The role of on and off transients in determining the psychophysical spatial frequency response

Abstract Two experiments were run to investigate the contrast sensitivity of the visual system to sinusoidal gratings of variable spatial frequency when the on- and offsets of their presentations, lasting about 500 msec, were either abrupt or gradual. In the first experiment, it was found that a presentation having abrupt on- and offsets, relative to one having gradual on- and offsets, increased the contrast sensitivity at low spatial frequencies but left unaltered the sensitivity at high spatial frequencies. This result is consistent with previous findings which indicate that visual channels preferring temporally transient stimulation are predominantly tuned to low spatial frequencies. In a second experiment it was found that the increase in contrast sensitivity at low spatial frequencies is primarily due to the leading abrupt onset; i.e. at low spatial frequencies a presentation having an abrupt onset and gradual offset increased contrast sensitivity relative to a presentation having a gradual onset and abrupt offset. This result suggests that the spatiotemporal frequency response of the visual system, like its temporal or spatial frequency response, must be specified not only in terms of a modulation transfer function but also in terms of a phase response.