Further Observations on the Spiral After-Effect

Abstract

was given two trials (expansion after-effect on Trial 1, contraction after-effect on Trial 2), the after-effect being projected onto the spiral itself. Two more trials (expansion after-effect followed by contraction after-effect) were given, with the after-effect pro- jected onto a photograph of the spiral, 3 in, in diameter and 27 in. from S, thus subtending the same visual angle as the spiral. The order of presentation of the pro- jection fields was counterbalanced. The mean duration of the after-effect projected on the spiral was 20.19 sec.; when projected on the photograph, 15.69 sec. The mean dif- ference is significant by c test (p < .05), suggesting a relationship berween the duration of the after-effect and the distance of the projection field. That the shorter distance of the projection field rather than the movement from one projection field to the other is the important factor is suggested by the following observation. Several Ss spontaneously reported that if, when the after-effect had ceased, using the photograph as projection field, they turned and looked at the spiral, they again saw the after-effect. If they looked again at the photograph they did not see the after-effect. Experiment II.-Ss were 12 males whose mean age was 16.58 yr. Each was given two trials, using the photograph of the spiral as projection field. Six Ss were given an expansion after-effect followed by a contraction after-effect, for the others the order was reversed. The mean duration of the expansion after-effect was 20.23 sec.; for the con- traction after-effect, 16.0 sec. The difference bemeen these means is significant by I test (p < .01). This finding agrees with that of Spitz, i.e., the expansion after-effect was seen after shorter periods of stimulation (6 sec.) than the contraction after-effect which required 13 sec. stimulation (4). This is so, if the after-effect can be regarded as a result of the continuation of excitation set up by the rotating spiral, since it would then be expected that the method of producing the after-effect which resulted in a longer after-effect would also require shorter periods of stimulation to produce an after-effect.