How many times has the co-author of The structure of the human motion detection system by Andrea J. van Doorn, been cited?
21989
Jan J. Koenderink et al.: Bidirectional Reflection Distribution Function of Thoroughly Pitted Surfaces. (1996) authored by Jan J. Koenderink. Sylvia C. Pont and Jan J. Koenderink: Reflectance from locally glossy thoroughly pitted surfaces. (2005) authored by Jan J. Koenderink. Harro M. G. Stokman et al.: Color Measurement by Imaging Spectrometry. (2000) authored by Jan J. Koenderink Jan J. Koenderink et al.: The "shading twist, " a dynamical shape cue. (2012) authored by Jan J. Koenderink. Jan J. Koenderink alternativeName Jan J. Koenderink. Sylvia C. Pont et al.: Mixing material modes. (2012) authored by Jan J. Koenderink. Jan J. Koenderink alternativeName Jan J. Koenderink.
[INST]
Utilize the provided context information, to generate answers to the following question: 
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Question: How many times has the co-author of The structure of the human motion detection system by Andrea J. van Doorn, been cited?
Context: Jan J. Koenderink et al.: Bidirectional Reflection Distribution Function of Thoroughly Pitted Surfaces. (1996) authored by Jan J. Koenderink. Sylvia C. Pont and Jan J. Koenderink: Reflectance from locally glossy thoroughly pitted surfaces. (2005) authored by Jan J. Koenderink. Harro M. G. Stokman et al.: Color Measurement by Imaging Spectrometry. (2000) authored by Jan J. Koenderink Jan J. Koenderink et al.: The "shading twist, " a dynamical shape cue. (2012) authored by Jan J. Koenderink. Jan J. Koenderink alternativeName Jan J. Koenderink. Sylvia C. Pont et al.: Mixing material modes. (2012) authored by Jan J. Koenderink. Jan J. Koenderink alternativeName Jan J. Koenderink.
[/INST] Answer:
