In our standard suppressive models, suppression is maximal for a DS subunit only when the output signal from the delayed EX unit saturates at zero, which is often for a brief instant. In addition, the time at which the output signal from the delayed EX unit is zero must coincide with the timing of spikes from the nondelayed EX input to the subunit. This precise timing is not often achieved in the standard versions of our models, so maximal suppression is seldom obtained, resulting in significant input to the DS units in the suppressive models for antipreferred direction motion.

Stronger selectivity can be achieved by retuning the network model so that the firing rate of EX units is more strongly modulated by a sinusoidal grating stimulus, i.e. higher F1 for the same DC. In DS_Pre_Sup.2, we have retuned the LGN, IN and EX populations so that EX units have a stronger F1 response to drifting gratings, as well as a lower spontaneous rate, which results in less background input to the DS population. We have also eliminated the background input to the DS units, so that at rest DS units are further below spike threshold than in the standard suppressive model. These changes produce DS cells with a much higher DI.

Another factor that contributes to the strength of the suppression, and thus selectivity of DS cells, is the duration of the DS interaction. This explains the additional discrepancy between the DIO's obtained with the DS_Pre_Sup and DS_Post_Sup models, since the duration of the DS interaction in DS_Pre_Sup is much shorter (3 ms) than in DS_Post_Sup (20 ms).

Stimuli

Direction series, drifting gratings	Please get a Java compatible browser to see this.
Drifting gratings at 0 and 180 degrees	Please get a Java compatible browser to see this.

Results

Direction Tuning:

Data for all populations

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Preferred and Antipreferred direction gratings:

Data are presented in separate viewers for each DS unit, including spikes from all input EX and EX_D units, as well as the strongest weighted LGN/LGN_D and IN/IN_D units presynaptic to that DS cell.

Data for connections presynaptic to:
DS 0_0	DS 0_1	DS 1_0	DS 1_1
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