Figure 4.

Inverted-U dose-response with changing DA levels. (A) Plot showing average firing rate summed over all neurons in layer 3 in a single trial. When DA levels were varied from low to high, we saw changes in the firing rate of working memory neurons that were consistent with those found experimentally. When DA levels were low, D1 receptors were only weakly activated causing an increase in the strength between columns (i.e., between non-preferred inputs). This lead to a degradation of spatial tuning as can be seen by both preferred (column encoding 0° in the model) and non-preferred (column encoding 90° in the model) columns showing high firing rates (left). The firing rates of preferred direction neurons vs. non-preferred direction neurons during the delay period were not significantly different (p > 0.1; t-test). When DA levels were high (right), all inputs to neurons in the PFC network were partially blocked due to D1 receptor over-stimulation, leading to a decrease firing rate to both preferred and non-preferred neurons (p > 0.1; t-test). When DA levels were optimal, preferred neuron firing rates were higher than non-preferred neurons as is characteristic in successful working memory traces (p < 10⁻⁸; t-test). (B) Experimental results obtained from Vijayraghavan et al. (2007); Arnsten (2011) showing a similar inverted-U with varying DA levels.