Background Simple visual search task

1. Psychophysics of search for multiple targetsEndel PõderTallinn Pedagogical University, 25 Narva Road, Tallinn 10120, Estoniaand University of Tartu, 78 Tiigi Street, Tartu 50410, EstoniaE-mail: ep@tpu.ee Background Simple visual search task A display with N objects is presented. Observer has to decide whether the target object was present or not. Simple SDT model (e g Shaw, 1980, Palmer et al, 1993) Each object is represented by noisy internal variable with different expected value for target and for distractors. Variables are independent of each other and the number of objects. Observer says “yes” (target present) if at least one internal variable exceeds selected criterion. Results  The difference between visual search for a single and for multiple targets is somewhat larger than predicted by simple SDT model.  The effect of number of display set-size is almost independent of number of targets, and consistent with SDT model. Question Can a similar SDT model be applied to search for multiple identical targets (e g Sagi & Julesz, 1985; Trick & Pylyshyn, 1993)? Methods 4, 8 or 16 dots were presented for about 100 ms. Observer had to decide whether there were M or M-1 brighter dots among them. M was varied from 1 (usual search task) to 4. Luminance difference between bright and dim dots was varied in order to measure psychometric function. Conditions were varied across blocks of trials. Four observers took part in the experiments. Model Generalisation of the “max rule”. Observer says “M targets” if at least M internal variables exceed the criterion that is selected to maximize the percentage correct. For example, the probability that at least two largest values of K targets and N-K distractors exceed the criterion can be calculated: where PT and PD are the probabilities that a single target (or distractor) doesn’t exceed the criterion. Target and distractor distributions were assumed to be Gaussian with equal variance. Discussion Humans are less efficient in discrimination of multiple target objects, as compared with SDT model. It can be explained either by a second stage of processing that is unable to use all information present at the first stage, or inhibitory interaction between multiple targets, or unrealistic assumptions of the model. References Palmer, J., Ames, C. T., & Lindsey, D. T. (1993). Measuring the effect of attention on simple visual search, Journal of Experimental Psychology: Human Perception and Performance 19 108-130 Sagi, D. & Julesz, B. (1985). ’Where’ and ‘what’ in vision. Science, 228, 1217-1219 Shaw, M. L. (1980). Identifying attentional and decision-making components in information processing. In R. S. Nickerson, Attention & performance VIII. Hillsdale, NJ: Erlbaum, 277-296. Trick, L. M. & Pylyshyn, Z. W. (1993). What enumeration studies can show us about spatial attention: Evidence for limited capacity preattentive processing. Journal of Experimental Psychology: Human Perception and Performance, 19, 331-351.