P3-13: Dynamics of Unconscious Contextual Effects in Orientation Processing

• Main Authors: Isabelle Mareschal, Colin W G Clifford
• Format: Article
• Language: English
• Published: SAGE Publishing 2012-10-01
• Series: i-Perception
• Online Access: https://doi.org/10.1068/if723
• ISSN: 2041-6695

Contextual effects abound in the real world. A classic example of this is the tilt illusion that results in a target's orientation appearing repelled from that of a surround. However the opposite effect can also occur (the indirect tilt illusion) and the target's orientation appears shifted towards that of the surround. These orientation biases are believed to result from different mechanisms with the indirect TI requiring input from higher cortical areas. We designed a novel reverse correlation technique to investigate this. The stimulus consisted of a concentric annular surround (outer diameter 7.8°) containing a 2 cpd grating that could have one of 12 possible orientations abutting a circular patch (diameter 1.8°) where a vertical 2 cpd grating was presented every 2 s. The surround was refreshed on each frame (11.7 ms) and each orientation had an equal probability of being selected. The observers' ( n = 5) task was to report after each target presentation whether it had appeared tilted clockwise or counterclockwise of a subjective vertical. All observers displayed a strong direct tilt illusion with this novel technique. In the majority of the observers ( n = 4) we also obtain an indirect tilt illusion. Both illusions occur reliably and over a similar time course (roughly ±60 ms around the time of target presentation) despite the lack of conscious access to the surround orientation. These results support the role of a single mechanism underlying orientation biases and refute the need for higher-level conscious processing of the context.