ORIGINAL RESEARCH ARTICLE published: 25 September 2012 doi: 10.3389/fpsyg.2012.00355 The sensory components of high-capacity iconic memory and visual working memory Claire Bradley 1,2 and Joel Pearson1* 1 The School of Psychology, The University of New South Wales, Sydney, NSW, Australia 2 Ecole Normale Supérieure de Cachan, Cachan, France Edited by: Early visual memory can be split into two primary components: a high-capacity, short-lived Peter Neri, University of Aberdeen, iconic memory followed by a limited-capacity visual working memory that can last many UK seconds. Whereas a large number of studies have investigated visual working memory for Reviewed by: Paolo Martini, University of Warwick, low-level sensory features, much research on iconic memory has used more “high-level” UK alphanumeric stimuli such as letters or numbers.These two forms of memory are typically Nicola J. Van Rijsbergen, University of examined separately, despite an intrinsic overlap in their characteristics. Here, we used a Glasgow, UK purely sensory paradigm to examine visual short-term memory for 10 homogeneous items *Correspondence: of three different visual features (color, orientation and motion) across a range of durations Joel Pearson, The School of Psychology, The University of New from 0 to 6 s. We found that the amount of information stored in iconic memory is smaller South Wales, Sydney, NSW 2052, for motion than for color or orientation. Performance declined exponentially with longer Australia. storage durations and reached chance levels after s2 s. Further experiments showed that e-mail: [email protected] performance for the 10 items at 1 s was contingent on unperturbed attentional resources. In addition, for orientation stimuli, performance was contingent on the location of stimuli in the visual field, especially for short cue delays. Overall, our results suggest a smooth transition between an automatic, high-capacity, feature-specific sensory-iconic memory, and an effortful “lower-capacity” visual working memory. Keywords: iconic memory, visual working memory, early visual memory, low-level visual features INTRODUCTION Single items of color, form and motion seem to be retained in Short-term sensory memory has been proposed to be the first visual working memory with great precision and show little decay step in forming more high-level and permanent memory stores over periods of up to 30 s (Nilsson and Nelson, 1981; Vogels and that support behavior. For example, was the arrow on a road- Orban, 1986; Magnussen and Greenlee, 1992; Blake et al., 1997; sign pointing left or right? Was it red or green? Choosing the Magnussen et al., 1998; Pasternak and Greenlee, 2005). These low- right answer to such questions could have a significant behav- level visual features seem to be remembered with high precision, ioral impact. This early visual memory can be split into two sometimes close to the resolution of perception when only a single primary components: a high-capacity, short-lived iconic memory item is held in memory (Blake et al., 1997; Magnussen and Green- (Sperling, 1960; Coltheart, 1980) followed by a limited-capacity lee, 1999; Magnussen, 2000; Fukuda et al., 2010). The stimulus visual working memory that can last many seconds (Pasternak features that affect perceptual detection and discrimination tend and Greenlee, 2005; Fukuda et al., 2010; Keogh and Pearson, to have little influence on the decay functions of the related mem- 2011). ory for a single item (Blake et al., 1997; Magnussen and Greenlee, Although these two types of memory may,by definition,overlap 1999). in their temporal decay profiles, they are often studied separately, Little is known, however, about the perceptual properties of and seem to have different properties. Iconic memory is thought iconic memory. Numerous studies suggest that the information to last only a few 100 ms after perception and is often consid- stored in iconic memory displays low-level characteristics. Specif- ered automatic (Neisser, 1967; Coltheart, 1980). Visual working ically, it seems to be degraded by visual masks (Averbach and memory, on the other hand, is considered a more effortful process Coriell, 1961; Spencer, 1969; Turvey, 1973; Phillips, 1974; Gegen- that tends to last many seconds and requires active maintenance furtner and Sperling, 1993) and may be dependent on spatial loca- of information (Pasternak and Greenlee, 2005; Awh et al., 2006; tion (Phillips, 1974; McRae et al., 1987). However, most research Fougnie, 2008). While iconic memory is thought to have a vir- on iconic memory utilized“high-level”alphanumeric stimuli such tually unlimited capacity, visual working memory is thought to as letters or numbers, which often carry semantic content. For have a severely limited-capacity that differs across individuals (see example even when color or luminance was of primary interest Brady et al., 2011 for a review;Vogel and Machizawa, 2004; Fukuda for investigation, alphanumeric figures were still used as stimuli et al., 2010). Here, we examined visual short-term memory across (Coltheart et al., 1974). a range of storage durations from 0 to 6 s that enabled us to study To the best of our knowledge no study has yet examined the both iconic and visual working memory with one set of low-level visual features of color, form, and motion within the same sub- visual stimuli. jects using stimuli without higher-level semantic content. Here, www.frontiersin.org September 2012 | Volume 3 | Article 355 | 1 Bradley and Pearson High-capacity iconic and working memory we sought to investigate iconic memory and visual working memory for the low-level visual features of color, orientation, and motion, without any potential influence from high-level semantic information in the stimuli. We report that color, orientation, and motion information can be stored in early visual memory. However, less motion informa- tion seems to be retained in iconic memory compared to color or orientation. Additionally, we found partial memory retention of 10 items at 1 s that relies on active attentional resources. Such active encoding and maintenance suggest the involvement of visual working memory for periods of 1 s or more. Likewise we showed a decay of retinotopic orientation information with longer retention intervals, suggesting ascension in the visual processing hierarchy with longer storage times. Together these results suggest a smooth transition from iconic to visual working memory mechanisms across retention times of 1 s. MATERIALS AND METHODS GENERAL METHODS Participants FIGURE 1 |Three types of stimuli were used in this study: (A) Gabor patches that could vary in orientation, (B) uniform hue patches that varied Five subjects took part in this study (three females, two males, smoothly from red to green, (C) dot motion kinetograms, moving with age 21–28, mean 23), four of which were naive to the purpose 100% coherence, that could vary in direction of motion. (D) A single trial of the study. All had normal or corrected-to-normal vision and had the following structure: after maintaining fixation, participants were gave informed written consent before taking part in the exper- shown an array of 10 stimuli for 250 ms (color or motion) or for 450 ms iment. The participants had normal color vision, as assessed by (orientation). From the offset of the initial array and with variable delay, a cue could appear. It would point at one of the 10 stimulus locations for pseudo-isochromatic plates for testing color perception (Rich- 100 ms. After an interval of 500 ms, a test stimulus would appear at the mond Products). Four participants took part in all experiments, cued location and the participant had to signal the direction of change in while one took part in only one experiment (Experiment 1, N D 4; feature space (two alternative forced choice). Experiment 2, N D 4, Experiment 3, N D 5). Apparatus type, the patches had a diameter of 2.5˚ and were evenly spaced The experiment was performed using the Psychophysics Toolbox s on a 15.6˚ diameter circle centered on a fixation point (bulls eye (Brainard, 1997) for MATLAB running on a MacPro computer. fixation point: 0.3˚ inner diameter, 0.5˚ outer) positioned in the Stimuli were displayed on a calibrated 200 SONY Multiscan G520 center of the screen. The edges of the color and orientation patches CRT with a monitor resolution of 1024 × 768 and a refresh rate of were smoothed with a Gaussian mask. They were displayed against 100 Hz. Responses were recorded using the right and left arrows a gray background. A thin black line (length: 1.8˚) was used as a of a keyboard. Participants sat in a dimly lit room, 57 cm from the cue to point toward one of the 10 locations. The size and positions screen, with their head on a head- and chin-rest allowing their eyes of items on the screen were invariant across all experiments. A test to be aligned with the center of the screen. stimulus was used, that had the same location and properties as Flicker photometry one of the 10 original stimuli apart from a modification in the Participants were presented with two alternating chromatic fields relevant feature that was tested. For each feature, the value of this (3.6˚ diameter disks): a reference field and a variable field (Wagner modification was adjusted in a pilot experiment so that perfor- and Boynton, 1972; Jiang et al., 2007). The two fields alternated mance with no memory delay was approximately 90%. This was at 25 Hz, creating a fused percept when the colors were equilumi- to ensure that the difficulty of the task in the initial condition was nant and a flickering percept when they were not. Participants were equal between features. asked to increase and decrease the luminance of the variable field, Orientation stimuli (Figure 1A) were Gabors with a 50% until the flicker disappeared.