Plasticity in the Structure of Visual Space

Experiment design. Each experiment run contained pretraining testing, training, and posttraining testing sessions. In the training session, we used synchronized, repetitive presentation of two-point stimuli to strengthen the lateral connections between two retinotopically tuned neuronal groups. Because the successful induction of synaptic plasticity requires the presence of direct connections between the two neuronal groups, there should be an optimal separation between the two-point stimuli for changing the lateral connection strength. At longer separation, the two neuronal groups would not be effectively connected; at shorter separation, the two neuronal groups could be partially overlapping; either way, the number of lateral connections involved and the net change in lateral connection strength would be less. In the testing session, we used a match-to-standard protocol to measure the perceived distance. Participants adjusted the physical separation of a dot pair in the untrained hemifield to match the perceived distance of a dot pair in the trained hemifield. The difference between the pre- and posttraining matches was taken to quantify the change in perceived distance and the effect of training.

Change in perceived distance depends on training separation. The induction of synaptic plasticity and the change in lateral connection strength should be dependent on the separation between training locations. We measured the effect of training for a range of training separations. We observed a maximal contraction in perceived distance when training at a separation of 4.4 degrees. The contraction declined when training at shorter or longer separations. Black line, group average; shaded area, SEM (n = 30).