Prediction of cortical direction selectivity tuning
Latency and absolute phase values were used to predict the degree of direction selectivity present in the afferents to cortex. All possible pairs of cells were formed at each of two age groups. The relative phase difference (phi in formula below) between the cells in each pair were computed at several temporal frequencies, and this phase difference was plugged into a formula for direction selectivity (based on creating a spatial phase difference equal to the temporal phase difference):
If DS was greater than 1/3, it was counted as direction selective. However, if the temporal resolution of either cell was below the test temporal frequency, it was counted as not direction selective. The poor resolution of cells from young kittens (see above) accounts for the falloff at 8Hz seen here.
As discussed by Feidler et al. (1997), appropriate hebbian mechanisms would tend to associate quadrature pairs, enhancing the small degree of direction selectivity found here. However, the underlying distributions described here also affect the result. Thus, we predict that young kittens have fewer cells that are direction selective at low temporal frequencies, despite the preference for these frequencies in the tuning of LGN cells. This result is due to the small variance in absolute phase across the population.
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Date created: November 5, 1997
Last modified: November 5, 1997
Copyright © 1997, Alan Saul
Maintained by: Alan Saul