Spatial-frequency bandwidth requirements for pattern vision.

Abstract

Visual resolution is an important factor which affects human pattern recognition. Dealing with degraded visual resolution is relevant to both normally sighted and visually impaired individuals. This thesis describes three studies that address human pattern recognition under conditions of low resolution and its linkage to real life visual activities such as reading.

Deficiencies of pattern recognition in peripheral vision might result in higher bandwidth requirements, and may contribute to the functional problems of people with central-field loss. In the first study (Chapter 2), we asked whether there are differences in spatial-frequency requirements between central and peripheral vision for pattern recognition. Critical bandwidths (i.e., the minimum low-pass filter bandwidth yielding 80% recognition accuracy) were measured for letter and face recognition. We found that critical bandwidths increased from central to peripheral vision for both letter and face recognition, demonstrating that peripheral vision requires higher bandwidth for pattern recognition than central vision. In the second study (Chapter 3), we asked how letter recognition is possible with severe reduction in the spatial resolution of letters. We addressed the question by testing the hypothesis that when spatial resolution is severely limited, the visual system relies increasingly on contrast coding for letter recognition. The size of the gap between contrast thresholds for detecting and recognizing letters was used as a marker for the extent of reliance for contrast coding. We found that as spatial resolution for rendering letters decreases, the system relies more on contrast differences. Letters are the fundamental building blocks of text. Besides single letter recognition, it has been proposed that the visual span, the number of letters that can be recognized without moving the eyes, imposes a limitation on reading speed. In the third study (Chapter 4), we investigated whether the bandwidth requirement for reading speed can be accounted for by the bandwidth requirement for letter recognition. We found that bandwidth limitations on reading performance appear to be closely associated with bandwidth limitations on the visual span, and also to a bandwidth limitation on letter recognition. Together, these three studies provide us with a better understanding of spatial-frequency requirements for pattern vision.