A Quadrantic Bias in Prefrontal Representation of Visual-Mnemonic Space

Single neurons in primate dorsolateral prefrontal cortex (dLPFC) are known to encode working memory (WM)
representations of visual space. Psychophysical studies have shown that the horizontal and vertical meridians of the visual
field can bias spatial information maintained in WM. However, most studies and models have tacitly assumed that dLPFC
neurons represent mnemonic space homogenously. The anatomical organization of these representations has also eluded
clear parametric description. We investigated these issues by recording from neuronal ensembles in macaque dLPFC with
microelectrode arrays while subjects performed an oculomotor delayed-response task. We found that spatial WM
representations in macaque dLPFC are biased by the vertical and horizontal meridians of the visual field, dividing
mnemonic space into quadrants. This bias is reflected in single neuron firing rates, neuronal ensemble representations, the
spike count correlation structure, and eye movement patterns. We also found that dLPFC representations of mnemonic
space cluster anatomically in a nonretinotopic manner that partially reflects the organization of visual space. These results
provide an explanation for known WM biases, and reveal novel principles of WM representation in prefrontal neuronal
ensembles and across the cortical surface, as well as the need to reconceptualize models of WM to accommodate the
observed representational biases.