Three-dimensional object recognition is viewpoint dependent

Each sequential matching trial (experiments 1a-e) consisted of the following sequence of events: blank screen for 1000 ms, fixation cross for 500 ms, object image for 200 ms, mask (consisting of random combinations of features from the line drawings or shaded images) for 750 ms, second object image for 100 ms, mask for 500 ms. The trial timed out 1500 ms later if no response was given. In experiments 1a, b and d, a two-key procedure was used, in which the participant pressed the 'V' key on the computer keyboard if the two images were of the same geon (even if shown in a different viewpoint), or the 'M' key if the two images were of different geons. In experiments 1c and e, a go/no-go procedure was used, in which the participant pressed the space bar if the two objects were the same or allowed the trial to time out otherwise. Participants in experiments 1d and e were informed after each trial of their response time and the accuracy of their response (as shown in Fig. 2, this feedback lowered overall response times, but had little impact on viewpoint effects). For 'same' trials, each of the three views of each geon was presented three times as the first object in a trial and three times as the second object, producing three trials in which the two views were identical, four trials in which they differed by 45°, and two trials in which they differed by 90°. In these and subsequent experiments, trials were presented in a different random order for each participant.

Match-to-sample experiments (2a-c) each consisted of 10 blocks of trials. Each block included an initial presentation of a target object for 20 s, followed by 18 test trials in the following sequence: blank screen for 250 ms, fixation cross for 500 ms, object for 150 ms, mask for 500 ms, time out 1500 ms later if no response was given. Participants memorized the initial target, then pressed the space bar if the test object on subsequent trials matched the target object (even if the viewpoint varied), or let the trial time out if the test and target objects did not match. Participants in experiment 2c received feedback of the same sort as experiments 1d and e. The target object was always shown in the 0° viewpoint. Every test block included three same trials in each of the three viewpoints (0°, 45°, and 90°) of the target object and 9 'different' trials (one for each of the non-target objects).

In experiment 3, participants learned labels (given in Fig. 1) for the 10 geons, then performed trials in which they verbally named test images as quickly as possible. Participants first studied a sheet of paper showing the 10 objects with their names, then performed 20 trials in which each object was shown twice, along with its name, on the computer screen. Four practice blocks of five trials with each object followed, in which participants saw objects without their names and spoke the names. Objects were always shown in the 0° viewpoint during practice trials. Participants then performed two test blocks of six trials with each object, distributed equally between the 0°, 45°, and 90° viewpoints. All practice and test trials consisted of a 500 ms blank screen, 500 ms fixation cross and an object that stayed on the screen until the participant responded or until 2500 ms had elapsed. Response times were recorded via the voice trigger, but accuracy was not recorded. (The experimenter observed the first few participants, and found that accuracy was almost always perfect.)