END STOPPING AND LENGTH TUNING IN PSYCHOPHYSICAL SPATIAL FILTERS

Psychophysical spatial filters or channels are usually modeled after s imple-cell receptive fields, although many cortical cells are end stop ped and length tuned. Using psychophysical masking, we demonstrate an analog to receptive field end stopping and length tuning in psychophys ical spatial filters tuned to a wide range of spatial frequencies. Spe cifically, masking is maximal when the mask is approximately 5-6 arcmi n longer than the target but is reduced when the mask exceeds this len gth, consistent with the propel-ties of end-stopped cells. The strengt h and the extent of psychophysical end stopping appear to be determine d by the filter's spatial-frequency tuning, but length tuning varies w ith target length. The latter implies that spatial filters tuned to th e same spatial frequency could have different length tuning and that t here is no fixed length-to-width ratio of filter size. Phase effects s uggest linear length summation but nonlinear psychophysical end stoppi ng, which suggests that both first-and second-order visual processing is involved in end-stopped spatial filters. (C) 1997 Optical Society o f America.