Response processing during visual search in normal aging: the need for more time to prevent cross talk between spatial attention and manual response selection

RESPONSE PROCESSING DURING VISUAL SEARCH IN NORMAL AGING: THE NEED FOR MORE TIME TO PREVENT CROSS TALK BETWEEN SPATIAL ATTENTION AND MANUAL RESPONSE SELECTION Authors: Elena Amenedoa∗, Laura Lorenzo-Lopezb, Paula Pazo-Alvareza. This is the peer reviewed version of the following article: Amenedo E., Lorenzo-López L., PazoÁlvarez P. (2012). Response processing during visual search in normal aging: The need for more time to prevent cross talk between spatial attention and manual response selection. Biological Psychology, 91: 201-211. doi: 10.1016/j.biopsycho.2012.06.004 This article may be used for non-commercial purposes in accordance with Elsevier and conditions for use of self-archived versions. Post-print (final draft post-refereeing) 2 Response processing during visual search in normal aging: the need for more time to prevent cross talk between spatial attention and manual response selection Authors: Elena Amenedoa∗, Laura Lorenzo-Lopezb, Paula Pazo-Alvareza. a Department of Clinical Psychology and Psychobiology, Faculty of Psychology, University of Santiago de Compostela, Spain b Gerontology Research Group, Department of Medicine, Faculty of Health Sciences, University of A Coruña, Spain ∗ Corresponding author at: Campus Sur S/N, 15782, Santiago de Compostela, Galicia, Spain. Tel.: +34 981 563 100x13916; fax: +34 981 528 071. E-mail address: elena.amen[email protected] (E. Amenedo). Post-print (final draft post-refereeing) 3 Abstract It is still not well known whether the age-related behavioural slowing observed during visual search is due to changes in the allocation of attention, in response activation patterns, or to a combination of both. To help in clarifying it, attention-related (N2 posterior contralateral; N2pc, and N2 central contralateral; N2cc) and response-related (Motor Potential; MP, and Reafferent Potential; RAP) event-related potentials (ERPs) were obtained in healthy young and older participants executing a visual search task. Age was associated with N2pc and N2cc longer latencies, earlier MP onsets and longer MP rise times. Lower N2pc, higher MP and lower RAP amplitudes were also observed. Results suggest that older participants need more time to allocate spatial attention onto the target (N2pc) and to prevent cross talk between response selection and attention direction (N2cc), and that they are slower and need higher cortical activation when preparing and executing correctly selected responses (MP). Post-print (final draft post-refereeing) 4 Introduction The visual search paradigm has been extensively used in laboratory studies to examine the basic properties of human visual selective attention (Luck and Ford, 1998; Luck and Hillyard, 1994a,b). In this paradigm, participants search for a predefined target stimulus that is randomly presented in arrays containing a variable number of bilateral distractor stimuli. In a typical single feature visual search task, like that employed in the present study, the target differs from distractors in a physical attribute (i.e. orientation change), and the participants are required to indicate whether the target is present or absent in each array of stimuli by pressing a pre-assigned response button. Previous electrophysiological studies of visual search in young humans have revealed a posterior negative-going ERP component that appears contralateral to the visual hemifield in which the target is located 200–300 ms after the onset of a bilateral stimulus array (Eimer, 1996; Luck and Hillyard, 1994a,b; Woodman and Luck, 1999). This component was first described as the N2pc (N2-posterior-contralateral) to indicate its polarity, latency range, and scalp distribution (Luck and Hillyard, 1994a), and it has been well-validated as an electrophysiological correlate of the focusing of visuospatial attention onto a target stimulus during visual search (Eimer, 1996; Luck and Hillyard, 1994a,b; Luck and Ford, 1998; Luck et al., 1997; Woodman and Luck, 1999, 2003). The N2pc is recorded with maximum amplitude over parietal-occipital electrode sites contralateral to the attended item (Eimer, 1996; Luck and Hillyard, 1994a,b), and it is primarily generated in lateral occipital–temporal regions (Hopf et al., 2000; Lorenzo-Lopez et al., 2011). There is considerable evidence that older adults are behaviourally slower in tasks involving visual search (Hommel et al., 2004; Madden and Whiting, 2004; McDowd and Shaw, 2000). However, several questions remain unsolved regarding the interpretation of these findings. Mainly, it is still not well established if they represent a slowing related to deficiencies in the allocation of attention onto the target, to deficient response activation patterns, or to a combination of them. During the period between the onset of the target in the visual field and the emission of a correct response, several processes related to both attention and motor mechanisms take place. Specifically, allocation of attention onto the target and motor selection and preparation processes both contribute to the reaction time (RT) recorded during the execution of the task, and any or both of them may be affected by age. Recent studies have helped in solving the first of the above questions by providing the first evidence that normal aging significantly affects the allocation of visuospatial attention itself during single feature visual search (N2pc component, Lorenzo-Lopez et al., 2008a) by delaying the time needed to allocate spatial attention shifts onto the target and reducing the attention resources deployed to it. More recently, these age-related changes in N2pc parameters have been associated with a significant hypoactivation of its occipital–temporal neural sources that is more marked in the right hemisphere (Lorenzo-López et al., 2011). However, it is still not possible to conclude whether the age-related behavioural changes observed during visual search tasks are also due to changes in the processes related to the selection and preparation of motor responses to the correct detection of the target. In recent years, an ERP component related to the prevention of cross talk between attention direction and manual response selection has been described (Oostenveld et al., 2001; Praamstra, Post-print (final draft post-refereeing) 5 2006; Praamstra and Oostenveld, 2003). This component, named N2cc (N2 central-contralateral) is recorded at central electrodes contralateral to the side of presentation of the target stimuli during visuospatial attention tasks, and it presents the same polarity and latency as the above-described N2pc component. Based on results from single cell recordings on non-human primates (Crammond and Kalaska, 2000; Shen and Alexander, 1997; Wise et al., 1996, 1997), from neuroimaging data (Connolly et al., 2000; Dassonville et al., 2001) and from ERP current source analyses (Praamstra, 2006; Praamstra and Oostenveld, 2003), N2cc has been proposed to reflect activation of the dorsal premotor cortex that is invoked to prevent the selection of a manual response depending on the location of the target stimulus in the visual field, and hence to ensure that the response selection is not biased by the direction of spatial attention (Praamstra and Oostenveld, 2003). More recently, the fact that this component is reduced in amplitude when advance information of target location is provided by a cue has added support to its functional interpretation (Praamstra, 2006). In visual search tasks, the target stimulus appears randomly interspersed with distractor stimuli in the right or left visual field in each trial. In these tasks, it is frequent to assign one hand to respond to the presence of the target and the other to its absence independently of its position in the visual field. Under such circumstances, responses executed with the hand ipsilateral to the visual hemifield where the target appears are facilitated. Thus the ability to select the correct response hand depending on the task instructions and not on the position of the target in the right or left visual field can be considered an executive function that helps inhibiting the vulnerability of response choice to be influenced by the direction of attention (Praamstra, 2006). In this regard, measuring the N2cc component makes it possible to explore the effects of aging on this executive function during visual search. Moreover, although the N2pc component has been well characterized in older samples (Lorenzo-López et al., 2008a, 2011), it was also examined in the present study in order to compare age-related effects on stimulus-related attention ERPs with those on response-related attention ERPs (Praamstra and Oostenveld, 2003). The observation of slower RTs in older populations is not limited to visual search tasks, but is also present in almost every study on aging. In fact, the ubiquity of this age-related effect has lead to influential theories of cognitive aging and to classical debates in this research context (Salthouse, 1996; McDowd and Shaw, 2000). Some recent ERP evidence has indicated that cortical dysregulation in motor activation patterns may underlie response slowing with age in choice reaction tasks (Falkenstein et al., 2006; Yordanova et al., 2004), although to our knowledge the mechanisms underlying the behavioural slowing during visual search tasks have not been examined yet. ERP research has shown that any task requiring an overt response to a stimulus activates electrocortical motor mechanisms immediately after or even in parallel with stimulus processing. Before a correct response is overtly executed, several ERP components associated to movement selection and preparation processes are generated in motor cortical areas. One of the most prominent ERPs associated with active limb movements is the Readiness Potential (RP; Deecke et al., 1969; Böcker et al., 1994; Shibasaki and Hallett, 2006). The RP develops through different components among which the most prominent for finger movements is a maximum negative deflection appearing just before the observable movement onset at contralateral central electrodes (Motor Potential, MP) followed by a positive deflection that is maximal around the overt movement execution (Reafferent Potential, RAP). The MP indexes motor generation of a selected response at contralateral motor cortex (Böcker et al., 1994). The functional significance of the Post-print (final draft post-refereeing) 6 RAP, although less well known, has been related to sensorymotor integration processes during response execution (Bötzel et al., 1997; Seiss et al., 2002; Szurhaj et al., 2006). Thus, measuring the MP and the RAP components elicited during correct responses to the target stimulus in a visual search task should allow examining motor generation and sensory-motor integration processes that take place between target onset and response execution, and their age-related changes. Finally, in order to examine the relations between the recorded response-related electrophysiological activity and the behavioural performance during the task independently of the effects of age, partial correlations (Baron and Kenny, 1986; Perry et al., 2009; Volkow et al., 1998) were computed between the N2cc and RP (MP and RAP) parameters (latency and amplitude values) and the execution data (RTs and hit rates). Methods Participants The original sample consisted of 17 young (10 females, 19.6 ± 1.9 years, range 18–24) and 22 older adults (11 females, 68.5 ± 6 years, range 60–84). However, four young (all female) and five older (4 female) participants were discarded from the original sample because of excessively noisy EEG motor-related activity that led to low signal-to-noise ratios for ERP-averaging purposes. Finally, data from 13 young (6 females, 20.08 ± 2.02 years, range 18–24), and 17 older adults (8 females, 68.29 ± 6.54 years, range 60–84) were tested. All participants were healthy well-functioning without a history of neurological or psychiatric disorders, had normal or corrected-to-normal visual acuity, reported normal colour vision, and were righthanded (Oldfield, 1971). None of the older participants had received a diagnosis of glaucoma or cataracts, and they performed the Mini-Mental State Examination (MMSE) (Folstein et al., 1975) showing normal scores (>28). Informed consent was obtained from all subjects and they were paid for their participation in the experiment. Stimuli and experimental procedure Recordings were made in an electrically shielded and sound attenuated room. Subjects sat in a comfortable armchair at 100 cm viewing distance from a computer screen with a black background and a continuously visible fixation white cross. They were instructed to maintain central fixation on this cross while they performed a visual search task consisting in detecting a target stimulus presented among an array of distractors that differed from them in its orientation (Fig. 1). On each trial a multi-element search array was presented composed of eight bars subtending a visual angle of 0.3◦ × 0.9◦, which were located at random positions within an imaginary rectangle of 9.2◦ × 6.9◦ of visual angle around fixation cross. There were always four bars in each visual hemifield. Three types of search arrays were randomly presented: homogeneous arrays, arrays containing a target defined by a change in orientation of one of the bars, and arrays containing a non-target defined by a change in colour of one of the bars. Homogeneous arrays (p = .6) consisted of eight blue-horizontal (RGB 0, 0, 255) identical bars. Orientation target arrays (p = .2) consisted of seven blue-horizontal bars and one blue-vertical bar. Colour non-target arrays (p = .2) consisted of seven blue-horizontal bars and one redhorizontal (RGB 255, 0, 0) bar. The orientation target and the colour non-target were equally likely to appear in the right or left visual hemifield and their location was unpredictable. Each search array was presented for 750 ms, followed by a variable inter-trial interval of 900– 1100 ms during which only fixation cross was present. The same feature (orientation) defined the Post-print (final draft post-refereeing) 7 target across all trials and the subjects were not informed about the appearance of the colour nontarget. All the stimuli and search arrays were created, presented, and controlled using the Presentation software application (Neurobehavioral Systems, Inc., version 0.76). The experimental session was divided into six blocks of trials, and several training trials were run before testing to ensure a good level of performance in both age groups. Each block consisted of at least 10 arrays containing an orientation target and 10 arrays containing a colour non-target presented to each hemifield, and at least 80 homogeneous arrays, to a possible maximum of 250 arrays in total. Participants were required to indicate as rapidly and accurately as possible whether the orientation target (a vertical bar) was present or absent in each search array by pressing a button with one hand for target present trials and another button with the other hand for targetabsent trials. Thus, the colour non-target arrays required the same response hand as the homogeneous arrays (target-absent). Response buttons were counterbalanced across subjects. As a result, in 16 participants (7 young and 9 older) the right hand was assigned to respond to target present arrays and the left hand to target-absent arrays, and in 14 participants (6 young and 8 older) the left hand was assigned to target-present arrays and the right hand to target-absent arrays. ERP recordings The electroencephalogram (EEG) was recorded with a NeuroScan system using scalp electrocaps (ECI, Inc.) with 30 electrodes placed at FP1, FP2, FPz, Fz, Cz, Pz, POz, Oz, F7, F8, F3, F4, C3, C4, T3, T4, PO3, PO4, FCz, CPz, CP3, CP4, T5, T6, P3, P4, FC3, FC4, O1 and O2 (10/20 International System). All the active electrodes were referred to the nose tip and grounded with an electrode placed at nasion. Vertical and horizontal electrooculogram (EOG) activities were recorded bipolarly from above and below the left eye and from the outer canthi of both eyes. Electrode impedances were kept below 10 k. The EEG signals were continuously amplified (10 K) and digitized at a rate of 500 Hz/channel, and filtered on-line with a band pass of 0.05–100 Hz. Data analysis 1. Behavioural data Reaction times (RTs) were on-line recorded for all subjects to the three types of search arrays in all experimental blocks (for orientation target and colour nontarget arrays, RTs to stimuli appearing in the right or left visual field were separately recorded). Only RT values associated with correct responses were considered for data analyses. Hit rates were calculated as the total percentage of correct responses with RTs no longer than 1100 ms. A common problem with using percentages as response variables is that the distribution of proportions may “pile-up” against 0 or 1, resulting in skewed data (i.e. non-normal distributions). Additionally this may lead to unequal variances. For meeting the assumptions of ANOVA, the original hit rates were firstly converted to proportions and then arcsine root transformed (Osborne, 2002). Mean correct RTs were compared across groups using a repeated measures analysis of variance (ANOVA) with age (young, older) and response hand (right, left) as the between-subjects factors, and search array (homogeneous, orientation right left target, and colour right-left non-target) as the within-subject factor. Original and transformed hit rates were also compared across groups using ANOVAs with age (young, older) and response hand (right, left) as the between-subjects factors. An alpha level of .05 was used for all analyses. Post-print (final draft post-refereeing) 8 2. EEG data All EEG data were off-line processed using Vision Analyzer software (Version 2.0). The EEG was digitally filtered with a 0.1–30 Hz band pass filter. Filtered EEG was segmented in epochs of 900 ms post-stimulus and 100 ms pre-stimulus to obtain attention-related ERPs (N2pc and N2cc, see below), and in epochs of 1000 ms post response and 1000 ms pre-response for the motor-related RP components (MP and RAP, see below). In all segmentation files, epochs exceeding ±100 V and those containing blinks, and horizontal or vertical eye movements were rejected and excluded from averaging, as well as epochs associated with incorrect or no responses. Because in previous research by our group no differential ERP effects related to attentional focusing were observed to colour non-target stimuli (Lorenzo-López et al., 2008a), only EEG segments associated with correct responses to orientation target stimuli were averaged for both attention ERPs (N2pc and N2cc) and motor ERPs (RP). Separate averages were obtained to orientation targets appearing in the right visual field (RVF) and to those appearing in the left visual field (LVF). This resulted in 4 attention-related and 2 response-related waveforms for each participant (see below). 2.1. Attention-related ERP data: N2pc and N2cc components. For the derivation of the N2pc component, difference waveforms for each participant were obtained by subtracting ipsilateral from contralateral ERPs relative to the target location in the right or the left visual field at posterior electrodes PO3 and PO4 respectively (see Lorenzo-López et al., 2008a,b). For the derivation of the N2cc component, difference waveforms for each participant were obtained by subtracting ipsilateral from contralateral ERPs relative to the visual field of attention following Praamstra (2006) formula that reads N2cc = [(C3 − C4)RVF attention + (C4 + C3)LVF attention]/2, where ‘RVF’ refers to the right visual field, and ‘LVF’ to the left visual field. This procedure extracts lateralized potentials collapsing the activity of both hemispheres (see Praamstra, 2006). In the resulting waveforms of both N2pc and N2cc components, mean amplitude values were obtained from 150 to 300 ms for the young adults, and from 200 to 450 ms for the older adults (these latency intervals were selected as the most representative for each group after visual inspection of the respective grand mean waveforms, see Figs. 2 and 3, and Lorenzo-López et al., 2008a,b). In order to test the effects of age on N2pc and N2cc parameters, peak amplitude, mean amplitude, and latency values of these components were entered into separate ANOVAs. For N2pc values, mixed ANOVAs with age (young, older) as the between-subjects factor and electrode (PO3, PO4) as the within-subject factor were run. For N2cc values, one-way ANOVAs with age (young, older) as the between-subjects factor were executed. An alpha level of .05 was used. 2.2. Response-related ERP data: MP and RAP components. RP averages to orientation targets were analysed at relevant electrodes at the contralateral motor areas (C3 for right hand responders, and C4 for left hand responders). Baseline was defined in these ERPs as a 200 ms interval between 1000 and 800 ms before the motor response (button press). Peak latency and amplitude values of the most negative displacement of the RP, the MP, were measured within the interval between stimulus presentation and response execution. Peak latency and amplitude values were also measured for the most positive displacement of the RP, the RAP, after the MP. The onset latency of the MP was calculated as the time when MP amplitude was 15% of its maximal value (cf. Mordkoff and Gianaros, 2000; Schwarzenau et al., 1998). The duration of the MP activation (MP rise time) was measured as the difference between MP peak latency and MP onset latency. MP and RAP peak latency and amplitude values, and MP onset and rise time values were submitted to separate repeated measures ANOVAs with age (young, older) and response hand Post-print (final draft post-refereeing) 9 (right, left) as the between-subjects factors, and target-response compatibility (compatible targetresponse side, incompatible target-response side) as the within-subject factor. An alpha level of .05 was used for all statistical tests. Whenever appropriate, degrees of freedom were corrected by the conservative Greenhouse–Geisser estimate. When necessary, post hoc comparisons were performed using the Bonferroni adjustment for multiple comparisons. 2.3. ERP-behaviour analyses. To examine the relationships between behavioural execution and electrophysiological activity independently of the effects of age, partial correlation analyses controlling for age (Baron and Kenny, 1986; Perry et al., 2009; Volkow et al., 1998) were executed between both RT and hit rates, and the latency and amplitude values measured for N2cc, MP and RAP components. Because no differences between RTs to right and left targets, and no interactions between response hand, age or array type were observed (see results), the mean RT values to both target stimuli were employed to compute partial correlations between N2cc and RT. In the case of motor components (MP and RAP) the amplitude and latency values obtained to compatible target stimuli were used for the correlation analyses. Partial correlations were considered significant at p < .05 (two-tailed). Results Behavioural results As can be appreciated in Table 1, the RTs were significantly slower in the older group for each array type (F(1,26) = 33.69, p < .0001, n2p = .56) irrespective of the hand assigned to respond to the targets (F(1,26) = .38, p = .54 n2p = .01). The effect of the array type was also significant on mean RTs (F(4,104) = 29.29, p < .0001, ε = .43, n2p = .53) showing the longest response times for orientation targets, intermediate for colour non-targets, and the shortest RTs for the homogeneous arrays (see Table 1). No statistically significant interactions were observed between age and response hand (F(1,26) = 3.11, p = .09, n2p = .11), between age and array type (F(4,104) = .58, p = .54, n2p = .02) or between response hand and array type (F(4,104) = .80, p = .44,= n2p.03). Moreover, target-response compatibility effects were not significant on RT values (F(1,28) = .61, p = .44, n2p =.02) in either young (target-response compatible: 500.55 ± 51.61 ms, incompatible: 500.50 ± 49.97 ms) or older participants (target-response compatible: 595.72± 55.61 ms, incompatible: 604.34 ± 42.07 ms). Effects of age (F(1,26) = 3.77, p = .06, n2p = .13) or response hand (F(1,26) = .16, p = .70, n2p =.006) were not statistically significant on hit rates (young right hand: 97.58 ± 2.68%, left hand: 98.55±.36; older right hand: 96.51 ± 3.23%, left hand: 94.50 ± 5.40%). No significant interactions between age and response hand were observed (F(1,26) = 1.28, p = .27 n2p = .05). The results of the ANOVA executed on the arcsine root transformed data revealed significant effects of age that indicated worse performance levels in the older participants (F(1,26) = 4.73, p < .04 n2p = .15). No significant effects of the response hand (F(1,26) = .15, p = .70 n2p = .006) or of the interaction between age and response hand (F(1,26) = 1.26, p = .27, n2p = .05) were observed on these transformed data. Electrophysiological results 1. Attention-related ERPs: N2pc and N2cc components Significant effects of age were observed on both N2pc (Figs. 2 and 4) and N2cc (Figs. 3 and 4) peak latencies (N2pc: F(1,28) = 37.69, p < .0001 n2p = .69; N2cc: F(1,28) = 16.56, p < .0001, n2p Post-print (final draft post-refereeing) 16 References Baron, R.M., Kenny, D.A., 1986. The moderator-mediator variable distinction in social psychological research: conceptual, strategic, and statistical considerations. Journal of Personality and Social Psychology 51, 1173–1182. Bocker, K.B.E., Brunia, C.H.M., Cluitman, P.J.M., 1994. A spatio-temporal dipole model of the readiness potential in humans. I. Finger movement. Electroencephalography and Clinical Neurophysiology 91, 275–285. 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