Ikkai, Mccollough & Vogel: Number of Items in Vwm 1

49 50 Visual working memory (VWM) helps to temporarily represent information from the 51 visual environment, and is severely limited in capacity. Recent work has linked various 52 forms of neural activity to the ongoing representations in VWM. One piece of evidence 53 comes from human event-related potential studies which find a sustained contralateral 54 negativity during the retention period of VWM tasks. This Contralateral Delay Activity 55 (CDA) has previously been shown to increase in amplitude as the number of memory 56 items increases, up to the individual’s working memory capacity limit. However, 57 significant alternative hypotheses remain regarding the true nature of this activity. Here 58 we test whether the CDA is modulated by the perceptual requirements of the memory 59 items, as well as whether it is determined by the number of locations that are being 60 attended within the display. Our results provide evidence against these two alternative 61 accounts, and instead strongly support the interpretation that this activity reflects the 62 current number of objects that are being represented in VWM. 63 64 65 66 Introduction 67 68 Our ability to represent information in an active state is facilitated by the visual 69 working memory (VWM) system. This system is capacity-limited, such that only a small 70 amount of information can be represented simultaneously. Neural measures of VWM 71 have provided critical evidence regarding fundamental attributes of this system. One form 72 of evidence comes from single-unit recording studies with monkeys. Neurons across a 73 wide range of cortical areas show a sustained increase in firing rate above baseline during 74 the retention period of VWM tasks (Kubota and Niki, 1971, Fuster and Alexander, 1971, 75 Funahashi, Bruce and Goldman-Rakic, 1989a); an effect often referred to as delay 76 activity. The delay activity in many cells have been shown to be highly sensitive to 77 properties of the remembered material such as its spatial position (Funahashi, Bruce and 78 Goldman-Rakic, 1989a, Chafee and Goldman-Rakic, 1998, Umeno and Goldberg, 2001), 79 and identity (Warden and Miller, 2007, Rainer and Miller, 2002), and is correlated with 80 behavioral outcome (Funahashi, Bruce and Goldman-Rakic, 1989b). Similar activity has 81 been reported in human neuroimaging studies showing sustained activations during the 82 retention period of VWM tasks in regions such as the prefrontal cortex (PFC), 83 intraparietal sulcus (IPS), lateral occipital cortex (LOC), and primary visual cortex (V1) 84 (Srimal and Curtis, 2008, Courtney et al., 1998, Postle et al., 2000, Curtis and D'Esposito, 85 2006, Ferber, Humphrey and Vilis, 2005, Harrison and Tong, 2009, Serences et al., 86 2009). Of these regions, the IPS has recently gained much attention because it is strongly 87 modulated by the number of items being remembered in VWM, but reaches an asymptote 88 once memory capacity is exhausted (Todd and Marois, 2004). 89 90 Similar evidence can be observed using human event-related potential (ERP) 91 recordings in which a large negative wave is observed over posterior electrode sites that 92 are contralateral to the position of the remembered items that persists during the retention 93 period. This Contralateral Delay Activity (CDA) is strongly modulated by the number of 94 items in memory, but reaches an asymptote once capacity is reached and is highly 95 Ikkai, McCollough & Vogel: Number of items in VWM 3 predictive of the individual’s specific memory capacity. (Vogel and Machizawa, 2004, 96 Vogel et al, 2005, Drew and Vogel, 2008, Robitaille et al, 2009). This suggests that the 97 CDA provides a measure of the number of objects that are in VWM. However, there are 98 two important alternative accounts of this activity that preclude such a conclusion. First, 99 CDA amplitude modulations may reflect the increasing perceptual demands of the 100 display. That is, as the number of items in the display increases, the perceptual difficulty 101 of the display also increases, and it may be these increased encoding demands which may 102 be the factor that drives increasing CDA amplitude rather than reflecting increased 103 memory representations. Second, the CDA may reflect a spatial indexing process that 104 represents the number of locations that are currently being attended. All previous CDA 105 studies have used displays that have confounded the number of objects with the number 106 of positions. In the present study, we seek to evaluate these two alternative accounts of 107 this activity in an attempt to determine what aspect of WM performance the CDA 108 reflects. 109 110 111 112 113 114 Materials and Methods 115 116 Overview 117 In the first experiment, we will test whether the CDA is modulated by the amount 118 of perceptual effort required for the display rather than the number of items in memory. 119 To do this, we will compare the CDA amplitude for arrays of items that are presented 120 either in high contrast or very low contrast while also manipulating the number of items 121 in the display. We expect that the low contrast displays will be much more effortful to 122 perceive than the high contrast displays, thus if the CDA is primarily sensitive to this 123 increasing perceptual effort then we would expect that low contrast items should produce 124 an increase in amplitude as compared to high contrast displays with the same number of 125 items. 126 In the second experiment, we will test whether the CDA is sensitive to the number 127 of objects in VWM, or if it is instead sensitive only to the number of locations that are 128 currently being attended. Here, we will attempt to decouple the number of memory items 129 from the number of attended positions by presenting the memory items as a sequence of 130 two arrays separated by 500 ms. We have previously shown that separating the memory 131 items into two 2-item arrays results in a “step-like” function for CDA amplitude: initially 132 amplitude is at the 2-item level, then quickly ramps up to the level of 4 items that were 133 simultaneously presented (Vogel, McCollough and Machizawa, 2005). Experiment 2 will 134 be similar, with the critical exception being that we will also present a condition in which 135 the items in the second array will be presented in the same locations as those in the first 136 array. If the CDA amplitude is determined solely by the number of locations, we would 137 expect that remembering 4 objects presented at 2 locations would be equivalent to 138 remembering 2 objects at 2 locations. Because subjects made a same/different response 139 for each array in the sequential conditions, on a quarter of these trials a change was 140 presented in each array. This resulted in four equally-probable trial types for the 141 Ikkai, McCollough & Vogel: Number of items in VWM 4 sequential conditions: array 1 same/ array 2 same; array 1 same/ array 2 different; array 1 142 different/ array 2 same; array 1 different/ array 2 different. Thus, detection of a change on 143 array 1 provided the subject no information regarding whether or not array 2 would have 144 a change. 145 146 Subjects 147 All subjects were between 18 and 30 years old, have normal or corrected-to148 normal vision, and no history of neurological disorders or color blindness. Subjects were 149 recruited from the University of Oregon community and were paid $10 per hour for their 150 participation. A unique set of subjects participated in each experiment, with 17 in 151 Experiment 1 and 18 in Experiment 2. Subjects with eye-blink or eye-movement artifacts 152 in excess of 25% of trials were excluded from further analysis. Two subjects in 153 Experiment 1 and three subjects in Experiment 2 exceeded this threshold. 154 155 156 Stimuli and procedure for Experiment 1 157 In all experiments, the stimuli were presented with Presentation software 158 (Neurobehavioral Systems, Inc., CA) on a CRT screen in a semi-dark room. Items were 159 presented within 4°x7.3° rectangular regions bilaterally, centered 3° to the left and right 160 of the middle of the screen. A black fixation cross was presented in the center of the 161 screen throughout the trial against the gray background. An arrow was presented above 162 the fixation point. Colored squares (.65° x .65°) were randomly chosen without 163 replacement from a set of seven colors (black, white, red, blue, yellow, green & purple). 164 Luminance for each color is shown in Figure 1b. On average, high contrast objects had 4 165 times as much contrast as low contrast objects (high = 42.81 cd/m, low = 10.75 cd/m). 166 167 The schematic of a trial is illustrated in Figure 1a. Subjects were instructed to 168 fixate the black cross from 80 cm of viewing distance. Each trial consisted of an arrow 169 cue (200 ms), memory array (100 ms), retention period (900 ms), a test array (1500 ms) 170 and the intertrial interval (ITI: 1000 ms). Subjects attended to the cued visual field and 171 remembered the colors of the memory array items. At the onset of the test array, subjects 172 responded whether the memory and test arrays were identical by a button press (same vs. 173 different). Subjects were instructed to make a button press as accurately as possible. Item 174 positions were randomized between the trials, with a constraint that no square was 175 present within 2 of one another. We used a 2 (set size: 2 vs 4) x 2 (contrast: high vs. low) 176 design, and all conditions were intermixed within blocks. All subjects completed a total 177 of 8 blocks of 100 trials each, resulting in 200 trials per condition. 178