Simultaneously Judging Contrast in Different Orientation and Frequency Bands

Abstract

Previous discrimination experiments suggest that suprathreshold contrast signals carried by orthogonally oriented gratings are combined when component frequencies are similar, but contrast signals are not combined across frequency bands regardless of orientation. We investigated neural and attentional processes underlying these findings in a concurrent-response paradigm. Test stimuli were composed of two superimposed sinusoidal gratings. In one condition, two 3 cycles deg⁻¹ gratings were superimposed at orthogonal orientations to form plaids. In the other condition, the component gratings were 3 cycles deg⁻¹ and 15 cycles deg⁻¹, both vertical. In each condition, each component independently took one of two slightly different contrast values, combined all possible ways to create four stimuli. On each trial, one stimulus appeared for 1 s. Observers made two contrast discrimination judgments, one on the vertical (or low frequency) component, the other based on the horizontal (or high frequency) grating. Highly correlated response patterns and the collapse of two decision axes into nearly one confirmed that observers were unable to make independent judgments of contrast on orthogonally oriented components of similar frequency. Analyses suggested that decisions were based primarily on a single neural signal representing the sum or average of the two contrasts present. When cues to discrimination were in different frequency bands, observers showed a marked inability to perform the simultaneous judgment task, choosing idiosyncratic strategies to maximise performance. Analyses indicated that the contrast information was processed through separate and independent pathways, but that information from the two bands was not simultaneously available to the observer.