Visual Memories Bypass Normalization

Abstract

How distinct are visual memory representations from visual perception? Although evidence suggests that briefly remembered stimuli are represented within early visual cortices, the degree to which these memory traces resemble true visual representations remains something of a mystery. Here, we tested whether both visual memory and perception succumb to a seemingly ubiquitous neural computation: normalization. Observers were asked to remember the contrast of visual stimuli, which were pitted against each other to promote normalization either in perception or in visual memory. Our results revealed robust normalization between visual representations in perception, yet no signature of normalization occurring between working memory stores-neither between representations in memory nor between memory representations and visual inputs. These results provide unique insight into the nature of visual memory representations, illustrating that visual memory representations follow a different set of computational rules, bypassing normalization, a canonical visual computation.

Keywords: normalization; psychophysics; visual memory; visual perception.

Fig. 1

Stimuli and example trial sequences from Experiment 1. Each stimulus (a) was composed of one of three different surround configurations at five different center contrast levels (10%–75% contrast). Example trial sequences are shown for the simultaneous (b) and the sequential (c) conditions. Observers viewed a center stimulus for 1,000 ms, which varied from trial to trial in contrast and orientation. In both conditions, observers were required to match the contrast of the probe to the remembered center stimulus after a 2,200-ms retention interval. During the simultaneous condition, the center stimulus was enveloped by a full-contrast surround stimulus, which had orientation content that was either collinearly or orthogonally oriented to the center. In the sequential condition, this surround stimulus was moved into the retention interval. After every interval in which a stimulus could appear, a counterphase flickering, full-contrast checkerboard masking stimulus was presented to reduce any lingering afterimages. Stimuli are modified for illustrative purposes.

Fig. 2

Results from Experiment 1. Perceived contrast of the center stimuli is shown separately for the (a) simultaneous and (b) sequential conditions. Observers’ estimates of the center stimulus contrast were near veridical (indicated by the dashed line). Data points reflect the apparent contrast estimates across all contrast levels, averaged over observers (N = 12), for the three different surround conditions (collinear, orthogonal, and no surround). Error bars denote ±1 SEM (note that in some cases the error bars are smaller than the data points). Schematics above the graphs illustrate the general experimental design. Normalization strength estimates (c) were derived from the normalization model. Parameter estimates illustrate the influence of the surround (collinear and orthogonal) on perceived contrast of the center stimulus for both the simultaneous and sequential conditions (see the Supplemental Material available online for additional parameter estimates). Error bars denote ±1 SEM.

Fig. 3

Stimuli and example trial sequences from Experiment 2. Stimuli (a) were composed of a center and a surround stimulus that both varied in contrast. Each component could be one of four contrast levels (10%–75% contrast). Example trial sequences are shown for the simultaneous (b) and sequential (c) conditions. The contrast of both center and surround stimuli had to be remembered, and after a retention period, observers were asked to match the contrast of the probe to either the center or surround that had been held in memory. Counterphase flickering, full-contrast masks were presented to reduce any lingering afterimages. Stimuli are modified for illustrative purposes.

Fig. 4

Results from Experiment 2. Perceived contrast of the center stimuli is shown separately for the (a) simultaneous and (b) sequential conditions. Data points reflect the apparent center contrast estimates across all contrast levels, averaged over observers (N = 10), for each surround contrast condition (10%–75% surround). Dashed black lines indicate veridical contrast estimation. Error bars denote ±1 SEM (note that in some cases the error bars are smaller than the data points). Schematics above the graphs illustrate the general experimental design. Normalization strength estimates (c) were derived from the normalization model. Parameter estimates illustrate the influence of the surround on perceived contrast of the center stimulus for both the simultaneous and sequential conditions (perception = blue; visual memory = red; see the Supplemental Material for additional parameter estimates). Error bars denote ±1 SEM