Acta Psychologica 72 (1989) 13-23 North-Holland 13 zyxwvutsrqpon EFFECTS OF RELATIVE POSITION AND SHAPE RELATION ON GAP DETECTION WITH GEOMETRIC FORM PAIRS * Garvin CHASTAIN Boise State University, USA Accepted September 1988 A letter presented parafoveally is better identified when flanked by a letter to its fovea1 side than when flanked by a letter to its peripheral side if the letters are different in shape. If the letters are similar, relative position does not affect identifiability. Whereas some articles in the literature suggest that this effect may not extend to geometric forms, results of the first experiment showed that it does. Results of the second experiment revealed that the strength of the significant interaction between relative position and shape relation remains constant over a range of eccentricities (2-20 deg were tested) extending above and below as well as to the left and right of fixation. Results are compared with predictions from a model that can account for the relative position-shape relation interaction. A letter presented parafoveally generally is identified more accu- rately when flanked to its fovea1 side than when flanked to its periph- eral side (Bouma 1970; Krumhansl 1977). Studies have shown that the effect depends upon the shape relation between the target and flanking letter; Chastain (1982a, 1985) found the relative position effect only with different(ly-shaped) letters; with similar letters the performance advantage on a target letter flanked foveally was absent. This relative position-shape relation interaction also was apparent but not statisti- cally significant in an experiment reported by Krumhansl and Thomas (1977). However, only a very limited range of locations was tested in the studies reported by Chastain and Krumhansl and Thomas, with the target between 1 and 3 deg from fixation and always to the left or right (never above or below). * This research was supported in part by a faculty research grant from Boise State University. Author’s address: G. Chastain, Dept. of Psychology, Boise State University, Boise, ID 83725, USA. OOOl-6918/89/$3.50 0 1989, Elsevier Science Publishers B.V. (North-Holland)