Douglas H. Lawrence

Two studies of visual search for word targets with controlled rates of presentation

The two studies reported involve the visual search of word lists for a target item when the rate of presentation is controlled and the words are presented tachistoscopically. In the first study, the target is differentiated physically from the filler items by being capitalized. When the target is the last item in a list, it is readily identified at all presentation rates, but when it is the first word or is embedded in a list, recognition accuracy is inversely related to presentation rate. In the second study, the differentiation between target and filler items is in terms of the presence or absence of category membership. All Ss at all presentation rates do significantly better on lists with an animal word as a target and a set of unrelated words as filler items than on the converse arrangement.

Resistance to Extinction as a Function of the Number of Delay of Reward Trials

It is well established that delay of reward during training increases resistance to extinction. There is lirrle agreement, however, concerning the controlling variables. Wike and McNamara ( 1957) suggest that resistance to extinction in this situation is directly related to the percentage of training trials on which delay of reward occurs. In their design, delay and non-delay trials were mixed in a semi-random manner. Emphasis on the percentage of trials involving delay, however, leads to the inference that unpredictability or uncertainty are the primary psychological variabIes involved in this phenomenon. This inference clearly has predominated in the early studies involving partial reward (Humphrey~, 1939). More recent work on partial reward effects (Festinger, 1961; Lawrence & Festinger, 1962) indicates chat the absolute number of nonrewarded trials, regardless of the percentage of rewarded trials, is the basic variable determining resistance to extinction. This finding follows directly from the dissonance reduction interpretation of partial reward effects formulated by these investigators. An extension of this interpretation to the delay of reward situation predicts that the absolute number of delay uials also should be the major variable in determining resistance to extinction. This should be true even though the animal can predict with complete certainty that a delay will occur on each trial. The present study is a test of this prediction. METHOD subject^.-Ss were 40 albino rats, 29 females and 11 males, from the colony maintained by the Department of Psychology at Stanford University. They ranged in age from 107 to 112 days at the beginning of the experiment. Appararus.-The apparatus was an L-shaped runway. The initial stem was 66 in. long, 7 in. high, and 5 in. wide. The first 12 in. was the start box. This stem contained 5 3-in. high metal hurdles spaced 8 in. apart. It led to a delay chamber 12% in. long, 11 in. wide, and 11 in. high. At the rear of the right wall in this compartment there was an exit chamber 5% in. aide and 5% in. long. It opened into a goal box 14% in. long, 5 in. wide, and 6 in. high. There were guillotine doors berween the start box and initial runway, berween the runway and the entrance to the delay chamber, and between the exit of the delay chamber and the goal box. The start box and runway were black, the delay chamber was unpainred, and the goal box was a medium gray. Running times were recorded by a Standard Bectric timer which started with the opening of the start box door and stopped when S broke a photo beam placed 5 in. inside the delay chamber.

An N-channel stroboscopic tachistoscope for typewritten stimuli

The n-channel tachistoscope described employs the principle of “stopping” a continuously moving list of words by means of stroboscopic flashes of light. It can present as many as 50 stimuli per trial at any rate between 5 and 40 items per second. Stimulus materials can be prepared on an electric typewriter. It provides only partial control over the various time parameters.

Temporal numerosity estimates for word lists

The Ss estimated the number of items seen when sequences of words were presented tachistoscopically at presentation rates from 4 to 16 words/sec. The degree of underestimation increased with increasing rates of presentation, and the maximum average counting rate was between 6 and 7 words/sec. In the second phase of the study, they were asked to recall as many words as possible from short sequences of words presented at rapid rates, as well as to estimate the number of words seen but not available for recall. The total number of words seen, as judged from these two reports, agreed closely with the counting rates. The number of words correctly recalled decreased with increasing presentation rates.