If attention moves in quantal jumps how can the mouse get his cheese? David Crundall Rm 315

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If attention jumps from one place to another, rather than moves in an analogue fashion, then how do we perform this task? Jolicoeur et al. (1986, 1991) tested participants’ abilities to perform a simplified version of this task. Two convoluted lines were presented with one of them passing through the point of fixation. The task was to say whether two targets fell on the same line or fell on different lines. The first target was always presented at the point of fixation.

A typical stimulus used in Jolicoeur’s studies: All of the second target locations fall on the circumference of an invisible circle, ensuring that all second targets are the same distance from the first target in Euclidean space.

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The line tracer slows down for hair-pin bends: (Jolicoeur, Ullman & Mackay, 1991)

The proximity of distracter lines affects how fast the line tracer can travel (Jolicoeur, Ullman & Mackay, 1991)

The line-tracing process appears to be automatic: Line/gapStrip/gap Curve/gap Curve/curve

Pringle and Egeth (1988) Results for response times to targets on the same curve:

So what mechanism underlies the line tracer? 1.A Pixel by Pixel operator? 2.A Bi-partite operator? 3. A Zoom Lens operator? 40% variance 44% variance 75% variance

To summarise: There appears to be a serial analogue movement of attention that traces lines. It is sensitive to curvature It is affected by the proximity of distracter curves It appears to be automatic rather than under strategic control

Scholte, Spekreijse & Roeflsema (2001): Does attention parse all curves pre-attentively? Or does attention strategically jump along the line to points of ambiguity (i.e. can a quantal movement of attention still account for line tracing)? Or does attention just trace the target line?

Findings from Scholte et al (2001) The lines are not pre-attentively parsed, as the crossovers should not increase RTs and colours should be reported equally as well from both lines. Attention traces the whole line and does not jump from one point to another, otherwise the colours on the target line should have been reported as poorly as those on the distracter line. There is leakage when lines cross, with attention leaking into the distracter line. Conclusion? Is it an analogue movement of attention, or is it something else entirely, related to object-based perception…

Most theories of visual perception assume a pre-attentive stage that initially parses a scene into candidate objects or proto-objects on the basic of Gestalt groupings of features (Bayliss, 1998; Rensink, 2000) Curvature and collinearity can be processed in parallel (Fahrl, 1991; Field, Hayes, & Hess, 1993) The line tracing operator is therefore too slow to provide any valuable input into the processing of natural scenes (Boucart, Delord, & Giersch,1994) Does the line tracing operator have a real role to play in parsing objects?

Gestalt grouping does not occur if attention is directed else where (Ben-Av, Sagi, & Braun, 1992) Some researchers now suggest that attention needs to spread across a visual scene to segregate objects (Roelfsema, Lamme, & Spekreijse, 2000) Scholte et al. (2001) have suggested that there are in fact two levels of object parsing: Base grouping Incremental grouping – a spread of attention through the line rather than an analogue movement But...

Houtkamp, Spekreijse, & Roelfsema (2003) did the same experiment but the colours appeared at varying SOAs after the lines appeared.

So is “line tracing” really “object parsing”? Attention doesn’t move like an analogue spotlight but spreads through the object instead.

Or does it? Houtkamp et al.’s secondary task may have led subjects to adopt certain strategies that favoured reporting the colours. Ideally we need a single task that can distinguish between spreading or moving attention ms Short or long SOA 100 ms 2000 ms

Conclusions Attention automatically moves along a line in order to reach a second target. Is this a special example of analogue movement? Or is it a spread of attention? Is line tracing used in real life, or is it a special case that can only be seen in the laboratory? Could it still be part of the object parsing process?