Figure 3.

The emergence of orientation and direction selectivity in L4. (a) L4 neurons receive spatially offset ON versus OFF input from dLGN, which imparts OS. (b) To optimally activate a L4 neuron, the dark portion of an edge needs to cover the OFF region and the bright portion the ON region, a configuration that occurs if the main axis of the edge is perpendicular to the axis that connects the center of the ON and the OFF subregions of the RF. Any other orientation of the edge would produce a suboptimal excitation of the L4 neuron. As a grating drifts across the RF, it produces alternating optimal (t1) and suboptimal (t2) excitation. (c) Examination of the peak excitation across orientations reveals an orientation tuning curve. Panels a–c adapted from Lien & Scanziani (2013). (d) Direction-selective L4 neurons receive spatially offset transient versus sustained input from dLGN. (e, left) A visual stimulus moving in the preferred direction will first cross the RF of the dLGN neuron with a sustained response and then the RF of the dLGN neuron with a transient response. By doing so, the excitation produced by these two inputs will sum optimally. (Right) When the stimulus moves in the opposite direction, the transient response will have already largely subsided by the time the stimulus crosses the RF of the dLGN neuron with a sustained response, leading to less summation. Panels d and e adapted from Lien & Scanziani (2018). Abbreviations: dLGN, dorsal lateral geniculate nucleus; DS, direction selectivity; L, layer; OS, orientation selectivity; RF, receptive field; S, sustained; T, transient.