Figure 7.

Formation of ODMs during Phase 1. (A) The methodology of constructing the ODMs is outlined here (see “Materials and Methods”). The synaptic conductance changes induced due to STDP at the geniculocortical synapses from the LGN neurons corresponding to each eye are tracked over time. If the E neuron in layer 4 has a stronger set of afferents from the LGN from the left eye compared to the right eye, then the E neuron is labeled as “left” and color coded as green in the ODM. The exact opposite scenario results in the E neuron labeled as “right” and color coded as red in the ODM. If there is a tie (as in the beginning), then the E neuron is labeled as “binocular” and color coded as blue in the ODM. (B) The sum of the synaptic conductances from the LGN neurons corresponding to the left eye is compared against the sum of the synaptic conductances from the LGN neurons corresponding to the right eye at each E neuron. Each of these neurons are labeled as “left” or “eye” as described above. Then the mean and standard deviation of the difference between left and right eye afferent synapses for the neurons (nleft) labeled “left” is computed and plotted in a semilog format with the ordinate plotted in log scale while the abscissa data is plotted in regular scale. It can be seen that the mean value of the difference increases slowly. However, the standard deviation of the difference is higher than the mean implying that many of the E neurons have a very small difference in synaptic conductances while a few have a much larger difference. Similar behavior was observed for the right eye as well. This measure shows that the ODMs are not really stable during the Phase 1 in our model. (C) Early ODM appears to have several binocular E neurons since all the geniculocortical synapses are initialized with the same synaptic strength and there have not been sufficient inputs to alter the synaptic strengths via STDP. (D) At the end of Phase 1, the E neurons in the ODM shows eye selectivity with an even split of neurons becoming selective to one of the two eyes. There are no more binocular neurons. The ODM, however, appears fragmented with no large contiguous areas of neurons showing preference to one eye and not the other. Instead it has lots of small contiguous areas of various sizes. This is due to random stimulation of the LGN neurons with background activity with no temporal or spatial contiguity during Phase 1. The STD and mean data in (B) also provides further support to this basic phenomenon during this phase. However, this improves during the second and third phases of development as shown in Figures 10, 12 when there is more structure in the input data.