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Transparent and paired dot fields

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Summary

Two dot fields drift coherently in different directions. The dots may be paired, meaning that each has a partner in the other dot field that crosses its motion path. This stimulus was used to demonstrate that the perception of transparency breaks down if the dots are paired within a critical distance, and that responses in some MT neurons are more strongly suppressed in the paired vs. unpaired condition (Qian et al., 1994).

Examples

• qian_pair.stm - 2018 Jan 5 - Movie (.mov) - Paired, oppositely moving dots, 0.4 deg path within 6 deg aperture.
• qian_rand.stm - 2018 Jan 5 - Movie (.mov) - Unpaired, oppositely moving dots, 0.4 deg path within 6 deg aperture.

Imperfections

When the dots overlap, they add and may saturate.

Parameters

• direction - (0..360) Direction of motion for dot field 1.

• direction2 - (0..360) Direction of motion for dot field 2.

• lifetime - (s) The duration of a coherently moving dot pair, before it starts over at a new position. In the first frame, lifetimes are randomly and uniformly distributed.

• distance - (deg) The distance over which dots travel in a full lifetime.

• offset - (deg) The distance between dots, orthogonal to the axis of motion specified by direction. Thus, for horizontal motion, this would be a vertical offset.

• ampl1 - (0..1) The luminance of dots in field 1. Or, ampl can be used to specify a single amplitude for both dot fields.

• ampl2 - (0..1) The luminance of dots in field 2. Or, ampl can be used to specify a single amplitude for both dot fields.

• dotsize - (pix) SD of Gaussian for drawing single dots.

• dotdens - (dots/deg^2) Density of dot pairs.

• seed - Randomization seed, used for both dot fields unless seed_unpair is greater than 0.

• seed_unpair - Randomization seed used for second dot field, if this value is greater than zero. Thus, set this to a positive non-zero value to create unpaired dot fields.

• bgval - (0..1) Background luminance.

• ...

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