Characterizing the attentional mechanisms that affect *perception
Attention affords individuals the ability to grant priority in processing to certain aspects of the vast amount of visual information that is present in the world. Covert attention allows the observer to pay attention to a location in the visual field in the absence of eye movements. Experimental research has shown that covert attention affects performance in a large variety of psychophysical tasks. This dissertation is comprised of research investigating how covert attention enables this change in performance by influencing visual processing.
To identify the earliest level of visual processing covert transient attention can affect, I studied how attention affects basic dimensions of visual processing, namely contrast sensitivity. The first study reports the benefits of spatial covert attention on contrast sensitivity in a wide range of spatial frequencies. Attention enhanced observers' contrast sensitivity even under conditions that excluded all variables predicted by an external noise reduction model. These results support the signal enhancement model of attention.
The fact that attention enhanced the contrast of the signal suggests that it influences levels of visual processing that establish contrast sensitivity. In order to tap into these early levels, I conducted two studies investigating if transient covert attention differentially affects contrast sensitivity and spatial resolution ‘performance fields’ (shape depicted by % correct performance at particular locations in the visual field). Although attention improved overall performance, it did not affect performance fields (Carrasco, Talgar & Cameron, 2001; Talgar & Carrasco, In Press).
In the final study, I investigated if covert transient attention could affect the mechanisms underlying contrast sensitivity. I used critical band masking to assess if covert attention affects the spatial frequency tuning of a spatial frequency channel. The results reveal that while covert attention doubled sensitivity, it did not alter any of the characteristics of the channel mediating the task. These results indicate that the attentional enhancement of contrast sensitivity is not a result of a change in the tuning of the spatial frequency channel. Therefore, the earliest level of visual processing that attention exerts its effect must be at the level in which different channels interact.