The Role of Frontal Eye Fields and Posterior Parietal Cortex in Primed Visual Search - Research Paper Example

The Role of Frontal Eye Fields and Posterior Parietal Cortex in Primed Visual Search Student’s Name Institution Abstract Existing studies have already established the presence of a network of brain areas which are triggered when someone carry out a difficult visual search task. The main nodes on the network of brain areas are rPPC and FEF regions. The two regions have been demonstrated to be engaged in the alignment of attention; however, the contingency that the activity of these regions has on each other is not clear. This study sought to investigate the role of FEF and rPPC in primed visual search. The results demonstrate that the FEF and rPPC regions are not only required to conduct a visual search task, but that the two regions work in collaboration to yield normal function. In other words, the study has demonstrated that there is a functional involvement between rPPC and FEF areas. Introduction The involvement of neural networks in visual attention have been widely implied to involve the eye to locations in the visual field and frontoparietal regions meant for guiding attention. The regions that are involved in this include posterior parietal cortex (PPC), supplementary eye fields (SEF), and the frontal eye fields (FEF) of the right hemisphere[Szc10]. One of the important tools for investigating the role of these regions in matters related to allocation and deployment of attention, spatial localization, the identification of distractors and targets, and the stimulus features in the visual search model. Although studies involving functional magnetic resonance imaging (fMRI) have indicated that right PPC (rPPC) and right FEF (rFEF) are co-activated together with visual search tasks, studies involving transcranial magnetic stimulation (TMS) have tried to described how these two regions are involved in visual search tasks. These studies have determined that the two regions are not only co-activated but they are critically involved, and some evidence has pointed out that they play different roles. For instance, the involvement of rPPC only takes place when the target appears in unpredictable or non-primed space, whereas the involvement of rFEF takes places irrespective of the target position[Ell14]. At the moment, it is not whether there is a functional connectivity or not between rPPC and rFEF. Studies have already established that involvement of rFEF in visual search tasks takes place before rPPC, resulting to the idea there that the two regions may be dependent on each other. Lane et al. (2012) have investigated the functional involvement of these areas in priming in complex visual search tasks. They reported that for all the conditions, spatial priming took place, and search times for primed trials were faster as compared to nonprime trials. When the target was at the nonprime location, simulation through any of the three regions increased reaction times as compared to the sham condition. On the other hand, repeating the target location only resulted in an increase in reaction time when simulating through the rFEF. In visual search for colour x colour conjunction stimuli, two colors cannot take the one spatial location. However, Wang et al (1990) reported that in searches for Color x Shape, attention towards the accurate color can be guided by a parallel color module, while the attention towards the correct shape can be guided by a shape module. The two sources of guidance combined, they direct attention toward the target. This aim of this study is to explore the role of FEF and rPPC in primed visual search. The study will expand the findings of Lane et al. (2012). It will build up on the work of Lane et al. to investigate how rFEF and rPCC respond to spatially primed targets using colour x colour conjunction stimuli. Hypothesis: TMS of right PPC, right FEF and lFEF will increase search times in relation to sham conditions. Method The experiment was based on Lane et al. (2012) except that colour x colour conjunction stimuli were used (Figure 1; Wolfe et al., 1990) and only the primed conditions were analysed. Participants Twenty-five participants (9 male), aged between 18 and 45 years (median age 23 years) participated in the experiment. They had normal or corrected-to-normal vision, scoring at least 20/25 binocularly on a Snellen near-field eye chart. In each experimental session, TMS was delivered to a different stimulation site as per Lane et al. (2012). In each session there were 14 participants. Some subjects participated in more than one session. No participants were excluded from the study and all gave informed consent in accordance with the Declaration of Helsinki (World Medical Association, 2013) and with the approval of the Departmental Ethics Advisory Group of the School of Psychological Sciences at the University of Melbourne. Apparatus and Procedure Stimuli were presented on a 21-inch CRT monitor using MATLAB running the Psychophysics Toolbox (Brainard, 1997; Pelli, 1997). Participants sat 57.5 cm from the screen, which was positioned at eye level, and used a combined head and chin rest to view the monitor. The participants viewed the stimuli head on, so that their head and trunk were aligned with the monitor. The testing room was darkened. Each trial commenced with a small, white fixation cross presented on a black background. The cross subtended 0.5° by 0.5° of visual angle, and was displayed at the centre of the screen. The cross was presented for 500 ms, followed immediately by the search array. This display comprised a 10 x 6 virtual array of possible stimulus locations, subtending 32° x 24. In total, there were 12 items per display. If present, the target was a square of which the left side was red and the right side was green. Approximately half the distractors were squares with a left red half and a blue right half. The remaining distractors were squares with blue left halves and green right halves. All the squares subtended 2° x 2° of visual angle. They all had a white midline of width 0.2°. The CIE coordinates were as follows: red = .62, .36; green = .34, .57; blue = .14, .07. Participants were required to determine as quickly as possible whether the target was present or not and indicate their response by pressing one of two keys on a keyboard. The stimulus remained visible until the participant made a response. Participants were requested to maintain fixation on the fixation cross throughout each trial, but eye-movements were not monitored. As in Lane et al. (2012), the trials were presented in blocks of 28 trials, 20 of which would contain a target. In six of these target-present trials, the target was repeated. The TMS pulses were applied in exactly the same manner as occurred in Lane et al. The data was analysed in the same manner as that described in Lane et al. except that, for the sake of simplicity, only the primed condition was analysed. Results For every condition, there were thirty-six primed trials. Figure 1 below show the search array that was used in the experiment. A comparison of search times for each region was done for every participant in every TMS condition. The response time for every target presentation and for TMS and sham reaction times at every site are shown in Figure 2. 0 ms----------------------500ms------[TMS pulse]----1000ms Search array of 12 items Figure 1. A small, white fixation cross was presented for 500 milliseconds, followed by the search array of 12 items that remained present until a button-press response was made. TMS was delivered at 10 Hz for 500 milliseconds from the beginning of array onset. Figure 2: A graph showing the mean Response Time (RT) in the rPPC, IFEF, and rFEF conditions for primed location trials, separately for the sham and TMS conditions. The effects of sham TMS on reaction times was evaluated. This was done using single factor ANOVA and the results were as follows (F1,36 = 0.959; P = .476). This means there were no significant differences. The results of the single factor ANOVA are as shown in Table 1 below. To examine the differences across the three sites, the sham TMS-effects for the primed trials was compared across the sites by the use of independent sample t tests. The results indicated that the sham TMS effect at rPPC was not significantly different to that at IFEF (t24 = -0.15; P = 0.90), or rFEF (t24 = -1.01; P = 0.50) and likewise the sham TMS effect at IFEF was not significantly different to that at rFEF (t24 = -0.97; P = 0.51). The results of the t test are as shown in Table 2 below. Table 1: Single Factor ANOVA Table 2: t-Test: Two-Sample Assuming Unequal Variances Discussion The findings of this study confirmed some of the results obtained by Lane et al. (2012) that rPPC, rFEF, and IFEF are functionally involved in visual search. The main objective of the study was to determine the role of FEF and rPPC in primed visual search. The there was considerable priming effect that was strong to TMS at all regions, in such a way that search time decreased over two subsequent presentations of the target at one and the same location (Lane et al., 2012). Therefore, TMS did not impact the priming effect as such. As a result, it would appear that complicated visual displays are not contributive to the presentation that these sites are involved or not involved in the priming effect as other previous studies had suggested, and using previous performance parameters like eye movements (Ellison, Lane, & Schenk, 2007; Ellison, Rushworth, & Walsh, 2003; ). As a result of the scattered nature of brain’s visual processing, it would appear difficult to locate one region in charge of the priming effect in the course of complicated visual search. On the contrary, the assessment of the involvement of each region in the processing of primed trails is significant evidence in the evaluating the specificity of the roles of these regions and their interaction. As a consequence, the important counterpoint in this study, similar to Lane et al. (2012), was the effect that sham TMS had on reaction times for trials in primed target location. The key finding of the study was that all the three regions were connected in relation to their involvement in primed trials. This is contrary to Lane et al. (2012) who reported that rFEF was not connected to rPPC and IFEF in relation to their involvement in primed trials. TMS over rPPC, rFEF, and IFEF significantly increased search time. As it was hypothesized, rFEF was involved in the search process and as expected TMS of rFEF increased time relative to sham condition, similar to the primed condition of Lane at al. (2012). In addition, the rPPC and IFEF as expected we are involved in the search process with the harder search task, despite the fact that the target location was primed. As mentioned earlier, the data analyzed in this hypothetical experiment was only from the primed condition. As expected, the TMS of both the rPPC and IFEF increased search times relative to sham condition. In other words, in the new search in this study, the TMS increased search times relative to the sham condition for all the three regions as expected. This indicates that all the three regions are critically involved in visual search tasks when the target location more predictable and repeated. On the contrary, in Lane et al. (2012) TMS increased the search times only for rFEF in the primed condition. Considering the relative timing of rFEF, IFEF, and rPPC involvement in conjunction visual search tasks, someone may presume that the communication direction is the reverse[How09]. Studies have already established that FEF are 80 milliseconds earlier in critical involvement of the processing of a visual task than rPPC[Kal081]. This is in line with their alleged roles; that is, FEF are involved in target detection, whereas rPPC is significant for translating the visual information into action. Activations initiated through fMRI indicate that when there is a decrease of electrical activity in rPPC, there is considerable decreased BOLD response in FEF areas than seen when sham tDCS is applied in the same regions[Ell14]. Furthermore, this effect is two-sided[How09]. This information not only indicates that there is a functional involvement between FEF and rPPC regions, but also that rPPCC communicates with both IFEF and rFEF[Kal08]. This study has demonstrated that there is a functional involvement between rPPC and FEF areas. References Szc10: , (Szczepanski, Konen, & Kastner, 2010), Ell14: , (Ellison, et al., 2014), How09: , (Howe, et al., 2009), Kal081: , (Kalla, Muggleton, Juan, Cowey, & Walsh, 2008; Muggleton, Juan, Cowey, & Walsh, 2003), Kal08: , (Morgan, Jackson, van Koningsbruggen, Shapiro, & Linden, 2013), Read More

On the other hand, repeating the target location only resulted in an increase in reaction time when simulating through the rFEF. In visual search for colour x colour conjunction stimuli, two colors cannot take the one spatial location. However, Wang et al (1990) reported that in searches for Color x Shape, attention towards the accurate color can be guided by a parallel color module, while the attention towards the correct shape can be guided by a shape module. The two sources of guidance combined, they direct attention toward the target.

This aim of this study is to explore the role of FEF and rPPC in primed visual search. The study will expand the findings of Lane et al. (2012). It will build up on the work of Lane et al. to investigate how rFEF and rPCC respond to spatially primed targets using colour x colour conjunction stimuli. Hypothesis: TMS of right PPC, right FEF and lFEF will increase search times in relation to sham conditions. Method The experiment was based on Lane et al. (2012) except that colour x colour conjunction stimuli were used (Figure 1; Wolfe et al., 1990) and only the primed conditions were analysed.

Participants Twenty-five participants (9 male), aged between 18 and 45 years (median age 23 years) participated in the experiment. They had normal or corrected-to-normal vision, scoring at least 20/25 binocularly on a Snellen near-field eye chart. In each experimental session, TMS was delivered to a different stimulation site as per Lane et al. (2012). In each session there were 14 participants. Some subjects participated in more than one session. No participants were excluded from the study and all gave informed consent in accordance with the Declaration of Helsinki (World Medical Association, 2013) and with the approval of the Departmental Ethics Advisory Group of the School of Psychological Sciences at the University of Melbourne.

Apparatus and Procedure Stimuli were presented on a 21-inch CRT monitor using MATLAB running the Psychophysics Toolbox (Brainard, 1997; Pelli, 1997). Participants sat 57.5 cm from the screen, which was positioned at eye level, and used a combined head and chin rest to view the monitor. The participants viewed the stimuli head on, so that their head and trunk were aligned with the monitor. The testing room was darkened. Each trial commenced with a small, white fixation cross presented on a black background.

The cross subtended 0.5° by 0.5° of visual angle, and was displayed at the centre of the screen. The cross was presented for 500 ms, followed immediately by the search array. This display comprised a 10 x 6 virtual array of possible stimulus locations, subtending 32° x 24. In total, there were 12 items per display. If present, the target was a square of which the left side was red and the right side was green. Approximately half the distractors were squares with a left red half and a blue right half.

The remaining distractors were squares with blue left halves and green right halves. All the squares subtended 2° x 2° of visual angle. They all had a white midline of width 0.2°. The CIE coordinates were as follows: red = .62, .36; green = .34, .57; blue = .14, .07. Participants were required to determine as quickly as possible whether the target was present or not and indicate their response by pressing one of two keys on a keyboard. The stimulus remained visible until the participant made a response.

Participants were requested to maintain fixation on the fixation cross throughout each trial, but eye-movements were not monitored. As in Lane et al. (2012), the trials were presented in blocks of 28 trials, 20 of which would contain a target. In six of these target-present trials, the target was repeated. The TMS pulses were applied in exactly the same manner as occurred in Lane et al. The data was analysed in the same manner as that described in Lane et al. except that, for the sake of simplicity, only the primed condition was analysed.

Results For every condition, there were thirty-six primed trials.

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