized filter via IR has been reported even with rotation of the filter (Thompson et al, 2008), showing that PPF does not impede IR eye-tracking (e.g., Cambridge Research Systems 'Eyetracker and Eyetracker Toolbox' using Edmund Optics polarizers; Thompson et al, 2008). However, eye-tracking equipment warrants testing with a PPF over the tracked eye to verify its compatibility with a PPF setup before developing the concurrent eye-movement recording protocol.…”