Amblyopic suppression is not explained by signal attenuation

Abstract

The current model of amblyopic suppression is that it follows as a consequence of normal dichoptic masking where there is an attenuation to the input from the amblyopic eye. We directly test this by measuring and compensating for the signal attenuation in a dichoptic-masking paradigm. Using a qCSF approach, we measured individual's monocular CSFs when the untested eye saw a mean luminance background and when the untested eye saw a band-pass filtered noise whose peak spatial frequency was matched to that of the test grating. Interocular suppression was quantified by the difference in thresholds occurring between these two conditions for each eye. To factor out any signal attenuation by the amblyopic eye, the contrast of the noise mask was always matched in visibility: it was set at five and ten times the threshold of the untested eye. Due to a contrast ceiling effect, we narrowed our analysis to the frequencies where this matched visibility for our noise stimuli could be achieved. We found that, for normals, the two eyes exhibit symmetric suppression. However, in amblyopes, the amblyopic eye exerted significantly less suppression than the nonamblyopic eye, even though the contrast attenuation for the amblyopic eye had been accounted for. Furthermore, the suppression from the nonamblyopic eye to the amblyopic eye was not significantly different to that found in the normals. Similar conclusions were reached for noise stimuli that were 10x threshold for normals and amblyopes. We conclude that amblyopia involves asymmetric interocular suppression not accounted for by the known elevation in contrast threshold of the amblyopic eye. The nonamblyopic eye exhibits the expected dichoptic masking suppression of the amblyopic eye but the amblyopic eye exhibits much less counter suppression of the nonamblyopic eye. This imbalance leads to dominance by the nonamblyopic eye under binocular viewing.

Meeting abstract presented at VSS 2016