Properties of spatial channels underlying the detection of orientation-modulations.

Orientation-modulated stimuli are thought to be processed via a two-stage process, the first stage involving the detection of the carrier by mechanisms in striate cortex and the second stage involving the detection of the modulation by way of integration of carrier-based information by mechanisms in extra-striate cortex. Much is known about the spatial properties of the channels underlying carrier detection but less is known about the properties of the channels involved in modulation detection. Using a discrimination at detection paradigm, we show that the mechanisms underlying modulation detection are tuned for envelope spatial frequency and orientation. The tuning of these channels is not substantially different from that previously described for carrier mechanisms, namely 1-2 octaves for spatial frequency and 30° for orientation. For orientation, these stimuli exhibit an oblique effect that is dependent on absolute carrier orientation, suggesting facilitative interactions between first- and second-stage processes.