Fig. 5.

Early MD attenuates the shift in cortical ocular dominance elicited by a later MD of the fellow eye. Measurement of VEPs from right and left V1 were made using cup electrodes placed on the scalp while animals viewed grating stimuli of varying spatial frequency that were phased reversed at 2 Hz (A). The experimental timelines of Prior and No Prior MD groups are depicted in B, with arrows indicating the ages at which VEPs were measured. To provide a reference for our experimental data (C), VEP power data from five visually normal animals revealed a rise in power from P27 to P56 (n = 2), then a decline in VEP power to eventually reach very low levels in adulthood (n = 3). This decline in VEP power from its peak likely derives from a thickening of the skull with age, as well as a coincident growth of the temporal mastication muscle that is positioned under our recording site on the scalp. Measurement of VEP power elicited by separate stimulation of the left (blue trace) and right (yellow trace) eyes across 4 spatial frequencies and a control gray screen are presented with reference to non-visual baseline power (open circles) for an example animal in the Prior MD group (D). VEP power was comparable and balanced between the eyes before MD was imposed at P30, but after 7 days of left-eye MD there was a reduction in VEP power elicited by stimulation of the deprived eye (E). Following MD, provision of 3 weeks of binocular vision restored a normal level of VEP power to the originally deprived eye, which was in balance with the fellow eye (F). Imposition of a second MD, this time to the right eye for 10 days at 8 weeks of age, produced an attenuation of VEP power in the deprived eye that appeared less extreme at this older age than the effect of the first MD (G). Measurement of VEPs from the No Prior MD group were balanced between the right and left eyes before MD was imposed (H). After 10 days of MD started at 8 weeks of age, there was a large reduction of VEP power measured for the deprived eye, and the non-deprived eye showed an apparent potentiation relative to age-matched controls (I). To compare the extent of the shift in ocular dominance across groups (J), the summed VEP power from each eye was used to calculate an ocular dominance index for each animal, and for left (solid symbol) and right (open symbol) V1. The effect of 10 days of MD on ocular dominance was 20% smaller in the group of animals that received a prior MD (n = 3) compared to those that did not (n = 4). Different symbol shapes in C represent VEP measurements from different animals using the 0.05 spatial frequency. Duplicated pairs in C represent measurements from the right and left V1.