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Paul T. Sowden, Emre Ozgen, Philippe G. Schyns; When a plaid is not a plaid: attentional modulation of spatial frequency processing. Journal of Vision 2002;2(7):283. doi: 10.1167/2.7.283.
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When an observer is cued to detect a sinusoidal grating presented at one SF their detection of an unexpected SF is impaired compared with when the same SF is expected (Sowden & Schyns, 2000, Perception, 29, s24). Further, these ‘expectancy’ effects are SF tuned consistent with their origin being top-down directed monitoring of early SF processing channels (Sowden, Özgen & Schyns 2001, Perception, 30, s91). These findings support the possibility that the task dependent selection of spatial scale in perception of faces and scenes (Schyns & Oliva, 1999, Cognition, 69, 243–265) also results from attentional modulation of spatial frequency (SF) processing.
Here we explore a direct analogy to the latter effects. Can expectancy effects cause orthogonal perceptions of the same plaid stimulus? In a sensitisation block observers were required to report the direction of tilt of a briefly presented (120 msec) single frequency grating (±30° relative to vertical) presented at luminance contrast threshold. The SF of the grating was either 0.5 or 8 c/deg. Prior to each trial an auditory cue signalled the SF of the forthcoming grating. Next in a test block, unknown to observers, plaid stimuli were randomly interleaved with sensitisation stimuli (1:3 ratio). The plaids were composed of the two single-frequency grating components combined orthogonally at the two different orientations (±30°). We found that observers tended to correctly report the direction of tilt of the cued plaid component (71% of trials). Importantly, observers never reported perceiving a two-component plaid but always perceived a single frequency grating.
Our finding of orthogonal perceptions of exactly the same plaid stimulus provides further evidence of top-down driven attentional modulation of SF processing.
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