The influence of fixation points on contrast detection and discrimination of patches of grating: masking and facilitation.
The use of fixation points (FPs) in visual psychophysics is common practice, though the costs and benefits of different fixation regimens have not been compared. Here we investigate the influence of several different types of FP configurations on the contrast detection of patches of sine-wave gratings. We find that for small targets (1 degrees), the addition of a superimposed central FP can increase thresholds by a factor of approximately 1.3 (approximately 2.5 dB) in comparison with no FP, and a factor of approximately 1.5 (approximately 3.6 dB) in comparison with FPs that surround the target. These results are consistent with (i) a suppressive influence on the central region of the target from a central FP, and (ii) facilitatory influences from surrounding FPs. Our analysis of the slope of the psychometric function suggests that the facilitatory influence is not due to reduction of uncertainty. Plausible candidate causes for the facilitation are: (i) sensory interactions, (ii) aids to ocular accommodation and convergence, (iii) a reduction in eye-movements and (iv) more accurate placement of the observer's window of attention. Masking by a central FP is not found for the suprathreshold task of contrast discrimination, suggesting that the masking effects of pedestal and FP do not combine linearly. This means that estimates of the level of masking produced by a contrast pedestal can depend on the details of the fixation point.
School of Life and Health Sciences, Aston University, Birmingham B47ET, UK.
Robert J
RJ
Summers
Tim S
TS
Meese
D015350:Contrast Sensitivity / Q000502:physiology*; D004192:Discrimination, Psychological; D005403:Fixation, Ocular; D006801:Humans; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010470:Perceptual Masking; D010775:Photic Stimulation / Q000295:instrumentation / Q000379:methods; D011594:Psychometrics; D011601:Psychophysics; D012684:Sensory Thresholds