Assessment of magnocellular (M) and parvocellular (P) pathways in strabismic amblyopes and normal subjects using functional magnetic resonance imaging (fMRI).

Abstract

Abstract: : Purpose:To determine the BOLD (Blood Oxygen Level Dependant) response to magnocellular (M) and parvocellular (P) biased stimuli in strabismic amblyopes compared to normal subjects using functional magnetic resonance imaging (fMRI). Methods:Ten adult strabismic amblyopes (amblyopic eye visual acuity range 6/ 9 – 6/60) and seven normal adult subjects underwent fMRI scanning in a 1.5 Tesla scanner. Scans were analysed using in–house software (1). Activation maps were produced using analysis of covariance.The M biased stimulus was a 60' black and white chequerboard reversing at 10 Hz . The P biased stimulus was a 10' black and white chequerboard, reversing at 2 Hz . The fMRI paradigm was an on/off design with a 21 second on and 21 second off period. The visual field 13 by 7 degrees. Monocular stimuli were presented at 5, 10, 20, 40 and 80% contrast via a mirror. Results:At 80% contrast the P–biased stimulus produced significantly less activation in VI in amblyopic eyes compared to normal (P<0.05). In addition fellow eyes of amblyopic subjects also showed a reduced BOLD response at 80% contrast to the P–biased stimulus compared to normal (P<0.05). At all contrast levels except 5% the M–biased stimulus produced significantly less activation from amblyopic eyes than in normal subjects in V1. Conclusions:These studies show abnormalities of cortical responses to both M and P biased stimuli in strabismic amblyopes compared to normal subjects. Abnormalities were found not only in amblyopic eyes, but also fellow eyes. These results are in agreement with our previous findings using motion and colour visual evoked potentials (2). Further study of M and P pathway abnormalities in amblyopic children may allow more effective and efficient treatment strategies to be developed in the future. References: 1. Bullmore ET et al.Statistical methods of estimation and inference for functional MR image analysis. Magnetic Resonance Medicine 1996;35:261–277.2. Davis AR et al. Electrophysiological assessment of magnocellular and parvocellular function in early and late onset amblyopes. ARVO 2001, Abstract No. 292.

Keywords: amblyopia