fMRI task parameters influence hemodynamic activity in regions implicated in mental set switching

Abstract

Mental set switching is a complex executive function that is required when the focus of attention must be altered in order to adapt to a frequently-changing environment. While there is generally acceptance that switching is subserved by a fronto-parietal network, there is a considerable lack of consistency across studies as to other brain regions involved in executing mental set switches. This functional magnetic resonance imaging study sought to determine whether paradigmatic design aspects such as stimulus complexity, motor response complexity, and stimulus ordering could account for the differences in reporting of brain regions associated with mental set switching across previous studies. Several brain regions, including the striatum and anterior cingulate, previously associated with mental set switching were found to be related more to resolving intra-stimulus interference conferred by increased stimulus complexity and increased motor response complexity than to executing the mental set switch. In considering stimulus ordering, defined as the number of non-switch trials preceding a switch trial, brain activity was not observed in the fronto-parietal regions typically associated with switching but rather in regions in the anterior prefrontal cortex, sensorimotor cortex, and secondary visual cortices. Our results indicate that these important paradigm design aspects that are theoretically unrelated to set switching per se should be balanced and controlled for in future experiments, so as not to obscure clear identification of brain regions truly engaged in mental set switching.

Figure 1. Partial schematic of the paradigm design and examples of stimuli and associated parametric terms

A partial schematic of the paradigm is depicted in the upper portion of the figure, and example stimuli are shown in the lower portion of the figure. Participants were given a verbal cue 1.2 seconds prior to stimulus onset as to which stimulus attribute to count (color, shape, size), and this cue remained on the screen throughout the entire trial. Participants then had 4 seconds to respond with the appropriate button press (1,2,3). ISI stands in for the variable interstimulus interval. Each stimulus was assigned an integer value (1,2,3) based on the number of dimensions, corresponding to its complexity. Stimulus A was assigned a stimulus complexity value of 1, since it did not differ on any dimensions. Stimuli B and C each contained two dimensions (B: size and shape; C: shape and color) and were assigned a stimulus complexity value of 2. Stimulus D had three dimensions (shape, size, color) and was assigned a stimulus complexity value of 3. To determine the response complexity, the number of potential valid responses were counted for each stimuli, not assuming a specific cue. For stimulus A, there was only one potential response option, as there was only one color, one shape, and one size presented. The only valid button press was 1, and stimulus A was assigned a response complexity value of 1. For stimulus B, there were two potential response options, as there was one color, two sizes, and two shapes. Valid button presses were 1 and 2, and stimulus B was, thus, assigned a response complexity value of 2. For stimulus C, there were three potential response options: one size, two colors, and three shapes (valid button presses were 1, 2, and 3). A response complexity value of 3 was assigned to stimulus 3. Finally, for stimulus D, there was again only one potential response option, as there were three sizes, three shapes, and three colors (valid button press was 3).

Figure 2. Behavioral data from Design Factor x Trial Type interaction

Graphs of the Design Factor x Trial Type interaction for the three design factors, where switch trial reaction times are shown in BLACK and non-switch trial reaction times in GRAY. For stimulus and response complexity, an overall slowing with increased complexity was observed. Of note, is that for trials with only one potential response option, there was an extinguishing of switch costs. In considering the number of preceding non-switch trials, there is an overall improvement in behavioral performance up to three preceding non-switch trials. For switch trials preceded by four or more non-switch trials, continued improvement is not observed, and switch costs actually revert back to the original levels of switch trials preceded by only one non-switch trial.

Figure 3. Main effects of switching

Regions with greater activity during switch trials are shown in YELLOW/ORANGE, and regions with greater activity during non-switch trials are shown in BLUE/GREEN. The t-maps have been thresholded at p < 0.05, corrected for multiple comparisons using False Discovery Rate (FDR). Color bars are expressed in terms of t-scores, and images are displayed in normal convention (left = left).

Figure 4. A: Parametric effects of stimulus complexity

Regions with greater activity for trials with high stimulus complexity are shown in YELLOW/ORANGE, while regions with greater activity for trials with low stimulus complexity are shown in GREEN/BLUE. B: Effects of stimulus complexity on switching. Stimulus complexity related regions with greater activity during switch trials are shown in YELLOW/ORANGE. Stimulus complexity related regions with greater activity during non-switch trials are shown in GREEN/BLUE. C: Parametric effects of response complexity. Regions with greater activity for trials with greater number of potential response options are shown in YELLOW/ORANGE, while regions with greater activity for trials with fewer number of response options are shown in GREEN/BLUE. D: Effects of response complexity on switching. Response complexity related regions with greater activity during switch trials are shown in YELLOW/ORANGE. Response complexity related regions with greater activity during non-switch trials are shown in GREEN/BLUE. All t-maps have been thresholded at p < 0.05, corrected for multiple comparisons (FDR). Color bars are expressed in terms of t-scores, and images are displayed in normal convention.

Figure 5. Parametric effects of number of preceding non-switch trials

Regions with greater activity for trials preceded by a greater number of non-switch trials are shown in YELLOW/ORANGE, while regions with greater activity for trials preceded by a lesser number of non-switch trials are shown in GREEN/BLUE. All t-maps have been thresholded at p < 0.05, corrected for multiple comparisons (FDR). Color bars are expressed in terms of t-scores, and images are displayed in normal convention.