Abstract. stimuli (see SNARC effect) have identified an attentional bias induced by numeric

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2 Abstract Previous studies investigating interactions between spatial attention and numeric stimuli (see SNARC effect) have identified an attentional bias induced by numeric stimuli in accordance with a mental number line model of processing. This processing model attributes orienting effects induced by numeric stimuli to long-term memory structures, such as a left-to-right number system in a given culture. The present study aims to investigate long-term memory may modulate attentional biases induced by the contents in short-term memory. In other words, can simply thinking about number induce spatial biasing effects? In this study, dichotic listening is used to measure of spatial attention in the auditory modality. Subjects were given numeric items to retain in short term memory during the spatial attention task, and prompted to recall the numbers verbally after each trial. Factors of number magnitude, load size, interval size between numbers, and presentation order are analyzed in terms of their effect on auditory spatial orienting. Auditory spatial orienting is assessed through a measure of laterality index a ratio of left/right ear responses indicating attention as allocated to the left or right side of space. Results demonstrated a clear influence of number magnitude and presentation order on auditory spatial attention, however these effects were highly variable depending on other factors present in each experiment. Overall, results suggest that in addition to SNARC based orienting responses, the orienting effects of items in short term memory on spatial attention may be influenced by novelty, interactions with language, and multiple neural mechanisms responsible for representing quantity information.

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4 Acknowledgements I would like to thank Edward J. Golob, Ph.D., for his mentorship and for providing the opportunity and the tools to complete this research. ii

5 TABLE OF CONTENTS ACKNOWLEDGEMENTS...ii LIST OF TABLES AND FIGURES...iv Chapter 1. INTRODUCTION METHODS RESULTS DISCUSSION...35 Appendix 1. TABLES AND FIGURES REFERENCES...67 iii

6 LIST OF TABLES TABLE 1: Experiment Design Overview TABLE 2: Summary of Results LIST OF FIGURES FIGURE 1: Cowan Model of Working Memory. FIGURE 2: Example Schematic of a Trial FIGURE 3: Experiment 1 Magnitude FIGURE 4: Experiment 1 - Other FIGURE 5: Experimetn 2 Sub-range FIGURE 6: Experiment 2 - Order FIGURE 7: Experiment 2 Sub-range x Order FIGURE 8: Experiment 2 - Magnitude FIGURE 9: Experiment 2 - Other FIGURE 10: Experiment 3 - Magnitude FIGURE 11: Experiment 3 - Order FIGURE 12: Experiment 3 Magnitude x Order FIGURE 13: Experiment 3 Sub-range x Order FIGURE 14: Experiment 4 - Magnitude FIGURE 15: Experiment 4 Magnitude x Load FIGURE 16: Experiment 4 - Load FIGURES 17 and 18: Summary of effects by experiment iv

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8 1 Introduction Past experiences improve our understanding of the world around us from moment to moment. Memories influence our understanding of events in the world as they transpire. For example, with respect to attention - hearing one s name called from across the room tends to draw one s attention towards that part of the room. In terms of cognitive systems, hearing your name triggers associations in long-term memory that ultimately cause a shift in attention. The likely reason this happens is that your name is personally significant. As similar shifts of attention occur in different people with different names, long-terms memory is clearly involved in guiding attention. Experiences are consolidated throughout life in order to build a repertoire of skills and known behaviors. Memory enables us to associate specific skills and behaviors with specific contexts. This context-sensitive repertoire of behaviors and skills affects both attention and perception. Attention and perception are modulated with respect to previous experience to ensure the selective use of behaviors and skills that were previously useful in that context. In this manner, the impact of experience on immediate cognition is in large part what enables predictive behavior. In addition to influencing immediate processing in terms of attention, experience influences lingering effects in short-term memory caused by external stimuli. Experience affects which features of external stimuli are retained in short-term storage otherwise known as working memory. Experience then guides not only our immediate processing of stimuli, but also our post-processing of events that occur in the world. For example, a

9 bilingual speaker of both French and English would have two distinct memory systems 2 dedicated to understanding either language. These two distinct processing systems for language help to promote the automatic transformation of verbal information into meaningful semantic structures upon exposure to a given language. If verbal stimuli are to convey meaning, they must first be referenced with relevant resources in long-term memory. Immediately after attending to a French speaker, memory systems that correspond to processing French are triggered and subsequently cause their meaning to linger in short-term memory. In this manner, not only is overt attention mediated by longterm memory, but so too are internal means of processing external information. By using experience to mediate which memory systems are activated in association with specific external events, resources that are relevant to the context at hand are selectively implemented to guide behavior. Previous experiences inform not only our future reactions to stimuli, but also our ways of thinking about and means of encoding those stimuli in the short-term. Ultimately if a specific behavioral or cognitive system is consistently useful in a specific context (such as memory for words in a specific language when focusing on speakers of that language), a learned association between that context and those cognitive tools is formed that no longer requires deliberate action to implement. Experiences in long-term memory directly influence which elements of external stimuli linger in short-term and working memory. Typically, features and memory systems that are useful for accomplishing a task tend to predominate over those that are poorer predictors of future events. Here we have laid out a clear dynamic by which overt behaviors and lingering processing effects are elicited in the context of certain external

10 stimuli. What has yet to be considered is the possibility of overt behaviors that are not 3 elicited by external stimuli, but rather in response to internal lingering effects in short=term memory. Specifically, behaviors associated with external stimuli may also be associated with features of those stimuli that linger in short-term memory retention. One such feature is novelty. Novel stimuli are strongly attention drawing, and attention is guided by experience with external stimuli, however novel items are by definition new to experience. Attention devoted to novel stimuli supports the notion cognition is guided by associations with both known external stimuli as well as features of those stimuli, which linger in short-term memory (such as novelty). Both external stimuli themselves as well as their lingering features may promote associated behavioral responses somewhat independently. If predictive behavior were informed solely by past successes, attention and cognition in general would be hyper-selective for stimuli and events similar to those that were previously seen. Such a system would eventually lead to rigid associations between behavior and external stimuli. Attention systems and behavioral responses associated with lingering effects of stimuli like novelty serve to dampen such rigidity. It is also worth noting that in the example of bilingual speakers, internal language representations would have distinct sets of corresponding behaviors and skills associated with them in memory. These behaviors associated with processing in each language may have been established over time by cultural differences (Zebian, 2005), or by idiosyncrasies within the lexical organization of that language (Ji, 2004). Thus it is clear that behavioral and cognitive systems can be directly associated with either a specific external event, or a specific internal event such as processing novelty or representations

11 in a specific language. These internal and external events both serve as contextual cues 4 that lead to the activation of behaviors associated via long-term memory. Ultimately these effects are all dependent upon interplay between memory, and goal directed cognition in that moment. In the next section this relationship of attention, short-term memory, and long-term memory will be analyzed in-depth using the embedded process model of Nelson Cowan (Cowan, 1999). Embedded processes model: Attention, Short-term memory, Long-term memory The Cowan Working Memory Model provides an especially apt depiction of the efficiency-oriented relationship between accumulated experiences and immediate processing first described above (Figure 1). In the presented model, information from long-term stores is temporarily activated in the form of short-term memory. This activation occurs selectively with respect to the focus of attention in conjunction with central executive goals and habituated associations. Attention is directly mediated by central executive processes which support voluntarily attention, however novel stimuli are also attention drawing. Though a number of systems contribute to which associations from LTM are ultimately activated in conjunction with current goals and stimuli these systems all contribute to a single behavioral output. This output exists in a combination between controlled and automatically evoked behaviors, which are derived from the sum of factors listed above. Cowan s model does an excellent job of characterizing how long standing habits are evoked through associations with current stimuli in order to guide

12 cognition. Unfortunately the model does not address the manner in which lingering 5 effects of stimuli in the short-term store may themselves evoke associated behaviors independently of the external stimuli. Behavioral associations with internal and external contexts usually serve to enable predictive behavior. Unfortunately when multi-tasking or task switching, we often retain information in the short-term store which have little or nothing to do with what is currently occurring externally. In such situations, lingering behavioral influences caused by retained information in short-term memory may serve to distract from other goals. For example, remembering a phone number while searching for a pen and paper. While additional items in short-term store do nothing to make a visual search easier, retaining phone numbers in short-term memory may itself be associated with behaviors that are counterproductive to the goal - such as only orienting attention to the area of the room where the phone is found. Multi tasking, or task alternating may elicit habituated behaviors relevant to the items being retained in short-term memory, but irrelevant, or perhaps even counterproductive for the task at hand. This suggests that behaviors associated with items in STM themselves may be detrimental to accomplishing goals except in the case that those items correspond to current external stimuli (Woodman & Luck, 2007). While it is clear that experience affects attentional behavior and lingering items short-term storage, what is less clear is whether items in the short-term store may also influence attentional behavior in accordance with long-term experience. Cowan s model asserts that executive processes can encode items in the short-term store independently of the focus of attention. If these items elicit associated behaviors similarly to items derived from the

13 6 focus of attention externally, then the relationship between attention and the contents of short-term memory is arguably bidirectional. Relations Between Spatial and Numerical Cognition Spatial and numerical cognition: behavioral studies Behavioral studies have indicated that cognitive and neural systems utilized during spatial and numerical cognition exhibit an inextricable relatedness. One phenomenon known as the spatial numerical association of response codes (SNARC) effect has shown direct connections between numeric stimuli and spatial attention, with subjects responding faster to low and high numbers with their left and right hands, respectively (Dehaene, Bossini, & Giraux, 1993). The conventional explanation for this spatial bias is the existence of a horizontal mental number line rooted deeply in long-term memory. In terms of directing attention, life experience would indicate that low numbers usually appear on the left and high numbers on the right side of space. Thus if stimuli consist of smaller numbers, behaviors associated with the left side of space will predominate over others. These long-terms associations between low numbers and the left side of space are derived from experience with a given culture s number system. In essence, small numbers are associated with a leftward shift in attention, but that leftward shift in attention is further associated with other behaviors such as a decreased reaction time in the left hand. Further studies verified the cultural basis for the SNARC. Studies in Iranian populations, which conventionally learn to read and count from right to left, have

14 reported an inverse SNARC effect (Zebian, 2005). The results of subsequent studies 7 precluded any purely motor-based explanation for the decreased reaction times (as counting on one s hands consistently from left to right would potentially cue these motor systems). One such study demonstrated that SNARC effects occur not only in accordance with a mental number line, but also when numbers are conceptualized on a clock face as times of day (Bächtold, Baumüller, & Brugger, 1998). Results demonstrating reversal of SNARC, along with other research demonstrating that SNARC effects appear when subjects are sufficiently exposed to any specific range of digits, supports the theory that the observed behavioral bias in the SNARC effect is directly related to associations with structures encoded in LTM to guide attention. Spatial and numerical cognition: Neurological disorders Behavioral and neuroimaging results imply that spatial and numerical representations utilize similar cognitive resources. Disorders such as Spatial Hemineglect and Gerstmann s syndrome provide strong convergent evidence with these hypotheses. Patients suffering from Hemineglect experience attentional deficits that prevent them from representing spatial information contralateral to a lesioned hemisphere (typically the right parietal lobe) (Vallar, 1993). Typical neglect patients perform poorly on tasks involving spatial skills such as line bisection (Jewell & McCourt, 2000). This deficit in spatial representation systems even affects recall of items in memory (Bisiach & Luzzatti,1978). Results in Hemineglect patients who were asked to draw familiar items produce especially peculiar effects; subjects did reproduce all 12 numbers on a clock, or

15 all petals of a flower, but depicted them as all one side spatially (Corbetta & Shulman ). Together, these results demonstrate that Hemineglect involves deficits in representation, rather than perception or memory exclusively. Hemineglect often involves lesions in areas that were also implicated in neuroimaging studies that investigated numeric cognition, concordantly; Hemineglect patients also demonstrate deficits in numeric processing. Hemineglect patients with right parietal lesions consistently guess too high if asked to estimate the midpoint between two numbers, evidence that representations corresponding to both the left side of space and small numbers share a common neural basis (Zorzi et al, 2012). Results in number bisection tasks also demonstrated an increase in bias as the interval size increased, which is consistent with studies demonstrating a correlation between right parietal activation and number interval size in humans performing calculations (Cutini et al, 2014; Pinel et al, 1999). Gerstmann s syndrome, though somewhat less common, provides additional evidence for a neurological link between spatial and numeric information. Typically Gerstmann s is characterized as a left posterior parietal (angular gyrus) lesion producing four primary symptoms; agraphia, acalculia, finger agnosia, and left/right confusion (Gerstmann, 1940). While this disorder is often concomitant with spatial neglect, worth noting is the connection between spatial confusion, calculation skills, and the ability to identify fingers. Studies in patients suffering from Gerstmann s support the conception that skills for processing numbers begin with using fingers to count (Fischer, 2005). Repetitive transcranial magnetic stimulation studies provide additional support for this notion. Repetitive transcranial magnetic stimulation delivered to the left angular gyrus

16 temporarily causes both finger agnosia and inhibition of number processing in healthy 9 individuals (Rusconi, Walsh, & Butterworth, 2005). Together, Gerstmann s and Hemispatial Neglect syndrome provide evidence for a bihemispheric model of numeric processing, in which exact calculation involving smaller numbers depends more heavily on left parietal regions, and less exact processing at larger intervals relies on resources allocated within the right parietal lobe. In the next section, these two distinct systems involved in number processing will be discussed in terms of neuroimaging results. Spatial and numerical cognition: Two systems for exact and approximate representation Along with the above behavioral interactions, imaging studies in both humans and primates have shown significant overlap in brain regions activated by a variety of spatial and numerical tasks such as reaching/grasping and addition/subtraction, and line bisection. (Hubbard, Piazza, Pinel & Dehaene, 2005). Despite this large degree of overlap, a number of distinct cognitive systems devoted to different types of numeric information have been identified. Gerstmann s and Hemineglect syndrome both affect number processing, but in distinctly different ways, and are associated with different patterns of brain damage. Gerstmann s Syndrome is associated with lesions in the left angular gyrus in the inferior parietal lobe, leading to issues with exact calculations, left/right directional confusion and finger agnosia. The concomitant nature of these symptoms suggests that there exists some similarity between processing directionality and exact quantities, allocated to the left angular gyrus. Hemineglect patients do not

17 demonstrate these symptoms, but do experience similar calculative deficits affecting 10 spatially distributed attention. Hemineglect patients have difficulty estimating the middle value between two numbers, typically in association with right superior posterior parietal damage. This deficit is enhanced by increasing numeric interval size (Zorzi 2012). Ultimately, this system is involved in processes that requires distributed attention in order to achieve approximate judgments, rather than focused attention leading to exact judgments as seen in Gerstmann s syndrome. Further studies demonstrated a clear dissociation between systems dedicated to processing small exact quantities and larger approximate judgments. One basic difference between small and large numbers is their occurrence in everyday language, as small numbers are appear with a higher frequency across a multitude of languages (Dehaene, & Mehler, 1992). Experiments with numeric tasks performed by bilingual speakers found that low magnitude quantities were most effectively manipulated in a subject s native language (Spelke & Tsivkin, 2001) suggesting some intrinsic connection between processing of small quantities and language processing. Regions in the left hemisphere are vital for language. Other studies have shown that left hemisphere language regions also have greater activation in to small, exact quantities (Dehaene & Cohen, 1997; Dehaene, Piazza, Pinel, & Cohen, 2003). Utilizing functional near infrared spectrometry to investigate neural activity related to the SNARC effect, researchers identified distinct bilateral patterns of activation dependent upon number sub-range and magnitude. Specifically, activity in the left angular gyrus was associated with culturally induced biases seen with verbally encoded numbers, and while parietal activity inducing SNARC

18 was largely bilateral, increasing the sub-range size lead to relative increases in right 11 parietal activity (Cutini, 2014). These results together suggest that manipulating sets of low magnitude, near sub-range numbers with consistency amongst other features would have effects on language processing distinct from number sets with higher degrees of complexity. Ultimately these lesion and imaging studies demonstrate bilateral parietal contributions to mental number line processing. Left hemisphere networks, specifically the angular gyrus, are implicated in processing small quantities that can be represented on fingers or verbalized. A right parietal bias is seen for larger quantities with high subrange, denoting a non-verbal approximate representation of quantity information. These left hemisphere associated small quantities are implicitly associated with verbal information, which can be explained by their prevalence in language. Numeric stimuli associated with the right parietal lobe are also associated with visual representations of quantity (Fias et al, 2003), and approximate systems that influence spatial attention, but do not directly interact with language. Ultimately, these two systems for exact and approximate representations of quantity have been associated with systems devoted to focused and distributed attention, respectively (Chong & Evans, 2011). Auditory Spatial Attention and Memory The Cocktail Party Effect Classic studies in auditory spatial attention utilizing dichotic listening protocols initially identified what has been termed the Cocktail Party Effect (Wood & Cowan,

19 1995), in which subjects presented with different spoken narratives to each ear (i.e. 12 dichotically), when asked to attend to a specific ear, are easily distracted by the presentation of their name in the unattended ear, and subsequently fail to recall information from the attended ear during distractions. This study provides an archetypal example of Cowan s model in the context of auditory spatial attention. When subject are asked to attend to only one ear, central executive functions direct attention towards that ear and away from the ignored ear in order to facilitate representing the attended stimuli. The elements of the story to be recalled exist as short-term associations with current goals and stimuli, which occupied attention previously. Though attention systems devoted to novelty detection may inhibit subjects from completely shutting off the ignored ear, shifting attention towards items that correspond with ones already in the short-term store (both in terms of story content, and what side of space they came from) is achieved relatively easily under most conditions, like a Cocktail Party. When subjects are presented with their name in the ignored ear, structures in long-term memory trigger associated behaviors involving shifting attention towards that stimuli, and take precedence over attending to the original story. Regardless of the subjects deliberate attempt to ignore the other ear, stimuli from that ear becomes attentionally relevant when referenced with structures in LTM. This effect implies that even under conditions where attention is occupied, if a certain stimulus is sufficiently associated with behaviors that govern attention, then attention will be shifted in a manner similar to past experience with that stimulus. In other words, the degree of focus required to attend to any external feature is a function of similarity between distractors and items that have been

20 13 attentionally relevant in memory. Ultimately, this study provides an excellent example of how shifting attention causes adjustment and reshaping of associated memories and behaviors allocated to short-term storage. Right Ear Advantage in Dichotic Listening Beyond the perceptual and behavioral biases that arise due to memory associated with number line or one s name, there exists a number of basal spatial attentional biases. Attentional biases that are not specifically due to past experience serve as an excellent tool for investigating biasing that are derived from experience. While the bias identified via the Cocktail Party Effect clearly arises due to associations in LTM, subsequent studies in dichotic listening have demonstrated basal perceptual biases that are conceivably independent of LTM associations. One such phenomenon known as right ear advantage has demonstrated a basal right ear bias for verbal stimuli (Studdert-Kennedy & Shankweiler, 1970). When presented with conflicting consonant vowel sounds (CVs i.e. Ka and Da ) dichotically, subjects perceive the stimuli only in the right ear approximately 30-50% more often than in the left ear. A general explanation for this effect can be found in the left hemisphere s aptitude for speech sounds in conjunction with the relative dominance of its inputs from the contralateral (right) ear. Experimental Goals and Hypotheses In fluid cognition from moment to moment, working memory exists as a conjunction between attention to current stimuli, and items held in STM. It is clear that

21 goal-related stimuli, such as numeric items in SNARC experiments, affect behavior. 14 What is less clear is the role of STM representation itself on cognition and perception, independent of task relevance. It is well established that items in STM can trigger LTM associations that may subsequently bias cognition. As the Cowen model characterizes STM and attention as arising in an activated subset of relevant LTM structures, it is then plausible that task-irrelevant items in STM may affect perception and performance on that task. More specifically; task irrelevant numeric items in STM could activate a left/right bias in spatial auditory-attention, via the competitive engagement of LTM structures triggered by both numeric content in STM, and the spatial-attention task. In the following study, dichotic listening and numeric STM loads were utilized to investigate the possibility of automatic top-down modulation of spatial auditory attention by task-irrelevant numerical items in STM. Subjects were presented with numbers in the auditory modality, then subsequently exposed to four pairs of conflicting dichotic CVs. Numbers in STM came in sets of two or four and were classed by a number of features including overall magnitude, order, and magnitude of difference. Upon presentation of dichotic CVs, subjects were asked to report which word sound they perceived; potential answers consisted of Da, Ga, Ka, and Ta. CV sound files were manipulated in order to ensure fusion into a single percept upon presentation to subjects. Following exposure to dichotic CVs, subjects were presented with a visual Recall cue prompting recall of the numbers presented at the outset of the trial. Recall ensured that numeric content was held in STM during exposure to and discrimination of CVs. Effects on spatial attention were analyzed by calculating laterality indices (LIs) for each subject. LIs were calculated

22 by dividing the difference between the number of left and right ear responses by the 15 total number, thus an LI of -1 would correspond to a subject perceiving only sounds in the left ear, while an LI of 0 would indicate an equal proportion of left and right ear responses. By calculating LIs across different numeric conditions (i.e. low vs. high magnitude numbers), the differential effects of numeric contents in STM on auditory-spatial attention were documented.

23 16 General Methods Subjects A total of 57 healthy Tulane University undergraduates were tested across four Experiments (Mean age = 19.8 ± 3.8; 30 women and 27 men). An overview of the four experiments with subject numbers is shown in Table 1. Pretest After providing demographic data and informed consent, subjects participated in brief surveys indexing musical experience, handedness, and cognitive failures. Subjects then underwent audiometric testing to probe for hearing loss. Subjects with hearing loss greater than 25dBs were to be excluded from the study. Stimuli and Procedures A schematic of a trial is shown in Figure 2. In each trial subjects were first presented with an auditory STM load of numbers. Then they heard four pairs of dichotic consonant-vowel sounds (CVs); one CV was presented to the left ear and one CV was delivered to the right ear. Lastly, subjects saw a visual prompt to verbally recall the memorized numbers. Load varied from 0 to 4 items depending on the experimental conditions. Subjects responded to 4 CVs/trial. Subjects were exposed to 2 to 4 blocks containing 15 to 18 trials lasting 26 +/- 4s each with load, and 18 seconds without load. No block exceeded 10 minutes in duration.

24 17 Within a block STM loads consisted of either number ranges 1-9 or Load size was consistent within blocks excluding experiment 4, which aimed to analyze the effect of load size within block. Possible features contained in a given STM load included number magnitude (magnitude - low, mid, high), interval size between the numbers (subrange narrow, moderate, wide), and the order in which they were presented (order increasing/decreasing). Auditory-number stimuli consisted of numbers 1 through 93 produced by an adult male voice (<1000 ms duration, 60 db nhl). Auditory-word stimuli consisted of consonant-vowels (CVs) also produced by an adult male voice, which differed in the initial stop consonant (/da/,/ga/,/ka/,/ta/; 250 ms duration, ~60 db nhl). CVs were presented to both ears in 12 possible conflicting combinations. Subjects were given 4 seconds to respond to each CV pair using a handheld response pad with four buttons; each one corresponding to one of the four possible CVs. Responses were used to calculate an index of subjects left/right ear bias similar to previous work with these stimuli (Yurgil & Golob, 2010). Following CV presentation, subjects were presented with a visual Recall cue - instructing them to verbally reproduce the numeric STM load from that trial. Vocal responses were recorded to check recall accuracy. Trials with failed recall were excluded. Subjects recall recordings and data from response pad inputs were subsequently analyzed. All experiments took place in a sound attenuated, and electrically shielded booth. Subjects were seated comfortably in front of a computer monitor wearing insert headphones, and holding a four-button response pad.

25 Data Analysis 18 Laterality indices (LI) for each condition were calculated for each subjects using the formula (# trials right ear - # trial left ear)/(# trials right ear + left ear). Previous studies have reported basal LI.2 to.5 (indicating subjects reported hearing stimuli 30%- 50% more often in their right ear, than their left) (Yurgil & Golob, 2010). Data were analyzed using analysis of variance (ANOVA) tests using factors of short-term memory number magnitude (magnitude - low, medium, high), number of digits in STM (load) interval size between the numbers in STM, termed sub-range (narrow, moderate, wide), and the order in which they were presented (order increasing/decreasing). Greenhous-Geisser corrections and Helmert contrasts were performed as indicated within the results of each experiment.

26 19 Results Experiment 1 Magnitude and Load Goal: Experiment 1 sought to understand the effect of representing low, mixed, or high numbers in short-term memory during dichotic listening, in terms of shifting auditory spatial attention. Methods Numbers in short-term memory on each trial consisted of low (1-4), mixed (1-9), or high (6-9) magnitudes. In each block of trials memory load consisted of either two or four numbers. Blocks were administered in counterbalanced order. Behavioral Results Average laterality index as a function of magnitude and load are shown in Figure 3. A 3 (magnitude: low, medium, high) x 2 (load: 2, 4 items) ANOVA test showed a significant main effect of magnitude on LI (F (2,34) = 32.8; p <.001). As shown in Figure 3 the main effect showed that low numbers had the smallest LI, high numbers had a larger LI, and mixed numbers had the largest LI. There was no significant effect of load, and the magnitude x load interaction was also not significant. Occasionally subjects responded to dichotic CVs with a CV that was not present in either ear, which was categorized as an other response. In terms of percentage other responses, a magnitude (3) x load (2) ANOVA test showed a trend for an effect

27 20 magnitude (F (2,34) = 3.4; p <.06). A Helmert contrast showed a significantly smaller LI for large relative to small and medium number magnitudes (p<.03). The proportion of other responses was significantly increased when subjects were representing small numbers in memory compared with mixed or high number loads as seen in Figure 4. Discussion Results indicate a clear interaction between items held in short-term memory and spatial perception of auditory events. Though the left and right number trials did show consistently opposite perceptual biasing effects, the mixed number trials showed a substantial increase in REA under both low and high load conditions. These results indicate that with consistent load size, low and high magnitude number sets evoke an auditory spatial bias that corresponds to a mental number line based processing model. Results from mixed number loads appear to induce a right ear bias. Interestingly, this increased LI does not differ significantly from LI values found when subjects were instructed specifically to attend to the right ear, as reported in previous experiments utilizing the same stimuli (Yurgil & Golob, 2010). One notable difference between low/high magnitude trials, and mixed magnitude trials, was the subrange of numbers appearing in a trial. Small and large magnitude sets were limited to numbers less than or greater than 5, and thusly low load pairs had a difference no greater than 3. Mixed magnitude sets could contain numbers anywhere from 1 to 9, and consequently had a significantly higher average distance between them. Subsequent

28 experiments were performed in order to isolate effects of sub-range and magnitude 21 separately. The process of representing and maintaining memory for items corresponding to both sides of a mental number differs fundamentally from remembering items corresponding only to one side. When representing multiple items centered around one location on a number line, the spatial bias associated with that memory acts as an associative cue for recalling those numbers. When items corresponding to both sides of space are represented, employing a spatial association with those items does not substantially simplify the features of the representation. In the case that spatial associations are no longer a useful cue for recall, spatial biases associated with numbers would potentially interfere with auditory spatial attention in a manner that yields to the basal REA seen in with phonemes. Beyond interactions with LI, numerosity seems to have a significant effect of perception of phonemes generally, evidenced for by the significant increase in other responses seen with low numbers. This result suggests that representations of low numbers affect word perception differentially than high numbers, and may interfere with representations of perceived phonemes in a way that higher numbers do not. Ultimately, the marked increase in LI associated with mixed magnitude sets is what prompted subsequent examination of the effects of number sub-range on spatial auditory attention.

29 Experiment 2 Magnitude and Sub-range 22 Goal: Experiment 2 sought to characterize the effect of sub-range on LI, as a putative explanation for the mixed results seen in experiment 1. Experiment 2 also exposed subjects to blocks containing load size of 1 item, in order to further characterize load effects. Methods Blocks that aimed to analyze mixed number effects utilized near (difference of 1-2), moderate (difference of 5), or wide (difference of 7 or 8) sub-ranges, as well as increasing and decreasing order pairs of numbers. Blocks designed to investigate load effects of 1 number/trial utilized numbers 1-9 similarly to in experiment 1. Overall range remained the same (1 to 9) in both types of blocks. Behavioral Results An ANOVA test examined LI with factors of sub-range (narrow, moderate, wide) and order (increasing, decreasing). There were no main effects of sub-range (Figure 5) or order (Figure 6). There was a significant sub-range (3) x order (2) interaction (F (2,32) = ; p <.01) (Figure 7). When at narrow and moderate sub-ranges LI was larger for increasing pairs; this relationship was reversed at the wide sub-range. A one-way ANOVA examined LI in the 1-item load condition using the factor magnitude (3), but unlike experiment 1 there was no significant effect of magnitude. Despite the lack of consistent relation between LI and magnitude, numbers around the midline did lead to a significant decrease in LI relative to low or high numbers as seen in Figure 8. Analysis of other responses using a one-way magnitude (3) ANOVA test

30 showed a significant effect of magnitude (F (2,32) = 5.628; p <.02). The proportion of 23 other responses decreased linearly with increased magnitude (F (2,32) = ; p <.001), as seen in Figure 9. Discussion Experiment 2 sought to further elucidate the effects of sub-range between numbers. In experiment 1, mixed number pairs were associated with an increase in LI, and mixed number sets had a larger sub-range than low or high number sets. Thus, an increased sub-range between numbers in STM was one putative contributor to the result seen with mixed numbers in experiment 1. While no significant change in LI was seen across sub-range or order of presentation, sub-range with respect to order did produce significant results. At narrow and moderate sub-range, auditory spatial attention seems to relate to directionality of the number set, with increasing pairs drawing attention further right. This effect of directionality appears to breakdown when numbers are farther apart on the number line, as wide sub-range pairs showed an opposite association, indicating a decreased saliency of the numbers directionality. One possible explanation for the inversion of laterality effects seen between moderate and high sub-range numbers is differential levels of association between the two numbers (as a function of how close together they are). Subranges and averages of number sets which fall closer together on the number line are easier to calculate and thus easier to associate with a single spatial location on that number line. Once the distance between the numbers is sufficiently large, encoding them

31 for retrieval through mnemonic association with a specific location on the mental 24 number line requires the implementation of more complex systems (Stanescu-Cosson, 2000). This explanation is in line with a processing model by which stimuli are encoded in a goal-oriented manner, where spatial cueing of numeric memory via association with a number line would putatively aid recall of near sub-ranges but be a less effective mnemonic cue for wide-range pairs. (Galfano, Rusconi, & Umiltà, 2006). While this data provides some information about the effects of sub-range alone, the implications of number magnitude on these results may not be fully accounted for as wide sub-range sets were consistently mapped onto both extremes of the number line (i.e. 1,9 or 9,1 ), while narrow sub-range sets could correspond to both the left and right side of the mental number line (i.e. 2,3 or 8,9 ). Due to this edging effect of wide sub-range numbers, subsequent experiments sought to control for both magnitude and sub-range simultaneously. Experiment 3 Magnitude, and Sub-range Expanded Goal: Experiment 3 analyzed the effects of both magnitude and sub-range of numbers in STM, this time using a range of numbers spanning from 6 to 93 (as opposed to 1-9). Methods Numeric load consisted of narrow (mean difference of 12.5), moderate (mean difference of 25), and widely spaced (mean difference of 37.5) pairs of numbers. Number pairs were again classed as low magnitude (average = 25), medium magnitude (average =

32 50) and high magnitude (average = 75) sets. Order of presentation was controlled for, 25 with blocks containing an equal number of increasing/decreasing pairs/feature. This protocol was designed specifically in order to investigate the effects of sub-range and magnitude independently from one another unlike experiment 2 in which wide subrange pairs necessarily corresponded to low and high magnitude portions of the number line. (Note: 3 subjects not included because of missing data due to errors on trials with a specific feature) Behavioral Results An ANOVA tested magnitude (3) x sub-range (3) x order (2) showed a main effect of magnitude (F (2,32) = 7.956; p <.01). The magnitude effect demonstrated a leftward shift in LI when subjects were remembering high magnitude numbers, and a strong REA shown for small and medium magnitude numbers (Figure 10). There was a main effect of order ANOVA (F (1,16) = ; p <.000), indicating greater LI for decreasing sets of numbers (Figure 11). The main effect of sub-range and all interactions were not significant, although there was trend towards a magnitude x order interaction (F (1,16) = 3.162; p <.07) (Figure 12). The absence of a sub-range x order interaction in experiment 3 can be contrasted with the significant sub-range x order interaction in experiment 2 (cf. Figure 7 and Figure 13). Analysis of other responses used a one-way ANOVA for magnitude, and the percent of other responses did not differ across the three number magnitudes.

33 Discussion 26 Experiment 3 sought to elucidate the effects of both sub-range and magnitude, without sacrificing information about either. In experiment 1, mixed number pairs were associated with an increase in LI, and mixed number sets had a larger sub-range than low or high number sets. Experiment 2 demonstrated that the mixed number results seen in experiment 1 could not be accounted for by the increase in sub-range alone. In experiment 3, main effects of sub-range were consistent with experiment 2. Despite both experiment 2 and 3 showing no effects of sub-range, Figure 13 shows that experiment 3 fails to show 2-way 2 (order: increasing, decreasing) x 3 (sub-range: narrow, moderate, wide) interaction. While clear sub-range effects were not present at any magnitude in experiment 3, the effect of magnitude itself differed greatly from previous experiments. Furthermore, order effects were consistent with wide sub-ranges in experiment 2, but not near or medium sub-ranges. Taken together, these results indicate that order effects are inversed upon expansion of their sub-range beyond subjects average operation span which is further evidence for differential encoding and processing of sub-ranges, which are close enough to count. In terms of percentage other responses, the significant increase in percentage of other responses in association with low numbers seen in experiments 1 and 2 was absent. Worth noting, is that the first two experiments utilized a much narrower overall range.

34 Experiment 4 Magnitude, and Load Combined 27 Goal: Experiment 4 was deigned in order to provide higher resolution investigation into load effects, as well as the effect of intermixed load size within a single block. Excluding the intermixing of load size within block, and the additional load types of 1 item, 3 items, and no load, experiment 4 was identical to experiment 1. Methods In experiment 4, numeric load consisted of low (1-4), mixed (1-9), or high magnitude (6-9) sets of anywhere from 0 to 4 members, depending on load. Experiment 4 was performed in order to investigate load effects with higher resolution, which was originally prompted by the unexpected results found in experiment 2, when subjects had only 1 number in STM. All trial types were intermixed within a block a notable difference from experiments 1 and 2, where each block consisted entirely of 1 load size. Behavioral Results A two-way ANOVA using 3 (magnitude) x 4 (load: 1, 2, 3, 4 items) had a main effect of magnitude (F (2,38) = ; p <.001) and a significant magnitude x load interaction (F (6,114) = 6.752; p <.001). Magnitude results are shown in Figure 14, and had greater LI for low magnitude numbers relative to medium and high. The magnitude x load interaction indicated a complex pattern of results (Figure 15). The association between LI and load was systematic at low and high magnitudes. At low magnitude numbers load increase led to decreases in LI, particularly form 1 item to 2-4 items.

35 Conversely, at high magnitudes increases in load led to increases in LI. For medium 28 magnitudes load and LI did not have a consistent association. A second analysis compared no memory load (0) to having a STM memory load (1-4 items) using a Helmert contrast (Figure 16) showed that the 0 item condition had a significantly smaller LI relative to the combined 4 load (F (1,19) = 6.566; p <.02). Discussion Experiment 4 was performed in order to better understand results showing differences in REA when subjects held 1 number in STM vs. 2 or 4 numbers. Beyond the effect of load magnitude within a specific trial, intermixing load sizes appears to have had an additional effect that ultimately yields magnitude results similar to both experiment 3 and the 1 number condition from experiment 2. In addition, the marked decrease in LI associated with the no load condition could be due to a number of mechanisms, as previous studies have associated increases in REA with increased memory load (Mondor & Bryden, 1992), as well as the fact that the absence of load would also eliminate the effect of verbal cuing on speech perception, by eliminating the dual-task nature of the experiment on those trials (Lamers & Roelofs, 2011). Magnitude results were consistent with experiment 3 in the low and high magnitude conditions, however; shifts in LI were revered in the medium magnitude condition. Similar to what was seen in experiment 3, experiment 4 showed no significant increase in other responses seen with low magnitude numbers. This result is significantly different from other responses found in experiment 1, despite the fact that

36 in both experiments, subjects were sensitized to the same range of numbers. This may 29 imply that differential encoding mechanisms are utilized when load size is variable, similar to when operating in the larger number span of experiment 3. In addition, load size itself is a component of recall, as evidenced by recall errors in which subjects provided the correct numerals, but did not produce the right amount. Comparing main results of experiments 1 through 4 Table 2 contains a summary of results by experiment. Figure 17 contains a summary of magnitude results by experiment. The different response patterns in experiments 1 through 4 will be contrasted in the general discussion. A summary of percentage other responses x experiment can be seen in Figure 18.

37 30 Discussion The overall goal of this study was to investigate if numeric items in STM may bias auditory spatial attention, with smaller numbers biasing attention to the left and larger numbers biasing attention towards the right. Experiment 1 showed a decrease in LI with small numbers, thus providing support for the original hypothesis. Experiment 2 tested the hypothesis that the large LI in experiment 1 with mixed trials was due to a greater sub-range in mixed trials. Results in experiment 2 did not support this hypothesis because that there were no significant effects of sub-range size. Experiment 3 was similar to Experiment 1 but used a larger range of numbers. Results on experiment 3 demonstrated a clear reversal in the effect of number magnitude on LI by showing a smaller LI in association with high magnitude numbers. Experiment 4 was performed to investigate the influence of short-term memory load ranging from 0 to 4 single digit numbers. Results were consistent with experiment 3. Experiment 4 did not demonstrate any consistent relation between LI and load size, but the effect of intermixing load types within block may still influence how magnitude affects LI. In the following section, the results of experiment 1 and 2 will be contrasted with those from 3 and 4 to characterize some key differences. Experiments 1 and 2 supported our initial hypothesis but experiments 3 and 4 examined a variety of other conditions and showed that the effects of numbers in short-term proved more complex than previously conceived. On a basic level, the hypothesis that numeric information in STM could bias auditory spatial attention was supported in all of the experiment, however; the manner in

38 which this bias occurs is dependent upon a variety of factors. In experiment 1 low 31 magnitude numbers reduced LI, while in experiments 3 and 4 low numbers increased LI. Experiment 2 utilized very similar protocols to experiment 1, and found no main effects of sub-range or order with these stimuli. Experiment 3 had substantially different attentional shifts in relation to number sub-range and order. Ascending presentation order lead to a decrease in LI in in experiment 3, while in experiment 2 this only occurred in the high sub-range condition. Ultimately, two distinct response patterns to numeric stimuli were evident in experiments 1 and 2 compared with 3 and 4. In experiments 1 and 2 magnitude effects reflect smaller LI s with low magnitudes. In 3 and 4 magnitude effects show smaller LI s for high magnitudes, and LI is shifted oppositely with respect to order at all sub-ranges. Additional effects on percentage other differed between experiments in a manner that suggests different patterns of interaction with language processing based on characteristics of items in STM. In the next section these key differences between experimental conditions will be related to cognitive systems devoted to both exact/focused and approximate/distributed attention that together are devoted to calculation and representation. The impact of these two systems on number processing will then be utilized as a possible explanation for the manner in which magnitude responses in 3 and 4 differ significantly from responses earlier.