Visual percepts modify iconic memory in humans

Abstract

Our visual system briefly retains a trace of a stimulus after it disappears. This phenomenon is known as iconic memory and its contents are thought to be temporally integrated with subsequent visual inputs to produce a single composite representation. However, there is little consensus on the temporal integration between iconic memory and subsequent visual inputs. Here, we show that iconic memory revises its contents depending upon the configuration of the newly produced single representation with particular temporal characteristics. The Poggendorff illusion, in which two collinear line segments are perceived as non-collinear by an intervening rectangle, was observed when the rectangle was presented during a period spanning from 50 ms before to 200 ms after the presentation of the line segments. The illusion was most prominent when the rectangle was presented approximately 100 to 150 ms after the line segments. Furthermore, the illusion was observed at the center of a moving object, but only when the line segments were presented before the rectangle. These results indicate that the contents of iconic memory are susceptible to the modulatory influence of subsequent visual inputs before being translated into conscious perception in a time-locked manner both in retinotopic and non-retinotopic, object-centered frames of reference.

Figure 1

(A) Poggendorff figure. In the upper panels, the line segment on the lower left side (a) appears non-collinear with the line segment on the upper right side with a rectangle (left); however, without the rectangle it is obvious that these line segments are actually collinear (right). In the bottom panels, the line segment (b) appears to be collinear with the other line segment with the rectangle (left); however, these line segments are non-collinear (right). (B) Sequential presentation of the Poggendorff parts in Experiment 1. Two line segments and a rectangle, all of which are components of the Poggendorff illusion, were presented sequentially. The duration of each component was 16.66 ms. (C) The magnitude of the illusion is plotted against the ISI between the line segments and the rectangle. The dashed line represents the point of subjective continuity obtained from the control condition. The letter ‘s’ denotes that the line segments and the rectangle were presented simultaneously. Asterisks denote significant differences compared with the control condition (p < 0.05 with Holm’s correction). Error bars denote the SEM (N = 7).

Figure 2

(A) Poggendorff parts on an apparent motion stimulus in Experiment 2. Two line segments and a rectangle were presented against a surrounding stimulus with an ISI of 100 ms. The rectangle and the surround stimuli were presented 1° to the left of the original location where the line segments did not abut obliquely on the rectangle. (B) The magnitude of illusion in the conditions in which the line segments and the rectangle moved with the surrounding stimulus (rectangle-surround), only the rectangle moved so that the line segment and the surrounding stimulus was stable (rectangle-only), and only the line segments moved with the surrounding stimulus (control). The dashed line represents the point of subjective continuity obtained in the control condition. (C) Poggendorff parts on a moving stimulus in Experiment 3. The surrounding stimulus moved from right to left at 20°/sec. Two line segments and a rectangle were presented against a surrounding stimulus with an ISI of 50 ms. (D) The magnitude of the illusion in the conditions where the rectangle was presented first, the line segments were presented first, and only the line segments were presented (control). The dashed line represents the point of subjective continuity obtained in the control condition. Asterisks denote a significant difference compared with the control condition (p < 0.05 with Holm’s correction). Error bars denote the SEM (N = 7 and 8 for Experiments 2 and 3, respectively).