Results

Target detection results

Mean performance was poor (Figure 3.8). The mean value of d prime never exceeded a value of 1, which corresponds to 69% correct for both yes

and no. trials. Performance was particularly poor when measured using the hit- FA rate since the average hit-FA rate was always below zero.

Individual performance was variable. Two of the participants (#1 and #2) responded to targets in both frequency streams, irrespective of the task instructions suggesting that they did not perform the task correctly. Across all conditions the average false alarm rate (56%) was actually greater than the average hit rate (35%). One interpretation of this low performance is that the training was not adequate. Another interpretation is that the task was just too difficult and no amount of training would benefit performance. It is interesting to note that despite the best performance, participant #3 reported some

difficulty following the task instructions. In particular, after the experiment, the participant reported that the word ‘high’ in the instruction ‘high sounds’ could be interpreted as high probability of occurrence, referring to the majority stream, instead of high frequency. Despite this comment, this was not reflected in the pattern of performance, but it was taken into account when modifying later versions of the task.

Figure 3.8 (A) hit rate-FA rate in % and (B) d prime of target discrimination performance for the four ‘attend’ conditions in pilot study 1. Bars indicate average across participants, symbols indicate individual performance. H: high-frequency-majority stimulus. L: Low-frequency- majority stimulus.

Spatial specificity of frequency-sensitive responses

The distribution of low- and high-frequency-sensitive responses that are shown by the incidence map broadly agrees with the scheme reported by Talavage et al. (2000). Figure 3.9 shows this distribution in four horizontal slices through HG. High-frequency-sensitive regions (shown in red) were located more medially in HG. Peaks corresponding to those reported by Talavage et al. (2000) are numbered 1, 2 and 4 in Figure 3.9. Low-frequency-

sensitive regions (shown in blue) were located more laterally in HG. Peaks corresponding to those reported by Talavage et al. (2000) are numbered 1 in Figure 3.9. The incidence map did not show much evidence for overlap between participants. It is not clear from the incidence map whether or not adjacent voxels represent a frequency-sensitive response from the same or from different participants. The impression gained from visual inspection of the individual results is that individuals activate small and coherent clusters of voxels rather than a mosaic of isolated voxels. However the pattern of responses is driven by participant #3, and to a lesser extent by participant #1. Participant #2 did not show any low or high frequency-sensitive regions at this particular probability threshold. The individual extents of activation are

reported later in Table 3.4.

Figure 3.9 Incidence maps showing frequency-sensitive response around the auditory cortex in pilot study 1 Incidence maps are overlaid on the average anatomical image of (A) participants #1 to #3 and (B) participants #7 to #13, and shown in four axial slices. The z value shown on the top left of each slice The high-frequency regions are depicted in red and low-frequency sensitive regions in blue. Orientation: R to L.. Numbers on the figures represent the high- (2 and 4) and low-sensitive (1) regions that Talavage et al. (2000) identified.

Attentional modulation

First, the beta values were plotted for peak voxels in regions 1 and 2 in the left hemisphere for participant #3 (Figure 3.10). This example was chosen

because the results were representative of the other hemisphere and the other two participants.

Figure 3.10 (A): Axial view (z=+6) of left part of the anatomical scan of participant #3, upon which are shown two high frequency sensitive regions (in red) and one low-frequency sensitive region (in blue). The contrast performed for the high-frequency-sensitive regions was: just listen high-frequency majority stimulus>just listen low-frequency majority stimulus (A>B). The reverse contrast was performed for the low-frequency-sensitive regions (B>A). These clusters are partly located in one of the subdivisions of HG (according to the probability maps

(Eickhoff et al., 2005). The grey bars in the (B) and (C) show the beta values (arbitrary values) for each of the six conditions for the peak voxels of region 1 (B) and region 2 (C). The red bars represent the 90% confidence intervals.

In region 1 (Figure 3.10b), there was a greater response when

participants were instructed to attend to the low-frequency targets in the low- frequency-majority stimulus (column 6) than when attending to the high- frequency targets in the same stimulus (column 4). The difference between them was significant at p<0.05. This attention-specific enhancement is consistent with the attentional enhancement hypothesis. Also consistent with the general suppression hypothesis, the results in region 1 showed a reduced response when participants were instructed to attend to the high-frequency targets in the low-frequency-majority stimulus (column 4) than when they were instructed to passively listen to the same stimulus (column 2). This difference

was significant because the error bars are completely non-overlapping. However, an important finding that is inconsistent with the attentional

enhancement hypothesis was that attending to the low-frequency targets in the low-frequency-majority stimulus (column 6) generated a smaller response than passively listening to the same stimulus (column 2). The attentional

enhancement hypothesis would predict that attending to the low-frequency target increases the response in low-frequency-sensitive regions compared to both passive listening (general enhancement) and attending to the high- frequency targets in the same stimulus (attention-specific enhancement). For the high-frequency region 2 (Figure 3.10c), there was also a trend for a greater response for attend high-majority (column 3) than for attend low-frequency targets in the high-frequency-majority stimulus (column 5) but the response size was smaller and so these differences were not significant.

The second exploration of attentional modulation plotted the beta values for regions 1 and 2 across all three participants. The centre of region 1 was defined by the coordinate x=-46, y=-20 z=7 mm which showed overlap in two out of the three participants. The centre of region 2 was defined by the coordinate x=-42, y=-19 z=1 mm which showed response only for one participant. The beta values were extracted for all voxels contained within a sphere centred at this peak and with a 6 mm radius. Figure 3.11 shows the mean results and individual responses for these two regions plotted across the six experimental conditions. The pattern of attentional suppression was generally very similar to that seen previously for the peak voxel in participant #3, most notably in region 1 (shown in Figure 3.11B). In addition, the previous

namely there was a greater response when participants passively listened to the low-frequency-majority stimuli (column 2) compared to when they were detection low-frequency targets in the same stimulus (column 6). None of these effects reached significance, because there was a lot of variability across participants. In the high-frequency region, there was very little effect of the listening instructions on the magnitude of the response. The results showed less variability than in the low-frequency region.

Figure 3.11 Beta values for all conditions in (A) high-frequency-sensitive region 2 and (B) low-frequency-sensitive region 1 for all three participants. Grey bars represent the average value, while the symbols indicate the values for each individual participant.

It is interesting to comment on the lack of association between target- detection performance and the effect of attention on the size of the response in regions 1 and 2. For example, participant #3, who obtained the best

performance scores, showed the greatest reduction in the low-frequency- sensitive region for the ‘attend’ compared to the ‘just listen’ conditions.

3.2.3 Summary

Pilot study 1 had four aims. The first aim was to design a mixed- frequency stimulus that was suitable for separating high- and low-frequency- sensitive responses in the auditory cortex. While high- and low-frequency- sensitive regions were identified around HG, there was not very consistent overlap between individuals and participant #2 did not evoke reliable

frequency sensitive activation. It is possible that the voxel resolution used to acquire the data was too coarse to detect the small volume of the frequency- sensitive activity. Higher voxel resolution can assist to avoid partial volume effects; a voxel that is relatively large in volume could contain tissue that does not contribute to the MR signal, which results in reducing SNR (Huettel et al., 2004). The second aim was whether the spatial location of these responses matched those found in previous studies of tonotopy. This was confirmed in participants #1 and #3.

The third aim was to investigate whether the task was effective for manipulating selective attention. The answer to that from pilot study 1 was ’no’, because two out of three participants did not seem able to conform to the task instructions and there were a relatively high number of false alarms. The

reason was either because the training was inadequate or that the task was too difficult. Inadequate training could also be responsible for the lack of

understanding of the task instructions. The issue of poor performance is investigated in pilot study 2.

The fourth aim was to investigate whether there was attentional modulation of the auditory responses when attending to different sound frequencies. In the high-frequency regions of the three participants, both feature-specific enhancement and suppression were generally confirmed. The low-frequency regions showed quite unexpected results. Specifically, there was a smaller response for the ‘attend’ conditions compared to the ‘just listen’ conditions. This result could reflect an effect of presentation order. For all participants, the ‘just listen’ conditions were presented in the first run of the fMRI experiment and the ‘attend’ conditions were presented in the second run. A smaller response in the second run than the first run might reflect adaptation to the sound stimuli over time, unrelated to the listening instructions. A solution to this problem would be to fully randomize the order of the conditions.