Opposite Influence of Perceptual Memory on Initial and Prolonged Perception of Sensory Ambiguity

Observers continually make unconscious inferences about the state of the world based on ambiguous sensory information. This process of perceptual decision-making may be optimized by learning from experience. We investigated the influence of previous perceptual experience on the interpretation of ambiguous visual information. Observers were pre-exposed to a perceptually stabilized sequence of an ambiguous structure-from-motion stimulus by means of intermittent presentation. At the subsequent re-appearance of the same ambiguous stimulus perception was initially biased toward the previously stabilized perceptual interpretation. However, prolonged viewing revealed a bias toward the alternative perceptual interpretation. The prevalence of the alternative percept during ongoing viewing was largely due to increased durations of this percept, as there was no reliable decrease in the durations of the pre-exposed percept. Moreover, the duration of the alternative percept was modulated by the specific characteristics of the pre-exposure, whereas the durations of the pre-exposed percept were not. The increase in duration of the alternative percept was larger when the pre-exposure had lasted longer and was larger after ambiguous pre-exposure than after unambiguous pre-exposure. Using a binocular rivalry stimulus we found analogous perceptual biases, while pre-exposure did not affect eye-bias. We conclude that previously perceived interpretations dominate at the onset of ambiguous sensory information, whereas alternative interpretations dominate prolonged viewing. Thus, at first instance ambiguous information seems to be judged using familiar percepts, while re-evaluation later on allows for alternative interpretations

Opposite influence of perceptual memory on initial and prolonged perception of sensory ambiguity.

Observers continually make unconscious inferences about the state of the world based on ambiguous sensory information. This process of perceptual decision-making may be optimized by learning from experience. We investigated the influence of previous perceptual experience on the interpretation of ambiguous visual information. Observers were pre-exposed to a perceptually stabilized sequence of an ambiguous structure-from-motion stimulus by means of intermittent presentation. At the subsequent re-appearance of the same ambiguous stimulus perception was initially biased toward the previously stabilized perceptual interpretation. However, prolonged viewing revealed a bias toward the alternative perceptual interpretation. The prevalence of the alternative percept during ongoing viewing was largely due to increased durations of this percept, as there was no reliable decrease in the durations of the pre-exposed percept. Moreover, the duration of the alternative percept was modulated by the specific characteristics of the pre-exposure, whereas the durations of the pre-exposed percept were not. The increase in duration of the alternative percept was larger when the pre-exposure had lasted longer and was larger after ambiguous pre-exposure than after unambiguous pre-exposure. Using a binocular rivalry stimulus we found analogous perceptual biases, while pre-exposure did not affect eye-bias. We conclude that previously perceived interpretations dominate at the onset of ambiguous sensory information, whereas alternative interpretations dominate prolonged viewing. Thus, at first instance ambiguous information seems to be judged using familiar percepts, while re-evaluation later on allows for alternative interpretations

Stimulus and results of Experiment 4 ‘Pre-exposure in binocular rivalry’.

<p><b>A</b>) We investigated the perception of binocular gratings to test whether the effects of pre-exposure reflect a general phenomenon among ambiguous stimuli, or whether they are specific to the rotating globe. When a leftward and a rightward tilted grating pattern are presented to the two eyes observers perceive them alternating for several seconds at a time. We used the paradigm presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030595#pone-0030595-g001" target="_blank">figure 1B</a>, with the intermittent viewing period lasting either 0.5 or 2.5 minutes and the test period lasting 50 seconds. In 50% of the trials the grating stimuli were swapped between the eyes at the beginning of the test phase (compared to the intermittent phase of that trial), to be able to dissociate the effects of percept-stabilization from those of eye-stabilization. <b>B</b>) The predominance of the alternative percept (<i>left</i> graph) and the ‘alternative eye’, i.e. the eye that was suppressed during the pre-exposure (<i>right</i> graph) at the onset of the test phase (numbers in grey shading) and during subsequent ongoing rivalry (±SEM; bin-width: 8.3 sec.). In line with the previous experiments in which we used the rotating globe (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030595#pone-0030595-g002" target="_blank">fig. 2E</a>), the predominance of the alternative percept during ongoing rivalry was increased after pre-exposure (<i>red</i>) compared with a condition without pre-exposure (<i>blue</i>). This increase was larger after long pre-exposure (2.5 minutes; <i>dark red</i>) than after short pre-exposure (0.5 minutes; <i>light red</i>). Rivalry at onset was not influenced by the duration of the pre-exposure. Pre-exposure did not affect the predominance of the alternative eye. In all conditions the predominance of the alternative eye was low initially and near 50% later on. <b>C</b>) Lower two graphs: Average duration (±SEM) of percepts that occurred between 0 to 16.7 seconds after pre-exposure (pre-exposed percept in <i>black</i>, left graph; alternative percept in <i>red</i>, right graph; without pre-exposure in <i>blue</i>). Upper two graphs: Same data, but now showing the average <i>difference in percept duration</i> between the conditions with and without pre-exposure. The effect of pre-exposure duration is better viewed with this correction, because the variability between the participants in the overall mean percept duration was rather large. The decrease of the duration of the pre-exposed percept is not influenced by the duration of the pre-exposure, whereas the duration of the alternative percept is longer after long pre-exposure than after short pre-exposure (in line with the result for the rotating globe, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030595#pone-0030595-g002" target="_blank">fig. 2C</a>).</p

Paradigm and results of Experiment 3: Intermittent and continuous pre-exposure.

<p><b>A</b>) We test the influence of the blank periods during the intermittent pre-exposure we compared intermittent pre-exposure with continuous pre-exposure. Both pre-exposure procedures included the same total amount of exposure to the stimulus (i.e. 0.4 minutes). To ensure stable perception during the continuous pre-exposure we used unambiguous stimuli. <b>B</b>) The predominance of the alternative percept at the onset of the test phase (number in grey shading) and during subsequent ongoing rivalry (±SEM; bin-width: 20 sec.) for the binocular-unambiguous (<i>left</i> graph) and monocular-unambiguous (<i>right</i> graph) pre-exposure stimulus. Averaged baseline measure in <i>blue</i> (without-pre-exposure). For both stimuli the predominance was larger after the intermittent procedure (<i>dashed red</i> lines) than after the continuous procedure (<i>solid red</i> lines) in a time-window ranging 20–80 seconds after pre-exposure. In the first time-bin (0–20 sec.) the reverse was true, mainly because the first pre-exposed percept lasted shorter after continuous pre-exposure than after the intermittent pre-exposure (see upper graph in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030595#pone-0030595-g004" target="_blank">fig. 4C</a>). <b>C</b>) <i>Top row:</i> The average duration (±SEM) of the first percept in trials that started with the pre-exposed percept (<i>black</i>) and trials without pre-exposure (<i>blue</i>). For the binocular-unambiguous (BIN) as well as the monocular-unambiguous (MON) stimulus the duration of the first percept was reduced after continuous pre-exposure and not after intermittent pre-exposure. <i>Bottom row:</i> The average duration (±SEM) of percepts that occurred between 20 to 80 seconds after pre-exposure (pre-exposed percept in <i>black</i>, left graph; alternative percept in <i>red</i>, right graph; no pre-exposure in <i>blue</i>). The duration of the alternative percept was increased, whereas the duration of the pre-exposed percept was not. Abbreviations: BIN = binocular-unambiguous, MON = monocular-unambiguous.</p

Results of Experiment 1: Ambiguous pre-exposure.

<p><b>A</b>) On the left: Predominance of the alternative percept at the onset of the test phase (i.e. identity of the first percept; see numbers in grey shading) and during subsequent ongoing rivalry (±SEM; width of time-bins is 30 sec.) in five conditions with increasing duration of the pre-exposure (from <i>yellow</i> to <i>dark red</i>). The <i>blue</i> line reflects the averaged baseline measure (without pre-exposure) for the 5 different durations of pre-exposure (statistics reported in the text were done on the individual baseline measures). During continuous viewing the predominance of the alternative percept was larger after longer pre-exposure durations and decreased over time. Such an effect was not present at the onset of the test phase. On the right: Predominance of the alternative percept for individual participants after a pre-exposure period lasting 4.3 minutes. Here, the predominance was calculated over a time-window of 0.5 to 3.5 minutes after pre-exposure. Within this time-window the group-data for this condition significantly differed from the baseline measure. <b>B</b>) The average duration (±SEM) of the first to the 25^th percept without pre-exposure (<i>blue</i>) and after 4.3 minutes of pre-exposure (pre-exposed percept in <i>black</i>, left graph; alternative percept in <i>red</i>, right graph). The duration of the pre-exposed percept was not increased, even while this percept was facilitated in the sense that it was likely to occur at stimulus onset. The results for the pre-exposed percept resemble the suppression hypothesis (proposing a ‘fatigue-like’ effect) more than the facilitation hypothesis (proposing a ‘memory-like’ effect) (see inset in left graph). Although the alternative percept was not seen during pre-exposure, its duration shows a clear increase after pre-exposure, which might relate to Levelt's 2^nd proposition (Levelt, 1967). <b>C</b>) The average duration (±SEM) of the percepts that occurred within 1.5 minutes after the pre-exposure (pre-exposed percept in <i>black</i>, left graph; alternative percept in <i>red</i>, right graph) or within the first 1.5 minutes of the condition without pre-exposure (<i>blue</i>). Data are shown for five different durations of the pre-exposure. The duration of the alternative percept increased when the duration of the pre-exposure increased, whereas the duration of the pre-exposed percept remained unchanged.</p

Paradigm and results of Experiment 2: Unambiguous pre-exposure.

<p><b>A</b>) The paradigm. The pre-exposure period had a fixed duration (4.3 minutes) and contained either an ambiguous globe, a globe disambiguated using binocular depth-cues (disparity) or a globe disambiguated using monocular depth-cues (contrast- and size-imbalance). The subsequent test period always contained an ambiguous globe. <b>B</b>) The predominance of the alternative percept at the onset of the test phase (numbers in grey shading) and during subsequent ongoing rivalry (±SEM; bin-width: 30 sec.) in the condition without-pre-exposure (<i>blue</i>; averaged baseline measure) and after ambiguous (<i>dark red</i>), binocular-unambiguous (<i>red</i>) and monocular-unambiguous (<i>orange</i>) pre-exposure. After pre-exposure the predominance of the alternative percept was increased during continuous viewing (but not at onset) in all 3 conditions. This increase was successively larger for the monocular-unambiguous, binocular-unambiguous and ambiguous condition. <b>C</b>) The average duration (±SEM) of the percepts that occurred between 0.5 and 4.5 minutes after pre-exposure (pre-exposed percept in <i>black</i>, left graph; alternative percept in <i>red</i>, right graph; no pre-exposure in <i>blue</i>). The increase in the duration of the alternative percept was successively larger when the pre-exposed stimulus was monocular-unambiguous (MON), binocular-unambiguous (BIN) or ambiguous (AMB). The slight decrease in the duration of the pre-exposed percept did not significantly differ between the 3 conditions.</p

Stimulus and paradigm.

<p><b>A</b>) The stimulus consisted of black and white leftward and rightward moving dots placed such that they represented points on the surface of a virtual globe. Depth was signaled by the sinusoidal speed profile of the dots, i.e. the dots moved faster as they were closer to the vertical meridian of the globe, thereby creating the illusion of a 3-dimensional globe in depth. The virtual globe was perceived rotating around its vertical axis, but the direction of the rotation was ambiguous: either the rightward or the leftward moving surface was perceived in front of the other surface. <b>B</b>) A trial started with an intermittent presentation period of variable duration (up to 4.3 minutes) during which the ambiguous globe perceptually stabilized. Subsequently, the ambiguous globe was presented continuously for a prolonged duration (up to 10 minutes). During this period perceptual alternations occurred every few seconds. The stabilized percept is referred to as the ‘pre-exposed’ percept throughout this manuscript. We investigated the effect of the pre-exposure on the durations of the pre-exposed and alternative percept, during continuous test period.</p